Rotating Weight - Fact or Urban Legend

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Comments

  • AidanR
    AidanR Posts: 1,142
    Actually, coloured compounds are usually slightly inferior to their black counterparts.

    Stop being a tart ;)
    Bike lover and part-time cyclist.
  • dennisn
    dennisn Posts: 10,601
    plumpy wrote:
    This has been great, heretics well and truly told, and I'm printing it off to show the wife. "Yes, they may be 700% more expensive than my RS10's but look, they're almost certainly 0.05, 0.5 or 1.5% faster!!"

    I really like the "almost certainly" part.
  • nickwill
    nickwill Posts: 2,735
    Rolf F wrote:
    Nickwill wrote:
    I've got a pair of Neutrons and they have been fantastic. I use them for hilly sportives and general Summer training. They are light and very durable. I think they are one of the most underrated wheelsets. The Ultras have, AFAIK, slightly lighter rims as well as the different hub. My impression is that the standard Neutrons offer more bang for bucks.

    Thanks for that - couldn't see much reason to go for the Ultras tbh - besides, the bike is black and white and there is too much red writing on the Ultras :lol:

    As a matter of interest, what wheels are you using when not on the Neutrons?

    I did have a pair of Open Pro Centaur wheels but they gave up the ghost. My Winter bike has a pair of Fulcrum Racing 7s bought on ebay. Needless to say. it's a great contrast when I return to the Neutrons. I've run the Neutrons for the last 4 years apart from the depths of Winter, and they have plenty of life left in them yet.
    http://lakescyclist.wordpress.com/
  • rick_chasey
    rick_chasey Posts: 75,660
    dennisn wrote:
    Chris James wrote:
    Rick Chasey wrote:

    *sigh*

    I fear I am misunderstood, or misunderstanding.

    I though the OP was whether the advantages of lighter wheels were fact or myth?

    The physics says it's a fact! ....!

    *SIGH*

    *SIGH*

    I do wish people wouldn't do this.

    Perhaps you would like to explain which bits of physics actually apply, including equations with sample workings to show what benefit is accrued by lighter wheels, and under which circumstances.

    That way we can judge if you actually know what you are on about, rather than just stating that 'it is a fact'.


    Great post. Really.
    OK, who's up for that challenge 'cause it sure ain't me?

    I thought it was pretty well explained before?No need for algebra. You can look up each described physical phenomenon and decide for yourself whether it is right or not? Fair?

    For a given energy, an object of lighter mass will accelerate more quickly than one of larger mass.

    Any talk of inertia is beside the point, because, ignoring any friction, inertia is just the form of the energy put into the object (like gravitational potential energy).

    Also, as mentioned before, wheels of lower mass at a given speed ( not velocity because it's constantly changing direction!) will require less energy to turn due to the preservation of angular momentum.

    If you're so keen on finding out what is genuinley fact, beyond what someone says on a forum, you'd probably learn more from doing the mechanics yourself, rather than moaning that other people haven't done it for you.
    _
  • AidanR
    AidanR Posts: 1,142
    I think that's a bit harsh - there's no moaning going on here really. It's just that some people aren't at all familiar with mechanics.

    Inertia is not besides the point. It may be just a store of energy, but how did it get that energy in the first place? From the rider. A wheel with a lower moment of inertia requires less energy for a given angular velocity, as you say in your next sentence. That said, once you're at a constant speed inertia is irrelevant (unless you need to stop in a hurry!), so having a really heavy rear wheel for an hour record attempt is pointless (especially on a fixed bike!), if that was the point you were making.
    Bike lover and part-time cyclist.
  • dennisn
    dennisn Posts: 10,601
    So what's your take on all this RC??? Me, I'm leaning toward the idea that with my reasonably light wheels(32 spoke, DA hubs, Mavics reflex rims, Conti Sprinter tires) I wouldn't notice any difference(on the flat or up a slope) if I changed to some sort of weight weenie wheel setup weighing 200 or so grams less(per wheel). I will admit to not having bought a, so called, light set of wheels but have thought about it. Don't race, so I haven't bought into the hype(whether true or false) that these wheels are the greatest.
  • Chris James
    Chris James Posts: 1,040
    Can't be bothered to do the mulitquote thing, but I see you have avoided my challenge of doing some worked examples.

    Lighter wheels accelerate more easily but, due to their lack of momentum are harder to keep at steady speeds - both are very small effects though - virtually infinitessimal compared to aerodynamic drag.
  • lastwords
    lastwords Posts: 304
    I am a qualified Mechanical Engineer and had to learn all this angular momentum stuff when i was at college it was some time ago and cant be arsed to dig out the info from my box in the loft. But as it has been said before the physics say it is easier to accelerate the lighter wheel.

    My experience of lighter wheels are with MTB's i bought a lighter wheelset and i could definatly tell the difference it just felt like the bike wanted to go, rather than me forcing it to.
    I also had some Maxxis down hill tyres which were great for the strength and resistance to punctures / tears etc but they were heavy and hard to get on and off the rims, so i swapped them for XC tyres of the same make and tread pattern and again the difference was noticable.

    But my average speed and overall times of my local route didnt suddenly improve with the lighter tyres / wheels, the bike just felt nicer to ride.

    My road bike is a carrera valour with very heavy wheels i have been riding it since june 09 and have just bought a new wheelset that saves 1.3kg in wieght i am yet to ride the bike with these wheels as the weather has been bad and i want to save them for better days, but im sure i will notice the difference.
  • plumpy
    plumpy Posts: 124
    All sarcasm aside, surely the elephant in the thread here is that whilst more expensive wheels may or may not be lighter weight, they should definitely have much better quality bearings. If I do lash out a grand on some new hoops, I'll be expecting to notice the lower friction as much as, if not more than, the lower weight?
  • AidanR
    AidanR Posts: 1,142
    Chris James wrote:
    Can't be bothered to do the mulitquote thing, but I see you have avoided my challenge of doing some worked examples.

    Lighter wheels accelerate more easily but, due to their lack of momentum are harder to keep at steady speeds - both are very small effects though - virtually infinitessimal compared to aerodynamic drag.

    Dude, come on, I went through the maths once already, and gave you all the formulas. If you don't believe me, do it yourself. I'm not a text book.

    Lighter wheels aren't harder to keep at steady speeds in the sense that it'll take increased effort on your part. You have your power, versus wind resistance and friction, and at a constant velocity these will cancel each other out. Yeah, you could have a big-ass flywheel of a rear wheel so that small variations in your power output would be smoothed out, but that's just making the system unresponsive - not really a desirable thing.
    Bike lover and part-time cyclist.
  • balthazar
    balthazar Posts: 1,565
    plumpy wrote:
    All sarcasm aside, surely the elephant in the thread here is that whilst more expensive wheels may or may not be lighter weight, they should definitely have much better quality bearings. If I do lash out a grand on some new hoops, I'll be expecting to notice the lower friction as much as, if not more than, the lower weight?
    If an elephant in the room, it's a small porcelain one on the mantelpiece. Bearing losses are off the bottom of the scale, and there is no trend toward less bearing resistance with fancy hubs – if anything, the reverse, as expensive hubs may use better sealed bearings, which have more drag. Either way, it matters not. And yes we've all heard of old-time trackies lubricating their hubs with oil...!
  • rolf_f
    rolf_f Posts: 16,015
    AidanR wrote:
    Rolf F wrote:
    But if the calcs posted here imply that a lightweight wheel has little benefit to most of us, what are the numbers that justify spending the money on the aero? Unless there is no wind or it his head or tailwind, the benefits of the more aerowheel are going to be compromised by crosswind.

    No point in even trying to calculate that - it's a fantastically complicated model that you'd need a lot of computing power to solve. These things are best measured in a wind tunnel, as the likes of Zipp do. Not that I'd believe everything they say as they're a tad biased ;)

    If you want to measure that kind of thing you'll need a still day and then do multiple roll-down tests with different wheel combinations. Anything other than a very still day and you're likely to get variations in wind velocity which will render the results useless.

    And, generally, if something isn't really measurable in the real world, then the effects are probably not that significant. Which implies that shallow rim wheels are probably the optimum solution for most given that they should weigh less than their more aero counterparts! Incidentally, around here there is very little flat so most of the time, anything that helps you climb is probably worth having.
    Nickwill wrote:
    I did have a pair of Open Pro Centaur wheels but they gave up the ghost. My Winter bike has a pair of Fulcrum Racing 7s bought on ebay. Needless to say. it's a great contrast when I return to the Neutrons. I've run the Neutrons for the last 4 years apart from the depths of Winter, and they have plenty of life left in them yet.

    So nothing more aero then for less hilly terrain?
    Faster than a tent.......
  • AidanR
    AidanR Posts: 1,142
    No, it is measurable in the real world, it's just that with any experiment you have to be careful with your variables, and when you're talking about changes in aerodynamics that means wind in particular. And that's a tricky one to control.
    Bike lover and part-time cyclist.
  • nickwill
    nickwill Posts: 2,735
    Rolf F wrote:
    Nickwill wrote:
    I did have a pair of Open Pro Centaur wheels but they gave up the ghost. My Winter bike has a pair of Fulcrum Racing 7s bought on ebay. Needless to say. it's a great contrast when I return to the Neutrons. I've run the Neutrons for the last 4 years apart from the depths of Winter, and they have plenty of life left in them yet.

    So nothing more aero then for less hilly terrain?

    It's all hilly round Kendal!
    http://lakescyclist.wordpress.com/
  • plumpy
    plumpy Posts: 124
    Cheers Balthazar. Just as I'm talking myself up to a sexy new Placebo Wheelset, you bring up the whole lose weight/get fitter thing again...[/quote]
  • balthazar
    balthazar Posts: 1,565
    plumpy wrote:
    Cheers Balthazar. Just as I'm talking myself up to a sexy new Placebo Wheelset, you bring up the whole lose weight/get fitter thing again...
    Get the wheels! Get them because you like them, because you can afford them, etc. You don't need them to make you go faster – who cares? Enjoying bike riding can include enjoying spangly wheels whose spokes glint in the sunshine... you don't need spurious claims about "performance benefit".
  • dennisn
    dennisn Posts: 10,601
    plumpy wrote:
    Cheers Balthazar. Just as I'm talking myself up to a sexy new Placebo Wheelset, you bring up the whole lose weight/get fitter thing again...
    [/quote]

    Ya, forget that. We don't want to have to work to go faster, we just want to buy it, and if you believe the ad men, all you have to do to reach the podium is throw done a wad of cash. A big wad of cash.
  • Escargot
    Escargot Posts: 361
    AidanR wrote:
    No, it is measurable in the real world, it's just that with any experiment you have to be careful with your variables, and when you're talking about changes in aerodynamics that means wind in particular. And that's a tricky one to control.

    Interesting approach Aidan.

    I did this calc for personal interest some time ago but from a purely mechanical viewpoint (I'm a mech eng) to calculate the difference in torque required to accelerate from a given speed to higher speed. The wheel bit is easy using M = I * alpha. I then used F = ma along with the gear ratios to work out the respective torque necessary to accelerate the rider and wheel from speed 'a' to speed 'b'.

    I don't think any of the physics is in doubt but think the important thing here is the "magnitude" of the difference gained by changing to lighter wheels. My calcs showed that the difference in torque required was of the order of 1.0% for a mass reduction of a few hundred grammes. I guess if you considered wind resistance etc. this would increase this percentage but still feel it's negligible for 99% of the people that ride here.
  • neil²
    neil² Posts: 337
    There's also the issue of the constant small balance-correcting steering oscillations around the headset which ARE continuous accelerations. Lighter rims = lower inertia, again. I wonder if this is why lighter wheels feel nicer when climbing?

    So I summarise that:
    - lighter wheels accelerate quicker in rotation (remember there are two of them)
    - lighter wheels also accelerate quicker in translation
    - there can't be any difference at constant speed
    - deep rims also reduce the rotational inertia for the same weight
    - steering is (literally) lighter for lighter wheels
    - braking will be better due to lower rotational and translational momentum

    You could always gaffa tape 100g worth of nuts evenly spaced around the rim on each wheel, ride 5km, take them all off and try it again. Then you are keeping the hubs tyres etc the same. If you reckon that the difference in (a) performance (b) aesthetics is worth an extra £500 on the wheels then go for it!
    The Riddler
    The GT
  • Chris James
    Chris James Posts: 1,040
    lastwords wrote:
    I am a qualified Mechanical Engineer ....

    So am I!

    As you say though, better wheels tend to feel nicer and better (invariably lighter) tyres roll better
  • Chris James
    Chris James Posts: 1,040
    AidanR wrote:
    Chris James wrote:
    Can't be bothered to do the mulitquote thing, but I see you have avoided my challenge of doing some worked examples.

    Lighter wheels accelerate more easily but, due to their lack of momentum are harder to keep at steady speeds - both are very small effects though - virtually infinitessimal compared to aerodynamic drag.

    Dude, come on, I went through the maths once already, and gave you all the formulas. If you don't believe me, do it yourself. I'm not a text book.

    Lighter wheels aren't harder to keep at steady speeds in the sense that it'll take increased effort on your part. You have your power, versus wind resistance and friction, and at a constant velocity these will cancel each other out. Yeah, you could have a big-ass flywheel of a rear wheel so that small variations in your power output would be smoothed out, but that's just making the system unresponsive - not really a desirable thing.

    Edit - it has just ocurred to me that you thought my post was aimed at you - it wasn't. It was to Rick Chasey.

    I am not disgreeing with you! I never said I didn't believe you.

    As you say yourself, take it to extremes and the heavier wheel - i.e. flywheel smooths out variations, it is just that a few grammes here and there make no practical difference either way.
  • rake
    rake Posts: 3,204
    stick with wrought iron dennisen.
  • disney
    disney Posts: 51
    I dont know about all the tech details I have read, but when I put on my Schwalbe winter tyres I thought I had put on a set of lead weights and i noticed that I was using 1 lower gear to go up the same last big hill before home.
  • careful
    careful Posts: 720
    All I know is that plenty of guys with much heavier wheels seem to pass me with the greatest of ease - especially up hills! Recently I was out on my best carbon featherlight and was passed by a bloke with a camping stove and sleeping bag strapped to his saddle. For a few minutes I thought about packing it all in, then I thought - what the hell - I enjoy it.
    Seriously though, unless you are a top racer looking for the slightest edge this weight weenie stuff is all nonsense IMO.
  • disney
    disney Posts: 51
    Remember the biggest problem apart from fitness is the weight issue. I ride with a friend who is 10 kilos lighter. Going up a hill he just dawdles whilst drinking tea and reading the paper all at the same time! You can guess my progress behind him.
  • Anonymous
    Anonymous Posts: 79,666
    careful wrote:
    All I know is that plenty of guys with much heavier wheels seem to pass me with the greatest of ease - especially up hills! Recently I was out on my best carbon featherlight and was passed by a bloke with a camping stove and sleeping bag strapped to his saddle. For a few minutes I thought about packing it all in, then I thought - what the hell - I enjoy it.
    Seriously though, unless you are a top racer looking for the slightest edge this weight weenie stuff is all nonsense IMO.

    Maybe to you, but I just like having a light bike.

    I'm probably the slowest person on this forum but hey, I like bikes and bike bling...
  • rhext
    rhext Posts: 1,639
    Having caught up with this thread again, re-reading all of the carefully constructed scientific arguments for and against, I now find my sympathies are with plumpy on page 3.

    Can someone pass me my credit card?
  • Harry182
    Harry182 Posts: 1,170
    rhext wrote:
    Having caught up with this thread again, re-reading all of the carefully constructed scientific arguments for and against, I now find my sympathies are with plumpy on page 3.

    Can someone pass me my credit card?

    Thanks for the succinct summary! Can't imagine there's any logical counter argument to plumpy's reasoning. Saved me a lot of reading. :D

    [Leaves to get credit card.]
  • DaSy
    DaSy Posts: 599
    Is it not the case that when climbing you are constantly fighting against gravity which is decelerating you as it tries to drag you down the hill, and you are constantly re-accelerating with each pedal stroke, so the small effect of lighter wheels is felt more on a climb?
    Complicating matters since 1965
  • Escargot
    Escargot Posts: 361
    DaSy wrote:
    Is it not the case that when climbing you are constantly fighting against gravity which is decelerating you as it tries to drag you down the hill, and you are constantly re-accelerating with each pedal stroke, so the small effect of lighter wheels is felt more on a climb?

    Absolutely spot on.

    Of course you are right technically, however (and this is a big however), if you upgrade to lightweight wheels you are only likely to save a few hundred grammes at best.

    Now if your bike weighs 8kg then 200g represents 2.5% of the total weight saved. This sounds okay but when you sit on top of the bike this changes everything.

    If you weigh 75kg then the 200g saving in wheel mass then becomes approximately 0.24% of the total weight. Please forget all the BS that's banded about regarding rotating mass as it's complete gash (not the physics but the applicability to road bikes).

    I don't know about you but I can't even tell the difference when I've drunk a bottle of water (about 500g) so sure as hell am not going to notice any benefit from lighter wheels during a climb.

    :D

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