Ask Josh Anything #007

I was re-reading the article on tire pressure and Roubaix and the optimization between pavement and pave. As noted a 5 psi drop of pressure is typically not a deal breaker, but in this optimization I assume the regular loss of pressure over the course of the race is taken into consideration as well? Since in this type of race each sector is important and different that may be a factor but what about the end of the race when you might really need those couple of watts in the velodrome? Sounds like a pretty complex problem but how is pressure loss during the race factored into the optimal tire pressure at the start of the race? How much is lost in a typical race? and is Roubaix a worst case scenario for the pressure loss over the day?

Awesome episode as always !

Couple of questions regarding the 105% rule and CRR.

1) 105% rule :

- does it matter where the widest section of the rim is relative to the tire widest section ? Firecrest for example has its widest section close to spoke bed but pretty far from tire and is actually narrower than most tire at brake track… creating kind of peanut shape more than an airfoil shape… so should the 105% rule be at brake track or external part of the rim in case of a disc brake rim since brake track does not apply (even if it means a toroidal shape that is actually 120% widh of tire width somewhere in the middle and 105% at brake track) or should it be 105% somewhere on the rim but it does not matter where?

- Zipp 858 NSW is a different beast with the 18h rim (front rim brakes) being 24.4mm max width and the 24h rim (rear rim brakes & front and rear disc brakes) being 23.7mm max width… meaning all tires even 700×23 are going to be wider than the rim with the 17mm internal width. So what do the Zipp engineers know which allow them something that does not respect the 105% rule at all ?

2) CRR

- when doing aero testing I’m trying to be as precise as possible regarding all parameters but it’s almost impossible to know what’s the correct CRR is on the road I use for testing. Do you have any advice to calculate CRR properly ?

Thanks again, looking forward to the next episode(s).

It’s possible to determine Crr (and CdA) for a bike without using a power meter, so it would be possible to do so for inline skate wheels without a power meter. It’s a hassle to do but if you’re desperate enough …

Hi all, a question for Josh, prompted by the final hour record podcast and the comments about Boardman’s frame I his Merckx record.

I love my steel bike. Old fashioned round tubes. Simple diamond shape. Caliper brakes. Is it worth my while having my old CX-ray Mavic Open Pro wheels rebuilt with something cheap and ‘aeroish’ like alu Kinlin 31. Will I feel more benefit from the aero gains or the rolling resistance improvements or perhaps not that much at all?

Thanks for keeping my brain ticking in my dotage.

“Oh no! Not again”

I would be interested in an update on pulley wheels since the Friction Facts data I have is getting old. Have the newer drivetrain offerings (eg. r8000) improved in this area (over the models tested in 2013) or can similar gains still be found by moving to ceramic bearings and/or OSPW? In summary,have these gains become more marginal? Thanks.

I have enjoyed every episode, so I hope you all continue to find this platform rewarding enough to continue for years to come.