Another word on bike fit

Okay maybe more than one word, here goes …

This all started out about a discussion of how frames are measured today. Back in the day I had a 57cm Klein Quantum. It may have been a tad large but I was more flexible then and I could ride it for hours on end with no issues as far as comfort goes. So basically the Klein fit well. Now fast forward 18 years and I am ready to buy a more modern ride. I look around and find a 56cm Motobecane. I think to myself, this is good, just a hair smaller so I should be more comfortable. Well for the most part it fit but I needed several millimeters more spacers under the stem to get the handlebars in a more comfortable position. Now I have a PedalForce RS in a 55cm frame size and it is nearly a perfect fit.

The reason I thought all these bikes would fit is that I thought and still do believe, that effective top tube measurement is one of the most important measurements you can have to compare frames. The problem came up when I started looking at frames with much more compact geometry. I was shocked to see that in some frame geometries it was suggested that I should be on a 50-54cm frame depending on the manufacturer. I would never ride a 50cm frame, or at least I thought so.

This forced me to look for a better way to "level the playing field" so to speak when it comes to how do I compare frame X to frame Y when they measure them differently. The answers require a little High School trigonometry and a few measurements that are usually provided. That answer is STACK and REACH.

Simplified, STACK is the measurement from the center of the bottom bracket to the top of the head tube. Mathematically represented it is:

STACK = BB_drop + (Axle_crown_ht + Head_tube_ht) * sin(Head_tube_angle)

REACH is the measurement from the center of the bottom bracket to the center of the head tube along the effective top tube line. Mathematically represented it is:

REACH = Effective_top_tube - STACK / tan(Seat_tube_angle)

Here is a picture for the more visually inclined:

Here is the reason these two measurements are important. STACK will tell you just how low the handlebars can go or just how much you will need to place spacers under the stem to get them comfortable. REACH will tell you just how stretched out or how scrunched up you will be on the bike with your preferred stem length. In my personal fit I always look for a 100mm - 120mm stem length.

Baring any medical or external forces, you are, as an adult, the size your going to be. Well maybe some of us might be getting rounder as we age. So saddle position is saddle position regardless of what bike you are on, and it should always end up the same distance from the bottom bracket regardless of frame size. Once you know your ideal saddle position remember those measurements and give them to any shop where you are thinking about purchasing a bike. They should be able to compare those with their existing specifications and tell you just how long (REACH) and how tall (STACK) any given frame is, either by formula or measurement. This way you are comparing apples to apples. If the shop isn’t interested in those numbers, run away.

These measurements are especially critical today as nearly all bike manufacturers have moved to compact and semi-sloping frame designs. You can no longer say I ride a 55cm frame since that measurement means nothing in today’s world. You can also not judge a bike fit by how much clearance you have to the top tube by standing over it.

Bike fit is still an individual thing, but with a little information and a bit of research and preparation you can be a better informed consumer. It also doesn’t hurt you to bring in your old bike and explaining what you like or don’t like about it when buying your new ride.

Even with this information, ride and fit only comes from being on the road, making small adjustments to saddle, cleat, handlebar and other adjustments. But hopefully this information will be of some value to anyone that is looking to buy a new bicycle.