Are run flat tyres a false economy...?

harveybobbles

I've noticed that working on cars with RFT's fitted that, while the tread in the centre of the tyre is still at 5/6mm, the shoulders of the tyre are down to 2mm or sometimes smooth.

This is obviosuly down to the thicker, reinforced side walls, which yes, are a good idea for the LARGE percentage of people who don't know how to change a wheel when faced with a puncture.

But I can't help thinking that this tyre with 6mm on is now scrap. where as a normal tyre with 6mm on would last another 2 years or more.

The main culprit I've noticed is the 3 Series BMW with 205 55 16's...

Strider590

They're also heavier, thus increasing the rolling weight and decreasing fuel efficiency.

My opinion is and always has been that if you don't know how to change a wheel, you shouldn't be driving in the first place.... So the only benefit of run flat tyre's should be to help keep the car under control in the event of a blow out.

System

Strider590 wrote: »

They're also heavier, thus increasing the rolling weight and decreasing fuel efficiency.

Carrying a spare wheel and tyre adds far more weight than any possible increase of weight for the four RFTs

I would not have expected a RFT to wear unevenly just because of the stiffer side walls, the tyre manufacturers are aware of this fact and design the tyre accordingly. Much more likely that the wrong tyre pressure has been used.

Having said that, I do prefer to carry a full size spare if at all possible. But, a car dealer recently advised me that adding a spare wheel would alter the car's environmental tax banding (CO2 rating) due to the added weight - not sure if I believe this or not ?

Strider590

BAA1 wrote: »

Carrying a spare wheel and tyre adds far more weight than any possible increase of weight for the four RFTs

Notice I said "rolling weight"

Do you know anything of centrifugal forces (G-force)? If not, then hold a house brick (or anything heavy) in one hand and spin around on the spot as fast possible, see how long it take for that brick to become too heavy and fly off into the neighbours cat (hopefully)
And then the other element is how much harder it is to spin a heavier object.
Compare two people, one skinny guy with a bowling ball and one hideously fat guy with a tennis ball.... When ask to spin around for 30 seconds, which one will complete the most revolutions?

Rolling weight has quite a substantial effect on how a car behaves. The faster you try to go the greater the effect.

Kilty_2

Runflats should be an optional extra IMO - making them standard and thus introducing implications if you want to change them for normal tyres sucks.

Personally I'd never bother with them unless they came on the car I was buying as standard and I didn't have a choice.

MX5huggy

BAA1 wrote: »

But, a car dealer recently advised me that adding a spare wheel would alter the car's environmental tax banding (CO2 rating) due to the added weight - not sure if I believe this or not ?

That is rubbish, true the manufacture removes the spare to save weight and therefore get a lower CO2 figure. But adding it back as an optional extra does change the CO2 output figure. Just like adding electric seats adds weight but does not change the CO2 figure.

Skoda charged me £55 for a full sized spare instead of a bottle of goo and a pump, which I recon is less than a new 17" bridgestone tyre would be.

andygb

RFTs are a pain the neck, I used to have them on a 5 series, and in the metric size, which was even more of a pain! They are more expensive, wear out quicker and do not seem ro ride very well on poor road surfaces - most of the UK!

harveybobbles

The problem with having a bottle of gunk/compressor is what happens if you have a blow out? Or slide int oa kerb or run over something sharp and you end up with a chunk missing from your tyre?

You then have to phone and wait for help from someone/recovery firm. Where as with normal tyres, I'd just pull over and pop my spare wheel on and get on with my journey...

System

Strider590 wrote: »

Notice I said "rolling weight"

Do you know anything of centrifugal forces (G-force)?

Yes I do.

Inertia can easily be calculated. It would be interesting to see if the added inertia caused by your idea that RFT tyres are heavier than their none run flat equivalents, would have a greater effect on fuel efficiency than would the added weight of the spare wheel and tyre. I do not know the answer and think the difference could well be negligable.
However, a far more significant impact on fuel efficiency would be the rolling resistance of the tyre, these Rolling Resistance Coefficient figures are available from tyre manufacturers.

neilmcl

harveybobbles wrote: »

The main culprit I've noticed is the 3 Series BMW with 205 55 16's...

That's not the experience I've had. I had a 320D with that size tyre for 4 years and tyre wear was always even across the tyre.

neilmcl

Strider590 wrote: »

They're also heavier, thus increasing the rolling weight and decreasing fuel efficiency.

Strange how the most fuel efficient standard fuel cars tend to be BMWs with RFTs as standard. And as far as not being able to change a wheel is concerrned a lot of these cars don't have space for a spare so that's not really option, in fact the only option would be standard tyres with a mobility kit.