Braking instability (996.2 GT3)
Discussion
Normally it’s either a worn diff or the diff set up. A classic cause is the plates being fitted the wrong way round. Although your car has the right hardware that would be my starting point. Can you explain in as much detail what the car is doing under braking? Are you trailing when the problem occurs or braking in a straight line is the instability from the front, rear or laterally?
Thanks chaps. Good shout about starting with the ‘cheapest’ first.
The diff was installed by Tech 9 a few years ago, at the time when they were looking after the late Sean Edwards’ race cars, so it ‘should’ be correctly fitted.
Edit to add, the car’s got Alcon front, OEM rear, RS29 all around.
Steve Rance said:
Normally it’s either a worn diff or the diff set up. A classic cause is the plates being fitted the wrong way round. Although your car has the right hardware that would be my starting point. Can you explain in as much detail what the car is doing under braking? Are you trailing when the problem occurs or braking in a straight line is the instability from the front, rear or laterally?
Straight line, instability at the rear, at various speed. It wasn’t much (big angle) and it wasn’t scary that required big correction, just a bit of ‘tiny, instinctive’ steering inputs.The diff was installed by Tech 9 a few years ago, at the time when they were looking after the late Sean Edwards’ race cars, so it ‘should’ be correctly fitted.
Edit to add, the car’s got Alcon front, OEM rear, RS29 all around.
Edited by BrotherMouzone on Sunday 6th January 21:06
As above, could be a number of things.
My 996.2 GT3 had all new suspension and geo by CG so I knew that was good when I started to get instability at rear under heavy braking. Turned out I was getting faster and tyre pressures in rear where too high. Pyrometer and adjustments fixed that.
Then, a year later, it started to happen again. This time it was worn diff plates and a rebuild fixed it.
Edited to add. Alcon front and OEM rear is perfect set up IMO.
My 996.2 GT3 had all new suspension and geo by CG so I knew that was good when I started to get instability at rear under heavy braking. Turned out I was getting faster and tyre pressures in rear where too high. Pyrometer and adjustments fixed that.
Then, a year later, it started to happen again. This time it was worn diff plates and a rebuild fixed it.
Edited to add. Alcon front and OEM rear is perfect set up IMO.
Always start with the basics and work your way from there. Get it up on a ramp check the tyres / pressures and give the suspension physical check over and make sure nothing is heavily worn or has excess movement. If nothing obvious is found inspect the brakes, condition of pads and checking you don’t have a binding piston or pad.
Move on from there would be my advice.
Move on from there would be my advice.
Pressures are critical as is tyre temp. On crappy road rubber on pressures and or temps that are too high, the car will be horrible under braking. Over inflated tyres will baloon and the contact patch get smaller. Even in cold ambient, I would not expect to get more than 2-4 flying laps before the tyres went off - BUT the handling characteristics would be horrible everywhere not just under braking.
As a guide, if i'm on cups on a normal clockwise circuit i'd start off cold at 1.8 bar on the loaded side of the car (passenger for RHD) and 1.85 bar on the unloaded side. The tyres will be at 2.1 bar all round within 2 laps - providing you've already warmed up the fluids before you go out.
As a guide, if i'm on cups on a normal clockwise circuit i'd start off cold at 1.8 bar on the loaded side of the car (passenger for RHD) and 1.85 bar on the unloaded side. The tyres will be at 2.1 bar all round within 2 laps - providing you've already warmed up the fluids before you go out.
Steve Rance said:
Pressures are critical as is tyre temp. On crappy road rubber on pressures and or temps that are too high, the car will be horrible under braking. Over inflated tyres will baloon and the contact patch get smaller. Even in cold ambient, I would not expect to get more than 2-4 flying laps before the tyres went off - BUT the handling characteristics would be horrible everywhere not just under braking.
As a guide, if i'm on cups on a normal clockwise circuit i'd start off cold at 1.8 bar on the loaded side of the car (passenger for RHD) and 1.85 bar on the unloaded side. The tyres will be at 2.1 bar all round within 2 laps - providing you've already warmed up the fluids before you go out.
Steve, I usually drive at 29/31psi (2.0/2.1 bar) front/back hot which works fine for me (interestingly don't know why I don't just stick to 30 psi all round!). However I had a strange experience late last year at silverstone. We had a dry morning (and so tyres had been dialled down to hot pressures) which later in the afternoon went wet. Lazily I just carried on driving and everything felt fine - enjoyed the more gentle wet laps too (wasn't torrential, but light rain)As a guide, if i'm on cups on a normal clockwise circuit i'd start off cold at 1.8 bar on the loaded side of the car (passenger for RHD) and 1.85 bar on the unloaded side. The tyres will be at 2.1 bar all round within 2 laps - providing you've already warmed up the fluids before you go out.
Sometime later I happened to notice my left side tyres - both front and back - had horrendous breakdown of the OUTSIDE edges. Can't believe they were disintegrated to unsafe and illegal levels. The centre and inside edges were fine with plenty of tread. I can only assume that the wet portion of the day resulted in lower tyre pressures but as they had been preset for a dry track they were basically running extremely low and overheating (parts of the track were probably drier) - plus the grinding effect of a wet track rubbing the surface away.
Just a note of warning to everyone else - if your day turns wet after a good period of dry running, do not forget to INFLATE your tyres back up again instead of just enjoying the drive as it is
Edited by LaSource on Monday 7th January 12:04
My guess is that the tyres lost heat over lunch and dropped back, the rain in the afternoon prevented them from getting up to temp again and the side walls started to deform because the pressures were too low. As you say, always re inflate your tyres after a day on the track or this will happen as you drive home!
Steve Rance said:
My guess is that the tyres lost heat over lunch and dropped back, the rain in the afternoon prevented them from getting up to temp again and the side walls started to deform because the pressures were too low. As you say, always re inflate your tyres after a day on the track or this will happen as you drive home!
Yes one always re inflates at end of day before driving home - it was the intra-day change that caught me out.Update on this....
Had the car serviced, plus new front trailing arms and geometry done in February, also fitted Cup2s.
Since then did the RS day at Oulton, then Anglesey TD, Wales road trip and a few blasts at local roads. No instability on TDs and on road to report.
Had a play with its adjustable dampers today and settled with one of the recommended settings from Tech9.
Car is so sweet to drive and I love it.
Had the car serviced, plus new front trailing arms and geometry done in February, also fitted Cup2s.
Since then did the RS day at Oulton, then Anglesey TD, Wales road trip and a few blasts at local roads. No instability on TDs and on road to report.
Had a play with its adjustable dampers today and settled with one of the recommended settings from Tech9.
Car is so sweet to drive and I love it.
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