Additional Tire, Wheel, and Drive Considerations

			Wheel Offset or "Backspacing". This is a frequently-overlooked condition that can occasionally make you a little nuts. Or maybe a lot nuts, depending. It usually occurs when you have to change a set of tires on a car with limited clearances between the tires and the body. Hmm. Same width and diameter - you actually measured them - but the new ones hit, rub, are uncomfortable close, whatever. Huh? A bit of information you may not be aware of: even though they may have exactly the same tread width, wheels from different manufacturers can often have significantly different total widths.
Figure 1.11, above, shows what can happen when you inadvertently put a set with greater backspacing on a car originally equipped with a narrower total width package. In the illustration, dimension "X" refers to the distance from the upright or axle bearing/bushing to the body. As you can see, it's possible for a tight or bare-minimum tire-to-body clearance to suddenly disappear, even though the tire "section width" hasn't changed. The solution? Beyond squishing the body out of shape to gain some clearance (not that anyone we know has ever done that right?), replace the tires with a set that have equal or lesser backspacing than the tires the car was built with. Which, of course, means that you have to know what your current hub dimension is. You do know that, don't you? If not, you might want to make a habit of measuring the hub dimension of every set you use. Why every set? Just in case things change while we're not paying attention. While you can safely assume that every wheel of a given manufacturer's production run is turned out to the same dimensions and tolerances (except when the old "Zero Defects" sign falls off the wall and into the tire machine), you shouldn't assume that those dimensions will never change. Material stock cost and supply, manufacturing efficiency changes, new or altered design - a great many things can cause dimensions to change during the design life of a tire assembly. None of these things necessarily drive a tire manufacturer to label their tires "New! Improved!," and they really don't change the basic advertised dimensions of the tires, either. A nominal 1.00" x .300" tire can still measure correctly, even though the entire assembly may have been slightly to significantly modified. Due to those same conditions, I have hesitated to print a list of the backspacing dimensions I've measured on the various wheels I've worked with. I know what I have, but I don't really know what you have., nor do I know that this morning the guys at Acme Rocket Slot Car Wheel & Tire Corp decided to change a few procedures and dimensions for reasons unknown to us mere mortals. The only thing I can say with any confidence is what I've already encountered. But what the heck, right? So here's what I've got:

Wheel	Backspacing	Don't send me any hate mail if the wheels you measure differ radically from these\ numbers. Provided, of course, that you do occasionally measure them. The point is that it's probably wise to pay a little more attention if you've got one or more cars where this may be a problem. Remember that while a narrower-track wheel may prevent a body interference problems, it may simultaneously create other clearance problems you may not anticipate. The tire-to-endbell clearance in certain sidewinder configurations is one that comes to mind, for example. Understanding what fits and what doesn't is a good thing to know before, rather than after, you have to replace a set at the track just before qualifying.
DRS (large hole)	.140" - .141"
DRS (small hole)	.137" - .138"
Koford (old, w. spacer)	.157" - .158"
Koford (old, w.o. spacer)	.137" - .138"
Koford (new? 6-hole)	.141" - .142"
Pro Track	.130" - .131"
Pro Track (big hub)	.130" - .131"
Sonic (slot)	.140" - .141"
Sonic (small hub)	.136" - .137"
Unknown (5 plunge mill back)	.142" - .143"

Unusual Tire Growth. Yes, that's right, growth, not wear, not shrinking, not turning to home-made dog **** the way they all do after a while. This one has been bothering me for a few years, and it still bothers me, mostly because I don't have any real answers. Lots of questions, lots of messing around, but still no answers. Basically, it can be described as a relatively sudden and radical increase in natural rubber tire diameter and width, accompanied by a drastic drop in durometer reading. "Radical," in this case, is something in excess of 2%, and "sudden" is between 4 and 5 passes. 2% growth may mean that a .300" wide tire only grows .006", almost negligible for traction purposes, but it also means that a 1.010" tire grows to 1.030" or more, a difference that can have a direct effect on performance. I've pulled tires off cars after a few number of runs that actually grew by more than .035". This has happened to the tires I make, as well as the tires of at least four different manufacturers/suppliers of commercial tires. I've been asking racers around the country about this for some time, and have yet to have one say "Oh, sure, that happened to me too." For that reason (and a lot of destroyed tires), the conclusions I've come to have little to do with who makes a tire and how they are run, but maybe a lot to do with where they are run and with what they are glued. Not being a rubber chemist, I suspect it's possible for some glues to react with other glues or substances usually used in shutdown areas in a manner in which the combined substances can "relax", for want of a better term, the cellular nature of the tire material to some degree. This being 2000, if I say something like "If you use this tire glue and your track uses this other stuff in the shutdown area, it seems your tires can turn to **** really quickly," I would be a) pretty accurate regarding at least one combination, and b) most likely pilloried, sued, or otherwise harassed and humiliated by the two manufacturers who make the stuff. No thanks. So I'm not going to say that, being the prudent person I am. What I will say is that you should periodically check the true diameter of your tires, preferably off the car and with a digital caliper, during their use. Just in case. If you come up with anything interesting, let me know, and we'll share it with out little friends.