[ake109]

I keep hearing and reading that if you swap your say 205 tyres to 235 tyres, the wider width of the 235 saps your fuel economy but gives you more grip.

OK, first the fuel economy due to rolling resistance bit:

Assuming the same inflation pressure AND rolling radius, shouldn't the 205 give the same contact patch area as the 235? Newton's law on equal and opposite reactions and all that. In fact, with the same rolling radius, the 235 will have a thinner profile and thinner profile tyres IIRC use higher inflation pressures. So the 235 actually has a SMALLER contact patch area.

Ditto for the 'grip' part. With a smaller contact patch, how does the 235 tyre give more 'grip'?

Unless somehow, the shape of the contact patch matters more than the area of the contact patch?

What gives?

[JZeezSTi]

Very soon there will be a couple of the really knowledgable Wheel and tire guys posting here and clearing up some of this.

[snorky]

a 235 tire is wider than a 205 tire. a 235 tire will therefore have a larger contact patch. a larger contact patch will offer more friction. More friction = more grip.

[Rum]

Originally Posted by snorky

a 235 tire is wider than a 205 tire. a 235 tire will therefore have a larger contact patch. a larger contact patch will offer more friction. More friction = more grip.

Yeah exactly - OP: how do you figure the contact patch would be the same on a tire that is 30mm wider?

[ake109]

Originally Posted by snorky

a 235 tire is wider than a 205 tire. a 235 tire will therefore have a larger contact patch. a larger contact patch will offer more friction. More friction = more grip.

AHA! Thats the part where unless my understanding of physics is totally screwed, is a common fallacy!

A 235 is wider then a 205 - Yes.

But 'Contact Patch Area' x 'Inflation Pressure' x 'Number of Tyres' = Weight of Car!

So unless your car weighed more, or you run lower pressure, the _wider_ contact patch is compensated for by the _shorter_ contact patch to give you the SAME AREA.

So a 235 tyre running 35 psi actually gives a SMALLER contact patch compared to a 205 running 34.5 psi.

It still gives a wider patch, but thats what I am asking. Is a _wider_ contact area much more desirable then a _longer_ contact area?

[ake109]

Just to follow up since some of you are saying - Wider tyre = More Contact Area.


Imagine a car weighing 1000lbs

It has 4 tyres with 10 psi inflation pressure.

Contact Patch Area MUST = 1000 lbs/4tyres/10psi = 25 square inches

It doesn't matter if you are using 195 or 205 or 325 tyres. As long as the car weights 1000lbs and runs 10psi, the Contact Patch Area MUST = 25 square inches.

[Rum]

Originally Posted by ake109

AHA! Thats the part where unless my understanding of physics is totally screwed, is a common fallacy!

A 235 is wider then a 205 - Yes.

But 'Contact Patch Area' x 'Inflation Pressure' x 'Number of Tyres' = Weight of Car!

So unless your car weighed more, or you run lower pressure, the _wider_ contact patch is compensated for by the _shorter_ contact patch to give you the SAME AREA.

So a 235 tyre running 35 psi actually gives a SMALLER contact patch compared to a 205 running 34.5 psi.

It still gives a wider patch, but thats what I am asking. Is a _wider_ contact area much more desirable then a _longer_ contact area?

1) Think about the weight on each tire while cornering.
2) your formula seems to be missing a bit about the cross-sectional shape/sidewall stiffness of the tire, which I'm sure would affect the "inflation pressure" that the tire sees at the contact patch.

Interesting points you bring up though, hmmm. WHat you say adds up, but it doesn't sit right in my head.

[inpuressa]

regardless of psi, width, etc, all tires have an optimal contact patch. This depends heavily on the weight of the car. You can give a narrower tire less psi to make "look" like a bigger contact patch, but you then sacrifice sidewall stiffness and create rotational drag (every tried to ride a bicycle with a flat tire?). Having a wider tire will give you that contact patch while minimizing the downsides.

Having a wider tire also means you should have a faster engine to rotate the extra mass. Car will be heavier, and need that extra grip. Smaller (lighter cars) tend to have less HP, so they don't need such a wide tire (eg. miata). Like stated in the op, tires have an optimal contact patch, so do cars have an optimal width for tires. The balance is tricky. Just my 2cents.

[RaceComp Engineering]

The OP is right....contact patch area is "roughly" the same with a wider tire but the shape is different, and that has a large influence on grip.

- Andrew

[blindfold]

Originally Posted by RaceComp Engineering

The OP is right....contact patch area is "roughly" the same with a wider tire but the shape is different, and that has a large influence on grip.

- Andrew

right, say a 225/45/17 on a 17x7.5 wheel would have a narrow but longer contact patch versus a 255/40/17 on a 17x9 would result a wider but "shorter" with the overall area being the same due to pressure per area. Once you brake, accelerate or corner, the "wider but shorter" will elongate and I believe the wider patch gives you the extra grip needed in the direction of travel. That is my theory and it could be 100% wrong.

[inpuressa]

Mazda6Tech - Are Wider Tires Better?

sure, flame me for getting this from a mazda site

and a whole thread dedicated to this:
Automotive tire/wheel engineering - wider is better? or not? size of contact patch arguement

Gotta love them snow tires used in WRC.

I run RC touring cars, and interestingly enough, 24mm tires produce more grip than 26mm...

[oscargarza88]

hm... ok ive never thought of this but im confused now...
so in the mazda link isnt it saying that a skinnier tire has a bigger contact patch?
So in conclusion a wider tire has more grip right??

[inpuressa]

one of the big factors seem to be the relation between the rim and tire width.

So if I put a 225 and replace my stock 235 using the same rim, will I get better grip?

[DrPhyzx]

Originally Posted by RaceComp Engineering

The OP is right....contact patch area is "roughly" the same with a wider tire but the shape is different, and that has a large influence on grip.

- Andrew

+1

You would also naively think that contact patch size doesn't matter if you are working from high-school physics first-principles (all that matters is coefficient of friction and normal force). To first order that is true too, but then there are all sorts of higher-order effects that come in and make 10-15% difference (which is a big difference... like between 0.85g and 0.95g on a skidpad!). One thing a wider tire does is let you run a softer compound at the same pressure without overheating.

[frostyrock7c]

Originally Posted by DrPhyzx

+1

You would also naively think that contact patch size doesn't matter if you are working from high-school physics first-principles (all that matters is coefficient of friction and normal force). To first order that is true too, but then there are all sorts of higher-order effects that come in and make 10-15% difference (which is a big difference... like between 0.85g and 0.95g on a skidpad!). One thing a wider tire does is let you run a softer compound at the same pressure without overheating.

Exactly! It's not always an immediately obvious benefit, sometimes it simply allows for more tolerance, i.e. more surface area = better heat displacement. There are also other aspects of a tire besides it's width that aids in grip... compound, pattern, sidewall.