Gents,

I have the OE Tamana 19's on my 2010 XKR, which are 8.5" front and 9.5" rear. Current tires are 255 front, and 275 rear.

If I can't sell these stock wheels for fair money and then get some aftermarket 20's that are also wider, I will keep these stockers. Painted metallic dark grey, they are really nice.

I'd like to not only add some rubber to the rear for added grip (this thing has zero, both off the line and on corner exit during spirited driving), but to widen the track a tad too, for visual reasons. I will be doing spacers, but will wait on them until I figure what is the widest tire that I can run without bulging and causing a loss of contact-patch and odd tire wear.

I know that I can go to a 265 on the front and a 285 on the rear with no problems, but would a 275 work on the front and a 295 on the rear with no loss of rubber to the ground and weird inner-tire wear due to bulge?

Thanks.

 

I would stay within the manufacturer's recommended rim width range for the tire you want to use. Looking at the tables on the Tire Rack site, you can run a 285 Pilot Super Sport on a 9.5" rim, but not a 295. In that tire, they don't recommend wider than a 255 on an 8.5" rim. Other tire models might have different ranges and of course there are differences between nominal sizes and measured section and tread widths. Interestingly, the PSS has two 255/45-19 tires, one with a 9" tread width and one with a 10" tread width (also with a lighter max load, for Porsches).

 

I put a 285 on the back but stuck with stock in the front for steering feel.

 

i'm with you. it looks like 255/285 is the all you can get away with, if you want to keep things right.

 

plus, these rigs have so little rear grip off the line and in any part of a corner once back on power (sometimes even before if your corner speed is on the limit), that there is no need for more front grip. just the opposite. wish I could put a harder front bar or softer rear on this thing (or both haha).

 

How old are the tires that came with it?

Just a change to new tires and higher grip compound should make a much bigger difference than a wider tire.

 

No (sane) amount of rubber can keep the back end of these things stuck-- just too much torque. I have 305s, can still light them up at pretty much any speed in any gear. OK, perhaps a 4.2 XK, but XKRs? - nope.

 

They are new and soft-ish compound. I need soft AND wider. I'm also doing this for the visual component too, haha.

 

yeah, unless you are running track day rubber, this is probably true. either way, I want some wider rubber for the visual factor, and to be able to run less thick spacers when i buy them.

 

From my college days:
“Friction is independent of area, but directly dependent on weight”
So theoretically no matter how wide the tire, if the compound is the same, you will get no more grip.
The trick is to use a softer compound, and use the wider surface area of the ‘foot print’ to compensate for the higher wear rate.
The only site I know of that might help is Tirerack, and look at the test data for maximum G pulled on a constant radius turn.
Regards,

 

The weight coefficient of traction is a vector, not a constant. Weight is a force "down", but when a tire rotates, the contact patch moves nearly directly horizontal and to the rear. The effect of 'weight' is not even close to a linear value.

 

“Friction is independent of area, but directly dependent on weight”


That was an interesting concept. I had to read about it, because it did not make sense to me. But it does now.

 

Most people don't know this, but you can 'weight' your car by measuring the actual contact patch area and then multiply that by the PSI in the tires. For each tire, that is, and then you'll have a fairly accurate four-corner weight. Sidewall stiffness does play a factor, but it is still fairly accurate.

 

So how would you measure the contact patch?

 

L x W. I've used four sheets of paper tucked around between the tires and the floor. Measure where the sheets don't fit under.

 

Cee Jay,
Yes of course ‘weight’ on the tire changes with the conditions, i.e. under acceleration the weight will increase, under braking the weight will decrease, but that doesn’t change my basic statement. Grip (friction) will change according to the weight on the tire.

The tire doesn’t care where the weight comes from, hence inverted wings on high performance cars first seen (in the U.K.) during the 1967 BOAC 500 at Brands Hatch on the winning Chaparral 2F. After that everyone jumped on that ‘mass free weight’ bandwagon.
Regards,

 

I thought I'd add a bit of explanation of tires in case anyone is interested:

The friction model describing earlier in this thread is sometimes referred to as Newtonian physics. It is reasonable accurate for solid materials sliding on solid surfaces, but is far less accurate for pneumatic tires. The coefficient of friction between a tire and the road surface is created by a combination of mechanical interlocking of the rubber tire surface and irregularities in the road surface and molecular adhesion between the rubber and the road surface.
The value of the friction coefficient varies continuously while in use as follows:
Lowers with added load – if your tire and wheel weigh 20 pounds and the coefficient is 1.0 with the wheel and tire on the road surface with nothing attached it will take 20 pounds of lateral force to move the tire sideways. If you attach it to the front of your car adding 980 pounds to the load, the coefficient of friction will reduce to a value that is likely somewhere between 0.900 and 0.850.
Rises and then lowers with temperature. A coefficient of 1.0 at 80 deg F might rise to 1.1 at 250 degrees and then fall off to 1.0 at 350 or 400 degrees.
Rises and then lowers with camber angle. This means the effective camber angle for a tire at any instant which will vary according to the suspension geometry and suspension droop or bump at each wheel, and the accompanying amount of body roll.
Rises and then lowers with tire inflation pressure.
Tire tread blocks also distort individually from shearing stresses created by cornering force. The friction force each tread block can produce is reduced by this type of distortion and is minimized by reduced tread depth and by tread consisting of a few large tread blocks across the tread rather than many smaller blocks. One of the big advantages of slicks and DOT track tires is they reduce this tire squirm because they have fewer wide tread blocks or no tread blocks.
And finally, the tire contact surface with the road constantly changes due to elastic flexing of the tire as the wrinkle caused by the tire flattening at the contact area distorts the sidewalls and the tire contact surface. This distortion wave runs radially around the circumference of the tire carcass as the tire rotates. The magnitude of this effect is controlled by the applied vertical and lateral loads, and the tire construction, configuration, and inflation pressure.
Suffice it to say it’s complicated. All of the effects described will be somewhat different from one tire product to another. If the subject grips you, you can learn quite a bit about it in books such as Carroll Smith’s “Tune to Win” or Paul Van Valkenbugh’s “Race Car Engineering and Mechanics”. Smith has impressive racing credentials in car engineering and team management. Van Valkenburgh is an engineer and journalist.

 

Yes, it is complex.

I had to learn different tire modeling data sets when doing mods for PC games as a teenager.

It is the difference between raw friction coefficient and additional grip from interaction with the surface that allows a non ABS car to stop shorter, most ABS, especially older ones, cannot account for how to get to peak force.

 

just put 315 35 20s in the back of xkr a little rub here and there but can fix by a little trimming. I like the look but taking some heat from others
Nittos 555

 

2003 convertible