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Old 09-07-2015, 03:09 PM   #1
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Default Twitchy car (tail happy problem)

Hey guys,
Trying to understand what is wrong with my car. Car is very tail happy, very difficult to catch when it starts to oversteer, especially in auto-x. On the track I put back on the rear swaybar (oem 1.6 swaybar), the car feels more neutral, but tail happy in corner entry. Quick transitions and slippery surface make it worst. Been trying to adapt my driving to it, which still gives me pretty good results, but drove a very well setup car yesterday (thanks Joyrider! VVT, Xidas, 15x9 Nt01, Torsen, aero) and there is definitely something wrong with the balance of my car...

I have a few ideas on what might be causing the problem, just want your opinions. Car is 1990 miata, 1.8 swap, open diff,949 no aero.

Setup:
Suspension: koni race shocks (I believe I have the ones with the really short rear shocks), NA top mounts, 700-400lbs/in springs. Koni front bump stops, fatcat 36mm rear bumpstops, 1.125 hollow front bar, no rear bar.
Alignement: -3.0deg front camber, 4deg caster, -2.5 rear camber, 0 toe all around.
Ride height: 4in pinch weld front, 4.25in pinch weld rear.
Wheels/tires: 15x8 +25 konig wideopen, 205-50-15 rivals

Thanks for your input,

Ben
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Old 09-07-2015, 03:27 PM   #2
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Usually it would be a combination of stiffening the front and softening the rear to trend towards understeer. Sounds like you've already done a fair bit of that.

My guess is you're running out of rear shock travel and hitting the bump stops back there hard enough to break traction. Change rear tophats to get more travel? Trim the rear bump stops? Run lower tire pressure on the rear? Soften the rear shock settings relative to front?

Also, smooth is fast.
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Old 09-07-2015, 03:32 PM   #3
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Yeah, thought about that, but I'm afraid I will loose the very little bump travel I have with the rear shock I have, and the bump stops are all ready shorts... lol.
With my car, I HAVE to be smooth, or else I loose it. Yesterday, with Joyrider's car, I could just do whatever I wanted to it, and it would take it beautifully...

I think a LSD would help a lot, and I'll also try to run the FSB to the stiffer setting. After that I'll play with damping...
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Old 09-07-2015, 03:36 PM   #4
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Maybe try raising the car. I went from around your ride heights to more like 5" ride height and it got a lot better.
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Old 09-07-2015, 03:39 PM   #5
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^ Good catch. Didn't notice that before, but the suspension definitely needs some room to work -- especially with NA tophats.
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Old 09-07-2015, 04:13 PM   #6
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No, it doesn't need to go higher. I ran those exact shocks with 700/450 springs at those ride heights with similar camber/toe numbers and that front sway bar, and the car was essentially perfect. The 8041 RACE shocks have super short bodies, so running them with NA tophats is what you should be doing - running NB hats lifts the shock shaft up even further and trades droop travel you desperately need for bump travel you can't use. I actually ran a bit more rear sway at one point (MSM RSB) as I thought the car pushed a bit.

OP, if you want something to try, you can drop the rear end 2 turns at a time and see what that does, but I'm leaning towards a loose alignment bolt or some other deflection that's causing this, since I ran a very similar setup with great success for a long time. It almost sounds like dead tires - how old is the rubber and does it sit out in the winters?
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Old 09-07-2015, 04:35 PM   #7
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Thanks for your inputs guys!
Sav: Rake was an other one the list I definetly wanted to play with, just don't want to change too many things at the same time...haha. Alignement was checked 2 weeks ago, nothing is loose. Tires are on their 2nd summer, stored inside during the winter, maybe 8000 km, 2 lapping nights and maybe 60-70 slalom runs. About 3-4/32in left on the thread. They are not new for sure, but still look and grip decent.
Dropping the coils 2 turns just drop about a quarter in at the pinch weld?

Ben
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Old 09-07-2015, 10:35 PM   #8
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with that mount setup and bump stops dont those rear shocks bottom out internally before hitting the bump stop? Do your adjusters still like turn?
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Old 09-07-2015, 10:38 PM   #9
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Adjuster work fine, pretty sure they dont bottom internally... I know for sure the fronts don't.
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Old 09-08-2015, 12:28 AM   #10
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I had a similar issue where the car rotated too quickly on corner entry to mid corner. Eventually I eliminated the issue by making sure my rear bumpstops were shorter than the front. I'd suggest getting a ground control 45 mm bumpstop and cut it down to about 30mm and see if that tames the rear.

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Old 09-08-2015, 02:18 AM   #11
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If you still have the standard rear bumpstop that came with the shock, try putting that back in. I ran unmodified bumpstops with those shocks and had pretty good results.
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Old 09-08-2015, 08:45 AM   #12
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Tires dead. Been there, same setup.
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Old 09-08-2015, 10:59 AM   #13
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The more I think about it, dead tires seem plausible... 2 years ago, when I was running tires that were certainly dead, the car was doing pretty much the same thing as it is doing now... I'll still try to balance it out, but new tires will be coming in next year.

For those who ran this setup, do you think upgrading to 15x9 with 225-45-15 Nt01 or maxxis RC1 will be too much for these spring rates?

Thanks!

Ben
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Old 09-08-2015, 11:00 AM   #14
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double post... sorry
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Old 09-08-2015, 12:48 PM   #15
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Quote:
Originally Posted by wannafbody View Post
I had a similar issue where the car rotated too quickly on corner entry to mid corner. Eventually I eliminated the issue by making sure my rear bumpstops were shorter than the front. I'd suggest getting a ground control 45 mm bumpstop and cut it down to about 30mm and see if that tames the rear.
My car did this at ICAR, lowered the rear 1/4'' solve the problem for me. I didn't play with the suspension setup all summer long... only tires pressure.

Car is awesome and do what you want it to do ! I beat all my track times since ditching the turbo and having so much problem those last 2-3 years... now need more seat time and enjoy the car.
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Old 09-08-2015, 01:48 PM   #16
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Quote:
Originally Posted by bigben View Post
The more I think about it, dead tires seem plausible... 2 years ago, when I was running tires that were certainly dead, the car was doing pretty much the same thing as it is doing now... I'll still try to balance it out, but new tires will be coming in next year.

For those who ran this setup, do you think upgrading to 15x9 with 225-45-15 Nt01 or maxxis RC1 will be too much for these spring rates?

Thanks!

Ben
Same setup except 350lb rears. Bumpy as hell local tracks. Works well with 205 sm7 /225 rs3. Get New rubber and carry on. Loose is fast.
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Old 09-11-2015, 04:41 PM   #17
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Quote:
Originally Posted by bigben View Post
The more I think about it, dead tires seem plausible... 2 years ago, when I was running tires that were certainly dead, the car was doing pretty much the same thing as it is doing now... I'll still try to balance it out, but new tires will be coming in next year.

For those who ran this setup, do you think upgrading to 15x9 with 225-45-15 Nt01 or maxxis RC1 will be too much for these spring rates?

Thanks!

Ben
Should we take my wheels and tires and drive to Ottawa to have some data this weekend... we could spank some Ontarians in the process...
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