Lighter wheelset question

Discussion in 'Cycling Equipment' started by gr8outdoorz, Aug 10, 2009.

  1. gr8outdoorz

    gr8outdoorz New Member

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    I have a rookie question. By switching to a wheel set that is approx. 275g lighter, how will that affect climbing? A little easier or a lot easier? The question stems from hearing a couple people discussing "rolling weight" saying it had a greater effect on feeling the weight of the bike when climbing. I am curious to weather there is any validity to this concept.
    Thanks,
    Carlton
     
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  2. alienator

    alienator Well-Known Member

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    The idea that wheels with lighter weight (or more specifically, lower moment of inertia) makes noticeable performance improvements is, well, myth. It's been shown mathematically just how small the performance changes are, even with large differences in MOI.

    Our bodies aren't very good sensors and are too easily influenced by non-physical things.
     
  3. gr8outdoorz

    gr8outdoorz New Member

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    So I would see a negligible difference in climbing and spending the money on a lighter set would not be worth it?
     
  4. BikingBrian

    BikingBrian New Member

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    Exactly. Any benefits would be psychomological;). That money would be better spent on a PM (if you don't already have one). There are very few short cuts to getting faster; train more.
     
  5. Dietmar

    Dietmar New Member

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    The difference would be very small, yes. Keep in mind, this is 275grams off of a total weight (bike + rider & gear) of maybe 80-90kg or more, so that's about 0.3% or so. Further, the actual performance benefit of 0.3% less weight will be even less than that, at least if you're going faster than 10mph. On average, you're probably looking at less than a tenth of a percent in performance gain, which is really within the noise of the variability of all the many other parameters that affect your performance.

    Whether or not that's worth it is a different question, though. Some people still find those newer, lighter wheels "noticeably" better subjectively, and for some people 1000 bucks is peanuts compared to that feeling...
     
  6. alienator

    alienator Well-Known Member

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    Well, worth is a personal value. If the wheels get you more excited about riding, then they could be worth it, but don't expect to see big changes on the computer. I had a set of wheels that together weighed 880g. Now I have two wheelsets that weigh in at 1380 g and 1385 g. I can't really tell any difference in performance of the wheels on steep climbs (12%+), even though the difference in weights was 505 g. That's a 1.1 lb difference. The magnitude of the difference in moment of inertia is at least as much between the wheelsets.

    If I were going to buy another wheelset (keep in mind there are a lot of mountains to climb 'round these parts), I'd:
    1. Get a durable set of wheels
    2. Get a set of wheels that really lit my fuse.
    Aero wheels are a much better purchase than lightweight wheels.
     
  7. gr8outdoorz

    gr8outdoorz New Member

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    Thank you very much for your advice. I already love to ride no matter what the wheel set and fully understand climbing will get easier the better my endurance and overall shape. I had just had "heard" my original statement from a couple and wondered if it was valid or people talkin out their azz. Again, thanks for all advise!
     
  8. swampy1970

    swampy1970 Well-Known Member

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    Lew Racing Wheels or Lightweights?

    ... or were you just making that up?
     
  9. alienator

    alienator Well-Known Member

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    LEW VT-1's.

    I have to admit I didn't stay at a Holiday Inn last night. For anyone that thought I did, that story was made up, except for the part about the female wrestlers, the badminton racquet, and the 3-in-1 oil.
     
  10. krulle

    krulle New Member

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    mathematically you can accelerate and decelerate faster with lighter wheels. It is proven on cars, and it should apply to on bikes.

    This is due to a lower inertia.

    But climbing is not about accelerating, but about keeping the pace ... so you would then be better of with some heavier wheels (they are less likely to decelerate ...)

    light wheels will only give you a good advantage in sprints.
     
  11. alienator

    alienator Well-Known Member

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    Mathematically a log 2.000000001 m long is 1 picometer longer than a log 2 m long. Does it matter? Mmmm, no. Just because something is mathematically larger doesn't mean that size difference is significant. When doing science, you have to keep in mind the context, and you have to apply things properly. First, lighter does not equal lower moment of inertia. MOI is a function of mass AND mass distribution. Second, the difference in acceleration between bicycle wheels of varying MOI is very small, even when the difference in MOI is large. Farting might make a bigger impact than a lower MOI.

    Mark McM, at Weight Weenies addressed this issue and produced an equation of motion for a bike (a differential equation) that accurately calculates the effects of varying MOI on acceleration. The net result? The differences in acceleration are essentially negligible. Here it is: Weight Weenies • View topic - Rotating Mass... . In his post he applied his model to climbing, but the math is exactly the same with sprinting. If the sprinting is on level ground, you just set the slope term, S, to zero.

    In short, the idea that light weight wheels make a significant performance difference is a myth. They can make a big psychological difference, but that's got naught to do with the physical reality.
     
  12. krulle

    krulle New Member

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    Sorry, I don't see your point here. The calculation you reffer to says the same as I said.

    Heavier wheels can even be better while climbing => check the calculations

    There is one important thing to mention if you fall back on the calculations: it is a steady situation = steady pedal rate!

    If you would rewrite the calculations to an acceleration-situation, you would see that weight is important overthere...

    I've seen in many times on race-cars, lighter tyres give better acceleration times.


     
  13. noonievut

    noonievut New Member

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    I recently purchased some handbuilt wheels. While weight was part of the equation, I think the following ended up being much more important:

    - after a Q&A with the wheelbuilder he learned of my riding style, needs, and we determined what would suit me
    - the wheels are well built, will need less maintenance in the future and can take a pounding (they have already, though you wouldn't know it)
    - they come with great service (at no extra charge)
    - and with regards to climbing, the wheels are stiffer (though not any less comfortable than my previous wheels) and I have noticed an improvement in climbing

    Good luck.
     
  14. alienator

    alienator Well-Known Member

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    Oh? You did the numbers? Really? What did you use to solve the diff eqn? And again, you miss the context and the relevance of the differences. The differences are very small.

    Steady state? Really? Do you know someone who pedals in perfect circles, i.e. applies exactly the same force at every point on that circle? No, you don't. Cycliist typically pedal ellipsoidally when it comes to the application of force to the pedals. Therefore, since the force varies around the pedal cycle, there are accelerations. Go back to physics 101 and review the mathematical definition of force. At any rate, none of that matters because.........wait........wait..........wait.................here it comes......................it's an equation of motion! It's valid for steady state and non-steady state situations. Do want to know how to make that equation represent a steady state? See that dV/dt on the left? That stands for.......here it comes again..............acceleration. If you make that 0, guess what happens? That's right! There's no acceleration. It then is an equation representing a steady state! La voila!

    To show just how much you don't understand the physics of what you're talking about, answer these questiions:
    1. How big is a car wheel/tire combo's MOI compared to a bike's?
    2. How fast does a car accelerate (Remember that dV/dt thing?) compared to a bike?
    Again, you keep losing a sense of the scale with which is being dealt. Next you'll likely be alleging that if a spoke breaks, wheel pieces fly out in every direction because that's often what happens when a turbine blade breaks in a jet engine.

    You haven't even put a number to how much acceleration changes with changes to a car wheel's MOI.
     
  15. krulle

    krulle New Member

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    The topic you mention has some resolutions itself in it ...


    What are you trying to say here? Is there any cyclist intrested in the accelerations on one pedal-move?

    I am talking about accelerating from 0 to 40 km/h or from 35 to 60 km/h ....

    The calculations are not ment for that ...

    about 12 kg per wheel, and a total mass of around 400 kg (including the driver)

    so that will be more or less the same as a bike


    Ok, there you have a point, mucht quicker, but it is then also an over-powered system that should have less effect from inertia.


    1kg per wheel could be a difference of 0,3 seconds from 0 to 100km/h (2,7s to 3s)

    so less than 10 % weight-difference resulted in more than 10% extra time from 0 to 100km/h


    BTW the difference only has to be 1 inch to win a sprint ;-)
     
  16. alienator

    alienator Well-Known Member

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    Wrong. It's an equation of motion. It is perfectly applicable to the situation at hand.

    So, when you did your little race car experiment, did you actually do measurements? Did you actually make sure to hold variables constant between tests and account for those variables that you couldn't hold constant?

    As for the numbers, Mark McM's numbers show there is very little difference in performance between various MOI. The difference is on a level that is likely undetectable by the rider (especially given that with optimum aero wheels, the aero contribution is an additional 0.5 mph at 25 mph, a difference in velocity that requires a bike computer to be revealed, i.e. it isn't discovered by the seat of your pants).

    The thread wasn't about racing, but since you bring it up, might you explain, then, how it is that pro riders can win races....sprints, even.....on 7 kg bikes with unlight wheels when they can ride 6.8 kg bikes with their choice of light wheels? Technically your inch argument is just subjective flotsam. If you're going to argue for an inch, then you also have to discuss the differences in Crr as result of the paticular path a rider follows and the random surface structure of a road (implying a random Crr); the effects of varying meteorological conditions for each rider; possible aero effects resulting from position in the spring leadout and throughout the rest of the sprint; rider's energy status at the end of a race; varying inflation pressures for tires on the different bikes; and so on.

    When racing you often make choices because those choices might help, not because they definitely will. You make choices because said choices might have very few downsides and are therefore, in your judgement, worthy risks.

    None of that has anything do with the marginal benefits of low MOI wheels.
     
  17. krulle

    krulle New Member

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    OK, I won't be able to convince you, and I won't put any more effort in it.

    But just think about the fact that many competitions restrict the weight off bicycles ...



    About the race-cars (not that it matters here): I was involved in the measurements ( I needed different data), and it was done on 2 identical cars. First time the second car was slower, (with the heavier tyres), 2nd time they had identical tyres and the second car was faster ...

    It all happened in the timespan of 1 hour, and there where no special weatherchanges, no other changes at the cars, same pilots per car ...
     
  18. 531Aussie

    531Aussie Well-Known Member

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    This topic drives me nuts even more than Kirby Palm's BS crank length formula of 21.6% of inseam.

    .
    The 'importance' of rotating wheel weight on bikes is the biggest scam being perpetrated by bike companies at the moment; probably even more than bike (and component) weight (at least bike weight actually means something when riding up hills).

    Automotive analogies can't be used here, because of the massive differences in car wheel and motor revs, relative to weight, even in 'non-race cars'. For example, if a 1500kg car is generating 300,000w, that's a ratio of 200w-per-kg. Compare that to a 75kg guy on a 7kg bike, who 'might' generate 1000w stomping out of a corner in a crit: that's 12.2w per kg. Can you see the difference? :D

    Bike wheel aerodynamics and stiffness are far more important that the mythical external rotating mass, and these can be sacrificed by light, box-shaped rims.

    My fastest-feeling wheels are my uber-stiff, 2.2kg sets with 30mm-deep rims. I've tried many lighter wheels (sub-1500g) which feel as though they "spin up" way slower because they flex like shid!
     
  19. Peter@vecchios

    [email protected] New Member

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    Mine are a set of really nice Campagnolo Delta box section rims laced with 36 double butted spokes. I made a set with left hand rear Revolution spokes and fronts, and it felt much 'softer'.

     
  20. slyjackson

    slyjackson New Member

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    The stiffness of your rear wheel will make you a better climber. If you could somehow shave 275 grams of your existing wheel set, I will bet my next paycheck that hill will be just as hard.

    A stiffer wheel in the back without flex will get you up that hill a lot quicker and it will be noticable whether its 275 grams less or 275 grams more.
    A heavier wheelset will have somewhat of an advantage on the flats. How much don't know. There are others arguing this point through out this thread.

    Just my $.02 worth.:D
     
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