Physics of Bicycles

[Peter.Bridge]

I was interested in where the energy to power a bike goes and the various bike design parameters, not really specific to just electric bikes e.g.

1) weight - at first I was thinking this only really applied to hills, but I'm realising that there are very few completely flat roads and even on a slight incline weight makes a difference (although I think in most cases the weight of a rider is the most obvious place to make weight savings!)
2)tyres - so the rolling resistance of tyres must be a big factor - I have changed over from mtb tyres to more road based tyres and I feel that this must make a big difference
3)Wind resistance - this seems to become a bigger factor , as you go faster (or stronger wind ). So racing bikes encourage a riding position that is more aerodynamic ? I watched some cyclists doing a time trial and they were hurtling down a very long but not hugely steep hill and the cyclists had their elbows, wrists and hands horizontal on the handlebars, pointing forward and then head down and tucked in
4) Gearing - obviously you need the gearing so that you can pedal at your optimum rate at the speed/incline you are on
5) Mechanicals ? Are the mechanicals of a road bike much more efficient at transferring the pedalling power to the rubber on the road than a mtb or is the speed difference on road explained by the weight / tyres / gearing/ more aerodynamic seating position ?

 

[saneagle]

My thoughts:
1. Wherever there's an upward incline, the energy to go up it increases approximately 1% for each 1kg weight you have, assuming bike and rider weigh about 100kg. When you go downhill, the additional weight increases your terminal velocity and helps overcome any effect of headwind.
2. The effect of rolling resistance isn't massive. You can Google it to find out how much it is. You can feel the difference between narrow smooth high pressure tyres and knobblies. Fatbike tyres are ridiculous. The tyres make a difference to wind resistance too, which is unnoticeable at low speed, but at 20 mph, there's a big difference.
3. Wind resistance is the biggest factor. It's a square relationship with speed, so not much at low wind-speed, but substantial when you go fast. It's what limits your max speed down a hill. Clothing makes more of a difference than anything. Pro cyclists shave their legs.
4. Most bikes have gearing that suits normal riding, but if you want your fastest journey times, you need gearing that enables you to pedal with full power downhill.
5. The mechanicals are more about saving weight and cutting down air resistance.

I have two roadbikes. One is a medium sized Lightspeed titanium with very thin rims and tyres, the other is a large size Giant carbon fibre with 25mm tyres. The Giant is a really comfortable bike. You hardly feel bumps, and I was doing up to 80 miles a day on it. The Lightspeed is very rigid, It breaks your back when you run over a frozen pea, but it blitzes the Giant on best times, even though I don't ride that fast, as we have a lot of hills. I can see from tracking that it's very slightly faster uphill because it's about 1kg lighter, but it's much faster downhill.

 

[Deleted member 16246]

As Saneagle says. Good points made.

For me, not riding fast anymore, I'm less affected by wind resistance than fifty years ago, but tyres are fundamental. Thinner, high pressure tyres roll so much more easily than mountain bike tyres or even those for hybrid road/ rough path use. I can not understand the trend for those ridiculous fat tyres. I think some of them are fatter than those on 125cc motorbikes. The energy loss involved in rolling them down the road must be significant. Even swapping from a road bike to a mountain bike you can immediately feel the difference. Mostly, it is about tyres.

The OP is completely right about weight savings being more a rider issue than one for the bike. The other thing is that rider weight loss is very much cheaper than losing it through buying bike parts made from exotic materials. Better for the rider's wellbeing too.

 

[Az.]

4) Gearing - obviously you need the gearing

 

[AntonyC]

These are both popular:

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Powercalc vs ebike.ca simulator

I recently became aware of a cycle power calculator, Powercalc, shareware with an obnoxious nag screen, from http://www.machinehead-software.co.uk/cycling.html It is for bicyclists but includes power in watts required under specified conditions. The ebike.ca simulator has to calculate the load...

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If limited gear range means choosing between pedalling downhill and your preferred cadence uphill, you usually gain more time by optimising uphill.

 

[guerney]

Looks a great lark until that chainwheel (how many teeth does it have anyway?) hits a speed bump.

hills

There's a 23% increase in turning force where tyre meets road, when switching from a 26" wheel to 20"... therefore if increasing amperage to your "250W" manufacturer rated motor doesn't result in enough power for your hill climbing needs, transfer the kit to a bike with a 20" electrically driven wheel, or smaller. Bit of a stopgap measure until the DaFT change the law, if. My 20" wheeled bike eats hils for dinner What wonders a "500W" rated motor could work, would be more interesting for larger wheeled dudes who struggle with hills.

The other thing is that rider weight loss is very much cheaper than losing it through buying bike parts made from exotic materials. Better for the rider's wellbeing too.

That's a tall order when you're too wide. Of course it's soooo enjoyable putting weight on. No pain no loss, but it's far easier spending money on expensive lightweight components...it's a false economy in the long run, which many people could do with on a regular basis IMHO, provided it's slow jogging.

 

[Deleted member 16246]

That's a tall order when you're too wide. Of course it's soooo enjoyable putting weight on. No pain no loss, but it's far easier spending money on expensive lightweight components...it's a false economy in the long run, which many people could do with on a regular basis IMHO, provided it's slow jogging.

Yes - too easy to enjoy (over-enjoy) your food and drink. I know the feeling. I need to lose about 10 pounds to put me back in the green on the BMI calculator. Mind you, The measure is a rough one. BMI does not suit every body type. If you are of a robust build - I'm not talking fat - you will have an over-weight BMI. No rugby player or sturdy builder will be in the normal weight bmi category. Muscle is heavy, so if you do physical work, you will be heavier than the muscle free office worker who drives to work.

That can be a psychological excuse though - I mean self delusion. 'I am well built', can mean obese in some cases.

I need to lose 10 pounds. I know it. The problem is; when to start doing it. ....

I know you have done it - you said so.

I used to run a lot. My best half marathon time was 1hr 18, but that was when I was about 40. I would NOT recommend jogging for anyone who is 'heavy'. The forces on your knees, ankles and hips are hugely amplified by running. Have a care. Many of my contemporaries have agonising joint problems.

Brisk walking involves vastly lower impact forces. Running involves joint shocks that are three times body weight. Walking is half of that. You need to do it twice as long, but it will have the same overall health effect as far as heart health, diabetes control and general wellbeing is concerned.

Cycling will I would think mean very much easier time for joints, but you need to go a long way to use up calories.

 

[guerney]

Running involves joint shocks that are three times body weight.

I used to jog about a lot when I was young. I jogged so much, some people even said "Guerney, stop jogging".

I silently tai-chi glide along, like the ghost of a long dead ninja assassin, breathing almost normally. You know you're slow jogging right, when you can sneak up on cats. Before I lost weight, cycling made my knees hurt - slow jogging doesn't cause any joint pain, because slow jogging minimises joint jolts thusforth:

 

[Deleted member 16246]

I think our species evolved to walk and could run when we had to to escape or to catch prey.

We also evolved to live about forty years by which time most of us had left some self supporting children behind. Most of us would probably not be crippled by worn out joints at that age. Now we live a lot longer, but that is nothing to do with evolution, it is because we artificially prolong lives with medical care.

 

[flecc]

Now we live a lot longer, but that is nothing to do with evolution, it is because we artificially prolong lives with medical care.

And modern housing, heating, clean water supplies and sufficient food, which together took us to 70 years being commonplace before modern medical care.
.