OPTIMAL FAT BIKE
CHAINLINE AND “Q” FACTOR
*** currently under construction ***
For 1x12 systems, this is the distance between the centerline of your frame and the centerline of your chainring. (labeled as “CL” in the diagram below)
Benefits of a properly configured chainline :
reduction in chain, chainring and cassette wear and noise
reduction of dropped chains, especially when back pedaling in largest cassette cog
additional clearance from tire sidewall
optimal maximum tire and rim sizing for any given fat bike frame
A term coined by Grant Peterson, during his tenure at Bridgestone Bicycles. In general, a narrower “Q” factor on a bike, allows a narrower pedal stance, which is better for power transfer, and handling. For some, narrower “Q” also reduces strain on the knees and other ligaments and muscle groups involved in pedaling. “Q” stands for “quack” and is coined from the wide stance and waddling gait of ducks, according to Wikipedia. All bikes benefit from a lower “Q” in better cornering performance.
For everyone, there will be a point where an increasing “Q” factor on a bike becomes wide enough that it negatively affects handling and comfort. In general, this usually has a greater (negative) effect on shorter riders -or those width a shorter inseam, and less of an issue for taller riders with wider hips. It is a debatable topic for Fat bikes, as some find it noticeable, while others find it perfectly fine, and actually more comfortable than road bikes.
Referring to the diagram above, total “Q” is the distance labeled as “W5” from both the driveside and non-driveside cranks arms, to the centerline of the frame, plus the width of both crank arms. In other words, the distance from the outside edge of the driveside crank arm, where the pedal attachment location is, to the opposite side of the non-driveside crankarm. For Fat bikes capable of handling 5” wide tires, this number ranges roughly from 200mm to 220mm. For Fat bikes with a 3.8” (true) tire maximum width, the total “Q” may be as low as 185mm, and use a slightly narrower bottom bracket frame spacing.
Let’s put it this way, there is not a Fat bike out there, that has a narrow 185mm “Q”, that can also take 26 x 5” (true) width tires on 100mm rims at 30PSI, with a chainstay length under 460mm. It’s pretty much impossible to do, unless you remove both chainstays, and somehow make a frame stiff enough without them present. Even then, the chain will borderline be hitting the tire in a 1x12 system, and there is no mud or snow clearance. Any collection of mud or snow on the tire sidewalls, will deposit itself on the chain, and quickly foul up the rear derailleur. (Look for that design in the future, possibly using two 90degree wormdrives, that will replace part of the chainring on current SRAM DUB and RACEFACE CINCH DN NW cranksets).
SRAM DUB TECHNOLOGY
RideFATbikes.ca carries the complete line of SRAM DUB Cranksets for your MTB, PLUS or FAT bike
(Click on any of the cranks, to zoom in)
WHAT IS THE BEST SETUP FOR FAT BIKES?
Simple, the narrowest “Q” for the widest tires you are going to run on any particular Fat bike frame, and still have proper mud/snow clearance, along with smooth shifting and chain operation, over the type of terrain you ride and race on.
It would take a long time to dive into all the combinations, for all the major Fat bike frames, wheelset/tire, and crankset/chainring configurations - so we are going to focus on only the cranksets we use in building our 2018/19 FATBACK and FOES RACING Fat bikes. You can apply this information, and research the best setup for whatever bike you have, or are looking to build.
The chart above illustrates the latest FALL 2018 FAT BIKE crankset data from both SRAM (DUB) and RACEFACE (CINCH), using a standard BSA100 threaded, or PF121 press-fit bottom bracket standard. (170 OLD / RF169 compatibility). The Fat bikes we custom build by FOES RACING and FATBACK BIKES, both use the BSA100 standard.
Highlighted, is the overall “Q” factor of each crankset, the total crankset weight (including bottom bracket assembly), and the trail rating of that specific crankset.
Note that the SRAM (DUB) cranksets all have a 66.5mm chainline, and RACEFACE (CINCH) cranks have a 68.5mm standard chainline, that can be increased to 75mm if the chainring is flipped.
A wider chainline, allows wider tires to be installed (26 x 4.8” on 80mm rims for example), and is especially useful on 1x12 systems shifing into a 50T large cassette cog, when there is a shortened chainstay (440mm or less).
You can clearly see the balance created when comparing :
BEST “Q” for C/S to Tire/Rim Ratio
If you can hit the following numbers, you are at about the best obtainable scenario for each test condition : (minus mud/snow clearance requirements)
MINIMUM FRINGE FAT BIKE CONFIGURATION
Total “Q” = 189, with chainstay length at 430mm, bottom bracket at BSA73/PF168, and wheelset/tire combo of 26 x 4” on 80mm rims at 30 PSI inflation.
OPTIMAL “NO COMPROMISES” FAT BIKE TIRE CONFIGURATION
Total “Q” = 203, with chainstay length at 440mm, bottom bracket at BSA100/PF121, and wheelset/tire combo of 26 x 5” on 100mm rims at 30 PSI inflation.
The Fat bike optimal “Q” factor boundary is between those two ranges. If you are well above those numbers, (like Q > 220) then your bike is doing the duck waddle…and it might be time to look into some upgrading.
CHAINLINE and “Q” FACTOR Design Variables
There are at least 7 variables frame designers have to deal with when maximizing on only 1 or 2 categories below, before they lose chainline, or the (FAT bike category) tires start hitting the frame :
Total “Q” factor
Minimum C/S (chainstay) length
Maximum tire width and height
Maximum rim width and height
minimize bottom bracket width
minimum heel strikes to the frame
There are bikes that reach the first zone, like dedicated PLUS bikes that have been modified to use Fat Bike tire widths of 26 x 3.8” or 4.0” (on 70-80mm rims) to break into the bare minimum region of the FAT race category. But these same bikes cannot also work with 26 x 4.8” to 5.05”+ tires on 100mm rims. Their frames are not set up to do both, and start to fail their optimal configuration as you put wider 4.6” tires on 26 x 80mm rims on the bike, and force them into the frame.
NAILING FAT TIRE MAX SIZES, LOW “Q”, MINIMUM C/S LENGTH, and COMPATIBLE with the best 2018/19 SRAM DUB and RACEFACE CINCH crankset
Two Fat bikes that nail the second optimal boundary configuration are the FATBACK SKOOKUM and FATBACK RHINO FLT.
Best “Q” factor with a BSA100 bottom bracket
accepts all major low-Q cranksets, including the best by SRAM DUB and RACEFACE CINCH series
440mm C/S length