Bike weight vs Body weight

[DieselnDust]

Bike weight is over rated - every body weighs a good 6-10 times the bike.

 

We lose bike energy for bling/ego but lost body weight is a much cheaper way to lose weight and I'd say most cyclists have a few kg to lose here and there...

 

Also - the only difference that weight makes when riding a bicycle is energy lost due to wind resistance. Of course that is a theoretical statement and is a little different in the real world but it is true none the less.

 

 

Not sure what you mean by this. Can you explain.

Drag is measured in grams and equated to Newtons or Force to calculate the power absorbed in overcoming drag.

But its not the same as weight as they have different force vectors.

Total weight would influence rolling resistance more than drag.

unless you mean that more weight is higher inertia to overcome drag? If so then inertia requires acceleration and acceleration requires force. More mass requires more force (f=m.a)

[Eldron]

Not sure what you mean by this. Can you explain.

Drag is measured in grams and equated to Newtons or Force to calculate the power absorbed in overcoming drag.

But its not the same as weight as they have different force vectors.

Total weight would influence rolling resistance more than drag.

unless you mean that more weight is higher inertia to overcome drag? If so then inertia requires acceleration and acceleration requires force. More mass requires more force (f=m.a)

 

I meant in terms of climbing - if a million riders of varying weights riding at the same speed all climb a hill and descend to the same height on the other side they will all be in exactly the same place doing the same speed on the other side (if done in a vacuum).

 

The issue is wind resistance - it's not linear so the heavier riders (with larger surface area) will lose a little speed the whole way.

 

Of course there will be a little energy lost in tyre friction for the heavier riders.

 

Just an interesting way of looking at bicycle/rider weight.

[The Ouzo]

I meant in terms of climbing - if a million riders of varying weights riding at the same speed all climb a hill and descend to the same height on the other side they will all be in exactly the same place doing the same speed on the other side (if done in a vacuum).

 

The issue is wind resistance - it's not linear so the heavier riders (with larger surface area) will lose a little speed the whole way.

 

Of course there will be a little energy lost in tyre friction for the heavier riders.

 

Just an interesting way of looking at bicycle/rider weight.

uhm maybe I'm missing something.

when those riders are descending, gravity has an effect and the heavier riders will pick up speed quicker than the lighter ones.

Gravity will also play a role going up hill, but you mentioned they were climbing at the same speed.

[Eldron]

uhm maybe I'm missing something.

when those riders are descending, gravity has an effect and the heavier riders will pick up speed quicker than the lighter ones.

Gravity will also play a role going up hill, but you mentioned they were climbing at the same speed.

 

Starting speed at the same not on the climb.

[Captain Fastbastard Mayhem]

I meant in terms of climbing - if a million riders of varying weights riding at the same speed all climb a hill and descend to the same height on the other side they will all be in exactly the same place doing the same speed on the other side (if done in a vacuum).

 

The issue is wind resistance - it's not linear so the heavier riders (with larger surface area) will lose a little speed the whole way.

 

Of course there will be a little energy lost in tyre friction for the heavier riders.

 

Just an interesting way of looking at bicycle/rider weight.

There's also the matter of gravity, which essentially means the heavier rider needs to do more work (watts) than a lighter rider in order to overcome the effect of gravity on a heavier body. 

 

Aaaah, I get what you're saying. But no, not really. In order to do that, the difference in average speeds between the riders would need to reverse on the way down. 

 

IE: if rider A is doing 10kph up the hill, and rider B does 5kph up the hill, on the way down (with equal distance) rider B would need to be going double the speed of rider A in order to finish at the same time.

 

Without wind resistance that may be possible, but the difference in wind resistance is not as large as you may think, and wind resistance is not a function of weight, either. Even in a vacuum, they'd accelerate at the same rate and therefore if the guys freewheel from the top of the hill, the rider that gets to the top first will always win. 

[Jehosefat]

Gravity will also play a role going up hill, but you mentioned they were climbing at the same speed.

 

Yeah, in order to climb at the same speed the heavier riders will need to be putting out more watts as well...

[DieselnDust]

If the two riders descend in a vacuum then yes they will descend at the same speed assuming all losses are equal.

The heavier rider will take longer to come to a stop at they will have more kinetic energy at the same point on the course.

The heavier rider also starts with more potential energy (m.g.h).

Add in an atmosphere and the picture isn't as clear cut. To a point the bigger rider is at a disadvantage but there is an intersection where the kinetic energy over comes the air resistance more and the two riders speed starts to equalise. For each coeffiecient of drag there is an upper velocity limit in free fall conditions. The smaller rider may not get there before the heavier rider.

[Eldron]

There's also the matter of gravity, which essentially means the heavier rider needs to do more work (watts) than a lighter rider in order to overcome the effect of gravity on a heavier body. 

 

Feck - I forgot to say all riders use the same power. Hahaha doh.

 

Gravity is irrelevant in the equation - all energy gained as potential energy by the heavier riders is returned on the downhill.

 

Edit: Don't work and hub at the same time!

[Wayne pudding Mol]

It's so much more fun buying light components than it is giving up beer - in fact I've just got back from a business trip and I have a very light box from Schmolke waiting  - I'm going to open a beer knowing that the weight I'll gain will be mitigated by my new lightweight stem and handlebars

Yes I know I'm kidding myself but imagine how much faster I'm going to be on my light bike when I eventually commit to the diet

[Captain Fastbastard Mayhem]

Feck - I forgot to say all riders ride at the same power/weight. Hahaha doh.

 

Gravity is irrelevant in the equation - all energy gained as potential energy by the heavier riders is returned on the downhill.

Still doesn't get away from the fact that the difference in speeds needs to be reversed on the way down. IE if you're twice as fast on the way up, I need to be twice as fast on the way down, assuming the distance up and down are exactly the same. Wind resistance isn't a function of weight, just aerodynamics. Also, absent wind resistance and all other frictional forces, they'll go down at the same speed anyway. It's just that the bigger dude will have more kinetic energy, momentum & inertia due to the increased mass. 

[Eldron]

Still doesn't get away from the fact that the difference in speeds needs to be reversed on the way down. IE if you're twice as fast on the way up, I need to be twice as fast on the way down, assuming the distance up and down are exactly the same. Wind resistance isn't a function of weight, just aerodynamics. Also, absent wind resistance and all other frictional forces, they'll go down at the same speed anyway. It's just that the bigger dude will have more kinetic energy, momentum & inertia due to the increased mass. 

 

All riders will start and end up at exactly the same speed in the exactly the same place if they climb at the rate power.

 

The lightest rider will get to the top first but gain the least potential energy. The heaviest rider will get to the top last but gain the most potential energy.

 

All potential energy will be returned and everyone will be in exactly the same place, doing the same speed on the other side of the hill.

 

Edit: Check my edit - I removed the "weight" portion of the power/weight equation.

[DieselnDust]

All riders will start and end up at exactly the same speed in the exactly the same place if they climb at the rate power.

 

The lightest rider will get to the top first but gain the least potential energy. The heaviest rider will get to the top last but gain the most potential energy.

 

All potential energy will be returned and everyone will be in exactly the same place, doing the same speed on the other side of the hill.

 

Edit: Check my edit - I removed the "weight" portion of the power/weight equation.

 

 

ah I must have misunderstood you. Thought it was a race from the top only. Laws of Conservation of momentum and conservation of energy dictate that they must finish together if they started together.

we know in practice this doesn't happen due to the losses, of which air resistance is one

[Captain Fastbastard Mayhem]

All riders will start and end up at exactly the same speed in the exactly the same place if they climb at the rate power.

 

Nope. Heavy rider would need to go twice as fast over the descent, if he climbed half the speed of the light rider, due to same power but double the weight. Simple. 

 

The lightest rider will get to the top first but gain the least potential energy. The heaviest rider will get to the top last but gain the most potential energy.

Yes, but that's just PE & KE. PE & KE are functions of weight and velocity. Heavier rider has more PE / KE purely due to being heavier. At same V / H, heavier rider has more KE / PE.

 

All potential energy will be returned and everyone will be in exactly the same place, doing the same speed on the other side of the hill.

Nope. They'll only be at the same place if heavy descends at double the avg speed of lightie (assuming half the weight) due to the DISTANCE they need to cover.

 

Edit: Check my edit - I removed the "weight" portion of the power/weight equation.

 

 

If they had the same velocity at the end, the heavy dude would simply have double the KE of the light dude (assuming 100kg vs 50kg) due to the difference in mass. KE = 1/2 MV^2

 

Even if they were the same weight, and one climbed faster than teh other, the other would need to descend faster than the one to get to the end at the same time. It's a simple case of calculating the required avg speed in order to complete the distance in the same time. Nothing to do with potential / kinetic energy. 

 

If they end at the same speed, at some point the gap between them is constant, and he will never catch the light guy. If the heavier rider never goes faster than the lighter rider, then he will never catch up on the descent. If they both reach max speed as soon as they start descending, heavy rider will STILL be behind the light rider by the same amount of time they were apart at the top of the hill. 

 

 

[Eldron]

ah I must have misunderstood you. Thought it was a race from the top only. Laws of Conservation of momentum and conservation of energy dictate that they must finish together if they started together.

we know in practice this doesn't happen due to the losses, of which air resistance is one

 

Yup - I only had half a brain on thehub while I was working and left out several key factors :-)

 

When I was mucking about with the maths I found it interesting that the only difference weight plays when climbing and descending is in wind resistance. Scientifically obvious but it messes with the emotional logic. 

 

Edit: And some rolling resistance of course. Mechanical resistance will be minimal.

[Captain Fastbastard Mayhem]

Yup - I only had half a brain on thehub while I was working and left out several key factors :-)

 

When I was mucking about with the maths I found it interesting that the only difference weight plays when climbing and descending is in wind resistance. Scientifically obvious but it messes with the emotional logic. 

if power to weight is not the same on the climb, and the heavy guy does not finish fast enough to overcome the diff in avg speed up to that point, PE doesn't mean a thing. He will HAVE to go faster than the lighter guy on the descent. 

[Captain Fastbastard Mayhem]

Unless we're talking completely different things and you're ignoring the time differential between them at the top. Even wind resistance doesn't mean as much, as without it, and assuming no pedalling etc they'd descend at the same speed. Heavy would still be behind, if he was behind on the climb,.