Friday, June 14, 2013

Aero for slower riders

A quick chart today for future reference whenever that classic online nonsense argument about aero benefits only being for faster riders, or that aero only matters above a certain speed....

Let's set people straight now: Aerodynamic improvements benefit riders of all speeds and power outputs. But who gains the most benefit?

Whether a slower rider should be putting time/energy/effort/resources into gaining or buying an aero improvement when they might perhaps be better focussed on losing weight and training more (or harder or smarter) is a moot point. Really, though, such an argument is a false dichotomy. Why not do both?

The other consideration of course is if you are going to chase an aero improvement, then there are two main ways to achieve that:

  • improved aerodynamic positioning, or
  • improved aerodynamic equipment
But again, this is not a case of one or the other. It's quite OK to do both and train better. You know, one could train to improve fitness, work on gaining a better aerodynamic position, and treat themselves to some nice aero wheels, or move from using a road bike with clip on bars extensions to a time trial bike. This is not an either/or scenario.

If you are a back/middle of pack rider, then some bling wheels are not going to make you the next world champion, so some perspective here is warranted but the rationale for why you are looking to improve your performance is a matter of personal choice. If you want to be faster, then you do all the things you can given the constraints you have (time, money, knowledge, rest of life factors etc). And we are talking about people riding in competition-like events, not your cruiser to pick up some milk at the local shops (let's be sensible here).

If you are just happy with participating rather than competing, then sure, what does it matter? If you just like having nice equipment and have the money to spend, heck, go for it. Enjoy yourself.

But let's get the physics out of the way with a chart to quickly summarise the situation with an example.

The chart below plots the time taken to complete 10km on a flat road with no wind at various power outputs, from a modest 150 watts, through to a solid 350 watts. Other assumptions are shown on the chart, but changing the parameters really doesn't change the basic principles here. Click on the chart to see a larger version (right click to view in a new tab/window).

There are two lines, showing the reduction in time to complete the 10km as a rider's power output increases. No surprises there, more power with all else the same, you go faster.

The two lines also show the difference between a rider with a coefficient of drag-area (CdA) of 0.30m^2 and 0.27m^2 (a 10% reduction). That's roughly the sort of reduction in CdA you might expect going from standard low profile spoked road bike wheels to specialist aerodynamic wheelset, or riding on the tops to riding on the drops.

Under that are the blue columns, which represent the time saving over that 10km by reducing CdA from 0.30m^2 to 0.27m^2. As you can see, the slower less powerful rider saves more time in absolute terms than the faster more power rider. However, when expressed as a percentage of time saved, they are nearly equivalent savings, with the faster more powerful rider making very slightly better gains in percentage terms.

Now of course some parameters do change under some conditions, e.g. cross winds can affect the apparent CdA to differing degrees at different speeds, so in those situations, a faster rider may benefit a little more in percentage terms, but in general, there really is no physical reality to the old myth that aero only benefits the faster rider, or that óne needs to ride at X km/h to see benefit.

Pithy Power Proverb:
The largest absolute time savings from a given aerodynamic improvement are made by the least powerful/slowest riders.


Unknown said...

While I see your point in terms of time saved over a fixed distance. The use of a fixed distance for the test skews the result toward the slower rider. His test is over a longer time period than the more powerful rider so he has more time to benefit from the decrease in resistance.

When looking at the speed benefit in terms of kph, the slower rider gains only 68% of the speed increase compared to the more powerful rider.

cshansen73 said...

From the data supplied, going from 31km/h to 32km/h is barely significant, going from 42.75 km/h to 44.25 km/h is a significant increase. Speed increase is much more relevant than the time decrease

Alex Simmons said...

@ unknown:
No, this does not skews things at all since time to complete a fixed distance comprises the majority of such events. The only exceptions are the hour record and the 12 and 24 hour records, which represent a very small minority of such events, and are dominated by individual time trials and triathlons.

Alex Simmons said...

Not sure I really understand your point. Over a fixed distance, a speed increase is exactly the same thing as a time decrease. They are equally relevant.

Sure, the 350W rider gains a speed increase of 1.45km/h, and the 150W rider gains a speed increase of 1.00km/h. But that's because the relative improvement over their respective starting speed is about the same, which is my main point.

IOW if you make a 10% improvement in your CdA, then you can expect to improve speed by a little over 3%, no matter how powerful you are.

But this is missing the real point, which is that *all* riders can see a time/speed benefit from an aero improvement, it is not only the domain of the most powerful, and you don't need to be doing 40km/h (or whatever) before gaining a worthwhile improvement in speed.

Such lines of reasoning are fallacious.

Unknown said...

As stated in my original comment, your point that aero improvements are good for all riders, both fast ans slow is well made.

I mention the different perspective of time vs speed not because to the 1 hour or 24 hour record times but rather for many riders that compare average speeds on their routes. This is a common method of comparing rides for recreational cyclists.

Possibly the best point for those riders to take away is that it is more rewarding to compare rides using elapsed time rather than average speed. For slower riders a reduction in time of one minute is a smaller increase in speed than a one minute reduction in time for the faster rider and therefore he can see small improvements more readily by comparing time rather than average speed.