Rolling resistance

User avatar
peter
Posts: 1039
Joined: Sun Nov 25, 2007 7:39 pm
Location: sydney

Rolling resistance

Postby peter » Wed Nov 08, 2017 9:49 pm

Well, for a long time I was under the impression that rolling resistance increases as the tyre gets wider. Today I learnt that it is the opposite. Narrower tyres deform more and therefore more energy is wasted which causes increase in rolling resistance.

However, any advantage in reduced rolling resistance is offset by increased rotational mass. It seems 28 is the sweet spot at the moment.

User avatar
Derny Driver
Posts: 2049
Joined: Wed Nov 21, 2012 12:18 pm
Location: Wollongong

Re: Rolling resistance

Postby Derny Driver » Wed Nov 08, 2017 10:26 pm

peter wrote:... It seems 28 is the sweet spot at the moment.


I really love this sort of thing, you know, bike science. Its a bit like global warming in that a whole industry has now grown up around it and thousands of people are employed in development and marketing of new sciency stuff. Fascinating.
And these amazing breakthroughs in the lab are now being embraced by the rank and file bike riders.

Some cyclists can feel the speed difference between a 28 and a 23mm tyre. The 0.007 watts saved means they get home from riding feeling quite refreshed.
Some cyclists can tell the difference between a latex tube and a butyl one, just by riding. Latex is lighter and faster you know.
Some cyclists can feel the speed difference between a 1500 gram wheelset and an 1800 gram wheelset. Lighter is noticeably faster.
Some cyclists can tell the difference between ceramic bearing hubs and steel bearing hubs. Ceramic is faster of course.

But back to the 28mm tyres, I guess I would have to buy new wheels in order to get the aero effect wouldnt I. And would the new wide wheels and tyres clear the rear chainstays? Oh damn, probably not. Im going to need a new bike. Speed certainly is expensive, who would have thought?

User avatar
peter
Posts: 1039
Joined: Sun Nov 25, 2007 7:39 pm
Location: sydney

Re: Rolling resistance

Postby peter » Wed Nov 08, 2017 10:37 pm

I think they do have to come up with new theories/trends in order to make new sales. :D

I have 28s now, but I want 35s/40s/50s. The rims can't take anything wider than 28, then there's the brake/frame not designed to take wide wheels.

It seems the increase in disc brake adoption is to have enough clearance for wide wheels.

User avatar
trailgumby
Posts: 12808
Joined: Sat Jan 03, 2009 5:30 pm
Location: Northern Beaches, Sydney
Contact:

Re: Rolling resistance

Postby trailgumby » Wed Nov 08, 2017 10:48 pm

Disc brake road bikes: there is one marketing ploy to make me spend more that I agree with. Long overdue.

User avatar
Alex Simmons/RST
Expert
Posts: 4353
Joined: Fri May 02, 2008 3:51 pm
Contact:

Re: Rolling resistance

Postby Alex Simmons/RST » Thu Nov 09, 2017 7:01 am

peter wrote:Well, for a long time I was under the impression that rolling resistance increases as the tyre gets wider. Today I learnt that it is the opposite. Narrower tyres deform more and therefore more energy is wasted which causes increase in rolling resistance.

However, any advantage in reduced rolling resistance is offset by increased rotational mass. It seems 28 is the sweet spot at the moment.

Increased rotational mass is hardly an issue to be concerned with. It's only a factor when you are accelerating and the differences are tiny, several orders of magnitude less than the main resistance forces on a bike. I'm not sure why this myth persists.

The main trade offs on tyre width for those interested in the top end of performance (e.g. time trial) is the balance between rolling resistance and aerodynamics. The latter was the benefit with going narrow however with improvements in rim design and matching tyres with rims the aero disadvantage on wider rims/tyres was reduced for riding in non-zero yaw conditions. It depends also on the nature of the surface you are riding on. On a smooth indoor velodrome where yaw is very low, narrow still works very nicely, provided it is a top quality tyre/tube.

And keep in mind the differences in Crr of various tyres can also be managed by changing air pressure, IOW don't over or under inflate your tyres and this can swamp any differences due to width. As a guide, using the max rated pressure of a tyre will more than likely be over inflating and result in increasing Crr.

As to what difference a reduction in Crr can make, well the formula for the power demand due to rolling resistance is exactly the same as that for overcoming a positive gradient slope, with the Crr value replacing the gradient value. IOW Crr is like adding a small positive gradient slope to your ride. e.g. a good quality road tyre on a reasonable country road might have a Crr of 0.005, which is equivalent to riding up a slope of 0.5%. Put better tubes in and you might then have a Crr of 0.0045.

It's a small difference, not noticeable by the rider but does have an effect.

As a rule of thumb, each 0.001 change in Crr is equivalent to ~1 second per km in speed.

If interested you can see the impact of changes in power demand using a calculator I built:
http://www.aerocoach.com.au/power-from-speed/

ausrandoman
Posts: 719
Joined: Fri Apr 03, 2009 8:33 pm

Re: Rolling resistance

Postby ausrandoman » Thu Nov 09, 2017 7:50 am

peter wrote:Narrower tyres deform more and therefore more energy is wasted which causes increase in rolling resistance.


No, narrow tyres need higher pressure. Therefore the tyre deforms less on bumps and rough surfaces. This means that the rider and bike get jolted upwards. The energy required to jolt the rider upwards comes from forward motion, so each bump slows you down. A bigger, fatter tyre at lower pressure absorbs bumps better so the rider and bike keeping going on a level path.



peter wrote:However, any advantage in reduced rolling resistance is offset by increased rotational mass. It seems 28 is the sweet spot at the moment.


It's time to kill off the rotational mass zombie pseudo-science.

g-boaf
Posts: 8684
Joined: Mon Sep 26, 2011 6:11 pm

Re: Rolling resistance

Postby g-boaf » Thu Nov 09, 2017 8:14 am

As for the whole rolling resistance thing, I honestly cannot pick any difference between 23mm or 25mm tyres. The more notable differences would be from upgrading ones own power output.

What you do feel with 25mm tyres is more comfortable ride on very bumpy road surfaces.

trailgumby wrote:Disc brake road bikes: there is one marketing ploy to make me spend more that I agree with. Long overdue.


Unless you are riding the brakes all the time on massive descents or riding in the worst, most miserable conditions with carbon wheels, I cannot see the real point.

All of the bikes I ride have just normal rim brakes (either Dura Ace 9000, Ultegra, SRAM Red Aero-Link or Magura RT8) and they all have great stopping power.

billy70
Posts: 165
Joined: Tue Jul 23, 2013 9:30 am
Location: Perth

Re: Rolling resistance

Postby billy70 » Thu Nov 09, 2017 8:43 am

Alex Simmons/RST wrote:Increased rotational mass is hardly an issue to be concerned with. It's only a factor when you are accelerating and the differences are tiny, several orders of magnitude less than the main resistance forces on a bike. I'm not sure why this myth persists.

....

If interested you can see the impact of changes in power demand using a calculator I built:
http://www.aerocoach.com.au/power-from-speed/


Love it, brilliant as always Alex. Thank you!
Image

User avatar
gorilla monsoon
Posts: 3477
Joined: Tue Aug 26, 2008 10:45 am
Location: Lake Macquarie

Re: Rolling resistance

Postby gorilla monsoon » Thu Nov 09, 2017 9:09 am

Derny Driver wrote: Some cyclists can feel the speed difference between a 28 and a 23mm tyre. The 0.007 watts saved means they get home from riding feeling quite refreshed.
Some cyclists can tell the difference between a latex tube and a butyl one, just by riding. Latex is lighter and faster you know.
Some cyclists can feel the speed difference between a 1500 gram wheelset and an 1800 gram wheelset. Lighter is noticeably faster.
Some cyclists can tell the difference between ceramic bearing hubs and steel bearing hubs. Ceramic is faster of course.


I am not in that elite group, sadly. :(
Won't climb, can't sprint.

Roger Ramjet: Giant CRX3
Lady Penelope: Avanti Cadent
Barry Allen: Specialized Sirrus Expert

User avatar
peter
Posts: 1039
Joined: Sun Nov 25, 2007 7:39 pm
Location: sydney

Re: Rolling resistance

Postby peter » Thu Nov 09, 2017 9:25 am

Fair enough, rotational mass is a clever marketing ploy to get people to pay more for less. It’s much harder to reduce weight at the wheels, so they have to sell the amplifying effect.

Rim brakes do have a mechanical advantage, it would require a lot more force to brake near the center of wheel.

User avatar
Derny Driver
Posts: 2049
Joined: Wed Nov 21, 2012 12:18 pm
Location: Wollongong

Re: Rolling resistance

Postby Derny Driver » Thu Nov 09, 2017 9:30 am

g-boaf wrote:
trailgumby wrote:Disc brake road bikes: there is one marketing ploy to make me spend more that I agree with. Long overdue.


Unless you are riding the brakes all the time on massive descents or riding in the worst, most miserable conditions with carbon wheels, I cannot see the real point.

All of the bikes I ride have just normal rim brakes (either Dura Ace 9000, Ultegra, SRAM Red Aero-Link or Magura RT8) and they all have great stopping power.

+1
I find it odd that people are embracing the disc brake thing. On the one hand most riders would think nothing about spending a couple of thousand dollars more on a bike's groupset to save 500 grams. Then the marketing gurus tell us we need disc brakes but they will add half a kilogram or more to a bikes weight - and that's okay. Weight weenie saving has been the marketing push for decades, and now as we hit the point where there are practically no more weight savings to be found on any components, then suddenly its okay to add half a kilogram to a bike, more than okay its now desirable and fashionable.
I find it ludicrous that a rider might feel that their light carbon wheel equipped bike does not stop well with rim brakes in certain conditions, so instead of putting some good aluminium rims on it for a loss of 200grams, they buy a disc brake bike which is heavier and a set of carbon disc wheels which are 200grams heavier than their non disc equivalents. I might be stupid but I cant get my head around that.

User avatar
peter
Posts: 1039
Joined: Sun Nov 25, 2007 7:39 pm
Location: sydney

Re: Rolling resistance

Postby peter » Thu Nov 09, 2017 9:31 am

In addition to comfort, surely wider tyres provide more grip and therefore improve handling and by extension safety?

I think given the same type pressure, the contact patch areas are the same regardless of the tyre width, the shapes of the contact patch are obviously different.

User avatar
P!N20
Posts: 440
Joined: Thu Jul 22, 2010 6:50 pm

Re: Rolling resistance

Postby P!N20 » Thu Nov 09, 2017 9:48 am

I think we need another rim brake vs disc brake thread.

RobertL
Posts: 336
Joined: Wed Feb 17, 2016 3:08 pm
Location: Brisbane

Re: Rolling resistance

Postby RobertL » Thu Nov 09, 2017 10:34 am

P!N20 wrote:I think we need another rim brake vs disc brake thread.


NOOOOOOOO!!!!!!

User avatar
peter
Posts: 1039
Joined: Sun Nov 25, 2007 7:39 pm
Location: sydney

Re: Rolling resistance

Postby peter » Thu Nov 09, 2017 10:50 am

I do notice a lot more road oriented bikes now come with hydro disc brakes, definitely more common than say 3 years ago.

User avatar
Mububban
Posts: 1032
Joined: Wed Feb 20, 2008 12:19 pm

Re: Rolling resistance

Postby Mububban » Thu Nov 09, 2017 12:12 pm

This site has some interesting info for comparison:

https://www.bicyclerollingresistance.com/
When you are driving your car, you are not stuck IN traffic - you ARE the traffic!!!

User avatar
biker jk
Posts: 5810
Joined: Tue Nov 17, 2009 6:18 pm
Location: Sydney

Re: Rolling resistance

Postby biker jk » Thu Nov 09, 2017 1:22 pm

Derny Driver wrote:
g-boaf wrote:
trailgumby wrote:Disc brake road bikes: there is one marketing ploy to make me spend more that I agree with. Long overdue.


Unless you are riding the brakes all the time on massive descents or riding in the worst, most miserable conditions with carbon wheels, I cannot see the real point.

All of the bikes I ride have just normal rim brakes (either Dura Ace 9000, Ultegra, SRAM Red Aero-Link or Magura RT8) and they all have great stopping power.

+1
I find it odd that people are embracing the disc brake thing. On the one hand most riders would think nothing about spending a couple of thousand dollars more on a bike's groupset to save 500 grams. Then the marketing gurus tell us we need disc brakes but they will add half a kilogram or more to a bikes weight - and that's okay. Weight weenie saving has been the marketing push for decades, and now as we hit the point where there are practically no more weight savings to be found on any components, then suddenly its okay to add half a kilogram to a bike, more than okay its now desirable and fashionable.
I find it ludicrous that a rider might feel that their light carbon wheel equipped bike does not stop well with rim brakes in certain conditions, so instead of putting some good aluminium rims on it for a loss of 200grams, they buy a disc brake bike which is heavier and a set of carbon disc wheels which are 200grams heavier than their non disc equivalents. I might be stupid but I cant get my head around that.


Say I want to run an aero wheelset and wider tyres for comfort then the answer is disc brakes. I'm happy to sacrifice a bit more weight for aero wheels and wider tyres. The example you give of lighter alloy rims will sacrifice aero (moreover, making these shallow rims as wide as the disc brake wheelset rims would make them heavier).

User avatar
CXCommuter
Posts: 1867
Joined: Thu Feb 14, 2013 12:18 pm
Location: Lane Cove NSW

Re: Rolling resistance

Postby CXCommuter » Thu Nov 09, 2017 1:31 pm

I thought rim brakes are a form of a disc brake? :oops:
Image

User avatar
kb
Posts: 2308
Joined: Sun Nov 13, 2011 3:22 pm

Re: Rolling resistance

Postby kb » Thu Nov 09, 2017 2:50 pm

Derny Driver wrote:...
But back to the 28mm tyres, I guess I would have to buy new wheels in order to get the aero effect wouldnt I. And would the new wide wheels and tyres clear the rear chainstays? Oh damn, probably not. Im going to need a new bike. Speed certainly is expensive, who would have thought?

On the other hand, if you go on a diet and lose some weight you also deform a tyre less. Who would have thought? ;-)
Image

BJL
Posts: 466
Joined: Fri Nov 30, 2012 6:45 pm

Re: Rolling resistance

Postby BJL » Thu Nov 09, 2017 8:17 pm

Alex Simmons/RST wrote:Increased rotational mass is hardly an issue to be concerned with. It's only a factor when you are accelerating and the differences are tiny, several orders of magnitude less than the main resistance forces on a bike. I'm not sure why this myth persists.


Rotational mass is a factor as it has more effect the further away from the centre of rotation. Wheel rims and tyres are the furthest away from the centre of rotation than any other part on a bicycle. Static weight on the other hand, unless you're already at the point where you can lose no more weight, then paying thousands to lose 500g off a frame when you could stand to lose a few kgs is just lazy. I can save 600g off my bike by leaving one bidon at home. But that won't make as much difference as saving 600g off the rims and tyres. Yet people spend money on things like carbon bottle cages! Now that's a mystery to me.

And it's interesting that you mention 'acceleration' here since that's exactly what you are doing when riding a bicycle. Even on a flat road, you are working against the 'deceleration' that occurs when you stop pedaling for any length of time. You are actually 'accelerating' the entire time, even if you are just maintaining a steady speed. Any increase in gradient such as climbing hills and mountains amplifies this.

User avatar
baabaa
Posts: 696
Joined: Sun Apr 19, 2009 8:47 am

Re: Rolling resistance

Postby baabaa » Thu Nov 09, 2017 8:31 pm

P!N20 wrote:I think we need another rim brake vs disc brake thread.

https://youtu.be/Jj0uBQ7j5c4

Discodan
Posts: 172
Joined: Sat Jul 23, 2011 3:50 pm

Re: Rolling resistance

Postby Discodan » Fri Nov 10, 2017 7:54 am

BJL wrote:And it's interesting that you mention 'acceleration' here since that's exactly what you are doing when riding a bicycle. Even on a flat road, you are working against the 'deceleration' that occurs when you stop pedaling for any length of time. You are actually 'accelerating' the entire time, even if you are just maintaining a steady speed. Any increase in gradient such as climbing hills and mountains amplifies this.

I'm sure the physics purists will chime in any moment now and rip you a new one but just "no". If your speed is constant then you are not accelerating by definition; you still need to apply force to overcome resistance (which is what I think you were getting at) but it's not acceleration and is independent of weight which is what the topic was about. It takes no more force/energy to keep a 1,000g wheelset spinning at 20kph as it does a 2,000g wheelset assuming no aero differences between them
Image

User avatar
Alex Simmons/RST
Expert
Posts: 4353
Joined: Fri May 02, 2008 3:51 pm
Contact:

Re: Rolling resistance

Postby Alex Simmons/RST » Fri Nov 10, 2017 8:15 am

BJL wrote:
Alex Simmons/RST wrote:Increased rotational mass is hardly an issue to be concerned with. It's only a factor when you are accelerating and the differences are tiny, several orders of magnitude less than the main resistance forces on a bike. I'm not sure why this myth persists.


Rotational mass is a factor as it has more effect the further away from the centre of rotation. Wheel rims and tyres are the furthest away from the centre of rotation than any other part on a bicycle. Static weight on the other hand, unless you're already at the point where you can lose no more weight, then paying thousands to lose 500g off a frame when you could stand to lose a few kgs is just lazy. I can save 600g off my bike by leaving one bidon at home. But that won't make as much difference as saving 600g off the rims and tyres. Yet people spend money on things like carbon bottle cages! Now that's a mystery to me.

And it's interesting that you mention 'acceleration' here since that's exactly what you are doing when riding a bicycle. Even on a flat road, you are working against the 'deceleration' that occurs when you stop pedaling for any length of time. You are actually 'accelerating' the entire time, even if you are just maintaining a steady speed. Any increase in gradient such as climbing hills and mountains amplifies this.


I have actually done the physics, have you?

Keep in mind a couple of basic physics principles:
- conservation of angular momentum
- conservation of energy

If a heavier rimmed wheel is harder to accelerate (and it is, by a tiny little bit since it will have a higher moment of inertia), then that very same principle means it's also harder to slow it down.

So once such a heavier wheel has been accelerated up to speed, it won't decelerate as quickly, meaning any speed variation resulting from a variable power input is actually reduced. This is why flywheels are used in engines of various sorts, to smooth out the variation in velocity due to pulse like power application.

Keep in mind that the rotational kinetic energy of the rims is ~2% of the whole bike + rider KE, and so a small change in rim mass means <0.2% change in total system KE. Several orders of magnitude.

Then there are some simple real life examples that demonstrate there is very little variation in forward velocity during a pedal stroke, such as riders in a team pursuit that ride a few centimetres from the wheel in front while the riders are at difference phases of their pedal stroke, or a rider riding behind a derny yet doesn't experience major variations in the position relative to the derny's rear guard.

I have actually taken the time to work through all this stuff in some detail:

Here I examine the issue of wheel rim mass (and any aero trade offs):
http://alex-cycle.blogspot.com.au/2014/ ... ts-ii.html

which was an update of this earlier piece:
http://alex-cycle.blogspot.com.au/2013/ ... parts.html

This one examines in detail the issue of variability in velocity during a pedal stroke.
http://alex-cycle.blogspot.com.au/2015/ ... ocity.html

This one then combines the above two factors together being the acceleration scenario with variations in velocity.
http://alex-cycle.blogspot.com.au/2015/ ... ocity.html

User avatar
Alex Simmons/RST
Expert
Posts: 4353
Joined: Fri May 02, 2008 3:51 pm
Contact:

Re: Rolling resistance

Postby Alex Simmons/RST » Fri Nov 10, 2017 8:24 am

Discodan wrote:
BJL wrote:And it's interesting that you mention 'acceleration' here since that's exactly what you are doing when riding a bicycle. Even on a flat road, you are working against the 'deceleration' that occurs when you stop pedaling for any length of time. You are actually 'accelerating' the entire time, even if you are just maintaining a steady speed. Any increase in gradient such as climbing hills and mountains amplifies this.

I'm sure the physics purists will chime in any moment now and rip you a new one but just "no". If your speed is constant then you are not accelerating by definition; you still need to apply force to overcome resistance (which is what I think you were getting at) but it's not acceleration and is independent of weight which is what the topic was about. It takes no more force/energy to keep a 1,000g wheelset spinning at 20kph as it does a 2,000g wheelset assuming no aero differences between them


That's essentially correct, however the issue being considered is the power application of a bicycle rider is not constant but varies during the pedal stroke (in a sinusoidal pulse like manner, i.e. two downstrokes per crank rev). So it then becomes an issue of what sort of speed variation results. It turns out they are tiny because the inertia of the whole bike + rider system is large.

The primary factor though is that once you do increase the velocity of a heavier wheel, it doesn't slow down as quickly either.

User avatar
trailgumby
Posts: 12808
Joined: Sat Jan 03, 2009 5:30 pm
Location: Northern Beaches, Sydney
Contact:

Re: Rolling resistance

Postby trailgumby » Fri Nov 10, 2017 8:45 am

ausrandoman wrote:No, narrow tyres need higher pressure. Therefore the tyre deforms less on bumps and rough surfaces. This means that the rider and bike get jolted upwards. The energy required to jolt the rider upwards comes from forward motion, so each bump slows you down. A bigger, fatter tyre at lower pressure absorbs bumps better so the rider and bike keeping going on a level path.

This^^^

AKA "Suspension losses". It is also why 29er wheels conserve momentum so noticeably better off-road - the smoother axle path absorbs less of your kinetic energy.

There are limits, though. Fatbikes - which you would think should be even better on that basis - aren't any quicker and the signficant additional weight of the rubber could be a factor, along with handling issues from the undamped movement from having such a big bag. Some riders report understeer being a problem, although causes for this could be anything from geometry to poor technique rather than necessarily a function of the impact of tyre width on handling dynamics.

Who is online

Users browsing this forum: 49Vanguard, Cycleops70, familyguy, Google Feedfetcher, uart