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Spoke question for Engineers (and Woof).....(8 posts)

Spoke question for Engineers (and Woof).....Bruno S
Dec 30, 2002 11:43 PM
What is the speed of the air flowing around the spokes when riding at 20 mph ?

If I put my bike over a car and drive at 20 mph the speed of the air flowing around the spokes will be 20 mph. While riding, the spokes are not only moving forward at 20 mph but also rotating at XX mph. Does the air then flows around the spoke at 20 mph + XX mph?

If this is true, the spokes face much greater wind resistance than any other component with a similar frontal area and thus a low count, bladed spoke wheel would be the way to go.

BTW I'm just trying to justify my new Ksyriums in other way than looks :-)
Correct, sort of . . .micha
Dec 31, 2002 3:38 AM
At, say, 25 mph ground speed, the spoke extending straight up from the hub to the rim whips through the air at 25 mph plus rotating speed (works out to almost 50 mph). However, the spoke extending from the hub straight down to the rim is at a virtual standstill in terms of air speed. All other spokes travel at air at speeds between those two extremes.

In short, fewer spokes work. Get those Ksyriums :-)
re: Spoke question for Engineers (and Woof).....Woof the dog
Dec 31, 2002 12:34 PM
Ksyrium spokes are too fat and the rim is too shallow. I've read it somewhere that round spokes actually had lower air disturbance (i don't remember the name of that factor) than Ksyrium spokes. The real reason people who know like Ksyriums is their durability and stiffness. Good wheels for training thats for sure. Weight is not that great either. My aerohead/durace/revo wheels weigh less than Ksyriums. (1550 versus 1600 actual weight). I am assuming you have clinchers. Sorry to rain on your parade, but Ksyriums is hype just like Heliums were.

I guess that some spokes would be directly behind the rim and parallel to the ground (in horizontal position basically), so they will not be contributing to drag as much, right? I think they would also be moving your speed, not twice the speed like upper spokes directly between the fork blades or 0mph like those below. It seems that rim depth makes more difference than anything out there. Why? i donno, thats what they say :-)

Look, now i do sound like a know-it-all! hehe.
the pleasure of crappy componentsishmael
Dec 31, 2002 1:21 PM
Not that Kyseriums are crappy but hearing how they arent that light or aero means I'll be going faster on my next set of wheels
Woof, you got the right notion.micha
Dec 31, 2002 1:36 PM
The spokes are never moving twice the speed or standing still RELATIVE to the GROUND - you're absolutely right about that. They are cutting through THE AIR at different speeds, that's all.

Imagine yourself going 20 mph in a 20 mph tailwind - your airspeed would be 0 mph. Turn around and pound into a 20 mph headwind at 20 mph: your airspeed would be 40 mph.

There are some complications in the spoke speed - for example, the nipple end cuts through the air much faster than the hub end. Simple arithmetic - the nipple has to travel the rim circumference ( rim inside diameter x 3.14) in a certain amount of time, while the head of the spoke only has to travel the hub flange outside circumference in that same amount of time.

I do believe that fewer spokes make you faster. But they can also make you much,much slower - like when one of them breaks and throws your rim out of true to the point where you have to get off the bike and call for help. Been there.
what are you sayingishmael
Dec 31, 2002 3:39 PM
As I see it, once the spoke has reached 3 O'clock its not fighting the wind nearly as much anymore, and it IS practically standing still. Once it comes back around to 9 O'clock then it is whipped up and fighting against the wind. And I imagine the end of the spoke COULD be going twice the rider's speed. I'm tempted to say it depends on the speed the rider is going and the size of the wheel but then again maybe neither of these matter. I dont see why you bother explaining headwind vs tailwind airspeed.
Confused againKerry
Dec 31, 2002 5:26 PM
At the top of the wheel (12 o'clock), the spoke is traveling exactly twice the speed of the bike. At the bottom (6 o'clock) the spoke is dead stopped relative to the ground and has zero velocity through the wind. Max/min drag is close to those points but you have to take into account aerodynamic shadowing and interactions with the forks. Not a simple calculation! General rule is that reduced spoke count matters down to 24-28 spokes and doesn't matter nearly as much below that. This is based on wind tunnel testing.
re: Spoke question for Engineers (and Woof).....brider
Jan 2, 2003 7:56 AM
The top of the wheel (at 12 o'clock position) is moving relative to the ground at twice the ground speed of the rider (or the read out on your cyclo computer), and varies at a linear rate to zero at the tire contact patch. So the spokes at the top are not exactly moving at twice the rider speed, but it's pretty close (unless you've got some DEEP rims).

And aerodynamic drag generally goes as a square of the speed (at general bike speeds). So the spokes at the top are generating A LOT of drag. That is why you see all the airfoil section carbon wheels (like the HED3) with the airfoil section to take most advantage at the top of the rotation.

But also remember that this is considered "dirty" air (it has already passed the tire and rim), so it's not an easy calculation. Fewer spokes will cause less drag. Thinner spokes will cause less drag. As for the interaction of rim shape and the decrease in drag for particular spoke configurations -- I'll leave that to the real wind tunnel geeks.

But for me, I use 36 spokers for training, heavy gage, on old Mavic Open 4CDs, with 23mm tires. Then I put on the lighter stuff for racing, or use the Specialized Tri-Spokes for TTs.