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    Wheel Selection – Wheight vs. aerodynamics

From:
KarstenKB Dec 13, 2005, 1:08 PM

Question:
Okay, physics junkies, wheelset info

I'd like to get some feedback on the effects of weight vs. aero for different wheelsets. I think it's pretty well accepted that when given a choice between aero or weight, opt for aero unless severe climbing. But what about various trade-offs for wheelsets? Let's take two wheelsets: Zipp 404s and a Renn/HED3 combo. Three things to consider: price, aero, weight. Price goes to disc/HED3 combo. Probably the more aero combo would be Renn/HED3, but the 404s are still pretty darn aero. Weight goes to the 404s (1685g in clinchers) while the Renn/HED3 in clincher would be 1980g. I know, tubies would be lighter, but let's say clicher. If you can only afford one wheelset, the disc/trispoke combo would be best for flat to small rollers. But at what point (type of course) would the 404 be better? What about IMWI, IMLP, Silverman, Great Floridian, etc. Does the 300 extra grams really make that big of a difference for these courses? Silverman, probably. But I don't know much about WI or LP. I would guess disc/trispoke would be fine with the small rollers of Great Floridian (although I've never ridden the course). I know that it's 300 grams where you feel it the most, on spinning wheels, but on shorter (HIM, OLY, and Sprint) would you even notice the 300 gram difference? Just curious to see what those of you who are really into this think.

Opinions are welcomed, but they are also a dime a dozen. If anyone can show me the calculations, I'd be impressed.


Josh:
This is a difficult argument that has many facets and honestly none of us fully understand all of the interactions well enought to model them 100%, and probably never will. But here is what we know. Clearly nobody wants to trust wind tunnel data from a manufacturer, but the stuff we have on the web was completely taken and graphed by the wonderful folks at Texas A&M with us only there watching it. There is no possible manipulation here, and to improve the results this was the first known bicycle wheel test where every wheel was run a minimum of 3 times and averaged to take out any anomolous data (and it does happen). So you have my word that it is not bogus marketing material, and if you want you can always contact the guys and gals at A&M and they will verify.

Having said that, there are changes in tunnel protocol that have changed the apparent aero properties of some wheels over the years. The whole lenticular vs. flat disc thing got started when the lens disc guys (and a prominent university) started testing wheels at various angles by starting at 0 and slowly yawing out to 30 taking measurements along the way. This is how aircraft wings are tested and makes a lot of sense. However, with aircraft wings they then start at some higher angle and come to zero to check hysteresis. Doing this in the late 90's with wheels (it adds time and cost to the testing) we noticed that the lens discs would not track their initial data as well on the return. What was happening was that the discs from 0-30 was maintaining relatively attached flow, thus generating lift on the leeward side which appears in the data occasionally as negative drag. This would not happen every time, but many times. However, if you start the wheel at 30 and yaw to zero, the flow is detached and does not reattach to nearly 0 degrees, so the data looks much more like that of a flat disc. Once there is a bike and rider in the way, there is enough airflow disturbance that the flow cannot ever reattach so the 30-0 trip much better mimics the real world performance of the disc.

Of course, when all of this was being learned we really started to see bigger differences between discs and frames. Good example, the Trek TT frame was designed when the team were using lens discs and hence has pretty wide seatstays to keep the disc and seatstay boundary layers from coliding. A cervelo P3 on the other hand has very narrow seatstays that work ideally with a flat disc and can cause air damming when used with some lens discs. This effect is now seen to be greater than the lens vs. flat shape debate. The dimples simply help the air remain attached longer to our disc, thus somewhat mimicing the 0-30 attached flow scenario of the lens disc, and also the dimples lower the wattage to spin. The effect is small, but on the order of a few watts, which for some folks is not small at all.

As for deep vs 3, 4, 5 spoke wheels the question is really where do you want to be aero. We have now developed 4 shape revisions of the 404 in 5 years to best the Dupont/Hed3 wheel, and with the last two generations of 404 are faster than them at most real world wind angles. The 404 will be faster from 0-22 degrees, while the 3 spoke will becomes faster at 22+ degrees. The 808 is an entirely other beast, and when we first published our aero data showing the wheel to mimic a disc to 12.5 degrees we were really criticized for making so bold a claim.

Fortunately Tour magazine in Germany did their own wind tunnel test and found essentially the exact thing (in their test the 808 was actually slightly FASTER than the disc between 0 and 12 degrees!). This is not a factor of depth so much as shape, which really comes from much of what we learned doing the 404. Both of these wheels, however, are less aero than the H3 at high wind angles, but it is our feeling that it is these high wind angles (read: strong cross-winds) where people are least likely to actually ride the products so we have thus far not developed a wheel using these shapes with any more surface area as it is of no value at the lower wind angles and only adds speed at wind angles where very few people seem to be likely to actually use the wheel.

OK, having said all that. Weight is important, but is generally always trumped by aero. The analytic cycling calculator helps out with this, although we are now understanding better from our work with CSC and Phonak, that on steeper grades weight is more important than these models show. The theory is that since nobody really makes constant power, the dead spot in the pedal stroke allows for a slight deceleration that then requires a re-acceleration during the power stroke. These fractional acclerations really can sap power, and weight definitely affects this quite considerably, although aero has a small affect as well.

The smoother you pedal the more accurately the analytic cycling model becomes, but anybody using and SRM or a stroke scan on a computrainer will know that most of us are anything but round. This is partially why Lance's high RPM stroke kills Ulrichs low RPM power stroke on steep climbs the faster stroke is smoother and has fewer losses due to this fractional accleration issue. This is the stuff we are really trying to understand now so that we can have better answers in the future, but the advice we give people right now is that for everything short of hillclimb TT'ing, get the most aero stuff you can ride, and if you have the money buy the lightest version of what you want. You really can't have it all in every situation (which is why we now have 7 wheelset options), but you can have most everything 90% of the time. :-)

For more information on rim shape and aerodynamics check out our engineering white paper on rim shape:
Rim Shape:
http://www.zipp.com/RimShape/tabid/103/Default.aspx