Brisca F1 Stox racing has evolved into a highly sophisticated and technical sport over the last decade, and now more than ever tyres play an important if unexpected role in deciding the outcome of a race. Chassis have become more or less standardised and ninety nine pet cent of top drivers use ' big Block'' Chevys for power. Apart from the skill of the driver, how to get that power on to the track and keep the car on the line boils down to the one thing that touches the track - The Tyres.

Racing Tyres are not allowed in Brisca racing so drivers have to use tyres made and produced for general sale to the public. Despite the fact that radial tyres are generally used for all road cars nowadays these are not suitable for Stock Car Racing. Radial tyres give much greater mileage for road cars and better grip but because of their much softer and thinner sidewalls when used as severely as being thrown around a 350 metre oval track with 500bhp trying to break traction they tend to wallow and roll, and when they break traction they tend to do so without warning. Thus crossply tyres tend to give more control under racing conditions, and their thicker sidewalls are more capable of sustaining knocks and cuts before they burst.

Having decided on crossply tyres what is available? The tyres have to be generally 15inch diameter and these are getting scarcer as road cars nowadays tend to have smaller wheels.
Using a conventional L.D. differential of 5.7 to 1 ratio and racing in top gear all the time the final gearing is accomplished by the choice of tyres for the rear axle. A few years ago 8.15 x 15 tyres were fine for most of the engines then being used but with the advent of the 454 Chey which could pull better but did not need to rev as hard as some of the smaller engines, a larger tyre in circumference was needed.

To the layman tyre size marking is a bit baffling but on crossply tyres 8.20 x 15 means 8.2 inches width and a 15 inch internal diameter -- width being the tyre width not the tread width when inflated. Tyres are produced in different aspect ratios, and this means that tyres having approximately the same width have differing heights relating to the width; giving different circumferences but approximately the same width. So 8.20 x 15, 800x 15 and 8.15 x 15 are all about the same width but vary in circumference and therefore alter gearing, so 820 x15 gives a higher gearing for larger tracks - Belle Vue - Sheffield etc than an 8.00 x 15 which gives higher gearing than an 8.15x 15. Technical isn't it? All these three sizes were produced mainly by Dunlop and Avon for use on large cars such as Rolls Royce and Bentleys, the Dunlop tyres being much favoured by Stocks racers. The town and country pattern in these sizes have proved particularly difficult to obtain as they were mainly produced for export to countries such as Saudi Arabia where they have many Rolls Royces but not many roads! 6.70 x 15 is another commonly used size particulary the front of a stock car. This is about the same height as an 8.15 x 15 but narrower. 6.70 x 15s used to be made for a number of Commercial Vans on the roads but even these are becoming very rare. As I work for Goodyear I have on occasions in the past been able to get them to make several hundred town and country remoulds in this size especially for use for Stock Cars.

For shale tracks two types of tyre are used. When it is wet or there is a lot of shale on the track a town and country tyre is required for grip and traction.
If there is little shale on the track and it is dry, a road pattern tyre with ordinary tread is preferable - for it creates less drag and allows the tread to operate as it was intended, the hard dry shale approximating rough tarmac.

Tarmac tracks are a different proposition. The best tyre for a dry tarmac track is bald - as smooth as possible - this gives the greatest contact like dragster slicks. But get a little rain during the race and it's like driving on ice because bald tyres do not disperse any water. So its back to tyres with tread for damp tarmac, and if the track is really wet -- town and country tyres to move as much water as possible. When a tyre is new it is not much good for racing. It has to loose its squareness and become rounded to get the best adjesion, as we always travel in a left hand circle or should do! When drivers qualify for an important event like World or Semi Finalsthey will take a new set of tyres, race them just sufficiently to make them just right. then keep them for the big night. A tyre used on the offside rear of a Stock Car wears very quickly and used on a dry shale track will be nearly bald in 3 meetings it can then be used on tarmac but will need another couple of races before it is just right, and then maybe one meeting and its through to the canvas and scrap always assuming that it hasn't blown before this stage. Last season Frankie VVainman got a Remould firm to put a slick unpatterned tread on to 8.20 x 15 casings, and now a lot of drivers use these for tarmac as many more races can be had before they are worn out but they take a lot more races from new before they are properly run in very often drivers will use a smaller size tyre on the nearside rear of the car to that on the offside. This has the effect of making the car always want to turn left and helps negotiate the bends. So you can see there is a lot more to tyre than the average fan thinks and Stocks don't operate on the principale that anything will do. When you think some tyres cost £40 and can be scrap ater a first bend skirmish with a bumper £15 for winning a heat isn't much is it?

Original article was by Bob Tanser from the June 1980 Stock Car Magazine.