Riz

How would you compare it to your 1M, since they are both sort of cheaper parts bin versions of their bigger brothers?

Crazy Eddie

Great insight Thanks Pete !!

Petevb

Interesting results at my second autocross Saturday. Conditions were cool, mid to high 60s and windy, with quite a long time between runs. This changed the car's balance dramatically.

In the morning starting from 28 psi front, 32 PSI rear I was unable to build heat and tire pressure. Understeer was quite bad with the rear bar full stiff and the front medium, and the car felt skatey and unstable. I dropped the front bar to full soft, but this didn't cure the understeer, and after 5 runs I had only built 1 psi of pressure and was slow.

Afternoon I tried dropping cold tire pressures- 25 psi front, 29 rear. This made a huge difference. Grip was up from the first run, the car balance shifted to neutral, and the tires built heat and pressure. After 3 runs the tires had gained 4 psi- 29 front, 33 rear, and times were over a second quicker.

I'm not sure if the car understeers with low grip, if the rear tires built heat faster or what, but the shift in balance was dramatic for such a small change. It may go some way to explaining why some complain of terminal understeer and others don't? All I know is that I couldn't dial out understeer with the bars in the morning, while I could in the afternoon. I'd suggest those with balance issues play with pressures...

Ochocoronas

I have yet to Auto X my GT4 but in my 981Cs I found even tier pressures gave me the best results. PSI was all over the place depending on temp, course and course surface. I was running the stock Pirelli tires. Did you adjust anything other than the sway bars?

Beantown Kman

Petevb, always good to read your comments. One important point about the GT4 that I have not seen mentioned in any forums is the larger rear wheels that come as OEM. These 11" wheels are substantially larger than what has been supplied on previous 981's and 987's while the fronts have not increased at all. With wider wheels and tires, and more rear grip, the handling is obviously going to change, with more understeer. I think it is likely we will all be moving to 19" x 10" wheels for the track once the wheel manufacturers figure out what offsets will work. That should help the handling balance. And this car doesn't have so much power that we would need to be concerned about breaking the rear end loose under hard acceleration.

It's interesting that the chronic understeer this car suffers from did not surface as an issue during the reviews by the motoring press. I assume the Portimao circuit has mostly high speed, sweeping corners.

Tire grip, when the tires are cold, is greatly affected by tire pressure. I, too, have found hat starting with what some would consider shockingly low pressures has worked extremely well on cold days.

I do not believe the understeer people are experiencing is anything to worry about. With all the adjustments available for camber, tires and tire pressures, sway bars and settings, etc., this will soon become a non-issue. New sway bars with a wider range of adjustment may be all that is needed. In the meantime, we will have to make the best with what we've got.

Jenner

In the Motortrend review the understeer was so bad they said they thought the car was broken!

Beantown Kman

Right! I forgot about that one. And isn't it interesting that this review was done on a VERY tight and small circuit? But I don't think the issue came up at all, for anyone, at Portimao.

Jenner

I would agree with you except for Jason's comment about the GT4's he drove in Europe...was it the car or the track? I have to think they drove them in multiple conditions street/track and would have noticed under steer somewhere along the way no? Jason's MT review above noticed the under steer on the big sweeping figure 8 in the mid corner. Watch the video again the sweepers on the figure 8 are HUGE.

I know how Americans (both legal system and driver skills) are viewed by Europeans and would not surprise me one bit if the setup/handling is dialed in for additional under steer for the US market. It would not be the first time this has occurred on a US spec car vs ROW.

Petevb

Alignment is stock. Last run of the morning I tried bumping the front tire pressures up- ~34 psi all around. That was a step in the wrong direction.

I played with track precision for a couple laps, just starting to figure it out. Sloppy, but you can definitely see both understeer and oversteer. It'll log pressures, etc too, so I suspect it will be a good tool to troubleshoot balance issues when I figure it out:



I don't believe swaybars or any parts are different between US and euro cars. Given that just a 3 psi change made a huge difference in balance I'm suspicious that the car's setup is very sensitive...

sapman

Did increasing the front tire pressure make it too prone to oversteer?

PorscheG96

Pete, your experience is consistent with morning conditions @ Marina where the cold sandy surface is unforgiving but grip improves significantly in the afternoon. Dropping pressures is a good call to encourage deformation and build a little heat in the tires.

Your PTP plot appears to show corner entry understeer and corner exit oversteer which tells me that GT4 would benefit from adjustable shocks. I'd throw more rear compression at it and decrease front rebound...that'd be a riot.

GrantG

Or softer front swaybar setting (I think he had one more setting to try there - some people even disconnect a drop link to lose the whole front bar's effect)...

Petevb

Nope, still understeer.

Quite possibly the surface changed, though temperatures didn't come up much at all- we were overcast call day. I was also second run group in the morning, so others had cleared the surface...

I agree adjustable shocks would be the ticket for AX, but I suspect there's still a lot to be gained with driving style, alignment, etc. The oversteer was a lot of fun, much more so that the last course. And grabbing 1st into the tight stuff is child's play with rev match- seriously fast.

GrantG

Great news - looking forward to trying that on slow mountain switchbacks!

Beantown Kman

Sweet!