Tyres

Yes it is. According to the MoT tester's manual, reasons for rejection are:

• different types of tyre on the same axle

• crossply or bias-belted tyres on rear axle and radial tyres on front axle

Not only because the MOT manual says so. X-plys on the back and Radials on the front is also dangerous.

Alex

Thanks for the assistance - she failed anyway on a worn swivel... Back in for Retest on Monday.

Any idea why? In a skid or something?

nigelH

Any car will be unpredictable with a mixture of tyre types, but that doesn't explain why one combination is allowed for the MoT and the other is not. My guess is this: radials grip better than x-plies. Therefore, a car with radials on the front and x-plies on the back will tend to oversteer in a corner and be liable to spin right round in an emergency stop, whereas a car with radials on the back and x-plies on the front will tend to understeer in a corner and remain relatively stable under heavy braking. But it's only a guess. Anyone know better?

"Tyre Types

It is illegal to mix tyres of a different construction (cross-ply; bias belted or radial) on the same axle. Cross-ply and bias-belted tyres are seldom used on production cars, and are not widely available in the UK. Cross-ply and radial tyres should never be mixed on the same vehicle. Where a mix is necessary, radial tyres MUST only ever be used on the rear axle and cross-ply tyres on the front. This mix of tyres will produce 'over steer' whereas the opposite will produce 'under-steer'. (Over-steer refers to the car turning more tightly into a corner than it is steered; under-steer indicates that the vehicle turns at less of an angle than it is steered). Of the two conditions, over-steer is generally accepted to be easier to control. "

Taken from

I suspect this is also why manufacturers recommend keeping the best (i.e. least worn) tyres on the rear axle.

Now I've read that properly, they must surely mean it the other way around?

Thanks both. nigelH

Surely they must. I thought the opposite effect would be true, i.e. radials on the back would make a car understeer due to the greater grip at the back end. I'm prepared to be corrected on that one. But to say that "over-steer is generally accepted to be easier to control" is total bollocks. It is if you are Michael Schumacher [1]. But for the average driver, a mild understeer is much the safer setup. If you overcook it into a corner, it will gently run wide until you ease off. If you get it badly wrong, you are still facing forward. An oversteering car will need opposite lock and a delicate right foot, and has a tendency to bite back hard if you get it wrong. This is why almost all "family" cars are designed with slight understeer (and it's a characteristic of FWD as well, which is convenient).

If this is what ROSPA genuinely think, rather than just a copyist's error, then they are a duller lot of arses than I gave them credit for.

[1] Top Gear, and MS track testing the new Ford Mustang (I think). Round a long bend at over the ton, opposite lock all the way, controlled with one hand on the wheel while looking over his shoulder and talking to the cameraman in the back seat. I'm not a big fan of Schumi, but it was an awesome bit of driving.

If we are talking about "involuntary" breaking out, then I don't agree.

Back end breaks out - car sideways, skill and experience required to correct, danger of sideways slide into bus queue, risk of rollover if car hits a kerb or ditch.

Front end breaks out - car facing forward, natural tendency to tighten steering and ease off usually corrects problem, and if the worst happens, you hit a wall with the bit of the car that was designed to handle it - crumple zones, airbags etc.

What did Mercedes do when the new A-class kept failing the "moose test"? Designed in a little mild understeer.

If we are talking about doing this on purpose, then I agree entirely. One of my favourite cars was a 2.8 Capri (RWD, LSD*), which went in every way but a straight line in the wet. Its favourite direction was sideways.

*The transmission, not the reality-enhancer.

Always better to have the back end break out, than the front.

I like that bit about "Cross-ply and bias-belted tyres are seldom used on production cars, and are not widely available in the UK"

Unless of course you have a landrover, in which case X-Ply tyres are commonly available.

Alex

Richard Brookman vaguely muttered something like ...

Heheheh, had a Triumph Dolomite that also liked it sideways a tad .. ;)

Very good manners though, and very, very easy to control ..

In news: snipped-for-privacy@4ax.com, Alex blithered:

If the back end goes it is probably recoverable if the front end you're gone!

I think it's the other way around. People are so used to FWD cars which understeer that they no longer know how to handle (RWD) cars which oversteer.

With oversteer it is possible (with the correct skills) to recover the situation, wheras if you are in an understeer position, and the car is understeering into a brick wall at the side of the road, there is absolutly nothing that can be done. You have to back off and hope that the car will return to the desired line.

Just because people are not capable of handling oversteer doesn't mean to say it's better to have understeer. But then I think that skid-pan training and motorway driving are just some of the things should also be included in the basic driving test, driving standards are far too low these days.

Alex

Yebbut yebbut, that's the whole point. Cars are generally built for the mass market, and the mass-market driver CBA to learn the skills necessary to drive an oversteering car in safety. Understeer is usually gradual and predictable, whereas oversteer usually happens quickly and is pretty disastrous if allowed to develop beyond a mild tail-out posture. Hence the mega-millions of small family hatches with FWD that will push out on a corner if driven hard, but which react predictably and safely to the driver's natural reaction to tighten up and back off. Remember the days of the big old Ford Zephyrs and Austin Cambridges that would spin 180 deg and carry on backwards at the first sign of hard braking in the wet? I even managed it in a Morris Minor once, *and* I was expecting it! Modern cars are much safer, even if less "fun".

I agree about the training - we let people onto the roads with only the most basic skills. Given the number of accidents caused by aggression and/or impatience (as opposed to lack of skill), psychometric testing might be a useful addition too. To be fair, though, the roads are for everyone, not just the elite, and we have to accept that some compromises have to be made.