Tyre pressure - low profiles

[r3ckless]

i run 205/45/17 and have 38psi on all 4

[JohnL]

low profiles usually require a few psi more than "normal" tyres

It seems to be common practice for tyre and car manufacturers and retailers to recommend higher psi for tyres with lower profiles than for tyres with higher profiles (i.e. "normal" tyres). However, I tend to think this is more to do with protecting the wheel rim and (shorter) sidewall from impact damage than it is with the notion that higher profile tyres work better at lower psi than tyres with a lower profile. If anything it could be argued that tyres with softer sidewalls require higher pressures to prevent excessive sidewall deflection under load (and taller sidewalls behave as if they were softer even if the construction is the same other than the sidewall height).

Tyres with tall / soft sidewalls tend to give a soft ride but poor steering and handling characteristics. Such tyres also tend to be quite sensitive to psi, and increasing psi (to significantly over what most people tend to use with them) can substantially improve steering / handling at the expense of some deterioration in NVH.
Tyres with stiffer and/or quite low profiles tend to be rather less sensitive to psi, so much so that it's quite common for low profile tyres to be destroyed because the ('average') driver couldn't feel that they were nearly flat...

Keep in mind that the percieved market demographic for most higher profile tyres (and the cars to which they are likely to be fitted) will be for proverbial mother's picking up the kid's from school and doing the shopping, and ride comfort will be considered a high priority and reflected in the recommended psi. The percieved demographic for lower profile tyres will be at least somewhat more handling oriented, so this requirement will be at least somewhat higher on the priority list and reflected in the recommended psi...

but beware if you have crap tyres and you over inflate them.... can end up with eggs, sorta like when you kick a basketball really really hard

What?

[JohnL]

...... what he recommends, as he works for Bob Jane.

Some people who work at tyre retailers understand tyres quite well, but IMO, not many...

[RaZZa137]

It seems to be common practice for tyre and car manufacturers and retailers to recommend higher psi for tyres with lower profiles than for tyres with higher profiles (i.e. "normal" tyres). However, I tend to think this is more to do with protecting the wheel rim and (shorter) sidewall from impact damage than it is with the notion that higher profile tyres work better at lower psi than tyres with a lower profile. If anything it could be argued that tyres with softer sidewalls require higher pressures to prevent excessive sidewall deflection under load (and taller sidewalls behave as if they were softer even if the construction is the same other than the sidewall height).

Tyres with tall / soft sidewalls tend to give a soft ride but poor steering and handling characteristics. Such tyres also tend to be quite sensitive to psi, and increasing psi (to significantly over what most people tend to use with them) can substantially improve steering / handling at the expense of some deterioration in NVH.
Tyres with stiffer and/or quite low profiles tend to be rather less sensitive to psi, so much so that it's quite common for low profile tyres to be destroyed because the ('average') driver couldn't feel that they were nearly flat...

Keep in mind that the percieved market demographic for most higher profile tyres (and the cars to which they are likely to be fitted) will be for proverbial mother's picking up the kid's from school and doing the shopping, and ride comfort will be considered a high priority and reflected in the recommended psi. The percieved demographic for lower profile tyres will be at least somewhat more handling oriented, so this requirement will be at least somewhat higher on the priority list and reflected in the recommended psi...



What?

tyre and car manufacturers give you pressures recommended to give even tyre wear and get most use of tyre but if you want to drive hard or track use for a normal tyre you require the higher pressure for the load.

low profiles on the other hand need the higher pressure due to the fact that they don't have the high profile to absorb impacts such as pot holes

when i was talking about crap tyres and overinflating, i was referring to 205/40R17 that i have with no brand name and im pretty sure the mechanic who fitted them didn't check the pressure he put in because when i went to check them it was somewhere around 50+ psi and after awhile the car started vibrating and when i took the wheels off and had a look, the tyres weren't round, like when you kick a basketball and it makes an egg on it, that was what my tyres looked like

[vinnY]

so you're talking about a tyre wall bubble.. when it weakens in a particular section?

[RaZZa137]

well yes except mine wasn't on the sidewall it was on the tread the whole way round the tyre

[JohnL]

tyre and car manufacturers give you pressures recommended to give even tyre wear and get most use of tyre but if you want to drive hard or track use for a normal tyre you require the higher pressure for the load.

The idea that high tyre pressures typically cause excessive / rapid wear in the middle of the tread is a hangover from the days of cross-ply tyres (which have rather flexible treads due to the lack of reinforcing steel belts). With modern steel radial tyres it typically takes very excessive pressures to cause centre tread wear (because of the inherant tread stiffness of a radial ply tyre).

However, even with radials, the wider the tread happens to be the more prone it may be to becoming 'crowned' due to higher psi (i.e. it's easier to flex / bend a long stick than a short one), and this leading to somewhat increased wear in the middle of the tread. Fitting a wider tyre to a narrower rim also tends to 'crown' the tread to some degree, possibly contributing to increased central tread wear.

So, a radial tyre is more likely to wear in the middle if:
A) It's quite wide
and
B) The wheel rim is relatively narrow (for the tyre)
and
C) The pressure is quite high

But, if the tread is relatively narrow and / or the rim is not too narrow for the tyre, then 'C' will have relatively little effect (with most radial tyres, there are bound to be some exceptions, though I haven't yet come across one...).

low profiles on the other hand need the higher pressure due to the fact that they don't have the high profile to absorb impacts such as pot holes

This has nothing to do with how ell the tyre will work, and what pressure will result in optimal handling / grip. It is however one of a number of very good reasons to avoid excessively low profile tyres...

Some higher profile tyres need higher psi in order to remain stable under load. In my experience, higher pressures are often essential for reasonable stability if the tyre is fitted to a rim on the narrower end of the tyre manufacturers recommended wheel fitment range.

An increase in rim width of only 1" can make a profound difference to the behaviour / performance of a given tyre. As an example; a 195 tyre fitted to a 5.5" wheel tends to require quite a lot of pressure to remain adequately stable (and responsive) on the rim, but on a 6.5" rim the same tyre tends to be much more stable (and responsive), even if the psi is substantially lessened.

when i was talking about crap tyres and overinflating, i was referring to 205/40R17 that i have with no brand name and im pretty sure the mechanic who fitted them didn't check the pressure he put in because when i went to check them it was somewhere around 50+ psi and after awhile the car started vibrating and when i took the wheels off and had a look, the tyres weren't round, like when you kick a basketball and it makes an egg on it, that was what my tyres looked like

Be careful of assuming that a given experience with a specific tyre will hold true across the board...

[DC4Integra98]

Thanks for all the advice!

I ended up pumping all tyres to 36 psi. However.... I've seen a loss in fuel economy. Could be because it revs much better now.

[vinnY]

if anything the fuel economy should go up
afaik theoretically there should be less rolling resistance

[DC4Integra98]

what i thought too. Probably me being a lead foot thinking i've gotten a power boost lol

[flipfire]

Try 38psi, i run it on 205/40/17's. It usually goes down a psi every week.

[JohnL]

I ended up pumping all tyres to 36 psi. However.... I've seen a loss in fuel economy. Could be because it revs much better now.

The drop in economy won't be caused by the raise in tyre psi. By definition higher psi will reduce rolling deformation of the tyre (such deformation causing internal molecular friction / heat generation in the rubber and various plies etc) so will lessen heat generation and loss in the tyre (that heat energy originating from burning fuel...), i.e. higher psi lowers rolling resistance and somewhat improves economy because the tyres generate (and dissipate) less heat.

You're either driving the car slightly differently or some other co-incidental problem has cropped up.