Triathlon SBR Magazine Winter 2019 | Page 51

GEAR » WHEELS f you have done the training and have perfected your position on the bike to be the most aerodynamic it can be while still being able to get the maximum power that your training has produced through the pedals – then you are ready to start looking at making your bike as fast as possible. The first place to do this is in the wheels. If your goal is to start dropping weight on your bike, rotational weight is worth more. Grams in the wheels, pedals and cranks have more value than grams in the saddle, handlebars or frame. In fact, if a frame is made lighter at the expense of rigidity and performance, you would rather it be heavier. Wheels have an additional factor in that their aerodynamics plays a bigger role in the overall slipperiness of the bike. More than any other component. Yes, much more than the frame. I IT'S ALL ABOUT THE AERO There are a number of considerations when choosing a fast set of wheels. We are assuming that we are talking about wheels for a non-drafting cycle leg in a triathlon. So essentially a time trial. Straightaway we acknowledge that aerodynamics is more important than weight. If we were doing a draft-legal event, or a cycling bunch race, we would favour weight more because we would be constantly accelerating and decelerating throughout the race. The weight of the wheels, and specifically the rims, play an important role in how easy that will be. Generally though, deep-rimmed carbon wheels will not be heavy so weight should not really be a big concern. Next we need to decide on the front and back combination. The most aerodynamic combination is a disc wheel at the back and a deep-rimmed wheel on the front. Firstly we need to dispel some myths about disc wheels. It is simply aerodynamics that makes them faster. A disc wheel does not drive the bike along or provide momentum as many seem to think. In the early days of disc wheels some manufacturers experimented with weights inside the wheel that were thrown outwards towards the rim once the wheel was turning in the hope it would provide momentum. So once the wheel was up to speed, it would help to maintain that speed. This turned out not to be the case, however, so now all carbon disc wheels are simply trying to cheat the drag and turbulence created by revolving spokes. DISC FACTORS There are completely flat discs which, while being a bit more slippery, are more of a handful in cross winds. A lens- shaped disc wheel has a slightly bigger frontal area but is more stable and more aerodynamic in crosswinds of varying degrees. In real-world conditions, a disc is normally never a problem as far as wind goes though. It is bolted firmly into the back of the bike and most of our weight is holding it down. So apart from in truly extreme conditions, a disc wheel is always rideable. One factor to consider is that disc wheels with solid cores are very, very rigid and unforgiving. So on rough surfaces the road vibration is unfiltered up to the rider and this can contribute towards overall fatigue, far outweighing the aerodynamic benefits. On rough roads it is better to go for a deep rimmed but than in an enclosed velodrome. Any turbulence felt on the front wheel affects the handling and the way the bike feels and responds to your commands. Riding a very deep-rimmed front wheel in extremely windy conditions can be harrowing. Especially in the aero position with your elbows doing the steering. Now front wheels vary in depth from 30mm to 100m. The deeper the rim, the shorter the spokes and the more aerodynamic the wheel will be. The deeper the rim, the more surface area there is for Njord to get his hands on and pluck you from the direction you are trying to travel in. Just like the shape of a disc wheel affects it’s handling, different rim designs also make a difference in the stability of the front wheel. More rounded, aerofoil- shaped designs handle crosswinds much better than flatter shapes. Current wind-tunnel testing also suggests that there is such a marginal aerodynamic difference between a 90mm and a 50mm rim depth, that the shallower rim should always be the first choice because the energy cost of fighting the bike is far less than with the deeper rim in all but the stillest wind conditions. Let’s be honest, when have you ever done a ride with absolutely no wind at all? THE MOST AERO COMBINATION IS A DISC WHEEL AT THE BACK AND A DEEP RIM ON THE FRONT spoked wheel, where the spokes will absorb a lot of the road noise. On the back we can use a 90-100mm deep rim without any consequences to the handling of the bike. There is still the hole in the middle for the worst crosswinds to blow through and we have the spokes to make the ride less harsh. Most time trial frames nowadays do a great job of hiding the back wheel from the wind anyway so, without looking up any data from wind- tunnel testing, I would wager that there is a very small aerodynamic disadvantage between a deep-rimmed back wheel and a full disc when slotted into a heavily fairinged time trial frame. AT THE FRONT The front wheel is where things become more tricky. The front wheel is the vanguard of your assault against the wind. It is the first thing the air has to move around and is therefore the most important aerodynamically. Ideally we would want a disc on there, but that would be a disaster anywhere other IT'S WORTH IT Before we finish, let’s get back to some mythbusting. It has always been theorised that it is not worth buying aerodynamic wheels unless you are able to average a certain speed. Firstly, unless your route is completely pancake-flat, there are going to be periods of higher speed descending. So even if your overall average for the ride is less than 30kph, I am sure you will spend some time going over 40kph. Another, connected myth, is that there is only an aerodynamic advantage at high speeds. Yes, that is true if you consider 20kph a high speed. As soon as we start moving and having to push the air aside to do that, paying attention to our aerodynamics will help us. Wind-tunnel research has demonstrated that pro cyclists climbing a mountain like Alpe d’Huez at 20kph still benefit from aero wheels, and even sitting on another rider’s wheel, which brings us to another point – we are still drafting if we are closer than 10m on a climb. But that is a topic for another article… 51