different profile tyres front and rear

colino

Again, anything outwith + or - 2.5% is a no-no, but then again some people will advocate putting bargain basement parts on their cars just to keep them chugging along, but in the next breath recommend 4 wheel laser wheel alignment for the same jalopies. (I'm sure you will know the difference the wrong rolling radius tyres will make to the caster angle).

Spicy_McHaggis

Joe_Horner wrote: »

If something happened that wasn't the OPs fault (other car hits him) then it wouldn't be a factor at all.

If something happened that was the OP's fault, or partly, then they might want to reduce payout either by any difference in premium that they would have charged, or in proportion to any increased premium. So, for example, if telling them about the tyres would have added £100 to his £500 premium then, on a fault claim for £1200, they might try to settle for £1000.

A dummy quote, or phoning his insurers for a quote based on "any mods? yes, non-standard rear tyres" should give an idea of whether there'd be a premium hike. The chances are that, if he actually spoke to his insurer, there'd be no change in the premium because the change in the tyres won't increase risk in any meaningful way.

Only his insurers can say for certain though.

I don't see this as a mod or non standard part, it's a matter that the wrong size tyres are fitted. If both axels had the same profile on the same size rim I can't see an issue. But this is a case on one set of wheels being bigger than the other.

In all honesty if you asked a bog standard insurance company call centre, would I be covered. It wouldn't surprise me if the answer was no.

Joe_Horner

colino wrote: »

[...](I'm sure you will know the difference the wrong rolling radius tyres will make to the caster angle).

Since the OP has these fitted on the backs, which don't have caster, absolutely none.

I'd also love to know where you got this "over 2.5% is a no-no" from. On whichever wheels the speedo works off - usually, but not always, the driven ones - that's a good rule of thumb, but only to ensure that the speedo remains reasonably accurate. You will not find any difference in handling for that sort of change!

Iceweasel

colino wrote: »

Again, anything outwith + or - 2.5% is a no-no, but then again some people will advocate putting bargain basement parts on their cars just to keep them chugging along, but in the next breath recommend 4 wheel laser wheel alignment for the same jalopies. (I'm sure you will know the difference the wrong rolling radius tyres will make to the caster angle).

The OP's rear tyres are around 3.4% bigger than the fronts.

I seem to be getting a roasting here for being precise, picky or pedantic. - as opposed to those who suggest that everything will be fine, don't worry about it.

I felt it was right to inform the OP that there could be an insurance implication due to the mis-match - and the possible technical reasons why.

If the OP gets written confirmation from his insurance company that fitting those size of tyres on the rear axle, with the standard ones on the front, is perfectly acceptable, then everything will be peachy dandy.

For those who have never had to go through weeks of discussion with an assessor about 'undisclosed modifications' then all I have to say is 'Ignorance is Bliss'.

Every single company asks that you inform them of modifications - but some folks don't seem to appreciate what an assessor might deem to be a modification.

Better to tell them and find out its OK rather than just assume because it's a 'small' modification it doesn't matter.

I'm no 'doom and gloom' merchant - I'm a realist.

Been there, done it, and didn't get a T-shirt.

Joe_Horner

Spicy_McHaggis wrote: »

I don't see this as a mod or non standard part, it's a matter that the wrong size tyres are fitted. If both axels had the same profile on the same size rim I can't see an issue. But this is a case on one set of wheels being bigger than the other.

In all honesty if you asked a bog standard insurance company call centre, would I be covered. It wouldn't surprise me if the answer was no.

plenty of cars have different sizes - including diameters - between front and rear axles. It used to be a stock-in-trade for the modifying brigade before then decided that taking your exhaust off over speed bumps was somehow more cool. The only technical disadvantage is that you either carry two spares or can only fix punctures on one axle.

Spicy_McHaggis

Joe_Horner wrote: »

plenty of cars have different sizes - including diameters - between front and rear axles. It used to be a stock-in-trade for the modifying brigade before then decided that taking your exhaust off over speed bumps was somehow more cool. The only technical disadvantage is that you either carry two spares or can only fix punctures on one axle.

It's still used quite a lot in racing:

So it's ok for me to ignor the speed ratings too, as long as it's 70mph I'm ok?

colino

I'm not sure what is funnier, thinking the 2.5% variability parameter is to do with speedo inaccuracy or that having different size wheels on the back doesn't affect the caster angle.
They are right, never try and teach pigs to sing.

Joe_Horner

colino wrote: »

I'm not sure what is funnier, thinking the 2.5% variability parameter is to do with speedo inaccuracy or that having different size wheels on the back doesn't affect the caster angle.
They are right, never try and teach pigs to sing.

Ok, please do enlighten how raising the rear hub centre by 10mm (which is the only direct effect the bigger tyres will have) will affect the front suspension geometry to any meaningful extent?

I only ask because there is no caster at the back - camber, yes, caster, no. So you must be talking about changes to the front from that massive 0.23 degrees (assuming a 2.5m wheel base) change in attitude.

I'm always happy to learn.....

Joe_Horner

Spicy_McHaggis wrote: »

So it's ok for me to ignor the speed ratings too, as long as it's 70mph I'm ok?

From a safety perspective, and assuming you're going to keep reasonably to the speed limits, yes. You've been very hard pressed to find a tyre for general use that's less than S rated (113mph) for the past 30 years or so - even most remoulds are S rated now - and that rating will take legal UK speeds "plus a bit" all day, every day, regardless of what you've fitted it to.

In sizes used by more exotic cars, you'll probably struggle to find anything rated less than H (130mph) and if you're regularly driving round above that there's a good chance you won't have your licence long enough for the tyres to break.

colino

Joe, poor old Joe, you really have no clue on engineering at all. For people who may be following you, imagine for example, the extremes of jacking up cars in the 70s and 80s, arrived at by extensions on the cart springs on the rear, shoving cars rear ends up in the air.
Any clue how this extreme change to, amongst other effects, screwed up the dynamics of the chassis?
No, you probably don't.
Keep on giving, "advice" to people online who will take the lowest cost option, but remind yourself that googling answers leaves you none the wiser.