Q-Factor and Bike Fit : The basics and what you need to know
A number of years ago there was an idea floating around that gained a lot of momentum that a narrow q-factor was more mechanically efficient because it made the bike stiffer and more aerodynamic and from this an assumption started to follow that a narrower q-factor was also more biomechanically efficient for the rider
- this was around the time that the BB30 bottom bracket standard was popular and it was sold to us that because the spindle of the crankset that fit through the larger bearings was bigger in diameter and shorter in length so it was going to be stiffer than a traditional crankset
- turns out this was wrong because the combination of the crankset and the narrow bottom bracket actually was less stiff than other standard with wider bottom bracket shells and external cups
- this is why the wider bottom brackets, like the BB386, BBRight are more in favor now
- turns out this was wrong because the combination of the crankset and the narrow bottom bracket actually was less stiff than other standard with wider bottom bracket shells and external cups
- 73 mm BB can fit a 168 mm crankset
- three primary factors
- bottom bracket shell width
- crankset
- pedal
- a wider q-factor is necessary the wider the rear tire gets because the crank arms have to be clear of the chainstays as they turn
- the simple answer to the "should I aim for a narrower or wider q-factor?" is it depends
- on a typical road bike the majority of riders won't notice much difference in the small variance in q-factor that we see among most road bikes (somewhere in the 140s)
- in fact most won't have an issue as we move up to a typical mountain bike that supports up to 2.5" tires (150s to low 160s)
- problems can arise when we move into plus bikes (3" tires and greater) (mid 160s and 170s) and continues into the fat bikes that can easily move into the 180s up to 200
- note that many people, myself included, have no trouble with wider stances
- Bike Fit
- in the bike fit space there are a number of ways to solve a bike fit problem by increasing the q-factor on mny road bikes usually with longer pedal spindles
- from a bike fit standpoint there is actually a very likely scenario that a rider's will improve their mechanics by having an unequal q-factor at times
- we're all asymmetric and almost no one sits square on their bike so a serious case could be made that having one foot further away from the center line of the bike is necessary
- how do I look at it?
- I look at the infrared data of how the rider's LE mechanics are going, how is their knee tracking? what type of hip movement is occurring? ankle mechanics? and a huge driver is the orientation of their foot -- is it toe down at certain points? how much float do they take advantage of?
- in my experience a rider with more external rotation at the hip and eversion of the foot and ankle (essentially someone who toes out more, and perhaps they walk more "duck footed") will absorb wider q-factors better. The more toe-in and tightly adducted the rider's mechanics, the greater likelihood that they may choose a narrower width
- I look at the infrared data of how the rider's LE mechanics are going, how is their knee tracking? what type of hip movement is occurring? ankle mechanics? and a huge driver is the orientation of their foot -- is it toe down at certain points? how much float do they take advantage of?
- SIMPLE: there's a reasonable chance that a smaller rider would need a narrower q-factor but it often can be managed with cleat changes or a carefully selected crankset and pedal
- can we test to see what the best q-factor is?
- hanging test: https://www.ncbi.nlm.nih.gov/pubmed/24392768