Tuesday, 2 May 2017

Which One You Need To Be Faster? Aero Bike VS Lightweight Bike

Assuming we all want to go as fast as possible, we first need to consider the point at which the advantage of reducing aerodynamic drag begins to beat lighter, less aero equipment. The biggest obstacle as cyclists we’ve got to overcome is aerodynamic drag which accounts for 80-90%. Kinetic Energy, Drivetrain Friction, and Rolling Resistance make up the remainder. 

Of the aerodynamic drag, the bike accounts for about 20% (includes frame/wheels/components.) Whether you’re drafting in a pack or not, the more the rider can reduce drag, the faster the rider will be able to ride, or the fewer watts he or she will need to generate in order to maintain the same speed as compared to a non-aero bike. If all else is equal (position, conditions, components,) a rider on an aero road frame will travel farther during a set time-interval on the same effort than a rider on a non-aero bike.

Jean-Paul Ballard is an aerodynamicist, engineer and co-founder of Swiss Side with over 14 years’ experience as a lead engineer in Formula One teams. According to Ballard: “Aerodynamic drag becomes the largest resistance a rider must overcome from 15kph onwards. Aerodynamics definitely trump weight at an average speed of 25kph and we can show it clearly. At Swiss Side we have developed some very powerful simulation tools for calculating time over any course for any configuration.” 

Citing a specific example, Ballard explains: “We ran a simulation for a 120km ride with 1,000 metres of climbing and an average speed of 25kph. If we take an 80kg rider, on an 8kg bike, we get an average power output of around 150W. Now we can compare a five per cent improvement in aerodynamics on the bike with a five per cent reduction of weight from the bike. This comparison represents the typical difference you would see between a lightweight set of climbing wheels with a set of aero wheels.” So which configuration saves more time on this course for this rider? The five per cent aero improvement saves 3min 30sec; the five per cent weight improvement saves 20 seconds.

Is there any situation in which a lightweight bike is a better choice? Weight matters on a bike – we’re not trying to dispute that. However, typically when choosing between an aero road bike and a non-aero road bike the decision is based on two bikes that weigh close to the same. By close, it’s safe to generalize, round up, and assume no more than 2lbs, which is a pretty significant difference, especially if taking into consideration similar bikes (ie comparable components, price, etc.) 

The thing about weight is that we often look at it in terms of the bike alone, and we can’t, it’s not as simple as that. It should be looked at and evaluated as a complete system: Bike + Rider. As a system, weight becomes a lot less significant. A 150lb rider on an aero road frame that weighs 6.25% more than a standard frame is only 1.2% heavier as a system than its lighter weight clone (150lb rider on 17lb bike vs a 15lb bike.)

Consider the fact that weight becomes an issue on sustained climbs of 5-6%, even when climbing on a heavier bike, you’ll tend to benefit more from the aerodynamics rather than the weight savings. According to Ballard: “In all cases except a mountain top finish and an uphill time trial, the aero bike will always be the faster choice. Even when drafting in the peloton, the drag reduction is still measurable and important.”

To contextualise this, Ballard suggests that for pro riders, “The average gradient of the entire course where weight becomes more important than aerodynamics is 7.5 per cent.” For an amateur, “It lies around 4.5 per cent because they are slower.” These are generalisations and the exact numbers will depend on the individual, but in essence all the evidence suggests that you have to be doing an uphill time trial or a summit finish to justify opting for the lightweight, non-aerodynamic bike set-up. 

Bigham suggests another scenario where lightweight can triumph: “If you are an amateur not constrained by the UCI limit, and you can have a 5kg bike, then lightweight could be worth going for, especially if you are not the best descender and the speed at which you descend is not limited by aerodynamics, but skill.”

Best for you 

What works for you will be pretty personal; it’s down to your size and weight, and what sort of riding you’re doing. Generally, lighter riders will probably want to pay a little more attention to bike weight, since it will be a bigger proportion of the total, and frame stiffness is less of a consideration, since it won’t have to cope with huge torque loads. 

What kind of riding you do is, however, more important. If you’re mainly interested in sportive riding where your main objective is an overall time, always go for the aerodynamic option. However much time you might try to spend in a bunch, you’re essentially riding a time trial and, even on a very hilly course, aero will almost certainly win out.

If you’re a road racer, weight is more important, because the hills are usually where races are won and lost. There, you need to be able to climb competitively or you’re out of the game. But exactly how you balance weight and aero will still be dictated by your riding style. If you’re going to win anything other than a summit finish, you’ll still need the speed that only aero can give you to hold off the bunch, or to get the last half-inch in the sprint. You probably want a bike that complements your strengths, since that’s where the wins will come from.

Cycling Weekly magazine, 02/2016

Ride On!

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