In Part 1 I discussed the basics underlying bike fit and how frame size plays in. This provides an understanding of how bike fit works. It’s an important baseline understanding, but doesn’t lead to any answers. In part II I’ll cover a few questions that will help you come up with some answers. Here are a few factors to consider and questions you can ask yourself that will lead to a more informed determination of the correct frame size for you.
How tall are you? This is the first, the easiest and most important question to answer. This will usually narrow your choice down to one of two frame sizes. Unfortunately, if this is the only thing you consider it won’t help you finalize that decision. It does narrow the field a lot though
I’m 5’9” so I’m unlikely to ever be on an XL or XS. This narrows my choices to S, M, or L.
How are you proportioned? Proportional height refers to your unique ratio of leg length to torso length. And impacts cockpit as well as saddle position.
For example a friend and I are both the same absolute height (5’9″), but her legs are over an inch longer than mine and her torso proportionally shorter. Two years ago I rode a Scott Spark 900. She now has a 2014 Spark 900 in the same frame size. My saddle was slammed forward and I had a 90mm stem with 720 bars. She needs a setback post (for her longer legs) and both a shorter stem and narrower bars (since her reach is less). The same frame size worked for both of us, but I guarantee you that the ride feel is NOT exactly the same. Consider this when you think what size frame you want.
Another example. BMC offers a medium short and medium long in some frames (short and long refer to different top tube lengths). This isn’t common. My friend rides a medium short, but it feels VERY scrunched up to me.
So, when you hear someone say they’re 5’11” on a medium frame, and someone else say that they are 5’10” and on a large, and both think they have perfect fit it may be that their proportional height is very different. But wait there’s more…
How do you plan to ride this beast? Will you be in the saddle a long time pedaling for miles? Will you spend most of your time out of the saddle as you charge the downhills? DH bikes and XC bikes naturally are designed to fit very different as they are ridden very differently, but you need to consider this for your bike.
I plan to use my Mach 429 sl for endurance riding and racing, but I also love looking for all the optional jumps and drops in the trail and plan to take each and every one of them. Consequently I want my bike to fit both when I’m seated pedaling for an hour climb but also when I’m charging in Moab. If I was selecting this frame with predominantly downhill focus, I might have selected a large frame: the longer wheelbase is more stable at speed but forces a stem shorter than I want which would make climbing a little harder.
How do you want the frame to ride – what’s your style? This question mostly impacts wheelbase and cockpit setup. I like to think of this as whether you ride like the Hulk or Muhammad Ali.
- Hulk SMASH! The Hulk wants a bike that can charge straight ahead as fast as possible and plow through everything! This is in no way derogatory or related to body size – the hulk is damn effective and some small people I know ride like this. They tend to prefer a longer wheelbase and shorter stem and wider bars (even on an XC bike).
- Muhammad Ali famously floats like a butterfly and stings like a bee. He’d probably ride similarly and prefer a fit that favors a nimble, flickable, precise feeling. Generally this is a little shorter wheelbase, but slightly longer cockpit. I’m not talking about a 110mm stem here. The difference between a 60mm stem and an 80 or 90mm stem can be dramatic.
I’m more like Muhammad Ali. I prefer a shorter wheelbase and a little longer cockpit. A good friend who rides more like the hulk chose a larger frame and added a 60mm stem.
Back to the points in space
I introduced this concept in Part I. Take a look there if you need a refresher. Here’s my answers to the questions above and how they impacted my thoughts on the three points:
- Height: I’m 5’9”.
- Proportional height: I’m relatively evenly proportioned with slightly shorter legs to torso.
- Ride type: I’ll use this bike for endurance riding and racing and light trail riding, which includes 50-125 mile races and a 6 day stage race.
- Ride style: I like a nimble ride. I don’t do the Hulk well at all.
This let me know how I wanted to “fit” on my bike. I can ride bars between 660-760, but wanted something closer to 720-740. I didn’t want an XC length stem (90-110), nor did I want a stubby enduro stem (50mm). I wanted something close to 75mm. To achieve this the medium Mach 429sl would allow me to correctly hit my three points in space (get the proper fit) to prevent overuse injury. Because I plan to spend so many hours seated pedaling, I still plan to get a professional fitting (likely Retul). This won’t impact frame size but will impact saddle position and stem length slightly. I’ll turn the cranks well over a million times this year so a couple millimeters will matter.
The devil is in the details
The specific geometry numbers are VERY important. My medium Pivot Mach 429sl fits nothing like the medium Ibis or BMC that I’ve ridden. Take a look at the table below.
|Bike (all medium)||TT||STa||Reach|
|Pivot Mach 429sl||609.6||71.9||413mm|
Many folks will only look at the toptube measurements. By this measure the Ripley does look shorter, but the Pivot and BMC look similar, so you might expect them to fit the same. When you add seat tube angle into the mix things look different. The reach (which accounts for STa and TT) is actually significantly shorter on the Pivot than the BMC and yet still longer than the Ripley. The medium Ibis Ripley was WAY too small for me. The medium BMC Fourstroke takes a very different path and felt extra-long. A medium felt more like an extra-large.
Now let’s look at some other numbers.
|Pivot Mach 429sl||Medium||609.6||71.9||413mm|
The reach is nearly identical on all of these frames even though the STa and TT are slightly different. The difference is in frame size. So, knowing my height, proportions, ride type and ride style I could select any of these three and get the fit that I want (frame size in any case.) Of course this doesn’t take into account wheelbase or that fact that these bike may ride quite differently.
If possible try to demo a bike. If you do demo a bike make sure you consider how it’s set up (stem length, handlebar width, saddle position). If it’s not possible to do a demo, then do you best to answer the questions above, determine the ride and handling you want and make the best informed decision possible.