Roadbook — sheet 03
What rim depth do you actually need?
The short version: if you are not sure, buy something between 40 and 50mm. Almost everything else on this page is about the cases where that is the wrong answer.
30–40 mm
Shallow
The high mountains — summit finishes, hairpins, thin air
Climbing, crosswinds, all-weather riding
The lightest option and by far the most forgiving in a gusting sidewind. On a sustained climb this is where the weight saving is genuinely felt. The trade is real but modest: against a 50mm rim you are giving up a handful of watts at 40km/h — perhaps 20–30 seconds over a flat hour, and nothing at all going uphill.
Best forMountainous terrain, exposed coastal roads, winter bikes, lighter riders.
40–50 mm
All-round
The bergs — short, steep, relentless, all day
The default for most riders
The reason nearly every brand centres its range here. Deep enough that the aerodynamic benefit is measurable on any road above about 30km/h, shallow enough that you are not fighting the front wheel on a descent. If you are buying one wheelset to do everything, and you are not certain you need otherwise, buy in this band.
Best forRolling terrain, one-wheelset riders, first carbon upgrade, anyone unsure.
50–65 mm
Fast
The flatlands — echelons, crosswinds, cobbled sectors
Racing and flat, sheltered roads
A genuine advantage in a fast group or a flat road race — enough to matter over 40km. Modern wide, blunt profiles in this band handle far better than the 60mm rims of a decade ago, but you will still notice a strong crosswind, and you will notice the extra 100–150g on a long climb.
Best forRacers, flat and rolling terrain, riders who hold 35km/h+.
65 mm +
Very deep
The race of truth — alone, unpaced, flat out
Time trials, triathlon, flat courses only
A specialist choice. Fastest in a straight line at speed, and the least pleasant wheel to ride on a windy day or a technical descent. Excellent for its purpose and poor at everything else — which is fine, as long as it is your second wheelset rather than your only one.
Best forTime triallists, triathletes, flat closed-road courses.
Three things that matter more than depth
Internal width. A rim designed around a 28mm tyre will be faster and vastly more comfortable than a narrow rim of the same depth running a 25mm tyre at 100psi. Tyre and rim are one aerodynamic system, and the tyre is the bigger half of it.
Front-wheel behaviour. Crosswind stability is overwhelmingly a front-wheel property. This is why several makers pair a shallower front with a deeper rear — you get most of the aerodynamic gain and almost none of the handling penalty.
Your actual speed. Aerodynamic drag scales with the cube of velocity. At 25km/h a deep rim is doing very little for you; at 40km/h it is doing a great deal. Be honest about which of those you ride at.
Shallow entries worth a look
Observation list
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