During the design stage of the TR7, all aspects of the cars performance were taken into account by BL and the product was a good all-round car capable of being modified in many ways. It was decided that 185-70X13HR tyres could cope with the power outputs of the various models, whether it was the standard TR7 at 105bhp, the Sprint at 127bhp, the US spec 8 at 134/148bhp or the UK spec 8 at 155bhp. I am a firm believer, rightly or wrongly, that this choice of tyre is quite adequate for cars being driven on a day to day basis.
However, the reason for uprating an engine is to increase its performance, both in speed and acceleration to enable the car to be driven in a more spirited way. This is where superior tyres and wheels come into their own.
The output of a Sprint engine can easily be increased to 140bhp, likewise the V8 is only mildly tuned at 200bhp. In both cases we are looking at serious competition with large modem saloon cars and sports cars whose wheels are shod with somewhat larger rubber than ours.
I do not profess to be an expert on tyres and wheels, but I have been looking through a number of articles and have put together the following which may help to explain some of the mysteries surrounding this subject. Figs 1 and 2 show a typical wheel and tyre in cross-section from which we can get the following dimensions.
1) Dimension J - this is more commonly known as the rim width. The important thing to remember is that the measurement is taken from bead to bead, rather than from edge to rim edge.
It therefore represents the true rim width. On the TR7/8 this dimension is 5.5 inches (140mm) hence the term 51/2J.
2) Dimension Ji - this is the overall width, or section width, of the tyre when inflated to its correct pressure. Which when considering a standard TR7/8 tyre would be 185mm (7.25 inches).
3) Dimension C - this is the distance from the wheels inner rim edge to the hub mounting face. For the TR7/8 this is 3.875 inches (100mm). In combination with the J dimension it determines
how much of the wheel protrudes inboard and how much outboard of the hub face.
4) Dimension N - this is the nominal diameter of the wheel, in our case 13 inches (330mm).
5) Dimension T - this is the overall diameter of the tyre/wheel combination.
