tyre help please

[mjones16]

I have 17 inch alloys on my mk2 that are 7J wide, i currently have 205 fronts and 215 rears.

However after going V6 i have been told to change to a wider tyre size.

Can anyone tell me the maximum tyre width for a 17 inch rim that is 7J wide and the max tyre size for a 18 inch rim that is 7.5" wide?

cheers
Mark

[Gary Symons]

http://forums.twobrutal.com/showthread.php?t=16248

http://www.tyres-pneus-online.co.uk/equivalency-tables-advice.html

[PaulM]

Mechanically, probably about 225 to 235 wide and for a 7.5 rim 235 to 255. Not sure if these fit under standard arches tho

[mjones16]

Cheers mate

So is a V6 ok with 225 wide tyres then? or do i ideally need wider still?

Cheers
Mark

[PaulM]

I'll be runnning my V8 on 225's to start of with, so I guess they'd be a good start on the V6

[mjones16]

cool

Is the V8 got enough traction mate? have you been out in the wet yet?

cheers

[nikwills]

My Mk1 V6 has mk3 wheels with Goodyear Eagle F1 195/50 15's all round, seem to have plenty of grip during the summer months, I have another set 15" mk3 wheels with 215 rears for the winter but I haven't put them on yet.....I quite like the quick turn in on twisty b roads with the narrower tyres, quite how much I will enjoy it once the slippier winter weather comes in is another thing......

[PaulM]

I was always told that grip was defined by force applied/ unit area.
So if you go too big you'll end up with less grip ! more so in the wet,

Let the arguments begin !

[biteme]

I was always told that grip was defined by force applied/ unit area.
So if you go too big you'll end up with less grip ! more so in the wet,

Let the arguments begin !

Indeed! F=MA, so in the water, you want a narrower tyre, in the dry you want a broader one.

Look at the ladies in the WRC. Narrow unicycle wheels on the snow/ice. Huge garden rollers in the dry tarmac.

I've found a sneaky way around it though. I don't drive the Supra in the wet, if I can help it. (I stick to the 245-40-18 shod BMW lol)

[Jim-SR]

i run 195's on a 7" rim, 205 is the most balanced tyre, 215 is probably the sensible maximum. you dont want the tyre mounted like a mushroom with the sidewalls bulging out. 7.5" youd probably be looking at about 205 minimum, 215-225 in the middle and 235 maximum. personally id go 205 on 7" and 215 or 225 on 7.5". if you go too narrow then the tyre can become less forgiving on the limits, as i find occasionally with 195's on a 7" rim, especially in wet conditions.


with regards the wet tyre discussion - tyre width in the wet is somewhat of a misnomer. the tread area is more important. you could have a 10" wide tyre, but if it had massive grooves all over it then the actual tread area would still be small compared to a thinner tyre with less groove area. provided the grooves are capable of removing all of the water so that the actual tread is in full contact with the road then going narrower will reduce grip. a thinner tyre will typically apply more pressure to the tread, and thus it will dig further into the water toward the road surface. but grip is still proportional to the amount of rubber actually touching the road. so there is a balance somewhere in between. if you stuck 135 width tyres on for example instead of 195's, youd most probably find the 195's had more grip. go to 185's however and you might find that they had more.

but tread design is more important than tyre size. id run the same size road tyres wet or dry, and buy a tyre that is strong in both wet and dry conditions, but in the UK it makes sense to favour wet conditions more and have a set of Toyo R888's (or similar) mounted on a spare set of rims for summer fun!

WRC snow tyres have metal studs in them, they are seriously grippy on ice! they are thin to increase the pressure acting on each of the studs so that they penetrate further into the ice. if the tyre was really wide then the load would be spread out and they would be less effective. they arent a like-for-like comparison to road tyres. although in ice youre pretty much screwed regardless of tyre size. thinner is usually better in ice or snow though.

[keri-WMS]

There are more variations to the way grip works than people imagine:

1 - Dry road, normal driving = "static friction" (where area x pressure x coef of friction doesn't work normally as.....there is no "friction" as nothing is sliding!).
2 - Wet road, normal drivng where a lot of the "static friction" vanishes due to the lubricated surface, and "mechanical grip" has more of an effect. This is the rubber physically deforming/keying into the lumps/holes in the road rather than tearing chemical bonds etc, and is why motorsport "wets" are REALLY soft (to the point of falling apart if you load them up in the dry).
3 - Wheelspin!!!! This is the ONLY time where the normal pressure x area x coef of friction formula works properly, ie as the surfaces are moving against each other you actually get FRICTION!

I'm not an expert or tyres etc and am happy to be proved wrong, but this is my understanding of the principles.

There's an easy way to prove the "static" grip verses "sliding friction" thing where bigger non-sliding tyres = more grip. In a skinny tyred car you can throw it into a slide and it happens sort of smoothly, the cornering or braking without lockup forces are similar to those found when the car is locked up, or going sideways.

Now think about how a car with wide tires "feels", when you lock up or the back steps out it really "goes" and you feel like you're on ice....and if/when you regain a slide it "snatches" back. The difference is that all the extra "static friction" a wide tire has disapears as soon as it starts to slide....where a skinny tire has much less static grip to start with.

Of course there are heaps of other factors, like the pressure on a small area provided by a skinny tyre can force water etc out and get grip where a wider tyre starts to float etc, and low profile tyres have less complience resulting in less "chuckability" in most cases....and a tyre spinning will warm up and get stickier etc etc etc.

[smccullie]

If your needing new tyres give me a msg i will get them at cost for you :thumbsup: