A Jim of All Trades

Getting Aero

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This year I made a few changes to try to improve my bike aerodynamics and this past weekend I did some testing to see how much of a difference different gear actually makes. This post is going to show the results of my testing to see how much of a speed difference deeper wheels and an aero helmet make on the bike. I’ll note that this topic has been covered in great depth by countless others, so if you’re into triathlons and bike gear this won’t be news to you, but hopefully you’ll still find it a little interesting. If you’re reading this I’m going to assume you’re at least somewhat familiar with the importance of aerodynamics in triathlon and cycling, so I won’t elaborate on that here.

Last winter I was researching different things to improve my speed outside of training more. Wanting to get more involved in the sport, I wanted to make an upgrade that would hopefully make me a bit faster. I came across this article and, aside from working on my bike position (more on that below), I decided to buy an aero helmet, a used Giro Advantage 2 that I got online for $50 shipped.

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Through the year I kept my eyes open for some aero wheels as well and came across a Shimano RS80 C50 front wheel on craigslist, then shortly later picked up a Giant P-SLR 1 rear wheel on eBay.

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Not being a matching set they do look a little silly together on the bike, but they’re both well regarded 50mm deep wheels, and combined they “only” cost me $250, which is pretty cheap for an aero wheelset (I couldn’t have gotten a matching set for that price), so I’m quite pleased with them and don’t care how they look. There are better, deeper, lighter, etc. wheels out there, but for my budget these are just fine. I’ll also note that they actually might be a tad lighter in weight than my basic Bontrager Select wheels.

Here’s an older pic of the bike with the Bontrager wheels:

IMGP0694xs

When using this aero stuff I definitely *feel* faster on the bike, but I wanted to quantify how much of a change they actually made. So on Saturday I did some testing…

There is a stretch of road not far from my house that’s just a hair under 2.5 miles long. On the far end the last half mile goes uphill at a 4% to 5% grade, but otherwise it’s relatively flat. I use this stretch of road for interval work from time to time and it takes about 10 minutes to get there, which works for a short warm up.

On Saturday morning I did this course three times. Each time I pedaled easy to the stretch of road, then:

  • First did it with my “non-aero” Bontrager Select wheels and wearing my normal helmet.
  • Second with my aero wheelset, still wearing the normal helmet.
  • Third with the aero helmet and aero wheelset.

Recording this all with my Garmin 305 watch, I did the 2.5 mile stretch as hard as I could on the way out, turned around at the top, then did the stretch as hard as I could back. I counted each time out as a lap on the watch and each time back as a lap, excluding the turnaround. I then pedaled easy back to my house and changed the wheels or helmet for the next run. Everything else stayed the same. I wore exactly the same clothes, kept the same stuff on my bike, and even topped off my water bottle each time so all other factors would be as consistent as possible between the runs. At the end of this post are some other notes about things that could have potentially affected the results, though I think they’re minor.

Here are the results:

Aero testing chart

Having to go uphill at the end of the “Out” segment really hurt the time/speed of that leg, but going down helped on the way back, which is why there is such a difference between the two segments.

Here are some of the key take away points, combining the “out” and “back” times:

  • The aero wheels were 20 seconds – 2.4% – faster than regular wheels.
  • Adding the aero helmet was 21 seconds – 2.6% – faster than the aero wheels with a regular helmet.
  • The aero wheels and aero helmet combined were 41 seconds – 5% – faster than the regular wheels and regular helmet.
  • My heart rate stayed within 3 beats per minute for each run, which would suggest a pretty consistent level of exertion.
  • I think the reason the top speeds weren’t always recorded during the fastest run had to do with me jumping the gun a little with my effort vs. when I started the watch. If I was a little eager with jumping on the pedals, it would give me a bit of a head start and more time to reach top speed before I settled into the normal effort for the ride.

I had mixed feelings about these results. I was hoping for more than a ~0.5 miles per hour difference with each upgrade and have to wonder how much the margin for error is since testing was based on effort. Still, I feel pretty confident that I put the same amount of effort into each test, this still shows an improvement with the aerodynamic equipment, and is pretty consistent with the linked study. I’m especially pleased and surprised with the third test; my legs were feeling pretty fatigued by then and I didn’t feel like it went well. I still gave it all I had, but was sure that the time wouldn’t be any better than my previous test. Had my legs been fresh I wonder if I could have done better yet.

I also multiplied the combined times by 5 to project the overall improvements for a 25 mile (Olympic distance) ride. Sure, I wouldn’t be able to maintain this effort for 25 miles, and a lot of other factors would come into play, but it gives a rough idea about how much time savings these changes would give you in an actual race.

25mi testing chart

Summarizing for a 25 mile ride:

  • Aero wheels would save 1 minute 40 seconds over standard wheels.
  • Aero wheels and an aero helmet would save 1 minute and 45 seconds over just aero wheels and a regular helmet.
  • Using an aero helmet and aero wheels together would save 3 minutes and 25 seconds over regular wheels and helmet.

Again, this is just multiplying the times out so isn’t overly realistic, but still shows that there are time savings to be had. The article I linked to listed a 67 second savings with the aero helmet alone and roughly a minute savings with the aero wheelset, so my calculations aren’t that far off.

Between the two, if there’s one big take away, I’d say that getting a good aerodynamic helmet should be done before spending money on wheels. It yields the same improvement or more, and helmets are a lot cheaper than wheels. Helmets can be found used for $50 or less, and new ones are available for as low as $80 and up to $200 or more. Decent new wheelsets on the other hand typically start close to $1,000 and only go up from there. Even if you put together a used set like I did, the price will still be greater than most new helmets. How much a helmet helps is also dependent on your body positioning and how well you hold your position and head over a race. But when considering price, and assuming you can get a decent fitting helmet that compliments your position, and that you are comfortable holding your position, there is a much greater combined value and benefit in getting the helmet.

Talking about gear is interesting and the subject of a lot of different debates, but actually, the human body accounts for about 80% of drag on the bicycle (I’ve read this multiple times over the past couple years but do not have a source handy, sorry), so getting in the most aerodynamic position as possible will yield the greatest improvements. To that end, last winter I lowered the elbow pads on my bike as low as I could and moved the extensions as close together as possible, while still allowing room for a bottle between my forearms. I did not do before and after testing or take pictures, so don’t have any data to show how much of a change it made, but it did definitely reduced my frontal area in the wind, which is a good thing. I think I could comfortably get a little lower yet and maybe bring my arms a bit closer together, but I would need to buy a new bar to accomplish this, and am done spending money on upgrades for this bike for a little while. I’m comfortable and satisfied with how I fit on the bike for now.

Hopefully you found this interesting and helpful. Thanks for reading!

 

Here are some other quick notes about possible factors in the different times. I don’t think these played a huge part, but they should be mentioned:

  • It would have been more scientific and accurate to use a power meter and ride at a consistent power level each time to see how much the speed/time changed, but I don’t own a power meter and I found it too hard to maintain consistency by cadence or heart rate, so decided to go by what I felt was the highest effort I could sustain for the intervals. There is room for error with this approach, but it’s what I did.
  • The tires on the Bontrager wheels are Michelin Pro 4 Service Course and the tires on the aero wheels are Michelin Pro 4 Service Course Comp, which have slightly less rolling resistance, but both sets of tires have quite a bit of wear and neither are considered especially fast tires, so I don’t think this should have affected the results much. They both have the same types of tubes.
  • The Bontrager wheelset has a 12-27 cassette while the aero set has a 11-28 cassette. The gear spacing in the middle gears is nearly identical though, so to keep everything equal I didn’t use the largest cog in either wheelset, and didn’t use the smallest in the aero set – the second smallest is a 12 tooth gear, so it’s the same as the smallest on the Bontrager set. This is important because I could have used the smallest gear when using the aero wheels, but didn’t for the sake of equality. Meanwhile, I wouldn’t have been able to sustain the 11 tooth gear long with the Bontrager set, so I know I left some speed on the table when I was riding with the aero gear.
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Written by Jim

September 6, 2016 at 7:01 am

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