GT350R Springs for GT (high rate, minimum lowering)

[CrazedAntelope]

Hey Guys,

I wanted to give my initial feedback based on some posts earlier in this thread. I just had a shop install my BMR SP085 (980lb/in) in the rear along with my BMR spherical RLCA bearing and the FP toe-knucle bearing. I kept my front spring rates the same at 250in/lb on my ecoboost.

Old Rear Setup:
BC racing Spring + spring perch (784lb/in)
Stock RLCA bushing and toe-knuckle bushing
BMR CB005
Rear Eibach swaybar set to medium
JRZ shocks at +6compression, +5rebound

New Rear Setup:
BMR SP085 rear spring (980lb/in. Sits about 3/8" lower than the BC spring)
BMR RLCA bearing and FP toe-knuckle bearing
BMR CB005
Rear Eibach swaybar set to full soft
JRZ shocks at +3compression, +6rebound (manufacturer reccomendation for street)

The old setup was quite excellent for the street manners and confidence inspiring over bumps, but tended to understeer on corner entry and prematurely oversteer while exiting a corner. (Something I could only feel on Autocross)

With the new springs and bearings, the car was pretty much undrivable on the previous shock setting. The car was very uncomfortable and my back was hurting by the end of my 15 mile commute. I had to turn the compression way down and now it's sort of comfortable. Now it seems to be a little floaty over larger bumps in the road. However, overall handling balance is much improved and it seems I can get on the power earlier on corner exit (combo of increased confidence in the rear, less deflection of the bearings, and the softer swaybar offset by stiffer springs).

Of course, the JRZ's have the spherical mount at the top and bottom which everybody has been raving about, so there's no lack of articulation there. I did mention the new BMR springs sit lower, but I think I still am good on suspension travel. Attached is a picture of my compression shock travel when the car is resting on its own weight. It seems like I have 1.75" of travel compression before hitting the bumpstop.

We'll have to see what sort of shock tuning I can do from here. Maybe I'll add a small spring spacer just for better rear geometry, and to get that much further away from the bumpstop. But I'm still not completely sold on the new setup... these are stiff springs and I've removed/locked out quite a lot of mushy rubber from the rear suspension. Maybe I'll have to suck it up and drop down to the Steeda Ultralite Rear spring which is 880lb/in instead.

Please let me know if there is anything I could be overlooking! Does removing a lot of rubber mean I can run softer springs or does that mean I should run stiffer springs?

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[Norm Peterson]

Wheel travel will be a bit more than 1.9-ish, I think more like 2.5".


Norm

 

[CrazedAntelope]

Wheel travel will be a bit more than 1.9-ish, I think more like 2.5".

Thanks for the response Norm. So you're saying I should have about 2.5" of upward travel before I get into the bump stop?

Also, take a look at the silver nut at the bottom of the ruler. I probably have 1.75ish " of travel currently.

 

[Norm Peterson]

The exact amount of free travel will depend on what the actual motion ratio for the shock is. I was guessing 0.75(-ish), which would give 2.4" at the wheel for 1.8" at the shock (more simple numbers for illustration only).


Norm

 

[CrazedAntelope]

Ah, I see your point. I would assume 2.4" upward wheel travel is adequate. But hopefully some folks more knowledgeable than me can chime in and confirm or deny.

 

[BmacIL]

Ah, I see your point. I would assume 2.4" upward wheel travel is adequate. But hopefully some folks more knowledgeable than me can chime in and confirm or deny.

Well the rear spring motion ratio is 0.48. The shock will be near 0.75 as Norm said.

 

[Norm Peterson]

Well the rear spring motion ratio is 0.48. The shock will be near 0.75 as Norm said.

Thanks. "Shock" is what I meant to type the first time but found mostly the wrong letters for.


Norm

 

[CrazedAntelope]

WOW. This setup does great on the road course! I think [MENTION=10281]BmacIL[/MENTION] hit the nail on the head with a stiff rear spring and softer rear swaybar idea. You can really put down the power a lot easier.

I'm probably going to drop down to a 200F/880R rate setup just because the car rides a little bit rough still on the street. I just don't go out to the track enough to justify the ride. For some reason, I feel more comfortable turning the dampers up on a softer spring than down to near 1-2 clicks on a stiffer spring.

 

[keltymd]

Need to go more than that. You need to trim at least 5/8". This car uses its bumpstops a lot in stock form. It helps with the bouncy feeling if you trim more than you lowered.

I hope my issue is found. I developed a clunk in the rear on rebound. Checked everything and swapped back to pp shocks and mounts the issue went away. [MENTION=25806]SteedaTech[/MENTION] being the great ones at service they are mailed me a box and label to send my mounts and shocks to them and found one of the shocks was in fact bad. Today they are shipping my setup back to me with a new shock so I should have it this week. Gotta say A+ to steeda for support.

 

[BmacIL]

WOW. This setup does great on the road course! I think [MENTION=10281]BmacIL[/MENTION] hit the nail on the head with a stiff rear spring and softer rear swaybar idea. You can really put down the power a lot easier.

I'm probably going to drop down to a 200F/880R rate setup just because the car rides a little bit rough still on the street. I just don't go out to the track enough to justify the ride. For some reason, I feel more comfortable turning the dampers up on a softer spring than down to near 1-2 clicks on a stiffer spring.

Have you tried the billet rear shock mounts? These quite significantly improved the ride and took out a lot of harshness and oscillations. The other recent thing is tires. I had on Sumitomo HTR ZIII's and just went to Bridgestone RE760's. The difference in ride quality is shocking to say the least. My car rides beautifully now...better than the PP stock while having ~50% higher wheel rates. It's a thing of beauty. Now, the RE760's are not a track tire, but for a summer performance tire that can be tracked, they're fantastic. Their direct replacement is the new Firestone Indy 500.

 

[CrazedAntelope]

Have you tried the billet rear shock mounts? These quite significantly improved the ride and took out a lot of harshness and oscillations. The other recent thing is tires. I had on Sumitomo HTR ZIII's and just went to Bridgestone RE760's. The difference in ride quality is shocking to say the least. My car rides beautifully now...better than the PP stock while having ~50% higher wheel rates. It's a thing of beauty. Now, the RE760's are not a track tire, but for a summer performance tire that can be tracked, they're fantastic. Their direct replacement is the new Firestone Indy 500.

My shocks are JRZ. They have the spherical ends at the top and bottom of the rear shock. Other suspension mods in my sig.

You're right, tires make a huge difference. I am on Nitto NT05 with a 275/40R18. The ride quality on my aftermarket wheels is noticeably worse than on my stock PP wheels with 255/40R19 with MPSS (my winter set here in Cali).

 

[BmacIL]

My shocks are JRZ. They have the spherical ends at the top and bottom of the rear shock. Other suspension mods in my sig.

You're right, tires make a huge difference. I am on Nitto NT05 with a 275/40R18. The ride quality on my aftermarket wheels is noticeably worse than on my stock PP wheels with 255/40R19 with MPSS (my winter set here in Cali).

Yeah I would say most of what you're experiencing is tire harshness. What bars do you have on the car?

 

[CrazedAntelope]

Yeah I would say most of what you're experiencing is tire harshness. What bars do you have on the car?

Eibach. Front set to medium stiff and rear set to full soft.

 

[BmacIL]

Eibach. Front set to medium stiff and rear set to full soft.

I've found that most of this car's harshness is from the rear. The front can kinda just take whatever but the rear is very sensitive. Even the Eibach bar on full soft is a lot stiffer than the PP bar. I would consider dropping the Eibach front to soft and swapping back to the PP rear, perhaps. It won't help double-wheel/heave bumps but any single-wheel events should be quite a bit less harsh and your balance won't be much different.

Turning the rebound damping down a little also helps over big bumps where the front gets light.

 

[CrazedAntelope]

I would agree with you that harshness comes from the rear. I'll try turning the rear rebound down. Currently at +3 compression and +6 rebound. I'm just afraid of being oversprung and underdamped. At this point I have a bunch of springs (4 sets) at my disposal it's just a matter of swapping in and out.

The car felt awesome confident and comfortable on the street with 780 rear spring rate and the shocks at a much firmer setting. But I'm sure the current setting outperforms it in HPDE. I'm hoping the 880 rear will be a happy medium.

A lot of this is just to satisfy my own curiosity

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