I did that last weekend after my front training wheel blew and it was fine. Not sure if it made any aero difference but I would suspect that front wheel is more important to have aero than the back wheel.
Reason why I’m asking if that I am considering doing and Ironman with my non-aero powertap wheel for pacing but could still use the aero front if there is any advantage there.
doing an ironman like that would be risky, they tend to have wind since they are mid day and often exposed.
put a cover on that PT wheel
edit: to clarify its fine aerodynamically, but the handling can be tricky.
one deep wheel is better than none. deep front/regular rear is better than deep rear/regular front from an aero perspective.
For your IM get a wheelbuilder disk cover. Run your powertap wheel as a disk.
The bike won’t handle as well with a strong crosswind.
Buy a disc cover for your powertap wheel and you’ll be all set.
I used that combo for Kona in '06, since I wanted my PT rear wheel, and the cover/disks are not allowed there.
Went with 404 up front (it’s what I had at the time), and the regular DT Swiss rim in back.
It was fine, but I was very, very lucky that it was a relatively tame wind day that year.
On a blustery day, the deep front/shallow rear combo could be a handful.
Assuming the IM you are doing isn’t Kona, there is no reason not to have a disk cover on your PT wheel. None.
Go order one from Wheelbuilder.com right now.
You’re welcome.
Just to add (again) to this idea/thread. Any advice on the best wheel TO COVER? Right now running on my 9-speed a Mavic Open Pro and on my 10-speed a Rolf Venctor Pro (covers on both). Is there good bang for the buck to be had in changing the hub? I’m thinking I’m giving an advantage away to those Zipp folks.
doesn’t really matter which one you cover. pick the lighter one for a few extra milliseconds.
Just to add (again) to this idea/thread. Any advice on the best wheel TO COVER? Right now running on my 9-speed a Mavic Open Pro and on my 10-speed a Rolf Venctor Pro (covers on both). Is there good bang for the buck to be had in changing the hub? I’m thinking I’m giving an advantage away to those Zipp folks.
the handling can be tricky.
I’m curious to hear why you and sxevgan mentioned handling. As far as I know, a bicycle steers about the rear tire contact patch. Aerodynamic side pressure on the rear wheel is generally balanced about the contact patch and consequently cannot exert a steering moment. So, help me understand the concern about handling. Are you thinking that flow interaction with the legs results in unbalanced side pressure?
Thanks,
Jim
I think Jackmott and Tom A do a good job explaining the scientific side.
I just know from experience. My 50mm wheel doesn’t bother me at all, but if I run my firecrest 808 front with PT rear, it feels squirly.
My wife did a local du earlier this year. She flatted her rear wheel and the sag vehicle gave her a spare wheel without a cover. Winds were around 30 mph sustained that day. She said up until she flatted the bike felt fine. When they took her disc away, she could hardly keep control.
a disc wheel in the rear tends to help as it moves the total side force more towards the center/rear of the bike rather than the front.
so rather than just the front of the bike pushing aournd and steering, the whole thing will just lean a bit.
the handling can be tricky.
I’m curious to hear why you and sxevgan mentioned handling. As far as I know, a bicycle steers about the rear tire contact patch. Aerodynamic side pressure on the rear wheel is generally balanced about the contact patch and consequently cannot exert a steering moment. So, help me understand the concern about handling. Are you thinking that flow interaction with the legs results in unbalanced side pressure?
Thanks,
Jim
I ran this exact set-up at Vineman last weekend. Mavic Cosmic Carbone SL (52mm) front with Mavic Pro in rear with Powertap SL+. I wanted the power data more than I wanted the aero benefit of my other Carbone SL wheel.
a disc wheel in the rear tends to help as it moves the total side force more towards the center/rear of the bike rather than the front.
so rather than just the front of the bike pushing aournd and steering, the whole thing will just lean a bit.
This is true if you are interested in the distribution of side force about the bike+rider center of mass or center of pressure from the side. But it doesn’t make sense to me with respect to the rear tire contact patch, about which the bike rotates/steers. From that perspective, the side force on the disc should cancel. In other words, half the side pressure would be in back of the contact patch and half in front and the net contribution to steering should cancel.
I’m sure you’ve thought this through, so what am I missing? Jack? Tom?
Thanks,
Jim
I don’t really understand what you are getting at.
the rear wheel is fixed in the frame, with most of your weight on it.
the wind wouldn’t be moving the rear contact patch anywhere whether or not the side force is equal on the front and rear half of the disc.
i suspect the “handling better with a disc” has to do with the physics of bike steering and how lean affects it.
I don’t really understand what you are getting at.
Okay, sorry. Bicycles steer about their rear contact patch. That is, the rest of the bike pivots about the rear tire’s contact patch. In the extreme case, imagine that you turn the front wheel at a 90 degree angle and the bike would rotate in a circle about the rear tires point of contact with the ground. Does that help clarify?
Jim
Just like the others, I say it’s a good option…and for a little cash, you can cover the rear wheel and have a great, great setup…with power. Do it.
yes, i understand that.
i think what you are getting at, is you are imagining the disc wheel helps handling by resisting the rotation about the contact patch?
I don’t think that is what is going on, I think it just makes the bikes leaning more predictable and easier to control.
I don’t really understand what you are getting at.
Okay, sorry. Bicycles steer about their rear contact patch. That is, the rest of the bike pivots about the rear tire’s contact patch. In the extreme case, imagine that you turn the front wheel at a 90 degree angle and the bike would rotate in a circle about the rear tires point of contact with the ground. Does that help clarify?
Jim
i think what you are getting at, is you are imagining the disc wheel helps handling by resisting the rotation about the contact patch?
No, I was suggesting that the disk must have approximately zero net torque and therefore no effect on steering because there is approximately equal side force in front and behind the contact patch.
I don’t think that is what is going on, I think it just makes the bikes leaning more predictable and easier to control.
Okay, got it now.
Thanks,
Jim
it has to do with shifting the center of pressure towards the rear, away from the more easily turned front wheel. reducing the pressure on the front is similar to what HED and Zipp(& Flo) have done with the new rim shapes to lessen the forces applied to the front wheel by transient(inconsistent) side winds. John Cobb had an article on the old Blackwell Research site that is now gone, but here’s another attempt to explain it:
I don’t really understand what you are getting at.
Okay, sorry. Bicycles steer about their rear contact patch. That is, the rest of the bike pivots about the rear tire’s contact patch. In the extreme case, imagine that you turn the front wheel at a 90 degree angle and the bike would rotate in a circle about the rear tires point of contact with the ground. Does that help clarify?
JimJim, this “pressure center” thing is a myth that has been circulated a long time and it doesn’t make sense. Bicycles don’t steer around the rear wheel any more than the front wheel. Look up “Ackerman” for a visual.
First, the bicycle is made up of TWO parts, each with a wheel on the ground and with the steering axis connecting them, forward of both wheels.
A side wind will push on both parts so that the steering axis is forced to the side, initiating a steering angle on BOTH wheels, ~20x greater in the front. This will initiate a lean/roll into the wind from the countersteering. Obviously the front wheel aero steering torque and trail are overriding everything else here.
Second, the side wind will push the bicycle away from the wind, initiating a lean/roll opposite to the steering induced lean/roll. This is where the rear disc helps. The side wind resultant force will be greater (but slightly lower?). The two phenomenas will balance out better from a handling perspective.