Rotating Weight - Fact or Urban Legend

dennisn

Bear with me on this. After reading a post yesterday(can't find it today) I went down to my basement and looked at my bike hanging on a repair stand. Walked over and BARELY touched the rear wheel with my little finger. It started to move. No real force at all. Next I pushed one of the pedals down(little finger again). The wheel started to turn with only slightly more force than before. I continued pushing and it seemed that it took even less effort to keep the wheel turning once it got started. To me this sort of asks the question
"If it takes so very very little effort to start a wheel moving and, apparently, even less to keep it moving what is the advantage of saving 100 or 200 grams on a wheel? Now I realize that an extremely heavy wheel(i.e. steel flywheel) would take a bit more to get it moving, but am not following how a couple hundred grams could have any noticeable effect on the average bike wheel. I sort of got the impression that it's not starting the wheel moving or keeping it moving that takes any real power, but that increasing the
wheel speed is where the most effort is required, if you can call it effort. You can increase it with the same little finger. Just a guess but I would think that the real effort comes from having to get my fat *ss in motion AND up to speed and that unless I'm running a really, really heavy wheel I wouldn't notice a 200 gram difference.
Am I making any sense??? If not just consider the source and move on.

rick_chasey

You were still putting in energy just to rotate the wheels, despite the fact the bike wasn't moving.

Thus, just the process of spinning the wheels, let alone propelling your body weight and the weight of the bike, requires some energy.

CarlosDSanchez

The reason you want lighter rims is for quicker acceleration. They require less energy to get them moving. Weigh close to the centre of the wheel effects this less. Also a lighter wheel will be easier to steer so should improve handling.

dennisn

Rick Chasey wrote:

You were still putting in energy just to rotate the wheels, despite the fact the bike wasn't moving.

Thus, just the process of spinning the wheels, let alone propelling your body weight and the weight of the bike, requires some energy.

I agree, it all takes energy but the difference between what's required to move you AND
the bike is many, many, many, times more than what it takes to move the wheel. So in my theory the power it takes to move ANY reasonably light weight bike wheel becomes irrelevant and unnoticeable. Remember that's IN MY THEORY and, well, you know me.

rick_chasey

^^ Agreed.

Rotating weight isn't an urban myth however!

You'd probably also need to factor in that despite the small levels of energy savings per pedal stroke for a lighter wheel, these savings are amplified so much in big long rides that hey can amount to a perceptable difference.

dennisn

CarlosDSanchez wrote:

The reason you want lighter rims is for quicker acceleration. They require less energy to get them moving. Weigh close to the centre of the wheel effects this less. Also a lighter wheel will be easier to steer so should improve handling.

I agree that it all takes some power to get all things moving. I guess I'm arguing that in the big picture of moving a bike and it's rider, that the power required to move a wheel(little finger with barely a touch) is not noticeable with the addition, or subtraction, of 100 or 200 grams on the wheel.

jibberjim

Rick Chasey wrote:

You'd probably also need to factor in that despite the small levels of energy savings per pedal stroke for a lighter wheel, these savings are amplified so much in big long rides that hey can amount to a perceptable difference.

Except that the lighter wheels will have less inertia so they'll slow down quicker too so you're really just moving the effort, of course if you're riding a crit and accellerating/de-accellerating regularly then that's what you want. On long rides though, there's not so much de-acellerating.

CarlosDSanchez

The difference between a "light wheel" and a "very light wheel" would probably be negligable, maybe a few 001's of a second per mile, but in a race those miles and 001's add up eh?

For a Sunday afternoon ride though, it make no difference, spending a fortune on wheels is just for bling effect.

dennisn

jibberjim wrote:

Rick Chasey wrote:

You'd probably also need to factor in that despite the small levels of energy savings per pedal stroke for a lighter wheel, these savings are amplified so much in big long rides that hey can amount to a perceptable difference.

Except that the lighter wheels will have less inertia so they'll slow down quicker too so you're really just moving the effort, ......

Good point. I think. :? :?

rick_chasey

dennisn wrote:

jibberjim wrote:

Rick Chasey wrote:

You'd probably also need to factor in that despite the small levels of energy savings per pedal stroke for a lighter wheel, these savings are amplified so much in big long rides that hey can amount to a perceptable difference.

Except that the lighter wheels will have less inertia so they'll slow down quicker too so you're really just moving the effort, ......

Good point. I think. :? :?

Surely for the same energy input, a lighter wheel would be spinning faster, and thus have same inertia than the slower spinning heavier wheel?

nickwill

I think the weight of the rim comes much more in to play when climbing steep hills of 20% and above. When you are struggling to turn the crank over, those extra few grammes will surely have a major effect.

Barrie_G

most of the effort, as has already been mentioned, comes from having to move the weight of you and the bike along the road, next time you're out on the bike find a nice flat section of road, get off the bike then put the bike over your shoulder and try to get back up to your cruising speed

It's easy to spin the wheel because the wheel isn't moving any mass, start trying to propell something you're going to have to put in more energy the greater the mass or the greater the speed the more energy you have to put in.

rick_chasey

Barrie_G wrote:

most of the effort, as has already been mentioned, comes from having to move the weight of you and the bike along the road, next time you're out on the bike find a nice flat section of road, get off the bike then put the bike over your shoulder and try to get back up to your cruising speed

It's easy to spin the wheel because the wheel isn't moving any mass, start trying to propell something you're going to have to put in more energy the greater the mass or the greater the speed the more energy you have to put in.

Haha, this has moved beyond what you actually notice in the real world.

It's a question of mechanics!


It's certainly not a myth, either way.

balthazar

It's a common miscomprehension that accelerating wheels demands much effort. Dennis's experiment makes clear that the effort required is minimal, whether your wheels are light or heavy. This is easy to confirm yourself, if doubtful. Physics isn't required, because this isn't about modelling a system or making predictions, it is simply a matter of comprehension: experiencing the force required to accelerate a given range of masses.

I wouldn't know how to make it more obvious to somebody (that they need not spend a lot of money on light wheels), than this experiment, or a variant. If they still want the wheels, then it's for other reasons than rapid acceleration.

AidanR

A rough calculation indicates that to accelerate a 70kg cyclist to 20mph requires around 3300 joules, of which 100 is involved in spinning up the wheels, or 3%.

This ignores the rotational kinetic energy of the crank, pedals, chain, cassette and the rider's legs. It also ignores wind resistance.

This assumes a relatively lightweight rim and tyre combo (750g in total per wheel, typical of an Open Pro clincher set-up). Saving 100g off each rim/tyre would save 15 joules per wheelset, a 15% saving in energy to spin up the wheel, but less than 0.5% when considering the overall body.

Note, the moment of inertia of the wheel is only relevant during acceleration and deceleration. Not hills.

These calculations are very rough and ready. They may also be wrong and I'm happy to be corrected!

markos1963

This is all blown into the weeds by the fact that 70% or so of the effort in cycling goes on beating air resisitance. This is why TT'ers spend money on aero wheels without worrying too much about weight and its the same for the bike but this weight penalty doesn't detract from the fact that a TT bike has the fastest average speed out there on the road.

TheStone

I'm sure I read that one of the world hour record holders deliberately had a extra heavy
back wheel (can't remember the name, I think a Russian who was later banned)

AidanR

As I said, lightweight wheels will help the most when accelerating. This is when moment of inertia has any influence. When accelerating, the wheel needs to be spun up to speed, so a light rim and tyre makes a difference (though a light hub will not). The whole wheel also needs to be accelerated forward just like the rest of the bike.

When going up hills, a lighter wheel will help too, but just in the same way as any other part of the bike, i.e. moment of inertia makes no difference.

When going along the flat at a steady speed, weight will make no difference (save for slight affect on rolling resistance, but this is negligible). The only things that will make any difference to your speed are aerodynamics and the rolling resistance of your tyres.

Of course when racing, lightweight wheels are of particular use in the mountains for climbers that like to accelerate sharply on severe inclines.

rick_chasey

TheStone wrote:

I'm sure I read that one of the world hour record holders deliberately had a extra heavy
back wheel (can't remember the name, I think a Russian who was later banned)

A rotating mass is always accelerating towards the point of rotation.

Choosing a heavier wheel over a lighter one for reasons of weight is stupid.

Ginjafro

Wheels make up a significant proportion of the total mass and weight of any bike. If you want a lighter machine it makes sense to start saving weight there - lighter rims, lighter tyres etc. A lighter bike is without doubt that bit easier to accelerate and get up hills, however, a lighter cyclist may also accelerate and get up hills quicker too, so reducing your own weight might be more effecient and cheaper, provided of course, you don't reduce your power to weight ratio in the process.
The other important factor to take into account is "resistance". If your tyres drag as much as a scooter in a peatbog or you've got a seriously bad headwind its hard work to keep up.
Also, some of the power we produce is lost through other inefficiences of the bikes we ride, such as the resistance and drag of the various bearings, linkages and even movement of frames. But, hey, its all about man (women too) and machine working in some sort of harmony and finding that little bit of extra......

dennisn

Nickwill wrote:

I think the weight of the rim comes much more in to play when climbing steep hills of 20% and above. When you are struggling to turn the crank over, those extra few grammes will surely have a major effect.

I suppose that the steeper the hill the harder it is to move the overall weight of the wheel
up said hill. Just like it gets harder to move yourself and the bike up the same slope.
However, the power required to rotate the wheel itself remains the same whether going up or on the flat(I think)?????

AidanR

Correct. In fact, if you ignore friction (which isn't related to the weight of the wheel anyway) it takes no power at all to keep the wheel rotating.

It's all about acceleration, baby.

dennisn

Ginjafro wrote:

Wheels make up a significant proportion of the total mass and weight of any bike. If you want a lighter machine it makes sense to start saving weight there - lighter rims, lighter tyres etc. A lighter bike is without doubt that bit easier to accelerate and get up hills, however, a lighter cyclist may also accelerate and get up hills quicker too, so reducing your own weight might be more effecient and cheaper, provided of course, you don't reduce your power to weight ratio in the process.
The other important factor to take into account is "resistance". If your tyres drag as much as a scooter in a peatbog or you've got a seriously bad headwind its hard work to keep up.
Also, some of the power we produce is lost through other inefficiences of the bikes we ride, such as the resistance and drag of the various bearings, linkages and even movement of frames. But, hey, its all about man (women too) and machine working in some sort of harmony and finding that little bit of extra......

Not sure if I agree that a 2 or 3 pound wheel and tire is a significant proportion of the total mass. Say 200 pound bike and rider and the wheel / tire makes up a bit more than 1 %. of it all. Doesn't seem that much in the overall scheme of things.
As to the resistance thing. This is a factor in moving the whole bike and rider but doesn't have anything to do with the power required to start, and keep, the wheel its self turning.
At least that's my theory or to put it better - that's my question. :? :?

rhext

I think the answer to this perhaps depends on whether you buy into the theory that (say) a few hundred grams weight saving makes a practical difference to a bike/rider combination that weighs many many times that. From my point of view, there's a limit to how much it's worth spending on reducing the weight of the bike when there's still so much scope to reduce the weight of the rider.

If you do buy into the theory that saving a couple of hundred grams makes a difference then, yes, it is more effective to lose it from the wheel-rims than anywhere else. To accelerate a wheel you have to impart it with linear momentum and angular momentum. To accelerate anything else, just linear momentum. To get a feeling for what this means, put your bike on a stand and instead of gently rotating the wheel, try and get it spinning as fast as you can as quickly as you can. You'll feel some real resistance then: and the lighter the wheel, the less that resistance will be!

Anonymous

I love the feeling of acceleration, particularly when climbing, of a light wheelset, and I lurve light bikes, but I genuinely think that tyre and spokes would make more of a difference than wheel weight.

IMO A heavy aero wheel > lightweight non aero on virtually all terrain. Unless you are on a very steep climb...

dennisn

rhext wrote:

I think the answer to this perhaps depends on whether you buy into the theory that (say) a few hundred grams weight saving makes a practical difference to a bike/rider combination that weighs many many times that. From my point of view, there's a limit to how much it's worth spending on reducing the weight of the bike when there's still so much scope to reduce the weight of the rider.

If you do buy into the theory that saving a couple of hundred grams makes a difference then, yes, it is more effective to lose it from the wheel-rims than anywhere else. To accelerate a wheel you have to impart it with linear momentum and angular momentum. To accelerate anything else, just linear momentum. To get a feeling for what this means, put your bike on a stand and instead of gently rotating the wheel, try and get it spinning as fast as you can as quickly as you can. You'll feel some real resistance then: and the lighter the wheel, the less that resistance will be!

Well, just to play devils advocate, I would argue that just because you "buy into" the theory of a couple hundred grams making a difference doesn't mean that the theory is true. Isn't that kind of like saying "well, I bought these because they are supposed to be fast - I believe what was said about them - therefore they are fast". I guess I'm a pretty skeptical person but I don't see that as a bad thing. Drives my wife crazy but other than that.......

dennisn

NapoleonD wrote:

I love the feeling of acceleration, particularly when climbing, of a light wheelset, and I lurve light bikes, but I genuinely think that tyre and spokes would make more of a difference than wheel weight.

IMO A heavy aero wheel > lightweight non aero on virtually all terrain. Unless you are on a very steep climb...

I still don't quite follow how a light wheel, say 200 grams less than a "normal" one makes any difference in climbing, no matter what the slope, when you're trying to move maybe 200 pounds uphill. You're working to move the 200 pounds up the hill. You must HAVE a wheel but the difference between a normal one and a light one is 200 grams or 1/2 pound(give or take). This difference in wheel weight makes up about .5% of the total you're trying to move. The wheel its self takes but the touch of a finger to make it rotate, whether it's a normal or a light one, so that power requirement, at least to me, seems pretty much nil. Almost every last bit of your power is taken up in moving the entire mass up the hill and at best the weight of a finger goes into keeping the tire rotating. I THINK???? I do agree that it is harder to move 200 pounds up a 15% slope than a 5% one.

Anonymous

dennisn wrote:

NapoleonD wrote:

I love the feeling of acceleration, particularly when climbing, of a light wheelset, and I lurve light bikes, but I genuinely think that tyre and spokes would make more of a difference than wheel weight.

IMO A heavy aero wheel > lightweight non aero on virtually all terrain. Unless you are on a very steep climb...

I still don't quite follow how a light wheel, say 200 grams less than a "normal" one makes any difference in climbing, no matter what the slope, when you're trying to move maybe 200 pounds uphill. You're working to move the 200 pounds up the hill. You must HAVE a wheel but the difference between a normal one and a light one is 200 grams or 1/2 pound(give or take). This difference in wheel weight makes up about .5% of the total you're trying to move. The wheel its self takes but the touch of a finger to make it rotate, whether it's a normal or a light one, so that power requirement, at least to me, seems pretty much nil. Almost every last bit of your power is taken up in moving the entire mass up the hill and at best the weight of a finger goes into keeping the tire rotating. I THINK???? I do agree that it is harder to move 200 pounds up a 15% slope than a 5% one.

I think they just feel more lively although I'm talking about, say 200g per wheel difference. Don't know about outright speed uphill. It would be best to compare using a powermeter but mine is on the back wheel so that would have to remain the same. I suppose I could try it with a generic heavy front wheel I have and a Zipp 404 tub front. Steepish uphill shouldn't make too much use of the aero benefit of the Zipp. I could try and keep at a certain power output and measure speed or try and maintain speed and look at the power required (the latter would, I think, be easier...)

I think the difference will be negligible if any in terms of climbing power. But the light wheels definitely feel more lively (I've already said that haven't I? I'm tired.)

Anonymous

There is a shortish steady hill near me where I could test this, do, say, 5 runs with each wheel at 12MPH.

I could also try and accelerate in a Fixed gear from, say, 12mph to 17mph uphill as quickly as possible in a particular gear to try and get some semblance of acceleration. This may be quite inaccurate though.

It would be interesting to see the results...

Well, in my opinion anyway. I really don't think there will be much, if any, difference.

dennisn

NapoleonD wrote:

dennisn wrote:

NapoleonD wrote:

I love the feeling of acceleration, particularly when climbing, of a light wheelset, and I lurve light bikes, but I genuinely think that tyre and spokes would make more of a difference than wheel weight.

IMO A heavy aero wheel > lightweight non aero on virtually all terrain. Unless you are on a very steep climb...

I still don't quite follow how a light wheel, say 200 grams less than a "normal" one makes any difference in climbing, no matter what the slope, when you're trying to move maybe 200 pounds uphill. You're working to move the 200 pounds up the hill. You must HAVE a wheel but the difference between a normal one and a light one is 200 grams or 1/2 pound(give or take). This difference in wheel weight makes up about .5% of the total you're trying to move. The wheel its self takes but the touch of a finger to make it rotate, whether it's a normal or a light one, so that power requirement, at least to me, seems pretty much nil. Almost every last bit of your power is taken up in moving the entire mass up the hill and at best the weight of a finger goes into keeping the tire rotating. I THINK???? I do agree that it is harder to move 200 pounds up a 15% slope than a 5% one.

I think they just feel more lively although I'm talking about, say 200g per wheel difference. Don't know about outright speed uphill. It would be best to compare using a powermeter but mine is on the back wheel so that would have to remain the same. I suppose I could try it with a generic heavy front wheel I have and a Zipp 404 tub front. Steepish uphill shouldn't make too much use of the aero benefit of the Zipp. I could try and keep at a certain power output and measure speed or try and maintain speed and look at the power required (the latter would, I think, be easier...)

I think the difference will be negligible if any in terms of climbing power. But the light wheels definitely feel more lively (I've already said that haven't I? I'm tired.)

A while back I was thinking of building a set of lightweight tubulars and giving the Colorado Bike Tour one last go around. Now, in thinking about it I'm not so sure why my "normal" wheels won't do the job just as well. After all the two of them only weigh
a pound or so more than the DT190 hub ones I was going to lace up. Opps, forgot, they have ceramic bearings and that must be good??? Right???

balthazar

NapoleonD wrote:

There is a shortish steady hill near me where I could test this, do, say, 5 runs with each wheel at 12MPH.

I could also try and accelerate in a Fixed gear from, say, 12mph to 17mph uphill as quickly as possible in a particular gear to try and get some semblance of acceleration. This may be quite inaccurate though.

It would be interesting to see the results...

Well, in my opinion anyway. I really don't think there will be much, if any, difference.

I think this scenario introduces obfuscating complexity to the matter: the original experiment of spinning a wheel up to high speed in your hand is more revealing. Try it with a light and a heavy wheel, and note the difference: particularly, how easy it is to spin any normal wheel up to 20mph-ish in 2 seconds or so, with your thumbs on the spokes. That's an acceleration rate much higher than you could manage cycling the bike – yet you did it easily with only your thumbs. You don't need to "time" or measure anything, to reveal that the efforts we are talking about are nothing to the power generated by two legs (quad power?!)