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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  • Kurt Roeder

    Hi, All,

    New listener and I binged on your Hour Record series. Great story.

    I’m surprised that sulfur hexafluoride (SF6) hasn’t come up in your discussions of tire inflation. I found an expired patent (US4513803A) that specifically mentions use of SF6 to prevent pressure loss in truck tires. It’s not poisonous and not flammable. It seems like it would be easy to get because it has other common industrial applications including use as the filler gas in double-pane windows.

    SF6 is a horrible greenhouse gas. Is this why teams avoid using it or talking about it? Still, it seems like it would be worth it for a race like Paris-Roubaix.



  • Christian


    Not sure if this is the right place to ask a question, but here goes! I come from a bike background and was specializing a bit in TTs for a few years and now have started doing some triathlons. During a TT I would use a good amount of pins to optimally place the race number so that it’s not flappy at all and out of the wind as much as possible, but during a triathlon people use the race number belts for the race number. They don’t need to be worn in the swim segment, they sometimes have to be worn in the bike segment (it depends on the race organizer) and always have to be worn in the run segment. The race number belt holds the race number via 2 strings that hang from the belt and slide through the top two corner holes on the race number. If they are worn on the bike then they would have to face backwards, and on the run they have to face forward (although most athletes move them over to one side of the front of the leg). From my TT experience it drives me bonkers to have the number hanging behind me flapping in the wind from the race belt, and even during the run it catches some wind (but at much lower speeds, of course). Is there anything to do to optimize the placement of the belt and race number?

    Thanks guys, I love the podcast!

  • David Ward

    This might be my new favourite podcast. Awesome!

    2 questions and a 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).


    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.

  • Andrew

    IMO there was a big marginal gain that should have been called out for the pursuit questions. The thing about track races are they are all measured on the black line while the rider will most likely ride a greater distance. Kim Geist’s coaching account had a great visual example of this on a 333 meter track ( Segueing back to the the hour record, I think Tony Martin talked about how much time he would have to spend on the boards to be comfortable riding the line he would need to ride while Wiggins had practically grown up on the velodrome and thus had a major advantage.

  • Jono Church

    I recently heard that apparently slick tyres still need to be oriented in the correct direction as the threads of the casing are woven in such a way that rolls best in one direction. Is there any truth to that? Are tyre casings made with that in mind or is it only tread pattern that has an effect?

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