Carbon Rim Depth Observation and ?

[madsciencenow]

So I've been watching more CX this year than ever and one thing I've noticed is that CX rims (I'm speaking of the rims I see riders on the UCI circuit and other elite series riding but not suggesting they are actually CX specific) tend to be in the ~30-50 mm range (I'm estimating based on how they look to me on my TV). I'm curious if there is an advantage in having a deeper rim that would likely be heavier than a shallow CF rim that weighs less?

I understand the argument for an aero rim but this makes little sense to me in a CX race where you rarely reach speeds where you would benefit from the aero rim and the downside of the extra weight would surpass the aero benefit in multiples if you consider spinning up the weight of a deeper wheel the number of times a CX rider would need to do over the course of an ~1h race.

[DRietz]

The conventional wisdom for deeper rims in CX is that a deeper rim is more likely to slice through deep mud and less likely to get mud/grass stuck in the spokes in such sections.

Is that actually a data-driven benefit? I doubt it, but pros will do what pros will do.

It's easier to see a sponsor's logo on a deeper rim...

[homagesilkhope]

Yep, DRietz has it. That's been the unproven thinking for roughly 25 years now (well, the performance part anyway). Remember Richard Groenendaal riding FIR Antaras?

[mhoward]

I think some people feel that a deeper rim and shorter spokes can make the wheels stiffer/stronger. And the weight penalty is not as important as it is on the road. Also at MvP and WvA speeds the aero benefit might not be insignificant.

[rothwem]

Pros go faster than you would think too during a CX race, the sprint will be ~30mph or so which is enough that aero makes a difference.

[madsciencenow]

Thanks guys. the shedding mud thing makes sense.

I'm still not convinced that even for MvP or Wout the aeoro would matter. When you think about all the stopping and starting the weight element would seem to overwhelm any aero benefit you might get for a few seconds here or there.

[Mark McM]

Originally Posted by mhoward

I think some people feel that a deeper rim and shorter spokes can make the wheels stiffer/stronger. And the weight penalty is not as important as it is on the road. Also at MvP and WvA speeds the aero benefit might not be insignificant.

A deeper rim only makes the wheel stiffer to a certain point. If the rim is very deep, then the rim itself can bend laterally (kind of in a folding motion). As far as weight vs. aerodynamics: Most of a cyclocross race is at a relative low speed compared to road (less aero drag), and there are more accelerations (more frequent turns and sharper turns). Also consider that the cyclist may have to pick the bike up off the ground multiple times (such as dismounting for barriers or to run up stairs). So overall, weight may matter more than aero drag in a cyclocross race.

[ridethecliche]

Just because weight may matter more than aero on a cross course relative to a road race doesn't mean that it is more important on a cross course period. Like someone else said, these races can be wicked close and can make a pretty huge difference in a sprint. I'd be curious about the math to spin up vs hold speed given that the wheels they're using are likely pretty dang light to begin with even in aero form.

Additionally, if a rim is deeper than the mud you're riding in, you're not as likely to actually have to plow through it (like you would with a box section or very shallow V), so there's additional benefit rather than just mud shedding i.e. not having to shed mud in the same way to begin with.

[oldpotatoe]

Originally Posted by madsciencenow

So I've been watching more CX this year than ever and one thing I've noticed is that CX rims (I'm speaking of the rims I see riders on the UCI circuit and other elite series riding but not suggesting they are actually CX specific) tend to be in the ~30-50 mm range (I'm estimating based on how they look to me on my TV). I'm curious if there is an advantage in having a deeper rim that would likely be heavier than a shallow CF rim that weighs less?

I understand the argument for an aero rim but this makes little sense to me in a CX race where you rarely reach speeds where you would benefit from the aero rim and the downside of the extra weight would surpass the aero benefit in multiples if you consider spinning up the weight of a deeper wheel the number of times a CX rider would need to do over the course of an ~1h race.

The 'extra energy required' to spin up a 50mm rim vs a 30mm rim, carbon rims, is lost in the noise. About 50 grams per rim difference...less than 2 ounces...less than the weight of a PowerBar.

BUT...Is a 50mm carbon rim more reliable than a 30mm carbon rim on a really rough 'cross course? And is it easier to put big decals on these sponsored rims?

[fuji160s]

aren't most of the aero benefits lost with the knobby CX tires?

[BRad704]

Related to the idea of larger sponsor logos... deeper rims just look more modern and "cooler". And we as cyclists are nothing if not wrapped in vanity.