Improving cornering....options please
p-car.com has a DIY that shows how it's done.
Three Wheelin'
Joined: Dec 2003
Posts: 1,632
Likes: 2
From: Milton Keynes, UK
Just to briefly rake up a slightly aging thread I gained a little more information regarding tyre pressures today.
I had a full geo adjustment done today and, as part of it, I asked if we could determine the correct tryre pressures for me to run on my somewhat non-standard 285/30x18 tyres. Given the discussions which had gone before I decided to try - against my better judgment I hasten to add - 36psi in the front (225/40x18) and 44psi in the rears.
Having gone for a "spirited" ride, on return to the workshop we measured the tyre temperatures with a pyrometer and found the inside edge of both rear tyres at 46 deg C (small negative camber) and the centre tread at exactly the average temperature of 43 deg C. This would, apparently, indicate that the running pressure is exactly correct.
So, I would conclude that all the doubters of Kevin's original statement should reconsider.
My geo adjustment has absolutely transformed the car. Having adjusted out most of the negative camber (just a little left) and taken much of the toe in out the car now steers much lighter and has a very sharp turn-in with very little understeer. Bear in mind that my car has the turbo fenders with narrow bodied suspension so runs 26mm (F) and 32mm (R) spacers so the idea of reducing camber was to compensate for the already increased track. Reducing the camber also creates a slightly larger contact patch and, hence, a little more traction/grip.
For anyone within striking distance of Atherstone (Warks) in the UK I can't recommend Center Gravity enough (www.cofg.co.uk). Chris Franklin is the absolute master - and a nice guy to boot!!! No affiliation you understand - just a very satisfied customer.
Regards
Dave
I had a full geo adjustment done today and, as part of it, I asked if we could determine the correct tryre pressures for me to run on my somewhat non-standard 285/30x18 tyres. Given the discussions which had gone before I decided to try - against my better judgment I hasten to add - 36psi in the front (225/40x18) and 44psi in the rears.
Having gone for a "spirited" ride, on return to the workshop we measured the tyre temperatures with a pyrometer and found the inside edge of both rear tyres at 46 deg C (small negative camber) and the centre tread at exactly the average temperature of 43 deg C. This would, apparently, indicate that the running pressure is exactly correct.
So, I would conclude that all the doubters of Kevin's original statement should reconsider.
My geo adjustment has absolutely transformed the car. Having adjusted out most of the negative camber (just a little left) and taken much of the toe in out the car now steers much lighter and has a very sharp turn-in with very little understeer. Bear in mind that my car has the turbo fenders with narrow bodied suspension so runs 26mm (F) and 32mm (R) spacers so the idea of reducing camber was to compensate for the already increased track. Reducing the camber also creates a slightly larger contact patch and, hence, a little more traction/grip.
For anyone within striking distance of Atherstone (Warks) in the UK I can't recommend Center Gravity enough (www.cofg.co.uk). Chris Franklin is the absolute master - and a nice guy to boot!!! No affiliation you understand - just a very satisfied customer.
Regards
Dave
Still plays with cars.
Lifetime Rennlist
Member
Joined: Jul 2001
Posts: 15,077
Likes: 264
From: Montreal
Kevin - a few comments on your problem.
1) Was your car designed for 18 inch wheels? On my 964 (1989) 18 inch wheels were not offered and as I recall were not approved. The 18 inch wheels were only acceptable in later models of 993. As I recall Poprsche issued a bulletin saying not to use them on the 94 and 95 models. A suspension change was made in 1996 I think.
2) Accelerating INTO a corner will make the car understeer. In an 911 you accelerate after the apex as you unwind the steering. A 911 can accelerate sooner than front or mid engined cars but only AFTER the apex.
3) If you want the car to turn in more easily, add negative camber. Track cars use negative 2.5 degrees. This will cause premature tire wear though. Negative 2 is a decent compromise for street/track and gives good turn-in. You can also reduce the toe in - going to zero toe in in front will help turn-in but the car will be less stable on the highway/motorway.
4) Stiffer sway bars, shocks and springs will make the car transition nicely but ride will suffer. Based on my rides, the PSS9 suspension kit offers great handling without too much of a compromise on ride.
5) 44 pounds cold is the suggested pressure for 18 inch wheels on the back of a turbo - which is quite a bit heavier than your car, especially in the rear. The high pressure helps prevent pinch flats due to the low profile of the tire. If you drive on smooth roads, try a lower pressure in back. I'd try 38 lbs and see how it feels. Certainly it was good on my 993 which is similar in weight to your 964. My 964 had 17 inch wheels so I cannot compare directly.
6) When checking tire temps use only a needle probe type. The infrared ones are handy but give erroneous readings. The measurement has to be taken after spirited driving with no more than a minute or so elapsed before the temps are measured. The inside edge will always be hotter because of toe in and negative camber. There should be no more than 6 degrees C difference across the tread from iside to outside.
Best,
1) Was your car designed for 18 inch wheels? On my 964 (1989) 18 inch wheels were not offered and as I recall were not approved. The 18 inch wheels were only acceptable in later models of 993. As I recall Poprsche issued a bulletin saying not to use them on the 94 and 95 models. A suspension change was made in 1996 I think.
2) Accelerating INTO a corner will make the car understeer. In an 911 you accelerate after the apex as you unwind the steering. A 911 can accelerate sooner than front or mid engined cars but only AFTER the apex.
3) If you want the car to turn in more easily, add negative camber. Track cars use negative 2.5 degrees. This will cause premature tire wear though. Negative 2 is a decent compromise for street/track and gives good turn-in. You can also reduce the toe in - going to zero toe in in front will help turn-in but the car will be less stable on the highway/motorway.
4) Stiffer sway bars, shocks and springs will make the car transition nicely but ride will suffer. Based on my rides, the PSS9 suspension kit offers great handling without too much of a compromise on ride.
5) 44 pounds cold is the suggested pressure for 18 inch wheels on the back of a turbo - which is quite a bit heavier than your car, especially in the rear. The high pressure helps prevent pinch flats due to the low profile of the tire. If you drive on smooth roads, try a lower pressure in back. I'd try 38 lbs and see how it feels. Certainly it was good on my 993 which is similar in weight to your 964. My 964 had 17 inch wheels so I cannot compare directly.
6) When checking tire temps use only a needle probe type. The infrared ones are handy but give erroneous readings. The measurement has to be taken after spirited driving with no more than a minute or so elapsed before the temps are measured. The inside edge will always be hotter because of toe in and negative camber. There should be no more than 6 degrees C difference across the tread from iside to outside.
Best,
Three Wheelin'
Joined: Dec 2003
Posts: 1,632
Likes: 2
From: Milton Keynes, UK
On this occasion we didn't do the corner balancing - we were just running out of time - although Chris does have all the necessary equipment to do it. We did check ride heights though. The cost was £233 inc VAT.
Regards
Dave
Thread Starter
Drifting
Joined: Jun 2002
Posts: 2,464
Likes: 25
From: colchester UK
Cheers Bob, I have about 1.5+ of neg all round, I'll be playing with some adjustable sway bars soon, and I'll also get a better opinion when I see Christer for a comparison.
Tyre pressures helped no end, and I know 18"s are unapproved, but we are where we are, and that was yesterday, and today we are here. I like them, and can forgive the twitchiness on tramlining.......I also prefer the stiffer sidewalls.
I might go for a better corner balance and geo setup once I have some powerflex bushes fitted to some spare arms, it's all part of my winter tweaking plan.
Cheers
kevin
Tyre pressures helped no end, and I know 18"s are unapproved, but we are where we are, and that was yesterday, and today we are here. I like them, and can forgive the twitchiness on tramlining.......I also prefer the stiffer sidewalls.
I might go for a better corner balance and geo setup once I have some powerflex bushes fitted to some spare arms, it's all part of my winter tweaking plan.
Cheers
kevin