effect of tyres on oversteer/understeer
01-19-2003, 12:43 PM
#1
Racer
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effect of tyres on oversteer/understeer
my car has 18 inch wheels with contisport contact N1 & ZR rated tyres , size 225/40 front and 265/35 rear, one of rears is more worn than rest, down to 2mm tread or so ,other rear is 7mm, both fronts are 7mm.
After doing some track days in summer I started to set pressures at 36psi all round. When it got to winter here ( cold + wet,-2 to +5C ,no snow ) I checked pressures - they had dropped all round to 33 at the lower ambient temp - reset to 36 front/38 rear cold. When they were at the reduced (33) pressure allround I had noticed some oversteer setting in at low speed turns crossing round the 'roundabouts' we get on UK roads. You know, you typically have to flick left followed by immediate flick right and then straighten up ,it was the right flick that gave the oversteer....
Right now the car seems to be driving 'different' by following tramlines in the road - usually the grooves made by heavy truck tyres - and still moving about a bit on turns especially in the wet.
Is it simply that I need to put the rear tyre pressures back up to 44psi handbook setting ? Or is it made worse by the worn RHS rear tyre ? And is the tramlining only affected by front tyres ?
After doing some track days in summer I started to set pressures at 36psi all round. When it got to winter here ( cold + wet,-2 to +5C ,no snow ) I checked pressures - they had dropped all round to 33 at the lower ambient temp - reset to 36 front/38 rear cold. When they were at the reduced (33) pressure allround I had noticed some oversteer setting in at low speed turns crossing round the 'roundabouts' we get on UK roads. You know, you typically have to flick left followed by immediate flick right and then straighten up ,it was the right flick that gave the oversteer....
Right now the car seems to be driving 'different' by following tramlines in the road - usually the grooves made by heavy truck tyres - and still moving about a bit on turns especially in the wet.
Is it simply that I need to put the rear tyre pressures back up to 44psi handbook setting ? Or is it made worse by the worn RHS rear tyre ? And is the tramlining only affected by front tyres ?
01-19-2003, 02:09 PM
#2
Burning Brakes
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You are using same pressure in the rear than in the front on a rear-engined car. The general concensus I have seen for the rear weight biased 996 is 32-34 on front and 34-36 on rear (which raise to about 10 psi more when hot). Porsche "recomended" is about 36/44 I think?
I personally find my C4 with 18" wheels (and ROWM030) handle best with about 32 front and 36 rear (if I did the conversion from Kpa units right.) Do you have 17" or 18", C2 or C4?
Rapid weight tranfers from side to side such as the left turn to right turn roundabout example you mention will tend to provoke oversteering even on a neutral-understeer balanced car such as the 996. I used to have an M3 (modded with a more oversteer setup) which would get really tail-happy on chicanes.
But how do you have only one of the rears worn out? do you drive on in circles in the same direction all day long ;-) or have you run with that tire underinflated for an extended period of time?
I personally find my C4 with 18" wheels (and ROWM030) handle best with about 32 front and 36 rear (if I did the conversion from Kpa units right.) Do you have 17" or 18", C2 or C4?
Rapid weight tranfers from side to side such as the left turn to right turn roundabout example you mention will tend to provoke oversteering even on a neutral-understeer balanced car such as the 996. I used to have an M3 (modded with a more oversteer setup) which would get really tail-happy on chicanes.
But how do you have only one of the rears worn out? do you drive on in circles in the same direction all day long ;-) or have you run with that tire underinflated for an extended period of time?
01-19-2003, 07:58 PM
#3
Rennlist Member
You must also remember that when it gets cold out, your Summer performance tires will lose alot of their grip, causing the car to handle corners differently than they would in the summer time.
