Weight vs Aero
Which is faster: A more aerodynamic or a lighter bike?
We examined this eternal question back in 2006, which is what's presented below. Recently we checked the same question with data from modern frames, and we can tell you the results are still within 1 per cent of these old numbers.
To answer the weight vs. aero question, we need to look at two issues:
1. What are the weight differences between aero and non-aero frames?
2. Does the aero advantage outweigh the weight advantage in various scenarios?
The weight difference between Cervélo’s new R5 road bike (with rim brakes) and S5 aero road bike is approximately 245g.
What does 245g really mean? Of course, we are not accelerating or climbing with just the frame. We also have the other bike parts and, more importantly, the rider, to deal with. If we assume a 7kg complete bike and a lithe 50kg rider, the 245g weight savings represent an overall weight savings of about 0.4 per cent. For a heavier riders, it’s even less.
On to the difference in drag between a good aero frame with narrow tubes and a frame with more voluminous tubes optimized for low weight. Similar to the weight difference, the drag difference is large between the frames, but much smaller when looking at the drag of bike and rider combined. For a rider on a road bike like the R5, a good coefficient of drag (Cd) — that is, the inherent drag related to any particular form — would be around 0.6. On a TT bike like Cervelo’s P5, it would be more like 0.5. But the bike alone accounts for 0.009 of this difference, or a savings of 1.5 per cent.
So what is more important, saving 0.26 per cent in weight or 1.5 per cent in drag? The answer depends on where you ride. On the flats, where aerodynamics can be up to 90 per cent of your resistance, saving 1.5 per cent in drag is the answer. But if you are one of those rare riders who only ride up crazy climbs like the Mortirolo (and take the gondola down), take the 0.26 per cent weight savings.
But where is the tipping point? How steep does a climb have to be before the weight savings trump the aero gains? The answer depends on the rider’s weight and speed, as a faster rider would encounter higher aerodynamic forces while the gravitational forces stay the same. For an average 250 Watt rider, the tipping point is around a 5-per-cent slope. For a good pro who puts out 400 Watts, it’s 8 per cent. Of course, other performance-enhancing factors such as comfort, fit and handling must be weighed against additional aero savings on the flats and on descents.