First impression of Formula Atlantic slicks

[heavy85]

I only ran one set of Avons but you are in the ballpark <1.5 front / < 1 rear - I kept taking out camber on bias slicks. That didnt work well at all with radial slicks. Would have to dig way deep to find pressure I ran. One thing is they were great last race of the year then after sitting over the winter were rocks the first race out ... like slower than old street tires. I couldnt make that much caster work (at least on 200 tw tires) as had an uncontollable oversteer in really tight slow corners.

[JMortensen]

Managed to get mine out this year. Wasn't sure I'd make it but managed to get out there Sunday to the last SCCA event.

 

The good: Nothing broke. Everything worked just fine, power steering, button clutch, the relatively untested T56, all worked flawlessly. Only issue was I lost the dust cap for the LF hub twice. A little tweaking with some pliers solved that problem.

 

The bad: The car was way too loud. They allow 96 dB at local SCCA events, mine was showing 102 on the meter. Tried to add a longer turndown to the side pipes, it fell off. We ended up just letting off near the meter and they let it go. I did floor it on my last run and I didn't think the sound guy would really get mad about it, but they basically said don't come back until it's fixed. I have mufflers that I made in an attempt to copy the Burns Stainless super quiet bullet one. I put two pieces of angle iron in the flow path at 90 degree angles to each other. I guess the idea is that the angle diverts the flow into the packing. Started it up yesterday afternoon and it seems a lot louder now than it did before the event. Guessing I blew all the fiberglass packing out. Going to need to do something else anyway. In one of my better moves, I used V band clamps on the both ends of the muffler so I could take off the tip and add a second muffler if necessary, or just replace the whole thing from the header on back. Thinking Supertrapps or maybe buy some of the longer classic chambered mufflers: http://www.classicchambered.com/classic/products.html.

 

The super wide Formula Atlantic rear tires that I spent all that time fitting to the car didn't really grip that well. I'm using the spec compound, FACS. They also have R25 which might be softer and work better, don't know how they compare. The tires actually felt worse than the 10" wide Yokohama slicks I used to run on this car 13 years ago. Mark Haag (zredbaron) was impressed with the turn in, but I wasn't. I thought it felt pushy on slow corners. I was able to get some heat into the fronts towards the end of the day and it was doing better, but the rears were quite a bit cooler. I didn't want to try and screw around with settings too much, just wanted to drive it and see what fell off, so I don't have much in the way of tech info. My non-calibrated hand thermometer says about 140 degrees on the inside of the fronts and 130 on the outside, maybe 110 on the inside of the rears and 100 on the outside. The tires weren't rolling off of the tread, but there did appear to be a little more wear on the shoulder than I would have liked. I could spin the rears at will for as long as I kept my foot down, which seemed strange seeing as how it is a light car with 4.11s. You'd figure the car would just accelerate up to the tire speed, but there just wasn't any grip to make that happen. Lateral gs weren't really there either. Overall our times were very even, and pretty disappointing.

 

I have droop limiters on the car, and had some unusual behavior, not sure if it was droop limiter related or not. There was the longest straight into a kink and a C-box immediately afterward. Several times I got all crossed up going into the box. Dialed the bias bar towards the front, and then was told that the inside rear tire was coming up 4 or 5 inches off the ground. No sway bar. Even if I didn't have a limiter on there, it should still have been off the ground as there isn't 4-5 inches of droop available. I would think this means that I need more front spring, but I already had a lot more heat in the front tires than the rear. Might have to raise up the rear to get the roll center higher to get some heat in the rear and find a different solution for the front.

 

The other: Apart from the lack of grip and the weird tire lifting, the car was fun to drive. It was neutral in sweepers, would pivot on slower corners and I could steer with the throttle. With the exception of that one spot on the track, I never felt like it was doing something other than what I asked it to do and it felt predictable just about everywhere. Mark said it was nervous under braking, but I only felt that way into that kink.

 

Got a lot of thumbs up on the road and the autoxers really seemed to like the way it turned out. So kind of mixed bag. Very happy that it didn't break and it went so smoothly other than the muffler thing, but not real excited about running those tires again. There are Avons available which are supposed to be better but Berget didn't have any when I ordered these. Other option would be to make a drastic size change in which case I'd probably go 315s in 17 or 18s, or 275/35/15, but that would make my body ridiculously wide, so I'd like to try to make this 14" wide rim setup work first.

[jt1]

Glad you got to run it this fall.  To complete your first event with only minor issues is a huge accomplishment!

 

Maybe the front LCA is going negative and jacking the front down?  Any travel indicators on the front shocks?

[74_5.0L_Z]

I will postulate that the lifting of the inside rear wheel is caused by the combination of the following:

 

1. The 14 degree SAI and ~ 7 degree caster cause the outside tire to rise relative to the body and the inside tire to drop relative to the body.  This takes weight off the inside rear.

     You can see how much this de-wedges the car in even a static situation if you have some turn-plates and wheel scales.

2. Large scrub radius.  This exacerbates the problems caused by problem 1.  The larger the scrub radius, the more the outside wheel will rise and the more the inside tire will drop.

3.  Super stiff springs.  This magnifies the problems caused by 1 and 2 above. 

 

So what is happening is that you are entering the corner and transferring weight to the outside front tire, which as you turn is rising relative to the body.  Meanwhile, the inside front tire is dropping relative to the body.  Essentially, the car tips about the diagonal line connecting the inside front to the outside rear.   If the rear tire does not have much droop available, then it will be lifted off the ground.

 

I know from your build that you have super stiff springs, lots of caster, and HUGE scrub radius.

[RebekahsZ]

johnc recommends like 9 degrees of caster in the suspension FAQ.  I'm going to be increasing my caster from 5 degrees to damn near as much as I can get even if I have to limit steering angle to keep the tires from rubbing the fenders.  Purpose of this is to help keep car straight in standing mile.  74_, are you suggesting that running high degrees of caster is a bad idea?  I think I can assume that since this is a straight line event that I shouldn't encounter the problems discussed above (unless of course I get into a spin).  Or is this really a problem when large caster angles are COMBINED with limited droop?

[JMortensen]

I think it's the scrub that really makes the difference, if that turns out to be the problem. I have 5 inches of scrub.

 

Thinking the next step will be to get some other slicks on it and try it, even if the fenders are way out past the wheels, and then if it works a lot better it's decision time...

[74_5.0L_Z]

Caster has some really good benefits on our cars and also causes some problems. 

 

First the good:

 

With our front struts we have a large Steering Axis inclination (SAI).  With factory settings, the cars have very little caster.  So, with the factory settings the outside front wheel first tends slightly toward negative camber when the wheels are barely turned and then swings toward positive camber as the wheel is turned further.  Similarly, the inside wheel will slightly gain positive camber as the wheel is turned.  Ideally, the outside wheel would gain a lot of negative camber and the inside wheel would gain a lot of positive camber as the wheel is turned.  Adding caster helps the camber gain a bunch.  

 

On my car I have 6.4 degrees of caster and -1.8 degrees of static camber.  When I steer the front wheels 5.7 degrees, my outside front wheel gain negative camber so that it has -2.35 degrees, and the inside front wheel gains positive camber to -1.1 degrees.  This change in caster is before the outside wheel compresses and the outside wheel droops.  The changes in caster from strut compression / extension will add to that from turning.  The change gets really significant for tight turns.  If I steer my front wheels by 30 degrees(essentially full lock) the outside wheel goes to -2.9 degrees camber and the inside wheel goes to +3.9 degrees of camber.    It has been shown that having caster about half of the SAI give pretty good caster gains with turn of the wheels.  So, for our cars 6-7 degrees is the rule of thumb.

 

Now the bad:

 

Adding a bunch of caster causes problems.  First, it causes the weight jacking / de-wedging problem that I talked about earlier.  The combination of caster and SAI combine to cause the front spindle to swing on an arc. Adding caster tilts the steering axis back toward the fire wall.  The tilt of the axis from caster causes the spindle to rise on the outside tire and fall on the inside tire.  So, the SAI and Caster effects are additive (but in a non-linear kind of way). The large SAI and caster magnify the rise and fall of the spindle pin.  If we could get an SAI of lets say 5 degrees and run a caster of 2.5 degrees, we would get the same level of caster gain as we do with our struts and 6-7 degrees of caster.  However, the low SAI spindle would have much less weight jacking associated.  Unfortunately, strut cars need large SAI for wheel clearance.  The only way to get low SAI like I propose is to convert to a SLA set-up.  On a strut car the weight jacking can be minimized by running the smallest scrub radius possible.  Theoretically, a zero scrub radius car would not have any weight jacking from the steering effects discussed here.

 

Next, adding static caster increases caster gain.  If the strut had no caster and the lower control arm were parallel to the ground, the strut would maintain zero caster for all of its travel.  As you add caster by tilting the strut back toward the firewall, bump travel will cause the caster to increase.  The more it is tilted back, the faster the caster will increase for a given bump motion.  On my car, I have raised the TC rod 0.5 inches and run 6.4 degrees of caster at ride height.  One inch of bump increases my caster to 6.9 degrees. 

The problem with the caster gain is bump steer.  The caster gain causes the outer tie rod to rise relative to the steering rack.  If the rack is in the stock position, then this rise of the outer tie rod end causes the virtual length of the tie rod to shorten.  This causes toe in on bump, and we all know that  toe in on bump is a very bad thing.  The problem of bump steer can be mitigated a little by raising the rack or by adding the bump steer extension to the outer tie rod ends.  But that is a Band-Aid and not the best solution.

 

I have found that I can almost completely eliminate the bump steer in the front end (for my current set-up) by going with a much shorter rack (~22 inches center to center of the inner rod ends).

Edited October 7, 2015 by 74_5.0L_Z

[RebekahsZ]

Damn this stuff is complicated!

[tube80z]

Dan has some good information about caster above.  I don't think that is the problem here but it could be adding to it.  I say that because we've had two cars running the FA rears all round that were setup very close to Jon's car and neither of them lifted tires when going into tight turns under braking or in transition.

 

After thinking about this for a while the car is rolling on a diagonal on the inside front to the opposite rear.  So we're already extending the inside rear tire.  If we're on the brakes then the rear will be very close to fully extended before we start cornering.  Now add roll mostly from the front end dipping on one side and steering induced ride height change and it's looking closer to what is described.

 

So how to fix.  I'm thinking we need to go stiffer.  As the rear tires are not up to the same temp as the fronts I'd go with more rear spring and if there's a lot of rake reduce that at the same time.  If that doesn't fix it then more front bar or stiffer front springs.  This will help reduce front roll, dive, and rear lift.

 

Cary

[JMortensen]

I thought higher rear RC would put more heat in the tires. I was going to jack the rear up a bit. Remembered that I had to add 3/4 spacers in front and also narrowed front track 1/4, so scrub is probably closer to 5.5ish.

 

When you turn the wheel the whole front end lifts up. Maybe that is to be expected, but there is no noticeable lean when things are static. maybe that doesn't mean anything, but that's what happens in the shop anyway.

 

Don't want to give up on this yet. The other guys running the big tires in the front also had cold front tires, and I couldn't get any heat in the rears. I found that odd. Everyone seems to think that Avons are better than Hoosiers in this application, so I think next move is to try and get some Avons and see how they do. If that doesn't work out, then I'll have some serious thinking to do.

[RebekahsZ]

Burnout water box? That'll get'em hot!

[tube80z]

Raising the rear will help it turn better and should increase rear tire temp.  But at the same time it will not put power down as well.  Adding spring will do both but hopefully won't hurt forward traction as much.  Add Avons and this will all probably change.  At least it runs and doesn't have any issues that need to be addressed outside of tires and initial setup.  

[heavy85]

I will postulate that the lifting of the inside rear wheel is caused by the combination of the following:

 

1. The 14 degree SAI and ~ 7 degree caster cause the outside tire to rise relative to the body and the inside tire to drop relative to the body. This takes weight off the inside rear.

You can see how much this de-wedges the car in even a static situation if you have some turn-plates and wheel scales.

2. Large scrub radius. This exacerbates the problems caused by problem 1. The larger the scrub radius, the more the outside wheel will rise and the more the inside tire will drop.

3. Super stiff springs. This magnifies the problems caused by 1 and 2 above.

 

So what is happening is that you are entering the corner and transferring weight to the outside front tire, which as you turn is rising relative to the body. Meanwhile, the inside front tire is dropping relative to the body. Essentially, the car tips about the diagonal line connecting the inside front to the outside rear. If the rear tire does not have much droop available, then it will be lifted off the ground.

 

I know from your build that you have super stiff springs, lots of caster, and HUGE scrub radius.

So basically it works like a solid axle gokart - use front geometry to lift the inside rear tire so it will actually turn.

[JMortensen]

Saw some pics of Dan's car and have been impatiently waiting for him to update the thread. Slow day at work makes it worse. Sorry Dan, for stealing your thunder.

 

Anyway, he's clearly got the same thing going on. Cary noticed that he doesn't have a lot of steering input going on in this pic, so the basic question is still "Why is this happening?" I was trying to think back to other Z's that lift rear wheels and I think the only other one that I recall doing this is... mine in the 90s. I was told that I was lifting inside rears going into slaloms and I had really soft springs 200/250 and no droop limiiters and about 5 degrees of caster. I assumed that it was happening because my sway bar was really badly bound up in the back.

 

Cary's solution is more front spring, but it really doesn't look like the front end is compressed that far. Since I already have a front tire hotter than rears issue, I'm still thinking stiffer roll bar and stiffer rear springs for my situation.

Edited October 21, 2015 by JMortensen

[74_5.0L_Z]

Yes,  I am experiencing the same problem that I spoke of earlier.  This was a really tight course where you had to brake pretty hard and then turn pretty tight. 

 

I am trying to decide what I want to do to settle the car down.

 

Currently, I have 500 lb/in front springs, 425 lb/in rear springs, 20mm front sway bar, no rear sway bar, and  Koni 8610-1437race struts all around.

The front suspension has the following static settings:

 

camber:  -1.8

caster:  6.3

toe: 1/8" out

front roll center on the ground or maybe slightly below

 

The rear suspension has the following settings:

camber: -1.2

toe:  1/8" in

The rear roll center is about 2.5 inches above ground.

 

Tires:  AVON 10.5/23.0-15  Cross Ply Slick (A11 compound) all around  22 psi rear / 27 psi front

Wheels 15x10 with 5.25" back space mounted with no spacers.  This gives me about 2 inches of scrub radius.

 

I have no droop limiters on the car at the moment (except the struts themselves).

 

The car really pitches around more on the bias ply tires than it did on the Hoosier A6 tires I was running.  The tire itself seems to deform much more.  When driving, I do not notice the tires lifting (front or rear).  When these tires are done, I plan to go back to a radial.

 

I am thinking about lowering the rear roll center, raising the front roll center, softening the rear springs, or adding a stiffer front sway bar.

 

Edited October 22, 2015 by 74_5.0L_Z

[tube80z]

Saw some pics of Dan's car and have been impatiently waiting for him to update the thread. Slow day at work makes it worse. Sorry Dan, for stealing your thunder.

 

Anyway, he's clearly got the same thing going on. Cary noticed that he doesn't have a lot of steering input going on in this pic, so the basic question is still "Why is this happening?" I was trying to think back to other Z's that lift rear wheels and I think the only other one that I recall doing this is... mine in the 90s. I was told that I was lifting inside rears going into slaloms and I had really soft springs 200/250 and no droop limiiters and about 5 degrees of caster.

 

Cary's solution is more front spring, but it really doesn't look like the front end is compressed that far. Since I already have a front tire hotter than rears issue, I'm still thinking stiffer roll bar and stiffer rear springs for my situation.

 

I was saying from the picture it looks like the front is compressing (Dan's pic).  From your info I'd go with more rear spring to keep it from coming up.  And since you don't have a rear bar perhaps a Z bar instead.  Another option would be more tire pressure.  Until we see pics it's shooting in the dark.  And then we have that issue of you running the wrong tires that won't work on the car

 

If I can ever get you down to Medford we'll see what it's doing and get it fixed.

 

Cary

[tube80z]

Yes,  I am experiencing the same problem that I spoke of earlier.  This was a really tight course where you had to brake pretty hard and then turn pretty tight. 

 

I am trying to decide what I want to do to settle the car down.

 

Currently, I have 500 lb/in front springs, 425 lb/in rear springs, 20mm front sway bar, no rear sway bar, and  Koni 8610-1437race struts all around.

The front suspension has the following static settings:

 

camber:  -1.8

caster:  6.3

toe: 1/8" out

front roll center on the ground or maybe slightly below

 

The rear suspension has the following settings:

camber: -1.2

toe:  1/8" in

 

Tires:  AVON 10.5/23.0-15  Cross Ply Slick (A11 compound) all around  22 psi rear / 27 psi front

Wheels 15x10 with 5.25" back space mounted with no spacers.  This gives me about 2 inches of scrub radius.

 

I have no droop limiters on the car at the moment (except the struts themselves).

 

The car really pitches around more on the bias ply tires than it did on the Hoosier A6 tires I was running.  The tire itself seems to deform much more.  When driving, I do not notice the tires lifting (front or rear).  When these tires are done, I plan to go back to a radial.

 

How much weight do you have on front?  Any tire temps?  You're pressures look too much to me for Avons.  Maybe you arrived there by experiment but we're usually down in the 19 to 22 range.

 

As far as springs go for the Avons I'd be 100 to 200 lbs/in more.  You're car is around 2500 to 2600 lbs. with you in it?

 

Cary

[74_5.0L_Z]

I started with the tires in the 18 to 20 psi range and the tire temperatures seemed to indicate that was the correct pressure.  Unfortunately, the car felt like it was falling over all the time and the tires were rolling under pretty badly.  So, contrary to the tire temps, I started adding pressure until the car felt more stable.

 

I've got the car down to 2580 full of fuel with me in it. I have 610 lbs on the right front and 638 lbs on the left front.

Edited October 22, 2015 by 74_5.0L_Z

[RebekahsZ]

Any chance this tire lifting problem is simply a matter of insufficient droop?

[74_5.0L_Z]

The car could use more droop in the rear.  Unfortunately, way back when I sectioned my rear struts, I shortened them too much.  

 

I have a couple of options to get some of that droop travel back.

 

1.  Lower the rear.  I can lower it about 1/2 inch before the spring loses all pre-load at full droop. Unfortunately, that gets me really close to rubbing the inside if the wheel well at full bump.

 

2. Add an aluminum spacer between the rear camber plate and strut tower to lower the camber plate mounting relative to the body.  Then also perform option 1 but lower the spring farther.

 

3. Get some new rear strut housings and start over.