Aero wheels question...

Discussion in 'Cycling Equipment' started by quenya, May 5, 2013.

Tags:
  1. quenya

    quenya New Member

    Joined:
    Jan 14, 2010
    Messages:
    411
    Likes Received:
    3
    So... I fancy myself a strong time trialist and am looking for advice on bang for the buck in aero wheels. I use a wheelbuilder aero jacket cover on my rear (powertap) wheel. I am about to get a 40 mm front wheel (set actually) to use for training and racing. In TTs I've used a 27mm 18 spoke Easton ea90 rim (mine), and borrowed Zipp 404, Zipp 1080, HED jet 90... My 2 best 10 mile TTs were on the Easton, my best 40 km on the HED. All at about 27-27.7 mph. My intuition tells me I will be faster with the 40mm front and quasi disc rear than the 27mm... But I wonder if anyone has any idea how many seconds could be gained over a 40mm rim to say a 60mm, a HED3, or HED 3D, or 85-90mm wheel?
     
    Tags:


  2. daveryanwyoming

    daveryanwyoming Well-Known Member

    Joined:
    Oct 3, 2006
    Messages:
    3,857
    Likes Received:
    97
    No doubt a 40mm front rim will help but realistically that is not a very deep aero section compared to your 27mm rim. There's various on line data on estimated time savings at fixed power or power savings at fixed time for different wheel depths but it's tough to evaluate as most of that is vendor data which should be taken with a grain of salt and it's hard to know how much of the data was taken with a full bike and rider vs standalone wind tunnel testing with the wheel alone.

    I'd guess something on the order of a ten to thirty second savings in a 40K TT at your speeds when going from the 27mm rim to a 40mm rim. That's based on some aero Chung testing I did a few years ago comparing a set of Easton EA70x shallow wheels to American Classic 420s (38mm). The speed increase was measurable for the same power but much less than when I swapped the front for a Zipp 808 or HED3. The 40mm rim should be much easier to manage in gusty crosswind conditions than a deeper front wheel but at anything other than near zero yaw conditions it won't be as fast.

    I'd race time trials with the rear wheel cover and the deepest front wheel you can afford unless the conditions are really ugly with strong gusty crosswinds and then stick with the rear wheel cover but perhaps use a shallower front wheel. The disk is always a good idea and can help stabilize you in cross winds as it's behind your center of rotation but a deep front can be a handful in some conditions.

    Personally my current TT setup is an older HED3 mounted with a narrow tire (Bonty Aerowing 19) up front and a Renn disc in the back mounted with a Continental Supersonic 20. It's the fastest TT wheelset I've owned and that includes a Zipp 808/900 tubular setup I ran a couple of seasons ago.

    You should google the various manufacturer's data, I know Flo has some new data up and the big guys like Zipp and HED always have data online that compares their various wheel depths. You shouldn't really try to compare across brands unless the wheels were tested in the same tunnel session but you can get an idea of how much less drag you might get and at what yaw angles as you go to deeper wheels. Most of the comparisons are made to a very basic box rim wheel so your current 27mm wheels are likely a bit deeper than the reference wheel already.

    Good luck,
    -Dave
     
  3. alienator

    alienator Well-Known Member

    Joined:
    Jun 10, 2004
    Messages:
    12,596
    Likes Received:
    160
    Looking at Zipp aero data, the difference in power consumed between a given rim and the next size larger rim is on the order of 0.3-0.5 watts at 30 km/h (18.6 mph) and 1.2-1.5ish watts at 40 km/h (24.9 mph). That data is from a Tour test. I think the test was done with the wheel by itself in a wind tunnel (I don't read German, so I can't say for sure. If someone does read German, I can post the pdf). I also think that number is an average over a range of wind directions. That would mean that the numbers would likely hold up okay for the front wheel but might have no relevance at all to rear wheel performance since that's heavily dependent on the rider, the rider's position, and the bike frame. I'd suggest using the calculator at Analytic Cycling's site to see what sort of differences in average speed you might expect.
     
  4. daveryanwyoming

    daveryanwyoming Well-Known Member

    Joined:
    Oct 3, 2006
    Messages:
    3,857
    Likes Received:
    97
    Yeah, it's likely not a lot of difference between any two adjacent wheelsets like a Zipp 303 at 46mm depth vs say something at 50mm or even 60mm depth.

    Here's one chart that compares some different wheels and different brands. I can't say how good the data is or how it was collected but I grabbed it from this article: http://cyclingtips.com.au/2012/08/where-is-the-value-in-a-carbon-wheelset/

    [​IMG]

    You can use that and the commonly used estimate that:

    50 grams of drag (measured at 30 mph) =~ 0.5 seconds per kilometer (at ~ 25 mph) =~ 5 watts (at ~ 25 mph)

    So if you put those together and take another swag that the difference between your current 27mm deep wheels and your new 40mm wheelset is similar to the difference between the Zipp 202 and the Zipp 303 the chart above would tell you they're basically the same at zero yaw so on completely calm mornings or direct headwind/tailwind riding. But get out to 10 degrees yaw and the differences are greater but not nearly as large as going to a much deeper rim.

    -Dave
     
  5. quenya

    quenya New Member

    Joined:
    Jan 14, 2010
    Messages:
    411
    Likes Received:
    3
    Thanks a lot for the replies. HED's website suggests that the H3 is faster in some situations, FLO says that their 60 front might be faster than their 90 (I think I'm fast enough to make use of the deeper wheel) and I'd read that Zipp dropped the 1080 from their line because the 808 firecrest
     
  6. quenya

    quenya New Member

    Joined:
    Jan 14, 2010
    Messages:
    411
    Likes Received:
    3
    ... is faster. D'oh. I hit reply by mistake. To be clear, the 40mm set on its way isn't meant to be a TT solution but general training and road racing, that I might use to TT from time to time. So, it looks like you both are suggesting that a jump up to say 60mm from 40 would not be an enormous improvement and I should be looking for an 808-jet 9 profile. I could use the deep wheel for good courses or, if I'm in terrible weather, go to the 40mm. Or, an H3 for any TT course. I understand the H3 is much faster with a 19mm tire than with a 23... Where would a 20mm veloflex record sit? Still aero or if its not 19mm it's not worth bothering with?
     
  7. swampy1970

    swampy1970 Well-Known Member

    Joined:
    Feb 3, 2008
    Messages:
    10,057
    Likes Received:
    185
    The faster you ride the more you should be looking at the low yaw angles. The H3 is one of the better wheels at low yaw. 19 to 20mm tires would be best but interestingly Wiggo uses 22mm tubulars on his. Odd. The 808 firecrest is a great rim but from some data that Zipp posted a while back it looked as though it wasn't as good at 0 to 5 degrees as its predecessor. The 404's are not a massive amount behind at low yaw angles and would double as good road race wheels too. The Hed Jet 6 and 9 look a good buy for the money. Tony Martin has been killing it on those the past year or so, then again, he could probably kill it on a set of 36 hole Mavic MA2's. Personally, if these were TT only wheels, for flat/rolling courses, I'd go with either the H3 clincher or Jet 9. Of the wheels with carbon braking surfaces that I've ridden, none have given me the warm and fuzzy when yanking on the anchors hard.
     
  8. quenya

    quenya New Member

    Joined:
    Jan 14, 2010
    Messages:
    411
    Likes Received:
    3
    I am partial to aluminum braking surfaces. I guess I'll keep an eye on e-bay looking for an H3 or something along the lines of the Jet 9 front...
     
Loading...
Loading...