On the flats at steady speed virtually nil. On a steep climb it depends on how much you weigh plus the weight of all your gear. Check out http://www.analyticcycling.com/ and plug numbers into their Speed Given Power calculator in the Static Forces On Rider menu. For a given power you can calculate speed on a given grade with assumptions about frontal area, air density, rolling resistance etc. You'll see a bit of difference on hills, especially steep hills but very little difference on the flats.azdroptop said:Just wondering? My new wheelset/tire/cog setup is 1/2 pound lighter than my old set up.(Bathroom scale weight) And I was just wondering what kind of benefit that equates to if any at all? Just curious.
Hi Dave,daveryanwyoming said:On the flats at steady speed virtually nil. On a steep climb it depends on how much you weigh plus the weight of all your gear. Check out http://www.analyticcycling.com/ and plug numbers into their Speed Given Power calculator in the Static Forces On Rider menu. For a given power you can calculate speed on a given grade with assumptions about frontal area, air density, rolling resistance etc. You'll see a bit of difference on hills, especially steep hills but very little difference on the flats.
-Dave
Spend some time fiddling with the calculators over there and you'll see that most of the rotational mass talk is just that...talk. It doesn't make nearly as much difference as many believe.azdroptop said:.... With all the talk of ratational mass and wheels I was just curious...
Ya know given your handle I'd think you'd take into account a total physics model of acceleration and the work of moving uphill against gravity. Yes the larger moment of inertia of a heavier tire/rim requires additional energy to accelerate over a lighter tire/rim or weight near the hub. But you also have to consider the energy requirements of linear acceleration. From that standpoint the moment of inertia differences between "heavy" and "light" racing wheels is signifigantly less than the inertial requirements of accelerating the mass of the rider and frame. It's too easy to look at a wheel in isolation and conclude a hundred or so grams of rim weight here and there will make a huge difference in the energy required to accelerate that wheel but add the rider and the rest of the bike and it's negligible.9.8mps2 said:At a steady state you are just taking that greater mass down the road with you. On a climb you are pretty much just carrying that mass up the hill with you. The big difference is realized when accelerating ( increasing the velocity of the rotating mass). And then your brakes have a greater workload in taking that energy away when slowing. Yes, you could "coast" better, but if you are trying to pare weight for performance you are either pedaling at strongly as possible or braking.
Saving mass at the hub is much less appreciated than saving at the periphery with lighter wheels/ rubber.
Like a stock trade on comission, a heavier wheel will cost you more each time you sell or buy in brakes or muscles.
If a suspension is involved, the suspension works better with less "unsprung weight" involved - that being wheel & swingarm or fork - mass not actually carried by the suspension.
Fair enough. I'm not knocking your model for moment of inertia, just the isolation of one fairly minor component of the entire system. I can't count the number of times I've heard a cyclist explain the evils of rotating weight without a thought to the bigger picture. I'm sure it sells a lot of light tires and rims9.8mps2 said:Quite right Dave. The handle is no accident, my name is Michael Patrick Simmons , and I am also no stranger to physics. What I was trying to do was teach, and give it in terms so that someone may easily glean the big picture w/o any mathematical masturbation.( since with the data provided neither you nor I can precisely quantify what benefit is gained .)
If azdroptop comprehends your model better than mine - OK - I have failed.
In climbing doesn't matter wether mass is rotating or not. Acceleration was the case when one rotational kg counts as two stationary kgs.wiredued said:The last time I calculated my 1/2 hour climb I think 10 lbs extra used 24 watts aprox. but rotating mass is the worst kind for climbing.
Say you climb around 5 m/s (18 kph) on a steepish hill, dropping 1/2 lb will save you: ~ 11 x hill gradient (%)azdroptop said:Just wondering? My new wheelset/tire/cog setup is 1/2 pound lighter than my old set up.(Bathroom scale weight) And I was just wondering what kind of benefit that equates to if any at all? Just curious.
azdroptop said:just wondering
And even these are optimistic in terms of power savings since they don't take into account the other forces we have to overcome and most of us can't hold 18 kph on a 15% grade.rmur17 said:Say you climb around 5 m/s (18 kph) on a steepish hill, dropping 1/2 lb will save you: ~ 11 x hill gradient (%)
5% grade ~ 0.6W
10% grade ~ 1.1 W
15% grade ~ 1.7 W
IOW, not a whole lot. Savings are directly proportional to speed so you can scale up or down as you see fit from the 18 kph basline.
edit: yes I've simplified the sine(arctan(gradient)) bit ... for these masses it's doesn't matter much.
Yeah, true enough if you're trying to get that last bit of TT performance that half pound may be noticeable but if you consider that same 250 Watt/ 80kg rider in a 5 mile TT up a 7% steady grade you'd get:NM87710 said:Not a whole lot is relative if you've ever lost a TT(they're not all flat) by a 1 or 2 seconds. Not saying one should or shouldn't ride light equipment - just depends how much you're willing to do to achieve your goals.
I specialize in simple things .daveryanwyoming said:Dang rmur, your much simpler slope only model is pretty darn accurate and saves punching a bunch of numbers into the online calculators.
Somehow I don't think 3.1W/kg is gunna threaten a podium spotdaveryanwyoming said:Yeah, true enough if you're trying to get that last bit of TT performance that half pound may be noticeable but if you consider that same 250 Watt/ 80kg rider in a 5 mile TT up a 7% steady grade you'd get...
Actually, that's entirely on topic as this entire thread has been about power savings as you drop weight. Given a steady power output it doesn't matter whether it's your weight or component weight or weight in your water bottles, but most of us have a lot more spare body weight to drop whereas the bikes are already pretty lean.J@co said:Slightly off topic, but if you're interested in saving power, I heard somewhere that if you maintain your same power output, you "save" 1.3% power for each kilogram of body weight lost. ....... I'm not sure if this applies to the bike and wheels somehow, although you are not likely to have a wheelset which is a few kg's lighter than another.....
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