Tyres

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Post by f00lzz »

jeremy wrote:Many cars including BX 16valve and some ZX including the mighty 1.9D have odd sized spares - which are of course only fitted to one wheel on an axle - with the proviso of being careful (BX 16 valve - standard - 195/60 x 14 - spare 165 x 14)

The Citroen CX in its early form had narrower back tyres than front!

Circumfrence of 185/65 x 14 tyre is approx 1810.6mm
Circumfrence of 185/60 x 14 tyre is approx 1752.45mm

Difference - approx 3.3%
Early Xants had an 'odd' size spare!
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Post by Kowalski »

Peter.N. wrote:I think safty is primarly down to the way you drive. If you push a car to its limits, anything out of spec is going to show up, driven sensibly, I dont think it is going to make a lot of difference. I would be quite happy on cross plies!
I don't think you would. Firstly the sidewalls aren't as flexible so the ride wouldn't be so good and secondly you'd find that during an emergency stop the grip wasn't quite so good as on your radials had been.
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Post by dnsey »

Wasn't the DS one of the first cars to use radials? AFAIK, they were an integral part of the suspension design.
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Post by f00lzz »

Kowalski wrote:
Peter.N. wrote:I think safty is primarly down to the way you drive. If you push a car to its limits, anything out of spec is going to show up, driven sensibly, I dont think it is going to make a lot of difference. I would be quite happy on cross plies!
I don't think you would. Firstly the sidewalls aren't as flexible so the ride wouldn't be so good and secondly you'd find that during an emergency stop the grip wasn't quite so good as on your radials had been.
Rolls Royce stuck with cross ply tyres for years after everyone else was on radial ply tyres because they were a more comfortable ride
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Post by handyman »

The reason RR ran old cross ply tyres for years was because the wall stiffness was greater than radial tyres. They needed the support to let the great hulking brutes get round corners. Early Shadows fitted with radial tyres on narrow, tubed, non-safety rims used to wallow mercilessly on cornering and suffer sidewall failures quite regularly, and that was factory specified!
RR was not known for its embrassing of modern or innovative technology otherwise it would have got around to using Citroens far superior suspension systems much earlier. Why did it take RR and Merc so long to buy Citroens systems under licence?

As regards non-standard tyres and smaller spare wheels, the spare wheels are rated under C&U Regs for temporary, limited range and speed use. Check your drivers hand book.

Fitting tyres with different ride characteristics, ie size, aspect ratios, weight loading, etc. will invalidate your insurance as they are non-compliant. They could even baulk on paying a claim if the tyres were not the same OE manufacturer. Read your policy.

I have a little experience of tyre manufacture for both road and off-road vehicles.
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Post by bxbodger »

Fitting tyres with different ride characteristics, ie size, aspect ratios, weight loading, etc. will invalidate your insurance as they are non-compliant. They could even baulk on paying a claim if the tyres were not the same OE manufacturer. Read your policy.
Isn't that a bit unlikely? They may not pay out if it turns out your tyre's speed rating was too low, but not o/e??

The o/e spec for my Triumph was either ye olde Goodyear G800 or Dunlop SP68-neither of which has been available for years!
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Post by lolingram »

Methinks your memory has become dulled over time... crossplies are bloody awful! My first recollections of Michelin X tyres on a Lotus 7 are still sharp after 45 years.
I think safty is primarly down to the way you drive. If you push a car to its limits, anything out of spec is going to show up, driven sensibly, I dont think it is going to make a lot of difference. I would be quite happy on cross plies!
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Post by fourwheelsteer »

I'd like to try a car on crossplies; but only on a car designed for crossplies. I imagine a CItroen on crossplies would be an absolute horror but an MGB might be quite nice.

As for the ride issue, I thought it was the radial belt supporting the tread that caused the harsh ride. On crossplies the underlying plies could move relative to each other (generating lots of heat in the process). The radial belt is practically inextensible so any distortions have to be accomodated by the rest of the tyre. I may have got that completely wrong but it is what I remember from reading articles about tyre characteristics in 1960s car magazines.
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Post by f00lzz »

fourwheelsteer wrote:I'd like to try a car on crossplies; but only on a car designed for crossplies. I imagine a CItroen on crossplies would be an absolute horror but an MGB might be quite nice.

As for the ride issue, I thought it was the radial belt supporting the tread that caused the harsh ride. On crossplies the underlying plies could move relative to each other (generating lots of heat in the process). The radial belt is practically inextensible so any distortions have to be accomodated by the rest of the tyre. I may have got that completely wrong but it is what I remember from reading articles about tyre characteristics in 1960s car magazines.
Contrary to some of the previous posts... you are correct!
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Post by jeremy »

Fourwheelsteer - yes you're right - so what is 'suspension designed for radials?'

There are a few differences - as you say the radial has an inextensible belt running under the thread - and this can cause shocks when it hits ruts running across the road (or a sharp pothole for that matter. Textile belted tyres were the worst and Michelins with steel belyts were the best as the steel would give more than textile. Decent cars have rubber in the suspension which absorbs some of this force - many did not.

You will have noticed that if you park a radial shod wheel on a kerb the tread stays flat - do the same with a crossply and it deforms round the kerb. A crossply will deform on ruts running down the road in the direction of travel - and so tend to wander. (Reason is smaller rolling radius on the 'high' side of the rut - tends to run up the rut) They were particular fun on catseyes - and at times would be quite difficult to get off them while overtaking.

The radial runs flat and so these effects are barely noticeable. - but this leads to another problem which is the angle of the tread and the road. The geometry of some suspension systems does not do a very good job of keeping the wheel vertical to the road - and whereas a crossply will deform the radial will grip - until the tread starts to lift off the road due to the angle!

Most fun is to mix them in the approved combination - radials on the rear - crossply on the front - and so you have the wonderful self steering properties of crossplys on the front and the straight running radials on the rear - and the car wanders everywhere!
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Post by Kowalski »

The big differnece between radial and cross ply tyres is that on a radial, the sidewall and the tread have vastly different amounts of reinforcement in them. That means that with a cross ply tyre your sidewall is as stiff as the tread but with radial tyres the tread is stiff but the walls can be made very thin, the stiff tread makes it resistant to twisting, but it can flex along its length to accomodate bumps. When radials were novel they had much thicker sidewalls than they do today, a modern radial is a much more comfortable tyre than a radial of 20 or 30 years ago when the comparison was made with cross ply tyres. Bear in mind that radials these days are tubeless and that was not the case when they first arrived on the scene.
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