Wider tyre vs Smaller rim size + bigger sidewall

[457892345]

So i was pondering wheel size choices and wondered what PH thought.

I currently have 245/45/17 Square setup on 8j width wheels and was wondering whether it would be better performance wise to swap back to 18 and go for 265/35/18 Square. My thinking is focused on bumpy, undulating English B roads and the like, not track.

The car is a modified 135i with 400-450 whp, birds suspension, m3 control arms etc.

I'd pick the same rim style and tyre compound which makes it easy to compare.

By changing I gain : +20mm on all four corners in width. (8.3% increase)

By staying the same I gain: More sidewall height (14%), 0.6kg per rim lighter (from manufacturers website), moment of inertia closer to spindle, probably gets up and stays up to temperature better in normal conditions or cold weather and then all of the other benefits that smaller rim brings like better acceleration etc.

Tyre replacement cost is about the same.

Certainly the 245 setup is better in the normal conditions of driving due to factors like tramlining and aquaplaning and much better for potholes/speed bumps and ride quality but what about at 8 or 9/10ths? Does the extra width massively outweigh the extra confidence and compliance of the bigger sidewalls + weight and everything else?

Edited by 457892345 on Tuesday 28th February 05:16

[V8LM]

I think using square tyres isn’t helping.

[kambites]

Can you not go to 265/40/17 or something - keep the sidewall whilst increasing the width? Alternatively maybe go 265/40/17 at the back and leave the front at 245/45/17; you probably don't need anything wider than 245 on the front of a smallish RWD car and widening the fronts will probably worsen steering sharpness and feel.

[Bobupndown]

V8LM said:

I think using square tyres isn’t helping.

[Kawasicki]

I wouldn’t increase the width of the tyre and keep the rim width the same. You may end up with more grip, but less precise steering/handling.

[gazza285]

For road use? I doubt you would notice any difference whatsoever, so whichever you like the look of. Personally I’d keep the money in my pocket.

[MustangGT]

gazza285 said:

For road use? I doubt you would notice any difference whatsoever, so whichever you like the look of. Personally I’d keep the money in my pocket.

Very much this.

I would stay on the 17s personally, with the state of the roads in the UK more sidewall = less chance of damaging wheels and suspension.

[457892345]

kambites said:

Can you not go to 265/40/17 or something - keep the sidewall whilst increasing the width? Alternatively maybe go 265/40/17 at the back and leave the front at 245/45/17; you probably don't need anything wider than 245 on the front of a smallish RWD car and widening the fronts will probably worsen steering sharpness and feel.

The tyre sizes at 17 inch for road compounds max out at 245 wide, its only by going to 18 that can increase the width. I thought it might increase the size overall too much to fit by going 265/40/18 as It looks a bit bigger than what I have on at the moment (which is already bigger than oem 225/40/18-245-35-18) but might be worth the work to make it happen if it would be better. The calculators make it seem OK?

Edited by 457892345 on Tuesday 28th February 14:37

[457892345]

Kawasicki said:

I wouldn’t increase the width of the tyre and keep the rim width the same. You may end up with more grip, but less precise steering/handling.

So stay same as I am now, thanks for the Insight, could you explain why increasing would result in less precision in steering and handling feel? I've never driven anything with wider tyres than this to know.

MustangGT said:

Very much this.

I would stay on the 17s personally, with the state of the roads in the UK more sidewall = less chance of damaging wheels and suspension.

This was my thinking when buying initially, I was kind of rushed by mot and worn out original oem tyres to make a decision quickly and now reconsidering as I get more power what's best overall. More than happy to stay with 17s if that's better in the real world.

Edited by 457892345 on Tuesday 28th February 14:37

[GreenV8S]

It's better to stretch a narrow tyre on a wide rim than the other way around. It's due to the way the tyre deflects under side loads.

[457892345]

GreenV8S said:

It's better to stretch a narrow tyre on a wide rim than the other way around. It's due to the way the tyre deflects under side loads.

I see, so you mean that I went with 8j for my 245 wide tyres but if It were you you would swap to 9j? I was under the assumption keeping wheel weight lower would be more beneficial than stretching the tyre over a wider rim but I did read about that practice before too and can see how it would make for less tyre movement in sidewalls.

[GreenV8S]

It's not so much the amount of movement but what it does to the tread. A wide tyre on a narrow rim tends to pick in the inside edge under heavy cornering, which exacerbates the effects of camber changes. A narrow tyre on a wide has the opposite effect.

[457892345]

GreenV8S said:

It's not so much the amount of movement but what it does to the tread. A wide tyre on a narrow rim tends to pick in the inside edge under heavy cornering, which exacerbates the effects of camber changes. A narrow tyre on a wide has the opposite effect.

I see, I did read up on tyre stretching and its effects on performance and it seemed to suggest a "0 stretch" or a sidewall that is in line with the rim to be best.

I think in my case as I went for a 245 with 8j I'm likely at a - 1 or so.

As I mentioned I was thinking more about reducing wheel mass and increasing amount of air in sidewall vs bumps more at the time but i will take your advice on board if decide to change and maybe go to 10j if by some miracle its able to be made to fit at 265/40/18

[MustangGT]

457892345 said:

I see, I did read up on tyre stretching and its effects on performance and it seemed to suggest a "0 stretch" or a sidewall that is in line with the rim to be best.

I think in my case as I went for a 245 with 8j I'm likely at a - 1 or so.

As I mentioned I was thinking more about reducing wheel mass and increasing amount of air in sidewall vs bumps more at the time but i will take your advice on board if decide to change and maybe go to 10j if by some miracle its able to be made to fit at 265/40/18

You also need to consider wheel arch clearance, both width and height if making a large width change.