The advantage of light bikes, and particularly light wheels, from a KE standpoint is that KE only comes into play when speed changes, and there are certainly two cases where lighter wheels should have an advantage: sprints, and corner jumps in a criterium.
In a 250 m sprint from 36 to 47 km/h to (22 to 29 mph), a 90 kg bike/rider with 1.75 kg of rims/tires/spokes increases KE by 6,360 joules (6.4 kilocalories burned). Shaving 500 g from the rims/tires/spokes reduces this KE by 35 joules (1 kilocalorie = 1.163 watt-hour). The impact of this weight saving on speed or distance is rather difficult to calculate, and requires assumptions about rider power output and sprint distance. The Analytic Cycling web site allows this calculation, and gives a time/distance advantage of 0.16 s/188 cm for a sprinter who shaves 500 g off their wheels. If that weight went to make an aero wheel that was worth 0.03 mph (0.05 km/h) at 25 mph (40 km/h), the weight savings would be canceled by the aerodynamic advantage. For reference, the best aero bicycle wheels are worth about 0.4 mph (0.6 km/h) at 25, and so in this sprint would handily beat a set of wheels weighing 500 g less.
In a criterium race, a rider is often jumping out of every corner. If the rider has to brake entering each corner (no coasting to slow down), then the KE that is added in each jump is wasted as heat in braking. For a flat crit at 40 km/h, 1 km circuit, 4 corners per lap, 10 km/h speed loss at each corner, one hour duration, 80 kg rider/6.5 kg bike/1.75 kg rims/tires/spokes, there would be 160 corner jumps. This effort adds 387 kilocalories to the 1100 kilocalories required for the same ride at steady speed. Removing 500 g from the wheels, reduces the total body energy requirement by 4.4 kilocalories. If the extra 500 g in the wheels had resulted in a 0.3% reduction in aerodynamic drag factor (worth a 0.02 mph (0.03 km/h) speed increase at 25 mph), the caloric cost of the added weight effect would be canceled by the reduced work to overcome the wind.
Another place where light wheels are claimed to have great advantage is in climbing. Though one may hear expressions such as "these wheels were worth 1–2 mph", etc. The formula for power suggests that 1 lb saved is worth 0.06 mph (0.1 km/h) on a 7% grade, and even a 4 lb saving is worth only 0.25 mph (0.4 km/h) for a light rider. So, where is the big savings in wheel weight reduction coming from? One argument is that there is no such improvement; that it is "placebo effect". But it has been proposed that the speed variation with each pedal stroke when riding up a hill explains such an advantage. However the energy of speed variation is conserved; during the power phase of pedaling the bike speeds up slightly, which stores KE, and in the "dead spot" at the top of the pedal stroke the bike slows down, which recovers that KE. Thus increased rotating mass may slightly reduce speed variations, but it does not add energy requirement beyond that of the same non-rotating mass.
Lighter bikes are easier to get up hills, but the cost of "rotating mass" is only an issue during a rapid acceleration, and it is small even then.
8 comments:
Well, I about to put this to test this year. I have saved pretty close to 2 lbs in rotating weight for this season. I am curious about how this is going to feel going uphill.
Hey Steve
I think it is only going to make you faster.
I hate snow!!!Sorry i had to throw that in.It makes me feel better to release my anger once and a while.
I lost 10 lb of body weight for last summer. It was much easier climbing hills. It didn't cost me a dime, just my regular night-snack habit.
Ten pounds of body weight would be like 10 grand worth of bike parts.
I came to the realization about 3 years ago that I was never going to be sub 185lbs again. 190-195 seems to be ideal for me for any of my athletic purposes. The last time I was 185 I was so cranky all the time. My new wheels did not really cost me anything. Any choices that I make in equipment come under very close scrutiny due to my size. At my size lighter is not always better. However If I CAN get a component that has no weight limit, is lighter, just as strong (or in this case the Mavic's should be stronger) then I am going to pull the trigger. Everything that I am doing this year is replacing components that have 4 or more years of use on them and could potentially be a liability because of it. That included my frame, front derailler, CK hubs, headset, Carbon handlebar. I replace tires every year. Running tires tubeless puts a lot of extra pressure on the bead. I put even more stress on that bead by being huge. I have peeled old tubeless tires off the rim at 40 km/h during a high speed corner. Not pretty. As for the hubs, well it was painful to sell them, but there was no way I was going to rebuild that set of hubs anywhere near 1750 grams and be as strong as the Mavic's. Unless I got me a set of Enve carbon hoops. I don't have that kind of money to blow thanks! Could I be lighter, yep. The last time I was though I was not powerful at all. I have been fast at 195 (last year provincials), and slow at 185 (like 5 years ago before all the injuries happened) So this year during my search for replacement components, I decided to weight weenie it and find stuff that was lighter than everything I was replacing. Total it all up and hopefully it equals more speed! Oh yeah and the engine is all good so far this year as well.
Steve
You riding with Olympia again this year.
Yep. Until my body says no more.
Sweet!!
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