The sport of triathlon loves it’s aero bikes. No doubt about that. After all, they look cool, they put us in an aerodynamic position, and they’re really fast, right? Right?
Well, they can be fast, but often I think athletes over-estimate how fast they make them.
Aero vs Power
Very aero triathlon bikes (and time trial bikes for that matter), and by this I mean bikes with non-round tubes that are generally made of carbon, are often built with proprietary front ends (bayonet-style head tubes and the like) and designed to create a very aggressive riding position, so many times they have very short head tube lengths. Short head tube means a low handlebar position that many riders might be able to reach, but not necessarily generate power from. If an athlete is positioned outside of their optimal range for power development then power output (and speed) goes down. Poor fit equals poor power. Period.
If the fit is jerry-rigged (we’ve all seen the triathlete on the $10,000 bike that has the stem jacked up) then at the very least the benefit of the frame isn’t being taken advantage of.
But let’s not go there yet — let’s assume a rider can assume the position that the bike frame offers.
What exactly is the benefit of the aero frame itself?
Of all the changes you could make to your setup — helmet, wheels, aerobars, skin suit, shoe covers, POSITION — the frame accounts for the least amount of benefit.
But let’s look at how much that benefit is. I pulled this chart and graphic from an article written in Velo News a few years ago done by the group at Aerosports Research looking in depth at the aerodynamic benefit of different components.
This graphic provides the number of seconds of improvement a rider would see over a 40 km (24.8 mile) time trial. This is exactly the distance of the bike leg for an Olympic distance triathlon. To be able to compare aerodynamic drag numbers, we have to always test at the same speed — in this case 28 miles per hour, which, yes, is pretty darn fast.
The benefit that an aero frame (this is in comparison to a round-tubed TT bike that also puts the rider in the tucked position on aero-bars) provides is a 30 gram difference in CdA which translates into covering the 40 km time trial course 17 seconds faster.
How can we put that improvement into perspective? First, just compare it to some of the other components….it’s definitely on the low end.
My next question then was How much power improvement do we need to make up 17 sec over 40km? Meaning, how much harder would I have to pedal to make up this same amount of time?
Let’s do the math. Let’s say you’re a 160 pound triathlete and your bike weighs 20 pounds. So the combined weight of you and your bike is about 180 pounds or about 80 kilos. You’re a pretty good Olympic distance athlete and your time for the 40 km bike portion is 1:07 (so you’re riding a smidge over 22 mph). This requires you to push about 208 watts for that 67 minutes. You’re debating whether to drop $5000 on a fancy shmancy new TT bike and wondering if you should pull the trigger. Let’s assume that the new frame will garner you the entire 17 sec of benefit (even though those measurements were based on someone going much faster than you — actually they would be pushing north of 350 watts! — at 28 mph).
Guess how many more watts you’d need to push to make up 17 seconds in the absence of the new frame?
4 watts. Actually no, 3.7 watts to be more accurate.
The smaller you are, you’d need even fewer to make up the difference.
Still think that $5000 frame is worth it?
Now this 4 watts is considering a flat course and a calm day. Certainly throwing in some hills and a stiff breeze changes the watts necessary to ride at this speed. But I’m not arguing that aerodynamics and cheating the wind isn’t important. My point is that that the aero triathlon bike isn’t helping riders as much as they think.
What’s important is that regardless of the wind, the power needed to overcome a relative wind speed doesn’t change. Relative wind speed is basically that no matter if you’re going 17 mph on a completely calm day it will require a given number of watts to hold this speed. It’s the same number of watts (for that rider) to ride 12 mph into a 5 mph headwind.
Additionally, that aero frame may be worth 36 grams of drag reduction at 28 mph, but a how many grams of reduction does it get you at 20 mph? Or 17 mph? Not 36 grams worth, that’s for sure. Still worth the $5,000? How about $3,000?
The aero position for the weekend warrior
Again, all the previous arguments were assuming that you can get into the position that the frame was built for. What does this position look like? It means that the rider can spend 95+% in the aerobars while having a reasonable number of spacers under the stem and aero pads (<30 mm under each) and a stem with less than 15° of rise (although a case could be made for one with under 10°) AND still generate consistent power throughout the entire race distance.
What happens when you can’t get that aggressive? What if you need to raise the stem up 15-20 degrees or add spacers under the stem? Are you still getting the full aero advantage of that frame?
I am no expert in a wind tunnel, but I have spent some time in them. One thing that becomes clear in that environment is that aerodynamics isn’t a 1-to-1 equation where everything is black and white. For example, it would seem that you should be able to take the fastest wheels and pair them with the fastest frame and have the fastest bike, but the truth is that it’s actually a much grayer proposition. Bike components (and riders!) interact in strange ways, such that you could have the best intentions of improving your aerodynamics by getting a set of aero wheels but they might not play well with your frame and the result is, if not bad then at least less than optimal.
Rider position is no different. It might seem logical that even if a rider has a bad position on their bike, the frame itself being more aero will provide benefit and may make up for the rider’s shortcomings in fit. The truth is that there’s a good chance it won’t. A frame’s aerodynamics are drastically effected by how the air comes off the front end of the bike and meets the rider and again how the air comes off the rider to meet the rear of the bike. Referring back to being able to generate consistent power, if a position is too aggressive and a rider has to sit up on the bullhorns to stretch their back, any small benefit the aero frame is producing is far out-weighed by the increase in drag from the entire body sitting up in the wind.
In cases when a rider has difficulty maintaining the position of their aero steed, it’s my firm belief they would be better off either saving their money and getting a cheaper time trial frameset, or building a custom time trial frame (and still probably saving money relative to the higher end TT and triathlon bikes). A custom bike, even a round tubed version in many cases will be a faster alternative for many triathletes and TTers.
Why? First of all, a custom bike can be designed to create a position for the rider that is the optimal intersection of their power generation as well as their aerodynamics. An athlete that’s in a neutral body position will be more powerful. Additionally, from a bike fit standpoint, a neutral custom-made position results in fewer dynamic deviations like “flailing knee” tracking issues and the rider sitting more squarely in the center of their bike** — both factors that could absolutely improve aerodynamics. Since the rider’s body accounts for most of the aerodynamic drag, optimal body positioning will always provide the most improvement in aerodynamics. Even the researchers in the article mentioned above note that “The biggest benefit of the TT bike is that is fixes you into an efficient aerodynamic position…” — NOT that it’s aero-shaped tubes cut through the wind drastically better.
Nuts and bolts on cost for something like this? If you want to stick with carbon as your frame material of choice in aerodynamic tube shapes, it’s still going to cost a fair bit — likely anywhere from $4,000-$10,000 for the frameset (frame and fork).
The whole point of this article however was to elucidate how little those aero tube shapes are benefiting you. If you opted instead for a steel or titanium custom frame you’d be spending much less — in the ballpark of $2,000 for steel and $3400 for titanium — or basically the cost of entry-level and mid-level triathlon frames respectively.
Aerodynamics can in fact be “free time” because you don’t have to work any harder and yet you go faster. I am a huge proponent of getting the most aerodynamic benefit you reasonably can and there are plenty of changes to explore. But the aero triathlon bike never quite made sense to me….it’s a huge cost with a pretty low up-side. Not great bang for your buck.
So explore your non-aero options — they could be cheaper, or more comfortable, or more powerful, or all of the above — with a chance that they could be more aero overall too!
**often the first compensation our body makes is the pelvis falling off to one side to find some stability, you can see an example of that here