Wheel Selection

by Exercise Physiologist Rod Cedaro

Once your bike has been sized, you're sitting on it correctly, pedalling it efficiently and enjoying optimal fitness, your next port of call is 'buying' some performance. On a limited budget, where are you going to get the biggest bang for your buck?

Without a doubt, it's in optimising the type and combination of your wheels. As former AIS cycling biomechanist, Brian McLean, notes,

'Wheels have a double effect on aerodynamic load because not only are they moving in a straight line through the air, but they are also rotating. That is, a certain amount of power would be required to spin the wheel at 40kph even if the wheel is stationary. This is caused by the rotation of the spokes in the air and is something known as the 'egg-beater' effect. The other aerodynamic drag force is caused by the wheel moving along the road at a given speed.' (1)

When looking at wheels, you'll find more gimmicks than you can spin a hub at. There's a myriad of wheel types - conventionally spoked, composite construction 3-5 spoke, flat or lens-shaped discs and any combination thereof. So what should you be looking for when shelling out (potentially) $3-8K on a set of racing wheels?

Deep Rimmed Bladed Spoke WheelFrom an aero perspective, your worst choice is a box shaped rim in combination with a conventional 36 spoke configuration. This will give you reliability, but you are disturbing so much air that you might as well be riding with brake blocks clamped on. If you are stuck with this sort of a set up, you can improve your performance by trading the conventional round spokes for either bladed (best option) or oval (less desirable) spokes. If the budget extends a little further, purchase something new and use these as your training wheels.

The next consideration is the number of spokes. As a general rule, if you can reduce the number of spokes whilst maintaining reliability you're moving in the right direction. Spoke type is also important (e.g. 18 conventional rounded spokes create more drag than 28 bladed spokes).

So how do you select an optimal combination of factors? Choose an aero deep carbon rim in combination with a lower number (16-18) of bladed spokes. In my opinion Campag's Bora wheels are hard to surpass, but I'm probably biased by my Italian heritage!

Composite WheelComposite 3-5 spoked wheels create less drag than conventional wheels. They have a lower number of aerodynamically-shaped spokes, which are usually found in combination with a deeper aero rim configuration. The best shape for a composite wheels spoke is 'teardrop' (wider leading edge in the direction of roll to a tapered back edge). The disadvantages of composite wheels are that they can be harder to handle in strong cross winds. Furthermore, they only offer a slight advantage over the top of the range conventionally spoked 16-18 spoke deep rim options discussed above. They can be considerably heavier than conventional wheels, and dragging extra mass over a hilly course can more than counter any aero advantages gained when cycling downhill or along flats.

Disc wheels produce the lowest drag because they don't have any spokes. Having said that, the best composite wheels on the market have drag coefficients that are comparable to discs. Yet discs have fewer handling problems in tough cross wind situations. Cross winds can actually be used to the advantage of the experienced time trialist (unless the wind is blowing at exactly 90º to the direction of travel). If the wind direction is slightly off 90º, a good cyclist can actually gain a 'sail' effect from riding a disc.

If a cyclist is riding directly into a headwind or with a tailwind, flat and lens-shaped discs offer similar benefits. However as soon as the wind direction changes, the lens shaped disc offers superior performance. So budget permitting, a lens shaped disc is better than a flatter disc.

Deep Rimmed Bladed Spoke with Disc BackSo where to from here?

That largely depends on the course over which you're racing. Over a flat 40km time trial, a cyclist's best option is the double disc configuration. However safety factors preclude triathletes from using this set up.

If the course is undulating to hilly and windy, I'd suggest deep rimmed, low spoke number and bladed wheels (e.g. Hawaiian Ironman). On a flat, fast 40km time trial in relatively still conditions, you can't beat a similar wheel on the front and a lens shaped disc on the back (in my opinion Campagnolo Ghibli is still the best disc on the market, but be prepared to pay through the nose).

All the best with your choices, and stay upright!

The Peak Performance Lab offers biomechanical analysis of your cycling technique and advice on your bike set up. To book call us on 07 3234 2600.

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Reference
McLean, B. 1993, Triathlon: Achieving Your Personal Best p. 89