RX-Ben

Hello. This is to anyone out there with some decent hp/tq numbers since from what I understand (from talking to AWR) it doesn't occurr on lower power cars.

Under hard acceleration, the rear tires on my T2 toe in and move forward in the wheel wells. This is evidenced by tire rubbing on the rear trailing arm (wheel is about 3+ mm away normally) and also by rubbing on the front of the fender (again, about 4+mm away).

Anyone else experience this? The rear end is solid- delrin all around and AWR toe links and rear camber links (replace rear dog bone links). My only guess is that something is giving at the hub, but I thought they were all pillowballs?

Everything is solid and just rebuilt. This also happens on my friend's 360+ hp/tq car.

Thanks
Ben

C. Ludwig

iTrader: (27)

I can see how it would happen. The delrin isn't solid. It will give. The tires are trying to accelerate forward and the chassis is trying to stay where it's at. Lots of bite and a good amount of HP and you get suspension flex. What does AWR say to do? There is a spherical bearing kit for the trailing arm mounts that IS solid. The hub has spherical bearings also. But again, I would guess, that you're using a delrin DTSS eliminator bushing. There is some give in that as well. Possibly look into a aluminum alternative? Make the sure the lateral links are in superior condition. Don't trust the fact that you can't feel any slop in the ball joints.

Carl Byck

Ben what spring rates are you running? How much squat are you getting? Ground controlJay) has more experience with high HP RX7s than AWR. AWR does mainly EP cars, so ~200-250rwhp max. Also, the cars they work on are lighter than my car, I do not know about yours, so spring rates I run are way higher than what AWR would recomend. Squat will give you all kinds of changes in suspension geometry, so consequently I am going to try ~600/450, 600/400 as a starting point. I think the BP AX guys use like ~500 all around. LMK. BTW, I am jealous, see the single turbo forum for the latest on my saga Carl

Carl Byck

Also, what tires, wheels, offsets, power, ride height, and suspension settings, dampers. You can PM me if you prefer it to be confidential, although I doubt anyone else is building what we are (other than three guys I know ) While we are at it, did you require the Toe adjusters to get proper toe, or are they simply for ease of adjustment? Mine are the old 5/8" bolts, and are going to be a real PITA to install. Carl

RX-Ben

At least it was only your turbo that was damaged. I had a freak lifter failure which toasted much of my LS1 and caused me to miss the One Lap race this year. It happened hours before I was to start the 800 mile drive to the start. I did get about 150 miles on the car and it is ridiculously quick and feels very good.

I'm running 400f/275r and it is very very docile. I have Koni 8610-1149s up front (race strut insert for a Porsche 914 app) and Koni 8241-1050 Sports in back. I could go to 500f/325r on the street with little problem, the shocks are very good. My friends who live nearby (with FCs/LS1 swaps) complain about the roads with their Tein Flexs, poly/rubber bushings, and lower spring rates but my car feels like a luxury cruiser in comparison (and I have delrin/solid links).
I think I will up the spring rate as necessary, but I need to get a number of track events under my belt before I go with a stiffer setup.

I don't know how much squat I have, but I'm guessing a couple inches. The car already isn't keen on hooking up (with 335s in back), so I'm trying to avoid going to higher rates if possible.

The car weighed 2680 with the huge and heavy street tires/CCWs, rear cage and massive front Stoptech rotors/calipers. I'd like to swap in fiberglass doors, but I don't think any exist that will fit the street/track requirements. I also have a couple things to add (Accusump, wing) which will push it over 2700.

Nothing special on the alignment- -1 degree camber in back, 0-1/16" toe in. I didn't do the alignment, but the AWR toe adjusters make things incredibly easy back there.

Tires are Michelin Sport- 335/35/17 (26" diameter) in back, 285/35/17 (26" diameter) in front. The diameters are so big that I'm stuck with stock ride height so I have frame clearance, but I'll eventually bite the bullet and have some tube work done around the front strut tower to allow the car to sit a couple inches lower. I have plenty of room out back to drop down, up front is tight though.
I don't recall what the CCW offsets are, as the rears were changed. I think the fronts are 5.75" and I run a 5mm spacer.

On the toe issue- I guess the delrin could be moving, but I'm thinking it is something else. My friend's LS1 FC has the same movement and he has the Mazda rubber in there (the comp stuff wherever it was offered). The delrin/solid links that I have should restrict the movement, but we have about the same toe in/forward movement. I guess the hub is the last place to look. Anyone ever replaced the *****? Is it tricky?

Ben

Carl Byck

You really need Damon here, but you are going to have to increase your rear rate. You are loosing probably 25% of your contact patch on launch due to camber gain(negative). This is where AWR comes up short on advise IMHO. Don't get me wrong, they are the kings, but high torque apps like the LS1 are twice what they are used to. Your rates are lower than what I ran on my street car(450/275). I would say you need to go to 16s for the track, and 17x11.5rear/18x10 front. that way, you can do a 285-30-18 in front witha 24.6diameter, less sidewall, and the same contact patch. This is what almost all the Supra track cars run. that will allowyou to run the car lower. At stock ride height you actually have no need for the AWR parts. BTW a Michelin Piolt soport in a 335-30-18 fits if you wanted 18s all around. I would look at going to the combo that I suggested in a Toyo RA1, the michelins should have no problem hooking 400ftlbs, hell my 285-40-17 Kumho MX street tires hold 450ftlbs no problem on my Supra, so you have other issues. Also the MX is a better street tire than the Michelins in terms of ultimate grip. But if you can run R compounds, the RA1s will last 8000 miles, and get better with wear, and would have no problem hooking 440ftlbb, even in a 315. Anyway, camber gain is killing you, go to some stiffer springs, and you will increase your contact patch. Carl

RX-Ben

As for the Supra hooking up- it has 1650 pounds over the rear (not moving), vs. my RX7 which has about 1250 lbs. That extra 400lbs helps the Supra a bit, no?

Also, the 4.1 gears don't help traction at all. On F-body cars (with similar weights on the rear wheels to the Supra), traction on street tires is a joke with the 4.1s and even with Nitto DRs they have issues hooking up (not to mention on cars with more serious head/cam swaps).
I ended up picking those Michelin Sports because of the rules for One Lap. I didn't realize that some of the wider 18s have such small diameters. Thanks for the tip and luckily 5zigen makes a cheap 18x9.5 so I can fit on the 285s up front. It'll only allow a 3/4" drop vs. what I have, but it is a good starting point.

I'm glad I got the AWR stuff because of the ease of adjustment and also the camber pieces replace weak/worn (rubber bushed) links in the stock suspension. I'm sure they'll be very handy in the future.

On the rear springs, I'll give it a shot- what rate would you suggest? 350? I'm kind of worried about going too high, since I'll have to up the fronts, and I'm not sure how the Konis will respond to a 25+% increase in the spring rate.

Thanks again...suspension is currently greek to me. I'm way up to speed on building the car and the engine, but the farthest I've gotten with getting a really good suspension understanding is buying and starting to read some books on the subject.

Ben

Carl Byck

500/350, you have got to eliminate the squat, or you will never hook. Have someone video the back of the car on launch, you will se a ~2-3* increase in negative camber as you launch. On a 335 you loose several inches of tire, like going from a 335 to a 275 just when you need more tire. Since the weight distributio is similar, I doubt the extra weight makes that much difference, i've heard that argument before, I think it has more to do with maintaining better suspension geometry on launch. Since you ae not dragging the car, I wonder why you need such soft rear springs for a race car? Granted softer is better to a point, but only if you can control camber gain(which you are not right now) I am running 450rear/600 front, and it feels good, of course I have 13" slicks on a 16x12. As for the Konis, generally +/- 20% is about it. What were they valved for? Once you settle on a rate, you can have the shocks revalved so that rate is the middle of your adjustment range. Also 18x9.5 is on the narrow side for a 285-30-18, usually 10-11" rims for those. race tires will tolerate a narrower rim due to a stiffe sidewall, if you can get them on the rim. You need to run fairly high pressures on the 285-30s, or they can come off the rim, due to the minute sidewall. Carl

christaylor

Just out of curiousity, what car is this on? 2nd gen? RX8? I saw you mention T2, wasn't sure if that was class designation or car designation...

DamonB

Thanks, but I'm pretty FC ignorant so I tend to be quiet when they are the topic of discussion

Carl certainly knows a heck of a lot more than I on what an FC wants to do but I bet he's right on the money about too much squat causing lots of camber gain and therefore making you tire contact patch smaller at launch. The few FC's I've paid attention to seem to have lots of camber gain over their complete motion of rear suspension travel.

You don't have to automatically go to stiffer springs though, try stiffening the rear shocks in compression first (just the low speed if you have that option). When accelerating hard the weight transfers to the rear and makes it squat. Shocks are used to adjust the time it takes for the weight to transfer, a stiffer rear shock will prop up the rear end for a few extra moments and let the car actually start accelerating and hooking up rather than just instantly busting the tires loose.

Figure out how much rear suspension travel you're using. Put a zip tie around the piston shaft against the shock body. Make a launch and then get out and see how far the zip tie was pushed up the shaft; that will tell you how much motion you're dealing with and if you really wanted to go crazy you could compute exactly how much camber change you're dealing with. If you're really unlucky you're using up all your travel and hitting the bump stops. Nearly anything is better than hitting the bump stops.

I told a friend months back who drags an IRS car that if it were me I'd experiment with slight positive camber in the rear so that when the car squats at launch the tires are actually at zero camber. He thought I was a nut...until he tried it. Pointless for a road race car though.

Keep in mind you've got big power and torque and traction will always be a problem if you don't manage the right foot. Weight transfer is a function of tire grip. The very fact that you're able to make the rear end squat so much means you have good grip but if the car squats and then immediately loses traction it very well may be due to camber gain and decreased tire footprint. Video from the rear would be a great idea.

I'd see what gains I got from the shocks and then I'd experiment with springs as Carl says. Sometimes there is more to be gained by going stiff and therefore more tightly controlling the camber of the tire, especially when you have really wide tires.

Carl Byck

Thanks Damon, I suggested the springs due to Ben being at what sounded like the limit of his shocks already. Good tip on the zip tie, that was what I was looking for, but came up with the video, since it will show gross camber change on a 335. I believe he also has single adjustables, this means he will be making other sacrifices as he stiffens the rear with the shocks. It is certainly the best starting point if he has not already done so. Carl

C. Ludwig

iTrader: (27)

Just re-read you original post and in regard to the toe change most of it has to be coming from the hub if there is indeed toe change ocurring. If the tire is indeed toeing in to the point it touches the trailing arm what that tells me is that the tire is moving relative to the trailing arm.

Having said that I just realized that I see rubber marks on the trailing arms of my ITS FC. But I would bet my last dollar it is produced by sidewall flex when cornering. Destroyed a brand new Hoosier on the front this way. It had around a 1/2" of static clearance to the strut but rubbed under dynamic conditions. If you're running a tire that large with minimal clearance it might not be a suspension problem but simply tire flex. Tire shape is not static. They will grow in diameter and flex from side to side.

RX-Ben

Thanks again guys. I'm heading back home in a couple weeks to rebuild the engine and get the car back on the road so I'll try the zip ties, shock settings, as well as stiffer springs. I'd really be interested in getting a camera back there to see what is going on.

I don't think the shocks are done yet in terms of what they can handle with their current valving, but I wrote my contact at Koni to find out for certain.

Honestly, I don't care much about traction from a standstill since the car will be used for autox/track days/time trials.

In the end, I think the problem is a combination of everything- gobs of torque at any rpm, crazy FC camber gain, and probably a little soft on the shocks/springs in back. Without a doubt traction is directly tied to my right foot, but I just want to make sure that I'm in the ballpark suspension wise.

Ludwig- you've really had tires flex out over 1/2" I wasn't expecting tires to be completely stiff, but that is crazy. Perhaps I'm not getting toe in/forward movement? It is just tire movement? I'll give Michelin a call and see what they think.

Damon- Does the FD suffer any similar suspension woes?

DamonB

Some crops showing sidewalls of my tires. They are 245/45/16 on 16x8 wheels. At rest the rubber sidewall sticks out past the wheel about a 1/4".

Tire 1 and 2 are the outside tires on two different days, Tire 3 are the inside tires from Tire 1. In Tire 1 and Tire 3 the car was pulling 1.3 g there and half the footprint on the inside tires is not touching the ground!

DamonB

Mostly it has some large rubber bushings and once they get old and soft you certainly get suspension pieces moving and banging around. My FD spends a lot of time on race tires and I wear out the stock solid pillowball joints in the rear suspension about every 2 years. Just think about it; we're running nearly twice the loads through these parts over sustained periods compared to what they see in only short bursts on the street. I honestly find it suprising that suspensions on street cars don't just plain break more often when tracked!

C. Ludwig

iTrader: (27)

Yes. Surprised the hell out of me. When I first built my car a few years ago it was on Kumho Victoracers. Very first event I flat spotted a tire in practice. Only dealer at the track was a Hoosier dealer. I bought two Hoosiers and mounted them in the front. The same size Hoosier was considerably wider than the Kumho but I had one finger width of clearance (1/2"?) behind the tire and strut. First lap of the race I get smoke and the burning tire smell. End the race with a nice groove worn around the circumference of the sidewall. That tire is now a TV stand and I can honestly tell people that tire helped me win the first race I ever entered.

Carl Byck

Obviously you've replaced your sway bar mounts, re broken suspension bits. I have about a half inch of clearance between my trailing are, and the tire in the rear.I am on Bias ply slicks, so I do not know what they will do. I may have to run higher than Ideal pressures, and perhaps go to smaller tires. I certainly hope not. Alternatively, I will make the car even wider, so I can run 13" slicks all around I really think making sure every stock bushiong has been replaced, in combination with damper/spring increas will solve your problem. In the front, I play with differnt length springs in order to move the spring perch away from the max section width of the tire. I chose the 24.5-13-16" tire, because the max section width exactly coincided with the max clearance on the trailing arm. That way I have another inch of rubber on the pavement(if it works under load), otherwise I'll use a 12" slick, and space it out a little. The BP guys mod the strut mountng abs to move the damper outboard about 1/2" at the bottom, so they can clear a 12" tire up front. Carl