Okay, perhaps we can generate good discussion from the Q and A that appeared in last year’s HED brochure…all been talked about before but they kind of summarizes everything for the not so informed…
Q.1 Is it true that you have to be riding above 20 mph for aero wheels to make a difference?
A.1 The faster you go, the more power you produce. Efficient use of this power depends on how good your aerodynamics are. The time savings provided by aero wheels will always be proportional to the power of the rider. It doesn’t matter whether you produce 300 watts or 150 watts. In fact, slower riders may not be going at the same speed as faster ones, but aero wheels actually save them an even greater amount of time over the same distance. The only exception is on hills, where speeds slower than about 11 mph aren’t sufficient to compensate for the aero wheel weight disadvantage.
Q.2 Can certain wind angles push a rider along?
A.2 Think of a guy in a rowing boat, using ‘x’ amount of power to move along. Then erect a sail the size of a disc wheel. The surface area of this sail isn’t enough to propel him without rowing as well, but it will make the rowing easier. This is how a disc wheel works at beneficial wind angles. You can’t stop pedalling but it does feel easier.
Q.3 I can only afford one wheel, so which one should it be?
A.3 The front wheel breaks the wind first, so this is the more important of the two. In certain conditions, a front wheel will save twice as much as the rear. However, it also depends on the model of the wheel, because a rear disc will provide at least as much performance as any other aero design on the front.
Q.4 Is aerodynamic performance more important than weight saving?
A.4 let’s look at a few figures. Assume a rider uses a conventional bike over a 25 mile flat course time trial. He weighs 150 lbs, his bike is another 20 lbs, and he his producing 185 watts of power. The aero drag with this scenario would be approximately 6.5 lbs using aero bars into a 2 mph headwind of between 0 and 5 degrees. With his standard bike, our rider’s time would be 1 hour 7 minutes and 38 seconds. If we reduced the weight of his bike by three pounds, that time would come down by just 3 seconds. However swap the wheels to an aero design and the drag could be reduced to 6 lbs. Even with the additional quarter pound weight of the new wheels, his time saving over 25 miles jumps dramatically to over 1 minute 30 seconds. No contest.
Q.6 How much time could I save in a 25 mile time trial by switching to aero rims?
A.6 It depends on the wheels. Using our 150 lb test rider in an aero tuck as a model, switching to aero wheels will typically save between 1 minute 30 seconds and 2 minutes over a flat 25 mile course. Using a deep front and disc rear, two minutes is a realistic figure. At the other end of the scale, using a pair of Jet V40 would be worth a saving of 1 minute.