Automotive Race car suspension Class

[Ranger Mike]

if you like this class ..VOTE
https://www.physicsforums.com/threads/engineering-award.962196/

i also support this forum with a small donation...i urge you folks to kick in the price of a quart of 50W Valovine or the like..price of 6 pack of Pabst!

click on INFO at top of this page and hit DONATE

 

[Wild Willie]

Something that's rolling around in my head
if you calculate and find the RC of both front and rear

How much split should there be between the 2 in height ?
How does this effect cornering ?

 

[Ranger Mike]

read post # 229 on page 12
mass centroid axis is shown on illustration 13

 

[Robby Johns]

Great read RangerMike!

 

[Surdge]

Hey guys I'm from south Africa the 3 link is pretty new and those who have it doesn't Wana help me I'm building a dirt car with 3 link and need help with the length of the bottom trailing arms and the angles thanx

 

[Ranger Mike]

welcome Robby and Surdge
3rd link stuff on post 81,253,261 and 707 but check out index for rear end stuff too

 

[Surdge]

Thanks u have been a lot of help just one more thing that I have that I can't seem to find is what angle do u use for the j bar

 

[Ranger Mike]

see index on page 1 ..i wrote a lot on Jbar

 

[Surdge]

Morning iv got a another question ur shock bracket witch angle do u mount it on

 

[Ranger Mike]

READ the posts ...do the research ,

 

[Robert Garner]

Excuse me if I break in here to join the conversation. I'm new here and will start by attempting to insert a drawing of a suspension linkage roll analysis

.

Robert

 

[Ranger Mike]

welcome, robert

 

[Robert Garner]

Thank you for the welcome. Please excuse me a moment while I attempt to load a photo on this site.
Robert

 

[Ranger Mike]

a great reminder of what this class is all about

 

[Robert Garner]

Great Video!
It didn't mention the front to rear roll problems and solutions by Maurice Olley, however I believe this came later.

 

[Ranger Mike]

Olley was absolutely instrumental having gone to GMC in 1930 from Cadillac, Rolls Royce before that. One unheralded but very genius people to make GM what WAS. not is..now. 2002 book Chassis Design by William and Douglas Milliken, published by the Society of Automotive Engineers goes into much of his work. Besides being key to Corvette development that Zora Duntov gets all the kudos for i think he had 40 some pattens attributed to him.

 

[BEI77]

@Ranger Mike, in the world of circle track asphalt late model racing, which would make the handling better, the current quick change rear end or a independent suspension rear end?

 

[Ranger Mike]

BEI welcome
the IRS vs solid rear suspension debate goes on..in my opinion for round track racing the solid quick change is the top dog. IRS tends to not preform as well as the dedicated solid axle setup on straight line basis. since you race on the front and rear straights a lot more than the time you spend in the corners, you want maximum traction. IRS promotes lost traction due to independent springing and the set up. plus you have no roll steer option if you need to dial in rear roll steer.
the extra cost and additional maintenance head aches do not merit doing it. IRS do not like front to rear weight transfer. IRS are great for road racing and cars with mid engine or rear engines. IRS not so good for nose heavy cars. You can dial in camber on each side with the solid axle set up. prop can on IRS but counter productive. you will be racing on a bumpy track so use the solid set up realizing the IRS will lose traction more when you go over bumps vs the solid set up

 

[Ranger Mike]

I put his together as racing tip to fine tune your chassis. after two hours on you tube i could not find a decent video showing the tip about taping steering wheel but it is minus 10 F and end of winter so i share some videos and maybe put a smile on yer face!
When you are at the track hot lapping, try this to fine tune the car set up. Hot lap at mid speed and note the steering wheels degree of turn at mid track. The mid track is usually marked on the wall.

If not, pick a point and use this mid turn location on the wall for reference. You should have the 12 o’clock position marked already for easy of centering.

Note the 12 o’clock of the steering wheel when you are at the mid turn wall location. Use a piece of yellow or masking tape to mark it.

Next thing to do is hot lap the car at you best speed and again note the same mid turn location and the new mid turn steering wheel 12 o’clock position. Mark it with green tape.

If the steering wheel hot lap green tape mark is more than the slow speed yellow tape mark, the car is tight. If the hot lap green mark is less that the slow speed yellow tape, the car is loose. Adjust accordingly.

One of the better videos I found on this






good demonstration of counter steering



do not use this technique on these cars!



continued on next page

 

[Ranger Mike]

tips for beginners driving on dirt



Not very good tutorial but good racing to watch in -10 degree F winter

what not to do on asphalt

 

[Ranger Mike]

what not to do on dirt


Finally..this is what happens when you got too many rules and enforce them!


gotta watch if you need a laugh…

 

[drobbie]

Ranger Mike,
WIth winter the time to learn, we all should have read through every thread here (I know I did) and I thank you for such great information.
For my self, we have been working on the geometry of my friend's late model stock car. (8 inch tires, Coilover suspension and limited HP) We plotted out the front end only to find the attached image. Of course, it horrified me based on the knowledge I gained here and other readings, and we are in the process of changing it all around to bring it more in line with what we feel will make it run better. What I'd like to understand is what characteristics the car would have theoretically experienced from last year when someone else set the car up. From my experience, this geometry looks so messed up I would think the car should slide across the race track because there is no vertical load on the RF tire.
Any insight or comments you could provide would be greatly appreciated. And as always, thanks for hosting this forum as it is invaluable in understanding car dynamics in a simple and easy manner.

 

[Ranger Mike]

Drobbie thanks fro the kind words.
you are correct. it would push going in and be loose off the corner. It is a good thing you caught this set up error. The closer you can get it to center line is good and ifin you can offset it to the right , the better it will handle. Make sure to check camber build. you want it going the right way on each side and keep the curve linear. Bump steer should be under 0.030" per inch bump. Put some rear roll oversteer in it if you can. Dont worry too much about the limited HP as the tires can not handle it anyway. So you want to dial in the car way better than the other guy! put a front to rear brake bias control o n the master cylinders and put in a front right to left brake fluid bias valve in so that you can crank in the left brake to snap roll thru the corner better. put on a ARB (sway bar) loader that you can crank in while hot lapping. this is good if you have long races and the fuel cell lightens the rear end . When budgeting , plan on the dampers ( shocks) being the most funded as this area will do more than any other in this over regulated race series. put a card board box in the race car hauler so the trophy don't get dinged up.

 

[Dirtracer8]

I have been successfully running a dirt stock car w the same rc numbers you found on your car Drobbie.
Not saying it’s right but w the right springs and shocks it can be very fast.

Different setups and driving styles take different things.

Ranger Mike helped me figure a lot of this car out via this forum.

 

[Ranger Mike]

Good job!
you are correct Dirtrace in that the driving technique means a lot on dirt. The dirt setup ( if i understand your setup) is to get a huge amount of body roll to slam the right front tire with lots of down force. You can get too much down force if you offset the RC to the right side. This is the same reason the asphalt big bar soft spring set up sometimes has left side offset. it is to kill off too much down force on right front tire. You got the front end tied down to very little roll with the BBSS set up and you rely on aero to push down on the right front in place of the chassis roll loading. The traditional right side offset is for cars that can not take advantage of the BBSS setup due to bumpy track and rules making the car sit up high due to ride height requirements. Figure on 3 inch roll max. Also the rules may mean that you can not use aero devices to increase down force. I have seen diagrams by many dirt racers that show the CG and Moment Center during cornering and no vector from the MC to the tire. It shows resultant force from the CG to the tire. Shows centrifugal force with a downward vector. Not correct.
We get down force on the right front tire many ways with many setups. Now you have the dirt super late model monsters roll steer with many inches wheel base change and way huge body roll. Back in the day we offset the dirt RC to 4 inch right side when we had very little body roll. Same with the BBSS setup. It evolves and you got to learn what works best. too much down force is as bad a too little. Balance it for the track you race.

 

[Dirtracer8]

Ranger mike-
Maybe you could explain a little on to why a car w rc to left of center seems to collapse on left front on breaking before entrance.

My thoughts are that the weight is trying to come off right front via roll center and is transferring into left front spring.

 

[Dirtracer8]

And yes, your right RM- big spring on left front, tie down on right front . Tall spring on left rear, High lead and freed up rear end w no binds.

Metric “stock car”

 

[Ranger Mike]

can you go int o more detail on collapse? i gott go to work but will read your reply asap..thanks

rm

 

[Dirtracer8]

So what I mean is that when I back off gas and start to trail brake the left front of car collapses. It highly loads left front spring . To the point that the nose has bottomed out on left side but not right.

The left front spring is 200 lbs bigger then right front.

I been trying to understand this.

I don’t want to change much cause the car is crazy fast. I been thinking about putting a very little amount of anti dive into it on left side to combat this

 

[Ranger Mike]

i would say your front suspension is too soft. Dampers (shocks) may need tweaked but ultimately if you pan cake you are too soft on spring rates. i think you are turning he car into ago kart suspension when hitting the droop limiters (bump stops). it has been run often in the past but i am not a fan

 

[Wild Willie]

May I cut in for a question
If you were able to relocate rear RC left or right on the rear of a car
Reason I'm asking working on a mini stock with a young driver
We got the front roll setup
Now we are looking at the rear
We have the option to move the roll center left to right
Ranger Mike your thoughts

 

[Ranger Mike]

every race car is different on the handling deal. you have spec tires and restrictions for cost savings...in racing! ha...cost savings...
anyway, rear RC location can impact traction on those hard spec tires. You can in fact have rear RC offset. Panhard bar vs J bar especially. As with anything you have to test and record data to get best set up for YOUR car. I would start off neutral. Remember that when RC is neutral or center lined 50% of car body rolls to the right in the turn thru this rear RC. Personally i would work on making Left rear tire hook up more to help drive off the turn. Rear roll steer anyone? collapsible bushings? mount the lr spring in front of the axle?

 

[Ranger Mike]

On a flat track, while cornering, weight and centrifugal force act on the vehicle. Technically cornering force is a component of the lateral force taken in the direction perpendicular to the wheel moving direction, i.e. considering the side-slip angle. For easier understanding we consider lateral force acting perpendicular to the tire rotation plane. Centrifugal (lateral) forces are perpendicular to the ground and have zero contribution to the vertical force ( down force) felt by the tires (other than left/right distribution). Corn 1.jpegWeight always acts along the global vertical gravitational vector.As the track banking increase from flat, the weight felt by the tires actually decreases (m*g*cos(banking)). But with banking there is now a component of that (global) lateral acceleration in the direction of the track ( m*a_lat*sin(banking)). We loose a little weight force but pick up force due to banking. Corn 2.jpegRegarding the MC illustration Corn MC, what’s missing?

 

[Ranger Mike]

I know I dwell a lot on proper RC location and understanding the whole concept. Never forget that over simplifying something is as bad as not understanding the big picture. Reducing the whole suspension movement to a single point Roll over movement model will help but you really have to start thinking 4-D. Do not get too caught up in the RC is the only single point action when cornering. This single point/rotation model is useful for getting raceable values for roll gradient, RC height, RC location , roll rates during chassis build.

What is missing in the previous post illustration?Jacking effects. The jacking force is an extra force operating in parallel with the spring and damper resisting the cornering forces of Weight and Centrifugal force described above.

This force comes from the tires in contact with the track, acting on the vehicle. This force acts directly on the ROLL CENTER (RC) opposing the above cornering forces. Corn rc.jpegJacking is a force acting on the body from the tire which acts against the body roll effect of the cornering force. It acts laterally. The jacking force at each tire vectors to the RC. It is equal to the lateral tire load multiplied by the angle to the RC.

Look at the open wheel illustration Corn 3.jpeg. From this it’s possible to understand that in cornering the outer wheel has more horizontal load than the inner one and therefore it's jacking force is usually greater. You have a large upward jacking force from the outer wheel and a lower downward force from the inner wheel - both acting on the body. The sum of the two is a net upward force (total jacking force) which lifts the body during cornering.Think 4-D. You have the race car moving in 3D over time. The car is pitching back to front as well as down and yaw to the right and you have the time element. It is the whole X,Y and Z axis thing and time.
I took the above from F1 forum that has posts from a lot smarter people than I.A more in depth analysis is made here by Erik Zapletal (username 'Z') on the FSAE forums which includes this nice image: https://www.f1technical.net/forum/viewtopic.php?t=24134&start=15

 

[Dirtracer8]

Ranger mike or anyone else that can answer this..

My question is about brake caliper location on left rear of race car.

I bought a used grand national read end to have set up as a spare last week. It came from down south.

Ok- so looking at left rear hub from the side (I’ll use a clock face to describe) the left rear caliper is mounted in the “9” position. it is on front of axle tube.
On the right side looking at hub it is mounted in the “4” position. It is mounted on the rear of axle tube.

What’s the thinking behind this?

Better yet, is there a reason for this design?

I don’t believe anything like this has been brought up here.

Maybe get everyone’s gears turning . Any thoughts welcome

 

[Ranger Mike]

The short answer is that some one had some schooling down there. Rear calipers were mounted on the rear of the axle because the Detroit auto makers were using open differentials that did not have the axel wrap we have when running a spool or locker rear end. The illustration ( from “Circle Track Suspensions” by Forbes Aird ) shows what happened when you weld the spider gears in the production rear end. Now if you have a real restricted race car series that limits brake bias valving and dictated caliper and rotor size but not the mounting location, you can swap the caliper mounts and gain a slight brake advantage. The good old boys done figured out that with the left rear caliper in the stock (rear mount) location the brake action will actually try to lift the left rear. The relocated front mount set up will try to pull the left rear down and add down force on that corner. Same thinking we see when the left rear spring is mounted in front of the axle.

Braking is one area I have neglected in this class so I have to get to work and start writing. The carry out is closed so got to wait folks…

 

[Ranger Mike]

Using Brakes to win a race. I learned that on a race course turning left and right, more races are won out braking the other guy than blowing by him with max horsepower. There are 9 left and 8 right hand turns and just not that many long straights to out power the other cars. So I had to study up on the subject. We could go scientific and evaluate kinetic energy required to stop a 2500 pound car at 120 MPH but that gets into
Energy (lb/ft) = .0335 x [(mph max)² (mph min)² } x gross weight (lb). kind of math. So I will summarize.
We want to use the brakes to stop the vehicle and help it turn ( left hand turn round track applications). When the whoa pedal is pushed we have rear to front weight transfer ( I know, I hate using the words but for visual it cannot be beat).
When the rear brakes lock before the front brakes we can spin out. When the front brakes lock before the rear brakes we will push like a freight train straight toward the wall until we lighten up on the brake pedal. This is why we have the brake bias bar and two master cylinders on a race car.

In the post above we see why some racers change the mount location of the caliper. The real hot set up is to use brake floater brackets that swivel on the axle tube. These mount the caliper and connect to the chassis by radius rods. The rod angle can be flat level with the track, angled up hill or downhill. ( photo from IMCA Modified Racing Technology by Steve Smith). Brake floaters remove the brake torque from the traditional suspension links and direct the braking force directly into the chassis. Uphill angle adds down force to the tire contact patch. The greater the angle the more tire bite. Too much angle and the car is hard to turn because the right rear tire has too much traction under braking. Too much left rear uphill angle can make the car loose under braking going into the turn. The left rear will be getting too much traction and the rear of the car will want to pivot on that tire.
Down hill angle results in lifting force on the tire under braking. Flat no angle set up results in no force acting on the tire contact patch. Good starting point is 10 to 15° uphill on both rear floater rods.

 

[Dirtracer8]

Thanks for the info RM!
You seem to always enlighten me. Ever time I dig into this car ( which came from down south) I find more and more- between their bushing choices and locations and modifications and now manipulating the brake system... the guys that built this car were very smart. It’s just evidence that it don’t take a chassis shop built car to win races this day and age.

Money can buy technology but it don’t win races ..
Knowing what your using and how to make it work is key.

Thanks RM

 

[Bandit21]

Hi Ranger Mike,

I have definitely appreciated reading through the thread from time to time.

I am sure I will have a few questions in the future. I have been avidly trying to learn to science and physics of circle track racing over the last couple seasons as I realize how much more there is to it..

I have been having a discussion with a fellow over a setup design for dirt racing. I have been accustomed to trying to perfect the balanced and more conventional setups myself so this is something I am definitely trying to wrap my head around as it isn’t exactly my philology yet but I try to stay humble and learn.

He has been saying that on dirt, using a longer, softer than usual left rear spring is a better setup and in his thinking is that the extra compression when released or decompressed in corner will generate more force back onto the left rear tire as there is still energy in the spring, he goes on to mention that he tethers or limits the right rear travel in doing this.

He also goes on to say that when you check both on a pull down rig & on scales when you jack the left side up 3 inches when compared to a stiffer conventional spring that may lose its energy it will show there is more force or weight still being applied to the left rear creating more bite.I can understand on a static or statically rolled car how that would happen.

My thinking is that when we take the car into an actual live dynamic scenario it may indeed do the opposite and the energy stored at the latent travel would actually transfer more weight to the right rear tire from decompression and by limiting the right rear travel would greatly increase right rear weight, thus creating more right rear drive than left, and not the other way around.

I just don’t understand why one would need a longer left rear spring at a lighter rate, if weight jacks are allowed which in this case they are. I get that a shorter, stiffer spring may lose more energy at the end of travel but from what I have learned the weight will transfer to the right tires no matter what left rear spring is involved as there is more involved. But why would one sacrifice a good balance of springs for the extra energy of a longer, softer spring. Doesnt the law of every action has a reaction apply and wouldn’t the energy transfer over to the right side?

Also the cars we are discussing for the most part are metric, which in my learning have a high roll center naturally, along with a suspension that is constantly in bind, which is why a stiffer left rear spring split is the norm and also a softer overall spring package. We have run anywhere from 200lr-150rr to 250-200. He was talking about running 100 20” lr. I understand the metric 4 link is basically in a category of its own but I would like to here if maybe this style of setup works in more advanced chassis and maybe that’s where he is getting this info from, as I understand metrics have limitations compared to more efficient rear ends

 

[Ranger Mike]

Unless i missed something in your post, is sure sounds like your friend took an ill handling metric car and made it into a solid suspension go kart. Any time you have solid stops and limit the suspensions ability to suspend, you have a cornering situation that goes to snap roll. The car rolls over during cornering and hits the stops, goes to a solid contact and slams the car to the right side. When your hit the stops or teathers, all other spring action is null and void. You continue to slide until you straighten the car out and now since the left rear is so soft a large percent of the weight settles on the left rear instead of both rear wheels. Now you are driving off the corner with the left rear tire that is over loaded. All this is masked because you are on a dirt track and and can can get away with a lot of chassis probelms with a good driver.

I am curious about this set up. exactly how much wedge is this guy running? You know there is a reason we run cross weight on left turn cars. You go buy Steve Smiths book " Street Stock Chassis Technology" .
Base all your chassis adjust on known results and know how and why these work, Keep asking questions..only way you will learn,,and win!

 

[Bandit21]

Unless i missed something in your post, is sure sounds like your friend took an ill handling metric car and made it into a solid suspension go kart. Any time you have solid stops and limit the suspensions ability to suspend, you have a cornering situation that goes to snap roll. The car rolls over during cornering and hits the stops, goes to a solid contact and slams the car to the right side. When your hit the stops or teathers, all other spring action is null and void. You continue to slide until you straighten the car out and now since the left rear is so soft a large percent of the weight settles on the left rear instead of both rear wheels. Now you are driving off the corner with the left rear tire that is over loaded. All this is masked because you are on a dirt track and and can can get away with a lot of chassis probelms with a good driver.

I am curious about this set up. exactly how much wedge is this guy running? You know there is a reason we run cross weight on left turn cars. You go buy Steve Smiths book " Street Stock Chassis Technology" .
Base all your chassis adjust on known results and know how and why these work, Keep asking questions..only way you will learn,,and win!

Thanks for the reply, after talking some more it sounds like the tether is just barely making contact, maybe not in the snapping fashion. I think he is basing this particular setup along the lines of dirt late models and spring stacking. His theory is that when the car reaching maximum hike on the left rear, there is still energy left in the spring, which will then push back down on the left rear axle and keep a constant load to the ground. I just feel this extra energy would result in the weight trying to transfer more instead of having a standard stiffer spring that may lose energy at the end of the travel. It sounds like a radical setup and maybe it does indeed work, but my thinking was it was adding more grip to the left rear on exit with the extended compression. I understand there is a lot more to the overall chassis dynamic that comes into play than just this spring.

I have read that dirt late models will use taller softer springs in the rear, but this is to Keep the left rear higher, I would assume to take advantage of the aero benefits those cars have. But with a Street Stock I know aero comes into play somewhat,but if we weren’t focusing on that wouldn’t that indeed take more weight off when transitioning. My thoughts may be flawed as well

Thanks I will definitely be purchasing the Steve smith books, I have read bob bolles theory, I know how he stresses the balanced setup but it would be good to learn some more. Thanks again

 

[Ranger Mike]

there is a big difference between the metric 4 link stock suspension and a full floating adjustable 4 link bird cage set up. Those wild 4 link have huge roll over steer and insane body roll. concetrate on your tire temperatures and get proper wedge and stagger.

 

[Hillbillyd]

Well Id just like to say its taken me quite a few days of reading this from the start to finish and would like to thank all those involved I am overwhelmed with information but I've now got a great opportunity to learn so far I've been reading Steve Smith IMCA Modified and its pretty much covers what i need to know but being on here helps me understand it easier. Thank Ill be looking and learning

 

[Hillbillyd]

How do I determine the ideal scrub radius for a race car running 1/4 mile dirt oval , double A arm front suspension, 9 1/4”wide radial tyres, power steering and 3 link rear, 95.5” wheel base” 61” wheel track. I’m currently in the designing stage. Thanks

 

[Ranger Mike]

ideal scrub radius for our stock cars racing left turns is 5 inch. They typically have between 3.65" to 6 inch. Below 3.6 does not provide enough feedback to driver, above 6 inch adds too much scrub wear on the tires.

 

[Hillbillyd]

Thanks Mike I’ll work on 5” and also what height would you recommend for my front roll center and Rc off set

 

[Ranger Mike]

read post 251 on
page 11 and post 691 on page 28
if you are stock front clip on flat track you want it about 2.5" up and 4 inch offset to right for flat to 10 degree dirt

 

[Hillbillyd]

thanks again Mike that's clearer now as i had read that earlier on and now can understand it better.
Is there away to private message you with some brief details on my build or detailed details I'm sure it will be easier for you to advise on what's best suited to my needs. Thanks again for your time

 

[Ranger Mike]

do you turn left or right down there?

 

[Hillbillyd]

counter clockwise so yes left