Automotive Race car suspension Class

[Signguy]

Just by watching the cars running it at the Derby that won and have it figured out already it was essentially moving like a dirt car, the left rear tire was traveling into the fender well, the rear of the car was rising and the left ear tire was just being pounded into the pavement. We all know modifieds with 8" tires are an animal at the end of the race but mot anymore with this setup. same thing with the super Late models. I will add they are running the rear calipers on the bottom of the axle tube also. Yet again this is things I've seen on dirt but am not familiar with dirt as we've always ran pavement.

Several of the faster cars this year were "conventional" three link cars (DQ'd winner, and pole sitter for sure). Looking at them from a distance it appears they run a little more angle on the lower trailing arms, and the upper closer to the axle tube center than the classic recommendation. With a more "drag car" like IC, I'm perplexed as to how they soak up the off-throttle reaction. In the past when I've tried that it was automatic wheel hop. Perhaps the high spring rates and advances in shocks.

The other part of the puzzle is the new take on front...lot's of LF anti-jacking while the RF stays very similar to old-school.

If I'm not mistaken, this is representative of a fast car at the Derby this year.

 

[Signguy]

Couple of quick observations. This Center pull suspension with J bar links combines the best of 4 link and 3 link suspensions. The 4 link is the best set up for dirt as you need maximum traction tuning. This center pull set up eliminates the lower links of the 3 and 4 link suspension. Less weight hanging below the axel and less garbage to get snagged up on a bad track. Everything is above the rear axel tube out of harms way.

Look at the Instant center angle. I do not have the hard numbers but it sure looks like the intersection of the two J bars and the pull bar is way out in front of the axle. Like 10 foot in front of the car! We want the angles of the rear suspension linkage to form an Instant Center (IC) as far forward as possible. This longer lever cushions the acceleration when the tires hook up. This set up does not have a shock damper on the top link. Too short an IC distance means we have a lot of angle change on both sides and the car will be real darty when you nail it. Longer is better and gives the driver more control because you don't have a lot of angle change as the shorter IC has. The top link angle can be zero to 5 degrees down hill for traction.See attached forces on rear axle -
It is easier to pull a chain than push it. Race cars love forces that directly pull versus forcers that push and this set up is strictly a PULL. All links are pulling on the chassis. We have more initial static angle on the driver side of 6 ° and the right side angle of 12 ° . This means the car in chassis roll will grow more angle in chassis ROLL as the right side angle reduces as the right side springs compress. This is a feature that 3 link has but to a much lesser degree due to the limitations of the lower links. About the only thing you could to bias the rear wheel loading was to off set the top link mount. With the J bar design you can dial in rear wheel load directly to each wheel.

This is HUGE!
Question for you to ponder- What happens when the rear end is in Roll? What happens to wheel base on left and right side?

RM, I've been watching this center-pull setup with keen interest since it first started winning (lots of aero on outlaw bodied cars). Firstly trying to figure out how to model it to understand if it's actually better in force management and location or just a shiny object (winners win, builders sell cars). On the modeling since it appears to give another element of freedom in the linkage. Also, one spring/shock assembly is mounted in front of the axle which helps on the off-throttle torques. So far, the best way can understand the IC effect is to model it like IRS, from the center of the axle tube and not from the tire patch. A second reason I lean toward that understanding is that the spring rates used in the competitive cars are very stiff. Another reason is that the actually-running setup I tried to model would up having the linkage intersection point looking like that of a Viper or modern Corvette, really high. Racers are using bumpstops in both rears with very high rates. Also, very low rear roll centers. Panhards under the driveshaft, 6"-7" RC height. So with the steep upward angles you get a lot of desired roll steer, but the bad potential for wheel hop checked by more freedom and dampening than traditional. I'm guessing, but my real question is how to appropriately model the forces.
As I noted to STT93, though, traditional 3 links are still running fast...but with a lot of angle in the links. No idea on the spring rates on those though. Those cars are on bumps on the front, so I'm educated-guessing 1500-2000+ lb/in front, 400-900 on the rear. Can super high tech shocks and high spring rates (less travel) make the difference on wheel hop issues with radical IC's??

 

[Ranger Mike]

Rear steer YAW
A rear wheel drive solid axel race car with a 3 link suspension has the ability to move up and down. That is pretty obvious when you put a floor jack under the race car.

The whole axel housing can ROLL as in turn entry. The housing can Pitch ( like you pitch a softball) on acceleration. What is not so obvious is the rear end experiencing YAW.

Yaw is Roll Steer. It is roll over steer or roll understeer. This is caused by rear linkages set at different angles. If the driver side link angle is 6 ° up hill and the right side link is set at 12 ° up hill in static the rear end will yaw as the car rolls in the turn. When you are at speed and enter the turn, the left side wheel base length will shrink and the right side wheel base will grow. Watch the left rear wheel move toward the front of the rear wheel well on a dirt super late model entering a corner at speed. Dynamic roll lover steer.

See photo below on insane rear end roll steer.

3 Link rear end in action. Yaw is not so obvious as the link angles are set very close to each other.

4 link in action – note wheel base changes


This video reall shows the gimbal effect.

see post 1200 on page 48 for more roll steer detail.
Roll oversteer allows a race car to roll around a corner without breaking traction. When forward grip stops, the tire starts to slide toward the outside. Without rear roll oversteer the car has to break traction to turn. Now you are truly dirt tracking. When you have roll oversteer it steers the rear end to maintain grip and you do not break traction. This concept has been out there for years but did not get popular on paved tracks until recently. Now you will start to see the left side wheel moving to the front more vs. the old set up. Now you know what too look for. Driver side rear wheel well.

 

[dontlift19]

I haven't seen it specifically discussed, but should caster change be kept to a minimum during suspension travel? I'm asking in regards to dirt oval racing. In classes that have to use the stock suspension but are allowed to relocate the upper control arm mounts this would mean matching the top view angle of the upper control arms to the lower control arms. Would like to hear y'all's input on this, thanks.

 

[Ranger Mike]

Happy New Year - read post #319 on page 13
worry about camber change more..my opinion

 

[Ranger Mike]

F1 2020 rules change
Teams limited to only $175 million per year
things are tuff all over!

 

[Cavemans SPA]

1meyra welcome
I assume you run paved tracks. Spindle height most common on super lates is 7.75 inch. It has lower Roll center which means less camber change through spring compression. Spindle kingpin inclination can be between 5 and 10 degrees. Five years ago 8 1/2 degrees was

That's a response to a post regarding spindle height.

Is there any effect on spindle height as it relates to tire circumference? For example, if race ready tire circumference is 80" the center of that is roughly 12 3/4" (80 / PI / 2).

In this case, would the ideal spindle height be as close to 12 3/4" as possible as long as you're desirable roll center can be achieved?

 

[Ranger Mike]

Spindle height is distance from upper ball joint to the center line of the spindle pin. The only bearing the tire diameter has is where it places the ball joint centers relative to the pavement. This is why you just about have to have a good software program to follow things like tire diameter and its effect on the spindel height , effects on suspension link angles that make instant centers that make Roll Centers.

 

[Cavemans SPA]

Makes sense Ranger Mike. We're using spacers on the top and bottom of the spindle to move the upper and lower ball joints to get the desired Y coordinates for roll center.

But got to thinking about what would be the optimal spindle height. The attached image shows the brown line is center of upper and lower balljoints and the purple is where you end up with spacers to get all points and match spindle snout to tire.

The brown and purple lines are going to move along the travel arc at differently, so to your point, you need to understand the dynamic affects of this stuff.

Is Performance Trends Suspension Analyzer still the recommended software these days?

BTW, thanks for sharing all your knowledge.

 

[Ranger Mike]

Thanks fro the input Cave
i like Perf Trends but there are many other softwares out there. I like sys Analyzer as it has dynamic graphics ti display where things are moving during roll and dive. price is good too...

 

[Garage Engineer]

Hi Ranger Mike
Question: I have noticed that on a (rear) solid axle sprint car or world of outlaw car that the right rear link on the z-link rear suspension is angled up-hill yet they run the upper link on the LR down-hill

Am I safe to "assume" that they are doing this to help generate anti-squat in the RR corner and thus bite the RR tire harder mid corner into the dirt but at the same time remove the anti-squat from the LR and let the weight just kinda of dump backwards from the RF to LR on corner exit?

Curious to hear your take on this

Thanks

 

[Ranger Mike]

welcome gar eng
think about what is happening on chassis roll. The RR link that was up hill is now level, the LR link that was down hill is now level, so what has happened to the wheel base on each side?

do not get caught up in the anti squat thing. It can help/hurt forward bite or braking depending on how weird you want to get.
no one let's the " weight " dump on its own. Racers are real control freaks and want to control everything ( ask the Ex) so compression/ rebound are very important when cornering

 

[Jtcox3000]

Ranger Mike, I’m at a loss on how to get the left front down. This is a new car, moved roll center left, raised motor up 1/2” and to the left 1”.

 

[LogicIndustries]

If the car won't turn at all, put a stiffer right rear spring and a softer right front spring in it and a limiter chain on the left rear. That's a big swing, but it'll definitely flatten out the car's attitude in the corner. Might need to make clearance for the RF tie rod against the frame horn if you soften the RF up much though, so bear that in mind.

If it's turning a little but still needs help, maybe just go with the stiffer RR and the limiter chain.

If the car is turning just fine as-is (hard to believe, but could be I guess), you could try just going to the limiter chain on the LR.

You could also try a longer shock on the LF or put an extension on the one you have to give you more droop. That won't do much good if the spring is already slack though.

You've got a lot more rear body roll than the front suspension can handle, so you fixing your issue is going to be mostly centered around the rear suspension.

 

[Jtcox3000]

LogicIndustries thank you for your response. We did limit the LR hike more, increased RR top rod angle, stiffened up RF and RR spring and problem continued. As the track dries out and gets slick, we lower RR top rod and the car gets extremely loose on throttle center out. Believe it or not, the car actually turns in well, but very difficult to drive in traffic. So, moving RC further left wouldn’t correct the problem?

 

[Jtcox3000]

LogicIndustries thank you for your response. We did limit the LR hike more, increased RR top rod angle, stiffened up RF and RR spring and problem continued. As the track dries out and gets slick, we lower RR top rod and the car gets extremely loose on throttle center out. Believe it or not, the car actually turns in well, but very difficult to drive in traffic. So, moving RC further left wouldn’t correct the problem?

I read a ton of great info from Ranger Mike and others on what too look at.

 

[LogicIndustries]

It might, but the car doesn't "feel" the roll center laterally in the rear like it does in the front, especially on dirt. The height and length of the bar (when you get to the REALLY short panhard bars, like 8"-10" long) just seems to matter a lot more.

If the car turns in OK but is loose off, you might want to look at your rear steer to make sure you're not breaking the tires loose in the center by not steering them enough (or conversely you could be rolled over so far that you've got so much steer that you can't null it out coming off the corner).

At the most basic level here as it is setup now, your car "wants" to roll more in the rear than in the front (front overturning resistance is higher than the rear overturning resistance), ergo the front rolls to where it is willing to go and then stops, but the rear keeps rolling over and drags the left front up into the air as it does so.

Some of this is just the difference in the amount of articulation the front suspension has vs the rear, and that can't really be "fixed" due to the rule package you're operating under.

If it were my car and the RF frame rail is not already down in the dirt, I'd soften up the RF spring until it started to hit the ground ever so slightly when you go through big holes, but did NOT hit the ground on a smooth track. You want to go as low as you can on that side without constantly slamming the frame into the dirt.

Then I'd stiffen up the RR spring until the LF tire just about touches the ground and then get the rest from the limiter on the LR.

The LF tire is never going to carry a lot of load on a dirt car, but it's not doing anything at all hiked up in the air like that, so IMO you need to concentrate on getting it down on the ground again and then see how your drive off is and tune your rear steer to get you through the center and off.

 

[Ranger Mike]

Thanks for the kind words, JT

What size ARB (sway bar)??
Can you tell me the basics…??

% rear weight

% left side weight

% cross weight?

Roll Center height and offset in front

In back?

I assume you have 4 link rear set up

What top and bottom rod settings on both sides?

How much rear roll steer are you running?
Now tell me the tire temperatures inside middle and outside each tire on each corner

 

[Ranger Mike]

Things to adjust at the track

It took me a long time to really understand the importance of Roll Center location and Offset. Like Gravity, you cannot see it but you better believe it is there and it works. Think not? Just drop that 8 Lbs. sledge hammer on your right toe!
Adjust your MIND.

By this I mean think 4-D. Everything happening on the race car is moving in 3 directions (X,Y and Z) and has a Time element to it. Imagine in your mind the xyz movement and the time period.
Lets review our Go Kart days. We first learned that we had to deal with things on a race track in 2-D. We were racing in an X,Y world and occassionaly hit the Z axis world when we lifted the left front kart tire off the ground. It gave us a clue that there was a 3rd axis in play going up and down in the Z axis. We also learned there was a thing called torsion or Twist caused by Roll forces around the X axis.
When we graduated to the real big boy race car hobby stocks with real suspensions we had a whole re-think coming. Things got complicated when the “pros” at the track threw terms like stagger and wedge and weight transfer around.

These guys really must know their stuff! Actually they only knew and had heard about how to band-aid an ill handling car. Here is how you get your 4-D mind right.
In racing it is all about tires, Tires, TIRES. Tire contact patch is critical to understand. Tire alignment ( toe in / toe out, camber, caster, bump steer, rear roll steer), Tire pressures (stagger) and Tire Temperature. You absolutely must know and memorize all things regarding proper reading of tire temperatures.
Tire temperature are taken on inside , middle and outside. Average tire temp is exactly as stated. You add them up and divide by 3. What is the average diagonal tire temp supposed to be rt ft to lf rear? What is left front to right front average supposed to be? What is proper right front to right rear average? You have to know these and a lot more , for correct chassis set up.In racing you must know and understand impact of vector forces that occur in the corner.

Momentum wants to keep you traveling straight. Tires through the steering links change direction while still gripping the track and force the car to change direction. We have added this resultant vector of about 45 degrees from straight ahead. Add to this mix the fact we have chassis roll from left to right loading the outside tires. Now we add the interaction of these forces going through the front and rear Roll Centers. Up front we have offset the RC to one side (right side by 3 inches on paved tracks). This offset adds down force to the right front tire. You do not want offset on a road course race car since you are turning left and right. You want to bias the offset in left turn only race cars to add more down force to the outside wheel. Too much offset and you lift the left front tire.



Down force quick tutorial - Take a tire ( mounted on the wheel of course) and stand it up on its wide tread. Now facing the wheel Grip both sides like you are going to put it on the wheel lugs. Slide it on the garage floor. Slides pretty easy, right? Now have Lumpy the fat neighbor kid, sit on the tire and try this again. Sliding a 40 pound tire/wheel was easy but sliding a 165 pound tire/wheel/ fat kid set up is way more difficult. This is what happens when you add DOWN FORCE. More grip.
In order to win you must learn about down force and its effects on the Tires on each corner. Down force ( aero spoiler settings , cross weight % ( wedge), ballast weight placement and rear 3 or 4 Link settings all effect down force. So do the springs, and ARB (sway bar). This is why I have an obsession with you understanding Roll centers. It is the first step in building the car to win. Ride height effects down force but is pretty much dictated by rules on how low you can go.Roll Center set up should be done in the garage not at the track. You are adjusting down force on the right front tire when done properly. Same with ride height settings. When you monkey with ride height at the track it screws with the Roll Center.
Where does 4-D come in – Dampers (shocks) - these 4 dampers manage how the force vectors work on the cars suspension. It is all about TIME. How much down force you want is stated above. Swap out springs , adjust wedge, stiffen loosen the ARB to get the proper down force as shown by tire temperatures. But you have to figure when you want the down force and remember it is controlled by shocks.You may want to add shock to the 4 link to keep in in rear roll steer longer than the other guy..hint..
I got to head to the plane to catch a race..later

 

[Jtcox3000]

Thanks for the kind words, JT

What size ARB (sway bar)??
Can you tell me the basics…??

% rear weight

% left side weight

% cross weight?

Roll Center height and offset in front

In back?

I assume you have 4 link rear set up

What top and bottom rod settings on both sides?

How much rear roll steer are you running?
Now tell me the tire temperatures inside middle and outside each tire on each corner

I will get you the info when I get to shop and pull up my setup notes for that night. Probably won’t be until Monday evening Ranger Mike. Thank you for all the help.

 

[Ranger Mike]

Ifin racing for 55 years don't kill ya, no off shore bug will!
Things we all done but as of yet have not killed us.
No worries

The good die young .. I got some time

 

[dontlift19]

Might not be the right place to ask this, but maybe ya'll could point me in the right direction. I'm trying to put together a budget data acquisition system to use on my dirt race car. For now I'm just going to log the travel at the front wheels and g force with a 3-axis accelerometer. But I'm a little confused as to where to place the accelerometer in the car. From the looking I've done, some just place it wherever is convenient, and others say that it has to be as close to the center of gravity as possible, but they don't explain why. I've even seen some people use 2 accelerometers, one in the front and back, to see which end of the car is gripping better. Intuition is telling me the center of the car, but that's going to take a little fabrication as there's no good place to mount the accelerometer in the middle of the car. The other problem I'm foreseeing is that dirt cars produce so much roll angle in the turns that axes of the accelerometer will no longer be pointing laterally and longitudinally. So the readings I get won't be true lateral and longitudinal g force. I haven't seen this talked about anywhere, but it seems to me you'd have to come up with some way to measure the angle of the car in the turns (with a gyroscope maybe?), then use some trig to combine the correct components of the acceleration vectors to get a true reading. Let me know what ya'll think, thanks!

 

[Ranger Mike]

you got too much other race stuff to worry about , concetrate on winning. Dont re-invent the wheel. Will cost way more trial and error $$$$$ than spending $ 1700 for what is already developed and working.
https://www.pegasusautoracing.com/document.asp?DocID=TECH00148
AiM EVO5 Data Logger, no display, 1.3 Meter Cable

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Features include:

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• 4 wheel speed input channels*
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Kit includes EVO5 logger, GPS08 module (1.3m cable), primary wiring harness with 37-pin Deutsch connector, and Race Studio 3 software.

Note: This is a data logger only. You must add the AiM GS-Dash Display (Part No. MC-232, sold separately) in order to display the data without downloading to a computer. No display is included with this logger. Use of a dash display along with GPS also requires the addition of the AiM 4-Port CAN Data Hub (Part No. MC-329, sold separately).

 

[dontlift19]

Thanks, I looked through the specs and installation instructions of that data logger and got both of my questions answered.

Even at $1700 that doesn't include suspension travel sensors, those are another $300 each. I just graduated college a few months ago and can't justify spending that kind of money right now. I'm going to try and put my mechanical engineering degree to use and make my own, we'll see how well that works out lol.

Do you have any experience with the external shock travel indicators? Seems like they might work okay for asphalt, but on dirt where the track isn't as smooth the travel indicators wouldn't be very accurate. I think the only way to get really accurate dive and roll numbers to use in a front geometry software is to use a data logger with suspension travel sensors.

 

[Ranger Mike]

i wrote a lot about travel indicators on dampers on this post. ifin you done have the big bucks to figure accurate dive and roll, you still get a lot of info from these. and i would point out on dirt you have 4 differing surfaces thought the night. from slushy to hard packed. so the analogy about paved track is 1/4 to 1/2 right..savvy?

 

[chop6259]

Might not be the right place to ask this, but maybe ya'll could point me in the right direction. I'm trying to put together a budget data acquisition system to use on my dirt race car. For now I'm just going to log the travel at the front wheels and g force with a 3-axis accelerometer. But I'm a little confused as to where to place the accelerometer in the car. From the looking I've done, some just place it wherever is convenient, and others say that it has to be as close to the center of gravity as possible, but they don't explain why. I've even seen some people use 2 accelerometers, one in the front and back, to see which end of the car is gripping better. Intuition is telling me the center of the car, but that's going to take a little fabrication as there's no good place to mount the accelerometer in the middle of the car. The other problem I'm foreseeing is that dirt cars produce so much roll angle in the turns that axes of the accelerometer will no longer be pointing laterally and longitudinally. So the readings I get won't be true lateral and longitudinal g force. I haven't seen this talked about anywhere, but it seems to me you'd have to come up with some way to measure the angle of the car in the turns (with a gyroscope maybe?), then use some trig to combine the correct components of the acceleration vectors to get a true reading. Let me know what ya'll think, thanks!

@donlift19 -- Check out Autosport Labs. They have a very economical open source daq system. I picked one up a few years back for around $500. It has about everything you will need minus the sensors. I used it on a dirt late model.

As far as the mounting point, the idea is to put it near the center of rotation of the car (not necessarily the cg point); however, I would recommend putting it somewhere near this point, but also where it is easy to get to so you can plug in/pull the data card easily. I generally mounted it next to the cockpit on the top deck during test & tunes, and in the electrical compartment next to the MSD box if we ever tried to sneak it in during a race. I wouldn't worry so much about the mounting angle, so long as you are consistent.

I have seen people mount it with a 4-5 degree angle so it is near level in the corner. Honestly, the g-force data will most likely not be of much use unless you have a very good signal filter. It is definitely not a smooth trace. I ran one linear pot at each corner shock, and one on the fifth-arm shock. I wanted to put one on the rack too but didn't have the funds at the time. The key thing is consistency. You will most likely end up using it as more of an "A-B" analysis tool rather than an analytical tool unless you have some kind of model to run the data in.

I wrote my own kinematic model in Python and would plug the linear pot outputs into see rear steer, four-link ICs, etc. One big benefit is to set up the segments on your track and use the GPS to get segment times. Then you can see what changes would impact times at different sections of the track.

 

[dontlift19]

Thanks for the info chop6259. I think you're right, the more I look into it the gps data isn't as useful as I thought it was going to be so I've sidelined that part of the project. But I've got the rest of it working and I'm only into it around $100.

Got one for you Ranger Mike, how to find the roll center on an asymmetrical triangulated four link. All of the stuff I've seen online is for symmetrical, never for asymmetrical. This is on a GM metric chassis. From a top view, the trailing arms are all symmetrical. But from a side view all 4 of the trailing arms have different angles. The upper trailing arms are fairly close in angle, within a couple degrees. The biggest difference is in the lowers, the left lower has around 15 degrees, and the right lower has around 3 degrees. The upper and lower trailing arms never form instant centers because their paths never cross, so I'm thinking it has something to do with taking an average instant center height between the left and right trailing arms but I'm not sure. Let me know what you think, thanks.

One of the articles I was referencing: https://www.onedirt.com/tech/chassis-suspension/finding-your-center-finding-your-front-and-rear-roll-center/

 

[Ranger Mike]

when you buy the software and input the susepsion settings it replicates your actual set up. So if you run a longer right side upper and lower A-arm it will show up and calculate the RC and other info. Same with elevation readings. All built in and well worth the $$$
also will absolutely work on metric frame!

 

[drobbie]

You are so right Ranger Mike, the software is great.
I'd like to expand a little on dontlift19's question but more so when designing a front end that might have unequal lowers. In reading the earlier posts you go over the design of a front end and the placement of the roll center a few inches to the right of centerline. What if you wanted to design the optimum front end for an offset chassis car? Would you even up the lowers arm and move the mounting points or center the mounting points within the chassis and use uneven lower arms. (Note: I haven't used my software enough to get a great grasp on what the impact will be so looking for a little head start)

Thanks!

 

[Ranger Mike]

Lets review. The reason we have non equal a-arms is for maximum placement of weight on left side. We offset the front roll center to the right is to load the right front tire with down force to get maximum traction.

When we off set the engine to the left (vs stock location) we make the lever distance between the engine camshaft center line (COG) to the front RC longer. More leverage. If we hook up too much on right front we may lift the left front tire (sprint car style) so we have to be careful when we go max route on anyone setting. In outlaw anything goes super late models you do it all. The longer a-arm thing will take more time to get RC right but when you go all out, it takes time and money. More restricted classes dictate the rules and what you have to do.

There are so many variables that your question is pretty hard to answer. Ifin you are designing the ultimate front end I would go with the Indy car design of both lower A-arm mounts being equal height above track ( parallel ) same as upper mount points. Go max left side weight

Max engine offset, driver offset even rear end housing change to shorter axel on left side. Low left and fast. Then tweat ROLL center offset and location and minimum of RC migration ( within an inch of static at max roll).

 

[dontlift19]

My question was in reference to the rear suspension, not front. I have roll center software that does rear suspension, but it will only work for a symmetrical triangulated four link. I can do it manually with a CAD program but I’m not exactly sure how because the instant centers for the upper and lower trailing arms never intersect. The article I linked has a diagram of the GM metric triangulated four link rear suspension in case you aren’t familiar with it.

 

[Ranger Mike]

Dontl
the artice lyou attached tells you how to calculate rear RC. Even though the 4 links do not meet in anyone plane, the RC height is easy to calulate. The RC location to vehile centerline is a midpoint between link mount points on top of rear axle.

My Circle Track Analyzer has a simplified 4 link rear suspension recommended for 4 link Chevelle.
You are pretty limited on adjustments with metric 4 link.
It is what it is and since you can not adjust it up or down, I would concentrate on adding as much rear roll steer as you can get to it. Hope this helps.

 

[Jtcox3000]

View attachment 257102
Ranger Mike, I’m at a loss on how to get the left front down. This is a new car, moved roll center left, raised motor up 1/2” and to the left 1”.

Man what a difference a year makes! Hey Guys, just wanted to thank you for the input you gave me on our dirt mod that was lifting the left front Really bad! We went from a mid pack race car to now winning our 7th race of the year. We won 2 track championships and a Sunoco summer series event this year alone. Couldn’t have done it without you guys and this awesome forum! And yes this is the same car!

 

[Ranger Mike]

dont forget the cardboard box to put the trophy in!

 

[Jtcox3000]

Ranger Mike, logicindustries you guys are the best! Couldn’t have done it without you both. So thankful for this forum, the information is here! just got to read it, understand it and apply it! Felt like a real dummy asking for help but now I’m so glad I did!

 

[Wheelman71]

Man what a difference a year makes! Hey Guys, just wanted to thank you for the input you gave me on our dirt mod that was lifting the left front Really bad! We went from a mid pack race car to now winning our 7th race of the year. We won 2 track championships and a Sunoco summer series event this year alone. Couldn’t have done it without you guys and this awesome forum! And yes this is the same car!

View attachment 270274

Congrats.

 

[Fiveothis]

Ranger Mike, logicindustries you guys are the best! Couldn’t have done it without you both. So thankful for this forum, the information is here! just got to read it, understand it and apply it! Felt like a real dummy asking for help but now I’m so glad I did!

Can you share with us what all changes you made from the recommendations, and what you think worked and what didn't work?

 

[Ranger Mike]

Glad to answer specific questions as they are asked.
The only dumb question is one that is not asked.

 

[Wylde]

Hey RM,

Every time I have a question I always revisit this forum (have intermittently frequented since 2014). I know you are a big proponent for the performance trends software and that is my next investment. While I'm budgeting the cash for this I do have a question.

While I was mapping out the rear suspension of my dirt late model I was thinking about instant centers. On the dirt cars we do have birdcages on both sides of the rear axle which is where my question lies. In order to calculate the instant centers of the rear end you have to use the lower bars for sure. Although, I'm not sure if you would use any other links to capture the instant centers. Both sides upper bars are angled in the positive direction so you would get no convergence other than aft axle. The lift arm is level at static so that wouldn't be the correct option either. The one given i have is that obviously we do have a positive anti-squat value on the left rear given that it hikes as soon as throttle is applied. Would you suggest just using the just lower bar to map out instant center? I've consulted all of the textbooks I have and have not found much in the way of instant center calculation for this scenario. I'm interested in the theory as it seems to help me conceptualize my direction, execution and optimization.

Also, do you have any reference that would help me understand the math of the instant center? I see a lot of articles referencing 100% but I'm not understanding what the base value and calculated value is that would yield your specific percentage.

I've done a ton of work mapping and optimizing lift bar (torque arm) application and optimization. This is the next piece to the puzzle.

 

[Ranger Mike]

appreciate the kind words. i am out of town and i n a snowstorm. ill get back soon

 

[Ranger Mike]

wyld
please read post 301 on page 13 and 1399 on page 56
i will fire up the computer and get details on what info the software needs when looking at rear end

 

[chop6259]

Hey RM,

Every time I have a question I always revisit this forum (have intermittently frequented since 2014). I know you are a big proponent for the performance trends software and that is my next investment. While I'm budgeting the cash for this I do have a question.

While I was mapping out the rear suspension of my dirt late model I was thinking about instant centers. On the dirt cars we do have birdcages on both sides of the rear axle which is where my question lies. In order to calculate the instant centers of the rear end you have to use the lower bars for sure. Although, I'm not sure if you would use any other links to capture the instant centers. Both sides upper bars are angled in the positive direction so you would get no convergence other than aft axle. The lift arm is level at static so that wouldn't be the correct option either. The one given i have is that obviously we do have a positive anti-squat value on the left rear given that it hikes as soon as throttle is applied. Would you suggest just using the just lower bar to map out instant center? I've consulted all of the textbooks I have and have not found much in the way of instant center calculation for this scenario. I'm interested in the theory as it seems to help me conceptualize my direction, execution and optimization.

Also, do you have any reference that would help me understand the math of the instant center? I see a lot of articles referencing 100% but I'm not understanding what the base value and calculated value is that would yield your specific percentage.

I've done a ton of work mapping and optimizing lift bar (torque arm) application and optimization. This is the next piece to the puzzle.

Keep in mind that on a dirt late model you have a live axle decoupled suspension, so the typical anti-squat calculations you would normally find in textbooks for 3-link suspensions etc. do not apply. You basically have four sources of anti-squat in a typical dirt late model utilizing a 4-link rear suspension.

The forces generated by the 5th arm lift bar, the forces generated by the left set of four bar links, the forces generated by the right set of four bar links, and the forces generated by the j-bar. The 5th arm forces are a direct response from axle wrap, so that reaction force is pretty straight forward. You then have to determine the instant centers for both the j-bar system of links and the left and right four bar systems of links.

The anti-squat from the j-bar system is generated by taking a moment about that systems IC from the lateral forces at each of the rear tires. The left and right four bar links are determined by taking a moment about their ICs as well. The force you use is the force that is generated at the axle tube by the rotating tire. This approach is only valid because the 4-bar links systems (via the rotating birdcages) are decoupled from the axle tube.

Keep in mind that these calculations on their own are pretty useless other than for conceptual purposes. They should be used in a large set of equations that define the vehicle as a whole, where the anti-squat forces are but a small part of the overall picture. As for your question about how to determine the 4-bar link instant centers. You would have to project the links onto a two dimensional plane (i.e. a sideview) and find the intersection of the projected lines from the top and bottom 4 link bar. It will be below the ground plane and behind the tire CP on the left rear for example.

 

[Ranger Mike]

excellent reply Chop
Anti squat is a production automobile parameter to try to get some performance out of the grocery getter car design and of little value in a tunable purpose built race car. All you have to know on Rear IC is longer is better.
I suggest you work on mapping the rear end rotation on roll and measure pitch and yaw as well. know how much each link changes the set up and learn WHY. Software on the rear end just uses the lower arm mount points and the older version i have does not address diet car set ups.

 

[UFO]

RM Thanks for all you do for this forum. There is no other with so much interesting info. I would like to give some of my thoughts on dirt late model suspension. Please refer to the drawing below. This is weight transfer to the rear on straight forward acceleration without consideration to the dampers and a few other forces.

With acceleration we only have to worry about forces from one end of the car, the rear. Unlike decell and lateral G forces which have resisting forces from all four corners. Similarly to lateral there are six major things to consider after the wheel base and the center of gravity height are established. They are the elastometric weight transfer, geometric transfer, which consists of the height of the forward drive force compared to the height of the center of gravity of the sprung mass, the jacking effect of the forward drive force, and the lift of the torque control device on the rear axle. The last is the aero drag height, center, and magnitude, and down force location, and magnitude. On a track that has o lot of speed change in the straight the aero will be the dominating forces at the end and the others will dominate off the turn where the acceleration rate is much higher. All will be varied dynamically with changes suspension height and velocity.

This straight forward acceleration may never happen to some short tracks, especially if you run the inside where the straight is rounded by running up to the wall in the center. We must normally consider a combination of lateral and longitudinal on most parts of the track, which makes things much more interesting.

We know we cannot change the total weight transfer to the rear without changing center of gravity height, wheel base, or acceleration. We cannot change them easily, but we can control, to some extent how much is handled by the left and right rear. On tracks that have a lot of bite you are not off the throttle much at all. On tracks that are low traction you are going from low throttle to high coming off the turn putting the rear suspension through a lot more travel with high anti squat. In that case I think anti squat is very important. That is because as you get on the throttle the mass of the sprung weight is moving away from the track and the opposite reaction is pushing the rear axle into the track. This does not last long, only as the frame is moving up but it does give a head start on the traction cycle. When you hit the throttle there is only static weight on the rear and as the car moves forward weight is transferred to the rear giving more traction and allowing more acceleration. This cycle keeps building off the turn, but can have a head start with anti squat before it starts to accelerate forward.

Aero down force also increases with velocity. With a late model having a high horsepower to weight ratio you can afford to lift that wide body and spoiler as far up in the air as you can, but now that is limited by the rules. Another help in the turns is the high degree of roll of the body which directs the aero down force on an angle, putting more on the inside tire. The late models have gone to a ton of dynamic cross weight by standing on the left trailing arms. That makes a lot of inside weight, traction, and drive. They then use a lot of rear steer to help turn and this also puts the car on an angle to the direction of travel, but the rear wheels more in the direction of travel not breaking traction as much. That helps maintain the forward drive, and also makes the body side panel deflect the air helping resist centripetal force. This setup is geared more to a slick track.

The drawing shows how links and torque arm effect loading on the rear axle and unload the springs with acceleration. The gain in down force on the rear tire is not in addition to weight transfer from acceleration but part of it. The drawing shows a static position, the instant center and force direction and magnitude are changing dynamically and constantly. These are the basic forces, there are more involved.

I think I ran on long enough for one post. Please let me know any opinions on this.

 

[Ranger Mike]

ufo
i got halfway thru your post above, and stopped. was replying to each item but it became readily apparent you have not read thru this long 62 page class. if you had you would have seen my posts addressing each and every aspect you chose to comment on. i suggest you read and revisit your post.

 

[berkeman]

ufo
i got halfway thru your post above, and stopped. was replying to each item but it became readily apparent you have not read thru this long 62 page class. if you had you would have seen my posts addressing each and every aspect you chose to comment on. i suggest you read and revisit your post.

One tip for @UFO -- You can use the SEARCH feature in the upper right corner to search just this thread for keywords or phrases. Just pull down the "Everywhere" default for where to search, and change it to "This thread".

 

[UFO]

Thanks for the tip Berkeman. That is a cool tool to get around on this forum.

RM, sorry I didn't mean to be repetitive, this is the first time I ever posted on a forum. I'm a slow reader and started reading this a long time ago and also have a good memory, but it's short so I don't remember it all. I did go back and review a few things.

I was just trying to respond to Wylde's post and had other info to help make my point. I also disagreed somewhat with the point that anti squat didn't apply much to racing, I think it does on late models on certain tracks. I didn't expect a response to everything. There were some points that I don't recall seeing in earlier posts like the angle of the side of the car to the travel path, the force change from roll degree, anti squat and traction cycle, distribution of left and right loading under acc., the difference in anti squat effect on a tacky verses a slick track, and dynamic cross weight jacking from standing on the rods.

RM what is your opinion on the center of gravity calculations using the whole car, I think it should be on the sprung mass only. On a solid axle car there is nothing you can do to change the transfer or distribution of the weight of the axles so if you want to know total transfer, they should be calculated separately. This should move the CoG up a little, this is the only mass rotating around the roll center. To get more accurate weight transfer calculations, CoG lateral movement, fuel slosh, and tire spring rate should be considered.

 

[jack action]

With acceleration we only have to worry about forces from one end of the car, the rear.

Ignoring the front suspension in acceleration? Tell that to a drag racer.

We know we cannot change the total weight transfer to the rear without changing center of gravity height, wheel base, or acceleration.

True.

That is because as you get on the throttle the mass of the sprung weight is moving away from the track and the opposite reaction is pushing the rear axle into the track.

You seem to think that the fact that the sprung mass is moving, that it is somehow adding some vertical force. It does not. (Any extra vertical force would come from the increase in CG height, which would increase the weight transfer)

When you hit the throttle there is only static weight on the rear and as the car moves forward weight is transferred to the rear giving more traction and allowing more acceleration. This cycle keeps building off the turn,

You seem to think that the weight transfer is slowly building under a given acceleration. It does not. It is instantaneous. If the acceleration is there, the weight transfer must also be there.

Also, the car doesn't need to move to have a weight transfer. You can have an acceleration at a zero velocity (car stopped) and the weight transfer will be there. Of course, with an acceleration, the car will not be stopped for long.

RM what is your opinion on the center of gravity calculations using the whole car, I think it should be on the sprung mass only. [...] This should move the CoG up a little, this is the only mass rotating around the roll center.

I think this come from the fact that you think that the weight transfer is influenced by the body motion. Again, it is not (other than CG height change). The unsprung mass is accelerated and it will contribute to the horizontal inertial force acting at the CG (creating a moment about the ground, where the equal and opposing traction force lies), which in turn will be compensated by a reaction moment coming from the axles, i.e the weight transfer.

The keyword here is moment. I think @Ranger Mike hates the term weight transfer because ... there is no weight transferred. There is a moment that compensates for (or adds to) the weight supported at each corner.

The fact that the suspension moves is only a matter of knowing in what proportion (% anti-squat, % anti-lift) does the weight transfer goes through the solid links of the suspension or through the unrestricted spring.

 

[Ranger Mike]

thanks Jack, was watching goat screw at Daytona..what a waste..
my hang up is as stated. I hate the term.

Please read post 563 on page 23 on CoG
228 on page 10
470 on page 19
676 on page 28 and
811 on page 33 - this one is my rant on weight transfer!On this post ifin you don't do the homework, you don't get the trophy.

 

[berkeman]

thanks Jack, was watching goat screw at Daytona..what a waste..

Yeah, no kidding. I gave up after a couple of hours -- looked like the rain would postpone it until today and also give tome for more car repairs...