Tyre pressures, simple eh?

[Garlick]

The old Lexus has been riding a bit firm since having the new rubber fitted, and as it has the 17 inch DHP wheels fitted, finding the correct pressures is a bit tricky. All handbooks, guides and tyre fitter info only states the 16 inch pressures for 1997 era cars so it's a matter of guesswork.

A call to Lexus UK meant I finally had the correct pressures and so I popped out at lunchtime to check and yep they were all a bit too hard. A small reduction in pressure across the board and all is well again.

Feel a bit of an idiot really. Such a simple task and one that's so often overlooked.

[Garlick]

Bugger, I've left the pressures in the car.......I'll dig them out later.

[Shmee]

doogz said:

What did they specify for the 17's then? It's something i've never quite understood, the car still weighs the same, and if the tyres is still the same width, and therefore needs the same air pressure inside to retain it's shape appropriately, why would the required pressure be different?

I'm guessing, but how about because there's less cubic capacity of air, there's less cushioning and therefore it needs to be harder?

[balls-out]

Possibly due to height of the sidewall (ie profile) and if the tyre has a larger outter radius, then the 'less curved' surface will form a longer contact patch with the ground.

[thinfourth2]

doogz said:

I'm not trying to sound smart, if i was, i'd know the answer, but do you understand the concept of pressure? The height between the inside of the tyre, and the rim of the wheel could be 15mm or 60mm, but if the pressure is the same, the "cushioning" is the same.

If the sidewalls are significantly bigger or smaller, this can have an effect, but on modern tyres that are generally upwards of 200mm wide, i'm not sure how much of a difference the sidewall makes, when the centre of the tyre tread is so far away from the vertical support, and the pressure there has to be such that it doesn't let the sidewalls take all the weight and the tyre wears around the edge.

Any tyre experts about?

Imagine a clyinder with an area of 10square inches and a piston 100inchs from the end with 10psi behind it push it in 1 inch and the pressure will rise not very much

Now imagine a clyinder with an area of 10square inches and a piston 2inchs from the end with 10psi behind it push it in 1 inch and the pressure will double.

So it has alot to do with volume also.

[Shmee]

doogz said:

Any tyre experts about?

I said it was a guess

[Garlick]

I have no idea what is happening here, all I know is my tyres were over inflated

As you were gents.

[collateral]

Often takes me a while to figure out the pressure I get along with. Seems like a lot of FWD cars recommend lower pressures on the back axle than on the front, but I prefer the feeling of equal all round.

The Puma manual states an extra pressure for 'when you are driving above 100mph for an extended period of time'...

[TheRoadWarrior]

Yup as you say Garlick, pressures are often ignored. However I do find they seem to make a lot more difference on some cars than others, my old MR2 turbo, widely know to be a tricky car on the limit never seemed to bothered by a few psi either way but the clio I drive now is really fussy- any more than 1.5 or 2 psi from the suggested pressures and you start thinking the tyres have gone completely flat or wondering if it needs an alignment, pop the correct pressure in and alls well again

[richardxjr]

Glad you sorted it Garlick. LS's are VERY fussy about tyre pressures

PS I'm going to guess @ 34psi f+r (which you'll want to up by 5 for sustained high speed motorway blasting)

[Baryonyx]

Getting your tyre pressures right is one of those must haves. The handling of my Volvo was much improved when I inflated the the tyres to the correct pressure after buying it. I check the pressures regularly because it has made a big difference to handling and ride quality.

[thinfourth2]

doogz said:

thinfourth2 said:

Imagine a clyinder with an area of 10square inches and a piston 100inchs from the end with 10psi behind it push it in 1 inch and the pressure will rise not very much

Now imagine a clyinder with an area of 10square inches and a piston 2inchs from the end with 10psi behind it push it in 1 inch and the pressure will double.

So it has alot to do with volume also.

Point taken, but instead of pushing it by one arbitrary inch, apply an equal load to each, as the car is doing to the tyre in this case.

Yes, pressure at a constant temp is directly proportional to volume, is that the answer to this question then? Is it as simple as that? Seems a bit wonky if Garlick has went up a wheel size, although i'm assuming that as a result of this, the sidewall height has reduced, as has the internal volume, and he's had to reduce the tyre pressures as a result. Sounds a bit backwards, no?

Ah but the car doesn't apply an even load as the road has bumps

[sebhaque]

I tend to check my tyre pressures every week - well, I give one of the cars a once-over - lights, tyres, fluids - every week.

It was surprising when I was popping a bit of pressure in my rear tyres a while ago in a busy shopping area. Plenty of spaces around if you're wondering whether I was holding someone up. The amount of people who looked at me as if I was performing open-heart surgery was surprising. A straw poll of the office car park shows tyres in various PSIs across the board, and the ones that you can't tell are all sitting on mismatched ditchfinders anyway, so I think it's safe to say they haven't been checked.

[GestapoWatch]

Stoopid question but if you buy different tyres from that which were on from the start will the pressure have to be different from that specified inside the door?

[TheRoadWarrior]

GestapoWatch said:

Stoopid question but if you buy different tyres from that which were on from the start will the pressure have to be different from that specified inside the door?

Not if they're the same size.

[SSBB]

collateral said:

Often takes me a while to figure out the pressure I get along with. Seems like a lot of FWD cars recommend lower pressures on the back axle than on the front, but I prefer the feeling of equal all round.

The Puma manual states an extra pressure for 'when you are driving above 100mph for an extended period of time'...

I think it's something to do with reducing tyre deformation at high speed which will overheat the rubber.

Higher pressures on the front is because of the weight of the engine, gearbox and diff'? I assume that is why pressures must go up when you load the boot.

[balls-out]

doogz said:

Any tyre experts about?

If there are, they're probably wondering how to explain a complex topic is words understandable.
I did a mechanic engineer degree and whlist studying friction, I tried to apply it to tyres.
I soon discovered that its horribly complicated as the rubber starts to behave more like a fluid at time than solid and my tutor didn't have a clue either. The basic modeling we were using was far too simplified to even start to explain tyre behaviour. So I give the black circles a kick once week and generally they seem to do the job.

[collateral]

SSBB said:

I think it's something to do with reducing tyre deformation at high speed which will overheat the rubber.

Higher pressures on the front is because of the weight of the engine, gearbox and diff'? I assume that is why pressures must go up when you load the boot.

Interesting. My Fiat stated more in the front but both my Pugs were equal all round (although the recommended 306 pressure seemed much too high). A mate has a new Fiesta and I noticed on the door sticker they suggest equal now too.

I assume manufacturer pressures are with a full tank of fuel - presumably as the car lightens from fuel burning away the pressures reduce?