[Closed] Rotating weight and 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)

correct

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.
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.

I thought the rotational inertia of rotating parts effectively doubled their resistance to acceleration, not tripled - but even so the actual mass is quite small compard to the total for bike + rider.

Someone will be along shortly to tell us that they lost 200g from their wheels and it made them 20 minutes quicker....

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?

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.

25lb bike. Lose a lb off with nicer tyres - seems a lot

Stick 180 lbs of bloke on it. Saving minimal really.

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 😉

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)

correct

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 direction

A formula for centripetal acceleration (I think)

F=(mV^2)/r

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?

Lower rotating mass is awesome for the traffic light sprint (watch the Bromptons go!) - am doubtful of its benefits off-road...

It's not just rotating mass - it's also wheel diameter, so 29ers should really suffer if it matters!

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!

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.

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 [b]slower[/b]. 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.

[i]heavier wheels will decelerate quicker than light wheels[/i]
No.

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.
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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.............

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.

http://youtube.com/watch?v=zhX01toLjZs&feature=related
http://youtube.com/watch?v=pG_kT565-XQ&feature=related

[i]It's not just rotating mass - it's also wheel diameter, so 29ers should really suffer if it matters! [/i]

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.

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)

by something huge like 30 seconds (a 30 minute climb I think).

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...

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...

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...

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...

I'm not desperate to learn the science behind it, I suppose it only really matters if it makes a difference when I ride. By the sound of it 80g or so in a tyre that gains in durability and puncture protection isn't going to kill me.

[i]significant gain in performance for the weght saving.[/i]

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....

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...)

Been done before - Crikey has yet to come out with the proof he claimed he had behind his [s]point[/s]opinion.

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.

difference is easily noticeable.

I've also found a noticable difference between lighter and heavier wheels... 'nuff said?!

[i]Consistency doesn't matter - the question is, would the lighter wheels give a faster time[/i]

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.
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).

[i]I've not done the sums but even if it's mainly psycological that that's enough for some.[/i]

..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...
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-Having said that, I use wheels on my road bike because they are 50gs lighter than those it came with...

[i]wrong. More inertia means decelerate slower. But intertia is only about changing speed. [/i][b]If the speed is constant[/b][i] 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.[/i]

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.

I'm not sure what you're trying to disprove crikey.

In terms of acceleration though, we must consider everything, cos that's what's being accelerated. Context.

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.

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.

Its all 'weight', at a distance from the axle.

But the greater the distance from the axle, the greater the angular momentum!

I would suggest that the more you use your brakes, the more rotational mass matters.

significant if you're racing, negligible otherwise

Significant to me - every ride is a race 😀

I'm not trying to disprove anything. I agree with the theory. I'm saying that in the real world, and specifically the real world of leisure cycling as practiced by me and you, that the benefit of reducing rotating weight as opposed to any other weight is overstated.
In addition, the reduction in weight as practised by me and you in the context of mountain biking and often asked about on here, has a negligble effect on our performance.
I do agree that psychological factors associated with perceived performance benefits are important.

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.

This would be fine if you were just trying to lift the bike over a fence but your trying to propel it forward as well as up by making the wheels go round in circles.

rs - he is right - the rotation of the mass can be ignored unless it is accelerating. going uphill at a steady speed nothing is accelerating

This would be fine if you were just trying to lift the bike over a fence but your trying to propel it forward as well as up by making the wheels go round in circles.

yes, but at constant speed, there is no acceleration, so inertia doesn't matter. It so happens that inertia and mass are directly proportional for non-rotating objects, and that rotational inertia is double that for objects where all the mass is concentrated at the same distance from the axis of rotation (like a wheel). When you lift something at constant speed, what matters is its mass, not its inertia.

I do agree that psychological factors associated with perceived performance benefits are important.

to which end it might be more useful to alter your beliefs creatively 🙂 For instance you could cultivate the belief that blue valve caps are faster.

Red bikes are faster - everyone knows that

Red bikes are faster - everyone knows that

I have a red rear hub and red nail varnish so I'm sorted 🙂

geek out here

www.math.usu.edu/~bryanb/math 2210/Rotational Inertia.doc

and here

www.analyticcycling.com/WheelsClimb_Page.html

Agreed, I'm quicker when I wear my new Raji gloves, and much quicker when I wear a cycling cap turned backwards instead of a helmet.

rs - he is right - the rotation of the mass can be ignored unless it is accelerating. going uphill at a steady speed nothing is accelerating

Well kind of...

When you're going up hill gravity is constantly pulling you down and your wheels are constantly wanting to decelerate. In order to overcome this deceleration you provide the acceleration force with your legs (at a constant speed these forces are balanced).
Unfortunately for us the inertia of the wheels is a contributing factor to what we are pedalling against. Thus at constant speed the rotating mass still has an effect on how much effort we have to put in.

EDIT: Also, if you think about it, the bike constantly accelerates and decelerates throught the pedal stroke. If yours doesn't you'll be given a new gold necklace in 2012.

Also, if you think about it, the bike constantly accelerates and decelerates throught the pedal stroke. If yours doesn't you'll be given a new gold necklace in 2012

Bingo. Out of the saddle on a big hill and roational inertia will make a difference.
Unless of cause your all roadies arguing the toss on a mountainbike forum

rs - your first link doesn't work and the second is starting from rest - so acceleration plays a part it is not steady state and there is no distinction between the effects due to rotating mass and static mass on the speed.

[i]Thus at constant speed the [s]rotating[/s] mass still has an effect on how much effort we have to put in.[/i]

Fixed that for you.

[i]EDIT: Also, if you think about it, the bike constantly accelerates and decelerates throught the pedal stroke. If yours doesn't you'll be given a new gold necklace in 2012.[/i]

Yes, but these 'accelerations' are miniscule and are [b]lessened[/b] by a heavier wheel....flywheel effect, remember?

[i]The advantage of light bikes, and particularly light wheels, from a KE standpoint is that KE only comes into play when speed changes, and there are certainly two cases where lighter wheels should have an advantage: sprints, and corner jumps in a criterium.[13]

In a 250 m sprint from 36 to 47 km/h to (22 to 29 mph), a 90 kg bike/rider with 1.75 kg of rims/tires/spokes increases KE by 6,360 joules (6.4 kilocalories burned). Shaving 500 g from the rims/tires/spokes reduces this KE by 35 joules (1 kilocalorie = 1.163 watt-hour). The impact of this weight saving on speed or distance is rather difficult to calculate, and requires assumptions about rider power output and sprint distance. The Analytic Cycling web site allows this calculation, and gives a time/distance advantage of 0.16 s/188 cm for a sprinter who shaves 500 g off their wheels. If that weight went to make an aero wheel that was worth 0.03 mph (0.05 km/h) at 25 mph (40 km/h), the weight savings would be canceled by the aerodynamic advantage. For reference, the best aero bicycle wheels are worth about 0.4 mph (0.6 km/h) at 25, and so in this sprint would handily beat a set of wheels weighing 500 g less.

In a criterium race, a rider is often jumping out of every corner. If the rider has to brake entering each corner (no coasting to slow down), then the KE that is added in each jump is wasted as heat in braking. For a flat crit at 40 km/h, 1 km circuit, 4 corners per lap, 10 km/h speed loss at each corner, one hour duration, 80 kg rider/6.5 kg bike/1.75 kg rims/tires/spokes, there would be 160 corner jumps. This effort adds 387 kilocalories to the 1100 kilocalories required for the same ride at steady speed. Removing 500 g from the wheels, reduces the total body energy requirement by 4.4 kilocalories. If the extra 500 g in the wheels had resulted in a 0.3% reduction in aerodynamic drag factor (worth a 0.02 mph (0.03 km/h) speed increase at 25 mph), the caloric cost of the added weight effect would be canceled by the reduced work to overcome the wind.

Another place where light wheels are claimed to have great advantage is in climbing. Though one may hear expressions such as "these wheels were worth 1-2 mph", etc. The formula for power suggests that 1 lb saved is worth 0.06 mph (0.1 km/h) on a 7% grade, and even a 4 lb saving is worth only 0.25 mph (0.4 km/h) for a light rider. So, where is the big savings in wheel weight reduction coming from? One argument is that there is no such improvement; that it is "placebo effect". But it has been proposed that the speed variation with each pedal stroke when riding up a hill explains such an advantage. However the energy of speed variation is conserved; during the power phase of pedaling the bike speeds up slightly, which stores KE, and in the "dead spot" at the top of the pedal stroke the bike slows down, which recovers that KE. Thus increased rotating mass may slightly reduce speed variations, but it does not add energy requirement beyond that of the same non-rotating mass.

Lighter bikes are easier to get up hills, but the cost of "rotating mass" is only an issue during a rapid acceleration, and it is small even then.[/i]

Nicked from Wikipedia...
http://en.wikipedia.org/wiki/Bicycle_performance

and your wheels are constantly wanting to decelerate

confused thinking. Force is required to lift the wheels (and every other part of the bike and rider), and this is a static gravitational matter unrelated to inertia.

Just because calulated energy input might be say 1% greater or lesser doesn't necessarily equate to 1% faster or slower results, it won't be a linear scale, especially when performing at or close to your limits.

Classic STW threaad this one - nearly as good as [url= http://www.singletrackworld.com/forum/topic/wobbly-wheel-after-one-weekend-accetable ]Wobbly wheels[/url] for folk arguing black is white and totally getting the wrong end of the stick.

How come SFB and I are on the same side and right? Most unusual 🙂

not sure what everyone else has said, probably something about not accelerating when going at a steady speed.
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friction is slowing your wheels down which you have to overcome by accelerating them, therefore you're always having to accelerate them unless those friction forces are overcome by gravity which they won't be if you're going uphill
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that's what I reckon

How come SFB and I are on the same side and right?

Newton sorted this out over 300 years ago, it's not exactly cutting edge 🙂

bakes - holding them at steady speed is not accelerating - Doh!

friction is slowing your wheels down which you have to overcome by accelerating them, therefore you're always having to acclerate them unless those friction forces are overcome by gravity which they won't be if you're going uphill

uh, if you're going at constant speed then, by definition, there is no deceleration! Yes, you have to overcome friction and gravity, neither of which are related to rotational inertia.

If your are trying to climb a "rough" hill (and on a bike thatcan be pretty much anything that's not smooth tarmac you will naturally try to maintain a steady pace (especially with a group)as your wheel overcomes every little lump and bump you have to input a little more power to spin the wheel back up to speed (There will be some degree of flywheel effect but the losses, especially at low speed will be quite substantial) A lighter wheel will feel easier to pedal especially if you pushing quite hard, but I reckon it comes down to rolling resistance (friction) more than anything.

it's impossible to hold them at steady speed though, especially on a mountain bike
especially with all the bumps, rocks, cracks, roots constantly slowing you down, even in small amounts, they are constant and will add up
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EDIT: what oliver said

by definition

definition means nothing

just imagining the speed i could go if I was always accelerating 😆

Bakes - but a heavy wheel with more rotational inertia will be less prone to slowing over bumps..............

hmmm, true TJ, good point
I'm not wrong though

it's impossible to hold them at steady speed though, especially on a mountain bike
especially with all the bumps, rocks, cracks, roots constantly slowing you down, even in small amounts, they are constant and will add up

no, they average out 🙂

definition means nothing

rghly kdlbcp er wbcvbbe rtmq eppt! (definition-free language)

just imagining the speed i could go if I was always accelerating

That would be the same speed as you can go, spun out, in your fastest gear, or faster, if gravity assisted.

The corrected perspective is - imagine how fast I could go if I wasn't constantly being subjected to forces that cause deceleration.

When you're going up hill gravity is constantly pulling you down and your wheels are constantly wanting to decelerate.

Yeah, but if your wheels are heavy they will have more inertia and resist deceleration more than light wheels.

On the steady climb described there will not be a difference, but when it comes to interesting MTB stuff; slowing, speeding up, switchbacks, swoopiness, the agility of lighter wheels will become apparent.

the agility of lighter wheels will become apparent.

or perhaps the skittishness ?

"just imagining the speed i could go if I was always accelerating"

That would be the same speed as you can go, spun out, in your fastest gear, or faster, if gravity assisted.

it goes without saying that to be always accelerating you'd need some other source of motive power than pedalling, and I imagine avoiding obstacles will become harder as you approach the speed of light...

yes but as I approach the speed of light I would also become close to infinite mass & everything would have to get out of my way or get crushed in my path 😈