A wheely big problem (mixing wheel/tyre sizes).  
A wheely big problem (mixing wheel/tyre sizes). - FotheringtonThomas
A tyre is near the legal limit. It's 14x60x195. I have a near-unworn, in-date, 13x?x175 tyre on a rim with the same PCD. If I were to run it on the back, opposite a 14x60x195, what issues might I noticees?

Edited by FotheringtonThomas on 24/07/2008 at 23:28

Tags: legal wheels and tyres used cars spare wheels

A wheely big problem (mixing wheel/tyre sizes). - Screwloose

Apart from the MOT fail sheet and the £60/3 points penalty notice?

Broadly speaking; the C+U says that tyres on the same axle should be the same size and construction.
A wheely big problem (mixing wheel/tyre sizes). - Manatee
I asume FT has no real intention of doing this but is wondering whether the car will want to go round in circles or spin at roundabouts.

Based on the fact that many manufacturers now issue pram wheels as spares, maybe it won't.

The offset might be a wild card though, even the undersized spares must run in the track of the standard wheel. And will the hub and brake fit inside the 13" rim?
A wheely big problem (mixing wheel/tyre sizes). - FotheringtonThomas
I asume FT has no real intention of doing this


I was seriously wondering about it - to be used for a week or so.

The rear wheels are not driven wheels. They carry less load than the front. The size difference is not great. The tyre is of the same type as the opposite one. This would not be driven hard or fast.

On the 'net, there appears to be no good explanation of exactly what problem there might be, apart from "Oh it's illegal" or "Oh I don't like the idea" or "Oh, you shouldn't do that", si I wondered what the explanation would be, given the above circumstances!
A wheely big problem (mixing wheel/tyre sizes). - Number_Cruncher
Ring your insurers, and ask them if the car is still fully insured after your proposed modification - I'm sure they could do with a laugh.

A wheely big problem (mixing wheel/tyre sizes). - FotheringtonThomas
Humph. So no-one has an explanation of exactly what problem is.
A wheely big problem (mixing wheel/tyre sizes). - Number_Cruncher
Humph.


Talk to your insurer.

It doesn't matter a hoot what anyone on here says about your proposed folly. What the insurers say counts.

Edited by Number_Cruncher on 25/07/2008 at 09:40

A wheely big problem (mixing wheel/tyre sizes). - spikeyhead {p}
It's possible that it will upset the handling though it's unlikely that most people would notice in every day driving.

Have an accident and it could well be down to you to prove that a car that you deliberately set up in a way that you knew to be illegal was set up in a way that didn't materially effect the handling. Never ever drive without insurance.
A wheely big problem (mixing wheel/tyre sizes). - mjm
Ignoring the legal/insurance angle. From the information given there is every possibility that the rolling radius of the 13inch wheel is different to that of the 14inch by a noticeable amount. The ?small? wheel is going to rotate at a higher rate than the other. The car may also have a slight list to the side with the small wheel. In all honesty, trundling along a quiet road at a sensibly low speed, I don?t think that you would really notice anything. Cornering, again at a sensibly low speed, I don?t think would be any problem. Braking would be interesting. Because one wheel is rotating faster than the other, it will receive more braking effort per given distance than the other. But, it?s smaller in diameter than the other so the braking force applied to the tyre contact area will be lower (I think). This may mitigate the speed difference. Locking the rear wheel(s) is a sure way to lose directional stability. If, and I repeat if, I had somewhere quiet to try it out, I would be tempted to, just to see what happened.
A wheely big problem (mixing wheel/tyre sizes). - FotheringtonThomas
This is more like the sort of discussion I'd envisaged - however, I'm unsure about the braking effect (ABS involved, too). It crossed my mind that the issue might be the same as when cross-ply and radial tyres are mixed on one axle (who can remember what *that* was??).
A wheely big problem (mixing wheel/tyre sizes). - mjm
You haven't given the aspect ratio of the 13inch tyre. The 14inch is given as 60%. If the 13 is higher, 65/70 then there may not be enough of a rolling radius difference to affect the abs, or indeed non abs braking. (I had forgotten that a Cavalier of that age had it.)

From memory cross plies and radials run at much different slip angles anw would cause some interesting moments. Also from memory cross plies lose grip(in cornering) much earlier than radials. (I suppose you would have to remember which side had the cross ply and react accordingly :))

Cross plies did tend to break away at a lower level of grip than radials but did so more progressively than radials, giving a bit more time to react. I do look at some modern stuff with big wide radial rubber bands for tyres and think that if they ever did lose grip in cornering then without some sort of "stability" electronics, the nearest wall/ditch etc "has" to be the final resting place.
A wheely big problem (mixing wheel/tyre sizes). - Number_Cruncher
Owing to the larger slip angles, cross plies on the rear while radials are fitted to the front is both an MOT fail and illegal.

A wheely big problem (mixing wheel/tyre sizes). - bathtub tom
Just to muddy the water a bit:

My Almera runs on (factory fitted) alloys with 195x55x16 rubber.

The steel spare is fitted with 185x65x15.

I believe they're almost the same rolling radius.

Is NC saying it'd be illegal if I ran on the spare?
A wheely big problem (mixing wheel/tyre sizes). - movilogo
195x55R16 = 16*25.4+2*195*0.55 = 620.9 mm diameter
185x65R15 = 15*25.4+2*185*0.65 = 621.5 mm diameter

Difference is only 0.6 mm which is negligible. So, as long as radii of both wheels are same, it should be fine.

If there is any problem, it must be mentioned in car's manual. They usually contain what sizes you might use.


A wheely big problem (mixing wheel/tyre sizes). - Mapmaker
Well, a spacesaver spare is often a completely different radius wheel - if not tyre. And it is illegal to drive on a spacesaver spare - further than the nearest tyre repair place.

On the other hand, the last I heard was that the police would never prosecute you for driving on a spacesaver wheel, as there is some European law issue that means nobody is certain whether a prosecution would actual work.

Now, you're not supposed to drive further than 50 miles? on a spacesaver, nor at over 50? mph.

I guess that because the wheels will have completely different moments of inertia, you will end up with a setup that is very unstable as the axle will not be symmetrical. So probably OK up to 40mph. At 70, the car would shake.

Surely not worth the risk for the £10 that a 90% worn second hand tyre would cost you. Put it on the back, no need to balance it (and no policeman will pull you for an unbalanced wheel...)
A wheely big problem (mixing wheel/tyre sizes). - Number_Cruncher
>>different moments of inertia, you will end up with a setup that is very unstable as the axle will not be symmetrical. So probably OK up to 40mph. At 70, the car would shake.

No. Somewhere along the line, you're getting *very* confused about moments of inertia and wheel balancing/shaking.

You're right about the spacesavers being very restricted in their use.

I know that a car won't pass an MOT with odd sized wheels/tyres (there is one very specific exception, but that doesn't apply here).

What I don't know (but can guess!) is how the OPs proposed modification will be viewed by his insurers.



A wheely big problem (mixing wheel/tyre sizes). - FotheringtonThomas
it is illegal to drive on a spacesaver spare - further than the nearest
tyre repair place.


I have never heard of this. I imagine it is "illegal" to drive on a "space saver" tyre *at all*, if it's a different size from the others.
A wheely big problem (mixing wheel/tyre sizes). - jbif
I imagine it is "illegal" to drive on a "space saver" tyre *at all*,


Some previous discussion here -
www.honestjohn.co.uk/forum/post/index.htm?t=14118&...f
www.honestjohn.co.uk/forum/post/index.htm?t=40629&...f

The Taxi trade is governed by quite strict rules on Construction and Use, and their Licences do specify exactly how space savers are to be treated. But different councils apply different rules, eg:

www.plymouth.gov.uk/hackey_carriage_policy.pdf
"If the spare is a half-size or space-saver wheel, then this type of wheel must not be used for the carriage of passengers unless it is to complete a journey already undertaken when the original wheel was replaced ...
If a vehicle is equipped with a space saver spare tyre, it may only be used to get the vehicle to a location for the tyre to be changed for a full sized tyre. It is not permissible to carry passengers using a space saver tyre ..

www.ncdc.gov.uk/media/adobe/n/o/TAXI_LICENSING_CON...c
(ii) Operators to only use space-saver wheels strictly in accordance with the manufacturer?s recommendations.
(iii) Operators having to use the space-saver wheel must go directly to the nearest garage to have the puncture repaired, or a new tyre fitted.
(iv) Fare paying passengers must not be picked up whilst any licensed vehicle has a space-saver wheel fitted.

www.middevon.gov.uk/media/word/4/7/Adopted_Taxi_Po...c
" Space saver tyres are only approved if accompanied by a method statement highlighting driver responsibilities with regard to the maximum permitted speed and the temporary ?get you home? use of space saver tyres.."


committee-web.gedling.gov.uk/aksgedling/images/att...c
"..the space saver is limited to use in an emergency situation only at a maximum speed of 50 mph .... Distance and mileage the space saver is used for is to comply with manufacturers instructions"
A wheely big problem (mixing wheel/tyre sizes). - yorkiebar
Absosloutely do not do it.

However, if you were stuck off a highway and it was the only way you could move the car you would do it without a thought I would think.

Doubt if it would make any real difference at low speeds, gentle driving and braking.

But the roads have other hazards (other people generally) so its not the area to try it and find out.

Please dont. If times are hard, obtain a 2nd hand tyre for a £10 note .

And before anybody complains about a 2nd hand tyre (not something I would generally suggest), do you change tyres when you buy a car? They are 2nd hand tyres too !
A wheely big problem (mixing wheel/tyre sizes). - Number_Cruncher
>>They are 2nd hand tyres too !

Indeed. I do tend to give them a very good looking over though!

A wheely big problem (mixing wheel/tyre sizes). - davidh
Nothing wrong with second hand tyres in principle - its the vendors you've got to watch!

I got a puncture once, thought I'd get it repaired only to be told the hole was too near the side wall to be legally repaired (I checked the rules later to see wether the tyre an was right) Oh dear thought I, I'll go and get a second hand one.

Got a second hand tyre, leaving my original unrepairable tyre with the secondhand dealer. Chuffed with my saving I went back a couple of months later to get another one only to find my old "unrepairable" tyre back on sale!

Always always check for signs of repair - I have been to loads of secondhand tyre places and they were ALL selling repaired dodgy tyres including some that had repairs to tha actual sidewall. These were like a vulcanised type repair and not a rubber plug.

On reflection I should have dobbed them in.

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