The rim is the backbone of a wheel structure. It connects the rider to the road through the tire, spokes, and hub. Being a rotating body its mass and aerodynamic properties play a significant role in the amount of power required to keep you rolling down the road. The following items should be considered when making your rim selection.
Rim Weight: Rotational inertia is sensitive to how far mass is placed from the center of rotation. The rim is the farthest wheel component from the center of rotation so its mass has the highest effect on inertia. This is the reason heavier rims and tires feel more difficult to accelerate quickly. Some wheels provide weight savings at the expense of strength and stiffness. Be sure to consider your strength, weight and riding style before selecting ultra lightweight rims. The durability trade-off is rarely worth a small difference in weight.
Rim Stiffness: A stiffer wheel will generally transfer power better and allow for higher spoke tension, which significantly improves wheel durability. Laterally stiff wheels generally feel more responsive and predictable on mountain roads, in the dirt or during race situations. Note there are two types of functional wheel stiffness; vertical and lateral. There is a trade-off between high vertical stiffness and ride comfort, while lateral stiffness is closely related to steering response and improves with increased rim width.
Rim Aerodynamics and Material: Aerodynamic efficiency is arguably the most important parameter of a road wheel. Its benefits far outweigh mass considerations of a wheel. A good example is the solid disc rear wheel used for time trial applications. The mass of a disc can be double that of a spoked wheel and still be a much faster race wheel. During short power sprints the disc wheel is often described as slow and flywheel-like; however once the disc is in motion it can reduce aerodynamic drag by over 60%. While aerodynamic efficiency generally improves with rim depth the primary trade-off of deep section wheels is increased crosswind sensitivity.
Leading rim manufacturers have made great strides in balancing issues of weight, stiffness, and aerodynamics through the use of advanced composite materials. Composite wheels are generally lighter, stiffer, more aerodynamically efficient and isolate road vibration better then their aluminum counterparts. Mountain bike and off-road applications employ composite materials to improve steering response while minimizing overall weight.
When deciding on a rim material it may be difficult to avoid carbon when aerodynamics are high priority. With other applications carbon can be a nice option for improving steering response and/or ride comfort, however price is the primary trade-off to be considered.
Spoke Count: Wheels with fewer spokes are generally more aerodynamically efficient. They are also subjected to much higher spoke stress since fewer spokes carry the same dynamic load. Generally higher spoke counts contribute to higher wheel strength and durability. For race wheels, you may want to consider fewer spokes, however there are few benefits to training on wheels with lower spoke counts and typically the result is reduced wheel life. Consider your body weight, intended wheel purpose, and riding style before deciding on the number of spokes. Also be sure to select rims and hubs with the same spoke count.
If you have questions please contact our expert staff for further assistance.