Rotating Weight - Fact or Urban Legend
Comments
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rake wrote:except on a hill the extra weight will bring you to an overall slower speed at the top after the momentum has gone.
Funnily enough, it won't. If you simply hit the bottom of a hill and freewheel as far up it as you can, a heavy bike/rider combination will actually get further up the hill than a light one! Operating in a frictionless vacuum, they'd both go exactly the same distance. But in the real world a heavier bike/rider combination will lose a smaller proportion of its kinetic energy to air resistance than a ligher one!0 -
that may be true but at the point where the bike has stopped and pedal power takes over the heavier one will be drug back more. im more on the heavier strength side myself than lightweight efficient and while i can roll well on most things big hills are where lighter riders win. i might fix a flywhhel to my back wheel/.rhext wrote:rake wrote:except on a hill the extra weight will bring you to an overall slower speed at the top after the momentum has gone.
Funnily enough, it won't. If you simply hit the bottom of a hill and freewheel as far up it as you can, a heavy bike/rider combination will actually get further up the hill than a light one! Operating in a frictionless vacuum, they'd both go exactly the same distance. But in the real world a heavier bike/rider combination will lose a smaller proportion of its kinetic energy to air resistance than a ligher one!0 -
rake wrote:that may be true but at the point where the bike has stopped and pedal power takes over the heavier one will be drug back more. im more on the heavier strength side myself than lightweight efficient and while i can roll well on most things big hills are where lighter riders win. i might fix a flywhhel to my back wheel/.
Yes, that's true. You have to spend more energy to drag the heavier bike up the hill! But the thread was initially about rotating vs fixed weights. If you do fix a flywheel to your back wheel, what you need to do is to rig up a braking system which sets it spinning when you want to slow down...... Then you could really zip along!0 -
Wheelspinner wrote:Entirely empirical observation here, absolutely no claims to scientific methodology whatsoever.
I changed tyres. Swapped out some Contis which weigh 240 gm each, and substituted a pair of Schwalbe which weighs 180 gm each. Same bike, wheels, tubes, and run at the same pressure as indicated on the pump I've used for ages. Rode my usual run from home, which is about 40 km round trip.
Astonishing difference in feel, but even more surprising is difference in SPEED. I knocked over a minute off my previous best time on this course, and I've ridden this loop a hundred times in all kinds of weather.
The bike simply felt MUCH faster, more lively and held speed a lot more easily. Rolling resistance can't be the whole reason, so that leaves rotating weight as the difference.
Have you ever changed the tyres on a car from one performance type to another ? Rubber can make a huge difference, despite it all looking black and soft
Some compounds are harder than others, which affects rolling resistance, grip etc.
I don't think many actually realise the effects that tyres have on ride quality (even though its pretty obvious). I realise we're not exactly TDF racers but that does not exclude us from observing the differences in feel.
If you look at motorsort races i.e. F1, rally, touring cars, moto GP etc. all can be won due to tyre choice. Maybe the variations are not so great on road bikes but you get my point.
Ultimately the math is there for a reason and the math proves that the difference between 200g is negligible in terms of the extra power/energy/whatever required to shift the extra mass.
If wheel mass was equal to the bike mass then maybe we'd have a valid discussion but when talking about 200g vs 80kg i.e. bike + rider, then there's no amount of reasoning (empirical or otherwise) that will hold any weight (so to speak).0 -
There has been a lot of talk about rotating mass and even mention of moments of inertia....however, there has been no dicussion on what the moment of inertia of a body is made up of, namely mass and "radius of gyration". It is the latter which appears to be key to this discussion?
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165 wrote:There has been a lot of talk about rotating mass and even mention of moments of inertia....however, there has been no dicussion on what the moment of inertia of a body is made up of, namely mass and "radius of gyration". It is the latter which appears to be key to this discussion?
165
Here you go
The overview sums it up quite nicely
http://en.wikipedia.org/wiki/Moment_of_inertia
In the case of a wheel the radius stays the same so it is true to say that even two wheels with the same mass will behave differently if the mass distribution is different.
However, as has been mentioned before, I don't think anyone is arguing with the physlics but it is the magnitude of the differences that are questionable.
The math for this is pretty simple. Why don't you give it a go on paper and see just how much torque you need to accelerate a wheel and then compare it with the torque you need to accelerate a rider + bike. I think you'll find that there is no real *key* to this discussion except for the fact it doesn't make any discernable difference.0 -
i find myself agreeing somewhat with esqargot . wheels weight has more impact than static on the the bike and rider, if the bike is going 20mph the top of the wheel rim is going 20mph faster than the bike or 40mph relative to the ground. but the overall amounts are questionable given the weight of the whole rig. spending thousands to save a second or two every mile is rediculous for amateurs. if you just like nice things fair enough.0
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What is also important to account for is that saving 50g on the rims will actually be more noticable than 100g on the frameset when accelerating (however, it does usually matter about the cost for each weight saving for most of us!)0
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sam_commencal wrote:What is also important to account for is that saving 50g on the rims will actually be more noticable than 100g on the frameset when accelerating (however, it does usually matter about the cost for each weight saving for most of us!)
I understand that the heavier things get the more "noticable" the effort required to move them will be. I just am not a believer in 50 or 100 grams being anything I would be able to preceive in any way. IMHO. :? :?0 -
dennisn wrote:I just am not a believer in 50 or 100 grams being anything I would be able to preceive in any way. IMHO. :? :?
Instead of all the talk, theory, and opinions sometimes disguised as fact, just TRY IT FOR YOURSELF ffs. Borrow some nice light wheels, and go ride.
If you find no discernible difference, walk away happy, pat yourself on the back and say "See, I was right! three times.
If you do find a difference, buy the wheels.
Simple, isn't it?Open One+ BMC TE29 Seven 622SL On One Scandal Cervelo RS0 -
Ok, so what's the answer?
It seems that most people debating based on their scientific knowledge say there is little point in paying for lighter wheels, yet those that have tried them in practice describe a noticeable difference?0 -
I think it depends on how far you're prepared to push your luck too. You hear about people saying they've got light racing tyres but most only go as far down as the likes of the Open Corsa or the GP4000 options.
Never heard any praise for the Supersonic tyres and tubes on this forum, now THAT would save some weight! Whether you'd get to the finish is another matter though. Or am I wrong?0 -
Aud Reekie MTB wrote:Ok, so what's the answer?
It seems that most people debating based on their scientific knowledge say there is little point in paying for lighter wheels, yet those that have tried them in practice describe a noticeable difference?
Tell people that sugar pills are the latest miracle painkiller and often as not they'll make a headache go away too.....
FWIW, I've upgraded both wheels on my bike, and I didn't notice a difference0 -
rhext wrote:Aud Reekie MTB wrote:Ok, so what's the answer?
It seems that most people debating based on their scientific knowledge say there is little point in paying for lighter wheels, yet those that have tried them in practice describe a noticeable difference?
Tell people that sugar pills are the latest miracle painkiller and often as not they'll make a headache go away too.....
FWIW, I've upgraded both wheels on my bike, and I didn't notice a difference
+1....... I've also used different weight wheels / tires and never noticed anything.0 -
dennisn wrote:rhext wrote:Aud Reekie MTB wrote:Ok, so what's the answer?
It seems that most people debating based on their scientific knowledge say there is little point in paying for lighter wheels, yet those that have tried them in practice describe a noticeable difference?
Tell people that sugar pills are the latest miracle painkiller and often as not they'll make a headache go away too.....
FWIW, I've upgraded both wheels on my bike, and I didn't notice a difference
+1....... I've also used different weight wheels / tires and never noticed anything.
Ha! You knew the answer all along and you've just been playing with us!0 -
My LBS (and some stuff I've found on the net) likes to talk about 'wheel rigidity' - i.e. more expensive wheels are more rigid - therefore less energy is lost.
Any thoughts on this?"I thought of it while riding my bicycle."0 -
cakewalk wrote:My LBS (and some stuff I've found on the net) likes to talk about 'wheel rigidity' - i.e. more expensive wheels are more rigid - therefore less energy is lost.
Any thoughts on this?
Thoughts are that the energy loss might be a little more snake-oil: deformation of any component soaks up energy, but unless the rebound is damped, you tend to get it back again.
I find that the benefit of rigidity is that flexy components don't feel good to ride on: if you stamp on your pedals and your front mech rubs, that's likely to be because you're flexing the bike. Flexible wheels are not necessarily going to feel good at 40MPH down a twisty road and they're certainly a liability offroad.
But to give an indication of how likely the 'rigidity' debate is to descend into the same type of 'faith-based' positioning as the rotating weight debate consider two of the most prized virtues of high-end frames: rigidity and compliance!0 -
rhext wrote:dennisn wrote:rhext wrote:Aud Reekie MTB wrote:Ok, so what's the answer?
It seems that most people debating based on their scientific knowledge say there is little point in paying for lighter wheels, yet those that have tried them in practice describe a noticeable difference?
Tell people that sugar pills are the latest miracle painkiller and often as not they'll make a headache go away too.....
FWIW, I've upgraded both wheels on my bike, and I didn't notice a difference
+1....... I've also used different weight wheels / tires and never noticed anything.
Ha! You knew the answer all along and you've just been playing with us!
Guess I did know all along(at least for me). I buy the fact that some notice, some don't, and some think they notice, a difference. For me, I think that the weight difference in modern bicycle wheels and tires is not all that great and noticing it is not something I feel, unless I am using really heavy rims and tires for some reason. And that hasn't happened
since aluminum rims became the norm.
As for rotating weight. As far as the physics go I believe weight makes a difference, yet it's way to small for myself to notice given today's wheels.
Of course, all this is based on me lugging more than a few extra pounds around, which I'm sure throws a big wrench into the gears of science.
In any case the debate has been informative and fun.0 -
i'd say from experience on the tyre compound/tread versus weight thing, i've got a heavy pair of large volume slicks for my mtbs, and some very light very knobbly mud tyres.
The knobblies are noticeably easier to spin up, like night and day, despite the extra rolling resistance, however the top speed is a lot lower. Everything else constant...0 -
O.K. So I'm convinced that, to move the extra say 500 grams of a not so light wheel set, up a hill, it takes around 1% of the total energy required to move the whole mass up the hill, rider, bike, wheels. This may or may not be noticeable to the rider, but it is real weight that must be moved. So, a very, very, small percentage of the total effort goes into moving
a slightly heavier set of wheels up the incline. If I'm wrong feel free to stop me anytime.
Now, in an effort to further my education, can someone give me a formula to do some sort of calc's on power requirements to get this extra 500 grams moving, or should I say rotating, get it up to speed, and keep it there, as opposed to the lighter wheel set? If this has already been done and I missed it or didn't realize what I was looking at(more likely) please refer me back to that post. Don't give me too much cr*p. I'm trying. :oops: :oops:0 -
For an 'ideal' wheel (where all the mass is concentrated into a rim of no thickness), the rotational kinetic energy is the same as the linear kinetic energy. So if you've got a bike/rider combination of 100Kg and you knock a Kg off the frame, you get a 1% decrease in energy requirement. If you knock a Kg off the wheel, you get a 1% decrease in energy requirement from the overall weight of the bike, and further 1% decrease in energy requirement from the wheel.....so it's twice as effective as far as acceleration is concerned to take weight off the wheel as it is from anywhere else. Real wheels will have a lower moment of inertia than ideal wheels, however, so the actual gain will be less than that! And the additional energy requirement only applies to that fraction of the work that you do related to accelerating your bike.[/i]0
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rhext wrote:For an 'ideal' wheel (where all the mass is concentrated into a rim of no thickness), the rotational kinetic energy is the same as the linear kinetic energy. So if you've got a bike/rider combination of 100Kg and you knock a Kg off the frame, you get a 1% decrease in energy requirement. If you knock a Kg off the wheel, you get a 1% decrease in energy requirement from the overall weight of the bike, and further 1% decrease in energy requirement from the wheel.....so it's twice as effective as far as acceleration is concerned to take weight off the wheel as it is from anywhere else. Real wheels will have a lower moment of inertia than ideal wheels, however, so the actual gain will be less than that! And the additional energy requirement only applies to that fraction of the work that you do related to accelerating your bike.[/i]
Exactly what I needed. I think. Oh h*ll, I don't even know what I think now. :? :?
Thanks.0 -
A normal road bike tyre weighs about 250g. A full water bottle weighs about 750g.
You have identical bikes. One has normal tyres and 2 full bottles. The other has no water bottles but 1 kg tyres on both wheels.
Would the bikes both perform the same?0 -
Evil Laugh wrote:A normal road bike tyre weighs about 250g. A full water bottle weighs about 750g.
You have identical bikes. One has normal tyres and 2 full bottles. The other has no water bottles but 1 kg tyres on both wheels.
Would the bikes both perform the same?
You are evil you know????0 -
Sorry.0
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Evil Laugh wrote:A normal road bike tyre weighs about 250g. A full water bottle weighs about 750g.
You have identical bikes. One has normal tyres and 2 full bottles. The other has no water bottles but 1 kg tyres on both wheels.
Would the bikes both perform the same?
...is that one of those 'tree falling over in empty forest' types of question?0 -
http://www.training4cyclists.com/how-much-time-does-extra-weight-cost-on-alpe-dhuez/
"Ever wondered how much difference it would make if you made your bike lighter? I guess most serious riders once in while have concerns about their bikes whether they are light enough. As previously described the bike weight has impact on performance – especially during climbing. Thus, I thought it was interesting to find out how much difference it makes if you ride a heavier bike up alpe d’Huez. This little trial was performed by the best rider I coach as a part of his preparation for Tour de l’avenir (U23 Tour de France) in September.
In this experiment he had to ride up Alpe d’Huez four times with different setups. He was supposed to keep a pace around 275w on all four rides. The test bike was a Pinarello Prince equipped with Shimano Dura Ace and SRM crank:
1. Normal bike + 1.8L extra water in tyres(!)
This setup was quite interesting and got quite a lot of attention the day before when he had a puncture..! It was possible to fill 900ml into each tyre. Reducing weights on wheels is more important than reducing weight on non-rolling equipment e.g. bike saddle.
2. Normal bike + 1,8L extra water on bike.
This setup also got some serious attention because he rode a relative fast pace compared to most riders visiting Alpe d’Huez. Thus, when he performed the trial, well-trained riders were trying to keep up with his pace because this setup looked so extreme.
3. Normal bike
This setup was a completely normal bike.
4. Normal bike, reduced tyre pressure only 3 bars.
The last setup was ridden with reduced tyre pressure to 3bars. This was a tough finish on the last ride up Alpe d’Huez this day.
Results:
1. 52.01, 275w
2. 51.34, 277w
3. 49.40, 278w
4. 50.38, 273w
1,8L extra weight costs 1.54min up Alpe d’Huez. This is a remarkable test that shows us how important weight savings are when you are riding on steep hills. Also it shows that weight savings on wheels might be more important than weight saving on non-rolling equipment. These trials were only possible because of his SRM Crank that made it possible to compare each ride up the hill. You can make similar trials to test your bike setup if you want to optimize your bike equipment. These tests can be extremely helpful for e.g. time trialists if you don’t have wind tunnel in the neighbourhood."0 -
^ I'm not shooting the messenger, but according to the online calculator below, the extra 2 watts the guy did on the second test (with the 900ml of water removed from each tyre and used in the botles) accounts for 21 of the 27 seconds cut, and I suspect some, or most, of the remaining 6 seconds was caused by the rolling resistance being effected buy the 900ml of water being in the tyres. There also, obviously may have been other factors effecting the different tests.
http://bikecalculator.com/veloUS.html
If I had to make a statement about this, I'd say that the test pretty much proves that rotating mass is a myth wehn it comes to a steady state climb!0 -
Bike calculator is woefully inaccurate itself! According to that, my 26.12 10 mile TT time from last year should have been a 22 minute effort.0
