First time trial: Disc wheel or Powertap??

[flapsupcleanup]

I'm considering my first time trial and a buddy has a TT bike he will loan me for a while which has a disc rear wheel. I could throw on my regular rear wheel (American Classic 420) with a PT but will lose some aero benefit. I am a long time PT user and know what I can put out pretty well.

Which setup would you guys go with? I tend to lean towards using the PT wheel.

 

[daveryanwyoming]

...Which setup would you guys go with? I tend to lean towards using the PT wheel.

Tough call, the disk is definitely faster if the course is relatively flat. How much do you rely on the PT for pacing? If it's your first TT are you more interested in your best possible placing or do you want to learn about your pacing and power capabilities for the duration?

FWIW, I've had similar offers to use friend's disk wheels but prefer to run my PT wheels for the pacing and for the insight it gives me. It definitely taught me a lot about pacing, especially in the opening minutes where I was going much too hard. I do run a wheel cover on my rear wheel: http://www.wheelbuilder.com/store/product.php?productid=16265&cat=0&page=1

Whatever wheels you go with on race day I'd definitely do some training on the TT frame with the PT wheel to figure out if your position is costing you too much power and to dial in your pacing, especially at the start.

Good luck,
-Dave

 

[AshesGlory]

Dave,
I am in the same situation as the OP. Did my first ever TT last week and decided to go with my 404/PT rather than a borrowed disk.
I have a couple of questions.
The disk cover is a very interesting option. Does your PT transmit ok through the disk cover? Are you using a wireless PT?
On Pacing, is your goal to iso-power for the whole race or is the first minute or two at a higher than average power (given a flat course with insignificant wind).
Dave.

 

[Bikeridindude]

I'm considering my first time trial and a buddy has a TT bike he will loan me for a while which has a disc rear wheel. I could throw on my regular rear wheel (American Classic 420) with a PT but will lose some aero benefit. I am a long time PT user and know what I can put out pretty well.

Which setup would you guys go with? I tend to lean towards using the PT wheel.

How important is the race? Actually, that's probably not important anyway if it's your first one. I'd say go with the PT for sure. Man, it's nice to know what you really can do (especially in aero position) as opposed to what you think you're doing. Hopefully there'll be more time trials to where you can try the disc wheel, but if you can afford to "throw a couple away", the insight gained from the powermeter is priceless.

 

[daveryanwyoming]

Dave,
I am in the same situation as the OP. Did my first ever TT last week and decided to go with my 404/PT rather than a borrowed disk.
I have a couple of questions.
The disk cover is a very interesting option. Does your PT transmit ok through the disk cover? Are you using a wireless PT?
On Pacing, is your goal to iso-power for the whole race or is the first minute or two at a higher than average power (given a flat course with insignificant wind).
Dave.

The cover is no problem for the PT signal as it's basically a sheet of plastic and doesn't actually cover the hub or the existing PT cover.

The idea is to go isopower or very near it on a flat course and the biggest thing that will kill that strategy is starting too far above your sustainable power for the distance. You can't help but go above your sustainable power for the actual standing start, but you want to get it under control right away. A strategy to intentionally go harder in the first few minutes would be a mistake and cost you big time by the end.

Use some of your training time to do mock standing starts for ten to twenty minute TT simulation intervals. You don't need an actual holder, just start with one foot on the ground but start in a reasonable starting gear like you would in a race and figure out how to get up to speed and into your aero bars quickly without digging too deep a hole. Try different strategies with bigger and smaller start gears and more or less time out of the saddle before settling into your race position. Review the files and pay careful attention to the first minute of each effort, the peak power you hit and how long it takes you to settle into race pace.

That kind of careful power data review showed me I was starting a cog or two bigger than necessary and hammering way too many pedal strokes out of the saddle before settling into race position and pace. Invariably I'd be fifty or more watts over my race pace power for a couple of minutes at the start and then have real trouble settling into my best sustainable pace. Learning to start a bit easier, getting into the bars right away and spinning up a couple of extra gears before settling into my race gear resulted in higher average power and lower overall times.

Good luck,
-Dave

 

[AshesGlory]

Dave,
Any idea what is the difference between the wheel builder aero disk cover and the CH Aero offering?
Thanks, Dave

 

[daveryanwyoming]

Dave,
Any idea what is the difference between the wheel builder aero disk cover and the CH Aero offering?
Thanks, Dave

No idea, I've never worked with the CH Aero cover. But they're just some lightly formed plastic shells that cover the spokes. I've seen good looking home made versions so I expect most of the commercial offerings are decent and similar. It is nice to have the covers precut for your particular rims and PT hub, that saves some time and ensures a snug fit.

Zipp published a report a while back discussing wheel covers vs. full disks based on wind tunnel testing. IIRC, their major findings were that flat disk wheels were in general better or at least more consistently reliable than lenticular disks which includes wheel covers. It seems that lenticular disks can work really well or can generate excessive turbulence based on the specific chainstay design and clearance of the bike in question where flat disks weren't effected as much by bike design.

The other big finding and important one for wheel cover users is that wheel covers can actually be worse than uncovered spokes if the edges are left loose. IOW, wind can get up under loose cover edges turning the spoke fairing into an air scoop. The answer is to tape the edges of the cover to the rim and to the hub with electrical tape to make sure you have no exposed and loose edges.

Good luck,
-Dave

 

[rob of the og]

I'm considering my first time trial and a buddy has a TT bike he will loan me for a while which has a disc rear wheel. I could throw on my regular rear wheel (American Classic 420) with a PT but will lose some aero benefit. I am a long time PT user and know what I can put out pretty well.

Which setup would you guys go with? I tend to lean towards using the PT wheel.

Definitely the PT for a first timetrial. It's a difficult event to get right, and it'll take a few goes before you know how to pace yourself. The PT will at least give you a bit more of an idea about how much it's supposed to hurt (it's supposed to hurt a lot... but there are different degrees of hurting a lot!)

 

[rmur17]

No idea, I've never worked with the CH Aero cover. But they're just some lightly formed plastic shells that cover the spokes. I've seen good looking home made versions so I expect most of the commercial offerings are decent and similar. It is nice to have the covers precut for your particular rims and PT hub, that saves some time and ensures a snug fit.

Zipp published a report a while back discussing wheel covers vs. full disks based on wind tunnel testing. IIRC, their major findings were that flat disk wheels were in general better or at least more consistently reliable than lenticular disks which includes wheel covers. It seems that lenticular disks can work really well or can generate excessive turbulence based on the specific chainstay design and clearance of the bike in question where flat disks weren't effected as much by bike design.

The other big finding and important one for wheel cover users is that wheel covers can actually be worse than uncovered spokes if the edges are left loose. IOW, wind can get up under loose cover edges turning the spoke fairing into an air scoop. The answer is to tape the edges of the cover to the rim and to the hub with electrical tape to make sure you have no exposed and loose edges.

Good luck,
-Dave

Do you have a link Dave?

 

[daveryanwyoming]

Do you have a link Dave?

Not on this laptop, but I'll see if I find a link to the piece.

-Dave

 

[rmur17]

Not on this laptop, but I'll see if I find a link to the piece.

-Dave

okay thanks!

 

[frenchyge]

Any idea what is the difference between the wheel builder aero disk cover and the CH Aero offering?

Wheelbuilder covers can be precut to the wheel in question (incl PT hub opening), can be drilled/undrilled (for taping inside deep rims), come with plastic screws for attachment, and are slightly more expensive.

CH Aero covers are pretty standard for shape, drillings, etc.; come with aluminum screws which don't break easily, have a thin strip of foam weather stripping around the edge to seal off against the rim and prevent wind infiltration, and are a little cheaper. I had to expand the hub opening to fit my PT, and also trim around the edge to keep it from touching the brake pads.

This is from seeing *one* set of each between my CH Aero set and a Wheelbuilder set ordered by a teammate. YMMV.

 

[swampy1970]

Zipp published a report a while back discussing wheel covers vs. full disks based on wind tunnel testing. IIRC, their major findings were that flat disk wheels were in general better or at least more consistently reliable than lenticular disks which includes wheel covers. It seems that lenticular disks can work really well or can generate excessive turbulence based on the specific chainstay design and clearance of the bike in question where flat disks weren't effected as much by bike design.

-Dave

Just playing Devil Advocate here... but would you expect Zipp to publish a similar paper if they found that lenticular disks were better than their flat disks?

From a discussion that Peter Keen gave after the 1992 olympics, he said that Lotus found that, in their specific application - ie The Lotus Bike, that a flat disk was better but that was only because of the frame, specifically seat "tube" design. In other applications a slight lens shape was better. Of course with massive carbon seat/chain stays being more prevelant these days, this finding may/may not be as applicable.