Technical FAQ: Shortening cranks for kids, saddles, and compatibility

Lennard Zinn tackles reader questions about, among other topics, adjusting cranks for children to use safely.

Adjusting cranks for kids

Dear Lennard,
My wife and I plan to do some touring with our kids over the next few years, so I picked up a triplet and have set it up with a “kid-back” drop crank for our smallest (almost 4 years old) stoker and bolt-on crank shorteners for our biggest (almost 6 years old).

Recently, after about a 12-mile ride, my oldest daughter complained about the cranks hitting her ankle. The crank shorteners move the pedals outward a good half inch on each side, not only causing interference but also significantly increasing the Q-factor.

So, my first question is: Do you know if such a high Q-factor is a concern for a growing body? Especially if we build our way up to significant miles (our goal is to do 40-50 miles a day for several days at a time)?

One option I have read about and would be comfortable doing that would solve both the Q-factor and the interference problem is just drilling and tapping the crank arms and shortening them permanently. My daughter weighs less than 50 pounds so I’m not very concerned about her generating enough torque to break a crank modified in such a way (please let me know if that is foolish thinking). However, this would mean making a new set of cranks every time she has a significant growth spurt. (They are good old-fashioned square drive so this would not be exorbitantly expensive but still seems sub-optimal).

I know you make custom cranks so I thought you would be the best person to ask. Is there another option for shortened, adjustable cranks that don’t increase the Q-factor so much? Or that don’t leave so much crank extending above the pedal at the top of the stroke where it hits the rider?
— Al

Dear Al,
I think it is a poor idea to subject small, growing kids to extended periods of ultra-wide Q-factor. Because of their short legs, the same Q increase that might angle our legs out slightly will splay their legs out at an extreme angle. And then, with the mileage you’re planning on, repeatedly turning their hips and knees through the pedal stroke for hours on end when the articular surfaces in their hips are bearing on areas they are not meant to because of the leg angles could do some damage. Of course, at that age, all of their synovial fluid is flowing nicely, and all of their articular surfaces are super smooth and nearly friction-free, but to subject them to such sub-optimal ergonomics seems like a gamble you don’t want to take. As in the photo you sent, her feet automatically come way in, since that is what her body naturally prefers, but constantly banging her ankles on the cranks is not nice, either.

There is nothing wrong with drilling and tapping the cranks shorter for small kids, as long as you drill the pedal holes parallel to the bottom bracket spindle. I used to do this on numerous cranks when my kids were small, and those bikes continue to get handed down to family and friends, never with any problems.

You need to use cranks that are not hollow and are wide enough to provide enough material around a hole drilled along its length. Since cranks are generally splayed-out for chainstay clearance, the pedal threads are not perpendicular to the crankarm. Thus, you can’t simply clamp the arm and drill it or you will end up with a pedal that precesses wildly around a changing axis, rather than around the bottom bracket, as the crank goes around.

Clamp the spider arms of the drive crank flat on the drill press platform (or, better, milling-machine table), and drill the new pedal hole straight. Then put the tap for the pedal threads in the drill press chuck to hold it straight, and turn it by hand (with plenty of cutting fluid). Similarly, with the non-drive crank, use the small flat on the back of the crank head where it meets the spindle as an index for holding that one perpendicular while you drill and tap it.

Another option is an adjustable crank with normal Q-factor. This one is great.
― Lennard

Matching proper saddle setback

Dear Lennard,
Your garage door method to match proper saddle setback is brilliant! To match proper setback on bikes with different saddles, do you recommend measuring from the wide point of the saddle, which differs from saddle to saddle, instead of the nose of tail? I have a Specialized Toupe, Specialized Romin, and Selle Italia saddle on different bikes, and each one has a different wide point, although they are all about 27cm long.
— Chris

Dear Chris,
Actually, I measure all of the saddles to the sit-bone contact of the rider. If they are not identical saddles, I definitely measure neither to the nose nor to the tail.

I feel for where my sit bones are contacting the saddle as I pedal. I mark these points and draw a line between them across the saddle with a paint pen; I measure out from the garage-door channel to that line. I do that on all of the saddles of all of the bikes I’m matching. And, if the readjustment of the saddle and handlebar position is significant, as I get closer to dialing in the position, I re-check (and re-mark if need be) the positions of the sitbones on the saddle, since that may change as the relative positions of the saddle, pedals and handlebars change.
― Lennard

Mixing drivetrain parts

Dear Lennard,
I have M980 shifters (XTR 10-speed), and I want to use a road crank, say an Ultegra FC6800 52-36 combination, so I’ll need a front derailleur flat bar style FD-R770?

My rear derailleur is XTR 10-speed (M986) with a 10-speed cassette of 11-34.

My question is, because Shimano specified it as an “11-speed crank,” would it be compatible to my setup?
— Paramount X

Dear Paramount X,
Yes, that crank will work fine with your chain and rear derailleur.

I’m not sure if that front derailleur will work with your left shifter. The SL-R780 would be the standard shifter for that front derailleur. Since you’re committed to it, you might as well give it a try and let us know how it works. If you have to get a left road flat-bar shifter, the additional expense relative to that crank you’re buying is minimal. As you probably know, Shimano does not use the same cable pull on road and mountain 10-speed rear derailleurs (so right hand SL-R780 and SL-M980 levers are not interchangeable), so it may not use the same cable pull on road and mountain front levers for 10-speed systems.
― Lennard

Dear Lennard,
Will an XTR 11-speed 11-40t cassette work with SRAM Force 1 long-cage rear derailleur and shifters? This would get me nearly the range of a SRAM 10-42 without upgrading my rear wheel to an XD freehub. Would I have to use the XTR asymmetric chain, or will any 11-speed chain work?
— Todd

Dear Todd,
Yes, that cassette will work with that SRAM Force 1 drivetrain. Any 11-speed chain will work, although I imagine that the XTR asymmetrical one might work slightly better, since it was designed specifically for that cassette.
― Lennard