How to Choose Your Next Tri Bike: An Engineer’s Guide (Step 3)

In this post we wrap up the decision process involved with buying the best triathlon bike for you–the way a scientist would. Once again, the eight steps are:

1. Define your goals
2. Assess your own performance
3. Narrow the choices by price and fit
4. Find the manufacturers’ white papers
5. Look for independent research and reviews
6. Get a test ride and weight measurement
7. Compare aerodynamic data
8. Weigh the intangibles and make a final decision

how to buy your next triathlon bikeIn Step 1 and Step 2, we covered goal setting, self-assessment, price, fit, finding the bike maker’s tech papers, and how to look for good independent bike reviews. Let’s proceed with Step 3.

Get a test ride and weight measurement

This step cannot be stressed enough because it’s the easiest one to skip. In today’s evolving e-market it is tempting to order your bike directly from the manufacturer online, complete with custom components and paint schemes. There’s nothing wrong with that, but don’t let these “extras” overwhelm your need to check the fundamentals. The optimum situation allows you to test the exact bike you’re going to buy. At the very least, ride that model in the correct size with at least a close fit. Your goal is to ensure you can ride the bike and enjoy yourself while doing it.

  • Bring the saddle you intend to ride to the store when you come in, as well as your shoes and pedals.
  • Make the test as close as possible to the real ride.
  • Try to get at least 15-20 miles in on it to see how it feels. If that’s not possible, get a good ride in before you go to the test. That way your body is already a little pre-fatigued and more sensitive to those aspects of the bike that may cause you problems as you approach T2 in a race. Once you’re 95 miles into your Ironman, it’s too late.
  • You’ll want to take the bike around a few corners and up a few hills as well to see how things work when you lean over and stand up on the pedals.
  • Before you leave the shop, see if you put the bike on a shop scale. This is becoming more necessary as bike manufacturers refuse to publish weight numbers for their bikes. By the time you reach this stage of the process, you will most likely find that the difference between the finalists on your list is too small to matter.

Compare aerodynamic data

Now we get to the final discriminator in the contest. By this point you should really be down to two bikes, and the choice may be somewhat difficult. Size, price, weight, even the paint jobs. There’s really no factor that makes you want one more than the other. We are looking for any “extra” that tips the scale, and that’s what the aerodynamic assessment really is. For all the emphasis thrust upon this aspect of modern bike design, it is still the least important factor in your choice. That’s not to say it’s insignificant, but it matters less than fit and comfort. Knowing that aerodynamics actually matter least is extremely helpful because it frees you from all the technical jargon of white papers on bike aerodynamics. You’re just looking for a small difference– something to tip the scales between your final two choices. To do that, you’re going to look through the white papers you have and simply pull some numbers.

How to read a white paper on the aerodynamics of a bicycle

A white paper on a bike generally breaks down into four basic parts: an introduction to aerodynamics and possibly carbon fiber, a lead-in on the research and development process used to build it, some drag numbers, and then a group of colorful charts comparing drag at different yaw angles. All you want to do is cut to the chase and pull the numbers.

First and foremost, get the drag measurements from the manufacturers’ own white paper. These will most likely be the lowest “best case” numbers you’ll see on the bike, and the ones you want to check for accuracy in competing brand papers. The most important value is the drag at “0-degrees of yaw.” “Yaw” is the angle used to describe the direction of the wind against the bike. A 0-degree yaw angle means the bike is facing directly into the wind. A 90-degree yaw angle means that the wind is blowing directly from the side. Different manufacturers produce varying charts on drag values with respect to yaw angle. It can make things a little confusing. All you need to look for is drag between 0 and 10 degrees of yaw. In FASTER, I explain why this is.

If the white papers you’ve found list specific numbers (and not a deceptive chart), then you can use the simple equations in the section on bike aerodynamics in FASTER to figure out which bikes have less drag.

But watch out for charts or graphs as the scale used on them might make the data look more sigificant than it actually is. For example, take two bikes and compare them at yaw angles from 0-10 degrees. Here are our totally made-up values.

Yaw Angle Brand X Brand Y
1 10 14
2 22 26
3 33 38
4 41 45
5 52 54
6 65 67
7 72 76
8 81 85
9 89 94
10 102 110

It doesn’t matter that the numbers are completely made-up. The chart trick works anyway. Notice how the difference between each bike’s “drag” is usually 4 or 5, and at most is 8. Is that a big difference? It depends on whose chart you read…

how to compare bike aerodynamic data

how to compare bike aerodynamic data

Presto! Same numbers, different graphs! The first chart makes it look like a very close race. The second one appears to show at least a small difference. Take a look at the scale on their vertical axes, though, and you’ll see what happened. The scale on the first chart is way out of proportion. While it looks like the two bikes are close, there’s actually no way of reading anything meaningful from it. The second graph has a better scale. You can at least make a better guess based on what it’s showing you. Be very careful of this sort of thing when you look at white papers. You won’t see it happen as dramatically as I’ve illustrated it here, but subtlety is a key element to the trick. When it comes to the charts, size and scale matter.

Once you’ve pulled what numbers you can from various papers, it’s time to make comparisons. Take the best and worst drag values for each bike at the different yaw angles. Since drag is measured at yaw angles in increments of 5 and we’re only concerned with the numbers up to 10-degrees, we’ll only deal with the 0, 5 and 10-degree marks. You’ll have four sets of data– a “best” and “worst” set of numbers for bike #1, and “best” and “worst” for bike #2. We’re going to run three quick tests by doing some simple subtraction. First, compare each bike against the other when they are at their “best,” or lowest drag values. Then compare each bike’s “best” to the other bike’s “worst.” For each yaw angle, look at each bike’s drag value and give each bike a point for every time it “wins.” If you’re concerned about how much better it is than the other bike, you can find more details on the relationship between drag and power in cycling in FASTER. Just to help you keep things straight, here’s a quick “tournament schedule” for your bike comparison.

The “bike math test”

Round 1: “Best” bike #1 vs. “Best” bike #2

Round 2: “Best” bike #1 vs. “Worst” bike #2

Round 3: “Worst” bike #1 vs. “Best” bike #2

You want to see how each one fares when it is at the greatest disadvantage. Again, there’s no single number that gives us an answer. The key is consistency. Does one consistently outperform the other’s drag values? If so, then you’ve found your tipping point in your decision. If not, then congratulations! You’ve reached the eighth and final step of our process.

Weigh the intangibles and make a final decision

You are in the fortunate situation of being able to buy one of two bikes without any regrets. To the absolute best of your ability and the available information, you’ve found the right bike for you. Just run a quick comparison about other items you did or didn’t like about each model. These are things that are personal to you. Does the top tube shape let you tape your race nutrition to it the way you like? Are there upgrades available that are specific to the frame? Does it require maintenance that you’d rather not get into? Is it designed to accommodate an electronic shifting upgrade should you ever decide to go that route? These are things that only you can answer, and if aerodynamics didn’t give you a compelling reason to buy one bike or the other they may be your final deciding point. Whatever the case, you can at least know you made a thorough effort of your decision.

Happy riding!

If you’re interested in getting faster, you’ll be fascinated by FASTER: Demystifying the Science of Triathlon Speed. In Faster, astronautical engineer and triathlon journalist Jim Gourley explores the science of triathlon to see what truly makes you faster—and busts the myths and doublespeak that waste your money and slow down your racing. With this knowledge on your side, you can make simple changes that add up to free speed and faster racing.


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