Mixing tyre sizes
Posted By : Ian Palmerino, 18-Mar-2005, 07:06pm
David
As far as i know you have to have the same size tyres on the same axle, im sure that was the law when i was a tyre fitter in my youth, but stand to be corrected.
Ian
As far as i know you have to have the same size tyres on the same axle, im sure that was the law when i was a tyre fitter in my youth, but stand to be corrected.
Ian
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Mixing tyre sizes
Posted By : Colin Lindsay, 19-Mar-2005, 08:58pm
I would have said that as with the crossply / radial argument the tyres must be the same size on the same axle, so that while you can have different widths between front and rear axles each pair of front and rear tyres must be the same size. Going by 'Wilkinson's Road Traffic Offences (sorry, it's the only volume I currently have to hand!!) the rules for motor vehicles also apply to light trailers. Regulation 27(1)
of the Road Traffic Act prohibits the mixing of types of tyre on the same axle and even restricts what tyres you can have on
different axles.
I would recommend that for safety reasons you should avoid mixing tyres on the same axle. This does raise the question of space saving spare tyres as fitted to modern cars which are half the width of a normal tyre, since they are designed to fit opposite a normal sized tyre on the same axle and therefore must be within the law up to a point - however the use of these is heavily restricted and they are more of a 'get you home' type of fix rather than a long term replacement. It's not so much the law, it's whether or not your Insurance company will pay out in the event of an accident, and we all know how they look for the smallest excuse in order to avoid any payments!!
Mixing tyre sizes
Posted By : Mike Crewes, 20-Mar-2005, 06:07pm
David
Sorry it's a bit late now, but having tyres of different sizes BUT the SAME construction on the same axle is both perfectly legal and perfectly safe, provided you don't change the size from one extreme to another.
There are very good reasons of physics why this is the case and as a collision investigator I could go on for hours, but I have already written several articles about this (http://www.tssc.org.uk/copshop).
Although all the above is true, a vehicle so fitted will fail the MOT Test, but don't ask me why!
Space saver wheels and tyres have their own set of regs which must be adhered to when they are used. They are usually printed on the wheel, which must be a different colour from the rest.
Hope the trip went OK.
Regards
Mike
Sorry it's a bit late now, but having tyres of different sizes BUT the SAME construction on the same axle is both perfectly legal and perfectly safe, provided you don't change the size from one extreme to another.
There are very good reasons of physics why this is the case and as a collision investigator I could go on for hours, but I have already written several articles about this (http://www.tssc.org.uk/copshop).
Although all the above is true, a vehicle so fitted will fail the MOT Test, but don't ask me why!
Space saver wheels and tyres have their own set of regs which must be adhered to when they are used. They are usually printed on the wheel, which must be a different colour from the rest.
Hope the trip went OK.
Regards
Mike
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Mixing tyre sizes
Posted By : Michael Davis, 22-Mar-2005, 10:47am
An interesting point that I've been trying to sort out myself. The local tyre place was no help on the matter either.
My van is fitted with 235/75 tyres but the spare is a 205/65. By my calculations (well, James's actually - http://www.mintylamb.co.uk/?page=tyre.htm) that's a difference of well over 3 inches in diameter. My first guess would be that that sort of a difference could well alter the balance of the vehicle.
Hmm, maybe something else to sort out once I get my van back on the road...-
Mixing tyre sizes
Posted By : Mike Crewes, 22-Mar-2005, 05:55pm
Michael
This equates to 43 mm tyre radius which is even less on rolling radius (due to tyre flattening at the bottom). Whether, or not this will upset the balance of your vehicle depends on the suspension set up of your van and how it reacts. One thing to note, however, is that if these two tyre sizes are fitted to the drive axle they will rotate at different speeds and make your diff work much harder.
The tyre tread itself, however, will provide the same amount of grip even though it is of a different size. This is because the weight of the vehicle remains the same.
This may be outside the parameters of safe tyre use, but then again ... I can see why your local tyre fitters were unable to help.
Regards
Mike-
Mixing tyre sizes
Posted By : Michael Davis, 22-Mar-2005, 07:07pm
Fair enough. Thanks.
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