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capt jack · Jun 26th, 2021, 17:02

And a final twist in the tail. After the tracing was re-done I didn't really get to use the car much above 50mph for a week or so. Then I had to make a quick motorway trip, and noticed that whilst the car behaved perfectly up to around 60mph, above that, and on a straight motorway, it had a tendency to pull very slightly to the left.

So I very carefully measured the tread depth on the two front tyres. The brand new on on the NSF was at 8mm, whereas the OSF was at between 5 and 6mm.

Thinking about a couple of posts in this thread I thought I'd try swapping the top front wheels around. And sure enough, there is a noticeable difference.

So it's a 16" wheel with 205/55 tyres. 16" is 406.4mm, 55% of 205mm is 112.75, times 2, means that the new tyre is a total of 631.9mm diameter. The other slightly worn tyre is 3mm worn, so 6mm smaller in diameter, making it 625.9.

That means the circumference of the new tyre is 1985.2mm, the circumference of the part worn tyre will be 1966.3mm. So for each revolution of the new tyre, the old one has to rotate 18.9mm further, so therefore it must either rotate a little faster, or if both wheels rotate at the same speed, the slightly worn tyre will cover less ground. Either way, the directional stability of the car is going to be affected.

I can only think that at lower speeds this difference isn't really noticeable, but at higher speeds it will make a noticeable difference. Add in a road camber, and the difference is magnified again.

When you think about it, if you have a 4wd car the importance of matching the wear is emphasised because over time mis-matched tyre tread depths, and hence diameters can damage the differential.

On that basis, for a RWD car, if you need to replace only one tyre, it'd be better to make sure that the two rears are matched in type and tread depth, whereas on a FWD drive car you'd be well advised to keep the fronts matched.

After all that Maths my brain is hurting, and to be fair, in a 25-yr-old car that is not really driven a sustained high speed for much it's all a bit theoretical.

Nevertheless, it's food for thought!

Jack

Jun 26th, 2021, 17:02	#14
capt jack VOC Member Last Online: Apr 20th, 2024 18:56 Join Date: Jun 2006 Location: Selby, North Yorkshire	Post Script And a final twist in the tail. After the tracing was re-done I didn't really get to use the car much above 50mph for a week or so. Then I had to make a quick motorway trip, and noticed that whilst the car behaved perfectly up to around 60mph, above that, and on a straight motorway, it had a tendency to pull very slightly to the left. So I very carefully measured the tread depth on the two front tyres. The brand new on on the NSF was at 8mm, whereas the OSF was at between 5 and 6mm. Thinking about a couple of posts in this thread I thought I'd try swapping the top front wheels around. And sure enough, there is a noticeable difference. So it's a 16" wheel with 205/55 tyres. 16" is 406.4mm, 55% of 205mm is 112.75, times 2, means that the new tyre is a total of 631.9mm diameter. The other slightly worn tyre is 3mm worn, so 6mm smaller in diameter, making it 625.9. That means the circumference of the new tyre is 1985.2mm, the circumference of the part worn tyre will be 1966.3mm. So for each revolution of the new tyre, the old one has to rotate 18.9mm further, so therefore it must either rotate a little faster, or if both wheels rotate at the same speed, the slightly worn tyre will cover less ground. Either way, the directional stability of the car is going to be affected. I can only think that at lower speeds this difference isn't really noticeable, but at higher speeds it will make a noticeable difference. Add in a road camber, and the difference is magnified again. When you think about it, if you have a 4wd car the importance of matching the wear is emphasised because over time mis-matched tyre tread depths, and hence diameters can damage the differential. On that basis, for a RWD car, if you need to replace only one tyre, it'd be better to make sure that the two rears are matched in type and tread depth, whereas on a FWD drive car you'd be well advised to keep the fronts matched. After all that Maths my brain is hurting, and to be fair, in a 25-yr-old car that is not really driven a sustained high speed for much it's all a bit theoretical. Nevertheless, it's food for thought! Jack