Stokemonkey compatibility

Stokemonkey was conceived as an accessory for Xtracycle's hitchless trailer, which in the early 2000's more or less invented the longtail cargo bike as a category. At this writing (2014), the most compelling installation targets aren't Xtracycle conversions as in the beginning, but purpose-built longtails like Xtracycle's Edgerunner, Yuba's Mundo, and Surly's Big Dummy. Actually just those three. Compatible front-loader cargo bikes include WorkCycles Kr8 (Monkey Kr8?), CETMA's Largo and Margo, and Metrofiets. Get one of these, and there's not much left to talk about, compatibility-wise.

Stokemonkey is designed to work with longtail bikes as described in the compatibility notes. In all other cases, fitting Stokemonkey means embarking on an engineering adventure whose success cannot be guaranteed. We can’t provide much if any remote support, nor will we extend our warranty for experimental installations. That said, Stokemonkey has been successfully adapted to a number of special vehicles including recumbent trikes, more than one velomobile, some pedicabs and bakfiets-style cargo bikes.

The rest of this document concerns compatibility with ordinary 26"-wheeled bikes converted to longtails using Xtracycle's FreeRadical hitchless trailer.

    1. Rear triangle clearance
    2. MTB-type handlebar diameter
    3. Throttle-compatible shifters and control layout
    4. Square taper bottom bracket spindle
    5. Chainstay clearance
  1. Recommendations
    1. Frame and fork material
    2. Brakes
    3. Wheels and tires
    4. Bike style
Rear triangle clearance
frame partsclearance
There must be room for Stokemonkey within your bike’s rear triangle. This is the part of your bike’s frame that is normally occupied by the rear wheel, but that the FreeRadical makes free for Stokemonkey. Most medium and larger-size frames have enough clearance. Most “compact” frames and those with unusually low seat stays won’t have enough. Note that the use of Albatross or similar swept-back handlebars (noted below) can permit the use of frames that would otherwise be uncomfortably large. Measure from the center of the cranks or bottom bracket shell, as indicated in the second diagram at right, to the nearest point on the seat stays, perpendicular to them. This line must be at least 13.5″ (34.3cm) for Stokemonkey to fit. If your frame is very close either way, discuss the situation with us.
MTB-type handlebar diameter
Stokemonkey’s throttle fits only 7/8″ (22.2 mm) outside-diameter handlebars. This is the kind found on virtually all except road bikes; “drop” bars have too large a diameter. You can, of course, change from road to the required bars, but you’ll need to change your brake levers, probably your shifters, and possibly your stem too if you do. Consider also that road bikes tend to make poor cargo bikes due to their low-volume tires and focus on light weight.
Throttle-compatible shifters and control layout
Good throttle ergonomics are important to safety, but may require part changes and experimentation to achieve. The throttle must be mounted so as not to interfere with your shifters or brakes, nor to be interfered with by them. The following discusses problems and suggests solutions that you should weigh for yourself, based on your own equipment and preferences.

The default thumb-style throttle is designed to be mounted on the left side of the bars, leaving your right hand free to operate the dominant (or only) shifter. Other throttle styles are usually available. Trigger-style shifters are often not compatible with the thumb throttle (not on the same side, at least). Bar-end, twist, old-school thumb shifter, or downtube shifters work. You may also choose to remove any left shifter, foregoing multiple front chainrings altogether, as the assist can diminish the need for very wide range gearing.

Here are 3 example setups known to work well:

  1. Swept-back Nitto Albatross bars with bar-end shifters. These wide bars are smart for the leverage they provide with heavy loads. They also make the steering feel lighter than narrower bars, which is usually welcome on Xtracycle conversions. And as most everybody outside of North America knows, bars that put your wrists facing each other are much more comfortable than straight ones. Mounted upright, they bring your head to a commanding, comfortable height, and can permit the use of larger frames whose top tubes would be too long with unswept bars.
  2. Thumb shifters, with whatever bars you like. Thumb shifters in friction mode, like bar-end shifters, and unlike “click” shifters, will be even quieter than the motor, and can often cure chain skipping problems related to finicky indexed shifting setups with Xtracycles’ longer control cables and chain run.
  3. Hub gears such as the Shimano Nexus 8, NuVinci CVT, or the superlative Rohloff Speedhub are particularly good matches for Stokemonkey. All gears are selectable with one hand, whether you are moving or not, and the hubs build into very strong wheels.
Square taper bottom bracket spindle
The cranks we supply mount to square-taper bottom bracket spindles, not the newer splined types. We usually stock suitable bottom brackets in various lengths if a substitution will be required; your local bike shop can handle this as well. You can also supply your own crankset if you really want to keep your splined bottom bracket, want different gearing, etc. A right-side spider mounted on the left is another possibility, but you’ll need to provide a second right pedal and use a thread-locking compound to keep it from unthreading during use.
Chainstay clearance
The motor drive chain’s lower run must clear the left chainstay. Most steel frames will have adequate clearance; many aluminum MTB frames might not. If it doesn’t, you will need either a longer bottom bracket spindle, a frame with narrower chainstays, and/or some chainring spacers. Refer to the photographs at right to check.

  1. Shift your bike into the outermost front chainring.
  2. As indicated in the first photo at right, pull the lower run of chain up toward the middle of the rear triangle, keeping it in plane with the chainring, so that it crosses the chainstay.
  3. Looking from above, as in the second photo, assure that the chain will clear the chainstay by at least 1mm, again assuring that the chain is not bending to either side as it leaves the chainring.

If the chain clears on the right side, it will likely clear on the left after the spider is installed there, because these things are generally symmetrical.


Frame and fork material
We like steel. Aluminum and other materials used in bike frames can be as strong or stronger than steel, but they tend to have fatter stay tubing, contributing to some of the clearance liabilities cited above. These can be worked around by moving the cranks outward, but then your pedals are further apart, so more of your pedaling energy contributes to the torsional flex that bothers some longtail users. Wider pedals also limit high-speed cornering clearance, and can lead to knee pain for some.

But the most important thing in favor of steel is that it tends to fail slowly, with warnings. Any material will fail if subjected to enough stress, which, frankly, is more likely to occur with bicycles used to replace 4-wheeled steel vehicles weighing tons than with others. Aluminum and carbon fiber tend to fail suddenly and completely. Steel flexes a great deal first, and then is more likely to bend, buckle or crack in a less catastrophic way. We ride Stokemonkey-equipped aluminum bikes without worry, but after a certain excess of cargo and speed, we’d feel better with steel, especially with passengers aboard.

If you intend to use Stokemonkey to haul hundreds of pounds up thousands of feet, you can’t have brakes that are too good, because you’ll be coming down. Large-rotor disc brakes are a good idea. Anecdotal reports suggest that rim brakes on heavily loaded descents contribute to blowouts.

If you do want to go with discs, note that your frame doesn’t need disc-brake tabs; they’re on the FreeRadical. You will need to replace your fork if it doesn’t have them.

Wheels and tires
The most frequent component failures with Xtracycle conversions are wheels. This is no doubt due to many peoples’ reluctance to devote top-quality bicycles to lowly cargo duty, but this reluctance really doesn’t make any sense when your and your passengers’ safety is involved. Stokemonkey will let you move loads that exceed the trailer’s 200lb chassis rating (not that you should). The typical limiting factor is wheel strength. We entrust our heavier, more precious loads to hand-built 24″ or 26″ wheels with 36 tight, heavy, stainless spokes, 3-cross, on downhill-class rims.

While mountain bikes typically are the best candidates for Xtracycle conversion due to their high wheel strength and frame stiffness, mountain bikes are usually sold with knobby tires that are silly for street or smoother trail use. Essentially the only time knobby tires are beneficial is when the riding surface is softer than the tires themselves. If you intend to use Stokemonkey in mud or sand less than half the time, please use high-volume smooth-tread tires for optimum traction, comfort, and quiet efficiency. We are partial to Schwalbe Big Apples, which render mechanical suspension somewhat redundant.

Bike style
Among the best candidates for conversion are mid to high-end steel mountain bikes manufactured before the late 1990s. The Specialized Rockhopper and Stumpjumper lines from these eras are good examples, as are Bridgestone MB-1 through MB-3 — there are many more. These bikes tend to be overbuilt, which is valuable in a load-bearing bike, and they can often be had for very little money. They are likely to come with thumb-shifters and meet all the requirements above. With some love and part swaps, they can rival in appropriateness new bikes costing 5 times as much.

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