Conventional wisdom has it that if you're in between sizes you choose the smaller frame. I teach tri bike fit to about a hundred retailers a year at our workshops, and when I poll these retailers, that's the nearly unanimous response when asked the question.
But it's the wrong answer. At least nowadays it is. At least in tri.
Why? A clue can be found in the way the latest generation of timed race superbikes is made. The Specialized Shiv, the Scott Plasma 3, Storck's Aero 2, Giant's Trinity Advanced, all have this feature in common: everything sticking up above the head tube top is pretty much gone. The stem, such as it is, along with the bike's steer column, plus any spacers, the stem clamp, it all sits at the level of the top tube.
Look at the Giant Trinity Advanced (pictured above) and it's easy to see this. The Specialized Shiv (below) is another example. The idea here is to get rid of everything but the frame.
Why? Because stems, steer columns, headset spacers, whatnot, are very little better in terms of strength, weight, aerodynamics, than they were a decade ago. In fact, aerodynamically, they may be worse, because steerers, stem sizes and shapes, handlebar clamp areas, are now larger than they ever were.
So, let's get rid of all that, and fill it up with frame material which, in contrast, is more aerodynamic, lighter, and stronger than ever before. End users have correctly interpreted the message of the past decade: Tri bikes are no longer a spec driven market, but a frame driven one. In 1995 the sort of question that might be asked was: "What's your cheapest Ultegra bike." Can you imagine asking that question now, in the context of a tri bike purchase?
Today, it's, "I want a P3," or a "Plasma 2," or a "Slice 5," or any of a number of other really nice bikes made by Felt, Kestrel, Blue, Specialized, Pinarello, or two dozen other companies. And here's my question to you: Do you know what bottom bracket gets fitted into any of these bikes, as original equipment? Or what pursuit bar? Or chain? Or cassette? Or brake caliper? Or rim? Or tire? No, you don't. Because that's not what interests you. The frame interests you. As it should.
So, today, if you're in between sizes, it means the frame is too low, or it's not long enough (or too high or too long). Right? So, if it's too low, what are we going to fill that area up with? Headset spacers? A stem pitched up in the air? Or frame material? Me, I choose to fill it up with frame material.
Likewise, if it's lot long enough, there's a better solution than a longer stem: namely, a longer frame.
But the problem a lot of users face is this: The frame size I'm looking for is about the right length, but it's too low. Or: the bike's too tall, and if I get it a size down it's not going to be as long as I want. What then?
I'm glad you asked.
My recommendation to you is this: When in doubt, fit the bike according to it's length, not its height (this, if we're talking tri bikes; if it's a road bike discussion, my answer might be different). The reason is, there is a lot more height adjustability available to you than there is length adjustability. You want to change a bike's length? You put on a longer stem. Or a shorter one. That's it, really, unless you resort to adjusting the aerobar's armrests back and forth.
But height! Well, now, here's where we have some wiggle room. First, there's stem pitch. As you might guess, I'm in favor of what we call -17° stems, that is, stems that protrude forward parallel to the ground. Look again at these photos of the Trinity Advanced and the Shiv. As you'll note, the way you get more frame up under you is to avoid having that stem stick up in the air like it's on Viagra. To make room for that happy stem you have to lower the entire frame. That's not my idea of a good solution.
That established, if the frame is too low for you—too short in the vertical plane—upturning the stem is one solution. Most tri bikes come today, original equipment, with a +/- 6° stem, that is, it's + or - 6° from perpendicular to the steerer, depending on whether the stem is flipped one way or the other. But since head angles are 73° (many are 72°), a it takes a stem that's minus 17° to get you a stem extension parallel to the ground. Bike makers spec these 6° stems because it makes the bike a lot more adjustable: just flip the stem and you either lose or gain about 15mm of height. If you flip a -17° stem, well, now, directionally speaking, it's a really happy stem, probably way too happy for your purposes.
So, if you can, buy a taller bike size and have your 6° stem replaced with a 17° stem. That'll yield a higher performance set up. Nevertheless, stem pitch is one way to fix the height problem.
I'm not a fan of headset spacers. But, you do have that option available as well. Just know this: Your bike manufacturer has also probably outfitted your bike with what I call the Mount Fuji headset top cap. This is the top cap that's 20mm tall, or taller, and, if you remove this top cap and replace it with a 5mm top cap, you can again start thinking about a frame the next size up.
Consider this: That 15mm difference in headset top cap elevation is probably near the elevation difference between two adjacent frame sizes in a manufacturer's size run. Ask youself: are you paying for a P3 or a B2? Or are you paying for a Cane Creek or FSA headset top cap? What turns you on? If it's the frame, then, by all means, buy more frame and less top cap. This, as long as the next size up frame isn't now too long for you.
But, it must be acknowledged, headset spacers, headset top caps, stem pitches, all accrue to give you height, if what you need is height. And there's one more thing that gives you height (or reduces height): the aerobar you choose. Today's aerobars are made with armrests that sit anywhere from 20mm to 70mm above the centerline of the handlebar clamp. This means that if you need height, just changing from (let us say) one model of Profile Design aerobar can gain or lose you 25mm, maybe 35mm. If you need to gain or lose height, a change in the model of aerobar will get you more elevation, or drop you lower to the ground, than all the other means of height-adustment previously mentioned herein.
If you add this all up, it's very easy to raise or lower the elevation of your rider position. It's quite hard to lengthen or shorten the position any appreciable amount, especially if you want the bike to handle well. Parenthetically, your proper stem length may or may not be what comes spec'd on the bike O.E. It's typically better to ride a tri bike with a stem 2cm or 3cm shorter than what you'd ride on your road bike, though a lot of bike manufacturers don't realize that. So, me, I ride a 12cm stem on my road bike, a 9cm stem on my tri bike, and both bikes handle very nicely and as they should.
One thing about these new superbikes that don't have stems anymore. They have one problem: They don't have stems anymore. You often can't adjust these bikes for length. There are exceptions to this, and, I'll note those exceptions in the future, as I write about each bike individually. But, length adjustability is important. While I ride a 9cm stem on my tri bike, I could ride an 8cm stem, or a 10cm stem or perhaps an 11cm depending on other geometric characteristics of the bike. I'd like to have the freedom to choose my stem length, and some of these bikes take away that freedom.
If you "flatten" a more traditional bike by choosing a size larger frame, and a low-profile headset top cap, and a -17° stem, you get something not so different than these superbikes in terms of vertical profile, but with the adjustability that stems give you. In fact, if you look at a 2010 Felt bike (pictured just above) outfitted with its Bayonet system—which is not thematically unlike using a traditional stem—it doesn't look so different than the other bikes pictured, does it?
Other bikes—Cervelos, Kuotas, Kestrels, Treks, what have you—as well as the prior iterations of Plasmas, Trinities and Transitions, all made with traditional stems, can be spec'd and built to look pretty much like this Felt pictured here. Just, start with the idea of replacing the headset top cap and stem that come with these bikes out of the box, and replace them with "flattened" versions. Then, you get more frame for your money.
I think you can see, then, that the trend today is to buy the larger frame, if you're in between sizes. Even if you're spot on a particular size, consider whether the next size up might not work better for you.