Like the muscle currently beating within your ribcage your bike frame is at the core of the performance of the collection of parts that make up your ride (or whip or whatever you like to call your bike). The right frame is key to position, handling, comfort and aerodynamic efficiency. Like your heart you need a good one in order to perform well in Cycling/Triathlon.
A common oversimplification is that “it’s the rider, not the bike” – this is unquestionably true… however, when we’re looking for peak performance there are significant marginal gains to be had in paying attention to the frame. Of course, bike company advertising would have you believe that every new model is 15% more aero (and 10% lighter, 12% stiffer and 8% more comfortable than the last) and the lack of real world evidence for those claims has led to some riders becoming a little jaded about the benefits of aero equipment. So I’m going to frame the data in ways that relate to the type of riding that you, my treasured readers, engage in.
One of the first things to realise is that manufacturers generally make gains about speed gains for speeds of 50kph (or 30mph which is 48.27kph). Aerodynamic drag does not scale linearly – that means that the drag at 50kph is not 25% more than at 40kph – it’s more like 85%. From the rider perspective this basically boils down to the fact that the faster you go the harder it becomes to keep getting faster. From an equipment perspective it means that when you see a manufacturer (or magazine test) claim a difference of 20 watts between one bike and another you need to scale it down. To the right is a handy chart that shows what a claimed advantage of 20w translates to at other speeds.
Obviously 20w is a big difference but if your target race speed is 30kph (a 6hr Ironman ride) it reduces to 4w which is going to be fairly hard to notice unless you perform formal testing. So definitely marginal gains there. Obviously for pro tour TT riders who are going 50kph + the gains are quite significant which is why they tend to be very well equipped.
Further to that – for a road cyclist you have to factor the impact of drafting. So we need to discount the aero advantages by another 30%-50%. Thus a frame (or any equipment) that makes claim to a 10w advantage becomes 2-3w for a road cyclist racing at 40kph – again very hard to discern.
For a road cyclist equipment gains only really become important at crunch points in the race – times like the sprint for the line. At sprint speed the aero advantages are magnified as the rider faces the wind alone and is travelling faster than 50kph so the gains go up. Hence a tendency for pro sprinters to use very deep wheels, skinsuits, aero helmets and frames. Last year I looked at the impact of frame choice for Jan Bakelants in Stage Two of the Tour de France – he staged a late breakaway to win by 15 metres. His Trek made a difference of 10m (0.7s) over 1.6km on his own – a tiny amount but one that netted him the yellow jersey (and thus fame and more money). Sitting in the peloton it would have made a minuscule difference, but in those last furious moments of the stage equipment choice made a crucial difference.
Draft legal triathletes don’t even have that final sprint to gain the benefit of more aero equipment – in that instance it is all about saving every last iota of energy for the run and being able to respond to pace changes with less effort.
By now you should be able to see why the fantastic claims in bike advertising often don’t shine through as significantly higher speeds in races. And that’s even without discussing dodgy testing practices. Lets now move on:
Show me the data!
Lets have a look at the impact of aero frame design now. There has been plenty of testing performed by manufacturers and a few independent tests by magazines. Lava magazine performed a very useful independent test:
Lava Magazine Frame Testing
In 2013 Lava Mag performed a test comparing a Trek Madone (the pre-KVF, totally unaero model) to a Cervelo S5. You can read the article here. For now I’m not going to discuss the TT bike (Scott Plasma Premium) that was included in the test.
I’ve plotted their data as the watts required to go 48.3kph. You can see that the Cervelo S5 is consistently 23-25w better across the range of wind angles than the Trek Madone. At 40kph that translates to ~15w average improvement.
There has been other testing that confirms the ~25w advantage for a Cervelo S5 compared to a standard round tubed bike at 50kph.
Comparing a Madone to an S5 is pretty much worst vs best case. What about when we’re looking for more subtle improvements. Last year Triathlete Magazine performed testing comparing a few top tri bikes and I re-analysed it as their conclusions were wrong. In that article you’ll see that the differences between top bikes are only a few minutes over an Ironman. But – if you’re looking for marginal gains why wouldn’t you choose the fastest option as a couple of minutes are not to be sneezed at.
That article also illustrates a common problem with claims about aerodynamics – over simplified modelling. Some brands focus on high yaw performance and make claims based on that but they’re not all that relevant for the faster riders except in very rare conditions.
I’m approaching the 1000 word limit (self imposed) for this series so, in conclusion:
There are measurable and sometimes significant differences in the aerodynamic qualities of frames. However, at the speeds that the majority of riders see the advantages can be hard to perceive. Like all the elements I’ll discuss in this series frame aerodynamics is a valuable part of the whole – collecting marginal gains that together add up to a marked improvement in performance.