Tyre Questions

[StDunko]

Any tyre experts out there?

I've just bought a car that came with run-flats from the factory, but the previous owner switched to 'standard' tyres. (Pirelli cinturato P7's, with lots of tread, on the front axle, and brand new Michelin pilot super sports on the rear axle. So neither sets will need replacing for a long time).

My questions are:

1. Do I need to be running different pressures in the standard tyres, to the recommended pressures for run flats?

2. The car has no spare wheel. Can anyone recommend an emergency repair kit, so I'm not stranded if I get a puncture? (Ideally one that doesn't ruin the c.£270 tyre, if used).

[Whitey Grandad]

1 hour ago, StDunko said:

Any tyre experts out there?

I've just bought a car that came with run-flats from the factory, but the previous owner switched to 'standard' tyres. (Pirelli cinturato P7's, with lots of tread, on the front axle, and brand new Michelin pilot super sports on the rear axle. So neither sets will need replacing for a long time).

My questions are:

1. Do I need to be running different pressures in the standard tyres, to the recommended pressures for run flats?

2. The car has no spare wheel. Can anyone recommend an emergency repair kit, so I'm not stranded if I get a puncture? (Ideally one that doesn't ruin the c.£270 tyre, if used).

Check with the manufacturers for tyre pressures. The wear patterns on the existing tyres will give you an indication of the correct pressure. Even wear all across is good. More wear in the centre is too hard. More wear on the edges is too low.

I would never buy a car without a spare wheel. I had two BMW 5 series estates which were great but when I went to buy another they didn’t and couldn’t come with a spare wheel. I asked how far I could go on a flat run flat.

“50 miles sir”

Towing a caravan?

“About 25 miles sir”

So I’m on a French motorway and I have a flat and I can’t even get to the next exit? No thanks. Add to that in France I believe you can’t even get these run flat tyres and have to wait to get them imported. And also in France both tyres on the same axle have to have a similar amount of tread.

So I bought a Mercedes E350 estate which came with a skinny spare and bought a full spec wheel and tyre that I could put in the boot.

I have also seen many cases of BMW wheels where the rim has become completely detached after a hard impact on the road. They are not suitable for British roads.

[StDunko]

The fronts are almost new, and the rears (different width and recommended pressures to the fronts) have only done about 40 miles in total, so it's way too early to see any uneven wear.

Plus ideally I want to get the pressures correct, from the start, because at £270 a corner, I don't want the tyres to start wearing unevenly (from either under or over inflation).

I just wondered if there was a rule of thumb that pressures should be the same for the standard tyres as for the  run flats, or perhaps that the standard tyres should generally be inflated to 10% more than runflats (due to weaker sidewalls etc... etc...?).

The only info I could find from the tyre manufacturers was the max PSI for that specific tyre, but that does not help much because the "recommended" pressure will be way below that, and varies from car model to car model (due to loading and if the car is FWD or RWD etc.).

[StDunko]

I have just spoken to an experienced tyre fitter, so I thought I'd share the response I received:

 

If the size/ratings of the 'non-run-flats' is identical to the 'run-flats' they replaced, then the new tyres should be inflated to exactly the same pressure as the run-flats they replaced, i.e. the recommended pressures shown on the plaque in the car's door jam.  (Obviously if for some reason this then results in tell-tale uneven wear patterns, for either over or under inflation, then the pressures should be tweaked accordingly).  

Most emergency tyre repair kits (the through the valve gel type kits) will fix most punctures (other than side wall problems) and allow the car to be driven to a garage to sort the problem, but in 99% of cases these kits will effectively ruin the tyre and require the tyre to be replaced (not ideal if the tyre is relatively new). I was therefore told the best solution was to just have decent breakdown cover (which I thankfully do), meaning you can either be towed to somewhere to fix the tyre, or apparently these days the AA and RAC often carry a number of different sized donuts that they'll fit, then follow you to the tyre place, and then remove.  

[Patches O Houlihan]

On 06/09/2023 at 20:39, Whitey Grandad said:

Check with the manufacturers for tyre pressures. The wear patterns on the existing tyres will give you an indication of the correct pressure. Even wear all across is good. More wear in the centre is too hard. More wear on the edges is too low.

We had a BMW with run flats from the factory, and swapped them for normal tyres to improve the ride (and save ~£100 a corner at the risk of having to use tyre foam)

With quite a lot of German saloons/estates you'll see the back wheels have quite a lot of camber (the rear wheels lean in at the top). This puts more pressure on the inside shoulder of the tyre. Our tyres always wore out on that inside edge first - not because they were inflated wrong, but because the car was designed like that.

When a car corners at high speed the outside wheels tilt because of the grip generated by the tyres, if you tilt that outer tyre inwards by design when stationary, then it becomes flat when cornering at high speed; more tyre surface area is presented to the tarmac and you have more grip. The downside is that you wear the tyres out faster.

Ideally you'd want the same brand and model tyre all round, but in this case I wouldn't bother changing. Both axles have quality tyres, with the tyres matching on an axle.