Whats the deal with tyre width?

How much does wider tyres really do?

I'm not a raving lunatic but I'm thinking the follwing:

If you run, say stock 245s and compare them to fx 275s how much of a difference does it really make for grip, and when?

I'm thinking that with the weight of the car pressing down on the tyre, a 245 and a 275 @ the same pressure must have roughly the same area of contact with the road, right?

Of cause there will be differences if you go the the extreme (fx 165 vs 315 or so) but when is it really the wider tyres do their job? I'm thinking that a wider tyre has the needed surface to grip when fx going out of a corner since most weight of the car is an the outer back wheel.

But what about normal driving and setting off? wouldn't a 245 put as much thread on the road as a 285 going off the line? Assuming pressure is the same?

wouldn't a 245 put as much thread on the road as a 285 going off the line? Assuming pressure is the same?

No, because the 285 tyre is 40mm wider.

CW would have some wise words I'm sure,

but from my experience, wide tyres make the car tramline and it feels all wrong on the raod, but they can be good on a trackday when everything is good and hot.

sup suspension geometry is not set up for wide rubber, so it's hard to get an optimum good setup.

wider tire less pressure; less grip per given area, but more area, so I think it will depend on the road conditions, look at therally guys, they go from tarmac monsters to loose gravel pram wheels.

wet day on wide tyres = disaster... dry day on wide tyres = cool:D

But surely if u have a wider tyre that more in contact with the road, so wouldnt it be better on a wet day? than a pissy small one.

Question, i have stock UK spec 17" all round with 245 on front and 255 on rear....whats the largest tyre i could fit on the rims?.

Cheers,

Si

But surely if u have a wider tyre that more in contact with the road, so wouldnt it be better on a wet day? than a pissy small one.

 

Question, i have stock UK spec 17" all round with 245 on front and 255 on rear....whats the largest tyre i could fit on the rims?.

 

Cheers,

Si

wider tyre = more surface area = more weight spread out = less force exerted on a wet surface

Put it this way, why do rally cars run very narrow tyres when they race on snow/ice???

Thinner tyres can cut a path through water easier than wide, as there tend to want to sit on top of the water ala aquaplaning which aint good.

the last set of 295's I had were unbeliveably bad in the rain. they used to aquaplane on mild surface water.

wider tyres are used on high power rwd cars due to the fact that on transverse loading the width of the tyre is useful for allowing the load to reach the road.

Especially under corner situations when power is being applied.

The contact patch say on 245 and 285 tyres would be VERY similar purely because contact patch is a function of car weight and not tyre radius or width.

This relationship is true to a point...... ie. Citroen 2CV tyres on the same weight as a Supra wouldn't provide the same contact patch as a 285 width tyre for example.

But it also comes down to heat cycles. A thinner tyre to give the same contact patch obviously has to compress a little more. This compression as the tyre rolls along the road generates sidewall heat. Too much of this causes tyre deformation etc.

Rally cars use thin tyres for two reasons.... one because as mentioned already it is easier to bite through a small amount of ice than it is to bite through a large amount..... and two because once you have bitten through the ice you can use a thin tyre to work similar to a train wheel.... ie you can ride the track through the ice.

Big GT race cars use wide tyres due to heat cycles which in turn affect tyre wear and also the corner loading; in order they can distribute an even load during acceleration.

Question, i have stock UK spec 17" all round with 245 on front and 255 on rear....whats the largest tyre i could fit on the rims?.

I have 275/40/17 on the back of my standard rims, I had 255's on the front at one point but prefer 245s

wider tyres are used on high power rwd cars due to the fact that on transverse loading the width of the tyre is useful for allowing the load to reach the road.

Especially under corner situations when power is being applied.

 

...........->

Great description AJI. It is much along the lines I was thinking. Exactly with the "to a point" in mind. Of cause a 2CV tyre on a soop wouldn't just compress to the same....

In the US they very often run 300mm and wider on their soops, but this is probably just for show then..

But why is it that Porsche then fits so massive tires to their 911's? Think 285 is stock and even was it back on the 993 C4S with "only" 285hp. Is this down to the Porsche actually being a great track car and should be capable on track?

But it also comes down to heat cycles.

I never knew that but it makes perfect sense. I have melted many a tire on a track day, they just turn to putty and treacle around. Compounds would also help with this too, but I guess you can't drive a hard compound on cold winter roads.

Thanks guys, i hadnt thought of it like that. Makes sense I might stick with my stock widths then as i was only gonna go bigger for the wet (but it would be worse lol). Hehe

Thanks guys, i hadnt thought of it like that. Makes sense I might stick with my stock widths then as i was only gonna go bigger for the wet (but it would be worse lol). Hehe

Then you're doing the right thing by not changing. Also, keep in mind that winter tyres always are narrower that summer tyres. Same principle as being in the wet or gravel

 

But why is it that Porsche then fits so massive tires to their 911's? Think 285 is stock and even was it back on the 993 C4S with "only" 285hp. Is this down to the Porsche actually being a great track car and should be capable on track?

You will notice on 911's that with the engine being in the back then to 'support' the cornering forces that this weight generates the wheels will have be quite wide... otherwise the other option is to run a large camber setting. (Good for corners but they'll wear out fast on straight road driving).

Camber on wheels is usually down to the cornering weight acting upon it..... ie. front engined FWD touring cars have a lot of front end camber..... but the BMW touring cars with them being RWD have a lot of rear camber.

With an engine acting over the wheels on the 911's their race cars also use a lot of rear end camber.

A thin tyre wouldn't give much feel for grip.... on the limit you'd get there in an instant and then you'd be over it without much warning. A larger width tyre would allow you to get to the limit and then you'd be able to feel more progressivly as you go over it.

But Porsche and its 911's have tuned their suspension and alignment settings over the years and they produce great handling cars.... much different to the death traps they used to produce in the early 90's !

But grip feel from tyres varies hugely depending on heat, road surface conditions, power delivery, suspension setup etc. etc.

This is why race teams spend £1000's on test days.

But for road driving just go with what you are used to... and are happy with. The Supra was designed to have a significant difference in front and rear tyre widths, so bear that in mind.

on the limit you'd get there in an instant and then you'd be over it without much warning. A larger width tyre would allow you to get to the limit and then you'd be able to feel more progressivly as you go over it.

If I can ask.... would this hold any truth with the supra on the road?

Does the tyre width have any bearing on progressive handling close to the limit, as getting feedback (warning) from RWD is so important from our cars for safeties sake if nothing else?

or is it all suspension setup, geometry and tyre choice...?

I thought it was tyre compound and design that mainly influenced breakaway?

Also a lot of tyre width choices are, I suspect, fashion more than practicality. Certainly true with rim size.

-Ian

mike - its a combination of all the mentioned things.... tyre width just plays an added part of the whole equation.

As for on the road... the difference between choosing 285 over 275 widths is marginal. Unless you are often playing around on the limit I'd dare say you'd struggle to notice anything.

But changing both front and rear tyre widths to either too similar width or too varied width you'd definitely notice that on the road. If for example you had 215 on the front and 295 on the rear...you may find that only the rear of your car tramlines for example.... or that the front end is providing too much grip in the wet compared to the rear.

But as long as the width difference is not too different to the stock setup the car should handle as designed.

Thanks...

I think tyre makes also have a big effect on handling; some seem a lot less directionally stable than others.

How much does wider tyres really do?

 

I'm not a raving lunatic but I'm thinking the follwing:

If you run, say stock 245s and compare them to fx 275s how much of a difference does it really make for grip, and when?

I'm thinking that with the weight of the car pressing down on the tyre, a 245 and a 275 @ the same pressure must have roughly the same area of contact with the road, right?

 

Of cause there will be differences if you go the the extreme (fx 165 vs 315 or so) but when is it really the wider tyres do their job? I'm thinking that a wider tyre has the needed surface to grip when fx going out of a corner since most weight of the car is an the outer back wheel.

 

But what about normal driving and setting off? wouldn't a 245 put as much thread on the road as a 285 going off the line? Assuming pressure is the same?

well I *am* a raving lunatic, and I run 335's at rear - no probs with handeling, its the best I have experienced in any car and I have driven many.

well I *am* a raving lunatic, and I run 335's at rear - no probs with handeling, its the best I have experienced in any car and I have driven many.

Hehe, but did you ever run standard width? If so what differences did you notice (that cant be explained by the placebo effect)?