Race car suspension Class

[tracksupra]

Good morning Mike and other fine gents. I just joined this forum. I have been road racing a 1995 Toyota supra in GTO/SPO for the last 3 years and love the car but would like to move on to building a tube frame version of it, lower weight and proper race geometry are my goals. I'm by no means an engineer so I rely on the expertise of the open forum concept. Having read enough to know to start the suspension how do I get the start points of lower and upper A arm mounting points and length of A arms on a clean sheet of paper?
Thanks Lou

 

[Ranger Mike]

welcome Track, do you have suspension software?

 

[tracksupra]

Noy yet, what would you suggest ?

 

[Ranger Mike]

see page one post #1

 

[Jared0020]

Mike we have thought about trying to go to Salem sometime. I'm guessing you run street stocks there. What kind of front and rear RC you need to run there.

 

[Ranger Mike]

welcome Jared..i am out of country and don't have my notes..we ran late model and super late model at Salem, indiana. ill message you later..

 

[Ranger Mike]

Fast high bank 1/2 mile tracks
When running the High banks you won’ get a lot of body roll so the camber change will be slightly less. You will have more down force applied due to the banking.With a lower front roll center less negative camber gain is produce on right front. Ideally on a super Late Model look for 1.5 inch RC height but this can be as high as 2.5 inch. A stock clip car will have a higher RC than a custom fab front end. If you have to run spec tires, a lower RC is required to create more down force on the outside tires. Rear RC would fall between 8 and 12 inch. You need stiffer springs on the high banks so add 25 to 50 # to front springs and 15 to 25 # on the rears. Use 1/16 toe out so as not to scrub the tires on the long straights.

 

[John LNZ]

Hi Mike. I'm working on some wheel rates - what do you think about measuring out to the ball joint or out to the tire contact patch to do the calculation? It seems the contact patch is more relevant than the ball joint to me, and would like your thoughts on this.
We seem to have found a nice balance for the front and rear spring rates for a grippy track however struggling on a flat slick track (as usual) and are going to reduce our spring rates but retain our roll couple. What would your advice be on reducing the front wheel rates by 20% and the rear at 20% to match - do you think this is a good move?
Thanks, John

 

[Ranger Mike]

John, I am sure you know the advantages of knowing motion rate and then wheel rate. We need to know these to find effective Spring rates. I like your strategy of reducing spring rates..same % front and rear..maybe 20% is too aggressive...? i like going 10% and sneaking up on handling gradually...just my opinion..your situation is unique in that it is a wild ride sprint car that puts the fear of God in Tony Stewart!

 

[drobbie]

Love the discussions here but I have a question concerning RC location. For an asphalt car I some saying it should be located a little to the left annd other saying is should be located a little to the right. Why one over the other?

 

[Ranger Mike]

hello drobbie
We discussed roll center location and height on post # 229 page 15, post # 691 on page 34, it all goes back to post #62..AKA the neighborhood fat kid theory of down force.
a thumb nail summary..we need down force to stick the right front tire. We can do this with body roll or aero down force. If we have too much down force we over heat the right front tire and push up the track..no stick!
We locate the front roll center to the right side to add down force on the tire.
If we are using the race cars body aero dynamics to cause down force then we have to move the front RC to the left of center to kill off down force.
There are some wacky set up that have the front RC on the left side in static set up but it migrates to the right side under roll and droop.

we need even more offset on dirt to stick the right front...

 

[Johnskx125]

Hey mike, what would be your honest opinion on fuel cell placement on a dirt race car, our only option is left middle right and and very low. Attaches to bottom frame rail. It is currently in the middle. Only 3 gallon fuel cell

 

[Ranger Mike]

welcome John, look at page 14 post # 272 and page 26 post # 505...three gallon is about 21 pounds won't change things much in t he whole scheme of things but if you want optimum?...read the posts...

 

[moparx12]

Mike,

I have a question for you don't think it has been answered yet. Why is it beneficial on a 3 link solid axle rear suspension to have the panhard bar mounted such that it makes a 15 degree angle from a top view to a line passing through the axle tubes? Also, why is that better than mounting a panhard bar on the back side of the rear-end just behind the quick-change case? I am referring to an asphalt super late model on a relatively flat track (5-6 degrees). Thanks

 

[Ranger Mike]

excellent question -Panhard bars are some times called track bars and when we are discussing them here we assume left hand turn round track.

Panhard bars, are designed to locate a solid axle side to side, or laterally. The panhard bar locates the rear roll center.

We have discusses rear roll center in previous posts but a typical race car has front and rear centers, which are determined by suspension geometry. An imaginary line called the roll axis connects the front and rear roll centers. A turning car experiences a outward inertial ( some call it centrifugal, wrongly) force at its center of gravity, which tries to roll the chassis around the roll axis.

As the myth goes “centrifugal force also transfers weight from the inside tires to the outside tires”. Actually we have Inertial force “torqing “ the frame toward the outside when entering a turn.

All else being equal, raising the rear roll center increases the rear roll stiffness and thus the percentage of the inertial force ( think transferred weight if you like ) that goes to the outside rear tire. That loosens the car. Over steer. Lowering the rear roll center has the opposite effect.

All panhard bars swing in arcs, which means that the roll center moves up and down with suspension travel, including chassis roll. If the panhard bar is mounted to the right side of the chassis then the rear roll center will become lower as the chassis rolls to the right in a left-hand turn. That loosen the rear end. Right-side chassis mounts are the most common on pavement, while left-side mounts are more common on dirt. With a left-side chassis mount, the roll center will rise as the chassis rolls to the right, tightening the car up.

Panhard bars can also push and pull the rear end sideways as the suspension deflects. If you run a short Pbar you get a lot of deflection. Longer Pbar minimizes this deflection. Lateral rear axle movement generally causes rear end rear roll steer. If the rear’s skewing increases the right-side wheelbase vs. the left, the car will have roll-over steer.

Just like a short Pbar inclined bars further increase the amount of axle movement if their inclination increases as the chassis rolls. For example, a short panhard bar connected on the left and inclined downward toward its attachment at the rear axle will pull the rear axle toward the car’s left side quite a bit as the car rolls right. This is common on dirt race cars. The panhard bar controls the amount of lateral axle motion, and the geometry of the other suspension links determines how much the rear axle roll steers. Raising the panhard bar where it connects to the chassis will increase the amount that it pulls the axle to the left as the car rolls. With most dirt suspensions, this adjustment will increase roll over steer, loosening the chassis up in the turns. The opposite is also true.

Inclined panhard bars also produce vertical forces that act on the car’s chassis and the rear axle at the bar’s attachment points.When a panhard bar is level, the rear tires’ entire lateral grip is transferred into the chassis at the bar’s end horizontally. But if the bar is inclined upward to the left as in dirt car inertial the forces transmitted into the chassis will have two components – one horizontal and another vertical.

On dirt cars with left frame mount of the panhard bar, this causes chassis roll because it pushes up at the car’s left rear. There will also be an equal but opposite force pushing straight down on the axle where the bar connects to it. These opposing vertical forces increase as the inclination of the panhard bar increases, especially with larger angles.

If the bar is connected on the right and inclined upward to the right, it will pull the chassis down on the right side, but it will also unload the axle with a vertical force up, where it connects to the axle.

With inclined panhard bars – and all bars incline at least a little as they swing through their arcs – the point where the bar connects to the rear axle is important. If it’s at the center, its vertical force will be equally divided between the two rear tires. If it’s to the right, proportionately more of the vertical force will go to the right rear tire. The J-bars commonly used on dirt race cars are generally mounted to the right of the axle’s center line and inclined upward toward their left chassis mount. That sends more downward vertical load to the right rear as the car turns. J-bars provide drive shaft clearance.

The amount of room you have will determine if you mount the panhard bar ahead of or behind the rear axle. Depending on how you mount it the axle to rotate about its centerline, moving the bar’s mounting point up and down under acceleration/deceleration. Do not forget your rear roll center will move due to the axle twisting under acceleration. It may lower if the bar is moved from the front to the rear. You have to take the time to find out where the rear RC migrates when you go from static to dynamic...this is where the chassis software comes in handy. As with the other variables affecting the rear roll center height, moving the roll center up will increase rear roll stiffness and thus loosens the car up.
I never was a fan of offset mounting. Non parallel non planar mounts of the Pbar as viewed over head. I like things square to the frame and 90 degree vectors and the like.

Summary - Panhard bars should be parallel to the ground when the car is at static ride height. Otherwise the axle will be forced to move diagonally. A Panhard bar will cause lateral movement y nature but you want to minimize this. One way is to use a longer the bar because you have less displacement. Never preload it. Attach the bar to the chassis on the right side for left hand turns. Your rear roll center is the point crosses the centerline of the car.

 

[Ranger Mike]

http://www.jayski.com

i recommend this site for finding out the true time the green flag drops. Also has good insight into NASCAR rules, penalties issued before during after the race.

 

[loganc_racer]

It has occurred to me that I may be misundestanding rear steer. Somewhere in all of the data I have viewed in the last couple months, I have in my head 1/8" as an amount of maximum rear stear. I have been measuring how much the axle moves as if to shorten/lengthen the wheel base. Should I be looking at it as how much the tires toe ?
Thanks, Logan

 

[Ranger Mike]

Rear steer is when the differential housing moves up and down and changes the wheel base. Growing wheel base means rear roll over steer. This action will not cause toe out in the classic sense because the rear end is one solid piece and the rear tire center lines will be the same during roll as it is static. The old notes from the Monte Carlo street stock I worked on says when I was simulating chassis roll ( 3 inches of right-rear bump and 1 inch left-rear droop), the metric chassis pulled the right rear tire forward almost 3/8 inch more than the left. This will tighten the car on corner exit or anytime the power is on. I think we ran 1/2 inch one time but was too squirrelly at power on.
I do know dirt late models run insane amounts of rear steer to hook up...

 

[loganc_racer]

That is exactly what I get, if i go 3" bump right, 3" droop left it goes to about 7/16". I have found that doop causes the least movement. when i put bump in it starts to move greater amounts. I trIed raising the ride height to make it better, and it does, but the ride height is probly going to be to high. What do you think about limiting the roll of the car to help minimize this situation?

 

[Ranger Mike]

I am not a big fan on induced roll under steer. If you can not run neutral a small amount of roll over steer is ok as you can dirt track it if need be. Pushing is bad.
If you are loose going in and push coming off then look at rear roll under steer as a cause. But this class sis so restrictive, you know better than I as to what you can adjust on your car.

 

[UmpWorld]

Ranger Mike.

Question. Somewhere out there, I found a formula that gives the equivalent spring rate when moving the spring 2" forward on a Torque Arm. I cannot seem to locate this formula anymore. It looked at distance from center of the rear-end to the center of the 5th coil spring on the torque arm (example 32" to 34"). Do you have the formula?

 

[DaveC426913]

860+ posts?
I think we need a whole new subforum dedicated to Race Car Suspension Class.

 

[Ranger Mike]

Ump welcome , regretfully I have no formula for you. Do you have photo of this application?
Dave C ...glad to assist the popularity of the whole physics forums web site...and thanks for putting up with my rants over these past few years...
rm

 

[Tom_K]

Ranger Mike.

Question. Somewhere out there, I found a formula that gives the equivalent spring rate when moving the spring 2" forward on a Torque Arm. I cannot seem to locate this formula anymore. It looked at distance from center of the rear-end to the center of the 5th coil spring on the torque arm (example 32" to 34"). Do you have the formula?

I don't have the formula but here is an online calculator that may help.

 

[Ranger Mike]

we discussed motion rate , wheel rate and spring rate calculations in earlier posts. Regarding torque arm spring mount position, the location most likely will not change the effective spring rate as much as the mount angle relative to the chassis. This change vs mount angle will not impact on handling as much as the leverage angle effects the traction. i will look over the notes once the car hauler door unfreezes (thaws out )

 

[UmpWorld]

Ump welcome , regretfully I have no formula for you. Do you have photo of this application?
Dave C ...glad to assist the popularity of the whole physics forums web site...and thanks for putting up with my rants over these past few years...
rm

Thanks,
Attached, are two pics of what I'm referring to. I don't have the software program. I copied the pics.

 

[Ranger Mike]

a picture is worth 1000 words..ok you use the formula on page 1 post 17 for wheel rate calculation. basically it is distance A / distance B squared.
we assume the coil spring is mounted straight vertical and not at an angle.Looking at your computer screen shot you attached in above post A=40" I estimate B = 43" ?
so 40 / 43 = .9302² or .8953
you multiply your coil spring rate times .8953
if you run a 350 # coil you really only have 302# to counter torque
if you move the coil toward the rear end by 2 inch now you have
38/43 = .8837 and you square this so you have .78
.78 times the coil spring rate of 350# = 273 #
or something like this...

 

[UmpWorld]

a picture is worth 1000 words..ok you use the formula on page 1 post 17 for wheel rate calculation. basically it is distance A / distance B squared.
we assume the coil spring is mounted straight vertical and not at an angle.Looking at your computer screen shot you attached in above post A=40" I estimate B = 43" ?
so 40 / 43 = .9302² or .8953
you multiply your coil spring rate times .8953
if you run a 350 # coil you really only have 302# to counter torque
if you move the coil toward the rear end by 2 inch now you have
38/43 = .8837 and you square this so you have .78
.78 times the coil spring rate of 350# = 273 #
or something like this...

Thanks Ranger Mike

At the same time you were writing this, I got a response from a chassis designer that said when moving the spring forward on the torgue arm, in order to keep the same effective rate, the formula would look like this:
(Old Rate x Old Length)/ New Length. So, if the current rate was a #300 and you were going to move the spring forward 2" (36"), the rate of the spring would need to be:
(300 x 34) =10200 / 36 = #283.33

I looked at post 17. I see a few differences in that my spring is the end of the arm in what ever location I place it at.

This forum board is awesome!

 

[Ranger Mike]

We are coming up to 350,000 views. My heart felt thanks … I wish I could take credit for this but such is not the case. As I said many times, all this can be learned on your own. It just takes dedication and commitment to stick it out. Once the light bulb comes on .. the rest is simple. One mentor in my life I wish to thank is a fellow.. R.E.L. ..he should have been Richard Petty’s adviser. He was for a brief period at a truck plant in Ohio. God Love him.. I sure as hell learned a whole lot.. and after doing major stupid stuff on the race team.

Dick..sorry about the ball pien hammer on the brand new race car paint job..
I still made it in the Army……dfamn
Still lived thru it….Don…Ronnie..Doug..Arnold eve n wanted to beat my ass..

Bottom line is ..ifin you love racing. No one can keep you out. But be advised young Parnelli…once in ..you can not get out..entirely..you will always be in it..

Btw its my birthday,,,bad typing

 

[Jared0020]

Can you explain Jacking Force and how to achieve the best way of doing it

 

[Ranger Mike]

maybe...after i get done wit h Tech inspection..after my bday...

 

[Ranger Mike]

i got to a get off line ..bday party going..just want to say thanks..racers...we risk life and limb for astupid trophy..work on a freezing concrete floor in winter..repair a p.o.s crtaftsman lawnmower to save tire money..show up at the kids school play smelling like 90 weight...church lady thinks you are th devil..

yeas i am a little ..happy.. tonite..remeber this

three people you never piss off...
Army Cook
You fav bartender

most important...

never ..ever ..piss off the Flag Man!

did we break 350,00 0 yet??

 

[Philthy]

Going back to my question on page 38 and replies on page 39 I have remedied the problem.
In the end I purchased Susprog3D software and drew up the front and rear. With the front I found that shortening the instant centre by lowering the top control arm did a few positive things. The dynamic roll centre (in tilt and bump) remained closed to the static R/C and the camber gain in bump + camber loss in droop improved. I reduced camber down from 4 degree neg to 3.2 degrees neg and then went racing. The instant centre ended up right in the centre of the opposing tyre with a 80 inch track width. The angle in the top arm meant I had to modify the arm so the rod end would not bind.
The on track performance was great, car was a dream to drive straight out of the truck and the tyre wear looked very very good. Podium result so I would say worth the effort. Thanks for the input, some of it headed me down the right course.

 

[Philthy]

Next question. The coilover spring angle on a live rear end? Is there an advantage to angle the spring so as to minimise the spring rate gap between the rate in bump and the rate in roll?
For instance my car has 550 lb spring that give a 550lbs rate in bump however the rate in roll is only 245 lbs.
If I angle the spring somewhat the bump rate reduces to 422lbs and the roll resistance rate increases up to 299lbs.
The new spring angle runs close to 90 degree from middle of coilover to roll centre (low rear roll centre on my car with watts link horizontal under the diff).
Makes sense to me, just wondering if it is worth the re-fabrication and whether it will reduce lap times as the car handles pretty good now...but maybe could be even better?

 

[Ranger Mike]

Philthy at this point i would say you are the best expert to decide on your car. You are there. You have reached that point where you can pretty well figure things out and set them up to win. If you know that much on spring rate, stay with your set up. I like the fact you run Watts linkage. Always liked those setups. A real bear to get right but once you did, it ran away from the panhard bar set ups. You spring setup sounds good. Glad i could be of slight assistance.