RALTech | Mass vs Aerodynamics

Since the birth of the bicycle until relatively recent times, the quest for the designer has been to reduce the bike's mass. With the exception of a few enlightened but misunderstood mavericks at various times throughout history, little emphasis has been put on design for aerodynamics.

This is not so surprising, really. Mass is easy to measure and appreciate. The first thing many people do with a new bike is to pick it up and remark how light it is. Aerodynamic drag, on the other hand is difficult and expensive to measure. What's worse, is it's not consistent; a drag reduction in one situation with a certain rider might not be the same for another. At least with mass, you know where you are, even though it is scarcely relevant.

To reduce the mass of the bicycle by 0.5 kg could increase the cost by £1000 or more, assuming a reasonably good starting point. All other things being equal, this would result in an improvement of just under 1 second in a 25-mile time-trial. How much would you need to improve aerodynamics to achieve the same effect? Approximately 190 mm² of drag area (C_DA). That's something like an inch or so of exposed brake cable. Tidying up your cable runs doesn't even cost any money - FREE SPEED.

One special consideration here, though, is body mass. Whilst an extra kilogramme of mass will have a small effect on speed, the associated increase in volume and surface area is far more significant, partly because the human body (and fat, in particular) is so much less dense than steel or aluminium, even than carbon fibre. Given the number of factors involved, it is difficult to determine an exact relationship between change in body mass and change in C_DA, but bounding estimates suggest between 1000 - 2000mm² per kilogramme. This effect should be added to the effect of mass alone. In other words, body mass is worth about five times as much as equipment mass in terms of time gained or lost.

This table can be used to estimate the effect any equipment or position changes will have on your performance. It also shows the relative effects of each change.

How to use the table:-
In the column on the left, pick the type of change you have made, then read across to see what equivalent effect this would have on other parameters.
For example, supposing you have identified a reduction in drag of 1 gram, this is equivalent to a power saving of 0.10 W or a time saving of 0.43 seconds over a '25'.

Baseline data used:-
Mass (rider + bike): 83 kg, C_DA (rider + bike): 0.220 m²,
Rolling Resistance: 0.0035, Power: 283 W to produce '25' time of 55:00.

More specific calculations using your own more exact data can be carried out using the spreadsheet below, which you can download and play with as you wish.