What road wheel rim depth do I need? That vital question will take some research to answer, especially in the era of carbon fibre. In the old days of alloy, rims were narrow and shallow, and weight was the prime consideration for all cyclists. But then carbon fibre took over because the new miracle material was lighter and more rigid than metal.
Easier to shape as well, so designers spent many hours in wind tunnels, perfecting the ultimate aerodynamic rim for road cycling. Streamlining was always important on the road, where air resistance and energy demand increase exponentially to sustain a speed above 15 mph. Average speeds of 50 kilometres per hour (30+ mph) are achieved in competition, to say nothing of breakneck alpine descents at twice that velocity. Aero rims play an important role in such high performance.
At first sight, seen from a distance, the modern pro peloton looks like it’s rolling on fat bike rubber. But these days, tires are still skinny – 23 or 30mm. A closer look reveals that deep carbon rims gave the impression of bulky tires. Those broad carbon hoops are the product of intense aerodynamic development and, in the overall cross-section, resemble a teardrop, rounded off at the leading end by the curved profile of a tire. An elongated teardrop section has different wind-cheating properties compared to a shallow rim. The cross-section may be V-shaped, with a sharp edge where it meets the spokes, or U-shaped, like a wine glass without a stem. There is a hybrid combination of both profiles, which some aerodynamics experts favour. The deeper the better, they say – and the ultimate depth is found on a rear disc wheel, where one might say the rim profile ends at the hub!
The pros and cons of deep rims
Let’s take a look at the pros and cons of deep rims. The advantages are obvious. They shave seconds off your best time against the clock or give you an advantage in a solo break. The greater the distance, the greater the energy saving and the less time taken. They also add stiffness to the wheel, which results in a more efficient transfer of power from the pedals. Sure, they may be heavier than shallow rims, but there’s no denying they cross the finish line first – in certain competitions, not all.
However, there are prices to be paid for the deep-rim advantages. The first is comfort. The extreme stiffness gives a hard ride. At times, the rider may feel he’s sitting on a jackhammer, not a saddle, especially with the aggressive geometry of a time trial machine. This may be tolerable for a solo event lasting an hour or so on level roads – but no professional would be happy to use very deep rims in a one-day classic or national stage race. However, in the cycling leg of a triathlon on a straight-level road in calm weather, an 86mm deep rim is a big advantage.
Now for the bad news. An ultra-deep carbon rim on the front wheel turns into a liability in gusty weather. Deep rims work beautifully when the wind is from ahead. But when a crosswind hits them on their broadsides, they respond like windmills, push the bike sideways and play merry hell with the steering. Performance suffers when a competitor must fight to keep the bike headed in a straight line. The lighter the cyclist, the more dangerous the effect. A petite lady should never use a deep rim in front – unless the day is utterly calm and windless. A strong crosswind gust could blow her off the road.
Shallow rims
Shallow rims are lighter, and even in the alloy, they are easier to accelerate, having less inertia, which can give them a 1/100 sec advantage over the final 200 metres of a sprint. On a course such as a major mountain pass, where the peloton climbs less than 15 mph, aerodynamics is not an issue. A shallow, light rim helps the cyclist in his fight against gravity. But it will give no advantage in the high-speed descent that usually follows a stiff climb. For a course that ends at altitude, such as the famous Alpe d’Huez in France, a deep rim profile would be a handicap. But on the level of battling a headwind, it far surpasses a shallow profile, even though it weighs more.
Which size of depth rim is suitable
Like so many other life choices, the right rim depth demands a compromise. For the club rider who enjoys Gran Fondo, a rim between 45 mm and 55mm deep is a good choice.
It strikes a happy medium between the extremes in comfort, aerodynamics and stability, especially amid a crowded peloton – even when a crosswind strikes. Such a rim will also offer greater durability for the rider who competes and trains on the same wheelset. The cyclist who lives and competes in a mountainous region could go shallower. If a heavyweight dedicated competitor is looking for the ultimate rim to be used on a flat course in calm conditions, he can go for 86mm at the rear – and something less adventurous in front of his course happens to cross a side wind.
How to choose suitable rim depth wheels from ICAN.
ICAN has performance wheel sets with a wide range of rim depths. This gamut of quality wheels caters to extremely specialized purposes – and more general cycling. No matter what two-wheeled discipline an enthusiast follows, the ICAN catalogue has a wheelset with a rim depth to satisfy every customer. There’s the Aero 35 disc for mountain climbers at $655, plus 50mm and 55mm versions. For the pro time trial competitor, Alpha 86 Pro costs $540, and this range is also available for the peloton in depths of 55, 50, and 40 mm. A good all-around choice for beginners on a budget would be Alpha 50 Disc Pro wheelset at $485.
