21/08/2020
Forty years ago, Jaguar’s then new XJ6 was regarded highly by the reviewers of the day and by buyers. It was a combination of luxury, speed, power, and superb ride quality. It would gently waft passengers along and absorb all but the meanest of road intrusions. The suspension was beautifully tuned and tuned to work on 15 inch diameter wheels and 60 series profile tyres.
It was that small yet potent wheel and tyre combination that leant itself to why the XJ6 was so highly regarded.
There is a point where the sidewall of a tyre, the section that runs from the outer diameter of a wheel to the section that grips the surface, no longer actually contributes to the ride quality due to the size of the wheel it’s wrapping.
Consider these two sets of numbers: 205/65/15 and 295/30/20. 205 is the width, the 65 is the percentage of width the tyre has for the sidewall, and the 15 is the wheel diameter. Ditto for the second set, however it’s clear that the 30 is, as described by some writers, a “licorice strip”. It’s thin in relation to the width therefore the absorbency factor of the tyre (leaving out the suspension componentry) will be far less than a 65 profile tyre.
Naturally, a brand’s engineers and designers will work together for the specifics of a particular model. A Jeep Compass, for example, will have 18 inch wheels on the entry level Night Eacle, and 19 inches on the top of the ladder Trailhawk. But the tyres will be of different profiles to provide the same “rolling diameter” and the ride quality will be slightly different due to the shorter height of the sidewall.
Drive something large, like a Grand Cherokee, and immediately the ride quality difference is noticeable.Not only will it ride on larger diameter wheels, the profile is much shorter. The SRT version has Pirelli P-Zero 295/4520 rubber, so less absorption.
And here is another reason why tyre choice can affect ride quality.Pirelli are renowned for performance tyres, so a P-Zero’s construction will be different to a tyre intended for a a family sedan. The rubber will have a different formulation, there’s more bracing in the sidewall to minimise flex and folding, so the ride quality of a similarly sized tyre not intended for a sporting application will be softer and more supple as a result.
The Garage Miami is your one-stop-shop for information about engines, services, wheels, and tyres. Give us a call on 07 5572 1711 so we can answer your questions.
