Tyre pressure ?

Mandrake · Post by **Mandrake** » 14 Dec 2005, 07:18

Ok, so I've bought a pair of 205/60/R15 Michelin XM1's for my Xantia, and so far so good, they seem to be working very well (not much chance to drive them yet, only through town) however I'm a bit unsure what pressure to run them at to give the best balance between ride and handling, and have normal wear patterns.

The owners manual says the original 185/65/R15 should be 33 psi at the front, and that 205/60/R15 at the front should be 35 psi. However the cars that they list having the 205/60/R15's fitted to such as the V6 are also heavier cars, and the weight plays a role in determining optimum tyre pressure too.

For now I have them at 35 psi, but I wonder if anyone has any comments on whether it is correct to have slightly higher pressure in the larger size tyre carrying the same weight...or whether I should put them back to the 33 psi that I was running in the original tyres...in other words is the size of the tyre or the weight of the car the main reason for the difference in recommended pressures in the book ?

The tyre fitter followed the plaque in the door that said 33 psi, and I bumped them up to 35 later on after I had left, and I did notice the ride was noticably firmer at 35.

Regards,
Simon

AndersDK · Post by **AndersDK** » 14 Dec 2005, 10:02

A classic problem.

Really I think you should test it the same way you're looking for suspension geometry problems : testpaint on the tyres - and then adjust inflation until you get the best overall evenly surface wear.
You already know how wear looks on under/over inflated tyres.

Post by **Peter.N.** » 14 Dec 2005, 13:01

Test paint? Thats a new one on me Anders, sounds like a very good idea though! I have met it on metal surfaces i.e. crownwheels and pinions, but never thought of applying it to tyres.

Simon. As you now have more rubber in contact with the road, to maintain the original resiliance you will theoretically need less pressure, but I am sure Anders advice is sound, you would have to test it in a fairly straight line as a lot of cornerning will scrub the paint from the edges.

jeremy · Post by **jeremy** » 14 Dec 2005, 15:26

Citroen seem to recommend rather higher pressuress for their tyres than other manufacturers. I'm not sure if this is necessarily true today but a quick glance through my 89 Glass's tech specs suggests that Fords were generally 26 (Cosworth 30 - 34) Austin Rover 26 - 29. Audi were in the low 30's, Nissan 26 - 29.

Which suggests to me that the things will run safely at many pressures and that its a matter of taste - comfort - handling choices rather than structural problems.

Having said that I test mine for overheating on a long run or when running with a heavy load by feeling them and I have had properly inflated but cheap/worn tyres feel hotter than I like on my BX TD.

I like Anders suggestion of painting the tread to ascertain wear - never heard of that one before!

A couple of pounds on Citroen suspension seems to make quite a difference to the transmission of road roughness.

JohnT · Post by **JohnT** » 14 Dec 2005, 15:56

Surely a tyre pressure is directly related to weight? If the car is the same weight and different walled tyres are added then the pressure remains the same? Have I got this wrong? but you can see my logic?

Kowalski · Post by **Kowalski** » 14 Dec 2005, 16:43

JohnT wrote:Surely a tyre pressure is directly related to weight? If the car is the same weight and different walled tyres are added then the pressure remains the same? Have I got this wrong? but you can see my logic?

Ok, here comes the physics...

Pressure is force divided by area. Your tyres transmit the force from the weight of your car onto the ground, this produces a contact pressure or ground pressure. The force on your tyres is fixed, its the weight of your car and you can't change that, so its the pressure decides the contact area (or vice versa).

Tyres are round and to get a contact area they have to deform, the bigger the contact area you want, the more they have to deform. The downside to this deformation is that the tyre has to flex as you drive a long and this generates heat, if you run your tyres at too low a pressure they'll overheat, at best the rubber will be heat treated, at worst they'll disintegrate.

The construction of your tyres won't affect the contact area, that is fixed for a given pressure BUT they will affect the shape of the contact area.

I've got a big old book of tyre data from Michelin, it gives tables for various tyres, load ratings at different speeds and pressures. In short, tyres can be run at a range of pressures but there are limits. In the wet its the contact pressure that drives the water out from between the tyre and the road so low pressures in the wet are bad that is partly why wide tyres aren't so good at resisting aquaplaning where as narrower tyres are.

np · Post by np » 14 Dec 2005, 18:07

After changing to 205/60/15 on mine,i put the recommended pressure in for the 205`s.However,when i went along the motorway,i found the car to be a bit "floaty" & unstable.Since then,i`ve stuck to the 185`s pressure of 33psi front & 30 psi rear,with no probs.

JohnD · Post by **JohnD** » 14 Dec 2005, 18:11

I had 205 Michelins on my 2.1 Xantia and I always ran them at 33/32. Also the Michelin website advised that the pressures should not be increased when towing a trailer.

Mandrake · Post by **Mandrake** » 14 Dec 2005, 23:48

Thanks everyone.

It looks like I shouldn't be raising the pressure at all then. I'll run them for a couple more days at 35 to get a feel for the characteristics (I've hardly had a chance to drive on them yet) and then drop them back to 33.

Regards,
Simon