Automotive Race car suspension Class

[Ranger Mike]

[QUOTE=rick7343;4032703

Center of gravity down... why are lower cars faster around the corners?

In general low cars turn better simply because the distance between the CoG and the RC is shorter. But the assumption is the weight tranfered thru the RC is going where it does the most good. Having RC located to the left is not going to plant the Rt ftont tire As EFFECTIVELY as having it plant when the RC id off set to the right side when turning left

 

[rick7343]

So would it nbe wrong to assume with a heavy car and only 8" tires that there is already enough weight on the rf tire to plant it?

The fast guys and most successful race shop all tell me their roll centers are left... they just won't tell me how much..lol

They said on some of their bump stop cars (late models)the RC is outside the car to the left to keep the front left down... so they are tying to elimnate body roll...

Both my outside tires get really hot and get greasy after a few laps, so I think I have too much body roll, so the moment arm on the front would need to be shorter to combat the roll, correct?
(I have tried a lot of rear springs to try and match the rear roll to the front since the rear has a short moment arm..)

We only have 2 nights left, and I doubt I will make tomorrow night... It is our anniversary and time is limited since they put me on shiftwork ... (I am not quite married to the car, my wife keeps telling me...)

The car is actually pretty good in the corners right now, I am underpowered compared to those darn crate engines... It just doesn't really feel "hooked"...

 

[rick7343]

Does moving the RC left have something to do with the angle between it and the tire contact patch relative to the RC height?
I think Mitchell wrote that the height of the RC doesn't matter as much as the angle between it and the tire contact patch...

So you could have a higher roll center,but moved to the left, and it would act similar ro having a low roll center?? There must be something or why do it?? It must change the action of the moment arm too by changing the angle of it also...??...

One of the fastest cars here in my class has the same front clip and a pair of 400 springs in it... so it is possible... Have to say, he is always adjusting his though...

 

[Ranger Mike]

400# spring guy it racing a go kart chassis which has just about zero camber change. I suspect rear end goes loose and he dirt tracks it out of the turn..his success I think is due to driver... Anyway

On way to Bahamas until Thursday

 

[rick7343]

Nice!

 

[rick7343]

I know you don't agree with the light front end, but it seems to be the coming way... (The shocks are fairly too btw...)

Maybe I should have spent more time with the 400's in mine, even slamming on the brakes it wasn't quite using all the suspension on the right front... I had the bar preloaded a lot and shouldn't have... To limited on time to test properly..

Eliminating some of the rear steer, and found my rear whell bearings loose too for some reason... they were new 150 laps ago.. (floater)

Have a good time in the sun...! (it wasn't hot and dry enough in the US??)

rick..

 

[TheSymbol]

Thank you for this excellent info :)

 

[rick7343]

Thank you for this excellent info :)

?? guessing you mean the thread in general...

 

[TheSymbol]

Yeah.
Was talking about the thread by RangerMike.
:)

 

[Carson76]

I know this is off topic but i have tried everywhere else to no avail. Does anyone know of a company other than Howe that can provide the pictured bracket? Howe has discontinued it.

 

[Ranger Mike]

Carso,,,look at

http://www.speedwaymotors.com/search-bracket-rear.html



or left hander chassis, Port City racing
speedway engineering..
the y all can whip up a copy if you give them good dimensions..

 

[Timaladd]

DIrt Late Model Roll Center Location

I was watching Lucas Oil Dirt Latemodel races I have DVR'd and allt he top cars had their RF tire out 10 inches. I assume they are doing this with longer RF control arms because you can only run 5in backspacing wheel on the RR without hitting the frame on most cars.

Please explain the theory behind this. No doubt the rollcenter is far to the left softening up the right front. It looks like the cars keep the RF pinned down all the way around the track. I have no doubt that most of our speed is in the front end. When i go to big races and watch all the pros unload and work on their cars, they always focus on the RF and have a big selection of RF upper control arms.

Thanks for your insight.

 

[Ranger Mike]

timaladd
welcome...good questions..
pls review page 16 post 251 and 255...
summary..wider makes for more cornering power..adding 2 inch length to right side A-ARM
will move Roll Center to the Right about a half inch on 1 inch dive,,,this puts more transferred weight on right front to turn car on corner entry

 

[willywipper]

Hello All,
I've been a long time student of the "Dirt Oval" game, and I've been reading this forum for some time now... and I must admit, a lot of this stuff goes right over my head. But occasionally I apparently jump up to your level, and something sinks in my old noggin. Thank You to all for taking time to share your insights, wisdom, and theories. Learning really IS fun !

I do the tires for a Super Late Model Dirt team (and I believe 100% in temperature monitoring). We've been struggling on our big, 1/2 mile track when it goes slick. The problems began when we upgraded to a "new and improved design" from one of the major chassis manufacturers. I am convinced there is different front end geometry in the "improved" design. The rest of the team is willing to let me take the lead on this, so I recently purchased Computerized Chassis Setup software by RST in Florida. Before we measure the cars (4 total) I'd like to play with the program a little. Unfortunately there is no "sample" data included with the software. I was wondering if anybody had numbers for a dirt late model, that would have the proper (competitive) geometry, that I could punch in. Or if it's already in this forum, please point me to it.

Again, I Thank everyone for their time.

 

[Ranger Mike]

willy..welcome... suggest you go to page 18 on Bench Marking the car..post 285 I think...I understand your want to play with the program and getting good chassis set up on dirt...I will look up my notes to see if I still have some...but.. and this is big...
there is very little to be gained by using another cars set up notes unless everything on your car is the exact same setup.
One different length A-Arm will throw the whole thing off as will a 1/2 inch taller / shorter ball joint mount. Ten degrees difference in coil over angle will screw up the wheel rate on that wheel.
Every car is different regarding the amount of weight transferred and the rate that this weight is transferred...AND THE LOCATION of where it is transferred too ( bad grammar i apologize).
I think if you bench mark the car and keep good notes on changes you will be able to tune it into the winners circle. You got to know the base line so you know what to change to make it handle.

The question from you post is – when the track is slick , is the car pushing going in ? Or not hooking up coming off the turn?

 

[willywipper]

Thank You Ranger. I understand your warning about looking at "other" car numbers. I guess I'm more interested in proving to myself how "little" changes can make big differences in maintaining proper geometry. Having an accurate set to start with would give me that option.

Answering the question: Push or Loose... it's hard to say. Every week it changes because of the "tip of the week" from well-meaning folks. That's why I feel it's important to get down to REAL basics... not what competitors/friends suggest, or even what the chassis manufacturer tells us. I'm headed to the shop this afternoon to try my hand at measuring one of the cars.

Thanks Again.

 

[Ranger Mike]

willy..i know exactly what you mean...everyone isan expert at the race track..the quite guy in the corner ( with all the trophys) understands what's happening and why the car acts like it does...you can't explain how roll center offset effects hadling to these rail birds...they over hear Joe Hot Dog say he runs a 500 pound rt ft spring and that becomes gospel..


Should we review what happens going into the turn?

 

[Ranger Mike]

Reading Tire Temperatures

Example 1 -You are in a passenger car..rear wheel drive V8 and you enter a left hand turn. You suddenly realize you are going too fast. You hit the brakes and turn the steering wheel to the left..the car continues to go straight ..to the guard rail...this happened to a friend of mine in a 440 CID Dodge Challenger..he died...
In this case, we have sprung weight being transferred to the front end, under braking and laterally. The weight is too great for the tires to counter and the tire tread literally shears. Classic Under steer or front end Push. A friend of mine maintains that if the driver had nailed the gas pedal he could have driven out of this fatality. That takes huge gonads ...we will never know.

Example 2- I was helping a friend of mine tear down an old barn. We dismantled it piece by piece and were hauling it to the city dump in an old Ford Ranger pick up truck. These were long wood beams that drastically over hung the bed by 5 feet! We really overloaded the bed so much so that I think we had about a 1/2 inch left before the rear axel snubs made contact. I took a load to the city dump and that thing was very light in the front end...very “ floaty”...I had to granny it around the turns even at 30 MPH which was about tops. The thing really did not want to turn in the corners. Classic front end Under steer or Push. Too much rear weight and not enough on the front end to plant the tire and turn the vehicle.


Now let's look at a stock car V8 class race car set up for asphalt short track Saturday night. Assume you did your home work and calculated the proper spring rates from Page 2 on this forum or your springs and shock package are “ close” to what the hot dogs are running. As the driver enters the turn he has a PUSH and the car is loose on exit.


Two things any crew chief better have in the tool box , next to the Rolaids, are Stagger tape ( page 27 post 426) and a Pyrometer..preferably one wit ha memory that will permit you to take readings and will display three reading s per tire. Racing is about data and adjustments and you need proper tools to measure what the car is doing. Tire temperatures are the only real way to tell what is going on and it is empirical. Learn how to use the tire pyrometer and make a habit of doing it correctly. I have a nice techy one that tell you to stab the right front tire inside, middle and outside then move to the right rear tire. The cursor ques you where to take the readings. Insert the sensor needle 1/8 to 1/4 inch into the tire at a 45 degree angle and be consistent. You wil be stabbing three places on the tire face. Inside edge ( not on the edge but an inch from the inside edge) the middle and the outside ( not the very outside..,an inch from the outside edge). When taking readings try to do it as soon as the drive comes off the track as these tires cool very quickly. Tune and test day is ideal as you can do it on t he track if you are solo ones in that session. Then jack up the car and take tire stagger readings right front first. Record the tire growth and this will tell you about stagger. There are a lot more tips I can go into but let's go to the heart of this post.



The optimal tire temperatures should be in a range of 190 to 240 degrees. On a short track it is normal for the outside edge of the RF tire & the inside edge of the LF to be 5 to 10 degrees cooler. This is because of the way the tires travel down the straightaway. On a larger track with longer straights, this spread will be even further. On an oval, the RF tire will have more negative camber, thus resulting in the inside edge of the tire contacting the track more than the outside edge giving you the higher temperature. On the LF you will run with more positive camber, so just the opposite holds true. While cornering these temperatures should even out if you have the correct amounts of camber & or weight transfer. The more camber you run, the higher these spreads will be. On a small track were you spend a lot of time cornering, you'll find the spread not as high. This is because your spending more time cornering than on the straights, thus distributing the temperatures across the face of the tire more evenly. If you try to achieve even temps across the tire you may develop a push. This is telling you that you have too much positive camber. Although the tire may be flat on the track, on a straightaway, the tire will not be flat on the track while cornering. Do not try to dial in the camber so the tire temperature is even across the front tire face.

The best way to read tire temperatures is to run 10 laps on a particular setup. Read the temps and don't expect to learn everything reading the temps only once. It will take a number of these sessions to sort everything out that is going on with the tires. Make sure your not locking up the brakes or making any sudden changes in your steering outputs. These will all create erroneous tire temperatures readings.

A tire with too much NEGATIVE camber will show an excessively higher temperature at the INSIDE edges. Lean the top of the tire out to the outside.

A tire with too much POSITIVE camber will show an excessively higher temperature at the OUTSIDE edges so lean the top of the tire in.

A tire that is OVER inflated will have a higher middle temperature than the inside & outside edges.

A tire that is UNDER inflated will have a lower middle temperature than the inside & outside edges.

A car with too much toe OUT will show higher temperatures on both INSIDE edges of the front tires.

A car with too much toe IN will show higher temperatures on both OUTSIDE edges of the front tires.

A RF tire that is HOTTER by more than 10 degrees over the RR indicates a tight PUSH (understeer) condition.

A RF tire that is COLDER by more than 10 degrees over the RR indicates a loose ( oversteer) condition.

A tire with the HIGHEST average temperature is the corner of the car that you should work on first.

A tire with the LOWEST average temperature is the corner of the car that you should work on second.

A RF & LR diagonal average that is the same or higher than the front & right side average indicates too much wedge.

A RF & LR diagonal average that is more than 10 degrees lower than the front & right side average indicates not enough wedge.

Lets look at a few examples.

RF
I----M----O
208--202--194
Indicates too much negative camber.

RF
I----M----O
194--202--208
Indicates too much positive camber.

RF
I----M----O
204--188--197
Indicates an under inflated tire.

RF
I----M----O
204--210--197
Indicates an over inflated tire.

RF
I----M----O
204--198--194
Indicates correct camber. Overall average temp is 198.6.

RR
I----M----O
227--225--223
Overall average temp. is 225.

If the RR & RF temp above came off the same car we have a very loose racecar. The RR is 26 degrees hotter than the RF. If this RR is also the hottest tire on the car, it indicates the RR is spinning and or sliding in the corners. We need to go to a weaker RR spring to keep more weight on this tire and prevent the wheel spin. This should cool this tire & tighten up the chassis.

Caution -Temperature Averaging works when you have a fast car and need to fine tune it. If your car is a log wagon and wallowing around like a whale cure the handling first.

 

[Ranger Mike]

Tire Temperature - 2

One thing I have really struggled with is this...

If the Right Front Tire is much hotter than the Right Rear ( by 30 degrees)...thus an under steer condition..pushing...the cure is to go to a softer Rt Ft spring rate...from all I read this weekend...the rt ft spring is too stiff and resisting the body roll and kicking the weight that wants to come forward, back on the rear tires..thus causing the rt ft tire to slide sideways in the turn...

all I can figure is that there is not enough weight being transferred up to the Rt Ft to plant the tire and turn the car...so going to a softer spring will keep more weight up front..

every other article says the right ft tire is doing "too much work"...

I think there is a better explanation and will have it later...I appreciate any input...

 

[Ranger Mike]

Weight Transfer - Humbug

Lets get in our Go Kart. The one with solid axels, West Bend 2 cycle engine and twin Tillotson carbs. If we race around the traffic cones we placed at the Wal-Mart parking lot we feel force trying to lift the inside (left) tires and plant the outside tires. If we can run fast enough and the tires are sticky enough to keep contact, eventually we would lift both left side tires for 100 percent weight transfer of UNSPRUNG Weight. But there is NO SUCH THINGS as "weight transfer". There is Momentum!


We are dealing with two kinds of force vectors when cornering. One is forward force, one is lateral ( sideways) force and one is lifting force due to body roll.

The tire is in contact with the track and pushes back on all of these forces. The tire sidewall acts like a spring to a small degree to soften the load being dumped on the tire. Downforce is transferred to the pavement until such time as the tire contact patch cannot longer maintain adhesion. When this limit is exceeded we have the tire sliding on the pavement. The tire pyrometer results will show this tire temperature exceeds the other tires temperatures by a large degree.

These cornering forces depend on the radius and speed your running, and your track width and center of gravity.

Now let's modify the go kart frame to have a center pivot for the front axle (the tube connecting both front spindles). If we modify the rear of the go kart frame to have a center pivot for the rear tube and mount the drive axle so it will work properly. Typically, both pivot points will not be same height and we have the essentials of our modern race car chassis. One problem. The chassis will flop to one side or the other due to gravity. By placing coil springs on each side of the pivot points front and rear we can “suspend” this arrangement, so the chassis is centered.

If we repeat the cone course test we note that we can attain a bit more speed before both left side tires lift. The springs dampen the fat kid weight transfer to a slight degree by increasing the time it takes for the transferred weight to finally cause tire shear. The suspension effects the percent of load the front takes versus the rear as well as left wide load vs right side load. Suspensions dictate where a certain percentage of the load going ( front or rear, side to side).

Lets really mess things up and put a heavier spring on the right side of the front of our go kart and try the cone killer course.
We find that we now have an under steer condition as the right front tire wants to wash out or snow plow when we get going fast. This is because the right front spring is resisting the downforce on t he tire MORE THAN the right rear. The right front ( compared to the right rear) is acting like a solid link and only the tires side walls are dampening the impact on the loading the tire contact patch.

We could go on with more examples like moving the rear pivot point higher ( ROLL CENTER if you have not figured this out by now).

By moving the Roll Center “ higher” (relative to the track surface) and assuming the Center of Gravity (CG) is above both ft. and rear RC, we have shortened the lever arm that is the distance between the CG and RC. This means you now have a shorter moment arm to counter Center of Gravity Inertia. Simply put, less load transfer will be going thru the coil spring and more so the cornering force must be countered by the tire side walls and contact patch. ( same as original unspring cart example). The higher RC will wash out first since the rear tire is overloaded more than the front tire which had a longer moment arm and could better control the fat kid weight.

This is a simplified version of weight transfer - HUMBUG!!! And we did not even get into diagonal weight transfer, sprung vs. unstrung weight, all the tech..was just my way of trying to explain why you lower spring rate when you have excessive tire temp on one end of the car...

 

[an1mal_69]

Hi Guys, slightly off topic this one, as in where the discussion is at present & not strictly being a race car (allbeit an offroad racer)

Im building an off road truck at the moment, I've already put together a 3-link setup on the front which ran airshocks. I'm re-designing it to run coilovers and also for better link geometry, at the same time I'm going to be 3-linking the rear running coilovers also.

The main issue I have at the moment is with the Panhard rod (trac bar?). Initially I was going to go with both PH rods running the same direction so they shift the same way over bumps etc, but looking closer its going to make for a much cleaner install if I run the rear opposing the front. When I thought about it a little more, wouldn't it be more stable when tipped over (side hilling) with them opposed.

I think any differences with regards to shifting track would be minimal with the PH rods pretty much horizontal at ride height, and due to an 8" side wall on the tyres.

Is there a definitive answer on this one or is it a compromise one way or the other?

Many thanks in advance, Nik..

 

[Ranger Mike]

welcome Animal
look at page 16 post 246..note location of the Roll Center..I would think on a left and right turn vehicle, you would want both to be as in line as possible. the height can vary but to me, the line up should be as centered as possible otherwise the load transfer would be biased...also make sure the differentail 3rd link is mounted as close to centered as possible..an old round track trick was to offset the 3rd link the same percentage as the perecnt left side weight...i.e. run 60 % lft side weight, and you have 60 inch rear track, mount 3rd link 60 x .60 = 36 inch to left of centerline or right rear tire..

 

[an1mal_69]

Thanks for the input Mike.. I'm getting there with my numbers.. This is my existing(ish) setup..

And this the proposed(ish)..

I'll probably aim for a bit more vertical separation at the axle end (top link up a bit). The main query however was running the PHrods 'crossed up' any thoughts on that?

 

[Ranger Mike]

i like set up 2 a lot better..less anti squat...roll axis a lot better...upper link still not centered but a lot better..
all in all..I think it should be a lot better than what you are running now

 

[an1mal_69]

Thanks!

Yea the top link is constrained by the chassis rail..pretty much runs along at the same angle & has maybe 80mm to slot into on upwards travel between the chassis and sump, I should have more scope for mounting on the rear. That's sparked another thought actually, is there any benefit/disadvantage of running the top links (front & rear) on the same, or opposite sides of the chassis? I'm guessing if you aim for rollover/understeer around 0 there's no real effect?

 

[willywipper]

OK Ranger... I benchmarked one of our cars... Dirt Late Model. My software is coming up with a rear RC of 11.9" at the center of the j-bar. And a front RC of 3.07" high and 12.15" right of centerline (using the center of the tire treads as centerline of car). So, now what ? Is this in line with what a late model should be? I'm thinking the 12" to the right should put a lot of weight transfer on the right front... but our RF tire is barely hotter than the LF... ? On the subject of temperatures... we see the inside edges (towards the differential) of both rear tires hotter (by 30 to 40 degrees) than the rest of the tire. I understand the RR hotter on the inside edge due to stagger (4 to 5") but shouldn't the LR "outside" (toward infield) edge be hotter, due to stagger. Is the LR getting too much side bite, leading to higher temps? Then the right rear is just sort of skating along, on TOP the surface?
Stagger's another question... the Rocket base setup is 4", so we start with that. The calculations only call for 2" min stagger for our big half mile... but how much does one of those LR tires grow down the straightaway? Could we be going into REVERSE stagger?... Driver feels he HAS to have at least 4" of stagger from past experience. And has won races with 5".

 

[willywipper]

Ranger Mike,
In your earlier post:

If the Right Front Tire is much hotter than the Right Rear ( by 30 degrees)...thus an under steer condition..pushing...the cure is to go to a softer Rt Ft spring rate...

from all I read this weekend...the rt ft spring is too stiff and resisting the body roll and kicking the weight that wants to come forward, back on the rear tires..thus causing the rt ft tire to slide sideways in the turn...

Would a sliding front tire build that much heat? According to Furney's book on Hoosier Tires, if you see 30 degrees hotter on a rear, it's likely due to a spinning tire. Since the fronts are not under traction load, I'd think the heat buildup would be a lot less. Unless the track is an almost constant turn...

Just thinking out loud.

 

[Ranger Mike]

Welcome Mali...until you know where the Roll Center is onm the car, I recommend you not change anything. The RC will change when you change spindles as the Ball joint height will change. Tube upper A-Arms are amust for serious racing. I think it is "safe" to buy adjustable upper Ball Joints ( meaning you can use them reagrdless of final settings.) bottom adjsutables ..would not waste $$ on this yet..until you measure the front end geometry to find out where you are.
Take the time to Benchmark the car and measure things..BEFORE you make any serious changes. This way you know how the car felt and performed before changes and can track the progress as you change things.
Since bump steer is so important, about all I can terll you is adjust this last , after you changes the spindles and keep it under 0.030 inch per inch bump. Also you should know what Ackermann you run.
Thanks for the kind words..
RM

 

[Ranger Mike]

I will try to better explain the importance of Roll Center (RC) offset since I have had a lot of private messages about this.
Some of you are harping on my over zealous fixation on knowing where your Roll Center is on your race car. My answer is that if you do not know, you are only band-aid racers and the path to victory lane will be along hard road. Trying to make a car handle at the track and reacting all the time gets old. You have to be PRO Active.

There is no such thing as "Weight Transfer". The only way to put more weight on the right front tire is to place weight bars on that corner. Everyone uses weight transfer but what really happens is the car has Momentum and Inertia. Momentum is defined as the tendency of a body to remain in motion. In simple words, momentum is your force or speed of movement. Inertia is defined as the tendency of a body to oppose the change in its position. In simple language, inertia is what keeps you going. Both of these are lumped together time and time again. Think of momentum as a straight line force that resists change in direction. So this fictious "weight" is traveling in a straight line until the car hits the corner. This force is then countered by many things like ARB (sway bar) and the cars springs, tires, Track surface. So I will continue to wrongly use the term weight transfer for the remainer of this discussion.

Rule number 1 – In a turn we have to deal with weight transfer. We can not change this fact. We can only control and direct where it is going. This weight transfer consists of lateral weight transfer and body roll. If we cut down the amount of body roll we increase the amount of lateral transfer and vise versa. Both must add up to the same total regardless of proportions.

Rule number 2 – We need vertical load on the right front tire to properly turn the car. Top illustration shows stock car production chassis as it came off the production line. Pretty good compromise for grocery getter. RC is stock height centered and 2.75 inch from the Center of Gravity. Note that 50 % of the weight of the sprung weight will transfer through the RC. This transferred weight is in the form of lateral weight transfer ( large black Arrow) and body roll of CG thru the 2.75 inch lever.

Lateral Transfer – The large black arrow is pushing straight on the RC which in turn is pushing straight through to the tire contact patch and to the track at a 15 degree angle.

Body roll transfer – We also have weight transfer due to body roll acting thru a 2.75 inch lever which will slam this weight onto the right front tire increasing traction.If everything was left the same and we lowered the RC height but kept it centered, we would have a longer lever between the CG and RC. We would need stiffer springs to counter the roll. We also would have less than 15 degree angle from RC to tire contact patch and would lateral transfer would be less.

Bottom Illustration shows effect of RC offset 3 inch to the right of the car. Now 55 percent of the left side sprung mass is transferring through the RC. We have increased the distance between the CG and RC. So we have more weight being transferred through a longer lever to the right front tire.

We have increased the angle of transfer to the tire contact patch to 22 degrees. This means we are more effectively countering the lateral weight being transferred and we have less lateral weight being transferred and more roll weight transfer ( longer 4 inch lever). Don't forget, both must add up to the total sum of weight transferred. The right front corner will “ jack up” on corner entry.
If we offset the RC too much to the right we will be carrying the left front wheel through the turns ( old sprint cars ran on three wheels most of the time).

If we have the RC offset too far to the left we don’t have enough body roll to stick the right front tire. The angle between the RC and tire contact patch is 12 degrees which means we should have less lateral weight transfer than the stock set up but we do not. Only 40 to 45 % of the sprung weight will be passing through the RC. Not enough to stick the right front tire and we will be shredding the right front tire tread because too much weight is LATERALLY transferred. We we won’t have enough lift on the left front tire to hook up the right rear tire on corner exit. Car will push going in and be loose coming off.

Summary – We need to lower RC that is offset to the right side to kill off jacking effect and reduce camber build. This means a longer moment arm from RC to CG but we can use the body roll transfer to properly plant the right front tire. Too stiff spring rates, huge ARB and the use of bump stops turn the car into a giant Go Kart. Wrong move. You need some degree of body roll. As with everything in racing, it’s a compromise.

 

[Ranger Mike]

Proper Roll Center

Note that the proper Roll Center location for traditional racing set up is about 1 inch above ground level and 3 inch to the right. This lower height kills off the jacking effect of lifting the right front tire ( we are now at 6 degrees angle). Lower RC helps kill off camber build too. The longer 6 inch lever arm from the CG to the RC means we have to up the spring rate on springs/ ARB. This is most common set up used on Saturday night bull rings.

Regarding the BiG Bar Soft Spring set up - ( primarily on flat tracks) We want the nose at minimum ride height at static and when we brake going into the turn we want to compress the front springs and tie the front down with high rebound shocks to hold it down. We want no air coming in under the front. We want to use the aero to push down the right front as opposed to the body roll used in traditional set up. Forget about body roll planting the right front tire. We want low RC to kill off anybody jacking. If we have to shift the RC location to kill off too much vertical load of the right front tire..we may have to move the RC to the left. The whole idea is to minimize body roll ( you are still going to get it no matter what) and tie it down until the middle of the turn. You want to keep the rear end high and aid the rear spoiler in the air stream to increase traction on exit. This means you have to run stiffer rear springs to keep the tires planted on exit. You really have to stay on top of camber and tire temps as you want max tire contact at full spring compression...that is to say...given you may have 1 inch max bump ( spring compression) on suspension travel you must make sure camber is such that you have a flat tire contact patch on left and right front tires at this point of travel.

The whole idea is to reduce suspension roll, increase consistancy of the set up which decreases response time.
Its all about the tire contact patch and if we can minimize susepension travel and maximize tire contact patch of all four tires better than the other racer..we win.

 

[willywipper]

So the roll center-to-contact patch has an ideal angle? is 45 degrees the best combination of lateral and vertical loading ?

 

[Ranger Mike]

Saturday night duck walk - why RC location matters

Thanks Willy...a 45 degrees angle between RC and tire contact patch would place the RC centered on the right front grease zerk of the right lower ball joint. You would have maximum jacking effect and you would lift the left front tire thru the turns and overload the right rear tire...like the old sprint cars used to do..classic ..but wears out the rt rear quick.
This would permit air to get under the front end and defeat the aero down force which is what the BBSS ( big bar soft spring) set up is all about.

If we are running a traditional set up and not the BBSS set up we want a balance of placing the RC so it loads the right front tire without jacking the right front.
We want the RC as low as possible to kill of the jacking effect and minimize camber build yet...plant the right front using body roll without lifting the left rear tire.

On dirt tracks we want even more RC offset was we are dealing with a slick track 75 % of the time ( dirt track goes thru 4 phases from slurry slick to hard packed over the course of the night depending upon how the track owner maintains the racing condition). That’s why you see the super late model dirt cars really twisting the body. They set up for slick and when the track dries and gets hard the cars really hook up toward the end.

Pretty much a balancing act.
My point is that you should know where the RC is and understand what happens when weight is transferred in a turn. If you do this, your chances of making correct changes on the set up at the track are a lot better. If you get the RC close, then chance are you can bring in the balance with ARB (sway bar) tuning, J-bar, shock, stagger and wedge adjustments.

Personal rant coming on - Too many times we all have been in the pits and hear “ its pushing like a freight train”. The poor guy driving does not have a clue as to how to properly counter this condition and ends up chasing his tail all night. In fact I would bet that 50% of the grid does not have a clue about the RC thing, If you have ever been to a Saturday night race and it was quack quack...you know what I mean. No one can pass on the out side, everyone is in line like a parade of ducks..the pole sitter wins. One race team has settled on the “ hot set up” and everyone else has copied it because it “ works”. And it may be off a mile..but nobody cares and no crew chief wants to make a change from the “ hot set up”. Then when some one does set up the car with proper RC and really hooks up and runs the out side groove all night and wins.., he is immediately accused of cheating. Crazy...but I love it!

 

[willywipper]

So front tire temps can tell us if we're loading the right more than the left... a little warmer means heavier load. If we pick the left front off the ground, it'll be cooler. ?thoughts?

Here's something new, Ranger. Hoosier just released its newest right-rear tire for us. It's 1" wider than the old style. Everything else is the same. What can I expect when we run it. Our track never goes hard, taking rubber... it always stays in the "shiny" stage, with little grip.

Thanks

 

[Ranger Mike]

if your rt ft tire is 10 degrees hotter than lft ft..ok..when it has average of all 3 temps 20 to 30 degrees hotter..time for a change...on rt rear tire, if you are running open wheel car..it may hurt to run wider tire...if you are in door slammer class go for it as wider is better..see track width posts 255, 264 and 322..

If you are not hooking up what you have now...it will probably be a waste of money..may even be slower...ifin it were me..try a set and see...we had a huge discussion in the Formula Car club one time about going to 10 inch slicks...would make for slower run times due to aero drag as we are limited to 2000cc and very strick engine mod rules..which really limits horsepower...but if I could cheat...would love um

 

[Fahlin Racing]

...Personal rant coming on - Too many times we all have been in the pits and hear “ its pushing like a freight train”. The poor guy driving does not have a clue as to how to properly counter this condition and ends up chasing his tail all night. In fact I would bet that 50% of the grid does not have a clue about the RC thing, If you have ever been to a Saturday night race and it was quack quack...you know what I mean. No one can pass on the out side, everyone is in line like a parade of ducks..the pole sitter wins. One race team has settled on the “ hot set up” and everyone else has copied it because it “ works”. And it may be off a mile..but nobody cares and no crew chief wants to make a change from the “ hot set up”. Then when some one does set up the car with proper RC and really hooks up and runs the out side groove all night and wins.., he is immediately accused of cheating. Crazy...but I love it!

Oh how I couldn't and still can't stand that type of thinking! Using the same setup makes for quite the boring race amongst competitors and fans.

 

[NaLLa8705]

Just read all 31 pages in this multi-year old thread. Signed up for a physics forum with no real interest in physics outside of making my race car faster...

And to say thank you for all your time in effort. How would you go about benchmark this front suspension?

 

[Ranger Mike]

Thank you and I am sure the other folks who have contributed to this discussion thank you as well..ref: bench marking stock suspension...if there is a way you can drive the car onto a full ramp lift ( the kind you drive on like those in muffler shops) and raise it..you can better measure the various mount points with car in normal ride height postition.

Alternative is to use jack stands and map the mount points as best you can then drop the car to normal ride height and try to get the outer BJ dimensions...

it is very difficult to do this wit hengine and all the shrouding in place...obviously..

 

[NaLLa8705]

What would be the upper "ball joint" point? The pivot point between pieces 6 and 5 referencing my previous picture? Does piece #6 move?

Here are some other pics to help.
http://images.rvsmods.com/var/albums/Cars/SR20/Suspension/P11%20vs%20B15%20Front%20Suspension.JPG?m=1342499417
(on the left)

http://images.rvsmods.com/var/albums/Cars/SR20/Suspension/P11%20Multilink%20Tunring.JPG?m=1342499423
EDIT: Did some more research, apparently the upright "third link" stays stationary as the wheel is turned.

So we have a rotation point on the top of the spindle and then two pivot points above that.

 

[Ranger Mike]

You are correct on upper Ball Joint at end of the strut..
Any good software suspension program should cover mcpherson strut design like you got. I hate these for racing purposes but you can trick them out.

 

[NaLLa8705]

Btw this isn't a strut design. The green, aftermarket piece in the top picture is a strut, but I am referring to the stock pieces. They are for two separate cars.

Once I get further along into my build I will report back with the bench marked front end.

 

[THORPE]

Quick question on anti dive on dirt oval
This is a Rc car btw.

I'm reading in circle track mag where anti-dive on right front and pro-dive on left helps promote turn in.

I accidentally got them backwards on my setup and fought a bad push all night when letting off at the end of the straight. I found my stupid mistake the next day.

From my understanding anti dive only loads the tire initially then the spring takes Ove from there.
So it's beneficial to load the rf tire initially into turn then try to get equal loading from setup through the rest of the turn?

 

[Ranger Mike]

Thorpe..it happens..but you found it!..great..pls read post # 314 on page 20.
Anti dive is used to keep the front end from diving when you are braking in a turn. Looking at the front end from the side of the car you will note different mount angles of the upper and lower A-arms. The problem is that you run into mechanical bind and start to lock up the pivoting of the A-arms. You run into large caster changes due to this as well.


Personally, I like a car with zero anti dive and like the suspension to be parallel with the track and minimal deflection. I try to minimize things like bump steer, rear steer, camber build. I like Ackermann and lots of it if the track radius warrants it. I like to use the suspension to tune the car. By this i mean I prefer to dial in the car with Spring rates and narrow it to optimum with ARB ( sway bar tuning) Then I really fine tune with shock adjustments. When anti dive and rear / roll steer and other factors come in I get confused and it takes too long to separate out what is causing what. If you can do all the heavy lifting in the garage when you are doing the initial build chances are you can dial it in a lot quicker at the tune and test. You had better not spend time at the race making major spring rate changes etc...if you do..you better go back top square one..my opinion

 

[missile07]

Ranger Mike...First of all I love reading all these posts & the help you have been offering us racers!

On your last posts I noticed you said you like to minimize things like bump steer, rear steer, & camber build...we run 4 link modifieds on dirt and we have been playing with cutting back on rear steer. Other than trial & error is there any info out there on evening out rear steer

 

[Ranger Mike]

Missile07 --Thanks for the kind words..pls. see post # 116 on page 8
When using a very trick 4 link rear suspension using roll over steer adds a big advantage on the dirt track surface. It allows the rear to roll into the corner without breaking traction. To review, when the rear rolls, if the outside wheel base grows longer than the inside wheel base we have roll over steer. This is because of the different in 4 link angles. I think if you start with left upper link at 13 degrees uphill, the rt upper link at 17 degrees uphill, and both lower links at 5 degrees down hill, it is a good base line. Make sure the rear end is straight and has not become bent thru wrecks and crashes.

 

[missile07]

We have been trying to figure out a way to properly measure at full drop the difference in the right & left. At full drop the left side is leading 3.5 inches more than the right. For the past couple of race nights we have shortened the right side bars up to 1 inch. Driver seems to think its helping him from the center off...but I am almost convinced that this is a band aid fix for something else.

 

[Ranger Mike]

i am n Kentucky today...will be back at home base tommorow. I think you have a roll understeer at center off. I agree that there could be something else off. Is is possibel to post the rear end settings on the 4 links?
What are the scale readings at each wheel? Have you read tires with pyrometer lately?

 

[missile07]

Well I don't have the settings right in front of me but I can post what I know...

Bar lengths

LR top 16"
LR bottom 12"

RR top 16" shortened to 15.75"
RR bottom 12" shortened to 11.75"

Bar Angles @ Ride Height

LR top 24-26 degree (chassis manufacturer recommendation)
LR lower 5 degree
RR upper 18-20 degree
RR lower 0-2 degree

As far as the wheel rates I would be lying if I said an exact number but he was running 120 lbs of LR bite & about 55% rear. The last night we were able to race I did pyro the tires and the RR was about 10-15 degrees hotter on both of the final two runs. Car hasn't finished out of the top 5 since June but just missing something.

 

[Ranger Mike]

not having my notes with me, i think you got just a tic too much roll understeer. My guess is to take 2 to 3 degrees out of top right upper link and read tires..if the temp drops a few degrees and car is better...may have found some of the problem...small changes ...dont do anything dramatic...

 

[THORPE]

Thanks for the reply. I'll start over on that anti dive and work on other factors as you suggest.
I'm goin way back and check more into roll center too. I moved mine down and to the right for last night. I got more turn in. I was carrying the left rear going in. I stiffened up both front springs and it help.

From what I'm starting to understand, lowering my RC even lower in the front will counter lifting the LR correct?

 

[Ranger Mike]

Thorpe ...great...now your getting it. When you dropped the ft RC you lengthened the lever between the CG and RC so stiffer springs were required to counter the body roll. You got more turn in becuase you used the weight transfer to plant the right front tire better and this gave you more cornering ability.. Excellent. The Lft rear was lifting at turn in BEFORE YOU CHANGED SPRINGS as expected because the right front was loading more and thus unloading the left rear. Also you reduced the " jacking effect" caused by the tire contact patch and RC that tries to lift the left front; hence left rear as well. Montitor the tire temps,,,LR spring change may be in order too? Your camber build should be reduced as a result as well which is a good thing so tire temps should tell you this too. Glad to hear about the change in handling..keep it going!