Different rubber size fronts vs back??

[wun911]

I like these weight saving ideas. (puts the flame suit on)

Anyone run like wider 215 at the back and stock 205 at the front on a 6.5 wide rim??
So I get more grip at the back where its needed. You dont need to the added weight of wider rubber on on the fronts. And you still get good responsive stearing.

Anyone run lower profiles on the fronts compared to the back so they get less side wall fex, and save some weight less moment of inerita due to smaller diameter etc etc!
I presume sidewall felx occurs more on the fronts than on the rear?? The spedo will still be accurate because its taken from the rear wheels??

In these situations have your alignment specs changed compared to oem stock setup?

[Adam_NAclubman]

215 on a 6.5 Ewwwwwwwwwwww

[wun911]

Oh is that dangerous??

You know the deal the most amout of rubber on the thinest rim possible to save weight...

[bruce]

Wun, come back to reality !

[Charlie Brown]

215s on a 6.5" rim will flop around like Granny's undies on a clothesline and cause even more grip problems.

If you have rear grip problems I’d suggest that the tyres you have on the rear at the moment are stuffed or you need to look at your suspension. Never run a different width tyre front to rear on a MX-5.

[mx52nv]

215s on a 6.5" rim will flop around like Granny's undies on a clothesline and cause even more grip problems.

If you have rear grip problems I’d suggest that the tyres you have on the rear at the moment are stuffed or you need to look at your suspension. Never run a different width tyre front to rear on a MX-5.

100% agree (except in nfs13b's case)

[Benny]

I'm running 215's on the back and 205's on the front, but I have 7' wide rims.

You have to be careful about the overall diameter fo the 215 tyre though, as some 215's are noticeably taller than the same tyre in 205.

I think 215's should be OK on a 6.5" rim, but you will notice they look a little "baggy", however, being baggy will mean that when you scrape the curb, the tyre will get scraped and not the wheel.

205's used to be factory fitted to 6" rims, so 215's on a 6.5" rim should be fine.
If you find them a little squirmy, add more air to stiffen them up.

[wun911]

Yeah im having a bad experiece with loss of grip on the rear wheels (stiff suspension + crap years + winter roads)

I always thought side wall profiles were responsible for side wall flex. I thought like 50 profiles gave side more side wall flex than 45 prfile.

So I should get 7 wide rims and stick 215 onto them? that should give more grip than a 205 on a 6.5 rim?

[stevesports]

under what conditions are the tyres losing grip wun? Also note that having a larger contact patch might affect the handling of the car.

[Adam_NAclubman]

You won't get a larger contact patch, just a different shape

[stevesports]

You won't get a larger contact patch, just a different shape

hmm? the way that i see it, if the tyre is wider, there is more tyre that touches the road, and as such a lrger contact patch.

[StanTheMan]

if you dump in a few bags of concrete. It will increase your contact patch.

The designers and engineers all say a wider tyre does not increase the the contact patch it only changes the shape.

with peeps like 13nfs who has a huge amount of power for a drag car. not sure how it works different. but its different.

I assume you have a pretty standard car. Satandradish power output. I'd be thinking about going back to 205 size Tyre but maybe with a softer compound or just a better quality tyre. & maybe change your geometry( wheel alliugnment) if you are having problems keeping the car on the road.

[StanTheMan]

Also if you do have more contact patch with the crappy weather in Melbourne you be more likely to aquaplane anyway.


*gone hiding*

[Alex]

I've been considering getting wider wheels for a while now but every time a thread comes up everyone seems to state "You won't get a larger contact patch, just a different shape" this statement (and ones like it) seems to imply that you need a larger area to get more grip. Then there is the whole tyre pressure issue that pops up every now and then so I decided to see how the area (magnitude) would change with pressure and came up with some numbers
Pressure (psi) / Area (cm^2)
28 / 12.70
29 / 12.27
30 / 11.86
31 / 11.47
32 / 11.12
33 / 10.78
34 / 10.46

So my contact patch will be ~20% smaller if I go to 34psi from 28psi, if grip levels are all about area then that means I'll pull ~20% less G's in my car around a corner, which would be a noticeable difference you would think. (forgetting that the coefficent of friction is not dependent on the area). I've messed around with pressures and found that while there is a difference I'm sure it's nowhere near 20%. (But there is a definite difference, especially in how the car handles/feels)
Which leads me to believe that the shape of the contact patch is more important that the actual area. Hence yes it will make a difference if you get wider wheels/tyres.

But IMHO if you are worried about grip the best way to deal with it would be to get better tyres, since different tyres have different compounds (hence coeffiecent of friction).
the following values were used for the data above
Conversion factor from psi to Pa = 6.895 x 10^3
Load = 250kg
Gravity = 9.81m/s^2
now to hope I did the calculations properly and didn't make a total fool of myself

[Fatty]

there is a good diagram in the "miata performnace handbook" explaining the way the contact will change shape, but not size, with wider tyres.