The real deal with deep rims
02-27-14, 09:22 AM
#76
commu*ist spy
They had full disk FRONT wheels back in the 80's. I have some videos of the old Red Zinger races in Colorado and the riders used them. But it was okay; Colorado isn't known for high winds.
Spokes act like fan blades and create a lot of turbulence/resistance. The fewer and the shorter they are, the better. I believe that was the thinking that led to the 650c wheels on tri bikes.
The aerodynamic benefits of deep rims only comes into play at higher speeds and when the rim is at least 40 mm deep. At least, that was the consensus argument on the last 20 or so threads on this subject.
But you can get 40mm deep carbon rims that weigh the same or less than the old 26mm aluminum rims. Have your cake and eat it, too.
Then there's the whole problem of the unaerodynamic shape of the rider.
Spokes act like fan blades and create a lot of turbulence/resistance. The fewer and the shorter they are, the better. I believe that was the thinking that led to the 650c wheels on tri bikes.
The aerodynamic benefits of deep rims only comes into play at higher speeds and when the rim is at least 40 mm deep. At least, that was the consensus argument on the last 20 or so threads on this subject.
But you can get 40mm deep carbon rims that weigh the same or less than the old 26mm aluminum rims. Have your cake and eat it, too.
Then there's the whole problem of the unaerodynamic shape of the rider.
02-27-14, 09:26 AM
#77
Senior Member
I think part of it is a fad, dropping 1500 on a wheelset for a few seconds? Cmon. Maybe I'll come back and read this later when I have time as I actually have to work today and not sit in an office spending half my time of BF like some of you I know do. :-)
02-27-14, 09:27 AM
#78
Senior Member
It is a funny looking frame. In fact, they are known as 'funny bikes'. The front wheel is smaller than the rear and instead of having a straight sloping TT they put the bend in. They were popular for time trials but not legal anymore as UCI dictates that front and rear wheels must be the same size.
02-27-14, 09:27 AM
#79
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I don't know the reasoning behind it. But I saw a study many years ago showing the aerodynamic gains from increasing rim depth. Essentially the curve was steeper at shallower rims and flattened out as rims got deeper.
02-27-14, 09:28 AM
#80
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It is accurate either way you look at it. The slower rider finishes faster over a fixed distance with aero than without than similarly equipped faster riders. It is just a statement of mathematical fact as calculated. When you translate that to a percentage of improvement, then you have to take into account that the faster riders are finishing 10 minutes faster than the slower riders. The time gain from aero, taken over the overall finishing time still grants the faster riders a larger 'gain' from the aero equipment.
'The slower rider gains more time' is not inaccurate. It very accurately states one part of a story.
'The slower rider gains more time' is not inaccurate. It very accurately states one part of a story.
02-27-14, 10:05 AM
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Am I reading the wrong numbers? I get much smaller than that.
For what it's worth, I'll bet that it's just round-off error that there's any difference at all. I don't know what their calculations were but ... using the most simple form of the drag equation, the ratio of times for the same distance should be the same for both the slow and the fast rider. Given that both have the same drag coefficient and other things being equal (which they're not in the real world).
For what it's worth, I'll bet that it's just round-off error that there's any difference at all. I don't know what their calculations were but ... using the most simple form of the drag equation, the ratio of times for the same distance should be the same for both the slow and the fast rider. Given that both have the same drag coefficient and other things being equal (which they're not in the real world).
02-27-14, 11:44 AM
#83
commu*ist spy
this is why they have tunnel tests. It's pretty much impossible to accurately determine the coefficient of drag on a person and a bike.
but here's an empirical formula for drag. there's some derivation of that in my old textbook somewhere.
where cd is the drag coefficient. If you decrease cd by getting more aerodynamic wheels, that decreases Fd (force of drag) linearly.
the other notable observation is that as v (velocity) increases linearly, Fd would increase by a factor of ^2. this is why having carbon wheels has a bigger influence at a higher rate of speed.
not that this has any practical use, but just some perspective.
but here's an empirical formula for drag. there's some derivation of that in my old textbook somewhere.
where cd is the drag coefficient. If you decrease cd by getting more aerodynamic wheels, that decreases Fd (force of drag) linearly.
the other notable observation is that as v (velocity) increases linearly, Fd would increase by a factor of ^2. this is why having carbon wheels has a bigger influence at a higher rate of speed.
not that this has any practical use, but just some perspective.
02-27-14, 11:54 AM
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this is why they have tunnel tests. It's pretty much impossible to accurately determine the coefficient of drag on a person and a bike.
but here's an empirical formula for drag. there's some derivation of that in my old textbook somewhere.
where cd is the drag coefficient. If you decrease cd by getting more aerodynamic wheels, that decreases Fd (force of drag) linearly.
the other notable observation is that as v (velocity) increases linearly, Fd would increase by a factor of ^2. this is why having carbon wheels has a bigger influence at a higher rate of speed.
not that this has any practical use, but just some perspective.
but here's an empirical formula for drag. there's some derivation of that in my old textbook somewhere.
where cd is the drag coefficient. If you decrease cd by getting more aerodynamic wheels, that decreases Fd (force of drag) linearly.
the other notable observation is that as v (velocity) increases linearly, Fd would increase by a factor of ^2. this is why having carbon wheels has a bigger influence at a higher rate of speed.
not that this has any practical use, but just some perspective.
