Rotating weight and climbing
Rotating weight is one of those cycling knowledge things that everyone talks about without thinking. As sfb says, weight is weight whether it rotates or not.Posted 7 years ago
The 'accelerations' that are used to suggest that rotating weight is important are very very small, and in the context of a chubby biker plus bike plus camelbak make next to no difference at all.
It may well double it, I think I picked up 3xs somewhere but could have got it wrong. There seems to be a lot of talk about lighter wheels and tyres and I can see that this would make a difference in a racing environment but for the more recreational rider does it make much of a difference?Posted 7 years agobreatheeasyMember
Suppose you could argue that the less good you are more stop and start your riding is, so technically rotating mass becomes more important.
In practice I don't think it really matters. You start worrying about grams on tyres, get the lightest you can find, than daren't ride the lines you were doing on the big ruffty tufty ol' heavy ones thus self defeating.Posted 7 years agoLionheartMember
200g off here per wheel but only 4 mins quicker – am I doing something wrong?
I have run different weight wheel/tyre combos on the same set up bike and it has made significant differences, currently run a Heckler with near DH wheel tyre set up – pretty difficult on every where but pointing downhill but with its XC wheels/tyres its great everywhere else. It is not just the extra weight but the rotational weight and unsprung weight make a noticeable difference – IMO 😉Posted 7 years ago
simonfbarnes – Member
One suggestion was that when climbing at a constant speed (not accelerating) it's dead weight that matters, not rotating weight (as you're not accelerating)
I'm not sure that is correct, the problem with climbing would be that gravity wants to make the wheels go in the reverse direction, therefore it requires effort to keep them moving at a constant speed. Therefore the effort need to move the rotating parts is still a factor.
Compare with riding on the flat and you don't really have gravity trying to take you in the opposite directionPosted 7 years agoRealManMember
A formula for centripetal acceleration (I think)
f is force, m is mass, v is velocity, r is radius.
I don't know if this would apply to this situation, but if it did, f is proportional to m, so double the weight of your wheels, double the force needed.
Maybe someone good at physics could confirm this/tell me I'm an idiot?Posted 7 years ago
the problem with climbing would be that gravity wants to make the wheels go in the reverse direction, therefore it requires effort to keep them moving at a constant speed. Therefore the effort need to move the rotating parts is still a factor.
I'm am particularly bad with maths/physics type stuff but I kind of understood it along the lines that a heavier rotating mass holds more kinetic energy and decelerates less readily, so it kind of balances out – I could be utterly wrong though!Posted 7 years ago
I'm am particularly bad with maths/physics type stuff but I kind of understood it along the lines that a heavier rotating mass holds more kinetic energy and decelerates less readily, so it kind of balances out – I could be utterly wrong though!
I would agree with that going downhill or even on the flat, but when climbing I think heavier wheels will decelerate quicker than light wheels and therefore require more effort to keep the wheels moving.Posted 7 years ago
Does it make a difference? I've just been reading a discussion relating to cycling and rotational weight on the internet and there seemed to be a fair bit of disagreement relating to the effect of rotational weight (heavy tyres/rims) and climbing (or cycling in general). As I understand it (and I probably don't understand it very well) static/dead weight (in say a tyre or rim) equates to 3x that when when rotating – heavier tyres require more engergy to accelerate to a given speed. However, I think that they decellerate more slowly too, so the difference in maintaining (not accelerating) a given speed between lighter and heavier tyres is negligable.
How does this work when climbing? One suggestion was that when climbing at a constant speed (not accelerating) it's dead weight that matters, not rotating weight (as you're not accelerating), other people seem to disagree. For the non-racer (where acceleration is important) does rotating weight matter at all?Posted 7 years ago
I think heavier wheels will decelerate quicker than light wheels and therefore require more effort to keep the wheels moving.
wrong. More inertia means decelerate slower. But intertia is only about changing speed. If the speed is constant then no force is required to maintain that speed, except for the vertical component against gravity – which is of course dependent on the static inertia only. So heavier wheels will only be harder for climbing due to their actual mass.Posted 7 years ago
heavier wheels will decelerate quicker than light wheels
The problem with this and other weight/performance issues is the context; that's the bit that everyone ignores. Theoretically, rotating weight makes a difference, theoretically you can model it and measure it.
But in the real world, over a two hour xc style ride the actual difference it makes is so small it gets lost in the noise…Most people would struggle to do a ride on different days, in different conditions in the same time. The time difference on different rides, the actual variable time it takes to cover the distance is far greater than the supposed theoretical difference that reducing rotating weight will provide.
As for the above, think about a flywheel; takes more energy to spin up but spins for longer and remember, wheels don't know they are going uphill…
Aerodynamics and rolling resistance are far more influential than rotating weight, but try selling lycra and semislicks to your average STWer………….Posted 7 years ago
As I understand it, mass adding to the inertia of the wheel is not going to effect climbing and more that the same mass being added elsewhere. However, as off road riding often involves more changes of speed, say in switchbacks, the easier acceleration and deceleration should be of more benefit than on the road. On the topic of weight – a pro british road rider shed about 8 punds and (because he maintained his power output) it cut his time by something huge like 30 seconds (a 30 minute climb I think).
Conclusion = weight of the bike important if you have <10% body fat already.Posted 7 years agob rMember
It's not just rotating mass – it's also wheel diameter, so 29ers should really suffer if it matters!
Its all 'weight', at a distance from the axle.
I've a few wheels/tyres etc and the heavy wheel/tyre/rotor setup is bloody hard work to pedal vs a more standard/lighter set-up.
And tbh there is nothing that I'd worry about going down on the lighter set.Posted 7 years agozaskarMember
PJay, why don't you write to an expert? or even a sport specialist university, hell you might get invited to a talk.
There will be things you won't know, hell I sometimes go back to GCSE and A'level books then journals.
If you are keen to learn then its even easier.
(Did race car geometry myself)Posted 7 years ago
so a 1 in 60 improvement for a 1 in 28 (assuming 12st rider and 28lb bike) weight loss ? That suggests there was a loss of power too…
As I'm busy and still at work I was going off the top of my head, hence the 'something huge like'… the figures dont matter, the point I was trying to make was that it was a significant gain in performance for the weght saving.Posted 7 years agoDickyboyMember
For the last time… 😆
Gravitational potential energy = mass x height x acceleration due to gravity – so any reduction in mass is directly proportional to the amount of energy you put in when hill climbing.
Kinetic energy = 1/2 x mass x velocity squared, however the kinetic energy of a rotating hoop (a wheel) is roughly equal to mass x velocity squared (ie twice as much as the non rotating parts) – but this only has effect when accelerating or decelerating.
If you are accelerating up the hill then 3x is probably correct, but then the mass is also working against you 2x already in that instance. The normal state would be to riding up a hill at a fairly constant speed so the energy input is pretty much just adding to the potential energy (plus rolling & wind resistance).
That's the maths – how much effect it has is approx 5% comparing 20lb bike plus rider to 30lb bike plus rider when riding up a hill, the jury is out as to how much effect that has in the real world…Posted 7 years ago
significant gain in performance for the weght saving.
The assumption being that everything else was unchanged…
Maybe he trained better too, maybe he had access to the British Cycling gurus, maybe he was less fatigued, maybe maybe maybe….
Weight reduction is significant at the upper reaches of cycling performance, but again, try convincing the average STWer that he doesn't need 3 litres of water in a camelbak to ride around a reservoir….Posted 7 years ago
Again context…. Your average rider, rather than Bradley the bastard son of Paul Weller, would be hard pushed to do a 30 minute climb consistently, and I would guess that the differences in each climb would mop up a significant proportion of that supposed gain…
Consistency doesn't matter – the question is, would the lighter wheels give a faster time (e.g. if you did 1000 climbs on each set of wheels with the same rider and bike – theoretical…)Posted 7 years agocynic-alMember
Been done before – Crikey has yet to come out with the proof he claimed he had behind his
FWIW my view is that it does make more of a difference – it's about moment of inertia. I've not done the sums but even if it's mainly psycological that that's enough for some.
I have 2 sets of wheels – 1 is olympics and light tyres, the other X3.1s and heavier tyres. 400gm of weight difference at the rim per wheel, difference is easily noticeable.Posted 7 years ago
Consistency doesn't matter – the question is, would the lighter wheels give a faster time
Theoretically yes, they would.
In the real world, there will be a significant overlap between light wheel times and heavier wheel times, such that the proposed performance benefit will be even less than is theoretically possible.
The weight difference will be of the order of hundreds of grammes; the weight of a water bottle, the weight of wet clothes as opposed to dry clothes, the weight of a muddy bike as opposed to a clean one.Posted 7 years ago
Lost in the noise….
crikey, the thing is all other things being equal, you will be somewhat faster with lighter wheels, if you get muddy with both light wheels and heavy wheels, the light wheels with mud are still lighter than the heavy wheels with mud. Really the same would apply for any component, lighter = faster (uphill at least).Posted 7 years ago
I've not done the sums but even if it's mainly psycological that that's enough for some.
..agreed. Performance in cycling has a large psychological component, and the power of the placebo effect is well known ( I'm most definetly not using placebo in any negative way; it works and is a provable thing).
But 400grammes? From a total weight of 100ish kg? 0.04%…
I think we will have to agree to differ, but I can't see how it makes as much difference as people claim…
-Having said that, I use wheels on my road bike because they are 50gs lighter than those it came with…Posted 7 years agoIanMunroMember
wrong. More inertia means decelerate slower. But intertia is only about changing speed. [b]If the speed is constant[/b] then no force is required to maintain that speed, except for the vertical component against gravity – which is of course dependent on the static inertia only. So heavier wheels will only be harder for climbing due to their actual mass.
This is the crux though. I suspect if you fit an accelerometer, you will see lot's of accelerations, of course you could just say they're insignificant, but I suspect it would need some proper experimentation to determine their significance in real world situations.Posted 7 years ago
the figures dont matter, the point I was trying to make was that it was a significant gain in performance for the weght saving.
significant if you're racing, negligible otherwise 🙂
how can speed be constant uphill without applying force anyway, if its harder to accelerate heavy wheels it should also be harder to keep them at speed when gravity wants to slow them down.
I said there will be force to lift the bike against gravity, and yes, this will be greater for heavier wheels due to their mass, however, the extra rotational inertia of the wheels does not affect the force needed to lift them. If it did, a spinning wheel (or anything else) would have to be heavier than a non spinning thing, which is not the case.Posted 7 years ago
The topic ‘Rotating weight and climbing’ is closed to new replies.