Wheel aerodynamics points of flow separation

Triathlon Forum
1

I’m currently revising for an aerodynamics exam and can’t help seeing cross sections of bike frames where I should be seeing pics of wing cross sections! Basically, I think I’m understanding the drag on wheels, but can’t say for sure.

  1. Presumably the drag on wheels starts to increase at a certain wind angle because the flow ceases to attach around the rear side of the wheel and so you’ve got the equivalent of stall?

  2. 0° yaw drag isn’t lowest for most wheels because the flow separates before the trailing edge of the wheel?

Are either of these assumptions close to the truth?

Presumably point 2 is why zipp use dimples; turn the flow turbulent to delay the point of separation. If this is the case, why do they put stickers on that cover the dimples?

And on a similar topic, why is there no aftermarket stick on thing (like a sticker!) with lots of holes to convert a standard deep section into a dimpled rim?

2

Hello Zebragonzo and All,

I like your sticker idea. I pulled off my Zipp stickers (16 grams worth) but a stick on dimples could be fun - in colors.

Would that run afoul of USAT rules?
“(e) There must be no protective shield, fairing, or other device on any part of the bicycle (including frame, wheels, handlebars, chain wheel, and accessories) which has the effect of reducing resistance. Aerodynamic carriers for food, water, and or cycling provisions may be attached to or be an integral part of the aerohandlebars
if they meet the following guidelines:”

Cheers,

Neal

3

Well /obviously/ this’d be for frame protection purposes!

4

Have you heard the Zipp podcasts? They are available on their website and go into detail about their design including why they use dimples.

5

I’ve heard some of them, but essentially the idea is to make the flow turbulent as then it sticks to the surface better. More skin friction, less drag friction (which is worse).

Keep the air moving in the layer next to the skin and we’re happy. Keep the air moving in a laminar fashion in the layer next to the skin and we’re really happy. If the air stagnates then things start to go wrong (skin friction slows the air down). Ideally the rim would be blowing air out of the rims towards the back of the bike as this would ensure that air sticks to the rim and keeps moving!

I’m meant to have spent the day revising for an aerodynamics exam…Well at least having read 50% of Joshatzipp’s posts I’ve gained lots of aero type knowledge!