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A short guide to carbon rim depth

April 22, 2018

 

With carbon wheels users now have many more choices: regular spoked wheels come in a variety of rim depths (from 20mm, 24mm, 30mm, 38mm, 50mm, 60mm, 88mm), but there are also rims with solid carbon spokes (5,3, even 2!) and of course full carbon disk wheels), so which is good for what and which depth to choose? 

 

This article provides a short look simplified look into the basics of the physics at work and translate it into the consequences felt by the rider for the main depths currently in use which are usually around 38mm, 50mm and 88mm.  But the general principles are valid for other wheel formats.

 

Everything else being equal, in general deeper profile  rims are:

1. Heavier

2. More Aerodynamic

3. More sensitive to side wind

4. More stable but more difficult to steer (they want to keep going straight).

5. More difficult to accelerate or slow down

6. Stiffer and noisier

 

 

From these general properties, one can deduct where their use will be best:

  • The more you need to go up steep hills, to slow down and accelerate quickly, not to miss an acceleration in a criterium, or a break in a road race and handle windy situation, you will go towards a lower profile, lighter wheelset. 

  • The more you are interested in pure aerodynamics and you intend to ride mostly alone at a steady pace in a point to point time race against the clock.

 

  • At one extreme, if you do a steep uphill time trial you may use a 24mm 990 grams carbon wheel set.  You do not care so much about aerodynamics (even the pros go up l’Alpe D’huez at “only” about 25Km/h!), but the almost 1Kg of weight difference compared to a deep section wheelset is huge in a steep climb.   You not want a heavy deep aero or disk wheel coming out of a hair pin on a 12% slope…

  • If you are in a criterium, a one day race with a hilly course or bad roads, a mountain stage in a Tour or a grand fondo with large climbs where you have to go up a lot (most of the time actually since going down will take a small portion of your time) , your best choice may be  a 38mm deep rim which is already aerodynamic but still very light. 

    • For this reason, 38 mm wheels are very good for riders that live in hilly or mountain areas. 

    • They are preferred and excellent for smaller and lighter riders that tend to climb well but are less powerful and more sensitive to side winds than heavier rider.

    • They will pass better over bad roads without beating you up (like in the spring classic with short steep hills and cobblestones).  

    • They are also preferred if you need very responsive wheels so you do not miss the break away, or if you expect a lot of pace changes like a criterium in a city course with many turns, since  a rider missing a break and having to close a gap alone instead of riding in a pack will pay a much higher aerodynamic penalty (tens of percent) than the benefit obtained by having a deeper and more aerodynamic wheel when riding alone (a few % maximum).

  • If you expect mostly flat and fast riding and you want a quite aerodynamic wheel with a reasonable weight penalty then 50mm deep rims are the new all-around wheels, especially for the pros who tend to spend most of the day above 40Km/h, unless they expect particularly windy or hilly conditions.  Powerful riders and sprinters will also tend to use them preferentially because they are interested in top speed, high wheel stiffness and also because heavier riders are less sensitive to side wind and a slight increase in wheel weight.

  • If your goal is the fastest end to end point, you would like to keep a steady pace, keep momentum on rolling terrain with no serious climbs and you do not care about sharp accelerations because you are riding mostly alone (for instance in a Time Trial or Triathlon) then you will want deep profile wheels like an 88mm or even a disk in the rear.   The aerodynamic benefit will be significant in fast and flat courses with calm weather, they will feel heavy and unresponsive as soon as you get serious climbs and if you are caught with significant side winds you may feel more like you are windsurfing than riding a bike…

Also note that even if the front wheel is more important from an aerodynamic standpoint than the rear wheel, it is also much more sensitive to weight, side winds and significantly affect bike handling, for this reason it is quite common to use a deeper wheel in the rear (which is loaded by most of the rider's weight and does not impact steering and handling as much) and a shallower profile in the front.  Depending on the conditions described above riders will use 38 in front with 50 rear or 50 in front with 88 rear or 88 in front with disk rear.  Whether it looks cool or ugly is already a different level of discussion that doesn’t have anything to do with performance (other than possibly some placebo effect ;) !).

 

 

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