Wanted to move this into its own thread, it is pretty interesting and this may go on a while.
Anyway the basic question is, can an out and back, or loop TT be faster with wind in some cases than with no wind at all. As we know from various wind tunnel results, drag can be a good deal less at medium and high yaw than at zero yaw.
But any wind will also mean more apparent wind speed when riding, at least some of the time.
So can you be net faster? I mentioned that we seemed to notice faster times at one of our local TTs that used to occur monthly in Austin when there was moderate wind at certain directions. So here is are the results from best bike split I ran on that course:
Out and Back
8.09 miles
250 watts normalized power limiter
Of course bestbikesplit’s physics could be wrong, the CdA vs yaw numbers could be unrealistic. But I believe analytic cycling’s dynamic wind page will get you similar results if you set it up using their wheel drag vs yaw inputs. I haven’t done it in a while because it is cumbersome to set it up.
As we know from various wind tunnel results, drag can be a good deal less at medium and high yaw than at zero yaw.
Define how those CdA vs yaw numbers were derived… and also define how they are being used to determine drag force in modeling programs.
For instance, when they yaw the table 10deg they don’t increase the tunnel speed to simulate a crosswind… they leave that the same. The apparent rider speed actually drops in that case. When you are riding on the road and you have a crosswind, the wind hitting your body is greater than when there is no wind.
So is the CdA value based on the tunnel speed and measured in-line drag force, or do they calculate the apparent rider speed and base it on that? I’m guessing they don’t do any calculation, which is why the dropoff in CdA with yaw is so large.
I do believe that many people can experience a slight benefit from a pure crosswind, but it’s a lot less than those tunnel numbers would indicate.
Good question, I was just using the default CdA vs yaw values that BBS picks for you once you enter a 0 degree CdA value.
So just now I referred to the thread where Damon Rinard indicated how to correct for drag force at yaw as measured in the tunnel, and used data from the Zabriskie-dummy on a P4
If I am doing this right (damon can maybe check me?) then the cda reduction at yaw in my test cases was less pronounced than zabriskie on a p4. It seems like the adjustment is so small you can mostly ignore it, even.
edit: (the cda conversion there is just approximated using the 50g drag = .005 CdA rule of thumb)
As we know from various wind tunnel results, drag can be a good deal less at medium and high yaw than at zero yaw.
Define how those CdA vs yaw numbers were derived… and also define how they are being used to determine drag force in modeling programs.
For instance, when they yaw the table 10deg they don’t increase the tunnel speed to simulate a crosswind… they leave that the same. The apparent rider speed actually drops in that case. When you are riding on the road and you have a crosswind, the wind hitting your body is greater than when there is no wind.
So is the CdA value based on the tunnel speed and measured in-line drag force, or do they calculate the apparent rider speed and base it on that? I’m guessing they don’t do any calculation, which is why the dropoff in CdA with yaw is so large.
I do believe that many people can experience a slight benefit from a pure crosswind, but it’s a lot less than those tunnel numbers would indicate.
On an out and back, a 90 degree cross-wind is clearly faster than no wind, at least theoretically. In my experience it’s pretty hard to replicate this on an outdoor course, though, because outdoor courses tend to have turns and also it’s rare that the wind is aligned correctly, so you usually end up with a headwind/tailwind situation which counteracts the effect. I did experience it once for sure, though. My CdA measured out at around 0.015 less than usual on a (fairly) straight out and back with a strong crosswind.
So sounds like the yaw/cda inputs for BBS need to be corrected values, if using wind tunnel data as an input, or you will get optimistic results. Since if it were doing the correction for us, my simulations wouldn’t have been faster with wind.
Ah this makes sense. We used wind tunnel data recently with overly optimistic results (though only about a 20 sec difference over a 70.3), but that was primarily at low yaw angles.
Let’s say you did a straight North-South TT. And the wind was directly from the East or West. So you saw the exact same apparent wind in both directions, meaning, assuming a roughly flat course - so that your speed is the same - then the yaw angles should be the same.
In that case, you’d obviously be faster with wind than without.
How much that changes as it becomes less of straightforward case of course, “depends.”
