Weight vs efficiency

[Pedalo]

How much does bike weight affect the range of a bike? I can't help thinking that an extra couple of kg can't make that much difference especially when considering it's only a small fraction of the combined weight of rider+bike. However lighter bikes do seem easier to ride so there's definitely something in it.

So as an example, if I had a bike weighing 20kg (and I weigh 80kg) and it had a range of 20miles, I managed to get the weight down by 2kg to 18kg, say by memoving the mudguards/rack, how much further might I expect to go?

Is it just a linear thing - 2% lighter(including rider) means 2% more range or is it more complex than that?

 

[daniel.weck]

I don't know the technical answer, but I can easily put on an extra 2 kilograms just by eating and wearing different clothes...

Seriously, I'd say that what impacts the range is mostly the pedal effort provided by the cyclist, and the hilly nature of the route. Weight is probably a negligible factor (within reasonable limits, of course...30+ kg unloaded bikes *will* have an impact on range ).

 

[flecc]

It is more complex as you've assumed it might be, and bike weight counts more. The difference is in the fact that bike weight added has no compensating factor, but our body weight does.

Our musculature develops to cope with whatever weight we are within reason, so if you gradually put on a stone in weight, your body through living with that increasing weight day by day grows the muscles to cope so you are still able to do what you normally do. It follows that your personal performance remains roughly constant over a range of moderate body weights, so those body kilos don't make too much difference.

By contrast, extra weight on a bike has no added way to cope with it, it's just extra work for you or your bike motor.
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[Pedalo]

It is more complex as you've assumed it might be, and bike weight counts more. The difference is in the fact that bike weight added has no compensating factor, but our body weight does.

Our musculature develops to cope with whatever weight we are within reason, so if you gradually put on a stone in weight, your body through living with that increasing weight day by day grows the muscles to cope so you are still able to do what you normally do. It follows that your personal performance remains roughly constant over a range of moderate body weights, so those body kilos don't make too much difference.

By contrast, extra weight on a bike has no added way to cope with it, it's just extra work for you or your bike motor.
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Thanks, that does make sense. I would guess that in my example above, if I was riding a bike but letting the motor do all the work, I would notice a 2% improvement in range. But if I was doing all the work and had the motor switched off I would notice more like a 10% improvement (before I fell over with exhaustion!).
For ebikes where the work is shared between motor and me, the range would be somewhere inbetween depending on how hard I pedal.

 

[Barnowl]

Mmmhhh. So if we cycle regularly shouldn't our leg musculature compensate for the extra weight of the bike - eventually. Not forgetting all that bric a brac I lug to work every morning. That 4 hour laptop battery weighs a ton.
A stiff breeze seems to present more of a problem though. It's strange how I get faster as the winter months approach despite the increase in air density.

 

[flecc]

Mmmhhh. So if we cycle regularly shouldn't our leg musculature compensate for the extra weight of the bike - eventually.

Yes, if you train to a higher workload it's possible your muscles can match that load. Unfortunately this mechanism works in the opposite direction too. If the bike weight reduces, the muscle power can be lost to suit over a period of cycling it, so reducing the bike weight could be self defeating. However, these only hold completely true if cycling is a dominant part of our waking life. For most their other activities will also play a major part in setting their muscle strength.

We are lucky enough to live in times when the thesis of muscle power matching body loading had been proven. Astronauts who live in space for a while in weightlessness become unable to walk on their return to earth due to their muscles wasting away, and for that reason they now have to do long sessions of vigorous exercise while in space to guard against that.

So if a bike's weight is reduced we do normally get the benefit in full since that causes no change in our strength, cycling generally being too small a part of our overall activity for the body to make a full overall adjustment. But if we manage to reduce our body weight we don't get that benefit due to the muscle strength adjustment to match. This only applies for a normal range of body weights, it doesn't hold good for extremes such as extreme obesity.
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[monster]

bike weight is insignificant if you are traveling at a constant speed. the only time you notice the weight is when you are accelerating from a stop. look at the speed and power calculator. you can add 20Kg to the bike and it will only change the top speed by 0.5mph at constant power.

 

[flecc]

bike weight is insignificant if you are traveling at a constant speed. the only time you notice the weight is when you are accelerating from a stop. look at the speed and power calculator. you can add 20Kg to the bike and it will only change the top speed by 0.5mph at constant power.

On the flat of course, but it's in hill climbing that it counts most and can make a real difference. Even on an e-bike I can tell if there's a little weight in the panniers on a 10% hill, and on steeper hills and especially when unpowered, weight becomes an increasingly big factor. As you can guess, I live in a very hilly area!
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[Barnowl]

I once encountered constant speed. Didn't last long though. Not just the hills - it's the constant stopping and accelerating (even on the flat).