To understand bike fit, you need to think about the size of the rider plus the way they ride and sit on the bike. Especially with mountain bikes, a lot of bike fit is impacted by riding style. It’s not enough to simply reach the bars and the pedals. The rider needs to feel balanced so they have optimal control of the bike in trail condtions.
The key measurements for my process are as follows:
- saddle position (distance from the bottom bracket to center of saddle)
- saddle set back (horizontal distance from the bottom bracket to the tip of the saddle)
- bar reach (horizontal reach forward from center of saddle to center of grips)
- bar drop (vertical drop or rise center of saddle to center of grips)
For saddle position, measure from the bottom bracket shell to the top of the saddle in the center. Ideally you should put a bit of tape on this spot so you can use it as your reference point for the other measurements.
For saddle setback, set a level between the saddle and the bars then use a plumb line to drop down to the bottom bracket. The measurement for setback is from the tip of the saddle to the center of the bottom bracket. Every saddle is different so you should also make note of the distance from the center of the saddle reference point to the tip of the saddle.
For bar reach, measure the distance to the center of the bars at the stem then measure the distance from that point to the grips in the horizontal line. This is because the center of the stem is not where your hands go. Different bars have different sweep and what really matters is the horizontal to the point where the grips are. You do not want to measure the direct to the bars because it will actually result in a larger number that is influenced by the bar width. It takes a pad of paper and some basic math but you can determine your bar reach and it is pretty critical in terms of picking the proper frame size.
For bar drop, use a board between the saddle and the bars. Stack material under one side or the other until the board is level. Once you have a level reference plane you measure downward or upward to determine the rise or drop the bars have vs. the saddle.
Some riders like their bikes with high saddles for full leg extension. Other riders prefer a lower saddle especially if they hit the technical areas hard.
The question of ideal fit is complex. Everyone is different. Personally, when riding off road, I use a position that is long and low with a short stem. This position helps me keep the front end of the bike light and makes me feel stable in rough areas.
With various stems and saddle positions, a bike that does not really fit can still work but it is not “ideal”. This can be a problem when frames are build in only three sizes to fit the entire population. Building with seven sizes allows me to have smaller steps between each size to obtain a fit that is almost as good as custom. Since I hand built each frame and don’t maintain a large inventory. This allows me to offer more sizes compared to the big bike companies because I don’t need to bring inventory over on container ships.
This week, I did some detail drawings of a few sizes for my standard frames with different component positions to show some of the range you can get. The following is my “large” frame. I enjoy thinking about position and playing with the CAD program gave me some insights into how to refine the standard sizes that I offer.
Consider my large frame size:
A really pretty small position could be achieved with the following setup. The position could even be made a little smaller with a swap to a seat post that does not include the standard 25mm setback in the clamp area.
A larger position is possible as shown here. An even bigger position could be made with a 120mm stem and sliding the saddle up and back a bit. Notice that I drew this with riser bars and an upward slope to the stem to avoid having a stack of spacers. Spacers generally work fine but in extremes can make the fork steerer tube weaker. If you need high bars for your bike consider a custom frame with a tall head tube or using bars with a bit of rise.
With my size range, I can fit *most* riders using standard sizings. Some riders *need* custom sizing or custom angles for specific reasons and that is also not a problem because I can quickly create a drawing that adjusts one of my standard sizes to meet their needs.