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PostPosted: Mon Nov 18, 2013 6:16 am 

Joined: Wed Jan 12, 2005 6:04 am
Posts: 558
Location: USA
Lately on our large group training rides there has been a trend towards these balls out drag race accelerations after each stop sign, and there are quite a few. Add to that rolling terrain, complete with sharp bergs within technical narrow country roads and you are going to expend a lot of energy. Even though the long flat sections make up a fair percentage of these rides some people have found they have adapted better by switching to smaller profile, hence lighter rims.

What I find interesting is that this wasn't the case before this change in tactics. People benefited from the deeper wheels here. We've were discussing how this isn't the case in racing conditions, even in crits, cause at least you can carry a lot of speed through the corners.

Just something to consider if you're thinking of taking that next step towards racing and are training in similar conditions.

PostPosted: Mon Nov 18, 2013 7:21 am 

Joined: Sat Feb 23, 2013 10:51 pm
Posts: 152
Looking for some sort of evidence, outside of anaecdotal, in the post to support the claim...

PostPosted: Mon Nov 18, 2013 7:36 pm 
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Joined: Tue Mar 10, 2009 5:27 am
Posts: 1244
Location: McCall, ID
Lighter rims will definitely make accelerations feel more responsive and lively, but does that correlate to real world speed increase? Thats debatable. Maybe a very, very small amount. But if someone ever uses the excuse, "Ahh I lost that sprint because I'm on my Deep Vs instead of my Alpha 340s", then they should receive a swift Chuck Norris roundhouse kick to the dome piece.


PostPosted: Mon Nov 18, 2013 8:16 pm 

Joined: Fri Dec 15, 2006 7:45 pm
Posts: 3968
Location: Natovi Landing
physics suggests zen is right. shallower and lighter rims do feel very different though which counts for a lot IMO

Stiff, Light, Aero - Pick Three!! :thumbup:

PostPosted: Mon Nov 25, 2013 2:38 am 

Joined: Tue Nov 11, 2003 9:47 pm
Posts: 2179
Location: Santa Cruz, California, USA
Here is a good discussion: https://en.wikipedia.org/wiki/Bicycle_p ... ght_wheels

The benefit of lighter wheels on accelleration is small, and could be equaled or outweighed by heavier but more aero wheels depending on how heavier and how much more aero.

PostPosted: Tue Nov 26, 2013 5:52 am 

Joined: Wed Jan 12, 2005 6:04 am
Posts: 558
Location: USA
Thanks for the math compilation reference Eric. It will be interesting to go through some numbers, thought it suggests wheel weight makes little difference.

The point I was trying to emphasize was the numerous changes in dynamics. Some days it smooth and fast while other days it's very choppy and fast.

The consensus here is that when you sum the effects of 10 to 12 very hard accelerations from stops signs along with the chaos from other sources (cops, vehicles and other cycling groups, etc.) yielding very dramatic changes in velocity, you may be better off with lighter wheels. Conversely, when it's smooth, with gradual accelerations to very high speeds then aero should be more beneficial. This is going with the assumption the power account is finite.

I am merely passing along observations from riders after the rides, though I would tend to agree. Under these extreme conditions heavier wheels can take their toll, or so it would seem.

PostPosted: Wed Nov 27, 2013 12:31 pm 

Joined: Sat May 12, 2007 3:40 am
Posts: 477
Location: Triange, NC
I would think that wheel stiffness could make it less clear on any perceived advantage. A stiff deep profile wheel when might accelerate faster than a softer, yet slightly lighter box rim wheel.

PostPosted: Wed Nov 27, 2013 8:36 pm 

Joined: Wed Oct 30, 2013 12:52 am
Posts: 301
If you have the opportunity - ride different wheels and tires and pressure. Its a lot easier and IMO more accurate than calculations.

Most calculations treat the bike and cyclist as if they go in a straight line and the bike is perpendicular to the ground at all times. I agree on the average the bike goes in a straight line and on average it is perpendicular to the ground, however there is lots of side to side movement in real riding and the rotational inertia of the wheel has bearing on that and it is changing constantly as the rider does mini turns right and left riding at 25mph just balancing the bike. Then there is the stiffness part. Energy gets scrubbed off in tire casing deformations, bumps, flex in wheels - without the rider even standing and sprinting.

PostPosted: Fri Nov 29, 2013 8:48 pm 

Joined: Tue Nov 11, 2003 9:47 pm
Posts: 2179
Location: Santa Cruz, California, USA
Unfortunately the "ride it" method also introduces rider perception into the equation... and that may mask the relatively small effects from lighter weight on accelleration or aerodynamics on speed.

I'd love to see some energy loss calculations from wheel deformation. My gut feeling is that most of it is in the spokes (especially on deep section carbon rims which are stiff due to their cross section) and the majority of spoke flex is returned- i.e. they act as springs.

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