How wide is too wide?

Dunno if it helps, but this is what I found for the rules in UK big sedan racing (where they put cheap XJ40s to good use):

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. 5.12.1 Road wheels must be no greater diameter than the original production or Group N homologated diameter plus 2” to a maximum of 18”, fastened to their hubs by the original number of studs/bolts. Centre locking wheels and their hubs are prohibited. Material is free.
However, a wheel of production diameter must be able to be fitted to the hub and rotated, as and when required. This effectively precludes the fitting of brake systems which are a larger diameter than the original wheel diameter will allow. 
Competitors must supply an original diameter cast, non split rim wheel at each meeting for the purposes of verification. 
Maximum permitted rim widths are: Class A 9.0"
 [Class A is 2689 cc and up, which means you!] . 5.12.2 Tyres
 The control tyre for the series is the Toyo Proxes R888 (GG Compound).
Toyo List 1A Tyres are also eligible for cars running to their own series regulations. No modification to tread pattern or depth is permitted.

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A quick Google search for "XJ40 racing" (because I thought I remembered they had a series going) got me scads of pictures of XJSs too, and of course THEY have a series. Duh. The Jaguar Enthusiasts Club here is probably a good place to start. Apologies if you've already done all this!

No such thing as too big (boobs) too wide (tires) too fast (cars) or too cheap (everything else).

An XJ-S is no lightweight - I don't think it is possible to fit a too-large tire on it without modifying the bodywork.

Well, these guys put 315's on there and seemed to like it:

http://www.magnusracingclub.com/LemonsJag.html

In reply to Curmudgeon:

You are nasty bastard. Where is the eye bleach.

Must offset some of the ugly...

Curmudgeon wrote:

Isn't that poopshovel's girlfriend?

The only real knowledge base I know of would be Jag enthusiast forums. That's how I picked gummies for my E30. Fortunately the guys on the E30 forums typically are actually knowledgeable and don't subscribe to the "wider is better" thing.

I ended up with the best skidpad numbers (using the free crappy android app) with 215/50-15 on my E30 with the way it was set up. That was amidst the thousands of young punk E30 kids who were asking how to fit 285's and rimZ under their "beamer" so they could handle better.

Or, maybe GRM mag could do an article on "what's best" and use your Jag as the test mule?

Even if it slows the car, there is the heat tradeoff. Doing an endurance event on street tires will beat the snot out of them, especially if your car has any weight or power or has a compromised (Lemons) suspension. Even if you sacrifice a few 10ths on your best lap, your overall average might still be better with wide tires that you aren't overheating. Our ~2200lb Lemons Saturn destroyed some street tires pretty quickly on Shenendoah in 95 degree heat with only like 100whp on tap.

I think the answer is: What is the biggest "sticky" 190+ TW tire you can fit? Does falken make super-wide stuff? I think Dunlop only goes to a 275, but its an 18. Shooting from the hip, I'd say 265/40-17. Should be around 25.5" tall, and fit well on a 10" wide wheel.

Seriously, 'how wide is too wide' is dependent on a lot of different things. While in a straight line, there will be additional rolling resistance from a wider tire on all 4 corners but that may be canceled out by being able to put power to the ground effectively. There will be additional resistance in a turn as the tire tries to 'twist', so it would seem to be a tradeoff (like so many things in life).

I think what you are after is reducing the drag caused by too wide of a tire on the front combined with the tire's slip angle, that 'bind' can get really bad. I have no idea of how to calculate that, wish I did. I'd like to see the calculator app Curtis mentioned too.

LeMons is a minimum 180 TW (I think ChumpCar is the same) and the most popular tires for that series are the Falken Azenis RT615 and Dunlop Star Spec. So the sizes those come in may limit your choices anyway.

mguar wrote: If I accept a tread wear of of say 210 rather than 190 there are many more choices available.. It will be really hard to get the weight of a XJ-S below 3000#s (since it starts out at 4200) That harder tire might gain me enough duration so a tire change won't be needed..

Not so much. The soft street tires handle the abuse much better than a harder compound. We ran ~400 TW tires on the front of our Saturn for about 4 hours, until we had melted all the tread right off them. The tread heats and just chunks away. And they are miserable to drive on. Teams with equally sized/powered cars are getting 2 full races out of the Dunlops. I would only consider Dunlop, Falken, and maybe the Contis if I were running a car with > 15" wheels.

And what I meant about the heat is that many teams go to a wider tire than necessary to resist heat. Given equal braking and acceleration loads, and only a slightly increased cornering load, the larger tire won't get as hot as a small tire (picture running the Jag around on 205s) will. Also running a wide wheel seems to eliminate flex which also contributes to heat. If I were to do it again with the Saturn, I'd be trying for 205s on a 7.5" or 8" wheel.

I really do want to do Lemons again.

In reply to Curmudgeon:

Why? Why? WHYYY?????!!!!!!!!

bravenrace wrote: In reply to Curmudgeon: Why? Why? WHYYY?????!!!!!!!!

In reply to mguar:

Everything's a tradeoff or compromise, unfortunately. IMHO, most races are won in the turns and under braking, not in straight line acceleration. For that reason I would lean toward maximizing my cornering power at the possible expense of ultimate straight line speed.

You bring up an excellent point about scrub radius, that would probably have a greater 'drag' effect in turns as compared to rolling resistance or slip angle. Like this: the less the tire tread has to deform to accomplish a given objective, the less rubber friction. So for that reason, if a wider tire would cause the scrub radius to get way off, then I would lean toward a narrower wheel that would correct the situation. A side benefit is that if the tire has to deform less it won't heat up as fast and that could be a big factor with a big heavy powerful XJS.

Of course the rear tires don't have to do quite as much crazy stuff as the fronts so they can be wider.

It's early and I haven't had any coffee (which may be the central item), but I'm having trouble picturing how increased scrub radius causes additional tire drag on a moving vehicle.

Increased scrub radius provides a lever arm between the steering axis and tread centerline and thus causes more kickback when one wheel hits something or grip is uneven, causes increased jacking from kingpin inclination or caster, and causes wheels to move fore/aft a bit instead of just steering in place.

But I'm just not able to picture additional rolling drag based on scrub radius...

When a tire's in a turn the inside and outside of the tire will travel different distances in the same time, same reason there's a differential in the back but it's not possible to have a differential in a tire. So the tire has to 'give' in some way; sidewall deflection, sliding across the pavement, etc, that's why a tire can wear in a 'cone' shape.

I do know that the Abomination was harder to push around the shop in a turn with the tires offset more to the outside (this moved the scrub point closer to the inside of the tire). when I had another set of wheels made, I changed the offset to get the scrub point closer to the center and the car was easier to push after that.

The way I visualize it: the more of the tire that's outboard of the scrub point, the more 'lever' there is as the tire is being turned. Move the scrub point to the middle of the tire, the 'lever' decreases.

Or that's my theory anyway. There's an excellent chance that I am dead wrong.

On that, the screwy Ackerman angle of the stock steering also had a HUGE effect on drag in turns. That had a lot to do with why I went through all the trouble to move the rack etc. The wheel offset thing was done after the Ackerman fix.

Ackerman influencing drag (especially when pushing a car around) makes perfect sense to me.

And offset influencing ackerman makes sense, because the tire's center is now elsewhere, so the nifty tire center line from the diagrams that goes through the point smack between the rear wheels is now moved aside, though I'm not sure how far relative to a meaningful ackerman adjustment.

From here perhaps it just turns into one of those "theory vs practice" things, and I need more practice