I was just reading through some of the technical stuff over at bicycelsports and came across a watts & wheels article. It said that at about 25mph it takes around 35watts to spin a 32spoke box rim, about 15watts for a deep V, and 5watts for a disk. Assuming(?) that it takes 35watts to spin the front and 35watts to spin the rear wheel we have a total of 70watts. Now, if I put on a rear disk at 5watts and a deep V front at 15watts, I have a total of 20watts. So with aero wheelset I get an additional 50watts, right?(70-20). When I plug these numbers into the analytic.com power/speed calculator(I only changed slope to 0, used 250W and 300W) I see a time savings of about 4minutes for a 40K, or about 1.6mph increase in speed. It also seems that an increase of 50watts would be huge for those with lower power outputs. The numbers seem too big, what am I missing?

Those numbers are way too big. I never believed them. This stuff helps, but not that much.

Using Cobb’s drag numbers and numbers from someone else’s wind tunnel testsing (can’t remember who) and conventional wisdom from other posters I arrived at the estimation that changing from box rims to a Hed3 (or equiv.) + disc would add .5 mph to a rider averaging around 25mph.

Possibly the watts would work out if the extra 50 watts were not applied to speed with 100% efficiency. Remember, there will be some lost in drivetrain, road and other friction, increased wind resistance, and most importantly if your legs suddenly output 50 more watts then you would get considerably less applied to the bike.

It is interesting to put the pacer on a computrainer at the same avg. watts as me and then compare between his speed and mine.

Remember, however, that the rear wheel sees disturbed air, while the front sees relatively “clean” air. So, the question we need to ask is, “How many watts does it take to spin the rear wheel when it is on the back of a bike with a rider on it.” Those numbers on Bicyclesports may be the wheels in a fixture, and not on a bike…

Does anyone know if those quoted drag numbers take into account the usual locations of those wheels? In other words, in the disturbed air of the rear of the bike, is the difference between std (35 watts) and disc (5 watts) really 30 watts, or does it take maybe…15 watts to spin the std in the rear and 2 watts to spin the disc?

Hope I am not trying to overanalyze…

Philbert

Good point philbert, that may be the ticket there. I’m pretty confident in my 1/2 mph analysis. I arrived at it through data from 2 separate testers and was confirmed through anecdotal evidence from posters on this forum.

This is a different way of looking at it than I normally have as I’ve always tended to think of time gained over a given distance than with speed gained. Here is Jim Martin’s take on this. http://home.hia.no/~stephens/aero.htm

You are correct, I intended to say that the pacer will always be faster because it has a 100% efficient pedal stroke. This is in the manual somewhere. There will be a little lost power in the drivetrain as well.

Tom, I hate to nitpick, but any power lost in the drivetrain will be invisible on a computrainer since the power is measured at the rear wheel contact patch.

Mike, at least you will be sure to see improvement! LOTS of improvement…for a long time! BWAAaaaHAHAAHAHAAaa! (Gotta love these torture machines, don’t you?)