What tire pressure?

[MountainBikeMike]

Re: What tire pressure?

Accurately set the manufacturer's recommended inflation pressure, then fine tune to your liking.



This will obviously differ from vehicle to vehicle, but generally, tyre underinflation becomes unsafe when:

- the handling and stability of the vehicle is adversely affected

- the inflation pressure is so low that the tyre cannot safely support the weight of the vehicle, since the load capacity [kg] of a tyre is proportional to inflation pressure (up to its maximum rated load capacity)

- the tyre overheats

All of which may lead to internal (cannot be seen unless the tyre is unmounted) or external structural damage.



If you want more grip, you should consider purchasing a set of road tyres that puts a disproportionately high emphasis on grip (at the cost of treadwear), such as the Bridgestone Potenza RE-11A, Dunlop Direzza II, Yokohama Advan AD08R or equivalent.

Cheers but that's all the obvious stuff. Why I'm getting at is; would I be wise to drop down 2-4 psi for a cruise day for a bit of extra safety, or would the differences likely be marginal.

As you said, to low + corners = un-supportive tire walls

I always keep thinking moto/mtb we drop the pressure to reduce rebound speed of the tire over bumps, and let them conform to the ground. And dragsters etc run low. Different for street cars I guess!

[Diesel_vert]

Why I'm getting at is; would I be wise to drop down 2-4 psi for a cruise day for a bit of extra safety

Unless I've misunderstood your statement, it's generally accepted that increasing a tyre's inflation pressure increases overall safety on paved public roads, not by decreasing it.

... or would the differences likely be marginal.

In terms of load capacity, you have a bit more leeway than usual, since you've fitted 235/40 R18 95W XL tyres (as opposed to the stock 225/40 R18 92W XL tyres).

As I stated before, I recommend accurately setting the manufacturer's recommended inflation pressure, then fine tune to your liking. I like to take a methodical approach to things (motoring enthusiasts are notoriously susceptible to confirmation bias), but that's just me.

But if you just want to see what a 2-4 psi reduction feels like, just deflate them, go for drive, and see if you like it (though preferably with no one else in the car to minimise your liability - just as a precaution).

[MountainBikeMike]

Re: What tire pressure?

Unless I've misunderstood your statement, it's generally accepted that increasing a tyre's inflation pressure increases overall safety on paved public roads, not by decreasing it.



In terms of load capacity, you have a bit more leeway than usual, since you've fitted 235/40 R18 95W XL tyres (as opposed to the stock 225/40 R18 92W XL tyres).

As I stated before, I recommend accurately setting the manufacturer's recommended inflation pressure, then fine tune to your liking. I like to take a methodical approach to things (motoring enthusiasts are notoriously susceptible to confirmation bias), but that's just me.

But if you just want to see what a 2-4 psi reduction feels like, just deflate them, go for drive, and see if you like it (though preferably with no one else in the car to minimise your liability - just as a precaution).

What I'm getting at is that with less pressure there is a bigger patch of rubber touching the ground- potentially more grip. The 'spring rate' and rebound of the tire is also decreased meaning when a bump or object is hit it the air doesn't deflect the wheel as much, it absorbs. The harder the pressure in the tires the more the suspension will do the work absorbing, and vice versa.

Will do try. But I ask because the effects such as wear happen over time, not just in a trial period. By the the tires are fekukked.

But lets finish it and ill make it easy (higher or lower or tie for answer):

In general/on average, would people tracking their street cars run slightly more, less, or same pressures on the track than manufacturer spec?

[wai]

I take it that grip would be enhance with a few psi less, for the sake of additional tire wear, but at what point does it become unsafe, un-fuelly, and drastic tire wear?

Tyre pressures do not affect grip. Grip is controlled by the coefficient of friction between tyre and road. The tyre pressures will affect the feel as a low tyre pressure will allow the sidewalls to flex too much. Sidewall flexing will also increase the temperature of tyres. Low tyre pressures will also increase edge wear.

High tyre pressures are not preferred either as this can increase centre wear. The ride will also be harsher as there is no flex in the tyre sidewalls. The tyre sidewalls are a significant item in the vehicle suspension.

The ideal situation is where the entire tread is evenly in contact with the road. This will ensure the best ride and life.

One way is to inflate the tyres to a nominated pressure and then check the contact with the road. It is easily done, particularly after you have driven a bit. If you see darker edges on the tyres, then this means that the edges have not been in contact with the road surface. Drop the pressures so that the dark edge is not there or only just visible.

If you have to go really low for tyre pressure, then it is a clear indication your tyres are too wide for the load or "over tyred". Overheating of tyres is not really an issue on normal roads. Excessive flexing can cause de-lamination and separation of the tread from the carcass which is why you do not want to go for really low pressures.

Now that you know this, you will see the darker edges of the tread on just about every vehicle you see parked.

[MountainBikeMike]

Re: What tire pressure?

Tyre pressures do not affect grip. Grip is controlled by the coefficient of friction between tyre and road. The tyre pressures will affect the feel as a low tyre pressure will allow the sidewalls to flex too much. Sidewall flexing will also increase the temperature of tyres. Low tyre pressures will also increase edge wear.

High tyre pressures are not preferred either as this can increase centre wear. The ride will also be harsher as there is no flex in the tyre sidewalls. The tyre sidewalls are a significant item in the vehicle suspension.

The ideal situation is where the entire tread is evenly in contact with the road. This will ensure the best ride and life.

One way is to inflate the tyres to a nominated pressure and then check the contact with the road. It is easily done, particularly after you have driven a bit. If you see darker edges on the tyres, then this means that the edges have not been in contact with the road surface. Drop the pressures so that the dark edge is not there or only just visible.

If you have to go really low for tyre pressure, then it is a clear indication your tyres are too wide for the load or "over tyred". Overheating of tyres is not really an issue on normal roads. Excessive flexing can cause de-lamination and separation of the tread from the carcass which is why you do not want to go for really low pressures.

Now that you know this, you will see the darker edges of the tread on just about every vehicle you see parked.

Good explanation. Will keep a lookout.

[Diesel_vert]

What I'm getting at is that with less pressure there is a bigger patch of rubber touching the ground- potentially more grip. The 'spring rate' and rebound of the tire is also decreased meaning when a bump or object is hit it the air doesn't deflect the wheel as much, it absorbs. The harder the pressure in the tires the more the suspension will do the work absorbing, and vice versa.

That may be so, but on a public road, safety and liability concerns would normally place limits on what you can achieve through the adjustment of inflation pressures alone (especially underinflation, which affects the tyre's load capacity).

Will do try. But I ask because the effects such as wear happen over time, not just in a trial period. By the the tires are fekukked.

Given your current tyre size, it's likely that nothing much will happen, but life often tends to throw a curved ball or two when you least expect it.

But lets finish it and ill make it easy (higher or lower or tie for answer):

In general/on average, would people tracking their street cars run slightly more, less, or same pressures on the track than manufacturer spec?

The idea of lowering the recommended tyre inflation pressure on a road car fitted with road tyres on the track, is to counter the elevated operating inflation pressures experienced on the track caused by the sort of temperatures generated by constant maximum acceleration and deceleration loads, and consistently high speeds (centrifugal) and cornering (lateral) loads - forces which are generally beyond the envisaged operating scope of most road tyres.

On a track, you are aiming for a "hot" working inflation pressure, so for reasons of convenience, one may reduce the inflation pressure in the pit lane before joining the track proper (that is, if you have already experimented on previous sessions and know what the anticipated hot inflation pressure will be).

On a public road, you will generally not experience those sorts of elevated pressures and temperatures normally encountered on the track, so in your situation (mountain runs), there is no real justification of underinflating one's tyres.

Personally, I don't see any real pressing need for most people to deviate from the manufacturer's recommended inflation pressures, but again, if you feel the recommended inflation pressures are unsuited to your driving style, then start fine tuning.

It probably doesn't help to mention that one person's idea of "good" handling might not be the same as the next, but there we are.

[gavs]

What tire pressure?

Tyre pressures do not affect grip. Grip is controlled by the coefficient of friction between tyre and road. The tyre pressures will affect the feel as a low tyre pressure will allow the sidewalls to flex too much. Sidewall flexing will also increase the temperature of tyres. Low tyre pressures will also increase edge wear.

Not quite true wai. Yes, grip is controlled by the coefficient of friction over a specific contact patch but if you increase the given contact patch then you are afforded more grip. So many times I see this regarding 4x4's when people learn about tyre pressures and the amazing increase in grip you get. True, if you go too far then it becomes detrimental but a lower pressure does afford more grip.

[wai]

Not quite true wai. Yes, grip is controlled by the coefficient of friction over a specific contact patch but if you increase the given contact patch then you are afforded more grip. So many times I see this regarding 4x4's when people learn about tyre pressures and the amazing increase in grip you get. True, if you go too far then it becomes detrimental but a lower pressure does afford more grip.

Nah. If you increase the contact patch, there may be more tyre in contact with the road, but the contact pressure is reduced and they cancel each other out. Racing tyres are different to road tyres because they use softer rubber. They have to be wide to provide an adequate life. Narrow or wide, for the same rubber, there is the same grip.

A few decades ago, Alf Costanzo (hope I spelled that right) competed in the Alfa Sud series. Apart from using scrubbed road tyres, he used the narrowest tyres permitted as he wanted to get the tyres up to optimum operating temperature as quickly as possible. The commentators said he would be all over the track as he would have no grip. Well, he had as much grip as anyone as it was the same rubber compound. He actually had slightly better grip as the narrow tyres were up to optimum temperature sooner. Wear was not an issue as the races were less than 10 laps at a time.

I have done drawbar tests where an axle is fitted with different tyres (one wide and one narrow) with the same rubber compound, and the same surface. Both wheels have the same load on them. The axle is pulled from the midpoint, and both wheels unable to turn, will slip at the same time and by the same amount.

4x4's reduce tyre pressures as the tyre overpowers the ability of the loose surface to provide a stable base on which to drive. By increasing the contact area, the ability of the loose surface to provide a stable base is increased. Remember, that on a sealed road, it is the coefficient of friction between the tyre and the road surface that matters. In an off-road situation the tyre does not slip, but it is the unsealed road slipping internally. The loose road slips internally rather than the tyre not having traction on the surface which is why road material gets spat out.

[jeremy629]

I have been playing with my tyres pressures a little bit lately

Had them at 32psi. I measured how far I got from the tank of fuel to see how good it was. Then tried 35 psi. Another tank of fuel. Then lastly I have been driving at 40psi and surprise surprise (not) I get even better fuel economy. I am happy to have it set at 40 now though and not any higher. Also the car feels better over bumps. Maybe I have just been lucky not hitting any big ones?

[wai]

The higher the tyre pressures, the lower the rolling resistance. The trade off can be a harsher ride.

Just check that you are not too high as this will increase centre tread wear compared to edge tread wear.

[jeremy629]

Yeah will be keeping an eye on it :0) thanks wai

[MountainBikeMike]

Re: What tire pressure?

Awesome. what were the differences in consumption?

[Diesel_vert]

I too was a fan of inflating the tyres above the specified values on the placard (for the specified load). In my own experience, it felt that the wear rate was reduced with no obvious centre wear pattern (a near-uniform wear pattern more or less, notwithstanding any alignment problems) and that less throttle input was needed to maintain momentum on the straights, though I didn't maintain accurate records to see if it made a difference to fuel consumption.

However, whilst it made the steering feel a little bit more responsive, it also felt that the tyres weren't able to hold the line in a corner as tenaciously and were more willing to slide. In addition, it felt as if the car would bounce over bumps on the road, making it a bit more twitchy when encountering mid-corner bumps. It made for a fun ride when pushing it, but somewhat disappointing, all at the same time.

I've also noticed that the ABS tends to engage earlier and/or more frequently during very hard stops, which was just disappointing and annoying.

Having said that, I'm also definitely not a fan of inflating the tyres below the specified values on the placard (in order to keep excessive wear at bay and maintain load capacity) so as a compromise... I just stick with the values listed on the placard - who would've thought, eh?

Well, FWIW, these are only my own (non-scientific) observations and experiences. YMMV.

[MrDeathLad]

i always thought it was the tyre compound that made a differnce to resistance not tyre pressure. sure, pressure makes a difference to the contact surface area but a softer compound = more resistance/grip = more fuel consumption? please correct if i'm wrong

[MountainBikeMike]

Re: What tire pressure?

i always thought it was the tyre compound that made a differnce to resistance not tyre pressure. sure, pressure makes a difference to the contact surface area but a softer compound = more resistance/grip = more fuel consumption? please correct if i'm wrong

Hop in whatever pushbike you have at home right now, let the tires down till they sag a little under your weight, and come back from round the block a fit man you'll see it easier that way