Ask Josh Anything #008

We’re back with more proof that we’re willing to ask Josh practically anything bike-related…and that he’s willing to take practically any question seriously. In Ask Josh Anything #008, we talk about what gas you should fill your tires with, the challenge of planning for endurance ride air pressure loss, marginal gains and Kipchoge’s sub-2-hr marathon, which pedals are most aero and whether aero pedals matter (they do!), whether aero adjustments could make a difference in downhill MTB races (they could!) and —as always — much more.

Got a question you’d like to ask? Text or leave a voicemail at the Marginal Gains Hotline: +1-317-343-4506 or just leave a comment in this post!

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7 thoughts on “Ask Josh Anything #008

  1. “Back in the day” we used to use liquid latex to fill the gap between tubular tyres and the rim (which then set) is there anything in that? Did it cost more Crr than the aero gain? Also on the topic of aero tyres, there was a period with quite a few – zipp tangente, bontrager aerowing and R4 aero. Any thoughts on why they went away, will aero tyres come around again?

  2. This might be my new favourite podcast. Awesome!

    2 questions and a comment.

    Comment
    1) I have read in a few places that the high heat absorption of dark colours also means dark colours let go of heat faster and as a result, the wind convention effect at around 22kph or higher means dark colours are cooler than light colours at that speed (or faster).

    Questions
    1) Aero over weight.
    Using best bike split, this looks very clear. But when you add in the extra element of riding in a 50 rider size bunch on the flat…..are the aero gains of an more aero, but heavier wheelset or frame mitigated to the point where weight starts to matter more when you have a main climb in the race or course? Assuming you never take a turn on the front of the peloton. Does this explain why marquee climbers still stick with light bikes over aero bikes in the pro tour?

    2) If you have aero data on a wheel, tested in a tunnel, presumably on it’s own or as the front wheel, what is the general consensus of the type of aero performance reduction would you expect for the back wheel (behind a frame, churning legs and crank etc)? All companies would post aero data of a front wheel hitting the clean air, so you wouldn’t just double that for 2 wheels, or would you? Or does it vary so wildly that there’s not a typical range?

    Thanks for the shows! Love them.

  3. Another question….

    I see riders like Julian Alaphilippe and Jens Voigt ride with quite an obvious “fish tail” style with their front wheel. Do riders like this need to use wheels (and tyres) that are optimised for a higher average YAW than other riders? I could see them riding into a higher YAW than other riders on the same course, at the same time.

    Thanks for the great show!

  4. I have a follow up Keith Richards’ (not that one) question.
    Prior to Josh’s response that aero pedals and even shoe covers provide a gain, I had been of the opinion that the air was too turbulent with both the pedaling action and the air disturbance off the front wheel.
    Other variables I can think of is that when pedaling actions differ, it would effect the gains, for example some may point their toes towards the road when sprinting or riding out of the saddle. Is the aero gain manifest in making the pedal stroke slightly easier or is it the system as a whole in the Q factor?
    Loving the show.
    Marginally yours,
    Phil from Brisbane

  5. Thanks for answering my questions (and even my comment) in Ask Josh Anything #009.

    2 points.

    1. In regards to black letting go of heat more easily than white at higher speeds, Josh said:

    “Are we better to put it there and take it away easily, then to just never put it there at all.”

    The point is, at high enough speeds, black won’t “put it there” as much and will also let go of the heat from the rider better than white, thus making the rider cooler. Which, in hot climates, is ideal.
    It is an interesting discussion and it would be good to get to the bottom of this at some point….episode 068?

    2. My name is David, not Mark 😉

    1. The problem with the way Josh described it is he was only considering radiative heat transfer for both heating and cooling. This neglects conductive and convective heat transfer (as long as the helmet is moving). Assuming we’re only considering a steady state condition where the rider has been in motion for some time so that the helmet reaches a steady temperature means the rate of heat addition is equal to the rate of removal. On the heating side, the only mechanism to raise the helmet temp. above ambient is radiative;, however, on the cooling side all three mechanisms play a role with radiation being the least important and convection being the dominant mode. So it may be true that the black helmet radiates more heat away than the white one, but this doesn’t change the rate of cooling in any meaningful way since convective transport is the same for both.

      But the problem is not quite that simple. Considering our rider with a helmet heated by solar radiation moving at typical cycling speeds, there will be two boundary layers: 1) the momentum (velocity) boundary layer that we usually think of and 2) a thermal boundary layer where the temperature goes from the temp at the surface of the helmet, to that of the surrounding air. Depending on the exact conditions, the thermal boundary layer can lie either within the momentum boundary layer or outside of it. Also, depending on conditions, the two might be independent of each other; or for the more interesting case, they could interact. Without going into all the possible combinations, the one we have to worry about is the case where the thermal boundary layer alters the velocity boundary layer. Specifically, the case where the helmet is hot enough to generate a density change in the air so that hot, light air near the surface of the helmet induces a buoyancy force causing premature separation of the velocity boundary layer. We know that early separation causes an increase in drag, so for this case, the hotter helmet could be significantly slower. Of course, whether we see conditions where this could occur in cycling is the critical question. Without working out all the details, my instincts tell me it doesn’t. I think there is enough convective cooling that the temperature increase at the helmet is small enough that buoyancy effects don’t occur, but without looking at the actual numbers, I can’t be sure. Perhaps someone whose worked in this area more recently than I could chime in.

      (By the way, this triggered a flashback to my graduate transport class. This is very close to a question we got on our first semester final.)

      1. The last sentence of the first paragraph is not clear. When I wrote, “… since convective transport is the same for both.” I meant the heat transfer coefficient is the same regardless of color. Convective transport is proportional to the temperature difference between the surface and the free stream air multiplied by the heat transfer coefficient so since more heat is put into the black helmet, it will take a larger temperature difference to remove it, i.e., the surface of temperature must be hotter.

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