GR86 - Understeer Alleviation - RCE Tarmac 2 (7k square setup)

[Tomyyyu]

Having an issue of dealing with entry and mid corner understeer, especially when changing transitions from one element to another in Autocross.

Currently running in STR and want to see how I can work towards resolving it. As my experience has grown, I’m tending to desire less understeer and more oversteer to actually get the car to rotate.

Currently running:

• RaceComp Engineering Tarmac 2s w/ Vorshlag Camber Plates (7k square springs)

• Front Rebound 11 clicks from full stiff (out of 16)

• Rear Rebound 9 clicks from full stiff (out of 16)

• Front Compresison 10 clicks from full stiff (out of 12)

• Rear Compression 8 clicks from full stiff (out of 12)

• Perrin Front sway 22MM (full soft)

• Stock rear bar

• 17x9 245/17/17 - A052 Yoks - 28psi when hot (all four)

My alignment is:

• - 3.8 camber front

• - 2 camber rear

• +6.5 caster

• 0 toe in front

• 0.16 degrees of toe in in the rear

I can get my ride height and rake if needed (it’s around 5 inches from the ground to the pinch weld in the front with a slight rake towards the front). For preload it's whatever they came out of the box with. But thinking this issue might be resolved with a slighter larger aftermarket bar or stiffer rear springs

Car is a GR86 so it’s an OEM 15MM and thinking of jumping to a 16MM rear bar. I have tried playing around with the coilover settings (i.e., increasing rear rebound, decreasing front rebound, etc.) and still no luck on removing the issue.

Or jumping to a 8K rear spring (which I might go with for starters). Have had a few others drive it, confirming the issue at hand.


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[14stu]

You could try an alignment that gives you a bit of toe out in the front and zero out the toe in the rear. The front toe out should get you some more front turn in and the rear part will make the car rotate mid corner. A little toe goes a long way.

[strat61caster]

#1 issue I see is it needs more camber up front imho which is the big thing for mid corner understeer, you’re not using as much of the tire as you could. -4 is the minimum but based on what you’re describing I’d shoot for -4.5 if you can. #2 for me is I think that front bar is too stiff for oem rear - when I use the oe rear bar my front bar is ~272#/in, a lot of my struggles with the front evaporated when I matched the front and rear bar stiffness - the car moved in sync which I didn’t realize it wasn’t doing. This will depend on your spring and bumpstop combo so my bar settings are unlikely to translate 1:1. I’ve tried being that stiff on the front bar and didn’t like it, ymmv. I’ve got the 16mm rear Perrin bar that I like a lot, middle or stiff on that might match the front you’ve got now IF you’ve got no issues over bumpy pavement.

Softer front spring or bump stop could help for initial turn in, basically right now as you turn in the car kind of skates right? Some of that can be fixed with driving style, load the nose more progressively with the brake (or even a lift) before you turn in, make sure your turn in is progressive and not sharp and binary - having someone else who is competent drive your car and confirm the issue is a huge help. You might also have too much compression front and/or too much rebound rear preventing the weight transfer, this is something a legit hotshoe/alien can figure out in 3-4 runs.

iirc kw/rce throw a big stiff bump stop on the front and I’ve had issues with kw race springs before - make sure they’re not coil binding and check the front bump stop to see if it’s engaged when sitting still. Coring that sucker out to try and make it softer would be my first $0 attempt. Everything else looks in the ballpark barring damper settings which I have no feeling for but there’s a lot who check here who might be able to spot something there.

Respectfully I think toe change is a bandaid here, there is a grip imbalance that isn’t affected by toe numbers in a reasonable range. It may help, it may be what you like (I personally don’t like front toe out on this car), but the problem is both transient and steady state, it’s unlikely there will be a silver bullet and more likely 2-4 tweaks get you into the zone. Good luck.

Edit re read op - missed the confirming co drives and damper fiddling so that’s out. 8k rear spring might be the answer in the end for sure, especially if you’re happy with the front stiffnes. I’ve found this car to be relatively insensitive to spring rate changes.

Oh and I forgot rake, raising the rear will be the easiest solution around. Jack up the car, raise the perches two turns each and try it - the ride height change isn’t enough to change alignment - and it goes the way you want with less camber and less toe in which should free the rear up to rotate. If the current rake is flat or the rear is lower then the front (depending on where you measure) absolutely do this first.

[Tomyyyu]

Originally Posted by strat61caster

#1 issue I see is it needs more camber up front imho which is the big thing for mid corner understeer, you’re not using as much of the tire as you could. -4 is the minimum but based on what you’re describing I’d shoot for -4.5 if you can. #2 for me is I think that front bar is too stiff for oem rear - when I use the oe rear bar my front bar is ~272#/in, a lot of my struggles with the front evaporated when I matched the front and rear bar stiffness - the car moved in sync which I didn’t realize it wasn’t doing. This will depend on your spring and bumpstop combo so my bar settings are unlikely to translate 1:1. I’ve tried being that stiff on the front bar and didn’t like it, ymmv. I’ve got the 16mm rear Perrin bar that I like a lot, middle or stiff on that might match the front you’ve got now IF you’ve got no issues over bumpy pavement.

Softer front spring or bump stop could help for initial turn in, basically right now as you turn in the car kind of skates right? Some of that can be fixed with driving style, load the nose more progressively with the brake (or even a lift) before you turn in, make sure your turn in is progressive and not sharp and binary - having someone else who is competent drive your car and confirm the issue is a huge help. You might also have too much compression front and/or too much rebound rear preventing the weight transfer, this is something a legit hotshoe/alien can figure out in 3-4 runs.

iirc kw/rce throw a big stiff bump stop on the front and I’ve had issues with kw race springs before - make sure they’re not coil binding and check the front bump stop to see if it’s engaged when sitting still. Coring that sucker out to try and make it softer would be my first $0 attempt. Everything else looks in the ballpark barring damper settings which I have no feeling for but there’s a lot who check here who might be able to spot something there.

Respectfully I think toe change is a bandaid here, there is a grip imbalance that isn’t affected by toe numbers in a reasonable range. It may help, it may be what you like (I personally don’t like front toe out on this car), but the problem is both transient and steady state, it’s unlikely there will be a silver bullet and more likely 2-4 tweaks get you into the zone. Good luck.

Edit re read op - missed the confirming co drives and damper fiddling so that’s out. 8k rear spring might be the answer in the end for sure, especially if you’re happy with the front stiffnes. I’ve found this car to be relatively insensitive to spring rate changes.

Oh and I forgot rake, raising the rear will be the easiest solution around. Jack up the car, raise the perches two turns each and try it - the ride height change isn’t enough to change alignment - and it goes the way you want with less camber and less toe in which should free the rear up to rotate. If the current rake is flat or the rear is lower then the front (depending on where you measure) absolutely do this first.

Already have rake, so it seems it’s either the body is too low and needs to be raised. Like 949 says here or we need to first try the 8k rear spring and then if still needing more, I could try the rear bar 16 bar while keeping the front 22 bar.

https://www.949racing.com/supermiata...lignment-info/

To avoid making too many changes at once, I will:

1) 8k rear spring
2) try it at an event
3) if still needing more, raise ride height and ensure rake is correct
4) get it aligned again
5) try it at an event

Till I get it right. Last option is just go get Karcepts MCS package.


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[strat61caster]

I’d check the ride heights and rake first - it’s basically free aside from some bruised knuckles, much less time consuming than a spring swap. But a plan is a plan!

[Racecomp Engineering]

Your plan is good, though I'd definitely just check ride height first.

Plenty of people tweak spring rates on Tarmac 2s, especially for auto-x. Nothing wrong with trying 8k rears!

EDIT: or stiffer, plenty of range to work with.

- Andrew

[RT-BRZ]

Dumb question guy here; are there fundamental differences in the suspension for the newer editions of this platform? I ask because it seems that the standard for the '13-'21 version is 6k front/9k rear. Adding aero seems to push to either 8k/9k or 10k/12k depending on your style.

I only ask because 8k rear seems like a half-measure if you're trying to fix understeer.

[Tomyyyu]

Originally Posted by RT-BRZ

Dumb question guy here; are there fundamental differences in the suspension for the newer editions of this platform? I ask because it seems that the standard for the '13-'21 version is 6k front/9k rear. Adding aero seems to push to either 8k/9k or 10k/12k depending on your style.

I only ask because 8k rear seems like a half-measure if you're trying to fix understeer.

Wouldn’t call it a half measure. I’d call it developmental, I’d rather go 1k at a time. Try it, take notes, go another K etc. since it seems that these cars have wildly different coil rates and people range anywhere from 4K front to 9k rear.


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[Tomyyyu]

Originally Posted by Racecomp Engineering

Your plan is good, though I'd definitely just check ride height first.

Plenty of people tweak spring rates on Tarmac 2s, especially for auto-x. Nothing wrong with trying 8k rears!

EDIT: or stiffer, plenty of range to work with.

- Andrew

Hm, gotcha Myles didn’t bring up ride height. He mentioned either 8k rear spring or 16 MM rear bar.

I just happen to cross upon that article for ride height on 949 (https://www.949racing.com/supermiata...lignment-info/).

If I do ride height I’ll need to get another alignment right? Are you looking for the ride height to be the same requirements as 949:

140mm minimum for floor to pinch weld in front
And about 5-10 mm rake for rear (so 145-150mm) from floor pinch weld

Just since I did order the rear 8k springs from you guys based on his suggestion, but wondering if I should do the ride height adjustment and alignment before doing the springs.

Do I need an alignment for rear toe (I don’t have lower or upper control arms aftermarket in the rear) if I swap the rear springs?


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[Racecomp Engineering]

Originally Posted by Tomyyyu

Hm, gotcha Myles didn’t bring up ride height. He mentioned either 8k rear spring or 16 MM rear bar.

I just happen to cross upon that article for ride height on 949 (https://www.949racing.com/supermiata...lignment-info/).

If I do ride height I’ll need to get another alignment right? Are you looking for the ride height to be the same requirements as 949:

140mm minimum for floor to pinch weld in front
And about 5-10 mm rake for rear (so 145-150mm) from floor pinch weld

Just since I did order the rear 8k springs from you guys based on his suggestion, but wondering if I should do the ride height adjustment and alignment before doing the springs.

Do I need an alignment for rear toe (I don’t have lower or upper control arms aftermarket in the rear) if I swap the rear springs?


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I'd need to know what those measurements translate to in center of wheel hub to edge of fender (or at least mm lower than stock). I do think the 8k springs are a good idea either way.

I would plan on an alignment when swapping the rear springs or at least a toe check.

- Andrew

[strat61caster]

+/- 1/2” of rear height will not materially affect rear alignment, guarantee it.

[Tomyyyu]

Originally Posted by Racecomp Engineering

I'd need to know what those measurements translate to in center of wheel hub to edge of fender (or at least mm lower than stock). I do think the 8k springs are a good idea either way.

I would plan on an alignment when swapping the rear springs or at least a toe check.

- Andrew

So I have a 16MM bar from Perrin for the rear and the 8K rear springs.

Where would you recommend I start? With the spring, or with the bar?

I can try to get you the ride heights this week, just want to swap the 200TW to get you accurate readings.


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[RT-BRZ]

Originally Posted by Tomyyyu

Wouldn’t call it a half measure. I’d call it developmental, I’d rather go 1k at a time. Try it, take notes, go another K etc. since it seems that these cars have wildly different coil rates and people range anywhere from 4K front to 9k rear.


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Fair enough. You have way more patience for taking these things apart and putting them back together than I do.

I'm running 8k/9k with a Verus race splitter and UCW wing and the car seems like it's pretty happy there even with a stock rear bar and Hotchkiss front bar.

[Tomyyyu]

Originally Posted by RT-BRZ

Fair enough. You have way more patience for taking these things apart and putting them back together than I do.

I'm running 8k/9k with a Verus race splitter and UCW wing and the car seems like it's pretty happy there even with a stock rear bar and Hotchkiss front bar.

I only say that, if not, it’s hard for me to tell did the ride height fix it? Did the bar fix it? Did the spring fix it? Do I need more bar stiffness setting? Do I need more spring? We could be here for day. Lol

Also what size bar and what stiffness setting?


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