Advantages of 25mm tyres over 28mm

[recumbentim]

So it is well established that fatter tyres of near the same weight have less deformation and  therefore less rolling resistance. Last year I ran 28mm  conti 4 seasons for 11000 miles and would not go down to 25 or 23 again. I am heavy though You can get several light race tyres in 28mm eg Schwalbe Ultremo,s 220gms , but not very durable .
     But to get a tyre to be more comfortable and  be more rounded like a tubular , reduce rolling and wind resistance , we should be using them on wider rims . A 23mm tyre should be on at least 17mm internal rim width . Not 13 or 14 mm like most of us are using.
   So this year as I need a new set of wheels soon I will try a wider rim , Velocity A23,s  . Which should allow me to run 25- 28 mm tyres with the above benifits.
          Fact or Fiction , we will see???

[mattc]

PR3s seem to be in a different fragility league to PR2s. A new pair are still fine for something like a wet 1200 

(They are probably much more fast/grippy/sexy, but I'm too uncultured to appreciate such things.)

[EdinburghFixed]

I once swapped out my lowracer rear wheel from a 700x23 GP4000s to a 26" Kojak (32mm) to compare the results on the local '10'. I set a PB with the 26" wheel. The course isn't particularly smooth.

To me, I don't think this says much more than "other stuff is probably a lot more important than the difference between two fast tyres". I'd agree with some of the posters above who say that they've gone wider without noticing much of a penalty, but quite an increase in comfort.

A 28mm tyre has 50% more volume than a 23mm one, but you can still run it at 100psi (I never bother pumping a 23 higher than 100psi anyway, after much trial and error).

I have a pair of tubs with 25mm Corsa Evo CX's on them, ridiculously expensive and impractical. They are noticeably nicer though, so if you were in a position to use them (and could afford the spares)...

[Biggsy]

A 28mm tyre has 50% more volume than a 23mm one, but you can still run it at 100psi (I never bother pumping a 23 higher than 100psi anyway, after much trial and error).

The relationship between volume and pressure is very important.  The tension of the casing of a 28mm tyre at 100 psi is higher than that of a 23mm tyre at 100 psi.  You don't get the benefit of all the extra volume of a larger tyre if you over-inflate it.  In your case, though, I'd say your 23mm tyre probably was under-inflated rather than your 28mm (much) over-inflated.

[pdm]

2p worth:

I tend to run very robust tyres - I hate fixing p*n*t*r*s on the road.
25mm Marathon Plus tyres at max pressure roll as well and better than 28's or 32's at their mac pressure and are lighter.
I find my summer steel frame commuter faster than the winter one sporting bigger tyres and can manage up 26kph average on ocaasions through the Peak District on them - quite respectable and not any different from the Durano tyres (200g lighter) on the Audax bike.
My feeling is therefore to go for the most robust tyre at 25mm for good conditions and reserve the bigger 28mm tyres for dark, potholed, cruddy, heavy duty, winter work....
In this vein, I think the Durano Plus tyres are the best around for Audax, and M+ 25's for medium/long distance commuting and general riding.

[mrcharly-YHT]

If you are using M+ or Durano Plus, you will not get any of the benefit of efficient lightweight casings. In that case I agree with all of your comments.  Fatter M+ or Durano plus weigh significantly more than the narrower versions.

I regard those tyres as a special case though.

[EdinburghFixed]

A 28mm tyre has 50% more volume than a 23mm one, but you can still run it at 100psi (I never bother pumping a 23 higher than 100psi anyway, after much trial and error).

The relationship between volume and pressure is very important.  The tension of the casing of a 28mm tyre at 100 psi is higher than that of a 23mm tyre at 100 psi.  You don't get the benefit of all the extra volume of a larger tyre if you over-inflate it.  In your case, though, I'd say your 23mm tyre probably was under-inflated rather than your 28mm (much) over-inflated.

My technical nous isn't enough to understand the implication of casing tension - my naive assumption would be that if it matters at all, it only matters insofar as flexing of the casing will cause friction losses, and there would be less flexing with higher pressure, therefore lower RR.

I've never really understood what constitutes "proper" inflation, especially since one rider may be 65kg and another 100kg or even more. If you define "proper" as "lowest RR" you'd think that it would have to be different in either case - and also, that a wide tyre might not have lower RR if you dropped the pressure, in which case it wouldn't have been "overinflated".

I think.

[Biggsy]

(In all the following, I'm assuming there isn't a vast difference in casing flexibility between the two tyres.  But there's rarely enough difference anyway between two models to over-ride the huge volume factor of 28mm versus 23mm).

"Proper inflation" can be whatever you want*, but if you want the same effect in your wide tyre as you get in your narrow tyre, then your wide tyre has to be inflated to a lower number of PSI than what you have in your narrow tyre.

It's not just about rolling resistance.  Higher casing tension means the tyre flexes less, which makes it less comfortable.  A 28mm tyre at 100 psi is much harder than a 23mm tyre at 100 psi - as you can feel simply by squeezing the sidewalls with your thumbs.  It doesn't make sense to use the same pressure for both these very different sizes, unless you want very different effects from each of them.

ALL of the effects of an inflated tyre depend on the ratio between volume and pressure, not pressure alone.

* Though I suggest the ideal is a good compromise between rolling resistance, grip, durability and comfort.

[Exit Stage Left]

(In all the following, I'm assuming there isn't a vast difference in casing flexibility between the two tyres).

"Proper inflation" can be whatever you want*, but if you want the same effect in your wide tyre as you get in your narrow tyre, then your wide tyre has to be inflated to a lower number of PSI than what you have in your narrow tyre.

It's not just about rolling resistance.  Higher casing tension means the tyre flexes less, which makes it less comfortable. A 28mm tyre at 100 psi is much harder than a 23mm tyre at 100 psi - as you can feel simply by squeezing the sidewalls with your thumbs.  It doesn't make sense to use the same pressure for both these very different sizes, unless you want very different effects from each of them.

ALL of the effects of an inflated tyre depend on the ratio between volume and pressure, not pressure alone.

* Though I suggest the ideal is a good compromise between rolling resistance, grip, durability and comfort.

The opposite is the case, the greater air volume in the 28mm tyre means that the casing moves more for a given load, cushioning the ride better, as the displacement is a lower % of the chamber volume. A tyre of bigger section is more comfortable at the same pressure.

[Biggsy]

The opposite is the case, the greater air volume in the 28mm tyre means that the casing moves more for a given load, cushioning the ride better, as the displacement is a lower % of the chamber volume. A tyre of bigger section is more comfortable at the same pressure.

No it doesn't in this case, because the greater pressure is resisting movement.  A greater volume doesn't help if the pressure isn't low enough to allow it to be compressed by the same load.

[Biggsy]

Inflate a 28mm and a 23mm tyre to 100 psi.  It's harder to press your thumbs into the 28mm's sidewalls at this pressure.

[Exit Stage Left]

Take a bicycle pump, apply a load to it a full extension, when it is compressed to 100psi see what displacement an addition load generates. If you do the same starting from a fraction of the length of the pump the displacement will be less. Envisage the pump as a spring in a suspension system. A greater chamber volume will give more compliance than a smaller one.

[Biggsy]

The opposite is the case, the greater air volume in the 28mm tyre means that the casing moves more for a given load, cushioning the ride better, as the displacement is a lower % of the chamber volume. A tyre of bigger section is more comfortable at the same pressure.

Not same pressure in terms of PSI on the volume.  Same pressure in terms of casing tension.  It takes more PSI in a narrow tyre for the air to press out on the casing to the same degree that it does in a wide tyre.

[Biggsy]

Take a bicycle pump, apply a load to it a full extension, when it is compressed to 100psi see what displacement an addition load generates. If you do the same starting from a fraction of the length of the pump the displacement will be less. Envisage the pump as a spring in a suspension system. A greater chamber volume will give more compliance than a smaller one.

I don't get what you mean by that right now, but I know that it takes more force to pump one stroke into a 28mm tyre when it's at 100 psi than a 23mm at 100 psi.

I get the general logic of what you're saying, but think you are missing a factor.  Unfortunately I don't myself have a good enough formal understanding of science to explain it better, so I will have to rely on other YACFers to continue or conclude the argument.

[mattc]

Take a bicycle pump,...

I'm not saying who is right here, but a pump is quite a poor analogy to a tyre;
the deformation of the pump barrel will be negligible compared to a tyre.

The tyre is a more complicated thing - as you compress it,  the casing deforms with the increased pressure so as to decrease the air pressure.

[Exit Stage Left]

Assuming a roughly circular cross section, then if we give a value of 100 to the chamber volume of a 23mm tyre, a 25mm has a chamber volume of 118 and a 28 mm tyre has a volume of 148. There's an  18% increase from 23 to 25 and 25% increase from 25 to 28. I run 26s' myself.

[pdm]

If you are using M+ or Durano Plus, you will not get any of the benefit of efficient lightweight casings. In that case I agree with all of your comments.  Fatter M+ or Durano plus weigh significantly more than the narrower versions.

I regard those tyres as a special case though.

I quite agree

A good racing tyre (folding bead with some puncture protection and durability) weighs in at about 220g. The Durano Plus 25 weighs 380g and the M+ 25 weighs 580g.

With my weight of 100kg, the difference between 200g and 380g for the tyre is pretty small. The difference between 200g and 580g certainly is and the difference between 200 and 680g for the 28mm or 840g for the 32mm M+ certainly is felt.

I have found no detectable difference in rolling resistance if the tryes are run at their max rated pressures. The 28's are definitely a little more comfortable than the 25's, though... I make up for the difference by have a more comfortable frame/fork with the narrow tyres...

[Exit Stage Left]

A bicycle pump is different because the cross sectional area of the piston is constant. Whereas a bicycle contact patch varies. Logically the contact patch area should be the same size for all tyres at the same pressure. So 1 square inch for a load of 100lb, minus the amount of load it takes to deform the tyre that amount uninflated, that's with a static load. The amount of movement around that static deflection will depend on the chamber volume and the elasticity of the tyre structure. The more air, the more shock absorbing capacity, the air weighs something as well, especially at 7 atm, it would be interesting to weigh the difference.

[Reg.T]

it takes more force to pump one stroke into a 28mm tyre when it's at 100 psi than a 23mm at 100 psi.

That seems counter-intuitive to me. Ceteris paribus, adding a fixed volume of air (a pump barrell-full) to another container (a tyre) should be easier for a larger container, since it will result in a smaller increase in pressure.

Can anyone explain what other factors would outweigh this (marginal) effect?

[border-rider]

I don't get what you mean by that right now, but I know that it takes more force to pump one stroke into a 28mm tyre when it's at 100 psi than a 23mm at 100 psi.

My first thought was that the pressure is 100 psi in both cases, and all the pump sees is air at that pressure the other side of the valve so I was struggling with that, but of course it's not about the pressure the pump sees but how much work it has to do to *change* that pressure. 

The pressure of the air obv. is contained by the tyre, which stretches until the force it exerts back at the air balances the force the air exerts on it.  Pressure being force/area and the bigger tyre having the bigger area, the bigger tyre must be exerting more force at the same pressure. (At this point my brain is fading, but I imagine that the force is proportional to the % change in linear dimension of the tyre (being elastic in nature) as it inflates past the point at which all the "slack" is taken up. So I think the bigger tyre, having more force about its person, is under more tension and it takes more work to change its dimension. Acting against that is that, as Roger T says, the % change in volume will be smaller which should decrease the work needed, but the point about inflating a tyre is that the volume doesn't change linearly with the pressure so the two won't cancel out. I think.)

[tonyh]

About  1.2 g/cubic dm  X  0.14 cubic dm X 7 atmospheres (or is it 8?) = about 1.2 g per tyre?

Expected it to be more, probably made a mistake...

Hang on, isn't the chamber volume about 1400 rather than 140? (Eg 7 square cm X about 200 cm)

So, 10 or so grams of air per tyre. Maybe.

[Exit Stage Left]

The air is the springing medium. more air means more movement for a given transient load. We move into a more complex area when we consider suspension on bikes. It's also helpful to consider what bikes with small wheels are like without suspension.

[Reg.T]

A number of the points discussed here are covered by the Tyres page on Sheldon Brown: http://sheldonbrown.com/tyres.html

(Not that I'm claiming that that's 100% accurate)

[Biggsy]

The pressurised air is pressing on the inside of the tyre to resist the load pressing on the outside.

What we need to work out is the pressure per square cm on the contact patch of both the 28 and 23mm tyres when both are inflated to 100 psi and carrying the same load.  Can someone do this for us?

[Biggsy]

A number of the points discussed here are covered by the Tyres page on Sheldon Brown: http://sheldonbrown.com/tyres.html

I think it backs up what I've been claiming.

I regret not learning more science at a mathematical level to be sure of the details.