Subject: Re: Help!what's the aerodynamic difference of the two wheels? Posted by: Russ on January 30, 2003 at 15:40:27: In Reply to: Help!what's the aerodynamic difference of the two wheels? posted by johnlock on January 24, 2003 at 01:07:32:

Message: >I just got confused about the aerodynamic differences >between solid wheels and spoke ones.Anybody help? The greater advantage between a solid disc wheel is NOT aerodynamics, but pure simple 9th grade physics. Disc wheels have a much higher moment of rotational inertia than a spoked wheel. The advantage comes more from that than from aerodynamics. If you want to compare aerodynamics, you need to compare a spoked wheel to something like a tri-spoke set up. The reason has to do with the amount of lift your wheel's spoke's turbulence creates in the form of drag. As someone said in an earlier reply, spoked wheels have a leading and trailing edge. That causes your wheels to either create drag or lift. Aerodynamic tri-spoked and 4 spoked rims have somewhat of a wing shape to them to cut thru the wind and also to create a form of lift to actually propel you forward as well as controlling the turbulence left in the wheel's wake. The buffeting from the turbulence not exiting the wheels wake smoothly slows you down dramatically. However there is something called the Venturi effect that more or less dictates that if air is accelerated off the surface of an object, it will actually push that object forward. That is the same principle of lift on an airplane's wing. The wing shape of aero wheels speeds the air up by cutting thru it first, then accelerating it over the rest of the wheel in a nice smooth pattern. This is also the same concept that uses the vents in a helmet to make it more aerodynamic, and why a golf ball with dimples flies further than a smooth one. Now after all that, what you are asking about is totally different. You are trying to compare a disc to a spoked wheel. It's not the same thing. A disc has higher rotational inertia being a solid object. When you try to accelerate a disc, it takes more energy to get it up to speed (high rotational inertia = poor acceleration) but this works both ways. It also takes more time for a spinning disc to decelerate due to it's inertia. This is Newton's 2nd Law of Physics that states that a body in motion tends to stay in motion and a body at rest tends to stay at rest until acted upon by an external force. So the disc is a faster wheel at speed, but not because of aerodynamics, but rather due to inertia. So in real world terms, what does this mean? Well, if you are going to use a disc, and it's windy out, they perform poorly. Not because of drag, but if the wind is crossing, it hits the disc on an angle and pushes it all over like a sail. This is also more proof that discs are not actually more aerodynamic as is commonly thought. But if you are in a controlled environment like an indoor velodrome, or on a calm day, they can't be beaten by even the most aero wheels in the world. If you have a flat triathlon type course on a clear day, discs rule. But why, if it's not aerodynamics? The answer is simple physics. A disc has a high moment of rotational inertia. It takes longer to get it up to top speed, but once you do, the harder acceleration characteristics start to work in your favor... now the wheel is less efficient at decelerating and so it tends to stay up at maximum speed. The disc is therefore easier to maintain top speed on. This is an advantage only on a ride that you don't have a lot of acceleration and deceleration taking place. In the middle of a pack of rode riders where you are constantly changing speeds, a disc is terrible, but out on the open rode without a lot of twists, turns, or hills, it's perfect. That is why if you watch the Tour d' France you would notice nobody climbs with discs. On mountain time trials, or time trials that have a lot of turns, spoked wheels are simply better. In this case you would see riders using wheels like a Zipp 404 instead of a disc. Not only are they lighter weight, but lower inertia too. So now after all that techno babble, I hope you have a greater understanding of how choosing the right equipment at the right time can make or break your races. Too many people are confused by aerodynamics vs. inertia of discs and spoked wheels though, and your question is a perfect example of it. Not to pick on you, but I used to think the same way at one point, and I am sure many others do too. I hope this helps to answer some questions you have about wheels, as this is a really confusing subject, but you did ask a great question. Russ