When I began considering a post regarding mountain bike sizing and fit I read lots of other sizing guides online. They were all very long and very confusing, with lots of photos and diagrams, videos and verbal directions. After attempting to plow through several, I found that I didn’t get very far on any of them. Finally I decided I’d rather have an ill-fitting bike than having to suffer through the drudgery of trying to figure out the unnecessarily confusing and condescending tutorials.
(If you have not read any of my other cycling posts, then let me fill you in on my general perspective that cycling “experts” have a tendency to make every aspect of the sport as complicated as possible.)
OK, back to how to properly size a mountain bike:
The fundamental problem of bike sizing and fit is that all of our bodies react differently to different stresses. We are all physically unpredictable. A “properly” fitted bike, performed in consultation with a professional bike shop technician, might just cause you all kinds of issues and pains (knees, back, feet, ankles, etc), while an “ill-fitted” bike where you violate all the best advice might just work perfect. What works for most people may not work for you. My personal experience of 40+ years of bike riding has taught me that I'm not like "most people". What really works is trial and error, so any bike fit guide has to be taken with lots of grains of salt.
What size frame to choose?
A long winded, complicated diagram of bike anatomy is just too much information so let’s make it simple:
In general this is the size mountain bike frane you should be using:
• XS: bike frame size 13-14 in. Rider height 5ft to 5ft 4in
• S: bike frame size 14-16 in. Rider height 5ft 4 in to 5ft 7 in
• M: bike frame size 16-18 in. Rider height 5ft 7 in to 5ft 10 in
• L: bike frame size 18-20 in. Rider height 5ft 10 in to 6ft 1 in
• XL: bike frame size 20-22 in. Rider height over 6ft 1in
If you are in between sizes or you want to confirm the most-likely correct size do a Stand-over Test: standing over the bike with your legs on either side of the top tube. With your feet flat on the floor and your bike shoes on, measure the distance between the top tube and your crotch. The distance should be a couple of inches 1-2.
How high should the seat/saddle be?
Perform the Heel Test: sit on your saddle/seat with the cranks (the thing your pedals are attached to) in a straight up/straight down position. The proper height will have your heel just touching the top of the lower pedal (the pedal on the downstroke) with your leg straight.
How should the saddle be adjusted?
It’s most likely that the best fit will come from having the saddle level and in the middle position of the saddles rails (the things that attach your saddle to the seat post).
Next, handle bar position. When you ride your bike your arms should be slightly bent.
Everything should be comfortable. If your position hurts, change it.
Trial and error.
Oh and one final thought: perhaps the biggest reason you may have discomfort, aches and pains, on a new bike or after sizing adjustments is a long lay-off from cycling. OR lack of conditioning for cycling. You're out of shape! Every spring when I get back on the bike after the winter off, I'm not bike-fit and I have a hole bunch of muscles and joints that have not been used much during my ski season away from cycling. Invariably I hammer away on Day 1 and have lots of issues during my first month back, irrespective of bike fit.