Doctor Diesel

Flexible friends?

Doctor, I often read your words of wisdom in Diesel Car’s Used Car Buyer’s Guides, and your frequent warnings about buying the cars reviewed with large optional wheels and low profile tyres fitted, on the grounds of ride comfort and replacement cost. But is it all as simple as just the dimensions of wheel and tyre diameter, and wheel profile ñ i.e. how much yield there is in the sidewall? I’m sure that I’ve driven, or been driven in, comfortable cars on low profile tyres, and in uncomfortable ones on more conventional tyres. Isn’t it also very much about how stiff the road springs fitted are, and how strong the damping effect of the shock absorbers is? Are not today’s variable damping control systems available on many cars capable of offering ìthe best of both worldsî ñ sharp and precise handling when you want it, with the ësportí setting, and a soft ride with less precise handling, when you want a comfortable ride, perhaps when Grandpa and Granny are being taken out for the day? 


Peter Newhouse


I fear that you may have opened the proverbial can of worms Peter, but I certainly do respect your observations.


The picture is far from simple and, as you say, there are indeed comfortable low-profile tyres, and I have personally experienced the apparently strange phenomenon on test cars, where one fitted with 18-inch wheels and low profile tyres rides better (in terms of comfort and refinement) than one on 17-inch higher profile tyres. In this scenario, it is rare to encounter a situation where both cars driven are on the same brand and specific type of tyre though, and therefore it is usually impossible to compare like with like. All is never as it seems with tyres, unfortunately, and the tyre manufacturers keep a lot of information about tyre properties to themselves.


Major manufacturers supplying significant numbers of tyres to car manufacturers as OEM (Original Equipment Manufacturer) fitting will in fact often specify their own requirements, in terms of the properties, and therefore the rubber compounds and the carcass properties involved that determine their load index and speed ratings, and other characteristics. You may very well (unknowingly) be supplied tyres with a 225/45/17 94Y MO marking (replacing some 225/45/17 94Vs) that were specified for fitting to a certain new Mercedes-Benz model (hence the MO code), and which has different base properties ñ possibly for a Mercedes-Benz model with a comfort bias ñ from your original tyres. It also has a higher speed rating, which may infer other differences in construction from your 94V tyre. In this case the load index of 94 stays the same at 670kg, but the speed rating rises from 149mph (V) to 186mph (Y) and may involve some other significant differences. So you are not changing like for like! Just don’t ask me why though, in a land where the speed limit is 70mph, we would want to fit tyres with such exotic maximum speed ratings as we get (I guess that it’s to do with the unrestricted speed roads in Europe), or which so wildly exceed the maximum speeds of which our cars may be capable.


But that’s only half the story, because there are huge differences in tyre manufacturer approach to the critical tyre properties, some of which will be determined by tyre wear targets, rolling resistance and thus fuel economy, sharpness of steering response, noise transmission and insulation from road shock, all of which may be interpreted differently, and with the inevitable compromises, both within the ranges of tyres that any manufacturer offers, and between manufacturers.


So you are correct, and it should really never be stated quite so specifically, as I’m sure that I often do, that any 235/40/17 tyre will ride more comfortably than a 235/40/18, or less comfortably than a 205/55/16! But my observations regarding higher replacement cost certainly are. Tyre manufacturers just love to give car manufacturers good deals on new tyre costs, that then become temptingly priced options for new car buyers, because they get a handsome return when those expensive tyres (which often wear faster, or are more easily damaged, along with the alloys!) are replaced.


To all this, we can then throw in the extra complication of the road properties of run-flat tyres, which necessarily have stiffer sidewalls, to prevent the flat tyre rolling off the wheel, and the whole picture becomes even more complex. Suffice it so say that I’ve been in enough MINIs fitted with 18-inch run-flat tyres to think about catching a bus instead next time, or of booking a dental appointment in advance!


I think you’ll feel that I’ve said enough, but I would certainly love to be able to hit the tyre manufacturers with a Freedom of Information request that would oblige them to tell me more about the properties of the tyres that they make, and perhaps someone, somewhere, could let me know why my tyres are rated for 149mph, and why the total load capacity of them is 3,000kg, when the car tops out at around 112mph and weighs only 1,450kg, with a load capacity of just 600kg? Thanks for asking the question Peter!


Doc Diesel

One Response

  1. IMO a most of the effect is down to unsprung weight and not the tyres..
    A bigger wheel with lower profile tyre weighs more than a smaller wheel with higher profile tyre. The suspension on a certain model of car may well only be tuned perfectly to the weight of one wheel/tyre combination, and using a different one whether smaller or larger will reduce ride quality. Bigger wheels and tyres usually affect acceleration and fuel economy also. Bigger wheels have more mass further from the centre of the wheel, so more energy is required to start it turning (as well as stop it turning). This means slower acceleration and higher fuel consumption.

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