Tyre pressures

[Ningishzidda]

the weight distribution i have is 38/62 (just me on the bike, hands on the hoods). i normally put 10psi more in the rear tyre. this weight distribution explains the rear tyre wear, which is normally twice as fast as the front (and rear tyre also wears more due to the torque when riding out of saddle).

This is probably a good average over a ride, but weight distribution is not fixed.  It varies all the way from 100% on the rear (on a very steep climb where the front wheel starts to lift up) to 100% on the front (braking sharply when the rear wheel starts to lift up). 

I generally have a bit more pressure in the rear, but not more than 5-10psi difference.

Greater rear tyre wear is due to it driving the bike, not because it is bearing more weight.

Why don't you call them a 'wheelie' and a 'stoppie'. We'll know what you mean,,

 

[P Walsh]

I'm not sure that rear tyre wear is greater because the rear drives the bike. I think it is the extra weight. I have noticed that my touring bike has more pronounced rear tyre wear when I tour with heavy rear panniers.

It can be both.  Driving causes some scrubbing motion, and weight increases contact area (when you don't use higher pressure to compensate).

I do use higher pressure at the rear, so the extra wear is confined to the central bit of the tread.

The idea of having tyre pressure the same, front and rear, just seems wrong to me. If I sit on a bike with tyre pressures equal, and look down at the tyres, the front looks much less spread than the rear. So the pressures are equal but the tyre shape in the contact area isn't. I try to get the shape closer to being the same, front and rear.

[tom_e]

Greater rear tyre wear is due to it driving the bike, not because it is bearing more weight.

I'm sceptical. 

If we take a ballpark weight distribution of 60 / 40, then the weight difference on the rear tyre of my bike over the front is about 175N.
If I'm putting out 200W at 20mph (I'm not, but lets be generous) then the drive force at the rear tyre would still only be 22N. 

If my weight distribution is more like 55 / 45, then the weight difference is ~87N. 
But if we took a more sensible guess at my usage of 100W at 15mph, drive force would be only 15N.

Even in the most extreme combination of those values, the weight difference is four times the drive force.  In the other extreme, 10 times.

But since my rear tyre wears at about twice the rate of my front, either the wear is non-linear with weight, or the drive force has a disproportional effect as you suggest.  Or a bit of both.  I can't see an easy way of testing it in the real world.  Do we know anybody who commutes with just front panniers?

[Kim]

But since my rear tyre wears at about twice the rate of my front, either the wear is non-linear with weight

Do tyres wear the same way as roads?  (4th power of weight)

[Frank9755]

Greater rear tyre wear is due to it driving the bike, not because it is bearing more weight.

I'm sceptical. 

If we take a ballpark weight distribution of 60 / 40, then the weight difference on the rear tyre of my bike over the front is about 175N.
If I'm putting out 200W at 20mph (I'm not, but lets be generous) then the drive force at the rear tyre would still only be 22N. 

If my weight distribution is more like 55 / 45, then the weight difference is ~87N. 
But if we took a more sensible guess at my usage of 100W at 15mph, drive force would be only 15N.

Even in the most extreme combination of those values, the weight difference is four times the drive force.  In the other extreme, 10 times.

But since my rear tyre wears at about twice the rate of my front, either the wear is non-linear with weight, or the drive force has a disproportional effect as you suggest.  Or a bit of both.  I can't see an easy way of testing it in the real world.  Do we know anybody who commutes with just front panniers?

The direction of the force is the key thing, not the magnitude!  The weight is acting perpendicular to the surface while the motive force is acting in parallel. 
Are you more likely to wear a tyre by squeezing it, or by rubbing it? 

[Samuel D]

If we take a ballpark weight distribution of 60 / 40, then the weight difference on the rear tyre of my bike over the front is about 175N.
If I'm putting out 200W at 20mph (I'm not, but lets be generous) then the drive force at the rear tyre would still only be 22N.

Assuming your calculation is correct (I didn’t check), that’s the average driving force. But a pedalling human applies power in staccato bursts, especially when standing.

Which is another thing. When your 200 W are applied at 5 miles per hour, as they would be when climbing a steep hill, even the average tractive force becomes much larger. The peaks at the beginning of the ragged pedal stroke when honking up a steep hill would be an order of magnitude greater than your calculated average in a high-speed cruise.

When you’re climbing a hill like that you can actually hear the rear tyre scrubbing the road. I like the sound – it’s flattering to hear my power applied to the road – but it’s the sound of a tyre wearing rapidly.

[P Walsh]

If we take a ballpark weight distribution of 60 / 40, then the weight difference on the rear tyre of my bike over the front is about 175N.
If I'm putting out 200W at 20mph (I'm not, but lets be generous) then the drive force at the rear tyre would still only be 22N.

Assuming your calculation is correct (I didn’t check), that’s the average driving force. But a pedalling human applies power in staccato bursts, especially when standing.

Which is another thing. When your 200 W are applied at 5 miles per hour, as they would be when climbing a steep hill, even the average tractive force becomes much larger. The peaks at the beginning of the ragged pedal stroke when honking up a steep hill would be an order of magnitude greater than your calculated average in a high-speed cruise.

When you’re climbing a hill like that you can actually hear the rear tyre scrubbing the road. I like the sound – it’s flattering to hear my power applied to the road – but it’s the sound of a tyre wearing rapidly.

I don't notice any rear wheel slip on steep ascents on a heavily laden tourer, but I do notice the rear wearing a lot quicker than the front. I have swapped the tyres half way round a tour in the past.

[Biggsy]

I'm not sure that rear tyre wear is greater because the rear drives the bike. I think it is the extra weight. I have noticed that my touring bike has more pronounced rear tyre wear when I tour with heavy rear panniers.

It can be both.  Driving causes some scrubbing motion, and weight increases contact area (when you don't use higher pressure to compensate).

I do use higher pressure at the rear, so the extra wear is confined to the central bit of the tread.

Not just higher pressure in the rear than the front, but further and enough higher pressure would be needed to compensate for the heavy pannier load.

If weight is still a factor, it'll be in conjunction with the scrubbing from driving.  I suppose the weight makes it scrub harder.

[Kim]

The solution, obviously, is to have a front wheel that's smaller than the rear...

[tom_e]

The direction of the force is the key thing, not the magnitude!  The weight is acting perpendicular to the surface while the motive force is acting in parallel. 
Are you more likely to wear a tyre by squeezing it, or by rubbing it?

I take your first point, but it's not an either/or thing. The magnitude must also be pretty important for determining wear rate.  Not sure rubbing is quite right - more the difference between pushing straight down on a tyre and releasing, and pushing down at a 15 degree angle then releasing.

Assuming your calculation is correct (I didn’t check), that’s the average driving force. But a pedalling human applies power in staccato bursts, especially when standing.

That's a very good point.  I don't know how much it would apply as a percentage of typical use.  Potentially quite a lot.

I'd still like to think of a way of testing it which wouldn't take more effort than the answer is worth.  I'm thinking either a full lab rolling jig, or adding a fair bit of extra weight to one end of a bike and comparing to control for long enough to generate a measurable difference in tyre wear.  Which is a lot of effort.

[ElyDave]

I'm currently 70kg ish 1/10 of that would be 7kg or 14.5lb ish.  can't see the connection to anything in bars or psis that remotely resembles a tyre pressure.

7 bar resembles a popular tyre pressure: 100 psi.  It's lower than you and I use (for 23mm), but personal preference varies!

I weigh about the same and prefer 130 psi for rear 700x23, but that is a lot higher than necessary.

Right, so how do you get from the rule of thumb of 1/10 the weight of the cyclist, being 7kg to 130psi?

[Biggsy]

Right, so how do you get from the rule of thumb of 1/10 the weight of the cyclist, being 7kg to 130psi?

I don't.  I don't use the rule of thumb!  I'd use around 7 bar or 100 psi if I did.  It's T42's rule of thumb.

Don't get this, or am I being a bit thick?

Maybe you just need a cup of tea and a lie down. 

[rr]

Two thoughts on this: back wheels are often less true than fronts -- this will surely increase wear; front and rear wheels are differently "sprung" in the frame and so the differing tire pressures are only part of the comfort and grip equation.

[jsabine]

Right, so how do you get from the rule of thumb of 1/10 the weight of the cyclist, being 7kg to 130psi?

I don't.  I don't use the rule of thumb!  I'd use around 7 bar or 100 psi if I did.  It's T42's rule of thumb.

Maybe the rule of thumb's as simple as (weight in kg)/10 = pressure in bar?

I imagine it's likely to be a metric one given it was suggested by T42.

[Biggsy]

I'll put the kettle on.

[ElyDave]

I'm with it now.  I have Tomintoul, peated.

I don't follow that ROT either, although I'm not far away.

[Samuel D]

I'd still like to think of a way of testing it which wouldn't take more effort than the answer is worth.  I'm thinking either a full lab rolling jig, or adding a fair bit of extra weight to one end of a bike and comparing to control for long enough to generate a measurable difference in tyre wear.  Which is a lot of effort.

Less effort would be to think of cars. Front wheel drive cars go through multiple front tyres per rear tyre despite having only modestly more weight on the front (especially when loaded). Rear wheel drive cars, far fewer. The axle weights, being not very different in the two cases, don’t fully explain this. It must be down to the driven wheels being harder on tyres.

back wheels are often less true than fronts -- this will surely increase wear

Why?

[Biggsy]

More dishing and more weight means rear wheels are often slightly less true than fronts, and being out of true makes it wobble about.  But front wheels wobble more than that because of the way human beings ride bikes, so it's another red herring, I think.

[jhob]

I run my tyres at 80/80 (relatively unladen day audax, 73kg rider), which is pretty low for 25s.  I'll normally end up so there's a few more PSI in the rear than the front, giving the pump a couple of extra blasts so in reality there's probably at 2-3psi difference front to back.  That said I'm not sure what the precision of the pressure gauge on the pump is and 2psi could well be taken up in that error.

[Frank9755]

The direction of the force is the key thing, not the magnitude!  The weight is acting perpendicular to the surface while the motive force is acting in parallel. 
Are you more likely to wear a tyre by squeezing it, or by rubbing it?

I take your first point, but it's not an either/or thing. The magnitude must also be pretty important for determining wear rate.  Not sure rubbing is quite right - more the difference between pushing straight down on a tyre and releasing, and pushing down at a 15 degree angle then releasing.

What is important is the magnitude of the relevant force, which is the one acting at right angles to the tyre, essentially rubbing its surface off.

You need some downward force to keep the tyre in contact with the road, but not necessarily very much.  Imagine a really lightly weighted rear wheel, which would skid a lot rather than grip.  A well-weighted one may well have less wear.

The other thing that really wears a tyre is braking, where you have the opposite effect to that of pedalling.

Think of where you see (car) rubber marks on the road (ie extreme tyre wear) - it's where someone has done an emergency stop or a boy racer has revved his engine and smoked his tyres!  You don't get rubber marks from heavily-laden HGVs ploughing up the motorway at a steady speed.

[tom_e]

Think of where you see (car) rubber marks on the road (ie extreme tyre wear) - it's where someone has done an emergency stop or a boy racer has revved his engine and smoked his tyres!  You don't get rubber marks from heavily-laden HGVs ploughing up the motorway at a steady speed.

I'm not disputing this is true, no.  I suppose I'm just sceptical we generate enough power to make it the biggest factor!

There is usually quite a lot of difference in the weight distribution of front to rear wheel drive cars.  But yes, the power of the engine is sufficient to make a big difference too.

[Frank9755]

What else could be causing it? 
If weight, why should that actually make any difference to the amount of wear? 

[Samuel D]

A front car tyre (e.g. VW Golf) may carry close to 400 kg, which at a tyre pressure of 2.3 bar (33 PSI) produces a contact patch in the region of 170 cm squared or 340 cm squared for the two front tyres.

A bicycle’s rear tyre may have 50 kg on it, which at a tyre pressure of 8 bar (116 PSI) produces a contact patch of about 6.1 cm squared.

The car may routinely pump 50 horsepower into those 340 cm squared while the cyclist may routinely pump bursts of 2 horsepower into 6.1 cm squared and furthermore at much lower speed than the car (i.e. much higher relative tractive force). You can see it is the bicycle tyre that is the more stressed by these tractive forces.

That is borne out in my practice. Spinning my car wheels is rare, whereas I often momentarily lose traction at the rear wheel while pedalling up steep hills, especially in the wet.

The other thing that really wears a tyre is braking, where you have the opposite effect to that of pedalling.

While true, braking (rear-wheel skidding aside) doesn’t cause much wear much in practice because we do so little of it.

If weight, why should that actually make any difference to the amount of wear?

I think it must make some difference because there is always some scrubbing as the tyre’s torus-shaped tread is forced flat against the road at the contact patch, and a heavier cyclist must have either higher pressure (i.e. greater friction in that scrubbing) or a larger contact patch (i.e. more distortion and localised tread slip when flattening the torus). However I agree this causes minimal wear compared to tractive forces. It must, given observed tyre wear.

[Frank9755]

If weight, why should that actually make any difference to the amount of wear?

I think it must make some difference because there is always some scrubbing as the tyre’s torus-shaped tread is forced flat against the road at the contact patch, and a heavier cyclist must have either higher pressure (i.e. greater friction in that scrubbing) or a larger contact patch (i.e. more distortion and localised tread slip when flattening the torus). However I agree this causes minimal wear compared to tractive forces. It must, given observed tyre wear.

I thought that to begin with, but the more I thought about it, I couldn't see a reason why it would be true!  More weight gives you better grip and hence less skidding or scrubbing of the tyre. 
And, thinking about the forces, can a purely vertical force (ie the weight) act to 'push' rubber off the tyre?

[P Walsh]

Am I wrong to think that regularly losing traction of the rear tyre on steep hills could be a consequence of running the tyre too hard?