Help needed for budget fast road/track suspension build

[tekno]

Hello y'all, looking to build a half way decent suspension set-up for fast road and track driving while spending the minimum possible and make use of what I already got, here goes...





The car: '13 GT86, forged bottom end, boosted from a gtx2867 on a 1 bar (14.5psi) minimum setting. Current suspension setup, which I put together for daily street use and dirt road fun: Bilstein B6 shocks, Chuhatsu Multiroad lift springs, SPC front camber bolts, Pedders -camber/+caster top mounts. This year I'd like to do a few track days and local time attack competitions, and a few changes/upgrades are needed.
I'm thinking of keeping the B6 dampers as they are fairly new (put around 3000 miles on them) and pair them up with some stiffer springs. The plan is to use coilover conversion sleeves which I already have and adjustable coilover springs, for which I already found a supplier that has them in tapered shape and with the correct diameters to fit with oem style upper mounts. My questions are, what spring rate should I go for and what's the max spring rate these B6's can handle? Would 6kg/mm both front and rear be to much for them?

With the springs that I'm going to install, I'd like to keep the ride height as close as possible to stock, maybe even 1cm higher to gain some more bump travel. As for the rest of the setup, I already bought and need to install the Energy master poly-bushing kit and rear camber adjustable LCA's. Will be rollin' on 225 45/R17 DOT semi slicks, unless someone buys my 18" wheels in which case I'll get wider 17" wheels and go for 245mm wide tires. Not planning on changing the anti roll bars for now since most of my budget will get poured into the mandatory safety equipment.

I would also like to experiment playing with anti-squat/traction mod bracket, but in the EU, everything doubles it's cost by the time it gets taxed and shipped so it's not within my budget. Hhere's a long shot though: Anyone that has them installed would care to measure and share by how much is the TCA pick-up point lowered? I can fab them up for a fraction of the cost...

[Ohio Enthusiast]

Originally Posted by tekno

I would also like to experiment playing with anti-squat/traction mod bracket, but in the EU, everything doubles it's cost by the time it gets taxed and shipped so it's not within my budget. Hhere's a long shot though: Anyone that has them installed would care to measure and share by how much is the TCA pick-up point lowered? I can fab them up for a fraction of the cost...

Not sure if MCA published their offset, but the GPSports version of this lists a 60mm drop in the pickup point. You can also check the thread here for discussion about the MCA and others' traction mods.

[tekno]

Originally Posted by Ohio Enthusiast

Not sure if MCA published their offset, but the GPSports version of this lists a 60mm drop in the pickup point. You can also check the thread here for discussion about the MCA and others' traction mods.

Well, my guesstimate based on a crude photo edit is that the MCA's drop it by about 45mm.



IIRC, the bolt is 10mm so bolt hole most probably would be 11mm, 4x11=44

From the photos, GP Sports bracket seem to have a higher drop, also it looks like the pick-up location is pushed towards the front of the car, most probably so that the wheel doesn't gets pushed towards the rear...

If I won't get more clear information I'll probably fab the brackets with a 50mm drop and see how it feels.

Thanks for the link!

[Ohio Enthusiast]

Originally Posted by tekno

Well, my guesstimate based on a crude photo edit is that the MCA's drop it by about 45mm.

IIRC, the bolt is 10mm so bolt hole most probably would be 11mm, 4x11=44

Yep, looking at the pictures in the MCA site I tend to agree that it's quite a bit less than the GP Sport's 60mm. Your ~45mm feels about right.

Edit: actually, I measured my GP Sport mod (waiting to be installed) hole and it's ~12.5mm, so I assume the bolt is an M12. This would make the MCA drop closer to 50mm.

[tekno]

Originally Posted by Ohio Enthusiast

Yep, looking at the pictures in the MCA site I tend to agree that it's quite a bit less than the GP Sport's 60mm. Your ~45mm feels about right.

Edit: actually, I measured my GP Sport mod (waiting to be installed) hole and it's ~12.5mm, so I assume the bolt is an M12. This would make the MCA drop closer to 50mm.

Chers for that!

Just read this on the thread you suggested:

Originally Posted by Gforce

MCA traction brackets drop the front suspension pick up point for the trailing arms by 53mm. This leads to a 25mm drop from that point to the trailing arm pivot point at the LCA.

So anti squat angle is reduced by that difference, 2/3.

I suspect that some of the benefit of this mod lies in the reduction of toe change as well as reduction of anti squat effects.

[NoHaveMSG]

It sounds like you already have a pretty solid plan. My only question is why DOT semi slicks?

[tekno]

Class regulations of the time attack series I want to compete in.

[strat61caster]

iirc Bilstein B14 are valved stiffer then the B6/8 and come with 4.5k F/7k R springs, RCE Tarmac springs for Koni Yellow were 280/290#, somewhere between 3k-5k is probably the happy upgrade you’re looking for, I’d gamble on 3k/5k if it were my car with a pair of 4k’s to try in the back pocket. 5k square or 6k square would also be reasonable starting points. The car doesn’t seem to care much what springs you put on it as long as your ride heights and bump travel aren’t jacked and your dampers don’t suck lol.

[tekno]

Andrew @RCE suggested F-5Kg/R-6Kg max rates on these dampers. Since it's a bit more nose heavy, I ended up ordering a square set-up at 5kg/mmq for starters, I'll see how it feels and adjust from there.
I actually passed the occasion to buy a used set of B14's, at a very low price, but... I don't like the idea of less suspension travel, that`s also one of the reasons I wanted to stick with the B6's and not use lowering springs.

[ZDan]

Don't need the RCLAs, don't need the "traction mod", going higher is bad for track, lower is better. I ran ~4.5 front with ~5.4 rear springs with Bilstein B8s (same damping as B6, just less droop travel) lowered 35mm on Swift Spec R springs, which I recommend for you for best track performance. They are significantly lower though... RCE Tarmac springs "only" lower about 20mm which would be about as high as I'd wanna run at the track if concerned at all about lap times.
Oh, nevermind I see you'll be running coilover springs? I think 6 all around should work.
You will want front camber plates as front camber will be inadequate otherwise. I ran Raceseng plates, there are other options for non-coilovers (Verus I think, etc)

Run ~-3.5 front camber, rear camber from lowering (which again I highly recommend for track) will likely be in the -2.2 to -2.7 range which is fine. Zero front toe, zero to 0.1 degrees total rear toe.

Have fun!