Aero Disc Cover Effectiveness?

vykkio

Does the aerodynamic benefit of the disc cover compensate for the extra 13.3 oz of additional weight? This is my first season racing with a power tap + aero disc cover and my bike splits have been the slowest ever. Ironman CDA 17 minutes slower bike split than my next slowest and half Ironman (hilly course) 6 minutes slower. My running has not improved with the slower bike splits, either....in fact in longer races it is slower than ever as well, so the slow bike splits aren't the result of better pacing. On the other hand, I've done a couple of sprints this year and won my age group both times with one of the fastest female bike splits of the day. One race was with my disc cover on a flattish course and the other was without my disc cover on a very hilly course. I weigh 123 lbs and my thought is that that the extra lb on the rear wheel may be more detrimental to me than it would on a heavier athlete. Am I better off without my disc cover except on flatter courses? Or would I be better off bagging the power tap in races and use a light weight aero wheel?

Keith Wick

A disk will almost always be faster than a lighter weight wheel. I don't think there is any IM in America where you would be faster with a lighter wheel vs. a disk. To put it in perspective, I'm sure that during a race you lose more than 13.3 oz due to sweat. An easy way to make up this weight difference is to carry less stuff on your bike and use whatever is available at the aid stations.

As far as being slower, my guess is that you are racing smarter. However, this could point to a few things if you've finished with faster bike splits & run splits without these metrics and would quantify the races as well executed----is your FTP correct? are you running to your vdot and is your vdot correct? If you could give us more info on these, we may be able to help you figure this part of the equation out (but it's not the extra weight of the disk cover).

HayesSanborn

There are a lot of well established myths in cycling that spilled over to the Tri World.  Some of the most ingrained myths is lighter is faster...in time trialing, weight on the flats means almost nothing. I'm not talking about adding 40lbs or anything, but <5lbs. </p>

As the road starts to tilt up, weight plays a larger factor. 13 oz is still nothing though. Look at this way, on a 125lb rider + 20lb bike setup that is adding .5% to your weight. You can model the time on various grades at http://www.analyticcycling.com/

All story short. Always run the disk, while you might lose a second or two on some hills, you will make up for it on the downhill and flats. Use as deep a wheel in the front as your comfortable with....

I'm 145lb and have yet find a course that I won't use a disk + 808....

 

 

 

Alan May

Just to voice an alternative point of view... I talked to some of the engineers at HED and they suggested that a disk is only an appropriate wheel choice if you can hold 26 mph.

I haven't spent any time in a wind tunnel and although I have taken my share of engineering classes, I wouldn't claim to understand the complexities (factors such as various yaw and apparent wind angles, impact of rotation weight vs. static weight, elevation changes, rider size and body type, riders position on the bike, distance of event, speed range, clincher/tubular, etc.) of low speed aerodynamics for cycling.

I don't expect it is a straightforward question to answer... I'd be curious to get Steve Hed, John Cobb, whoever the guru is at Zipp, and any other academics who spend a lot of time in a wind tunnel in a room and hear them discuss it.

I think the wheel cover + powertap on a training wheel is a clever cost effective way to race with power, but other setups may be faster in various conditions.

-Alan

Keith Wick

I don't have the evidence, but I find it hard to believe the 26 mph is accurate. This is only two examples, but the 2009 tour had a 52 km time trial. Fabian C. won it in a time of just over an hour, so about 32 mph. Cadel E. finished about 11 minutes off FC's time, which brings him down to 27.24 mph. If a disk is only good for those that can hold 26 mph, then we're basically saying that only pros or CAT 1's benefit from a disk, and we're talking only bike racers.



I'd also mention that TJ Tollakson did a 4:33:xx at LP, which means he averaged 24.6 mph on the bike. If the 26 mph metric is accurate, then not even the pro triathletes would be benefitting from a disk.

Michael Byerts

I hate arguments from authority but Jordan Rapp has a pretty good engineering background, and every motivation to find out the fastest set up. He has said, repeatedly, that a rear wheel disc is the fastest option, in almost every conceivable situation. Here's one example:
http://forum.slowtwitch.com/forum/Slowtwitch_Forums_C1/Triathlon_Forum_F1/Rappstar_-_you_said_that_a_disc_wheel...__P2794486
One concern I have with relying upon what wheel manufacturers say about this is the obvious incentive to find and highlight the data points that support their products. I don't believe they are lying, just that they are telling the best truth for their employer.

Alan May

Thanks Michael for the thread... definitely interesting reading. I think I need to purchase a cover and take some data for my fitness/positioning/bike, etc.

Anyone have access to any of these academic journals... I'd be curious to read these...

The understanding and development of cycling aerodynamics
RA Lukes, SB Chin… - Sports Engineering, 2005 - Springer
... where d is the distance from the bottom of the rim to the top of the wheel, were woven between
the spokes and attached to the rim of the wheel, resembling a modern deep section rim.
Zdravkovich also constructed a disc wheel made from a standard bike wheel with sheets ...
Cited by 16 - Related articles - All 3 versions

Translational and rotational aerodynamic drag of composite construction bicycle wheels
M Jermy, J Moore… - Proceedings of the …, 2008 - Prof Eng Publishing
... Zdravkovich tested wire-spoked wheels and bicycle frames with splitter plates inside the wheel
rim, similar to a deep ... and translational drag in wind tunnel tests on a Specia- lized 3-spoke aero
wheel, and also on flat-rim and deep-rim spoked wheels, convex disc, and flat ...
Cited by 3 - Related articles - All 4 versions


this abstract was interesting... (older study but rotational speed had only a weak impact on wheel properties... so maybe data from elite athletes is applicable to middle-of-the-packers)

Aerodynamics of yawed racing cycle wheels
G.S. Tew, A.T. Sayers


Department of Mechanical Engineering, University of Cape Town, Rondebosch, 7701, South Africa
Received 15 September 1997; Available online 17 August 1999.

Abstract
Recreational cycling and professional cycle tours have prompted cycle components and accessories to be designed with drag reduction as a first priority. This is especially so in the case of bicycle wheels. This paper describes wind tunnel tests carried out on a number of designs of racing bicycle wheels ranging from a standard 32 spoke wheel, through wheels with aerodynamic rims and spokes, to a solid disc wheel to determine the axial drag and side force coefficients on the wheels and to make comparisons. The tests were performed at relative wind velocities (30–60 km/h) and yaw angles (0–30°) corresponding to those encountered by the racing cyclist. Each wheel was found to exhibit its own particular characteristic curve which was only weakly influenced by the wheel rotational speed. The aerodynamically designed wheels showed reductions in axial drag of up to 50% when compared with the spoked wheel. The corresponding side force however increased. Cognizance should be taken of local atmospheric conditions when deciding on the design of wheel to use in a particular event.



In my comments above, I wasn't taking the HED engineer's comment as gospel and clearly wheel manufacturers have points of view, potential biases and possible conflicts of interest as disseminators of information. To their credit, HED has spent a fair amount of time at Texas A&M's wind tunnel. I was trying to suggest that we are dealing with a complex system and generalizing to broad conclusions can be illusive particularly given the challenge of creating repeatable conditions for testing and the number of potential variables that could impact the results. I'd personally like to get better informed on the peer reviewed science and marry that with other team member findings, vendor input, and my own data.

vykkio

Thanks for everyone’s input. It sounds like the aero disc cover is the way to go!

@Keith - My FTP and VDOT are correct. My FTP actually dropped by 10 watts between the OS and Ironman CDA. However, I also lost 18 pounds during that time so my watts/kg did not change dramatically. My VDOT increased from 36 to 38.

My sprint triathlon races this summer have been my best ever thanks to training with the EN plans. Yesterday I placed 1st in my AG with the 6th (out of 242) fastest female bike split of the day followed by my best 5k in 25 years! It makes me realize that my slow bike splits for IM and HIM this year are less likely the weight of the disc and more of an endurance issue. More long endurance rides??? Perhaps strength training would be of benefit at my age as well???

jburbatt

I think that strength training would likely be a big help just generally. You'll find a lot of ENers who add in strength training, especially as they get older

Keith Wick

I found that it actually took me a few years to increase my endurance related to my FTP. I used to die at 2 hr 80-85% rides, but this year I made a big leap in that area and could easily hold that level for 3 hrs after some FTP intervals to warm up. My overall FTP didn't change, but that 80-85% became my sweet spot, where even during the warm up when not paying attention I'd automatically default to that range.

Keep working the plans and if you extend anything for volume, make sure it's in the 80-85% stuff.

Chris G

Posted By Alan May on 14 Sep 2011 04:06 PM

Just to voice an alternative point of view... I talked to some of the engineers at HED and they suggested that a disk is only an appropriate wheel choice if you can hold 26 mph.



I haven't spent any time in a wind tunnel and although I have taken my share of engineering classes, I wouldn't claim to understand the complexities (factors such as various yaw and apparent wind angles, impact of rotation weight vs. static weight, elevation changes, rider size and body type, riders position on the bike, distance of event, speed range, clincher/tubular, etc.) of low speed aerodynamics for cycling.



I don't expect it is a straightforward question to answer... I'd be curious to get Steve Hed, John Cobb, whoever the guru is at Zipp, and any other academics who spend a lot of time in a wind tunnel in a room and hear them discuss it.



I think the wheel cover + powertap on a training wheel is a clever cost effective way to race with power, but other setups may be faster in various conditions.



-Alan



which engineers at HED have you discussed this with?  How many engineers actually work at HED anyhow?

 

david ware

Slightly different question... I am riding Hed 3 front and rear with a Quarq. Doing IMAZ in 2 months. For that course am I better off staying with what I have, renting a disc or using a wheel with a cover? Thoughts from anyone? Thanks.

kjanracing

I think the HED 3's look cool. Stay with those.