Tyre Width Advice without rubbing

[Nictrix]

Can you explain this bit as I dont quite understand it. It doesnt make sense in my head.

I'l try here we go nice and slow
a 2 4 5 - 4 0 - 1 8 tyre fitted to a 8.5j rim will have a greater contact area on the tarmac than a 2 5 5 - 3 5 - 1 8 tyre .

Oooohh you made yourself look all clever by trying to spell it out but really it just looks the same with no explanation.
It was a simple question that I thought you might have been able to explain.
Surely a 255 tyre has 10mm more tread across the width of the tyre in contact with the road than a 245, so how does increasing the sidewall by 5% increase the contact area of the tyre?

Forget it, I searched the net and it seems it all rests on the width of the wheel.

[mr wilks]

I'l try here we go nice and slow
a 2 4 5 - 4 0 - 1 8 tyre fitted to a 8.5j rim will have a greater contact area on the tarmac than a 2 5 5 - 3 5 - 1 8 tyre .

Oooohh you made yourself look all clever by trying to spell it out but really it just looks the same with no explanation.
It was a simple question that I thought you might have been able to explain.
Surely a 255 tyre has 10mm more tread across the width of the tyre in contact with the road than a 245, so how does increasing the sidewall by 5% increase the contact area of the tyre?

Forget it, I searched the net and it seems it all rests on the width of the wheel.

Apologies if offended , it was meant with some humour
i can't give you the mathematical equation , i haven't the skills but basically the contact patch is bigger with 245 40 18 over 255 35 18 on 8.5j as it makes the patch longer so the patch area is larger but not necessarily wider , the same effect on 9.5j rims with 255 35 19 over 265 30 19 , you think wider is better but you actually decrease the amount of rubber on the road .
.
"mcbutler" also has a relevant point RE tyre pressures having effect on contact patch which isn't that surprising & no doubt manufacturers apply higher pressures when attempting to get better MPG that reduces contact area whereas lower pressure would achieve less mpg with a greater amount of tyre on tarmac

[engine killer]

a quick calculation, 245/40-18 vs 255/35-18 the circumference, 245 is about 55mm (2.68%) longer
255/35-19 vs 265/30-19, 255 is about 61mm (2.95%) longer

the percentage difference play a very very small effect on speedometer

[Nictrix]

Oooohh you made yourself look all clever by trying to spell it out but really it just looks the same with no explanation.
It was a simple question that I thought you might have been able to explain.
Surely a 255 tyre has 10mm more tread across the width of the tyre in contact with the road than a 245, so how does increasing the sidewall by 5% increase the contact area of the tyre?

Forget it, I searched the net and it seems it all rests on the width of the wheel.

Apologies if offended , it was meant with some humour
i can't give you the mathematical equation , i haven't the skills but basically the contact patch is bigger with 245 40 18 over 255 35 18 on 8.5j as it makes the patch longer so the patch area is larger but not necessarily wider , the same effect on 9.5j rims with 255 35 19 over 265 30 19 , you think wider is better but you actually decrease the amount of rubber on the road .
.
"mcbutler" also has a relevant point RE tyre pressures having effect on contact patch which isn't that surprising & no doubt manufacturers apply higher pressures when attempting to get better MPG that reduces contact area whereas lower pressure would achieve less mpg with a greater amount of tyre on tarmac

Not offended I was only looking for an answer
Now does this make a difference with the same tyres on a different width rim?
For instance the same tyres on a 9" rim would the 255 tyre have more in contact with the road than the 245 then?
Just thinking aloud about a 275 on a 10" rim, is it a better fit for the rim than a 265 which looks a little stretched?

20200918_162631.jpg (125.4 KiB) Viewed 314 times

[mr wilks]

@nictrix,
265 30 19 on 10j is as you point out "undersized" I would be on 265-35-19 or 275-30-19 either of which would increase contact patch over 265-30-19 & give rim protection + improve ride comfort

[Nictrix]

@nictrix,
265 30 19 on 10j is as you point out "undersized" I would be on 265-35-19 or 275-30-19 either of which would increase contact patch over 265-30-19 & give rim protection + improve ride comfort

Cheers

[mcbutler]

Can you explain this bit as I dont quite understand it. It doesnt make sense in my head.

I'l try here we go nice and slow
a 2 4 5 - 4 0 - 1 8 tyre fitted to a 8.5j rim will have a greater contact area on the tarmac than a 2 5 5 - 3 5 - 1 8 tyre .

Oooohh you made yourself look all clever by trying to spell it out but really it just looks the same with no explanation.
It was a simple question that I thought you might have been able to explain.
Surely a 255 tyre has 10mm more tread across the width of the tyre in contact with the road than a 245, so how does increasing the sidewall by 5% increase the contact area of the tyre?

I think your post here shows you understand as much as you need to. Wider profile gives more contact area, a greater wheel diameter does the same but only very very marginally.
The fact remains that BMW know best when it comes to tyres. More rubber will lower your aqua planing speed and reduce grip in the wet generally.
May be better in the dry but I am not a tyre/suspension etc engineer or developer.

[BadgerDog]

I'l try here we go nice and slow
a 2 4 5 - 4 0 - 1 8 tyre fitted to a 8.5j rim will have a greater contact area on the tarmac than a 2 5 5 - 3 5 - 1 8 tyre .

Oooohh you made yourself look all clever by trying to spell it out but really it just looks the same with no explanation.
It was a simple question that I thought you might have been able to explain.
Surely a 255 tyre has 10mm more tread across the width of the tyre in contact with the road than a 245, so how does increasing the sidewall by 5% increase the contact area of the tyre?

I think your post here shows you understand as much as you need to. Wider profile gives more contact area, a greater wheel diameter does the same but only very very marginally.
The fact remains that BMW know best when it comes to tyres. More rubber will lower your aqua planing speed and reduce grip in the wet generally.
May be better in the dry but I am not a tyre/suspension etc engineer or developer.

Actually that's incorrect. ANY PROFILE OF TIRE will have, by definition, the same contact area for a given level of inflation. That last bit is important.

So, lets do the math. For convenience lets say theres a 2000 pound car. Each tire by definition has about 500 pounds on it (assuming 50/50, etc etc). If you air the tire up to 50psi (again convenience of numbers) thats 50 pounds per square inch. The math show us that we have 10 square inches of contact area. Period.

A wider tire at the same inflation will have a different geometry to the contact path. Wider side to side, and shorter front to back, but still 10 square inches in my example. However, with a wider tire you CAN deflate a bit, giving you a wider contact area, without having the tire deform as much. I think thats where the myth of "wider tire means more contact area" comes from. You also change whether you have more traction front to back or side to side depending on how you change the geometry of the contact area.

Now, there may be some differences brought upon by the fact that at the very edge of the contact area you dont get the full pressure, it feathers out. And that region will be larger on a wider tire. So a wider tire probably does have SLIGHTLY more contact area as a result, but its pretty close to negligible in the grand scheme of things.