Optimal Tyre Pressure

[ludecrs]

Its funny you say that, because we commented about the rear tyres when the car was on the dyno.

I dropped the rears to 35 and the fronts to 32 on the w/e. Car feels much better, but is still a little twitchy at speed and under sudden braking. Think I'll drop the rears to 32 aswell and report back.

[jadatis]

for daily use
whats the best for 18x 7.5
215 front and 235 rear
35 front 40 rear profile?

Then you have to know some other things from the car and tyres.
from car:
1. maximum speed, or speed you wont go over .
2. Gross Axle weights front and rear. somewhere under the motorhood or one of the door-styles on a metalish plate ( in Europe, dont know how that is in Australia). Dont devide the Gross Vehicle weight by 2, the GrAW's added together are a bit more then the GrVW, and back is mostly more then front.

ands not always needed
3. The advice pressures of the original tires and car. Only needed to calculate back the loads that were used for the normal Advice pressures.
Seldomly given nowadays and if they are made with the American TRA system, they can also not be used.
4. The camber angle ( when wheels are placed like this /-\) when above 2 degrees. Seldomly the case, so mostly not needed.

From the Tyres:
1. Maximum load of tyre. On tyre side-wall in Kg, LBS or the Load-index.
2. Letter that stands for the maximum permissable speed with the Tyre.
For your car probably W ( 270km/h) or Y (300km/h).
3. Reference pressure or kind of Tyre. If you finds on the Tyre side-wall "at xxxpsi(kPa)"then that always is the reference pressure. So not the maximum pressure of the tyre, that is something else, and not needed.

Then the Question is what you mean with dayly use.
It is all about the actual loads that come on the Axles, thats the most important, but the most difficult to determine.
And you have to build in a reserve for los of pressure in time, ocacional higher load, inacurate measurement.
And you have to make 2 spreadsheets, one for every axle, because you have diferent tyre-specifications on front and back. Only if you want to know the advices for normal use, otherwise you can do with 1 spreadsheet and chance the tire specifications and pick the advices for the right axle.

If are not able to use my spreadsheet then give the asked data and I will give you a full report.

[STiR]

If you're running big wheels with a low profile tyre it's possible the side walls can have a really hard time on the dyno. I remember overhearing a conversation at my local tyre shop of choice about this as someone wanted tyres replaced under warranty.

[IEVAQ8]

Its funny you say that, because we commented about the rear tyres when the car was on the dyno.
.

only reason i said it, is coz last week i saw with my own eyes my mates car on the dyno with brand new tyres, inflated properly and all the rest of it, just go a really egged shape and then delaminate on the dyno, it wasnt pleasnt and very scary to imagine what would have happened should one of us been standing next to it like we usually would.....

[Redsuns88]

Check your manual on what tyre pressure to run at..

but typically i run 35cold psi on my 255/45/17 tyres and if it rains then i slightly increase the psi to ensure its pumping out the water

[SNKS2K]

I did one of those defensive driving courses and the instructor recomended to have the pressure at about 37-38 for daily driving for any wheel size.

Doesnt matter what size, as we are measuring the pressure, not the amount of air you pump into the tire.

[WhiteAP1]

Its funny you say that, because we commented about the rear tyres when the car was on the dyno.

I dropped the rears to 35 and the fronts to 32 on the w/e. Car feels much better, but is still a little twitchy at speed and under sudden braking. Think I'll drop the rears to 32 aswell and report back.

Whats your suspension like?? Have you had it set up and balanced by a specialist?

At speed the rear may not be able to squat down enough due to the suspension being too stiff, causing you to loose some traction and give you the feeling that its not that planted or stable.

Also generally if the car has more of a nose down stance, and overly stiff suspension, under breaking your rear wheels may not be getting forced down enough, causing a very slight lift when the weight shifts to the front, resulting in some loss of contact with the road from the rear.

If you havnt already, with you power and tire size id get your suspension set up by a pro.

[ludecrs]

^

It was done professionally. Took about 4-5hrs to do it, camber, toe, rebound, aligned etc etc. I had th same issue again yesterday too. Car just feels like it floats above 170km/h.

It happens before you even hit the brakes. At the top of 4th gear is about 180ish KM/h and the car is just everywhere... Not a nice feeling at all - nothing like it used to be - a go kart. When I lift off, it calms down ; when I brake, its just as horrible to drive.

Its booked in for its 120000Km service tomorrow (epic wallet killer) so I'll see what the boys have to say and go from there.

[JAP-S2K]

Maybe check the rear LCA bushes, infact check them all, as a torn rubber bush can cause wandering through excessive play/movement.

[WhiteAP1]

Maybe check the rear LCA bushes, infact check them all, as a torn rubber bush can cause wandering through excessive play/movement.

^