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

[ruturaj001]

One of the GR86 forum member has been running 7k front 9k rear on SS-R (ordered it that way from RCE) but on BRZ (1mm thinner sway bar in rear).

[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

I’m on 245/40/17 DWS06Plus versus the 245/40/17 A052s I run.

But I would say it’s 13.5” from center of rim to fender on all four on the all seasons. I would say maybe take 1/2” off max for the 200TW tires.


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[RT-BRZ]

Originally Posted by Tomyyyu

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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All good points. I'm running Shaftworks dampers so I have the rest of the "static" components set (springs, bars, etc.) and do the finer adjustments with the dampers. In my case, the manufacturer suggested the appropriate ride height and that's where I am.

Yeah, it could take awhile to figure everything out. Good luck working through it.

[cmiovino]

I'm coming into this a bit late with all the responses. I think your camber is generally ok. I run -4F/-2.5R, so you're at least in the ballpark. I also run 6k springs all around vs your 7k, but they're both square.

I was dialing mine in earlier this season and found more (stiffer) settings in the rear REALLY made a big difference. I was starting out using the recommended 10 clicks from stiff front and 8 clicks from stiff rear and the car didn't want to turn. Very understeery and hard to drive.

I did a test and tune playing with all the settings trying equal, stiffer, softer, and variations front to rear. I settled in that softer in the front and stiffer rear was the ticket. I'd personally try that before swapping out parts just yet.

I also went back to zero toe all around. With those settings it didn't need to toe in rear some people run.

In all your mods and settings, that's the glaring issue to me. Might want to try it before trying to swap in new springs. I think 7/9k springs are becoming more and more acceptable for autox due to the motion ratio in the rear. I feel like playing with the settings is a bit of a bandaid, but it works just fine for me.

[Tomyyyu]

Originally Posted by cmiovino

I'm coming into this a bit late with all the responses. I think your camber is generally ok. I run -4F/-2.5R, so you're at least in the ballpark. I also run 6k springs all around vs your 7k, but they're both square.

I was dialing mine in earlier this season and found more (stiffer) settings in the rear REALLY made a big difference. I was starting out using the recommended 10 clicks from stiff front and 8 clicks from stiff rear and the car didn't want to turn. Very understeery and hard to drive.

I did a test and tune playing with all the settings trying equal, stiffer, softer, and variations front to rear. I settled in that softer in the front and stiffer rear was the ticket. I'd personally try that before swapping out parts just yet.

I also went back to zero toe all around. With those settings it didn't need to toe in rear some people run.

In all your mods and settings, that's the glaring issue to me. Might want to try it before trying to swap in new springs. I think 7/9k springs are becoming more and more acceptable for autox due to the motion ratio in the rear. I feel like playing with the settings is a bit of a bandaid, but it works just fine for me.

Yeah tried going almost full soft compression on the front and full stiff rebound on the rear and the issue was still there. That’s why I concluded I can’t “out tune” the issue at this point with the A052s. I need a different solution, but definitely appreciate your input.


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[Breezio]

My initial setup was very similar to yours, but 1" Hotchkiss bar in front (on soft) and factory PP 16mm bar in rear.
I didn't really have issues with understeer per say, but the rear of the car didn't rotate like I wanted it to (if that makes sense)
Putting in rear subframe inserts and zeroing the rear toe helped a lot, also increased front camber to -4.3.
I'm very happy with the car now.

[ZDan]

1st thing I'd do is swap the stock front sway bar back in. Stiffer rear springs after that.

I'm running 7k front with 9k rear RCE SS-2 on my '23 BRZ with stock sways. It is admirably neutral, even around the long turn 5 at Palmer CW (aka "Remlap) where my old '17 PP suffered a *lot* from terminal understeer!

Having played around with spring rates in other cars, IMO going from 7/7 to 7/8 isn't worth doing, go straight to 9k rears.

FWIW I'm very camber limited up front due partly to the RCE/KW top-mount design and partly some built-in asymmetry with my car, so I'm only running -3.2 front camber (at the left front anyway, sometimes more on the right for CCW circuits). While I'd like to run -3.5, IMO even if pyro temps and wear patterns may suggest "more front camber!" you are already at a point where more front camber isn't likely to give any big noticeable changes to handling balance, but no reason not to try it...

Oh yeah, also worth mentioning I have to run a LOT of ballast, 150+ lb, which all goes in the spare tire well along with the spare from my old '17 PP, so my weight distribution is quite a bit les nose-heavy at 51.5%F/48.5%R. So YMMV at closer to 55/45 stock weight dist...

[steverife]

Originally Posted by Tomyyyu

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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I suppose my question would be.... If wholesale changes accomplished what you wanted, does it matter?

[Tomyyyu]

Originally Posted by ZDan

1st thing I'd do is swap the stock front sway bar back in. Stiffer rear springs after that.

I'm running 7k front with 9k rear RCE SS-2 on my '23 BRZ with stock sways. It is admirably neutral, even around the long turn 5 at Palmer CW (aka "Remlap) where my old '17 PP suffered a *lot* from terminal understeer!

Having played around with spring rates in other cars, IMO going from 7/7 to 7/8 isn't worth doing, go straight to 9k rears.

FWIW I'm very camber limited up front due partly to the RCE/KW top-mount design and partly some built-in asymmetry with my car, so I'm only running -3.2 front camber (at the left front anyway, sometimes more on the right for CCW circuits). While I'd like to run -3.5, IMO even if pyro temps and wear patterns may suggest "more front camber!" you are already at a point where more front camber isn't likely to give any big noticeable changes to handling balance, but no reason not to try it...

Oh yeah, also worth mentioning I have to run a LOT of ballast, 150+ lb, which all goes in the spare tire well along with the spare from my old '17 PP, so my weight distribution is quite a bit les nose-heavy at 51.5%F/48.5%R. So YMMV at closer to 55/45 stock weight dist...

Well I already got the 8K springs per RCE and I can always try 9k down the road.

For autocross we have a few who in the GR (1MM larger rear sway) drive with Redshifts (revalved BC coils) and had tons of issues putting power down with a 8/9k combo and a 19MM front bar on stiff. Their fix was lots of toe in, in the rear and a 22MM front bar.

Based on that I thought I’d keep the 22MM front bar, and try 7/8k. But I had one person also mention trying to put the stock front bar back on.

It seems I should definitely try the rear 8k spring, and possibly try the stock sway. The problem is an event happens each weekend and I treat it as a test and tune, so I could make 1 change at a time, or both (I guess).

Curious as to what settings you run front and rear for rebound and compression as well.


Edit:

The 2022 GR86 uses the 2017-2020 set up, 18mm front and 15mm rear.
The 2022 BRZ uses a new 18.3mm hollow front bar and a 14mm rear.



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[ZDan]

Oh yeah, caveat: I'm not an autocross guy...

Originally Posted by Tomyyyu

Well I already got the 8K springs per RCE and I can always try 9k down the road.

For autocross we have a few who in the GR (1MM larger rear sway) drive with Redshifts (revalved BC coils) and had tons of issues putting power down with a 8/9k combo and a 19MM front bar on stiff. Their fix was lots of toe in, in the rear and a 22MM front bar.

"lots of toe in, in the rear," ewwwwww....
Lots of rear toe-in has *always* sucked for me on every car I've intentionally or inadvertently tried it on. AWFUL, no turn-in, more nonlinear handling at the limit and in transitions, could actually become *very* unstable *in a straight line* in very wet/inundated track conditions! And then at the next corner would not turn in... Also incredibly rapid tire wear. For me, zero positives to running a lot of rear toe-in and a ton of big negatives...

It seems I should definitely try the rear 8k spring, and possibly try the stock sway.

Yeah, sounds like good approach. Honestly don't feel like just going 7k to 8k on rear springs is going to make a profound difference in balance but could be wrong.

The problem is an event happens each weekend and I treat it as a test and tune, so I could make 1 change at a time, or both (I guess).

Also could be that some layouts are more understeer-prone than others...

Curious as to what settings you run front and rear for rebound and compression as well.

Currently I'm halfway between RCE recommendations for SS-2 coilovers for road vs. track, 10 clicks out in front and 12 clicks out in back. Might crank in a couple of clicks in back as I feel like I'm a little too reboundy off of bumps/whoops on the street.

[Tomyyyu]

Originally Posted by ZDan

Oh yeah, caveat: I'm not an autocross guy...

"lots of front toe in, in the rear," ewwwwww....
Lots of rear toe-in has *always* sucked for me on every car I've intentionally or inadvertently tried it on. AWFUL, no turn-in, more nonlinear handling at the limit and in transitions, could actually become *very* unstable *in a straight line* in very wet/inundated track conditions! And then at the next corner would not turn in... Also incredibly rapid tire wear. For me, zero positives to running a lot of rear toe-in and a ton of big negatives...



Yeah, sounds like good approach. Honestly don't feel like just going 7k to 8k on rear springs is going to make a profound difference in balance but could be wrong.

Also could be that some layouts are more understeer-prone than others...

Currently I'm halfway between RCE recommendations for SS-2 coilovers for road vs. track, 10 clicks out in front and 12 clicks out in back. Might crank in a couple of clicks in back as I feel like I'm a little too reboundy off of bumps/whoops on the street.

Gotcha I’m autocross only. Lol

It seems another STR person also runs a 19MM front bar from Perrin on full stiff with 7/9k springs.

I’m running 22MM full soft. And it does make sense when you look at the %s.

Also for 9k in the rear, if the motion ratio is about 0.75 in the rear, that means that will be around a 7k spring in the rear. So I do think you are right on 9k versus 8k.


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[ZDan]

Originally Posted by Tomyyyu

Also for 9k in the rear, if the motion ratio is about 0.75 in the rear, that means that will be around a 7k spring in the rear.

Akshually...

Wheel rate (what matters) is spring rate x motion ratio *squared*, so with 9 kg/mm springs in back, wheel rate is 9 kg/mm * (0.75)^2 = 5 kg/mm

I don't get why so many suppliers give "square" spring rates for these cars...

[ruturaj001]

Originally Posted by ZDan

Akshually...

Wheel rate (what matters) is spring rate x motion ratio *squared*, so with 9 kg/mm springs in back, wheel rate is 9 kg/mm * (0.75)^2 = 5 kg/mm

I don't get why so many suppliers give "square" spring rates for these cars...

Hmm, looking up, this looks right, it is squared. I just started researching in to the topic and I agree that square spring rate doesn't make sense on twins. I was using @900BRZ 's blog.

https://900brz.com/posts/gr86-vs-brz

Looking at your numbers, we would be staggered by a much larger margin when trying to have square wheel rate. 60% stiffer in the rear.

[renfield90]

Random grab bag of thoughts. Sorry it's not more organized, it's late and I'm supposed to be working.

My old T2 settings on 450lbs/in front and 550lbs/in rear:
Compression: 4 clicks from full hard (all 4)
Front rebound: 10 clicks from full hard
Rear rebound: 11 clicks from full hard

You want a stiffer rear spring on this car than on the front to make the suspension "flat." If you go too stiff in the rear, a 200tw tire (and/or your talent) will not be able to keep up. For me, too much was at 600lbs/in. Look into "motion ratio" and "natural frequency" if you want to dig into this more. A spring rate change is not the first change I'd recommend though.

If you are riding on the bump stops, your spring rate is not what you think it is. A bump stop acts as a spring in parallel which means the total spring rate is simple addition of both rates. And since a bump stop's spring rate is variable and very progressive...good luck! Get off the bump stops.

You do not have enough front camber and that is absolutely contributing to your understeer. If you can't get to -4.5 with your current plates, get new plates. This is IMO the first thing you should try, followed by getting the car off the bump stops, followed by a rear spring change.

From there I suggest finding a suspension adjustment flow chart and following it. Everyone has their own philosophy for how to tweak their car, but we're all mostly following the flow chart which provides a menu of options, and from there picking our own medicine (or in some cases, poison). Get your mid corner straightened out first because sometimes that will fix entry/exit as well.