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Provided both rims are double walled aluminum, what advantages/disadvantages of having deeper 30mm rim vs more narrow 18mm depth?

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EDIT: I am not Tour de France type of rider, not setting world record in speed, more of commuter, most of the time i am cruising in the city streets and sidewalks.

Used to think smaller depth rim has less weight and this could be advantage for cruising (not high speeds) compared to deeper heavier aluminum rim also its more flexible so it could be advantage as well for bumps etc?

P.S. I do have a 700c retro skinny frame bicycle.

LilBro
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Strength, and image of aerodynamics. A deeper rim is stiffer and can get away with less spokes or sloppier build than a low profile rim. Aerodynamic wheels have typically deep rims, and all deep rims get part of the glamour, even though bad design can make a deep rim just draggy as a low profile box section rim.

ojs
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  • This answer almost suggest that the aerodynamic difference isn't actually real. That is not true. Professional racers do know why they choose heavier rims even in hilly stages. They do also have lighter rims available, they choose before every stage. – Vladimir F Героям слава Apr 25 '21 at 07:59
  • @Vladimir F I didn’t arrive at that conclusion when I read it. It’s quite clear that a well-designed rim will have aero benefits, but a poorly-designed one will not, even though it may LOOK aero. – MaplePanda Apr 25 '21 at 16:28
  • @VladimirF MaplePanda already spelled it out, but to reiterate: The deep profile rims you see in racing are incredibly lightweight compared to everyday rims. They are also selected for low wind resistance, you aren't going to see a H+ or Velocity Deep V in a high level race. – ojs Apr 25 '21 at 18:14
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Mostly aerodynamics. Generally the deeper the rim, the less drag a wheel has. High-end aerodynamic wheels have carbon rims with 80mm or more rim depth. On the extreme end you have disc wheels. Of course a possible disadvantage is weight. Another possible disadvantage is susceptibility to side wind (not really a problem with 30mm depth, but certainly a thing with 80mm depth on the front wheel).

The reason why wheels with deep rims have less drag is mostly turbulence. When you look at a bike head-on you don’t even see the wheels, since the tires are wider than the rims. So it’s not “traditional” head on drag. The spokes create turbulence and the upper, outer parts of a wheel are actually traveling at twice the bike’s velocity against the air. Making this area as flat and smooth as possible (i.e. deep rims, few spokes) helps reduce turbulence. It also helps when the transition from tire to rim is smooth, which is one of the reasons why modern road bike rims have gotten much wider and now generally smoothly attach to a 25mm tire without the tire bulging outwards. The teardrop cross-section of the rim also helps to delay flow separation.

I think deeper rims can also be stiffer (radially), allowing for fewer spokes.

This website shows a very strong correlation between rim depth and reduction in drag. Older version of the diagram here: https://www.hambini.com/bicycle-wheel-power-data-50km-h/

I think the large differences among low depth wheels are mostly down to spoke count, spoke type and to a lower degree shape (and width) of the rim. Would be interesting to plot cost, weight, stiffness and strength until failure on the same diagram.

Michael
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  • By drag you mean because rim profile is flat it hits the air and acts as "sail" slowing me down?

    I am not a "Tour de France" type of rider not trying to set a world record in speed, simply a commuter who trying to decide on a rim, currently i have 30mm deep old rim old and have option to buy 18mm flat rim as "upgrade?".

    I thought lighter 18mm flat rim would actually give advantage for slow riding because its weights less.

     – LilBro Apr 25 '21 at 06:09
  • Could you elaborate on what means "power average" in this chart or "power absorbed" in website you linked? – LilBro Apr 25 '21 at 06:58
  • @LilBro to describe it as a sail would of course be an exaggeration. I think the main reason deep wheels are more aero is that there is less turbulent flow within the wheel. — Whether aerodynamics or weight is the more important concern depends not so much on how much of a racer you are, but what terrain you ride. In flat areas, extra weight actually eats up much less power than bad aero does, even at relatively slow speeds. — “Power average” just means how much power you need to put in to keep the bike at 50 km/h. They say “average” because the momentary power always has some jiggle in it. – leftaroundabout Apr 25 '21 at 07:38
  • @LilBro See https://bicycles.stackexchange.com/questions/32551/how-much-faster-will-i-be-if-i-lose-10kg-from-100kg-all-else-equal Even at commuting spees aeorodynamics is more important unless your comute is all heavy hill climbing. – Vladimir F Героям слава Apr 25 '21 at 07:57
  • Is the difference seriously that much? That’s like a eighty watt difference. – MaplePanda Apr 25 '21 at 16:30
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    The rim in question looks a log like Mavic Ksyrium right at the not-so-great corner of the plot. – ojs Apr 25 '21 at 18:16
  • I picked the 50km/h chart because the difference is more noticeable. But even the 30km/h chart shows a very clear advantage for aerodynamic wheels. And yes, a 30mm rim is right at the bad end of this chart, an 18mm rim would probably be significantly worse. Of course for a casual commute at 20 or 25km/h it might not be relevant. – Michael Apr 25 '21 at 18:40
  • Added a paragraph why deep rims produce less drag despite not changing the frontal area of a bike. – Michael Apr 25 '21 at 18:50
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    I do not buy that simple "turbulence" explanation. Some manufactures even try to roughen the surface to induce turbulence, saying it will act as a gold ball. It is dubious, but so is the simple explanation that turbulence is bad. Turbulence can actually keep the boundary layer attached and avoid separation. Most of the drags comes as the form drag of the tyre+rim body. Some aircraft airfoils use vortex generators to actually decrease drag. – Vladimir F Героям слава Apr 26 '21 at 08:16
  • @VladimirF: I knew the golf ball surface would come up. Yes, some wheels use small dimples in the rim to reduce drag. – Michael Apr 26 '21 at 08:18
  • See https://www.youtube.com/watch?v=H8TZwJGrvFY The manufacturers really do that. Even if it does not work in practice it is not true that turbulence is the reason for the drag. Laminar flow induces drag as well. – Vladimir F Героям слава Apr 26 '21 at 08:20
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As others have said, the advantage of a deep-section rim is aerodynamics, at a slight weight penalty. The weight difference here (even rotating weight) isn't much of an issue unless you're at the limits of human performance.

For everyday riding, I'd be more interested in whether either or both rim comes with reinforced eyelets (these are brass inserts, and can be spotted immediately), which help prevent the spokes from pulling through and can improve the rim's durability.

Adam Rice
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