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Tyre wear - mileages

Posted: 9 Nov 2019, 12:49pm
by Mick F
I've just replaced the rear tyre on Moulton (again).

Mileage out of the rears is about 1,600miles. Could get more, but the tread goes and the rubber gets very thin, and once I was down to the canvass in a couple of places.

Mileage from Mercian for rear tyres is about 5,000miles.

Considering that Moulton's rolling diameter is 18.2" and Mercian's is 26.5" it means (by using Pi) that Moulton's circumference is 57" and Mercian's 83"
Works out as about (round numbers) 1.5 times bigger.

1.5 times bigger in circumference.
1.5 x 1,600miles = 2,400miles

Why do I get three times the Moulton mileage out of Mercian?
Is there something going on that tyre wear on smaller wheels isn't proportional to circumference?

Am I getting confused here?
Is my point pointless? :wink:

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 12:59pm
by Brucey
that is quite a big difference. I'd say there is probably something going on there, either to do with the wheel size/tyres or the way the bike is used.

cheers

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 2:18pm
by Mick F
I could have said that, before I even posted! :wink:
"Something going on" ............. what is it?

Continental Sports Contact 406-28 vs Vittoria Rubino Pro 622-23

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 2:22pm
by Chris Jeggo
One cause of tyre wear is rubbing contact between tyre and road, because arc length and chord length between two points on the circumference are not the same. If the two tyres (Moulton and Mercian) are the same width and running at the same pressure then the lengths of the footprints will be roughly the same, so the arc subtended by the contact patch will be larger for Moulton than for Mercian. The difference between arc length and chord length goes as the cube of the angle (for small angles). This is all a simplification, but I suspect that it is an important part of the explanation for your observations.

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 3:38pm
by fausto99
Chris Jeggo wrote:One cause of tyre wear is rubbing contact between tyre and road, because arc length and chord length between two points on the circumference are not the same.

Can you expand on this a bit? How do you define rubbing contact? How does the difference in arc to chord length affect the rubbing contact?

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 4:40pm
by Jamesh
Could it be that the smaller diameter wheel scrubs more as it had less inertia?

Rather like a hgv wheel scrubs when turning?

Cheers James

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 4:44pm
by reohn2
fausto99 wrote:
Chris Jeggo wrote:One cause of tyre wear is rubbing contact between tyre and road, because arc length and chord length between two points on the circumference are not the same.

Can you expand on this a bit? How do you define rubbing contact? How does the difference in arc to chord length affect the rubbing contact?

There's more tyre on the road at any given time on a smaller wheel,on the drive wheel that means the same friction over a great arc of the smaller tyre.

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 4:46pm
by Chris Jeggo
fausto99 wrote:
Chris Jeggo wrote:One cause of tyre wear is rubbing contact between tyre and road, because arc length and chord length between two points on the circumference are not the same.

Can you expand on this a bit? How do you define rubbing contact? How does the difference in arc to chord length affect the rubbing contact?

Try this explanation. The roll-out method of cycle computer calibration consists of marking a point on the tread of the relevant tyre, marking where it touches the road surface, rolling the bike forward one revolution, marking where it next touches the road, and measuring the separation of the two marks. If you do this with the bike loaded and unloaded you get two different results, because of the flattening of the tyre at the bottom. (So for best calibration you should be sitting on the bike.) Now think about a small patch of the tyre as the wheel rolls along. While the patch is out of contact with the road it is always the same length (measured along the circumference), but as it goes through the flat contact area it has to shorten and then lengthen again, which must cause some rubbing. I would not care to estimate how far a given point on a tyre rubs along the road surface, because we are in the real world, with doughnut-shaped tyres with complex elastic properties, not in a 2-D geometric world with perfect circles and straight lines, but there has to be a close relationship.

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 4:49pm
by mig
you're smoother on the mercian.

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 6:13pm
by Tigerbiten
What kills a tyre is the amount of slippage against the road.
The lower gears on the Brompton encourage this type micro/macro slippage going uphill.
As you live where it's hilly, the rear tyre suffers.
The rear wheel on my bent trike also suffers from this type of wear due to the lack of weight on it.
I've worn a new tyre out in under 1,000 mile due to a lot of hill climbing on damp roads.
This is the reason I use a Big Apple tyre on my bent trike, it has more rubber to wear off before reaching the carcass.

Luck ...... :D

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 8:07pm
by Mick F
mig wrote:you're smoother on the mercian.
This seems a good statement.

Why am I more smooth on a 700c 531c road bike, than on a suspended 20" Moulton?
Same geometry, same bloke in the same kit, riding the same roads.

Seems a good statement, but why three times the mileage on the rear tyres?

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 9:02pm
by foxyrider
i get a similar disparity between my Airnimal running 520 tyres and my big wheelers. It doesn't just run to tyre wear - i also seem to get far more punctures in the smaller tyres. :(

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 9:09pm
by NUKe
Mick F wrote:I could have said that, before I even posted! :wink:
"Something going on" ............. what is it?

Continental Sports Contact 406-28 vs Vittoria Rubino Pro 622-23

And there. Is the answer different tyres different compounds

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 9:13pm
by iandriver
Hmm, random theory with no basis in science or suggestions I know what I'm talking about.

Heat making the rubber softer in a smaller tyre.

Pressures and the contact patch size.

More brake bias towards the rear. Dragging the back brake, different braking strategy on the small wheeler.

Re: Tyre wear - mileages

Posted: 9 Nov 2019, 9:25pm
by Mick F
iandriver wrote:Hmm, random theory with no basis in science or suggestions I know what I'm talking about.

All I can say, is that I get three times the mileage on one bike than the other.

Big difference.