I posted this in another section on tyres but will also duplicate it here.......Some of us may remember in the Eighties, and at the time there were only SR or HR rated tyres available. The five speed TR7 strictly had HR rated tyres fitted so as not to invalidate insurance policies, even though it may have been entirely possible to get a car through the MOT at the time with the much cheaper SR rated tyres fitted. Only the 4 speed TR7 had those. Strictly speaking putting anything less than HR rated tyres on a five speed TR7 would be illegal in the UK unless you could get away with it by telling your insurance company. In fact few tyre manufacturers now actually make 13 inch HR rated tyres. All seem to make T rated tyres though, and because of this I still see many five speed TR7’s with technically illegal T rated tyres fitted!!
There are however at least half a dozen tyre manufacturers that still produce HR rated 185 70 13 tyres suitable for the original TR7 although none of the well known makes like Good Year or Michelin still produce them. Most seem to be of Japanese, Chinese or Korean origin. I was very reluctant to fit Japanese tyres on a Classic British Sports Car. I managed to maintain pedigree by fitting new tyres from the AMERICAN manufacturer Maxxis and settled for Maxxis MA-701
http://www.maxxis.co.uk/catalog/tyredet ... eSupport=1 I got these from the selection at
www.mytyres.co.uk
The rating of the tyre is not just speed it is the load rating as well, generally a higher speed rated tyre will carry load easier, going down a little in rating affects the cars stability more than its speed ability. It is not advisable for anyone to fit tyres of a lower speed rating. Original TR7’s were OEM fitted usually with Good Year G800S 185/70 HR 13 tyres. These had a load rating of 84 and treadwear rating of 130, temperature rating A and traction A. My new Maxxis tyres by comparison have the same HR rating, but load factor of 86, and a much higher treadwear rating of 400.
The load rating expresses how vertical loads are transmitted to shape changes in the contact patch of the tyre. Fit a tyre with a heavier load rating and you will experience slightly larger but quicker steering (i.e. you need more steering input to generate the same amount of lateral force but transient phase goes a little quicker). The positive flip side is that you can throw about longitudinal load much more agressively before the tyre saturates with load, and get more turn-in/oversteer behaviour. Too high and the whole tyre ceases to perform, because the contact patch is not getting squished as much onto the road surface
Conversely, with a lighter load rated tyre on the steering input you require to go round a corner reduces, but the transient time increases and you seriously hamper any kind of dynamic steering ability in the car. Plus the ride goes soft and cornering may be unstable because the sidewalls are too soft. Not a good combination.
A good site for information on tyres is
http://www.pneus-online.co.uk/tyre-tech ... dvice.html There is also some useful background information on each manufacturer by brand name at
http://www.pneus-online.co.uk/car-tyre/
In short, always stick to the manufacturer's original rated load - going slightly higher is ok, don't go low. Never compromise on speed rating. Remember that your tyres are simply the most important thing that you will ever fit to your car.
