Rear suspension redesign, Optimizing a semi-trailing arm suspension Options

[groot]

Finally, I'm actually constructing my rear suspension. This one's been in the works for a long time. I've had the parts, but lacked the mill/TIG time to make it happen. So, I'm starting with the 914 trailing arm instead of the 944 trailing arm (need the TIG for that one since it's alum).

Some background, the SCCA production rules state this under "Unrestricted Suspensions: Suspension Control":

2. Suspension pickup points on the chassis or structure may be relocated and/or reinforced. If such points are relocated to the driver/passenger/trunk compartments, such points and attendant suspension components shall be covered with metal panels.
3. The manufacturer’s original system of suspension, e.g., live axle, swing axle, MacPherson strut, A-arm, etc., shall be retained. The wheelbase of the vehicle shall not be changed or relocated in a fore/aft direction.
4. Suspension bushings are unrestricted. Adjustable spherical bearings or rod ends are permitted on all suspension components.
5. Any anti-roll bar, camber compensating device, panhard rod, watts linkage, and/or other suspension stabilizer is permitted. Attachment points of such components are unrestricted.

So, I can move pickup points, I have to run a semi-trailing arm suspension, but I can add a toe link.

The basic idea is that I can get the camber gain I want by playing with the location of the pickup points of the trailing arm to the chassis, but replacing the bulky semi-trailing arm with rod ends on tubes allowing easy control of static alignment settings. Simple geometry.... But, you're stuck with a crappy toe curve.

What suspension gives an excellent toe curve???? A real trailing arm (no toe change). So, allow the rear suspension knuckle to pivot in toe, but control that toe with an additional link (attached to the chassis in such a way that it mimics a trailing arm). This was the brainstorm that hit me one day.

I gathered all the parts last winter, but spent so much time in AZ for work I didn't get to do the work before the racing season crept up on me.

Here's the work I did this morning. That rectangle stock sticking out will be where I attach my toe link. The semi-trailing arm links will mount on the top and bottom of the knuckle (bearing carrier).

[brant]

this is going to be a great thread!

[URY914]

(IMG:style_emoticons/default/popcorn[1].gif) (IMG:style_emoticons/default/biggrin.gif)

[Brad Roberts]

We need to go over and figure out exactly how Kinematic toe works on the Boxsters/993's/996's

Let me show you a little secret:

928's were the first to have Kinematic toe (little to no toe gain in bump)

I'll find a better picture when I get home. This tube chassis has the 928 arms on it.



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[Brett W]

Guys, Kevin and I have beat this one to death. One thing that we disagree on is, the interpretation of the rules. I say run something like the Sheridan rear suspension so you don't have to deal with the roll center movements every time you adjust the suspension settings. The rules say you can't add any extra pivot points. I don't see any extra active pivot points.


One of the benefits of Kevin's setup is he can force a favorable toe change which unfortunately on our suspension designs is non existent. Our cars will toe when lowered radically. While decent on the street it doesn't provide good suspension control on a race car.

How much rear suspension travel are you planning in your setup?

[Brad Roberts]

I have been of the belief that you could "fake" the semi trailing arm and do some kind of inboard heim joint control arms that stress the tranny (like an open wheeled car) use the semi arm to make it look like you are using it.

Roger Sheridan does this with an empty strut tube in the front of his car and a Penske shock mounted in-board of the empty strut tube. The rules call for it to be in place but he couldnt get a decent shock for the strut.


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[Brad Roberts]

Not sure what your bottom arrow is pointing too, but Roger only runs one toe adjustment rod. The entire wheel carrier pivots on that lower spherical bearing.

His car passes SCCA GT2 rules with flying colors. Jim Blakewells rear arms are VERY similar to Rogers and the ones we worked on back in 1996.



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[Brett W]

As far as the Boxster stuff goes, it uses a rear strut suspension. Its elastokinematics move the rear suspension to toe in under braking and over time the bushings will wear and create the same problem under lateral force.

The 928 has the same issue. It requires bushing compliance which is fine on a street car, but a race car requires a higher degree of suspension control.

Sheridan runs two toe adjustment rods on his car. Check this next picture notice the upper and lower rods. Sheridans car is grandfathered in as is Blakewell's car. SCCA put a change in the rules that require the shock/strut to run inside the spring. SO all further cars built after, I think 1997 (I don't have my rule book here) ahve to abide to the new rules. Also newer cars built after a certain date can run any type of rear suspension, in GT. But back to Production racing.

When you adjust our rear suspensions you have to deal with moving the roll centers around. It will change the handling every time you adjust it. Now Finch likes to set the rear suspension where it needs to be and re-weld the stock trailing arms into position. Other than dealing with compliance in the rear suspension arms this is not a bad way to go. I would build a new arm all together and set it where you need to be. Problem with this approach is you are pretty much fixed as far as suspension settings go. Having adjustment is good which keeps bringing me back to Sheridan's rear suspension design.

[maf914]

What is he doing with the shift rod where it enters the side shift console at the tranny? Is that some sort of universal joint? (IMG:style_emoticons/default/idea.gif)

The shift linkage discussion on another thread made me notice this.

[Brad Roberts]

He is running a seriously modified Vellios shift kit on a 915. The Vellios kit utilized a stock 914/901 plastic cover for the shift console.

Roger's arms versus Finch method:

The Finch method is much easier and you will find it more in production cars than you will the Roger style of arm.

Something else of note: learn to drive. All this is great, but I have found that the pro's rarely know SHIT about the suspension. They can drive BRICKS if need be. Bontempi will school your ass right now.. with none of this trick shit..LOL He may go faster with this good stuff (I installed roller bearings in his car a fw years back) but I really dont think all this is needed in a production car. It just doesnt build the speed or G forces that say Blakewells or Rogers car can generate.

I disagree about the "compliant" rear suspension stuff on kinematic toe. I havent ran a GT3Cup car or GT3RSR with "rubber" in the suspenion (ever) Pretty easy to see what the suspension does on the alignment rack by loading and unloading the chassis. Physically watching it, the toe doesnt change front or rear. I *beleive* the toe does change in the rubber cars, but no where near that of the semi rear control arm of the 914.


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[groot]

Ok, just got back from the dentist (root canal), so I now can get back to work.....

Not to hijack my own thread... but.... Production racing is more about engine than either the driver or the suspension. The next most important thing is the driver.... then it's suspension setup more than design.

I won't list my driving credentials because I think it's a silly discussion. Who cares? I've always said that if I get to the point that I (as a driver) am not getting the most out of my car, I will put someone in it who can.

I am doing this because I want to and it may make my car a bit more driveable and a bit easier on the rear tires. This is why I'm running production in the first place. Finch's car runs on a pretty close to stock suspension and it's very fast. I was inspired by Finch's legal GT2 944, however.



Now, back to the discussion at hand. Sheridan's setup is great for setting STATIC toe. It does nothing for DYNAMIC toe.... (the curve). His setup suffers the same toe curve issue on all semi-trailing arm suspensions. My idea is to control toe dynamically.... it's not a new idea. I've heard of people using compliant bushings and a toe link to deflect them to achieve a similiar results. Sheridan could have taken the next step and still be legal in GT.

Brett-I've come around on the legality of that Sheridan setup....actually a discussion with Finch helped me see the light (IMG:style_emoticons/default/rocking nana.gif)


Now, back to (IMG:style_emoticons/default/welder.gif)

[Brad Roberts]

Roger gave up on SCCA GT2 ages ago. I dont think he ever actually ran an SCCA race.

You are correct. Figure out a way to control toe throughout the suspension range and stay within the rules.

Should be interesting to say the least.


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[groot]

A little progress.....

Maybe this will help some visualize my project. And... no, I won't cantilever the rod ends, I need to fire up the chop saw to make the other side of the bracket and that would wake the boy.

Picture 2 tubes coming from the upper portion of the knuckle and the same on the bottom. One tube from the top goes straight forward and mates up with one tube from the bottom on the chassis. And, the other two tubes mate up inboard of that at a different point on the chassis. Where these points are located define the camber curve. Then, there's that lonely tube that comes off of the rectangular tubing... that's the toe control link. That mates up at a different point on the chassis and it's location defines the toe curve.

This is still a semi-trailing arm suspension.

[bam914]

Kevin, How fast are you going through rear tires? Are you running a Lincoln locker? AKA a welded diff. When I had my EP car I had six tires. 4 430 GY's and 2 600 GY's. I ran the soft one for qualifing and the 600's went on the left for the race. I had 6 sprint races, one Solo I, a PCA driver ed and the SARRC championship, which I won, on the those tires. They still had just less then half the tread on them. Just wondering. I had no crazy rear setup. Just the stock attachment point. -3/4 degree camber and 0 toe and an open diff.

[groot]

Hey, Blake,

I'm not really wearing through the rears any faster, but during a race I put much more heat in the rear and the car gets loose after about 6 hard laps. It's enough to force me to consider going to a split compound or smaller tire size in the front. This project is just to get the rear suspension working as well as I can get it to work. From here, I'll start trying split compounds or sizes to get the balance right over the entire race.

I've been running a open diff, too.

[Brad Roberts]

What tire pressures Kevin? and who's tires?

I'm limited to Goodyear experience with the production cars.

We did a TON of tire testing with Goodyear after they moved the plant from the USA. We still have tires with no compound numbers on them that they gave us after testing. They *used* to give us tires before each National race with no compound markings. Kinda cool.


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[Brett W]

Something else of note: learn to drive.

I disagree about the "compliant" rear suspension stuff on kinematic toe. I havent ran a GT3Cup car or GT3RSR with "rubber" in the suspenion (ever) Pretty easy to see what the suspension does on the alignment rack by loading and unloading the chassis. Physically watching it, the toe doesnt change front or rear. I *beleive* the toe does change in the rubber cars, but no where near that of the semi rear control arm of the 914.

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I have to agree, seat time is the most important thing to a successful driver.

The GT3 uses a completely different suspension than the boxsters and Cayman. You should not see this compliance. It is not necessary. The GT3 is designed as a race car from the beginning. Hans didn't have to worry about grandma trail braking her GT3 Cup car turning into the church parking lot. But when she lift throttles mid corner he can't have the tail of her shiney Boxster convertible cruising around on her when she is headed to the grocery store. All production cars are designed with toe in under braking for stability. The 928 relied on elastokinematic deflection to create its toe in under braking. The front dog leg moved back and the rear dog bone actually bent under braking. Race cars do not typically have compliance designed in. It makes the cars harder to drive.

Kevin, I agree Sheridan's setup is for static settings only. Why he didn't design in a toe link, I don't know. Maybe Bertold could tell us if we knew where to find him.

Production is more for drivers where GT is more for builders. Kinda like the Mod class in autocross.

[groot]

I share my progress on my rear suspension and everyone tells me to learn to drive. ZZZZZZZZZzzzzzzzzzzzzzzzzzzzzzz


Brett, prod is more a builders class than IT, so it all depends on your perspective.

Brad, I've been running 430s. I haven't run 600s yet, but may resort to that in the rear next season. The weight this car runs at, I find it hard to believe a 600 compound is really the answer, but I'll try it. I've been running a 1.5-2 psi split to keep the rears under the car. Starting pressure varies between 18 and 22 psi depending on temperature. I had some good conversations with Loshak about some other things to try, like starting at a higher initial pressure.

I wish I had a tire testing deal on the race car, but for now I just order 100 at a time for work and chew them up on a daily basis..... Our team of 4 guys ate up 32 tires in one day a month ago....

[Brad Roberts]

Hang in there Kevin! My "learn" to drive goes out to those watching at home. All too many times I come across people who spend more time "engineering" and not driving. It wasnt directed AT you. We have a LOT of people who "watch" these threads.

Thanks for the info on the tires. Unless something changed with Goodyear, I could never see a 914 on west coast tracks needing a 600. "Maybe" Willow Springs at 120deg track temp and mostly FAST right hand turns, but we have always dialed that out with tire pressure splits.

Burning down the rear tires.. huh?? can you give me tire temp examples from notes?.. if tyou do have it.. include track temp/air temp/ and actual track you ran that day.


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[Brad Roberts]

The GT3 uses a completely different suspension than the boxsters and Cayman.

Then why are 3/4 of the part numbers the same? (early 99-05 GT3) and early 97-04 986/996

The alignments are performed the same

Kinematic Toe is done the same way

Completely different? No. Different? yes.. but not by much



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