Quick Pressure Question...

[htial]

I have 235/40ZR18s on front and 265/35ZR18s on back... ...now one friend is telling me they should be 28psi front and 30psi back... ...where another is saying 38psi and 40psi... ... The last few time I've checked them I've kept all four at 34psi... ...(the new auto air pumps are great for that)...

I'm sick of hearing these two go on and on about what is right... ...so does anyone else know what the should be???...

[vidospace]

Yes the eternal mystery! I know on the track you should set the presures quite high (around 40psi) but on the street (and in the wet) thats a bit much I believe. Especially if you have super hard suspension. Remember the tyres do need to move a bit to absorb bumps etc.

[Beer Baron]

nooo. on the track generally you will be lowering from your street pressure as they will rise with the added loads of track driving.

it's really down to the type of tyre, type of use, manufactures recommendation and personal preference as to what is best on the street.

as a general rule the more pressure you put in the 'sharper' it will feel handling wise. it will feel like it turns better and it will transmit more bumps etc to the cabin. over a certain point though traction will be reduced as the reduced compliance in the tyre means it will loose traction more easily.

less pressure will give you a little more roll and give a nicer ride with less bumps etc transmitted to the cabin. too low though and it will damage tyres sidewalls and will give poor handling and traction,

the trick is to find the sweet spot. I would start at around 30-35psi all round for street driving for most regular radial street tyres. adjust a little up or down within that range and see what you prefer. keep and eye on the wear too.

[blind_elk]

As you increase the pressure, you will increase the overall stiffness of the tyre, which will contribute to the ability of the tyre to get you around a corner. But, you will also decrease the contact patch area of the tyre. There comes a point where the added stiffness is overcome by the reduced contact patch, and the car no longer likes to go around corners.

If anything, you need higher pressures in the front tyres - this will have the effect of reducing any understeer. For your setup, I would recommend 38 front, 36 rear for general road use. But maybe check what the manufacturer recommends, and then basically ignore it. Add at least 2psi to what the manufacturer recommends, making sure you still have more pressure in the fronts.

In the wet, ADD pressure (about 2psi or so) - what this does is hold the tread in a more open configuration, allowing the tyre to more easily pump the water from beneath the tread.

[htial]

Thanks guys...

I guess I'll stick with what I've been doing and leave it at 34psi all round... ...Maybe pump the front up a little... ...Both mates agreed that the rear should have a little more in them because they are a wider tyre... ...But you all recommend less in the rear...