Tyre Pressure for 205/40/17 ?

[JBT]

I run 28-30 psi on 205/50/15 tyres, doesn't feel squirmy at all.

Any more and the car crashes and shudders over bumps.

Same here (currently 29psi) on 195/50/15 for normal day to day driving. Tried everything from 32-36 cold at the track and feel that 33 cold works best for me. Interestingly, the MX-5 race car owner/drivers I've spoken to also reckon around 33psi should be the pressure for track stuff on road tyres. The pressure rises by a good 4-5psi after 7 laps.

[Boags]

Any more and the car crashes and shudders over bumps.

It is important that you don't count your tyres as part of your suspension. If it crashes at the correct PSI, then have a look at your shocks and springs.

Your tyres are to hold the car to the road, you need the optimum pressure to do that, not the most comfortable.

Unfortunately, different tyres, profiles, car weights etc will change what is the "correct pressure".

Boags

[Charlie Brown]

Charlie Brown Wrote:

Crikey where did this info come from Probably the English Cricketer’s dressing room during tactic discussions.

I have worked in the tyre industry for over 25 years
In that time I have had to deal with tyre wear issues and tyre warranty claims for tyre manufacturers.
I have built and designed suspension systems for many varied types of sports cars and custom race cars.

Not trying to belittle your knowledge of the subject Kev or Yan but surely you must agree that the weigh of the vehicle must be taken into consideration when determining the tyre pressure.

For example the Lotus Elise weighs in at 900kgs (vs 1050 – 1100kgs for the MX-5) and runs Pirelli P-Zero tyres. The owner’s manual states tyre pressures of 23psi front, 24.5psi rear. Now I know the Elise is not a normal passenger vehicle but AAMI might still have kittens at 23psi.

The VZ SS Commodore weighs in at 1654kgs and runs 235/40 R18 tyres and 32psi (220kpa), the new Commodore with revised suspension changes raise the pressure another 4 psi.

As I said in my original post, car manufactures spend millions developing their cars suspension / tyre set-ups. They have far more knowledge of their product than us know it all amateurs.

Start with their recommendations and then vary up to suit your personal requirements.

[ducktape69]

this has turned out to be a very interesting topic

after having a bit of a fang last night with 35psi instead of the old 27psi, the car feels a LOT better around the corners and definately feels less squirmy and traction has definately increased.....

ill see how i go on 31psi later on

[Gazbo]

It is important that you don't count your tyres as part of your suspension. If it crashes at the correct PSI, then have a look at your shocks and springs.

Your tyres are to hold the car to the road, you need the optimum pressure to do that, not the most comfortable.

Unfortunately, different tyres, profiles, car weights etc will change what is the "correct pressure".

Boags

I agree, but running any higher than 30 psi makes the car feel as if bumps are throwing it off line, a lower pressure "feels" like it grips and handles the bumps better.

Mind you, I think I still have original springs and dampers after 167000 kms! My drivers side rear sits about 10mm lower than the passenger side.

Maybe the "correct pressure" for my car is 28-30 psi? What would you think?

Also, people running higher pressures to "save their rims" - wouldn't lower pressures transmit LESS shock through to the rims? (assuming you don't go too low)

[roddclarke]

Hahha, i always love topics like this, everyone has different opinions and lots of Heated discussions

Gazbo, regarding saving your rims, the higher pressure is more to stop bumps buckling the rims, not the shock.

But here is my opinion..
I run 36PSI all round in both my 205/45/16 and my 215/40/17 for these reasons:
1, it grips a lot better and feels a lot more stable to me around corners with out sidewall flex
2, at lower pressures (32PSI) the car wheelspins a lot more and slieds around.
3, I chose handling over comfort.
4, with my driving i get better tyre wear (even across the whole tyre with 1.75 deg neg camber) at lower pressure i got outside tyre wear.

All of this is my opinions and how it feels to me.. no seintific facts.

on the track with Do1's i run 34 HOT and a stiffer supention setting as i find that works best for me..

when i did a murcotts driver training, they asked everyone no matter what car you have to put the tyres to 36PSI and to run them between 32 and 38 for all normal cars.

i think its not just the weight of the car that affects the amount of pressure you need to put in, but the type and shape (profile) of the tyre
some tyres need more pressure to hold the right shape than others.

and my last thing, if you go too low the side wall has to flex a lot more each rotation and if the tyres are too low you end up with the same affect as repeatadly bending a bit of wire back and forth, it breaks...
and none of us want that to happen... but i think its only a reall risk for long driving with under 25PSI (ie 130KPH down the freeway to brisbane)

So all i can say is type out a few different pressures and see what you like..