N.B. Article by Chris_Bike, entered here by flecc
Flecc challenged me to write an article on what makes for an efficient bike and I’ve fallen for it! We are talking about human-powered bikes here, but most of the same principles will apply to power-assisted bikes to one degree or another.
The first thing to acknowledge is that bicycles are remarkable efficient machines. The track bikes that you will see flying around the Manchester or Beijing velodromes are probably transmitting nearly 100% or rider effort into forward motion. Even your poorly maintained and rusting mountain bike in the shed will achieve over 80%. So what we are talking about here is how you make sure that you get the best out of that remaining <20%.
I think that the significant influences can be divided into two categories, those that are rider-related and those that are mechanical or bike-related. Some, like rider position and wind-resistance fall between the two.
Rider-related issues.
The power that we deliver to the drive chain of a cycle is a product of the torque we generate by pedalling action and the speed (rpm) at which we pedal – known as cadence. The torque that we can deliver is determined by mechanical features like leg and crank length but, most crucially, by the strength of the relevant muscle groups. Importantly, at the sort of speeds that most of us pedal, torque doesn’t change much with cadence. So the faster we pedal, usually up to about 100rpm, the more power we put down (at very high cadences, torque actually reduces).
So the first thing to say is that faster cadences are more efficient than slower ones. This doesn’t mean to say that we should all pedal at 120rpm (although the UK Olympic squad all can, with ease). Cadence usually falls off with age, even in really fit riders. What this does say however is that the available range of gears we have (and what use we make of them) is important – pretty well all riders will be delivering more power at 60-70 rpm than at 30-40rpm. You can feel this on any hill, where it is easier to climb in a lower gear, pedling at higher cadences.
The second rider-related issue is that of position on the bike. Position can influence torque as our leg is basically acting as a lever, but the most important effect of position is on wind-resistance or drag. Serious racing cyclists go to a lot of trouble to adopt a maximally “tucked” position when riding into the wind alone (riders in a group can derive a great deal of shelter form other riders). As I write this, Emma Pooley has just won a silver medal in the Olympic Women’s Time Trial on a bike developed by Chris Boardman and British Cycling. As a small rider, she would have been really hard to get into a truly aerodynamic position and I suspect that she will have spent some time in a wind tunnel getting it right. We clearly can’t all do this but, in general, adopting a lower position on the bike will be more efficient that riding upright.
Bike-related issues.
Chains are a very efficient means of transmitting power and the drive chain is not usually a source of inefficiency. Keeping them clean is important as grit and other debris will introduce friction. A major reason for lubricating your chain is to keep the dirt out and chains should be cleaned and re-lubricated frequently. There is some evidence that the chain drives larger sprockets (gears) with somewhat more efficiency than it does small ones, but the use of small gears is usually determined by the cadence that we can comfortably maintain and that, in itself, will aid efficiency.
Derailleur gears usually have lower frictional losses than hub gears, but the reasons for fitting one or other to a particular bike are not usually based upon considerations of efficiency (durability and ease of maintainence being more common considerations).
Tyres can certainly affect frictional losses and, in my view are one of the most frequently overlooked components on a bike. There is often much concern about puncture resistance but rolling resistance (friction on the road) has an enormous effect on ride quality. Rolling resistance is influenced by the physical resistance of the tyre compound and the degree of inflation. I would always recommend inflating tyres towards the top of their stated range. Harder tyres do deliver a harder ride, but they are much more efficient. (Road bumps can be smoothed out by raked or suspension forks and suspension seat posts if required, these may have minor effects on overall efficiency, but not nearly as great as those caused by under-inflated tyres). Tyres that are “slick” (little tread) invariably roll better than those with a deep tread, and narrow tyres roll better than wide ones. It is a fallacy to assume that a tyre with a knobbly tread will grip the road better (as opposed to a muddy track, where knobbly tyres do deliver). There are plenty of nearly slick tyres that remained glued to the road, even in torrential rain – it’s the quality of the rubber compound that achieves this performance.
Weight.
The weight of a bike is an issue, most especially when riding up hills, but this is not a reason to start worrying about every last gram weight of your equipment. The difference in weight between middle of the range and top of the range equipment is sometimes the same as the chocolate bar in your pocket! If, like me, you are at least 10 kilos overweight, is the extra 200 grams on the weight of your frame really going to make that much difference? However, where I would defend weight saving is in wheels and tyres. It is rider effort that turns these and, on hills especially, rolling weight really does make a considerable difference. This is the reason, of course, why climbers get out their lightest carbon wheels for the mountain stages of the Tour de France. By the way, weight at the rim is more significant than weight at the hub (flywheel effect) so don't get too paranoid if you have a hub motor in your ebike!
Geometry.
The geometry of a bike (a combination of frame angles/sizes and how it is set up) certainly makes a difference to how it feels and sporty bikes with steepish seat and head tubes together with a short wheel base do feel “nippier” and more responsive. This is probably partly to do with the way in which human power is transmitted through the cranks, partly to do with sharper steering on a shorter wheelbase and partly because shorter frames flex less under rider effort. Whatever the reason, it’s true, as you can tell for yourself by going round to your local bike shop to test ride a few models. I confess that I find it hard to explain what is happening here in strict physical/engineering terms, but the effect is undeniable.
So that’s some initial thoughts on cycle efficiency. I am happy to be responsive to comments and will try to incorporate further thoughts that emerge (but I’m not going to get into an argument about whether light wheels are better on hills – I’m afraid they just are!)
__________________
Chris
1970's Bob Jackson, 1980's Pinarello, 1990's Alves Tandem, 2008 Cytronex Trek
Flecc challenged me to write an article on what makes for an efficient bike and I’ve fallen for it! We are talking about human-powered bikes here, but most of the same principles will apply to power-assisted bikes to one degree or another.
The first thing to acknowledge is that bicycles are remarkable efficient machines. The track bikes that you will see flying around the Manchester or Beijing velodromes are probably transmitting nearly 100% or rider effort into forward motion. Even your poorly maintained and rusting mountain bike in the shed will achieve over 80%. So what we are talking about here is how you make sure that you get the best out of that remaining <20%.
I think that the significant influences can be divided into two categories, those that are rider-related and those that are mechanical or bike-related. Some, like rider position and wind-resistance fall between the two.
Rider-related issues.
The power that we deliver to the drive chain of a cycle is a product of the torque we generate by pedalling action and the speed (rpm) at which we pedal – known as cadence. The torque that we can deliver is determined by mechanical features like leg and crank length but, most crucially, by the strength of the relevant muscle groups. Importantly, at the sort of speeds that most of us pedal, torque doesn’t change much with cadence. So the faster we pedal, usually up to about 100rpm, the more power we put down (at very high cadences, torque actually reduces).
So the first thing to say is that faster cadences are more efficient than slower ones. This doesn’t mean to say that we should all pedal at 120rpm (although the UK Olympic squad all can, with ease). Cadence usually falls off with age, even in really fit riders. What this does say however is that the available range of gears we have (and what use we make of them) is important – pretty well all riders will be delivering more power at 60-70 rpm than at 30-40rpm. You can feel this on any hill, where it is easier to climb in a lower gear, pedling at higher cadences.
The second rider-related issue is that of position on the bike. Position can influence torque as our leg is basically acting as a lever, but the most important effect of position is on wind-resistance or drag. Serious racing cyclists go to a lot of trouble to adopt a maximally “tucked” position when riding into the wind alone (riders in a group can derive a great deal of shelter form other riders). As I write this, Emma Pooley has just won a silver medal in the Olympic Women’s Time Trial on a bike developed by Chris Boardman and British Cycling. As a small rider, she would have been really hard to get into a truly aerodynamic position and I suspect that she will have spent some time in a wind tunnel getting it right. We clearly can’t all do this but, in general, adopting a lower position on the bike will be more efficient that riding upright.
Bike-related issues.
Chains are a very efficient means of transmitting power and the drive chain is not usually a source of inefficiency. Keeping them clean is important as grit and other debris will introduce friction. A major reason for lubricating your chain is to keep the dirt out and chains should be cleaned and re-lubricated frequently. There is some evidence that the chain drives larger sprockets (gears) with somewhat more efficiency than it does small ones, but the use of small gears is usually determined by the cadence that we can comfortably maintain and that, in itself, will aid efficiency.
Derailleur gears usually have lower frictional losses than hub gears, but the reasons for fitting one or other to a particular bike are not usually based upon considerations of efficiency (durability and ease of maintainence being more common considerations).
Tyres can certainly affect frictional losses and, in my view are one of the most frequently overlooked components on a bike. There is often much concern about puncture resistance but rolling resistance (friction on the road) has an enormous effect on ride quality. Rolling resistance is influenced by the physical resistance of the tyre compound and the degree of inflation. I would always recommend inflating tyres towards the top of their stated range. Harder tyres do deliver a harder ride, but they are much more efficient. (Road bumps can be smoothed out by raked or suspension forks and suspension seat posts if required, these may have minor effects on overall efficiency, but not nearly as great as those caused by under-inflated tyres). Tyres that are “slick” (little tread) invariably roll better than those with a deep tread, and narrow tyres roll better than wide ones. It is a fallacy to assume that a tyre with a knobbly tread will grip the road better (as opposed to a muddy track, where knobbly tyres do deliver). There are plenty of nearly slick tyres that remained glued to the road, even in torrential rain – it’s the quality of the rubber compound that achieves this performance.
Weight.
The weight of a bike is an issue, most especially when riding up hills, but this is not a reason to start worrying about every last gram weight of your equipment. The difference in weight between middle of the range and top of the range equipment is sometimes the same as the chocolate bar in your pocket! If, like me, you are at least 10 kilos overweight, is the extra 200 grams on the weight of your frame really going to make that much difference? However, where I would defend weight saving is in wheels and tyres. It is rider effort that turns these and, on hills especially, rolling weight really does make a considerable difference. This is the reason, of course, why climbers get out their lightest carbon wheels for the mountain stages of the Tour de France. By the way, weight at the rim is more significant than weight at the hub (flywheel effect) so don't get too paranoid if you have a hub motor in your ebike!
Geometry.
The geometry of a bike (a combination of frame angles/sizes and how it is set up) certainly makes a difference to how it feels and sporty bikes with steepish seat and head tubes together with a short wheel base do feel “nippier” and more responsive. This is probably partly to do with the way in which human power is transmitted through the cranks, partly to do with sharper steering on a shorter wheelbase and partly because shorter frames flex less under rider effort. Whatever the reason, it’s true, as you can tell for yourself by going round to your local bike shop to test ride a few models. I confess that I find it hard to explain what is happening here in strict physical/engineering terms, but the effect is undeniable.
So that’s some initial thoughts on cycle efficiency. I am happy to be responsive to comments and will try to incorporate further thoughts that emerge (but I’m not going to get into an argument about whether light wheels are better on hills – I’m afraid they just are!)
__________________
Chris
1970's Bob Jackson, 1980's Pinarello, 1990's Alves Tandem, 2008 Cytronex Trek
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