Is there a big difference in watts required to spin a 1000g wheel vs. a 900g wheel. At 25mph? At 20 mph?
depends on what you mean by “spin”. newton’s laws of motion say that there is no difference if the speed and resistance stay constant, because an object in motion will continue to move at the same velocity barring opposing forces.
on the other hand, trying to accelerate a heavier object (whether with angular or linear motion) is harder than trying to accelerate a lighter one. a change of 11% isn’t substantial, but could be measured and possibly even felt in certain kinds of events. for most triathlons, probably not so much.
the frictional resistance of both wheels is largely a function of the tires, not the wheel. the wheel bearings play a tiny role here.
that leaves air resistance, which is hard to cover without knowing which wheels you’re talking about.
…on the other hand, trying to accelerate a heavier object (whether with angular or linear motion) is harder than trying to accelerate a lighter one. a change of 11% isn’t substantial, but could be measured and possibly even felt in certain kinds of events…
Any inertia effects anyone claims to be able to feel is most likely just “placebo effect”…the accelerations of a bicycle wheel are just too tiny (even in an all out sprint) for any effect on the acceleration of the whole bike+rider system to be felt, or even measured.
http://www.biketechreview.com/...ive/wheel_theory.htm
…In summary, wheels account for almost 10% of the total power required to race your bike and the dominant factor in wheel performance is aerodynamics. Wheel mass is a second order effect (nearly 10 times less significant) and wheel inertia is a third order effect (nearly 100 times less significant)…
thats an interesting short paper. i’m suprised, though, to see him not mention wheel windup for crits. this has little relevance to triathlon, but i think that even with modern wheel designs, its still quite a contributing factor during sudden acceleration …
depands on it aero drag if you want the be included
and with with out roll resistance
basicly drag of bearing at this speed in less then 1 watt
also its impostant to know where the center of weight is
well its just a tiny differance forget the differance it is to little
.
thats an interesting short paper. i’m suprised, though, to see him not mention wheel windup for crits. this has little relevance to triathlon, but i think that even with modern wheel designs, its still quite a contributing factor during sudden acceleration …
Didn’t you see the column for the crit? If you click on the “Appendix D” link you’ll be taken to this plot that shows the instantaneous acceleration plot.
http://www.biketechreview.com/archive/appd.pdf
Those are some pretty teeny accelerations, aren’t they?
If you look in the table, reducing the overall inertia of either the front or rear wheel by 50%(!) only reduced the overall average power requirement by a measly ~0.02%. Do you know how hard it would be to reduce the rotational inertia of a wheel by 50%?
Based on that, I have to agree with this conclusion:
Wheel inertia effects in all cases are so small that they are arguably insignificant.
no, no, i was agreeing about the inertial data. but wheel wind up is a different effect. its not based on inertia, its a mechanical effect caused by lack of rigidity in the wheel structure. i would have expected that to show as a separate effect, possibly large enough to detect for crits and some road races.
have you ever seen a video of wheel being wound up by a powerful sprinter? it can be quite alarming …
no, no, i was agreeing about the inertial data. but wheel wind up is a different effect. its not based on inertia, its a mechanical effect caused by lack of rigidity in the wheel structure. i would have expected that to show as a separate effect, possibly large enough to detect for crits and some road races.
have you ever seen a video of wheel being wound up by a powerful sprinter? it can be quite alarming …
Umm…no. And I’d be awfully shocked if you could show me any.
the one’s i saw predate the current internet. i saw them at a frame builder’s store in bristol in the UK, and were taken with sprinters on fixed wheel bikes mounted on early stationary hydro/mag trainers. ah, and actually, now that i think about it, they weren’t videos, but rapid-series stills taken with an automatic camera. i believe that jobst brandt also mentions wind up, possibly with some real numbers too.
thats an interesting short paper. i’m suprised, though, to see him not mention wheel windup for crits. …
He was talking more about crits than anything else. Kraig is a roadie by nature.
… i would have expected that to show as a separate effect, possibly large enough to detect for crits and some road races.
…
So…what you’re saying is that the hub spins faster than the rim when a wheel is heavier? I doubt it; the tire would break loose from the road before enough torque was applied at the hub to actually stretch spokes. Because, in order for the hub to turn ahead of the rim, half the spokes would have to stretch. The other half would have to shorten, or simply bow out. The bowing-out is possible, I suppose. But, to stretch stainless steel…I don’t think so.
Anyhoo, angels on heads of pins and all that. Does a heavier rim require more power to ride around a crit circuit? Yes, absolutely. How much more power? Less than can even be measured under perfect real-world conditions. A flapping race number or a water bottle is vastly more costly, in terms of power “lost.”
So…what you’re saying is that the hub spins faster than the rim when a wheel is heavier? I doubt it; the tire would break loose from the road before enough torque was applied at the hub to actually stretch spokes. Because, in order for the hub to turn ahead of the rim, half the spokes would have to stretch. The other half would have to shorten, or simply bow out. The bowing-out is possible, I suppose. But, to stretch stainless steel…I don’t think so.
Dived back into my copy of The Bicycle Wheel …
- I don’t mean to exaggerate the effect of windup. Brandt notes that its a real behaviour, though likely inconsequential
- He estimates that it needs about a 250kg load on a 170mm crank arm with a 2:1 gear ratio to wind up a small flange hub by 1 degree.
Since 1500W @ 100rpm (a reasonable guess at an upper limit for a pro sprinter) equates to only about 180kg, its reasonable conclude that
the effect is very small in any circumstance and of no consequence for triathletes. - He notes that for radial spoking, its required for the hub to rotate ahead of the rim in order to trasmit force. This is, of course, a steady
state while driving the back wheel, though the level of windup varies on the force being applied. - According to his spoke charts, i believe that you’re slightly understimating the ductility of stainless steel spokes. Brandt’s diagram #69 shows
forces of 200kg stretching a typical spoke (DT, Wheelsmith, 2.0mm or 1.8mm, butted or not) by about 2mm. It only takes
about 300kg to get cloe to the steady state for DT spokes, the WS ones had an odd yield curve. Some spokes tested stretched
as much as 6mm without breaking. - I’m going to have think about my memory of those rapid-frame photo sequences that I saw.
I’m not smart.
Theoretically, I have two 1000g wheels to pick from. Which one do I want:
Wheel A: Has a 1 gram hub and a 999g rim.
Wheel B: Has a 999g hum and a 1 g rim.
I think the answer is obvious (B- right?) so isn’t location of the weight critical in the analysis? If this has been addressed and I missed it, see line 1 of my post.
Cheers,
Brian
It doesn’t matter where the weight is. Yes, the heavier rim takes more power to spin up.
Again, the question is: How much more?
**
The answer is: Less than you think. Less than you can even measure.
I don’t doubt that you can “feel” the difference in your feet. But you can feel a feather in your hand. That doesn’t mean you jump any less high when you hold one.
But you can feel a feather in your hand. That doesn’t mean you jump any less high when you hold one.
That’s a killer analogy. Finally makes sense. Thanks.
Is there a big difference in watts required to spin a 1000g wheel vs. a 900g wheel. At 25mph? At 20 mph?
i wanted to include a jpg with calc
so you could see how small it is
but it seems i cannot do that why??
anyway its less then a 1 watts its to tiny
even in acceleration from 35 to 45 km
between 1000 and 900 gram
i took a rider with 380 watts
he needed the same watts for both
the avg speed was the same
42.357
distance covered 0,448 meters
end speed the same
45.034 km/h
only he has reached it 0.1 second faster (38 to 38.1 seconds)
so the even in accl. when it counts the most it to tiny watt difference
remember that also the lighter the wheel the faster it stops
while the stored energy (kinetic energy is smaller)
all things the same of course only weight is the factor
hope this helps