The Q-Factor page - What is a bicycle Q-factor?
An Article by Warren Beauchamp
Q&A     Making it Wide    Making it Narrow    Examples   Crank Length    Conclusions
Questions and Answers

Q: What is this Q-factor thing?
A: Q-factor is a commonly used term which refers to the distance between the pedals.

Q: How do you figure out what your Q-factor is?
A: This distance can be easily measured by removing one crank arm and turning it around so both crank arms point in the same direction, and then measuring the distance between the outside of the crank arms. This measurement is normally in millimeters.
A less accurate, but tool-less method to measure the Q-factor is to measure from the outside of one crank arm to the outside of the seat or boom tube, do the same for the other crank arm, and then add these measurements to the width of the seat or boom tube. 

Q: Why do I care about Q-factor?
A: Usually because you are trying to minimize it in order to make the nose of a fairing smaller, sometimes to optimize it to better match it to your hip width in order to minimize a hip, knee, or ankle pain situation.


Making it Wide
It's easy to make it wide. Just get some MTB cranks and a wide bottom bracket (BB). If that's not wide enough you can always add pedal extenders...


Making it Narrow
Mostly what I'd like to talk about are methods of narrowing the Q-factor, because that's what many of us need to do when building a streamliner, or when wanting to keep ourselves in a nice aerodynamic package. The two things you can change to narrow the Q-factor are the cranks and the BB. Cranks are not typically rated by Q-factor, but MTB cranks are typically about 20mm wider than road cranks. BBs commonly come in widths between 107mm and 118mm.

When thinking about using a narrower crank, think about how many chain rings you will need. If you will need a triple chain ring, you probably want to stick with an MTB crank, if you only need two, you can use a road crank, and if you only need one chain ring you can use custom narrow crank, a specialized BMX crank set like the Odyssey Black Widow Euro (See Black Widow Lites cranks on DansComp.com) or an obsolete French crank set like the B.S.A. Nervar.

If you still need to get narrower, you can use a custom BB. Phil Wood makes a 96mm BB as a stock item, which requires a narrowed BB shell, and a special Phil tool to install it. 

If you still need a narrower Q-factor, you can hack a couple inches out of a perfectly good 107mm BB spindle and weld it back together, making it about 53mm. The Coslinger team used this technique on their Coslinger streamlinerThom Ollinger says he cut about 2 inches out of  the spindle. To make it work he used a hollow BB spindle, cut it, and pinned it together. He made a fixture to hold it straight and with the crank flats lined up, and then TIG welded it. He then built up the bearing areas with weld and turned them on a lathe, leaving about .020 on the diameter. After that he put the part between centers and ground the diameters to match the inside diameter of the bearings. There is only .060 between the two bearings. This allowed them to have a Q width, plus shoes width of 12 inches. While a BB narrowed in this manner is perfectly good for racing, the bearings are so close together that there will probably be a little play and reduced bearing life. Also available is a custom manufactured 61mm BB spindle and bearings from Phil Wood. Thom had Phil Wood make up a small run of these beauties. 

 

This custom Phil Wood BB spindle and bearings require a custom BB, but allow us to have a 4.1 inch Q factor with the Shimano cranks, for a total pedal box width of 13.1 inches. 


Q-Factor Examples

BB Crank Q-factor Total width with shoes
107mm  Campi MTB 171mm (6.7") 15.7"
107mm 

Shimano 105sc

151mm (5.9") 14.9"
107mm  Odyssey Black Widow Euro 130 mm (5.1") 14.1"
96mm Shimano 105sc 141mm (5.5") 14.5"
96mm Odyssey Black Widow Euro 119 mm (4.7") 13.7"
61mm Shimano 105sc 105 mm (4.1") 13.1"
61mm Odyssey Black Widow Euro 84 mm (3.3") 12.3"
53mm   Odyssey Black Widow Euro 76 mm (3.0") 12.0"
53mm   custom 64 mm? (2.5") 11.5"

Crank Length
Reducing crank length is also important toward making the nose area of your shell smaller. Cranks are typically 175mm. Even if you have the monster long legs, 165mm cranks are not going to hurt your power or cadence. Cranks down to 145 mm in length have been used to good effect in Streamliners by racers like Robert LaFleur in his Zebra bike . Cranks like the Odyssey Black Widow Euro are available down to 150mm in length. Cheaper MTB or BMX cranks, which are not
particularly light weight or low Q can be re-bent to acheive a lower Q. Re-bending cheap cranks is the method used by the George Georgiev of the Varna team to achieve low Q. Most higher quality cranks are not re-bendable. Changing from 175mm to 145mm cranks can save you over 2.5" in nose height! Using a 165 crank, and the BB height on the 'Cuda, which is about 12" above seat height, I found a pedal box height of 23 inches (size 11 feet). Shortening the cranks to 155 saves about an inch, so that would be around 22 inches. A 145 crank gets you down to 21 inches height.

Mechanical means can also be used to reduce the pedal box size. The Kingsbury team uses a complicated crank system called the K-drive to obtain an elliptical pedal path. This is probably as close as you can come to a linear drive system without all the power loss associated with linear drive systems. 

To make pedal box measurements you can get someone to watch you pedal on a trainer and do some measurements. Sean Costin made a video of himself to observe how he pedaled, which was how they optimized the Coslinger fairing to his exact pedal stroke. Alternatively you can just hang your shoes on your pedals, and start measuring. The top of the stroke is pretty easy. Almost everyone's stroke is with the toe straight up from the pedal at the top of the stroke. The bottom of the stroke varies more by BB height and personal pedaling style. If you pull on the pedals more, your foot doesn't hang down as far. Even though you can make your foot hang straight down at the bottom of the stroke, this never actually happens while you pedal, so make sure you measure this well, and you can potentially save a lot of pedal box height.

Summing it up
So, with off the shelf parts (96mm BB with a Shimano 105sc crank set), you can save 1.22 inches off the Q-factor of your typical recumbent crank set, and make the nose of your fairing that much narrower. With these 165mm cranks, this would give you a pedal box size of 14.5 inches wide, by 23 inches high.  If you want to go completely wild, you can make the nose of your streamliner over 4 inches narrower, and a couple inches less in height. That should be worth some speed!

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