Adjusting your cranks is one of the most important things you can do to improve your cycling performance. If you are not doing it right, then you will suffer from poor handling characteristics and possibly even crash or fall off your bike. The problem with adjusting your cranks is that there isn’t really any standardization out there; each manufacturer uses its own method to accomplish the task. What’s worse, some manufacturers don’t provide a tool for this purpose at all!
So how do you know what size crank arm to buy? How do you know if your current cranks are too short or too long?
The first thing you need to decide is whether you want to go up in chain ring size or down in bottom bracket size. There are two main reasons why someone might want to make such a change: 1) they have bigger feet than average (or prefer smaller feet), 2) their frame doesn’t fit them well due to geometry issues, etc.
If you’re just looking to get into mountain biking, then going up in chain ring size is probably the best way to go. Going up in bottom bracket size would require you to get a longer or shorter stem, which could potentially affect your handlebar position. However, if you plan on riding trails or racing on a regular basis, then going down in chain ring size may be better for you.
You may not need to adjust your cranks at all if you are just looking to get into mountain biking. If you are just riding for exercise, then this consideration probably isn’t even a factor. You can ride a fixie with any size chain ring or bottom bracket; it’s all about preference and what you get used to.
Every single bike brand on the market has different crank arm lengths. You aren’t ever going to be able to find a size directly conversion from one company to another, and there is definitely not an industry-wide standardization. Take a look at your cranks and measure the length (in centimeters) from the center of the pedal hole to the center of the crank bolt. Now you can compare it with the lengths of other cranks on the market that are already sized.
Most crank arm manufacturers provide a conversion chart that shows the various lengths and sizes available for their cranks. They will also let you know what tools you will need to make this adjustment. For example, if your cranks are currently 175mm long and you want to go down to a 170mm length, then you need to use a crank arm cutting tool. These usually cost $10 or less. Before you make any cuts, be sure to double check everything first.
If you’re going down a size (e.g. from 175mm to 170mm), then you will have to remove some material from the non-drive side of the crank arm (the side without the chain ring). If you’re going up a size (e.g.
from 170mm to 175mm), then you will have to add some material to the non-drive side of the crank arm. The easiest way to do this is with a crank arm cutting tool, otherwise you will need to take it to a bike shop.
Some crank arm manufacturers don’t provide conversion charts or recommend that you take their products to a shop for installation. They are most likely aware that amateurs like ourselves will buy these parts with the intention of installing them ourselves. If you’re determined to do it yourself, then it’s best that you also buy a crank cutting tool (just in case).
If you decide to cut your own cranks, then you will need to remove the bolts and washers. Be careful not to lose the washers because they aren’t always the same size, and they are definitely not interchangeable. Now you will need to measure from the center of the pedal hole to the middle of the crank bolt. Write this measurement down somewhere because you’re going to need it later.
If you are converting from 170mm cranks to 160mm cranks (or vice versa), then you will have to remove or add material evenly on both sides. You may want to measure out 10mm and 20mm lengths on a piece of paper and play around with different combinations before you make your final cuts.
Sources & references used in this article:
Influence of crank length and crank width on maximal hand cycling power and cadence by C Krämer, L Hilker, H Böhm – European journal of applied physiology, 2009 – Springer
Maximal muscle power output in cycling: a modelling approach by Y Yoshihuku, W Herzog – Journal of Sports Sciences, 1996 – Taylor & Francis
Personal perspective: in search of an optimum cycling posture by K de Vey Mestdagh – Applied Ergonomics, 1998 – Elsevier
The science of cycling: factors affecting performance–Part 2 by EW Faria, DL Parker, IE Faria – Sports medicine, 2005 – go.gale.com
The biomechanics of muscle contraction: optimizing sport performance by W Herzog – Sport-Orthopädie-Sport-Traumatologie-Sports …, 2009 – Elsevier