Bicycle Q Factor Definition - Ebike Mid-Drive Motor Q Factor List

Court

Administrator
Staff member
#1
Hi guys, In recent years I have been hearing more about Q Factor in the electric bike space because motorized mid-drive bottom bracket systems are often a bit wider than traditional unmotorized bottom brackets. This is relevant because it changes the stance width and may impact your hip joints and overall comfort. First, let's define Q-Factor.
  • Q Factor Definition: The distance between the outer edge of the crank arms, measured horizontally - parallel to the bottom bracket itself. Basically, the distance between the the two points on the crank arms where pedals are threaded in. The wider the tread or Q Factor, the wider your pedal stance will be because the stance width will be increased.
What is a good Q Factor width? Well, the most standard measurement I found was 175 mm. Many people recommend a narrower or standard Q Factor because it can reduce hip pain (people naturally walk straight vs. out at an angle), keep the bike riding straight and more stable vs. swaying you side to side as you pedal, and reduce pedal strikes by keeping the lowest point of the pedal stroke directly below the bike vs. further out and down when you turn. However, I have found that on most fat bikes, and even some plus sized tire bikes, the Q Factor should be a bit above average so you don't snag your shoe on the chain stays as you pedal. This happens to me frequently when riding the Specialized Stumpjumper FSR 650B non-electric bike.

Older bicycles tended to have narrower Q Factors because they only offered a single speed or were using a 1 x drivetrain (one chainring up front with multi-sprocket cassette). However, starting around the 1970's, there were more bikes coming to market with multiple chainrings up front, some using a greater number of sprockets in the rear cassette (which requires more space to keep the chainline straight), and others using larger mountain bike tires. If you have three chainrings and the top-mounted derailleur, it significantly pushes out to the right side of the bottom bracket which increases the Q Factor.

A similar thing happens when you introduce an electric motor at the bottom bracket, and this is why many of the earliest mid-drives were only setup to work with 1 x drivetrains (this is the case for the first and second generation of Bosch motors. Yamaha was one of the first companies to introduce a motor that could accommodate multiple chainrings up front, but their PW motor did not offer shift detection and could damage the sprocket teeth and derailleurs if not shifted thoughtfully. Here's an example of one of the first Yamaha powered multi-chainring ebikes I ever reviewed, the Haibike SDURO AllMtn RC. There has been some back and forth about the necessity of multi-chainring setups on electric bikes, because it adds complexity, weight, and may not be necessary given electric assist. I have noticed that the Q Factor of hub motor powered ebikes may also be wider than average because they tend to use fatter tires (even for city bikes) to improve comfort and provide stability.

bicycle-q-factor.jpg


Below is a short list of the Q Factor measurements for some of the most popular mid-drive ebike systems. Feel free to add updates and comments below, and I'll do my best to keep this list accurate. It seems that the actual Q Factor of a motor can be changed based on the crank arm positioning and style. That means, you may encounter a fat tire electric bike with a mid-motor that uses different crank arms that may be wider than what is stated below. The chainring may also be offset to keep the chain line straight and fit around the wider chain stays and tire:
  • Yamaha PW-X Q Factor Length: 168 mm
  • Shimano E8000 Q Factor Length: 175 mm
  • Brose Drive T, TF, S Q Factor Length: 179 mm
  • Bosch Performance Line Q Factor Length: 180 mm
 
Last edited:
#2
Anyone know of crankarms that can narrow the Q factor for use on the Bosch motor? Designed right it could be narrowed up and closer to 140ish like typical road/gravel set up. I have a Bulls Dail-E Grinder and the 4 cm wider foot position due to the 180mm Q factor makes it pretty different than my road and gravel bike setups. Anyone out there heard of a solution for this?