Tire sizing

[Shrike]

ok so the this has been bugging me for ages

tire width

is 245 wide on all 4 corners to wide for say an 800kg car?

Does the weight of the car effect how wide you can go?

Will 245 wide give you more traction compared to say 215 wide?

(all on a 4wd car so rim width is the same etc and a 50/50 power split)

[touge_ae101]

245 will always give you more grip than a 215 tyre due to larger contact area. i very much doubt you can get your car down to 800kg though so wouldn't be too worried.

run as wide as you can but keep in mind how hard it is to get any kind of turning circle with a tyre that wide when it would normally only fit a 195 or 205 tyre

[1I1]

Malcolm brought this up and it's been FAQ'd

viewtopic.php?t=39835

[Grrrrrrr!]

245 will always give you more grip than a 215 tyre due to larger contact area.

Until it rains.. then wider tyres can become your worst enemy as you aquaplane into the nearest wall/pole.

I'd be interested to see if Malcolm would like to update that post, I know he's learnt a f-ton more about tyres and vehicle dynamics since 2005..

[Al]

Does the 245 actually give you a larger contact area over a 215? Especially if you upsized wheels?

Wouldn't the contact patch narrow front to rear but increase in width?

[Anth_555]

why would it narrow ? only tire presser changes that. size of wheel dosent make a difference

[sergei]

The contact area will be pretty much in same ball park between 245 and 215. It will be different shape and all, but are very similar, that is because area is determined by pressure and deformability of the tyre and not width.

Regarding why wider tyres work better in most of the time, is partially because of heat dissipation, total area of the tyre is increasing overall with wider tyre.

[sergei]

why would it narrow ? only tire presser changes that. size of wheel dosent make a difference

Think about it. You pump 36psi => or 36 pound per square inch, if both of the tyres are at same pressure, and the mass remains the same, the contact area stays the same. Which means one thing - the wider tyre will have longer patch but not as wide as narrower tyre ( ).

In reality it is more complicated:

http://performancesimulations.com/fact-or-fiction-tires-1.htm

[Malcolm]

245 will always give you more grip than a 215 tyre due to larger contact area.

Until it rains.. then wider tyres can become your worst enemy as you aquaplane into the nearest wall/pole.

I'd be interested to see if Malcolm would like to update that post, I know he's learnt a f-ton more about tyres and vehicle dynamics since 2005..

Aw, that means I have to read it all again

I'll have a look through and see if I feel like anything I now know ties things together better, and whether that allows me to shed more light on the situation - my suspicion is that I'm not good enough at explaining complicated topics in forum postings to do it more coherently than the Racecar Engineering article does.

[KinLoud]

Sergei is right

Tyre contact surface area depends on weight on the wheel and tyre pressure

For constant weight and tyre pressure the contact area remains the same
- doesn't matter how wide the tyre is
- wider tyre = shorter contact patch
- narrow tyre = longer contact patch
- same contact area

However, a wider tyre can give more grip
- you get maximum grip at a lower slip angle
- as you increase the slip angle past the maximum grip angle the grip will decrease quickly (less forgiving)
- wider tyre will aquaplane at a higher speed than narrow tyre

Narrower tyre
- maximum grip angle is greater than wide tyre but not as much grip
- decrease in grip beyond maximum grip angle is slow and progressive (more forgiving)
- will aquaplane at a lower speed than wide tyre

[Grrrrrrr!]

Ken, how does a wider tyre resist aquaplaning more than a narrower tyre?

Sounds back to front to me. Wider tyre has to displace more water (more width X same speed = more area covered per second), and the water in the centre of the tyre has to be pushed further out to the side to go around the tyre.

[Shrike]

245 will always give you more grip than a 215 tyre due to larger contact area. i very much doubt you can get your car down to 800kg though so wouldn't be too worried.

run as wide as you can but keep in mind how hard it is to get any kind of turning circle with a tyre that wide when it would normally only fit a 195 or 205 tyre

i was just using 800kg as a point if reference
And Im not worried about turning circle as i used to run 215 35 18s on a 48mm offset and that's the biggist i could go even with rolled guards as and lower offset would stick out more and and wider tyres would hit the shocks Im going 67mm wider in the back and 37mm wider in the front (need to double check how wide Im going in the front)

[tsoob]

There is such a thing as over tyre'ing a car.