Ask Josh Anything #007

We're back with a new set of questions for Josh, digging deep into the topics super-nerd cyclists care deeply about. We talk about the products that intrigued Josh most at Eurobike, find out what size (diameter and width) he'd make road wheels be if he could start from scratch, speculate on whether the very aero-looking Ceepo Shadow-R is actually aero, consider marginal gains in gravel races, ask whether it's better to wrap cables together or keep them separate, ponder the age-old question of tubeless vs latex tubes, and...ponder marginal gains opportunities in inline skating. (Hey, rollerbladers need marginal gains love, too.)

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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  • Pierre

    Hi Josh, Hi Fatty, Hi Hottie,

    Another awesome Q&A !

    Here are a couple of questions for Josh for Q&A episode 8 (that I’ll obviously listen as soon as it’s out there !) on the topic of aerodynamics and rolling resistance :

    1) Rule of 105% is now very well known thanks to you Josh and has really helped a lot of people understand modern wheels design. There are two things I’ve always wondered though :

    - Many rims including Firecrest won’t have its widest part at the brake track area. It’s pretty common to see “aero” rims that are 105% the width of the tire pretty far away from the tire with a brake track (not a correct name when talking about disc brake rim but I know you get the idea) width that actually narrower than the tire, giving the whole assembly kind of a “peanut shape”… so even though the 105% rule is somehow respected, shouldn’t it be 105% as close as possible to the tire ?

    - On the same topic, Zipp which is a company very aware of that 105% rule launched 858 NSW with a max width of 24.4mm for the 18 holes rim (front 858 NSW rim brakes) and 23.7mm for the 24 holes rim (rear rim brakes and front/rear disc brakes), that’s with a 17mm internal width. Even a 700×23 GP5000 with latex tube at 90psi is going to be wider than 23.2mm (24.4/1.05) on that wheel and I don’t see anybody running something narrower these days. Do they know something we don’t ? Does biomimicry rim design or does HexFin ABLC dimples change the rules and 105% is not relevant in that case ?

    2) All the articles talking about rolling resistance on the Silca Blog (my favorite being : part-4b-rolling-resistance-and-impedance) are awesome and very eye opening, I do happen to do some aero testing on my spare time and it’s ok to plug in a number for Crr and just don’t change it as long as you don’t test anything wheels related and you always test on the same road on a same day, then you can compare your results for different setups. Doing that : fixing Crr, fixing mechanical losses, etc. is not a problem at all for comparisons purpose. Now my question is : if I wanted to get a CdA number I can trust (not just for comparisons between different setups but really to keep that number) what would be a good testing method for measuring actual Crr on my testing road ?

    Thanks a lot and looking forward to listening to the next show and many more after that !


  • Andrew Burt

    It had been a few years since I read the Friction Facts report on pulley/jockey wheels. Would improvements by the drivetrain manufacturers (SRAM/Shimano/Compag) with newer series have narrowed the gap with the aftermarket offerings? Ie is R8000 better than the Ultegra version tested at the time?

  • Eric

    Arguing against some wheel size is just silly. Take a custom bike that looks “perfect” that rides “perfectly” for some person (whatever their arm/leg/torso length). Scale that person up or down and the optimal should be a bike that’s scaled the same in all dimensions, including hub width to maintain bracing angles.

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