Apparently, both my wife and I have been contemplating divorce, so we agreed to buy a tandem MTB. This one-way-ticket to splitsville has so far been crash-free and deceptively enjoyable. While I have no misgivings about our future -for tandem bikepacking- I am looking forward to day-trips across the Pisgah gravel and low-tech trails. However, my wife suffers from periodic wrist pain, often from her day-to-day tasks at work or at home. This pain can be exacerbated during cycling, so I sought out a suspension stem for the stoker cockpit on the tandem which might take the edge off of unexpected bumps and road vibration. And, perhaps allow her to retain the strength to strangle me as I steer our marriage, I mean, bicycle towards unchartered waters.
As you might imagine, a tandem stoker suspension stem does not seem to exist. (If you've stumbled across this blog for bicycle or automotive tech, it's because you're looking for something that doesn't exist. Or you're lost. Or both.)
I freely waste a lot of my time, but I try not to reinvent the wheel. There are about a half-dozen different suspension stems on the market which could be modified for our purpose. I ran across a gently-used RedShift Shockstop suspension stem on PinkBike for a reasonable price. It doesn't offer a lot of travel, but online reviews from cyclists confirm that it's great at damping vibrations. RedShift's product-line generally appears well-engineered and of high quality, so adapting one of their Shockstop stems was more logical than modifying some junk from Am*zon.
RedShift Shockstop stem, with spare elastomers and tool.
After removing the bottom plate, you can see the orientation of the elastomers within the stem, which compress under vertical force.
The copious blue Loctite and press-fit of the pivot-bolt suggests RedShift does not intend the end-user to disassemble the stem. I was surprised to find a pair of Enduro bearings. Since I planned to reuse the pivot-bolt, I chose to reuse these bearings over the alternative: a simple silicone-bronze bushing. I neither trust the concentricity of the pivot-bolt, nor its durability within a bushing; the bearings address both concerns (and, it was good enough for RedShift to design it that way).
I will have to replicate this shape...
A tandem stoker stem mounts to the captain's seatpost. It has a tubular, sliding extension that allows the reach to be adjusted for the stoker. The design for this adapter is basically 'stick the bearing housing and cruciform extension onto a tube'. Perhaps cutting the RedShift internals and welding it to a tube would seem to be the obvious solution. However, I can't heat-treat aluminum in my garage, which otherwise would leave the weld and surrounding material soft as butter...a sure-fire ticket to an uncontested divorce.
So, start with a solid rod of aluminum round-stock. Turn it down to the O.D. of the stoker stem extension, hollow out the inside, and leave a solid chuck for carving out the internals of the suspension stem.
The large end will be milled to house the bearings and create the cruciform profile that locates the elastomers and limits the stem's upward and downward travel.
After mounting the billet onto the mill and indicating it on the DRO, I drilled a pilot hole for the bearings, milled this section flat (according to the final width of the bearing housing), and bored the bearing bosses to the necessary depth using the boring head on the mill.
I roughed out the shape of the cruciform on one side before flipping it over and repeating the process.
And, using the magic of time-travel, we fast-forward to the (near) finished shape.
(I forgot to take some pictures during the process)
What you missed is tapering the top and bottom profiles of the cruciform; milling around around the bearing housing; chucking the adapter back into the lathe to profile the transition between the "tube" and the bearing housing; and, a bunch of sanding by hand to smooth out all of the angles and sharp edges left by the mill and lathe.
After pressing the bearings in, and test-fitting the stem with the pivot-bolts, I had to massage the bearing housing a few more times to allow clearance within the interior of the stem so that it could pivot without interference. There's no CAD or SolidWorks models here, nor a CNC machine center...just a set of Vernier calipers, a note-pad, and a manual lathe and mill. The important dimensions are spot-on; the rest is 'good enough' since it can't be seen once the stem is assembled.
Here is the finished RedShift 'stoker' stem...
...and, mounted to the tandem.
This modification does increase the minimum length of the stoker stem; however, we still have some adjustability fore/aft around where my wife's handlebars were originally positioned. No riding time yet, but I envision it will take a few rides to test the different elastomers and dial in her preferred compliance.
If you look at the scope of RedShift's product line, you might notice that a lot of their products fall into the "I didn't realize I needed this" category. That's not to suggest they are gimmicky; rather, they've engineered novel solutions and/or elegant integration in areas where the competition is not. Although I suspect the market interested in such a stem would be quite small, it would take an engineer from RedShift less than 15 minutes to edit an existing CAD file and spit out a CAM file to CNC-machine this part. Small production runs, or even turning out individual units 'on-demand', could be priced accordingly. Until then, I'll keep tooling around in my garage.
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