zipp claims at its website that the 808 is as fast as a disc up to about 12 degrees of yaw angle.
My question is if the 808 mounted on a cervelo P2C/P3C, with the wheel close enough to the cut out in the frame acts as a disc in higher yaw angles, so that maybe, in the right frame, an 808 is as fast as a disc in almost any real life conditions, I mean 40-45kph and 10-20kph crosswinds.
alvaro
up
.
Jens of ST and I had a conversation about this exact scenario when he was in the store a couple months ago. Hopefully he can chime in too since he had some more info/sources than I do.
I believe that you don’t get as much benefit when you add a disc wheel to a P3C or P2C as you would if you added it to a bike that was less aerodynamic. The difference between an 808 and disc would be pretty small, less than when you compare them to each other by themselves in a wind tunnel.
I have no data to support my thoughts though.
There is data to support the inverse claim, though. I.e. the Softride shows maximum benefit from a disc vs. a spoked wheel. So I don’t think it’s unreasonable to think that a P3 will show less.
A frame with a well designed cuttout definitely allows the rear 808 to perform at its best, but from all the testing we have done and seen at high yaw angles the wheel/frame relationship is really only affecting the airflow over the front half or so of the wheel for the 808, but not changing much on the rear half of the wheel compared to a frame of similar design with no cuttout, whereas the frame with the cuttout can affect the airflow over the entire surface of a disc. Overall we have seen that the drag differential between the 808 and disc when tested alone matches quite closely to the relative differences when tested in a bike without a cuttout, and that the disc gains a slight edge in the bike with the cuttout by a few watts over what was predicted by the drag differential of the wheels on their own (and this advantage seems to be limited to the higher yaw angles), and of course different frames with different cuttouts all show different levels of improvement. I don’t have any of our data in front of me at the moment for complete bike testing, but from all my experience I would say that the cuttout generally favors the disc at higher yaw angles
josh
My data on this was all from field testing (i.e. 0 yaw). I’ve found repeatedly that on a P3, the rear wheel choice has little effect on drag. Even my Bontrager powertap wheel isn’t that much slower than a disk.
But then, in standalone wind tunnel tests, the differences between deep wheels and disks at 0 yaw is pretty small too. (see http://www.zipp.com/RimShape/tabid/103/Default.aspx for example). So this isn’t a huge surprise.
If you draw a diagram, it seems like the cutout isn’t going to help so much when the wind is hitting at 10 or 15 deg. But, I can only speculate, since I can’t test that objectively.
Jens,
Is all of your field testing done in windless conditions? I do this (I figured its more easily reproduced as no wind seems = no wind and once there’s wind - well thats a whole bag of worms…). It took me a while to figure out that I wasn’t seeing much of a difference in wheels since wheels aren’t all that different in windless conditions…lead me to think that race wheels were not worth that much (and I guess they aren’t if there isn’t any wind). P3 also. I guess you can still test positions, helmets…
Dave
if have tested a cervelo p3c with out a rider
and also with zipp disc(also others ) and 808
i found in all angles that disc where the best (this only on aerodynamic terms)
.
Josh,
okay here’s a followup question. If one was using an 808 with PT hub for the power side of things … and a P2C/P3C aero bike, does it make sense to apply a wheel cover to that 808 to make it as close as possible to a ‘disk with PT built in’. We’re talking real world wind conditions here.
I know there is some weight cost but on aero terms only?
tia
Jens,
Is all of your field testing done in windless conditions? I do this (I figured its more easily reproduced as no wind seems = no wind and once there’s wind - well thats a whole bag of worms…). It took me a while to figure out that I wasn’t seeing much of a difference in wheels since wheels aren’t all that different in windless conditions…lead me to think that race wheels were not worth that much (and I guess they aren’t if there isn’t any wind). P3 also. I guess you can still test positions, helmets…
Dave
Right, in my book there’s no such thing as a “steady wind.” If there’s any wind at all, it’s unsteady. Even a difference of 0.5mph in wind speed will completely screw up your results. I saw that in analyticcyling.com before, but didn’t fully realize it until I did a few tests in wind and got garbage data.
And yup, that’s the downside, a lot of the aero stuff doesn’t really start to show until you get some yaw. The effect of the following are fairly apparent at 0 yaw though:
– jens
Jens,
Do you think there may such a think as a ‘steady wind’ testing on a track?? Coggan said something to this effect–and it would make sense to me, since my Cda on the track is consistently higher than on the road. His comment about watching a streamer on the top tube while in the banking (or something to that effect) got me thinking that there’s more than just the g-force effect going on.
FWIW, I’ve tested a 404 with and without wheelcover on the track, and the cover was repeatably and measurably faster.
My drag was higher on the track too – same for Jack Watts, who posts on BTR, I think. Some of that was probably due to the cracks and the up and down as you come off the banks.
But also the wind doesn’t hit you straight on because you’re turning all the time. I’m too lazy to do the vector math, but I’m guessing you get a fairly small yaw in the turns – maybe a few degrees – I’m really not sure.
Just to confuse things further, if you’re on the banks, your wheels are moving faster through the air than your head too.
– jens
You have to take into account though that even on the track, yaw angles of up to 10 degrees can be found. So zero yaw on the road is not a very common scenario, even when it appears not to be very windy.
Just to confuse things further, if you’re on the banks, your wheels are moving faster through the air than your head too.
– jens
You also have a higher effective g constant, which drives up rolling resistance. To go from north to south (making a U turn) requires some application of force(over time) somewhere, and it has to come out of the bike’s momentum, and be replaced by your legs. Rolling resistance is where you pay for it. Lap after lap.
Hello All,
Perhaps you need to do the runs in each direction and calculate the coriolis effect even though is would be several magnitudes below the other forces involved on the track. However it is large enough to causes errors (If not caluclated) in long range artillery shell impact point. 
Cheers,
Neal