M3hunter

Hi Guys
I have '11 GT w/ Brembo package, came with 255/40R19 mounted on 9x19 wheels. I lower it with STEEDA Sport springs felt great, then I replaced rear tires for 275/40R19 Michelin Pilot Super Sport tires, car felt very stable, then I replaced the fronts with the same 275/40's, the car had a quicker steering, felt good. But after a long drive at 80-90mph I had to be very concentrated, since at the slightest movement on the steering wheel, the car wanted to change line. I kept the stock 35 psi on all wheels. Please advice

danzcool

I'd say if you're not comfortable with how quick the response is, it's time to look into and adjust camber and/or toe.
Neutral or toe out will result in quick steering response, so you probably want some level of toe in.
Higher negative camber numbers also affect steering response, but it's usually about how hard you can corner.
It's a balancing act and I can't tell you what numbers you will be comfortable with.

ModBargains

Is tramlining the issue? With my 275 fronts and 315 rears I tend to find some freeways make the car tramline worse. In most cases wider tires tend to go with the grooves on the roads a bit more than less sided widths. It's annoying, but the grip advantages on the track are significant enough for me to keep the fitment, plus it looks really nice on the car.

If that's what you're experiencing, then yes, I have the same issue as well.

Norm Peterson

I suspect that you're getting "twitchiness contributions" from both of the above-mentioned effects.

When you lowered the car, if you did not correct your camber, you're running at cambers that are somewhat further negative than before. But it's not just the camber that's making it twitchy - it's the fact that when the camber went more negative the toe moved in the toe-out direction. Even if it didn't move all the way into toe out, it'll still have a little less straight line directional stability than it had before the lowering.

Try measuring camber yourself just to see what you do have (there are several low-$ DIY methods of doing this). For mild lowering combined with average driving that's "slightly aggressive in the corners" you might not have to touch or adjust camber, and only a toe fix would make a difference.


Tramlining is also very likely, and wider tires tend to be worse than more moderate widths. The tire's actual construction also matters - some 275/40's will be worse than others - and I would imagine that the OE tires were developed with as little tramlining as could reasonable by achieved without throwing away too much performance.

A 275/40-19 tire is significantly larger than your OE 255/40-19's - about 5 Load Index numbers worth in terms of load capacity - so you don't really need to run them at the same inflation pressure(s) for that reason to begin with. I'd start by trying them at 31 - 32 psi, at least for longer driving stints at higher speeds. Maybe for the first couple of times at speed, don't run with the car all loaded up with passengers and trunk stuff.

I suspect that between a toe-only alignment tweak and a little less inflation things will quiet down. You may also notice that you gradually adapt to slightly quicker steering response and teach yourself how to "calm your hands down". Actually, that is something that took me two or three weeks to fully adjust to with my '08 relative to what I had been driving (chances are pretty good that my car was built with cambers more negative than what yours are at now, and that's where they've stayed ever since).


Norm

M3hunter

On my case I don't think my wider front tires are following the road groves.
After I lowered my car with stock tires, even with wider rear tires, the GT felt great, no negative issues at all.
At this time I'm travelling with family, so my car is loaded (litle luggage), I don't know if lowering my PSI will be OK under the present Conditions.
Guess that you are correct probably also with my wider tires in front changed my alignment, I noticed a faster steering at all speeds (quicker change of lines). Yesterday I drove about 3 hours but I was not tired, like on my other day, did not find any difficulty, probably I just only need to get use to it ( or on the contrary to take it to align). Steering remained centered and is not following road grooves.
Thaks Guys!

M3hunter

Hi Norm,
Then I guess with all my changes came something positive. The negative camber and the toe out will improve this car handling.
I will follow your advice with your PSI recommendations.

M3hunter

Hi Norm,
Do you think a front 31-32 psi combined to a rear 30-31 will be OK?

flash_xx

What wheels and with what offset are you using to get those sizes to fit? Or are they protruding from the fenders?

Norm Peterson

Should be. Strictly speaking, it depends on whether the tires are SL or XL..

If both tires are "Standard Load (SL)" or if both tires are "Reinforced" or "Extra Load (XL)", you'd only need about 29 psi in the 275/40-19 to match the load rating of a 255/40-19 at 35 psi. So anything above 29 would be OK load-wise and you could tune a pressure stagger specifically for handling balance. Pirelli and Michelin almost certainly follow these ETRTO tables. This is probably the most common situation.

It gets a little more complicated if the OE tires were SL and the new tires are XL, or if the OE tires were XL and the new ones SL. But there is a way to correct for this sort of difference.

If both tires are P-Metric (and I assume Standard Load), it appears that the new tires would not even need 28 psi. The TRA P-Metric tables are different from either of the first two. Generally, I won't suggest tire pressures much below about 28 psi where street use is involved.

I'm gonna claim that the time spent today was and will ultimately be worth it.


Norm

M3hunter

Hi Norm, my 5.0 handled great on the road with the lowered PSI that you recommend for the 275's I lowered all 4 tires, no more nervous steering, it felt like the stock steering feeling that I had.
The only thing that I noticed a bit more of hidroplaning over water compared to stock, but nothing out of control.
Thanks Norm, again you hited the bullseye.