# 问乔什任何东西＃008

### 22 评论

• Ryan Willcocks

Happy New year to you Josh and team,

A couple of quick questions on Thru-axles and Quick release skewers.

1. You mentioned in a number of Podcasts and interviews the marginals gains and loses in regards Quick releases levers and their edges, do you have list of quick releases you would recommend or just to use the lightest Theft proof ones? ie (PZ Racing CR5.3Q skewers), (Styx Aero skewers) or (ControlTech Race Skewers)

2. Another probably heated question in regards skewers, do you recommend Ti skewers for their lightness or steel for their strength?

3. The last question is in regards to Thru-axle recommendations, do the same rules apply for thru-axles edges on road disc bikes wheels or does the stiffness of the hollow axle outweigh the aero losses of the skewer lever?

Thanks

• Tony Geller

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.

• Tony Geller

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.)

• David Ward

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 ;-)

• JÃ¸rn Thyssen

Two questions:

Question 1:

In general, most people recommend lowering the tire pressure on wet roads compared to dry roads for increased grip. What does science say?

Question 2;

Lowering tire pressure will obviously increase risk of pinch flats, but what impact does the tire pressure have on non-pinch flats? Anecdotally it seems my club mates who ride with (much) higher tire pressure than me flats more often. FWIW, the typical cause of flats in Denmark is very small sharp flintstones.