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Car rolls over front tires?

 
Old 04-27-2018, 07:59 AM
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Default Car rolls over front tires?

I just had my first track day this year at Milwaukee Mile last night. The car performed perfectly for 1.5 sessions, then the last 2 I started to get some weird feelings in the car that I haven't experienced before. Just looking to see if someone has some input as to where to start once I get under this thing.

2003 NB, DIY billies with 450# front 300# rear. 1.125" tubular front with 949 Racing links and reinforcement blocks, ISC rear extended hats, 15x9 Advanti Storms with 205 Federal 595 RS-RR tires (about 6 track days on these). Brakes are stock sport brakes with basic ceramic pads, they had over half of the material left when I started, upgraded the brake lines this winter to the 949 stainless lines and bled the system with fresh fluid. I just had an alignment done with a slightly more aggressive street alignment as I daily this car as well.

Conditions were sunny and 60 degrees, dry. The main straight I will hit about 105mph then brake to 2nd gear for almost a 180 degree right hander. While hard braking in this and one other zone, I started to get the front end to shake, almost like the tires were slipping on the rim (never experienced this, but its the only way I can think to describe it), or ABS was kicking in (dont have abs). The pedal would not pulse like a warped rotor, but I will inspect brakes to see. I dont get this while lightly braking or on the street unless I try to lock them up. As I would go into this right hand turn, the front left tire felt like it was falling over itself. My car doesnt understeer, but it would.... almost bounce over the tire if that makes sense. ONLY on right hand corners. The hard left handers the car can slide or change direction without any fuss. I checked tire pressures, they were a bit low, but even increasing PSI the condition remained, pressures were even across the front.

I will check everything in that front corner for tightness, check tire wear, brake pads/rotors, but these are completely new conditions for me. These are sticky tires, but effectively stock brakes, and the 'falling over itself" seems odd for a tire that has never had issues before.

Recap: Only on hard braking zones the front end shutters, but not the pedal, and only on right handers the front tire seems to fall over itself. I have not had this issue in 5-6 track events before.
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Old 04-27-2018, 11:57 AM
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best bet is to check over all of your suspension for wear. bushings, balljoints, hubs. check for excessive play.

I know exactly the feel your describing... in my case it was in a truck with a worn out front end.
it might be hard to detect because the loads your describing are pretty high compared to finding something that is failing and noticeable on the street
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Old 04-27-2018, 12:12 PM
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What exactly were the hot tire pressures?
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Old 04-27-2018, 12:31 PM
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Last session I went out 28.5 cold on the fronts. I was watching the wear triangles to look for rollover, didn't seem to have any issue. The tires are 205s on 9" wheels, although they are a bit wider than a standard 205. Perhaps I should bump up to 32 cold, see if that helps. The tires have very stiff sidewalls, but I might just be a bit too low on air. However the issue only presents itself on right hand turns, not left. And I verified the fronts were evenly pressured.

I will check all control arms and everything when I get the car in the air.
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Old 04-27-2018, 12:57 PM
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Do you have the actual numbers for the alignment? I've been through this same issue on a FSAE car about 5 years ago, and I don't think I have notes on the exact fix, but I believe it was alignment related for us. If toe is zero, it can cause an oscillation because small changes in the contact patch can cause the lateral force at each tire to go left and right, sometimes out of sync with each other. If the alignment is slightly different left to right, it could magnify the issue turning one way, and mask the other direction. With toe out in the front, you'll always have a lateral force pushing outwards which helps stabilize the car. Similarly in the rear, even having 0.5 degree toe out was more stable than zero toe, for the same reason.

Otherwise as mentioned, check bushing for wear and bolts for play.
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Old 04-27-2018, 01:33 PM
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You're either smashing your bumpstops, making mid corner imperfections ugly, or you've already smashed your bumpstops into oblivion and are now jamming your tire into the body of the car.

Those are not sufficient spring rates for your uses.
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Old 04-27-2018, 01:49 PM
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Alignment is below. Im not a guru when it comes to this. I gave them flyin miata's NB spec and told them to try and get close.
Front: Left -1.1 camber, Right -.9 camber. 5.6 caster 4.7 caster. .01 and .03 toe

Rear: -1.7 and -1.9 camber. -.02 and -.03 toe.

Concealer: This is the same set up I did 5 track days on before with no issues. Bumps might be something ill check, but I have only 1 year on this set up. Would you recommend something like 500# front and 400# rears? This is a dual duty car, I didn't think stock billies take much more than 450# springs.
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Old 04-27-2018, 01:52 PM
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Need more camber and more springs. 5 track days with those spring rates probably beat up your bumpstops.

I'd recommend 550/350 minimum, 600/400 works fine on stock bilsteins as well.
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Old 04-27-2018, 02:04 PM
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How are those rates on the street? I find the 450/300 combo quite comfortable to drive around town and on the interstate.

So. Check all parts for being loose, bad bushings, hubs, etc. Look to increase spring rate, and most likely need to replace bump stops again.
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Old 04-27-2018, 02:07 PM
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They're fine. 600/400 got a little porpoise-y, but 550/350 is nice.

Everything is a compromise, though. You don't appear to be having any issues with street driving, so if street driving is that much concern, leave it alone.
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Old 04-27-2018, 02:10 PM
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Ha, this is true. Ill try to get it up this weekend or next week and give everything a good shake.
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Old 06-13-2018, 04:16 PM
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SOLUTION: I finally got around to digging into this. I moved this past month so the car has been sitting. I inspected everything on the front end of the car, torqued everything, looked for play, there were no signs of issues. I think a lot of issues I have been having have been due to the top 2 nuts on the shock bodies coming loose. I found this thread: Spec Miata Community: Diagnose this front suspension vibration - play along! after I went and discovered my rear shock nuts came loose (causing rattling noise on the track) and I checked the front shocks again due to my issue I documented here: https://www.miataturbo.net/suspensio...ignment-94232/

I took all the nuts off the shocks, re tightened everything and went for an aggressive shake down. Car seems planted again. Im not able to hit full track speeds anywhere around me, but it seems to have fixed the issue. I have a track event next week which will be the real test.
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