Race car suspension Class

by Ranger Mike
Tags: class, race, suspension
 Sci Advisor PF Gold P: 1,403 Again many of you have messaged me on advice on gaining an advantage “ legally”. I looked up my notes from flogging the old GM metric chassis during my door flapper days. I was into chassis measurements big time and wanted to MAP the rear end movement relative to ride height and was trying to understand what was happening when the car was in the turn. I found that by setting the chassis on stands and mapping the rear end movement using 3 inches of right rear down and 1 inch left rear up, the metric chassis pulled the right rear tire forward almost 3/8 inch more than the left. This was AFTER I loosened the bolts holding the trail arms ..etc... as the stock serrated teeth bound up the components when “ properly tightened”. Anyway, this movement when rolling into a turn, would have the effect of tightening the car on corner exit or anytime the power was applied. I learned three things here. 1. Stock rubber bushings flex and can be to your advantage. 2. I had to free up the components to swing free when race ready and replacing the inner metal spacer that had teeth with a smaller diameter non toothed spacer was a must. Also I replaced the stock bushing bolt nuts with lock nuts at could be run down tight and then backed off a turn to prevent bind. 3. As the venerable Stroker Mcgurk once said..” If some is good and more is better then too much is just enough “ so I went on to see how much flex I could get out of the rubber bushing. More on this later. You have to measure your car to find out where you are relative to where you want to go. If the rear axle pivots so the right-rear pulls forward, this will tighten the car. If the left rear pulls forward, the car will be looser. Roll steer can be affected by using very soft rubber bushings. I tried the old tire softener trick but it seriously and quickly deteriorated the bushing and was a mess. With softer bushings on the right rear lower trailing arm, you have roll steer loosening the car on entry. This would reverse with the power on and tightens up the car on corner exit. The soft bushing allows the right rear wheel to pull rearward under braking. With acceleration, the right rear will push forward and this tightens the car on corner exit. You can soften these bushings by drilling holes in them parallel to the bolt. This permits more compressibility.. Before you loosen up all your bushings, you better think about it. If you want more roll steer to tighten up the car on corner exit you would soften the bushings on the right lower trailing arm. This compresses the wheel base on that side during acceleration by pulling the right rear forward. It would then steer to the inside. Softening the left side lower trailing arm bushings would offset much of the right side compression. This would loosen the car. Softening both might be an aid to forward traction because the rear end bottom mounting points ( trailing arms) shorten up and thus move forward in the chassis adding weight to the rear tires and more traction. Beware- If you are using roll steer to tighten the car on corner exit, it may loosen the car on corner entry. So why do all this...so you have an advantage over the other racer who looks at other areas for an advantage, The rules say stock trail arms and mounting locations and in some cases, no eccentric bolts...so...think about it...you could get up to 1/2 inch movement that could make all the difference.
 Sci Advisor PF Gold P: 1,403 Happy New Year and you got the best present possible...yeah I agree. Shock location half way between the mount point and Ball Joint will be pretty in effective. The closer to the BJ the better and if rules say you got to run stock location on the shocks...run a racing shock. And you MUST run a shock ( damper) to complete the proper susension. In some cases we need to run a tie down shock to keep transfered weight where we want. Stock location is not excatly racing ideal but as a minimum you need a shock and automotive engineers spent millions on designing the set up and it is a compromise for ride comfort, performance and economy of manufacture. Stock shocks will not stand up to the abuse of racing. We run Penske 3 way shcoks and have to rebuild every year. The shock oil deteriorates and the thin metal shims wear out. You get alot of heat when you dampen the suspension and this is the main culprit.
 Sci Advisor PF Gold P: 1,403 Speedway Motors has a bunch of racing shocks specifically for your class..I like this one myself..affordable and who is going to claim your " used shocks" for $50.00..never seems to happen..make sure you paint them same as your original shocks and smear a lot of grease and dirt on them.... http://www.speedwaymotors.com/QA1-St...ing,31463.html  P: 2 Notice my postscar were gone? I gotwas deleted forand being a spammer somehow? Anyway now that im back up and running I can ask more questions. On the rear ARB will a aftermarket stock mounting one suffice? I have seen some that mount to the rear end with similar bushings as the front mounted on the housing then the ends hooked to the LCA ahead in the front with some adjustable links ie http://www.hotchkis.net/_uploaded_fi...455image_2.jpg . Since it doesn't say anything in the rules about it id like to try but I have a good feeling if I start winning that will be the first "gray" area fixed and that bar mounting system isn't cheap. Your thoughts?  Sci Advisor PF Gold P: 1,403 I would mount a stock appearing ARB on the rear as close to " stock " location as possible and I do not think they will say anything because the car came with one from the factory..right?  P: 2 You know I bet that ol wagon did "wink" :) thanks RM  P: 2 Hi Mike. A question for you please... We have a car that is still struggling for side-bite on a flat slick trac. I am considering reducing my front roll couple by either reducing my right front spring or lowering the rate of my swaybar. I figure that this should transfer more weight onto the right rear through the first part of the turn. Am also considering reducing the rebound on the left rear shock and increasing the gas pressure hopefully speeding up the weight transfer left to right. What would be your opinion on this? Thanks  Sci Advisor PF Gold P: 1,403 Welcome and thanks for reading the notes. I assume you want to hook up faster and better to drive out of the turn..right? Flat track needs a lot of stagger and this is what drives you off the turn. Don’t forget when you increase stagger you have to add cross weight to keep the same down force in the car. Spring change- As stiffer left rear spring will tighten the car from middle of the turn to exit by keeping the cross weight in the chassis. Side bite and traction usually are a factor of the rear lower trailing arms and we need to look at these. Are you running 3 link or 4 link? Torque link? Do you have spring loaded radius rod on the rt rear? Raising and lowering the front of the rear trailing arms impacts the amount of load placed on the rear tires under acceleration. The trailing arm up hill angle adds more load because the rear end wants to move under the chassis as it hooks up against these links. The uphill angle of the arm reacts against the twisting motion of the rear end and ultimately loads the tire. Classic axel thrust. If you want more right rear side bite move front of the rt rear trail arm up hill a few degrees. But if you wil be taking load off the left rear when you do this. Typical setting are 3 degrees on the left rear and 2.5 degrees on the rt rear. Watch out for roll steer and know what the change does when you change this. See post 116 on page 8 of this forum.  P: 1 Hallo Mr. Mike, I read your post and that was really helping. I just read about how to design good RC location for double wishbone suspension and I've been tried to design suspension for formula SAE. I assume that good RC location is 1 inch above ground (minimize jacking effect and non rolling overtunning moment) is that correct? That I don't understand is how to get optimize length for upper control arm (find inner pivot location on upper ball joint / find final IC location) so that my design with RC 1 inch above ground and initial fsva length approx 78 inch has optimize length for upper control arm. Thank's  Sci Advisor PF Gold P: 1,403 Hariss welcome and thank you..many people and racers have posted here...I think 1 inch abov pavement is a good start...I recommend the chassis software as a very good tool to get the proper height and location as well as seeing the RC migrate thru bump...it is worth the$ 100 or so...well worth the effort to get the checker flag! if not possible send me provate message rm
 P: 3 Ranger Mike, what do you know about the new "Weight Jacking" setup taking the asphalt late model series by storm. I notice the guys running it use short upper arms for fast camber change and a ton of lf upper angle. People running this have a ton of travel and I've seen upwards of 12 to 15 degrees of lf camber become 0 at mid corner. What is going on here that I can use to be successful?
 Sci Advisor PF Gold P: 1,403 TSCOTT, i am currently in Europe and do not have access to my notes. Will return in a week. I will be able to give better reply with notes but suspect the trend has some merit. How do you know the camber goes to zero at mid turn? any more insite on what and where the weight is jacking to?
 P: 3 I just know from observation that the lf stands up for proper camber because I've measured old tires in the trash pile from the top team doing this. It is suppose to load the lf i assume, because that car can turn really good. Just from basic observation they use a short rf spindle where the rf lower is all the way down and the rf camber change is very minimal while the lf is really tall where static angle on the lf upper is probably 25 to 30 degrees. I've also heard that the pin angles on these trick spindles are less than 5 on the rf and 15 on the lf. This team runs hillbilly on the front with a 2" bar and hardly any spilt in the back springs, sometimes hillbilly back there. We race Nascar late model stocks around nc and va
 P: 3 I think it is called "weight jacking" because when the side force is applied to the cg and the cars chassis wants to roll to the right but the tires pull to the left, the lf suspension jacks down on the spring because of the high angle in the upper
 Sci Advisor PF Gold P: 1,403 Long plane ride home...ok...when we swap in the taller left frt spindle we shift the front roll center to the left, which is what we want to plant the right front tire..a lot of previous posts on this a few pages back. Typical NASCAR spec late models run around 17 degrees upper A-arm angle while the outlaw series run 20 degrees rt top and up to 26 degrees lft ft top A-Arm. If you look at post 325 and 332 on page 21 at King pin inclination angle you can gain more insite. Bottom line is when going into a left hand turn you are pivoting on the scrub radius ( see post 325 i think). This makes wheel base longer on the right side. When in a left turn, Scrub lengthens the right side wheel base and tends to loosen the car. When you counter steer, it shortens the wheelbase and adds understeer. This is a driver friendly situation as it has a very stabilizing effect to the cars handling in driver feed back. The driver needs feedback in a turn because it tells him how heavy the tires are loading and when they are on the edge of traction. We need more scrub on the left side due to posative camber we run. In outlaw series we run all kinds of trick upper and lower A-Arm combinations but in restricted series you have to look at what is still " open" and go with radical king pin angles and scrub radius tricks to help the front pivot. If you look at the camber build on an outlaw super late model is really goes to zero at mid track on rt ft and darn near same on lft ft...remember..it is all about the tire contact patch.
 P: 7 tscott8: I've been digging into the setup you're shooting for myself. We don't have any locals at the tracks I run at using it, but at the invitationals the out-of-town cars run a setup like this and they are extremely fast. Considerably faster than our top regulars. I haven't found a whole lot of info on it either, mostly gathered info from eavesdropping and observing what they were doing from a distance, cause they certainly weren't going to show you, matter how nice you asked. I've heard terms used with it like "camber thrust", "jacking force, are the most common. I've been told a correct shock and bumpstop package is a must. I've experimented with copying this setup, but I can't say I fully understand it. I've recorded the fastest times our current chassis has ever turned, but I struggle with consistency, partly due to being a rookie in a SLM, but definitely because there's more to it than I understand. Anyone that could give more insight, either by posting on here or private email, I'm eager to learn and would be happy to share any info I have. Also thanks Ranger Mike for your help on this forum, it is a great source of information
 Sci Advisor PF Gold P: 1,403 Doc Hathaway wrote one great piece on this whole scenario. It is a must read. It is a mandatory read before you do another thing today. http://ismasupers.com/downloads/Tech...ations%204.pdf Everybody else sets up the car with your typical camber setting, caster splits and recommended “ hot set up” Kingpins and spindles. Some may even know enough to add the proper offset on the front Roll Center. The car gets around the track pretty good if you hit on the close spring rates and shock package. And you wil be competitive with every body else because you have copied every body else “ hot set up”...maybe better because you have the proper RC height and offset....but.. all this stuff is static set up where as the suspension moves and we are trying to use a static setup in a kinematic ( moving ) world. The guy who understands what is happening in this kinematic world will be able to set up and run a lot better. Basically the new " hot set up" is to run very soft springs that will hit the rubber bump stops ( post 220 on page 14 above) just when you make mid turn. You want the springs to take the max load before bottoming BEFORE the rear end comes around on you. This setup uses the King pin Inclination Angle to crank in caster to " weight jack" from the inside rear tire to the outside front tire to build proper camber. But, KIA adds camber to the LF and takes away camber from the RT FT unless you have accounted for this by compensating for it during suspension movement. Hence. the different KIA. Min camebr loss on RF by low KIA, where you want the LF to ad more Pos camber so larger KIA on left spindle. You want the max tire contact patch on both front tires at mid turn. Scrub radius and caster do this weight jacking. If you set up your particular chassis correctly, with differing Kingpin Inclination Angles (KIA) and scrub radius ANNNDDDDD you run softest spring rate to rubber bump stops, you will be using the transferred weight to the max to plant both front tires to turn the car. Don't forget the Roll Center offset which comes from the different spindle heights. One more thing. Static caster and camber does not tell the whole story. Depending upon your set up, different outcomes happen when we have a tire contact patch acting thru the scrub radius as the suspension moves. You must know what is happening at the tire contact patch while this whole compression / weight transfer is happening. Because there are so many variations of A-arm lengths, spindle / ball joint heights, spindle heights, etc..it is impossible to replicate the fast guys set up unless you have an identical chassis. You must understand the theory to make it work ON YOUR car! It is like it has always been. Tires, Tires , TIRES..and the guy who makes the max tire contact patch on all four tires a higher percentage of the time will run faster and better and win. hope I did not confuse you..clear as mud? rm http://ismasupers.com/downloads/Tech...ations%204.pdf
 P: 7 Thanks Ranger Mike, your response was definitely helpful, as was the article in the link. After reading I'm sure my caster setting was off on both sides, probably camber settings too. I've invested a lot of time and money into (attempting) to educate myself on the kinematic aspect of the sport, and admittedly have a long ways to go before I'm happy with my personal level of understanding of the chassis and dynamics involved to make a fast race car. Our fast and consistent regulars have new chassis set up by thier respective manufacturers, and they don't change anything on the car without calling them. There are few respectable cars that are set up by years of trial and error but not much understanding of what is actually going on. I don't want to be one of those guys, and I've come a long ways in the last few years, but there's a lot of aspects that I definitely don't understand. We did try to more or less copy what the super fast cars that came up a couple times a year were doing, hoping to eventually figure out what they are doing that works so good, and use it to my advantage as a regular until everyone else figures it out as well but try to stay a step ahead. We were close a few times, super fast and just trying to fine tune, and somewhere in the fine tuning throw something way off and struggle to get back. We did go as far as checking what was going on static and in dive, use performance trends software to map the geometry and a chassis r&d program to balance the chassis and set wedge. We have an awsome shock package, and more power than we could ever use at the tracks we run at. The biggest problem we keep running into is a corner entry push, and as we know a poor entry results in a poor corner overall. I've been told I drive in too hard, and that may be a portion of the problem, but we did get it to work really well a few times so I think a setup error is definitely more of the problem. To try to reduce the chance of it being a brake system issue, even though everything was rebuilt before last season, everything is new for next season except the pedal, and a package recommended by an extremely seccusseful team that races all over the country. We run 10 inch Hoosier tires, spline 1 7/16 3 piece sway bar, 10 degree spindles, bump steer set to near zero through the full range. Tire temps were always pretty decent, showing good use of the contact patch, but I've since learned about measuring the tire wear as well so our camber on the right may initially be a little low. Roll center about 10 inches left and 2 inches above ground, about 2 inches of movement through travel. Thanks again for all the info you put on here, it definitely helps. If there's anything else I should be looking into based on this info or if you need more info about the car is be happy to put it up.

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