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

[groot]

Maybe not "burning" the rears, but definitely getting them greasy in the middle of a long run. I usually start out running a slightly higher slip angle in the rear and as the tires get hot, that slip angle gets higher and higher.

I'm usually alone at the track, so I'm not religous about getting tire temps. My racing buddies are running SMs in regionals.

But, there's usually a 10-20 degree difference after a long run between the fronts (170-180) and the rears (190-205). After a short run, they're pretty close.

So, in my mind, once I get the toe curve under control, I'll re-assess the situation. I do believe a crappy toe curve builds heat more quickly.

After that, I'll decide to either go down a tire size in front (the 8") or change a compound at one end of the car. I've heard the R250 compound is not a great choice, but I don't have direct experience.

[Brett W]

My bad I was under the impression the GT3 used a weird 5 link setup where the boxster used the strut setup. See what happens when I only get to work on old stuff. Plus there are no Gt3s running in HSR, yet (that I get to work on).

Kevin this tire data may not matter to you, but we run a 430 on our RX7. For a long time we ran the EP tire before we went to a wider tire. We would see 175 at the center of the outside rear tires. I will usually start pressures at 20-24 depending on conditions and come out with pressures close to 30-31 by the end of a 30min test session. The car weighs 2200 and change, so you should take that into account. The driver doesn't say anything about the tires going away towards the end of the situation.

[Brad Roberts]

Kevin,

we used 250's for qualifying if we had more than 4 people show up for our class.

made some calls:

The west coast tracks (Laguna & Sears.. very similar in elevation change to Mid Ohio/technical) the cars performed best at 27-28 hot on R430's from a year ago. This is a race winner who qualifies on the poll in a legal car against 5-6 other Fp cars who show up. Multi time National winner.



B

[Thorshammer]

Okay boys!

I have heard some of the limited prep guys running the cantelever slick at higher pressures than normal, I can't run them that high. My normal starting pressure is near 17lbs which gives me a 24.5-27 psi hot pressure. As for qualifying with 250's. Good call, my budget does not allow for that. I have been using tire warmers and qualifying on the 430's. As for running 600's in the rear. I don't think thats the way to go. But you have to do what you have to do. I think Kevin is driving the car very well, and very hadr trying to make up for his lack of power. He ran 139's? at MO and I ran 1:36's (I think) and I probably had 30 or better hp on him. Once he gets his new MOTOR. He'll be fine, btu I do agree that the rear triling arms can be improved. All the rest of us are doing is working with the existing trailing arm, Where as Kevin is attempting to correct some deficiencies (sp) with all semi trailing arm designs, adn apply them to the 914 within the constraints of the rules.

BTW Kevin, Sargis car has a similar type deformable bushing and toe link for his trailing arm swingaxle spitfire. Many of the front running spits have now developed something similar. It has been through two protests and been declared legal, so I think you are fine.

I don't totally agree with you in one way, I really don't think EP is just a HP class, I think both of us can run at or near the front with the right engine and chassis combo. With the way my car is set up now, I think I can run with Pratt or any of those guys. Lowshaks Honda IS fast. it will be interesting what my car will be able to do against him with the new motor. Building a Runoffs only motor.

I think if you have yours (come on JAKE buddy, I think I can, I think I can) (just kidding, I know your working hard at it) you will have a good shot.

I really think one of the only drawbacks to what we have is getting fresh air to the carbs/airbox, legally.

Erik Madsen

[Eric_Shea]

Great thread Kevin. This will be great to watch.

Erik and I had a talk about control arms last month. I'll have a nice kit or complete arm for the more plebian enthusiast based around the stock unit and some of the things we discussed.

Eric

[groot]

Erik, thanks, but I didn't get into the 1:39s, I only managed a high 1:40.

Updates... Here's how I'm mounting the rod ends to the trailing arm. I've got to do this on the bottom of the knuckle, too. It's beefy, I know. The 2nd generation of this suspension will use alum 944 knuckles.

[Brett W]

Have you run an FEA on this setup?

[groot]

Nope... if I had, it surely would have been much less beefy.

[GTPatrick]

Food for thought on this subject folks. Don't know if the following have been previously discussed or not but here goes.

1) Has anyone given some thought to adapting a Porsche 911 rear trailing arm suspension to a 914 ? Maybe one from a competitive/racing 911 car ? Cost may be high but some of the ideas in this thread ain't cheap either .

2) What about a totally new handbuilt design that completely replaces the 914 trailing suspension pieces ? Maybe even 1 that uses some of the components from either a 911 or possibly a 914 ? Again costs and time for development rears its ugly head here too.

With all of these design modifications, won't some of them possibly have a builtin design flaw/weakness that the original suspension design did not either address or was ever previously conceived of ???

Just some food for thought here. Some of your ideas seem to be really neat but why not take it one step further ???

GTPatrick.

[Thorshammer]

GTP,

The SCCA Rules have some very strong limitations. Most of us can and would change the entire design, but we must keep within the rule structure. IE Semi trailing arm. One problem using the 911 arm is the position of the engine and where the 911 arm would need to attach. Remember the 911 arms attach in an area where the trans width is, we must attach them to an area where the engine width is. This makes the car way too wide when using 911 arms. So this probably won't work.

Also Kevin is trying to change his camber gain while minimizing toe change. which is critical for racing cars. changes in toe which are significant are not good. While an amount of camber gain is very good. We actually would like just a touch more than we have now. And without the amount of toe change.

We appreciate your comments about this. Keep em coming.

Erik

[GTPatrick]

Erik, . . . .

I thought that it would be a rather neat idea for a street/track car ro be used in AutoX , PCA DE events or some other track event. I figured that the SCCA wouldn't go for it.

But if someone had enough $$$ and a lot of time on their hands iat could give some of those rice burner cars a good run for their money.

[Brad Roberts]

Ha ha.. I have a friend who thinkgs along the same lines I do! We discussed replacing the whole damn thing..

Scary how close the Boxtser stuff fits under a 914. Cured all toe gian issues and camber gain!!

Not legal (IMG:style_emoticons/default/sad.gif)


B

[Brett W]

If you don't have to, scrap the semi trailing arms and go to a true upper and lower control arm setup. That is what I am working on. Super production here we come.

[groot]

Every suspension design has it's compromises... even a double A-arm setup. I believe it's a matter of optimizing whichever design you have or switching to one that accomodates your needs.

[groot]

Left wheel end complete.

Tomorrow I'm off to AZ for work for 2 weeks, so no more progess until I get back. (IMG:style_emoticons/default/mad.gif)

[brant]

kevin before you leave town for work...
can you hold it up to the chassis in place for those of us that are extremely visual?

brant

[roadster fan]

If you look at the third picture, I think it mounts in that orientation on the drivers side (we are looking from front looking towards back of car). It has an upper and lower link, and the toe link is the mount furthest inboard (left on picture).

the upper and lower links will run to traditional 914 pickup points and the toe link will run to an advantageous position on the chassis to minimize changes.

Hope I got it right........hope you can visualize it with that explanation. If not I guess we will have to wait for Kevin to return from AZ to correct me.

Jim

[groot]

You got it, Jim.... except for the part about using traditional pickup points. If I used the original points, I'd be stuck with the camber gain of the original design.

I was thinking about how to hold it up to show the idea better, but it will definitely make more sense when I have some tubes attached... and all my tubing is still full length.

[Series9]

Ha ha.. I have a friend who thinkgs along the same lines I do! We discussed replacing the whole damn thing..

Scary how close the Boxtser stuff fits under a 914. Cured all toe gian issues and camber gain!!

I'm planning on that exact conversion on my next personal project.

[byndbad914]

Kevin, are you running the toe link all the way forward? Any reason why not inboard? Here is a shot of my 5-link rear I just finished a few months ago, I used two lower arms inboard to set toe, the top will set camber (mild track changes won't throw off the other dims).



based on this drawing I found online at one of those supsension software sites


Don't quote me that this is the best design ever or anything like that - I just got the car fired up yesterday, so now I have to weld in the seats and get it all scaled and finally get a real alignment on this car and check all the bumpsteer. Visually (tho' how accurate is that, right???) I see no bump whatsoever through about 5" of motion and since I will only see maybe 2" of that, I suspect it will be really solid.