ROLL OUTFOR TRACK....23mm Vs 20mm

[BernardG]

Looking at buying new tyres for my sons track bike...... Conti Supersonics....... Come in 2 sizes
23mm & 20mm....

If i use the 23mm his rollout should be the same as he is not permitted to exceed 6m.....

If i use 20mm his roll out would be reduced slightly....not sure how much though....

Is the sacrifice for a faster 20mm tyre in roll out more beneficial as you have a faster spinning
tyre.....??????????????

Which one would be overall faster..

Bernard

 

[Server]

In article <[email protected]>, [email protected] says...
> Looking at buying new tyres for my sons track bike...... Conti Supersonics....... Come in 2 sizes
> 23mm & 20mm....
>
> If i use the 23mm his rollout should be the same as he is not permitted to exceed 6m.....
>
> If i use 20mm his roll out would be reduced slightly....not sure how much though....
>
> Is the sacrifice for a faster 20mm tyre in roll out more beneficial as you have a faster spinning
> tyre.....??????????????
>
> Which one would be overall faster..
>
> Bernard
>
>
>
Stay with the size that will give you the right rollout or change ring and cog to suit. Also watch
for the drop in track clearance with the smaller bag. The 23's are faster at the same tyre pressure.
You can go for a 20 front and 23 rear. My son is U15 too. He has clipped the banking with 23's in a
derby - no prob - so wouldn't like to go to 20's.
--
Mark Lee

 

[Etxy]

I heard a rumour that 23's have less rolling resistance then 20's? Of course, I heard this after I got myself a new pair of 20's. Would anyone care to verify/contradict this rumour?

 

[Andrew Swan]

Etxy wrote:
> I heard a rumour that 23's have less rolling resistance then 20's? Of course, I heard this after I
> got myself a new pair of 20's. Would anyone care to verify/contradict this rumour?

All other things being equal (N.B. on bikes, they rarely are), a larger diameter wheel has lower
RR than a smaller one. Don't ask me why, this is just a general principle of physics. Anyone
happen to know?

&roo

 

[Etxy]

Originally posted by Andrew Swan
Etxy wrote:
> I heard a rumour that 23's have less rolling resistance then 20's? Of course, I heard this after I
> got myself a new pair of 20's. Would anyone care to verify/contradict this rumour?

All other things being equal (N.B. on bikes, they rarely are), a larger diameter wheel has lower
RR than a smaller one. Don't ask me why, this is just a general principle of physics. Anyone
happen to know?

&roo

But by that logic wouldn't 25's be faster, 30's faster still... at what point does this theory stop working? I know for a fact that when I moved from 25's down to 23's my average went up, not down... we only see pro's using 23's, sometimes down to 20's.

 

[rayner]

Originally posted by Etxy
I heard a rumour that 23's have less rolling resistance then 20's? Of course, I heard this after I got myself a new pair of 20's. Would anyone care to verify/contradict this rumour?

20s should have a lower rolling resistance than 23s because of the reduced contact area to the ground, which would reduce friction and result in less resistance.

 

[rayner]

Originally posted by BernardG
Looking at buying new tyres for my sons track bike...... Conti Supersonics....... Come in 2 sizes
23mm & 20mm....

If i use the 23mm his rollout should be the same as he is not permitted to exceed 6m.....

If i use 20mm his roll out would be reduced slightly....not sure how much though....

Is the sacrifice for a faster 20mm tyre in roll out more beneficial as you have a faster spinning
tyre.....??????????????

Which one would be overall faster..

Bernard

Forgot to write this in my last post. Had the same problem when i raced juniors. If i can remember right the difference is about four centimetres which wont make any difference whatsoever. I personally ran the 20s after riding 23s in my first year and the difference was amazing not only in speed but also in the responsiveness of the bike (probably was in my head tho!). The sacrifice in roll out is well worth it With the supersonics be a little bit careful when moving slowly on a steeply banked veledrome as they dont stick too well at slow speeds. Hope that helped.

 

[Gemma Kernich]

"rayner" <[email protected]> wrote in message
news:[email protected]...
> Etxy wrote:
> > I heard a rumour that 23's have less rolling resistance then 20's? Of course, I heard this
> > after I got myself a new pair of 20's. Would
anyone
> > care to verify/contradict this rumour?
>
>
>
> 20s should have a lower rolling resistance than 23s because of the reduced contact area to the
> ground, which would reduce friction and result in less resistance.

Don't think so... rolling resustance isn't purely a function of contact patch, rolling resistance is
also affected by the stiffness of the sidewalls in clinchers. As 23's are more flexible (generally)
they come up better in rolling resistance tests. You make your own judgement on how much the air
resistance increases with a fatter tyre though Am sure there's plently about this one the web
somewhere... Gemma

 

[amirm]

True. The higher the TPI, the lower the rolling resistance for the same given size. As the overall RR is the product of few factors, it's hard to rate tyres in this respect based on the diameter only.

Besides, RR is effective during acceleration and not crusing. So this should not be confused with the speed. The larger tyre, giving larger circumference, will go faster at a set RPM, but if itsRR is greater, it would take longer and more effort to accelerate with.

Originally posted by Gemma Kernich
"rayner" <[email protected]> wrote in message
news:[email protected]...
> Etxy wrote:
> > I heard a rumour that 23's have less rolling resistance then 20's? Of course, I heard this
> > after I got myself a new pair of 20's. Would
anyone
> > care to verify/contradict this rumour?
>
>
>
> 20s should have a lower rolling resistance than 23s because of the reduced contact area to the
> ground, which would reduce friction and result in less resistance.

Don't think so... rolling resustance isn't purely a function of contact patch, rolling resistance is
also affected by the stiffness of the sidewalls in clinchers. As 23's are more flexible (generally)
they come up better in rolling resistance tests. You make your own judgement on how much the air
resistance increases with a fatter tyre though Am sure there's plently about this one the web
somewhere... Gemma

 

[Stu]

l didnt want to get into this but here goes
> > > 20s should have a lower rolling resistance than 23s because of the reduced contact area to
> > > the ground, which would reduce friction and result in less resistance.
if the tyres have the same construction and "if" the pressure is the same the the contact area will
be the same, only the shape of the contact area changes, the shape of the 20s being longer and
narrower(this mean more tyre deflection), so the 20 has a higher RR than the 23
> > Don't think so... rolling resustance isn't purely a function of
contact
> > patch, rolling resistance is also affected by the stiffness of the sidewalls in clinchers. As
> > 23's are more flexible (generally) they
come
> > up better in rolling resistance tests. You make your own judgement on how much the air
> > resistance increases with a fatter tyre though Am sure there's plently about this one the
> > web somewhere... Gemma
>
> True. The higher the TPI, the lower the rolling resistance for the same given size. As the
> overall RR is the product of few factors, it's hard to rate tyres in this respect based on the
> diameter only.
>
> Besides, RR is effective during acceleration and not crusing.
are you talking about roll out? or maybe the weight of the tyre? because RR is always there
> So this should not be confused with the speed. The larger tyre, giving larger circumference, will
> go faster at a set RPM, but if itsRR is greater, it would take longer and more effort to
> accelerate with.
no its RR(at the same pressure) will be lower

how much difference it really makes is anyones guess

maybe l just didnt get what you were trying to say and there is always the chance l am wrong

 

[amirm]

I'm confused now. Let's define the terms here. My understanding of the rolling resistance comes from my motoring background, and by what I read here, it seems people are referring to a different concept under the same name. Or maybe I'm wrong.

I guess one thing that can be easily confusing is the "rolling resistance" and the amount of resistance caused by "friction".

I still cannot follow why the changed shape of deflected area going from 23 to 20 (same construct; same pressure) can actually affect the rolling resistance of the tyre. Unless the reason is "more threads involved in the deflected area contributing to a higher resisting force".

About the effect of rolling resistance mainly during acceleration and not during cruising, I was trying to say that the rolling resistance (and not friction force) is like inertia taking a considerable deal to overcome at the time of speed alteration. Once the force is overcome, keeping the momentum requires a lot less energy. Friction, in contrast, is always there. By choosing a narrower tyre, we try to reduce friction drag mainly (of course with pressure adjustment). Rolling resistance of the tyre is rather a product of its construct.

This is my understanding which is in no way perfect. I'd like to understand this concept right, so any further light is welcome.

Originally posted by Stu
l didnt want to get into this but here goes
> > > 20s should have a lower rolling resistance than 23s because of the reduced contact area to
> > > the ground, which would reduce friction and result in less resistance.
if the tyres have the same construction and "if" the pressure is the same the the contact area will
be the same, only the shape of the contact area changes, the shape of the 20s being longer and
narrower(this mean more tyre deflection), so the 20 has a higher RR than the 23
> > Don't think so... rolling resustance isn't purely a function of
contact
> > patch, rolling resistance is also affected by the stiffness of the sidewalls in clinchers. As
> > 23's are more flexible (generally) they
come
> > up better in rolling resistance tests. You make your own judgement on how much the air
> > resistance increases with a fatter tyre though Am sure there's plently about this one the
> > web somewhere... Gemma
>
> True. The higher the TPI, the lower the rolling resistance for the same given size. As the
> overall RR is the product of few factors, it's hard to rate tyres in this respect based on the
> diameter only.
>
> Besides, RR is effective during acceleration and not crusing.
are you talking about roll out? or maybe the weight of the tyre? because RR is always there
> So this should not be confused with the speed. The larger tyre, giving larger circumference, will
> go faster at a set RPM, but if itsRR is greater, it would take longer and more effort to
> accelerate with.
no its RR(at the same pressure) will be lower

how much difference it really makes is anyones guess

maybe l just didnt get what you were trying to say and there is always the chance l am wrong

 

[John Retchford]

Originally posted by amirm
I'm confused now. Let's define the terms here. My understanding of the rolling resistance comes from my motoring background, and by what I read here, it seems people are referring to a different concept under the same name. Or maybe I'm wrong.

I guess one thing that can be easily confusing is the "rolling resistance" and the amount of resistance caused by "friction".

Forget about friction (that involves sliding) and acceleration effects. The rolling resistance being discussed here stems from losses within the tyre.

The area of the contact patch of a weighted bicycle tyre equals the downward force divided by the pressure in the tyre. This comes from the definition of pressure. For a constant pressure the area does not depend on the size of the tyre, but is smaller for higher pressure. For the same pressure, the smaller tyre needs to deflect more to achieve the required contact area.
This deflection of the tyre walls and tread as the wheel rotates is the primary cause of rolling resistance because, for practical tyre materials, it is a lossy process (hysteresis loss).

From the above it follows that a larger tyre (both wheel diameter and tyre width) would deflect less and have a lower rolling resistance than a skinnier one all else being equal. But all else is not equal. For the same pressure a fatter tyre will have higher forces in the tyre walls. The force in the wall is directly proportional to tyre width for the same pressure. So, for a fatter tyre, you could opt for a thicker wall, which increases hysteresis loss, or for lower pressure, which also increases hysteresis loss.

In summary:

Rolling resistance comes largely from hysteresis losses when the tyre deflects. It can be minimized by:
High pressure, which reduces the flexing of the tyre
Thin, flexible walls (needs high wall strength)
Material choice (cotton has lower loss than nylon or Kevlar, but is weaker)
Large diameter (both wheel and cross-section)
Absence of patterned tread (tread squirms and increases losses)

Several of these factors are mutually exclusive, so designers need to seek the "best" compromise. At the moment this would seem to be a smooth tyre with a thin wall and with a width of about 23 to 25 mm on a fairly large wheel. Pretty much the sort of thing you see on most bikes! Note that we are talking about rolling resistance here and not about aerodynamics.

Does this help or just confuse?

John Retchford

 

[rayner]

Originally posted by John Retchford


The area of the contact patch of a weighted bicycle tyre equals the downward force divided by the pressure in the tyre. This comes from the definition of pressure. For a constant pressure the area does not depend on the size of the tyre, but is smaller for higher pressure.

>wouldnt that make a 20mm tyre with recommended inflation of 170 psi have a smaller contact patch than its 23mm counterpart with recommended inflation of 140 psi?

From the above it follows that a larger tyre (both wheel diameter and tyre width) would deflect less and have a lower rolling resistance than a skinnier one all else being equal. But all else is not equal. For the same pressure a fatter tyre will have higher forces in the tyre walls. The force in the wall is directly proportional to tyre width for the same pressure. So, for a fatter tyre, you could opt for a thicker wall, which increases hysteresis loss, or for lower pressure, which also increases hysteresis loss.

> the original conversation was about two identical tyres in different sizes. This means that the wall thickness is identical. With the 23 being taller wouldnt this mean that the sidewall would deflect more?
John Retchford

 

[Stu]

> I'm confused now. Let's define the terms here. My understanding of the rolling resistance comes
> from my motoring background, and by what I read here, it seems people are referring to a different
> concept under the same name. Or maybe I'm wrong.
>
> I guess one thing that can be easily confusing is the "rolling resistance" and the amount of
> resistance caused by "friction".
>
> I still cannot follow why the changed shape of deflected area going from 23 to 20 (same construct;
> same pressure) can actually affect the rolling resistance of the tyre. Unless the reason is "more
> threads involved in the deflected area contributing to a higher resisting force".
yes thats it, they also have to deflect further l have been trying to find some links but havent had
a lot of luck http://draco.acs.uci.edu/rbfaq/FAQ/8b.14.html this is a start
> About the effect of rolling resistance mainly during acceleration and not during cruising, I was
> trying to say that the rolling resistance (and not friction force) is like inertia taking a
> considerable deal to overcome at the time of speed alteration. Once the force is overcome, keeping
> the momentum requires a lot less energy. Friction, in contrast, is always there. By choosing a
> narrower tyre, we try to reduce friction drag mainly (of course with pressure adjustment). Rolling
> resistance of the tyre is rather a product of its construct.
we sure are looking at RR from different points of view the way l see it RR=the energy use to deform
the tyre inertia=the resistance of the whole bike and rider to being accelrated

> This is my understanding which is in no way perfect. I'd like to understand this concept right, so
> any further light is welcome.
me to

 

[amirm]

John:

In the fact that your explanation deals with tyre construct, it is not different from what I assumed first, except that you explained it better. I learnt these when I was in the market to buy tyres for my 18" alloy wheels (for one of my previous cars) in year 2000. I mean all about the interplay between thread pattern, side-wall stiffness, tyre pressure, rubber material and the aspect ratio. In that business, choosing wheel/tyre combo requires attention to the weight of both tyre and rim (as the main unsprung weight) and the rolling resistance of the tyre. Here, talking about bikes, I usually hear people referring to the weight of the wheel/tyre as a component of bike's overall weight rather than its effect on the acceleration.

As you can see in the following post, RR can be used to refer to a technically different concept which explains the source of my confusion.

Thanks for the explanation anyway. Cheers.

Originally posted by John Retchford
Forget about friction (that involves sliding) and acceleration effects. The rolling resistance being discussed here stems from losses within the tyre.

The area of the contact patch of a weighted bicycle tyre equals the downward force divided by the pressure in the tyre. This comes from the definition of pressure. For a constant pressure the area does not depend on the size of the tyre, but is smaller for higher pressure. For the same pressure, the smaller tyre needs to deflect more to achieve the required contact area.
This deflection of the tyre walls and tread as the wheel rotates is the primary cause of rolling resistance because, for practical tyre materials, it is a lossy process (hysteresis loss).

From the above it follows that a larger tyre (both wheel diameter and tyre width) would deflect less and have a lower rolling resistance than a skinnier one all else being equal. But all else is not equal. For the same pressure a fatter tyre will have higher forces in the tyre walls. The force in the wall is directly proportional to tyre width for the same pressure. So, for a fatter tyre, you could opt for a thicker wall, which increases hysteresis loss, or for lower pressure, which also increases hysteresis loss.

In summary:

Rolling resistance comes largely from hysteresis losses when the tyre deflects. It can be minimized by:
High pressure, which reduces the flexing of the tyre
Thin, flexible walls (needs high wall strength)
Material choice (cotton has lower loss than nylon or Kevlar, but is weaker)
Large diameter (both wheel and cross-section)
Absence of patterned tread (tread squirms and increases losses)

Several of these factors are mutually exclusive, so designers need to seek the "best" compromise. At the moment this would seem to be a smooth tyre with a thin wall and with a width of about 23 to 25 mm on a fairly large wheel. Pretty much the sort of thing you see on most bikes! Note that we are talking about rolling resistance here and not about aerodynamics.

Does this help or just confuse?

John Retchford

 

[John Retchford]

Originally posted by rayner

>wouldnt that make a 20mm tyre with recommended inflation of 170 psi have a smaller contact patch than its 23mm counterpart with recommended inflation of 140 psi?

Correct. That is what I said.

> the original conversation was about two identical tyres in different sizes. This means that the wall thickness is identical. With the 23 being taller wouldnt this mean that the sidewall would deflect more?

They both need to produce the same contact patch area if the pressure is the same. This requires the narrower tyre to deflect more, which produces a smaller radius of curvature in the tyre wall. If other things are equal, the wider tyre with less flexure will have a lower rolling resistance. But as you note (and as I noted in my previous post), the narrower tyre can sustain a higher pressure and a higher pressure will reduce the contact patch area and thus the flexure in the wall and tread. The narrower tyre can sustain a higher pressure because the force in the wall is proportional to the tyre radius for the same pressure. For the same wall construction, this means that the wall stress will be higher for the wider tyre.

All this is just basic physics. The problems come when we want to quantify the effects. This requires experimental results and the experiments are difficult to perform. The only ones that we (my son, actually) have done involved comparing the rolling resistance at negligible speed (to eliminate aerodynamic drag) of bicycles fitted with 23 mm tyres and with wide (about 40 mm) but smooth tyres, both inflated to the maker's recommended pressure. The figures were about 2 newtons for the narrow tyres and about 4 newtons for the wide ones. Both these forces are very low and emphasise how efficient the bicycle is.

John Retchford

 

[rayner]

Sorry if im a bit annoying but im trying to get my head around all of this. So can it be safely said that a 20mm tyre inflated to 170 psi when compared to its 23mm counterpart inflated to 140 psi will have less rolling resistance due to less deflection because of the higher pressure?

 

[John Retchford]

Originally posted by rayner
Sorry if im a bit annoying but im trying to get my head around all of this. So can it be safely said that a 20mm tyre inflated to 170 psi when compared to its 23mm counterpart inflated to 140 psi will have less rolling resistance due to less deflection because of the higher pressure?

You are not being at all annoying. People should never be annoyed when yuou ask why - except with themselves when they feel they should be able to answer but can't. That is the position in which I find myself. I think we are all groping with these concepts together.

I think the things I have written so far are sound and based solely on th relationship between contact patch area and pressure and on the properties of the materials involved (the hyrestesis loss). Now you want to vary both the width and the pressure! To answer your question I think we need to be able to predict the change in the radius of curvature of the tyre wall as the piece we are considering goes from being above the contact patch to somewhere else on the tyre. It is not obvious to what that is, although it is probably well known in the tyre industry. Anyone know?

What does seem apparent is that we can not keep reducing the rolling resistance simply by increasing pressure, reducing the tyre diameter if it is necessary to limit the force in the walls. (Remember that the force in the wall is proportional to the tyre width.) The big breakthrough in Edward Dunlop's invention of the pneumatic tyre was the use of the internal volume as an air spring, allowing the tyre to easily roll over small obstacles and roughness in the road or track. Eventually this ability will be lost and the resistance will increase, although the point at which this will happen will be different between a road and a smooth board track.

As far as your question goes, this was a very long way of saying "Idon't know".

John Retchford