The guy at my LBS says that saving weight on the wheels is more important than saving weight on the bike or on me and that he thinks the value is 2 for 1–1 lb of wheel savings is the same as 2 lbs on the bike.
Does that make any sense? If so tell me why–he said something about rolling weight versus static weight but it didn’t sound to scientific to me.
If it is true, what is the correct ratio of value on the wheel vs. the bike?
Doesn’t really matter for a time trial or tri, but makes a little diff in road racing since you are slowing, sprinting, corning so much. A lighter wheel (rim really) will “accelerate” a little faster and certainly “feel” more lively. Ratio - I have no idea. 2:1 I highly doubt. I would say the most important is YOU (i.e. engine) losing weight as I’m sure the WORST Tour rider could beat an average joe on a heavy 1970’s Schwinn with Joe on a P3 Carbon with Full zipp wheels regalia.
Also, stiffness in the wheels matters as well when standing/sprinting to catch the pack/line, climbing, etc. Hard to have lightweight and stiff - hence the price of a very good set of these wheels (I would love some Zipp 202’s for example).
He’s right. The wheels have both a moment of inertia (rotating mass) and regular weight (gravity). It might even be more than 2-for-1, depending on where you are taking it off from (e.g. lighter hoop is better than a lighter hub).
Aerodynamics is still the most important feature though, especially for wheels. Zipp 808’s are MUCH faster than 202’s even though the 202’s are quite a bit lighter.
I have no idea of the ratio, but it is well documented that savings on rotational weight are the most efficient. I’m sure Slowman has an article on it in the archives over on slowtwitch.com.
I have no idea of the ratio, but it is well documented that savings on rotational weight are the most efficient. I’m sure Slowman has an article on it in the archives over on slowtwitch.com.
unless you’re going for an hour record 
Quite so!
The guy at my LBS says that saving weight on the wheels is more important than saving weight on the bike or on me and that he thinks the value is 2 for 1–1 lb of wheel savings is the same as 2 lbs on the bike.
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The guy in your bike shop does not know much about physics… (very typical in the bike business).
It is true that when you accelerate, it takes more energy for rotating weight since you have to increase tnanslational speed AND rotational velocity. In the case of non-rotating weight, you only have to worry about the former.
Simple physics shows that if you consider that all of the mass of the rotating body is concentrated at its circumference (which is of course not the case), energy wise, the energy required to accelerate one pound of rotating weight is the same as that required to accelerate 2 pounds of non-rotating weight (hence the commonly quoted 2 to 1 figure).
In fact weight is not concentrated at the circumference (the true answer is somewhere 1 and 2), and, even if it was the case, energy required for acceleration is very small compared to wind resistance, even if you were to assume a criterium course where sudden accelerations are often required. Weight is not that important to start with, and rotating weight only comes into the equation when accelerating. So the difference between 1 pound of rotating weight vs non-rotating weight over an Ironman distance is likely to be of the order of 1 second.
Francois in Montreal
Francois in Montreal - I see it your way.
How I see it - I imagine the force required to spin up a wheel with tire that weighs about 1000 grams as opposed to one that weighs 2000 grams. Then I picture the difference between each wheel as 100 plus pounds of downward force (my body and bike weight) are applied to the wheel as it contacts the gound. The 1000 gram difference doesn’t seem as big.
David K
Well documented, but mostly by anecdote. I’d give this a read, as it’s probably the most well-written piece I’ve seen on the subject: