club.liberty.asn.au

I'm currently on the factory tyre size (215 18s), and would like some feedback on how much extra grip I can expect by going wider - incl getting wider wheels because I'm limited by the factory SpecB rims atm. Is it a major improvement or just a small difference (eg going with STI rims and a nice wide Tyre for them)?

Because even though my tyres are pretty grippy Bridgestone S001s, now that I'm on E85 I'm really struggling for grip when launching hard on the street (especially with LC and my grippy Jim Berry clutch). I am weighing up the compromise between better acceleration that you get from smaller tyres, and better grip off the line with wider tyres.

All things being equal, you could do a really rough calculation and say... 215 to 235 is a 10% increase in grip because it's an extra 20mm -> (20/215)*100 = 9.3%.
If you get one with a lower profile it might be the same diameter as, or very close to, the original tyre.

You'd probably have a better increase by changing to a softer tyre compound like the Yokohama Advan AD08R (basically the same as the Bridgestone RE11). But then you have everything that comes with that like shorter life time (AD08Rs apparently only last 20-25k km).
I recently changed to AD08Rs due to the buy 3 get 1 free deal going this month. I can say they are definitely in the next league (car originally had Pirelli P Zeros) for grip and cornering due to the stiffer side wall, softer compound and bigger contact area. Maybe a touch noisier but I can deal with that.

If you do go wider, also consider you may have to roll your guards.

Cougz wrote:All things being equal, you could do a really rough calculation and say... 215 to 235 is a 10% increase in grip because it's an extra 20mm -> (20/215)*100 = 9.3%.
If you get one with a lower profile it might be the same diameter as, or very close to, the original tyre.

You'd probably have a better increase by changing to a softer tyre compound like the Yokohama Advan AD08R (basically the same as the Bridgestone RE11). But then you have everything that comes with that like shorter life time (AD08Rs apparently only last 20-25k km).
I recently changed to AD08Rs due to the buy 3 get 1 free deal going this month. I can say they are definitely in the next league (car originally had Pirelli P Zeros) for grip and cornering due to the stiffer side wall, softer compound and bigger contact area. Maybe a touch noisier but I can deal with that.

If you do go wider, also consider you may have to roll your guards.

Excellent, thanks for the info . Makes sense now that you mention it. 10% extra grip isn't much, so I'm glad you put that into perspective. Especially considering the price of new wheels and wider tyres being more expensive (and reduced acceleration).

Cheers, I'll look into the AD08Rs. They look very aggressive - I love it! Wow, to be on another level even compared to P Zeros is impressive. But yeah, shorter life will be the downside.

Yeah, I'll try to avoid going so wide that I have to roll guards, but I'll keep that in mind.

Cougz wrote:All things being equal, you could do a really rough calculation and say... 215 to 235 is a 10% increase in grip because it's an extra 20mm -> (20/215)*100 = 9.3%.
If you get one with a lower profile it might be the same diameter as, or very close to, the original tyre.

You'd probably have a better increase by changing to a softer tyre compound like the Yokohama Advan AD08R (basically the same as the Bridgestone RE11). But then you have everything that comes with that like shorter life time (AD08Rs apparently only last 20-25k km).
I recently changed to AD08Rs due to the buy 3 get 1 free deal going this month. I can say they are definitely in the next league (car originally had Pirelli P Zeros) for grip and cornering due to the stiffer side wall, softer compound and bigger contact area. Maybe a touch noisier but I can deal with that.

If you do go wider, also consider you may have to roll your guards.

I am also lead to believe wider tyres require more braking force to stop the vehicle also, (don't argue with me), take it up with those who espouse this theory/knowledge.

Ah, very true! I hadn't thought of that, but it has to be the case. Higher rotating mass requires more force to stop. Unless lighter wheels offset the heavier Tyre, but the total weight of both will also affect suspension behaviour (unsprung mass).

Cheers, more and more downsides for little improvement in grip. Getting stickier tyres seems to be better in many ways, instead of just going wider.

Also remember wider tyres have to evacuate more water.
Just get some Pilot Super Sports.

fedaykin wrote:Also remember wider tyres have to evacuate more water.
Just get some Pilot Super Sports.

Ah ok, good to know! And yeah, so many people rave about Michelin PSS, I should definitely consider them.

Bridgestone RE003 bloody good too - very grippy wet or dry, maybe not so long lived though, still reasonable life for the $ according to others.

As I understand it, all else being equal, wider rubber with lower profile tyres creates a wider but shorter longitudinal (front to back) contact patch. The higher air pressures necessitated by low tyre side-wall heights means of the tyre touches the road less longitudinally.

Conversely, the lower profile wider tyre has less heat build-up to dissipate (due to less sidewall flex as well as wider tread for cooling) so can be made of generally sticker/softer and therefore grippier rubber. Ride quality suffers of course due to less absorption available in the smaller sidewall.

But a wider contact patch means some better lateral grip at the expense of less braking/traction grip - mainly because the lower sidewall flex makes for higher slip angle (point where tyre bends away from the road under lateral load).

But really if you are just going from 215/45/17 to 225/40/18 it probably stuff all difference longitudinally and the rubber softness will have much much more impact on grip. 5% wider, 8% shorter sidewall.

Good read here: http://www.carbibles.com/tyre_bible_pg3.html

Another factor to help increase grip is to drop air pressure a little to fatten the contact patch - try that first.

muzza wrote:Bridgestone RE003 bloody good too - very grippy wet or dry, maybe not so long lived though, still reasonable life for the $ according to others.

As I understand it, all else being equal, wider rubber with lower profile tyres creates a wider but shorter longitudinal (front to back) contact patch. The higher air pressures necessitated by low tyre side-wall heights means of the tyre touches the road longitudinally.

Conversely, the lower profile wider tyre has less heat build-up to dissipate (due to less sidewall flex as well as wider tread for cooling) so can be made of generally sticker/softer and therefore grippier rubber. Ride quality suffers of course due to less absorption available in the smaller sidewall.

But a wider contact patch means some better lateral grip at the expense of less braking/traction grip - mainly because the lower sidewall flex makes for higher slip angle (point where tyre bends away from the road under lateral load).

But really if you are just going from 215/45/17 to 225/40/18 it probably stuff all difference longitudinally and the rubber softness will have much much more impact on grip. 5% wider, 8% shorter sidewall.

Good read here: http://www.carbibles.com/tyre_bible_pg3.html

Another factor to help increase grip is to drop air pressure a little to fatten the contact patch - try that first.

Some very good info there, thanks muzza!

Yeah, all a balance between comfort, grip and longevity.
I'll look into all the options, but don't want to give up too much comfort.

Cool, I'll have a read of that link thanks.

And yeah, air pressure is definitely worth a try as well. My pressures are stock pressures atm, so pretty low compared to what some other guys run.

Another aspect is price, 235 40 r18 are a spec shared by iirc ford and holden, so stepping up to an 8 or 8.5" rim has benefits beyond moar grip.
Eg S001 on the 3 for deal at $750 or less.
My next rim purchase may well be brembo clearing Method 18 or 17x8 for hard touring

bigBADbenny wrote:Another aspect is price, 235 40 r18 are a spec shared by iirc ford and holden, so stepping up to an 8 or 8.5" rim has benefits beyond moar grip.
Eg S001 on the 3 for deal at $750 or less.
My next rim purchase may well be brembo clearing Method 18 or 17x8 for hard touring

Ah ok, cool . Iirc they make the speedo error slightly worse, but I'd put up with that for a cheaper price .

Yeah I'm about 9% off true speed at 100kph, but I display adjusted speed via BtSsm on the stock nav.
That tyre spec on 18x8.5 +35 also clears the inner guard on lock with avo caster adust LCA's

bigBADbenny wrote:Yeah I'm about 9% off true speed at 100kph, but I display adjusted speed via BtSsm on the stock nav.
That tyre spec on 18x8.5 +35 also clears the inner guard on lock with avo caster adust LCA's

Ok cool - good to know. I sometimes bring up the GPS speed on the factory Nav Unit under the Vehicle Signal menu (I think that's what it's called).

Ah ok, nice!

Muzza is on the money.

Wider tyres do not increase the size of the static contact patch with the road. They widen the patch, but the patch is then shorter (front to back), giving an overall neutral impact. Tyre pressures have a bigger impact on contact patch size than on tyre width on a given vehicle.

However, that neutral impact has a positive and a negative that result in that neutrality.

The positive is the wider contact patch means that during cornering, when loads are lateral (across the car), the contact patch is "longer" in the same direction as the loads, and thus you do get "more grip". This is why wider tyres result in higher cornering g-forces.

The negative is that the contact patch is shorter front to back. This means that during rapid acceleration and braking (in a straight line), the contact patch is shorter in the same direction as the loads, and you get less grip. Wider tyres are also more likely to aquaplane due to the wider front edge of the contact patch - this makes tread design (in particular the aspects of tread design designed to vacate water fron the leading edge of the contact patch, and thus avoid aquaplaning) all the more important once water comes into play.

So why are wider tyres generally considered to be better? The short of it is that acceleration and braking rarely actually occur in pure straight lines, and that the very minor loss in a straight line is more than compensated for by the additional lateral capabilities - the ability to brake and swerve, with a mix of longitudinal and lateral forces, is enhanced by the wider tyres.

All this goes out the window, of course, as soon as you get off solid surfaces - driving in gravel or on dirt or in mud changes things entirely, and often narrower tyres are preferred in these circumstances for their ability to drive down through a soft surface and find a solid substrate on which to grip. Driving on deep sand changes things again, where you want to prevent the tyre from penetrating the top surface at all (as there's no solid substrate to work with) - pressures are dropped (to increase contact patch size).

Wow thanks Manaz! Nice detailed writeup .

It's definitely starting to make sense.

I read it on the weekend but totally forgot to reply....