Tuesday, July 14, 2009

Getting geeky with bike fit and geometry

My order for a new bike has been officially placed. You might have guessed this from earlier posts, but I ended up going with a new Trek Madone 6.5. Fancy stuff for sure and I'm really excited about it. In fact it's the first bike I'll have paid for since I was a junior in high school (and this year marked my 20 year high school reunion), so it was fun to put together exactly what I wanted. Well, within reason anyway.

Trek's new Project One site is fun to play with. It's possible to put together some absolutely garish designs, but there are also some nicely understated options. I couldn't help going with custom paint. It'll take an extra couple of weeks for the bike to come in, but luckily my friend John--nice guy that he is--offered to let me use his rain bike during the interim.

Ordering a new bike meant rethinking what size bike to order. For the past decade I've ridden a series of 58cm Treks. This means a bike with a 57.3mm top tube, which is a better estimate of the size of the bike.

Warning: The rest of this post has some pretty nerdy frame geometry/bike fit content. If this kind of discussion makes your eyes glaze over you can stop reading now. OK?

There are a bunch of factors that go into which size bike to get, so for you bike fit geeks out there, this post is for you!

Rider 3 is forever giving me a hard time about bike fit. It's all in fun though. He knows how important it is too. And I'll be this first to admit that I'm hyper-sensitive about my position. The reality is that I've had way too many injuries (four knee surgeries and counting, plus a dodgy neck), so I now have trouble riding a super-aggressive position. I also know first-hand about how much more I enjoy riding when I can finish a long ride and actually sleep through the night, without my achy neck waking me up at 3 a.m.

So anyway, I've been riding the same size bike for a long time, but my position has changed. For the better. I used to ride with a 15 cm drop from the top of my seat to the top of my bars. I now ride with a 7 cm drop. This is a huge difference, but in fact my back is flatter, I'm more comfortable and can actually breathe--something I've heard that comes in handy while riding. The 15 cm thing might have worked when I was 24, but it doesn't anymore. Plus, with my position now I can still get very low by, get this, bending my elbows.

Trek's bikes tend to put riders into a pretty aggressive position. For example, look at a 56cm Trek Madone compared with another relatively aggressive bike, the Cervelo. The top tubes are within 5mm of each other at 56 cm and 56.5 cm respectively. But the Trek "Pro Fit" has a 140mm head tube, vs, 160 on the Cervelo. That's a big difference, and is part of why, for example, I've been running a +12 degree stem (with no spacers) for the past couple of years.

For reference, a 56 cm Pinarello Prince has a 165mm headtube, while a Ridley Helium has a 175mm headtube. And a Trek "Performace Fit" has a 170mm headtube.

From Team Two Wheel
Fit isn't always about riding in a "slammed" position. Note the spacer stack.

Anyway, back to me. I've been on a 58, but spending some time on Trek's site, and with the boys at Two Wheel Transit, I started to rethink my options. I don't have a terribly high seat height (relative to my height) at 744mm. Here's a chart from Trek that shows some ballpark recommendations about frame size. (Ignore the green line and reference to John Burke. He is not me!)


So you see I could go either way (between the 58 and 56).

Now let's look at reach and stack height. Reach is helpful to study between frames. It's essentially a measure of the length of the front half of the bike--if you draw vertical lines through the bottom bracket and the headtube, then measure between them you have the frame's reach. This is more accurate than looking only at the top tube because the reach is independent of the frame's seat tube angle, which effects top tube length (as well as how far in front of or behind the bottom bracket you can adjust your seat), but not your reach to the bars.

Stack height, on the other hand, helps describe how tall the frame is from the bottom bracket to the top of the head tube. This time you'll need horizontal lines through the bottom bracket and along the top edge of the headtube. So when you're trying to compare your position on the bike from one frame to another, it's a more accurate than looking at head tube length indepedently because it removes the bike's bottom bracket height as a factor.

This picture should help clarify the last couple of paragraphs:

Trek offers two fit options for its frames, which is wildly helpful. The "Pro Fit" has a shorter headtube, while the "Performance Fit" is 3cm taller. I've been riding a 58cm bike in large part because the 56 had too short of a head tube, and it doesn't make sense to run a bike with 4 cm (or whatever) of spacers under the stem. I also used to need the longer top tube, but have shortened the size stem I use a bit over the years.

Other riders may need to look at stack height in a different way. These riders--typically tall guys with ultra-long arms--need to ensure they can get long and low enough. Rider 2 is someone who has to think about this. For a more extreme example take a look at Robert Gesink's bike. Notice the drop between the seat and the top of the bars? Contrast this with the bike of, say, Lance Armstrong.

OK, let's look at my comparison between the 58cm "Pro Fit" and the 56cm "Performance Fit," the two sizes I considered.

The 56cm Performance Fit gives me a stack of 576mm and reach of 387.
The 58cm Pro Fit gives me a stack of stack of 567 and reach of 400.

So the 58 Pro Fit would be 9 mm lower. But remember I'm currently using a +12 degree stem. So having a bit more stack is actually helpful. (Each degree of rise in a stem equals roughly 2mm in height.)

The 58cm Pro fit is also longer by 13mm. But I'm currently running a 110mm stem, so moving to a 120mm version is no problem and running it in the -7 degree position gives even more effective length. To show how it all comes together I plugged the numbers into this helpful stem calculator.
So in the end I ordered a 56cm Performance Fit Madone 6.5. I'll run a 120mm stem in the -7 degree position with 20mm of spacers below it (maybe a bit more or less depending on the height of the headset cap) and will end up with a nearly identical position to the one I have now. And in fact I've wanted to be a hair more stretched out and a tad lower than I have been, so on paper anyway, the bike should be perfect.

Wish me luck. The bike should be here in about 30 days.


  1. There is only one real bike fit 'system' out there....that is Retul. You should check it out

    nice post

  2. Yes, Retul looks interesting, although I haven't been to one of their fitters. I've done a lot of work with Andy Pruitt over the years--both for me and athletes I worked with. Retul's founder worked for Andy. Kit Vogel with BikeFit (who is a lot closer to home and often visits Spokane) is also excellent. Really, really good. And it's nice to work with someone with so much direct experience, plus a Ph.D in physical therapy. My only concern with Retul is that you're still dependent on the experience of the person running the system, and they may or may not have a strong baseline in fitting, understanding of anatomy, etc. Food for thought. I'd love to try a Retul fit sometime though.

  3. Enjoy the new Madone. You'll have no regrets because you did so much homework. Many happy future rides to you.

  4. Great post! I'm also trying to decide between a 56 or a 58. How tall are you and what's your inseam?

  5. Hi Anon:
    I'm 5' 11". Inseam? Not sure off hand. I'd start with your seat height and use the red and white chart further up in the post--the one below the pic of Chris Horner. Like I wrote in the post, my seat height is 744mm. If you're in Spokane the boys at Two Wheel could help you for sure. If you don't know your seat height you might consider getting a fit first. And of course the 58cm bike has a 13mm longer top tube. Again, you need to figure out what you need from a fit perspective then work backwards from there. Best of luck and let me know if I can answer any questions.