187
Tyre aspect ratio
David Knowles (Damask 1215) from Oxfordshire provides this useful insight to tyre aspect ratios. (May 91)

I have noticed a number of MGBGTV8s, and for that matter other MGs, fitted with various widths and types of tyre, some of which were clearly not suited to the wheels upon which they had been fitted. I decided to put together this workshop note explaining the meaning of the aspect ratio and the suitability of different tyre sizes, hopefully dispelling one or two myths in the process.

Aspect ratio
The aspect ratio of a tyre is the ratio of the "section height" of the tyre to the "section width", expressed as a percentage - see sketch. Traditionally radial ply tyres have nearly always had an aspect ratio of 82% and because this was so universal, it was never marked on the tyre. Thus for example the correct tyre for the MGBGTV8 is a 175/82 although the 82 is rarely referred to. Nowadays of course we see a proliferation of different "low profile" tyres with aspect ratios of 70, 65, 60 and even lower. An obvious attraction for MGB enthusiasts is the possibility of fitting a wider tyre and the commonest way of doing this is to substitute a 185/70 tyre for the standard 165.

SECTION WIDTH


Cross section of a tyre (Drawing: David Knowles)

Considering d
ifferent tyre sizes
Whenever a tyre of a different size and aspect ratio is being fitted, it is important to consider two points in particular:

Suitability - is the wider tyre suitable (and legal) for the rim size? This is a safety consideration.

Rolling radius - is the overall diameter of the wheel

and tyre combination (hence the "rolling radius") altered. If it is different to standard then the overall gearing ratio would be changed by fitting those tyres and the reading on speedometer will be wrong.

The accompanying table shows the various dimensions of the most popular 14 inch tyres suitable for fitting to MGBs and MGBV8s. As you will see, the 185/70 combination makes an acceptable substitution for the standard 165 tyre fitted to the MGB 1800 and was offered on the optional alloy wheels fitted to the later MGB 1800s, but it makes a less acceptable alternative for the 175 fitted as standard to the MGBGTV8.

How far can you go?

Most tyre retailers suggest a 195/70 for the MGBGTV8 and this is really the widest tyre you could hope to get away with using the standard 5 inch rims fitted to the MGBGTV8. The 205/70 tyre is even closer to the overall rolling diameter of the standard MGBGTV8 wheel and tyre combination, but it is too wide for the standard 5 inch rim and would cause other problems - fouling wheel arches and more fundamental steering geometry problems which are outside the scope of this note. Any tyre which is much wider than the standard width may potentially give rise to problems with fouling the bodywork, but this is less of problem with the MGBGTV8 (and also some of the later black bumpered MGB1800s) because of the increased ride height compared with the chrome bumpered MGB 1800. If you consider changing the wheel (to increase the rim width) one possibility is to invest in a set of Reliant Scimitar Dunlop wheels, which are the same basic pattern as the standard MGBGTV8 (alloy centres with steel rims) but a different rim configuration and offset.

I can see little point in going much further than that for an otherwise standard MGBGTV8 set up for road use, but racing MGBV8s are something else entirely!

Scuttle shake myth with 185/70 tyres
One story which has been bandied about is that fitting 185/70 tyres instead of the standard 175 tyre will increase scuttle shake (particularly with MGB 1800s) because either the structure of the car is not up to it or the suspension is not up to it. It is often suggested that the wider tyre is just too heavy, citing the alloy wheels fitted as an option on the later MGB 1800s as justification. I checked up on this and the tyre experts I contacted confirmed my belief that this is basically nonsense: a wider tyre may show up a poorly maintained front suspension but the difference in overall weight is really not that great. The principal difference is between a 165 and a 185 tyre which is equivalent to a ring of tyre tread 20mm wide!

I fitted Avon Turbosteel 185/70 to my MGBGT 1800 and can thoroughly recommend them as an improvement over the good 165 Michelins I had before. Turbo steels are fitted as standard in "175HR14" form to certain Rover SD1 models and I intend to fit some to my MGBGTV8 when the next tyre change is due. An important point for MGB 1800 enthusiasts made to me by a tyre distributor I contacted was that whereas the 185/70-14 and 175-14 sizes are still a reasonably popular fitment by car manufacturers, the 165-14 tyre is becoming much less common, except on some Peugeots and this may make the tyres scarcer at tyre retailers in years to come.

Comparison of tyre dimensions

All the dimensions above are for a 14 inch (355.6mm) diameter wheel and the dimensions in the table are in millimetres.