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#11
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Bumps and efficiency
SYJ wrote: Do bumpy road surfaces typically exact costs in riding efficiency? Yes... but it is difficult to tell how much. I've seen no testing that addresses this, but Bicycling Science discusses it briefly. They show an old chart developed by the army (for tank crew) that gives power absorbed by a person's body versus frequency and amplitude. An amplitude of only .1 inches corresponds to ~50W power loss at 5hz (10.7m/sec; 23.9mph... if the bump occurs once per wheel rev). Some approximate values from the chart are tubulated below: Amplitude (inches), Frequency (hz), Speed (mph), Power Absorbed (W) ..1, 4, 19.0, 3 ..1, 5, 23.9, 50 ..1, 6, 28.6, 100 ..25, 4, 19.0, 290 ..25, 5, 23.9, 1200 ..25, 6, 28.6, 1900 Clearly, the losses go up a lot with increasing frequency and amplitude. In a field test this would tend to mimic aero losses more than rolling resistance. In fact it would increase much faster with speed than even the aero losses. The difficult thing is relating this to a bicycle. I don't think I typically ride on any roads that are smoother than .1 inches every wheel revolution... so am I lossing 50-100W when riding at a good speed? Somehow I doubt it. Probably, considerable attenuation of vibration occurs before it reaches the rider's body. I would expect elastic parts like tires, stems, frames, seats, etc, to absorb little energy (ie they would flex with little loss). Still, .1 inches is not a lot of motion, and I'm sure I experience that much fairly often... on my hands and butt. If someone wanted to test for this, a large roller with a rough surface could be constructed, and the tire could be weighted by an actual person... ie have a bicycle with rider on top supplying the force. Or maybe someone with a good power meter could do a field test on different surfaces. I'm curious if it is more beneficial in a long ride/race where crappy roads are the norm to use 25mm tires vs. 23s. I'd think so... even 28s... and drop the pressure as low as you dare. A good quality tire suffers little degradation in Crr going from 120-80psi even on an extremely smooth surface, so I think you'd be faster... and that doesn't even consider the comfort issue! |
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#12
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Bumps and efficiency
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Bumps and efficiency
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#14
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Bumps and efficiency
On 29 Jun 2006 12:02:23 -0700, "SYJ" wrote:
Do bumpy road surfaces typically exact costs in riding efficiency? While doing a loop that I've ridden dozens of times, I noticed that my speed had dropped almost two MPH from my norm on a section that had recently been chip sealed (as opposed to the baby's butt smooth pavement that existed previously). I didn't feel especially cooked (at least, no more than usual), there didn't seem to be any wind, and I was able to ramp the speed back up to normal after turning off the choppy surface, all of which led me to question whether it was indeed the bumpiness that was affecting me. Does anyone have any similar anecdotes? Any studies of this? Was I dreaming? I'm curious if it is more beneficial in a long ride/race where crappy roads are the norm to use 25mm tires vs. 23s. It's more a matter of the vibration affecting the engine than any increase in rolling resistance. I think the fatter tires at lower pressures and tubular tires are advantageous. I have had a hard time going fast on bad buzzy road and cross riding on clumpy uneven grass. Familiarity helped a lot. I still work on that, and it was work getting used to pushing for speed on bad surfaces. Ron |
#16
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Bumps and efficiencyP.S.
From: (BigBen)
Assuming we're talking about same model of tyre, same tyre pressure, how would that be any better than decreasing the 23mm tyre pressure a bit - say, from 120 to 110 psi, for instance?? cheers, jbr Well, it would make for an even rougher ride for one, but the problem is not so much road vibration as it is control. A 23mm tire wouldn't be any wider that the gravel it's rolling over, and narrower tires tend to e harder to keep on a line, as the road texture constantly tries to direct it's direction of travel and the rider has to struggle more to keep the bike riding straight. - - Comments and opinions compliments of, "Your Friendly Neighborhood Wheelman" My web Site: http://geocities.com/czcorner To E-mail me: ChrisZCorner "at" webtv "dot" net |
#17
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Bumps and efficiencyP.S.
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