I've never covered the subject in any detail before, and it appeared that there's a distinct lack of even the most fundamental appreciation of what tires are all about! In any competition discipline - or in fact any road-going situation as well - the tires and control parameters used are so important to success, practically all the effort put into the chassis and suspension is to enable those four, relatively small (about the size of your average 'welly' foot-print) contact patches to give of their best. So something was desperately needed to address the situation.
As I said, the ultimate goal is get those four small contact patches to give of their very best. After all - they're all that stands between you and the 'scenery', and can be the difference between 'champion' and 'also ran'. It may also save you from investing a considerable amount of money in the wrong direction. So understanding a little of what they do is of paramount importance. There are no hard and fast rules as to which is best, and where. Different discipline, class, or formula rules often limit tire (and rim) size, compound, and even manufacturer. Read them excruciatingly carefully before splashing out on even the most apparent of bargains. If your rules are fixed, then that's what you have to use. Even then, some knowledge of what tires are about could give you that vital edge.
For those that have an open choice it soon becomes very clear that there's quite an array of choices, so where to begin? Trying to identify a tire and its construction, other than manufacturer, can be a complex problem. Some miss-guided folk would insist this isn't so - just read the information given on the sidewalls. However, over the years the tire walls have been littered with mysterious numbers in little boxes, supposed dimensions could be in inches or millimeters, or a mixture of both. They can refer to tread width, either inflated or not or when fitted to a certain rim width, or perhaps width across the fattest part of the tire. Or maybe the total outside diameter once inflated, or hypothetical diameter measured from wheel rim edge to tread face. And as far as diameter goes, is this the overall diameter, or that fore-shortened by measuring from wheel centre to ground where the tire's squashed? Several of the major tire manufacturers can issue you with a guide to what THEIR references mean, but are usually totally un-comparable with any other. Then slicks (tires completely devoid of tread) or treaded tires? A path through the jungle's clearly needed.
Unfortunately it's not as easy as asking the manufacturer or supplier. They may have intimate knowledge of the sphere in which they normally operate (F1, F2, etc.), and suggest a set up for you - but it may not be what you need. The alternative is for you to expand your knowledge on tires, decided what would best suit you then find them. So let us begin…
The main part of any tire is its carcass - this envelopes the whole tire except the outer layer of rubber that forms the treaded section (or not in the case of a 'slick'). It's essentially a flexible framework manufactured using various materials that include cotton, nylon, rayon, steel wire, and most recently Kevlar, often mixed, then coated with rubber. Very strong wire-cored edges form the 'beads' responsible for seating and sealing it to the wheel-rim.
What, why and how…
There are mainly two categories of carcass - cross-ply and radial. Cross-plies have various layers that crisis-cross each other at very carefully controlled angles. Radials are formed by the wall section layers going up, across, and down the other side at right-angles to the tread support band which runs straight around the circumference. Varying the layers - both in quantity and material types, bead construction, and rubber characteristics, tire designers can produce a plethora of types. These vary from something appearing to be a metal case covered in rubber, de-formable only with a heavy club hammer, to an extremely flexible one easily de-formed by hand. And all are spread across the spectrum from super-sticky treaded wet tires for small single seaters to rally tires capable of withstanding 140mph forest stages where rocks do their damnedest to tear them apart.
Cross-plies are considered as 'progressive' - meaning they break away (loose traction) relatively early but don't do it severely or instantaneously and remain very controllable by the driver. Illustrated in old racing footage by massive four-wheel slides round corners, largely because of the very stiff sidewall construction. Originally radials 'hung on' (maintained traction) much longer, but often broke away sharply and without warning. The carnage that could result was considered unacceptable, largely because of comparatively flexible sidewall construction. Then Michelin came up with a radial tire that eclipsed the cross-ply, and has been the benchmark for all developments since. Comparing a similar specified pair of tires, the cross-ply deflects less, will warm up quicker, but looses peak performance quicker. Consequently a radial user can generally go one softer on the compound scale, and that can be very useful.
Tread compound and patterns vary dependant on usage. Tarmac racers will generally use a completely bald tread (slick) with the softest compound to gain maximum grip in dry conditions. Or, to squeeze water from under the tire in wet conditions to avoid the possibly disastrous 'aqua-planning' caused by water build-up in front of the tire, a specially cut set of tires. Those racing on the 'loose' (read - 'anything that isn't stable ground') would use a variety of tires with varying depth and style/size of pattern cut into them to maximize grip and material/debris clearance.
Compounds also dictate the amount of 'grip' afforded. 'Grip' is an expression to encompass two separate effects - slip angle and cornering force. Slip angle is what happens once a tire has started to be turned into a corner. Basically, it is never going in the direction it's been pointed. The difference between the line the tire's pointing and the way it's actually going. Once the car's turning, the slip angle increases at a rate dependant on a complex mix of factors - tire carcass, tread pattern, suspension geometry, and surface type and condition. Within a limited range (approximately 1 to 6 degrees) cornering force is generally increasing appreciably, over this it starts tailing off until it reaches around 10 degrees where no more improvement is made. At this point some tires start sliding gradually side-ways, others loose the plot completely and slide sideways MUCH faster! A small amount of toe-out or toe-in will induce a small amount of slip angle in a straight line. Maximum slip angle is full lock but still going off the track! Cornering force is the grip a tire needs to exert to keep you from flying into the scenery, and is at maximum when properly inflated, at optimum slip angle, under its designed load on dry, smooth tarmac.
And so to the all-important contact patch mentioned in the opening paragraph. These four (and sometimes three on our venerable Minis!) small patches of rubber are all that keeps you on your desired path. At rest they are one, reasonably symmetrical shape. On the move, the shape alterations are legion under the loads they have to cope with. Radials and cross-plies have fundamentally different reactions and shapes when deformed by cornering loads. We've covered their general performance differences earlier.
We can have a piece of the action though, as they can be varied in a number of ways that we can control - fitment, inflation pressure, camber angle, and load. Fitting to wider rims will generally increase the contact patch, vice-versa on a narrower rim. Raising or lowering pressure can quite subtly affects it, so is worth experimenting with (say in 3psi stages from the recommended 'norm'). Camber angle can be used to counter tire distortion by adding static camber (usually negative), but large camber angles - despite giving the required result in a corner - may cause problems elsewhere. It can reduce straight-line stability, over-heating on the inner edge (possibly more than it can tolerate) leading to excessive wear and possible failure, and reduction in contact patch under braking. So a compromise is needed. Load refers to the weight the tire sees, and although adding weight to a racing car of any sort seems abhorrent this is usually affected by the use of anti-roll bars.
Knowing the Enemy…
Ignoring all the obvious dangers of nails, sharp stones, et al., HEAT is the biggest enemy. And that includes both 'not enough' and 'too much'. Tires are designed to work within certain temperature parameters (i.e. for race-cars on tarmac somewhere between 85 to 110 degrees C). Running tires outside their particular temperature range will severely affect optimum performance. The ONLY way to know where you are with this is to use a tire pyrometer (temperature tester), an invaluable piece of equipment that EVERYBODY should use. Pretty cheap and will help immensely in setting up suspension. This is looked at these more closely in the relevant articles on suspension set-up.
If your budget's tight, the best choice is a set of 'intermediate' tires - something that'll cope reasonably well in all weather conditions you're likely to encounter, rather than blinding in the dry and diabolical in the wet. It'll give you your best chance, and improve your driving skills the most through maximum 'track time'.
Initially for tarmac racing, radials are the norm. Mount them on rims that are an inch or so wider than the actual tire tread width as this will help eliminate instability when turning in to corners as the rim moves through the tire, deflecting the sidewall. Ideally, with the car stood on its proposed 'foot-wear', shoving on the chassis should show no more than _” deflection in the sidewall. For racing on loose surfaces, tires with far stiffer sidewalls (probably cross-plies) are used, fitted to rim widths similar to the tread width. Some oval racing is done on compacted clay - so somewhere between these two.
Go to several meetings of the discipline you've chosen and see what the rest of the field is using, and particularly watch them in action. It will speak volumes over what is said in the paddock!
Have a good look round at what the opposition is using. BUT, remember a great deal of talk about tires (like engines, suspension, etc.) is subjective and possibly bullshit. Each person has their own idea how they 'feel' a tire works. And it is how YOU feel a tire is working for YOU that is important, not what the other guy 'feels'!