Tuesday, May 28, 2019

Tandem restoration tidbits: Tripelizing the Stronglight 49D tandem crankset and one more cog

Peugeot Tandem TM 8 Col du Glandon
The original gearing of the typical 10 speed bike-boom tandem does not really
match todays expectations for loaded touring in an alpine environment.
Being produced in 1980, the Peugeot tandem TM 8 is a classic ten speed bicycle, with 52/42 chainwheels at the front and a 14-28 cassette in the back. And what is already a fairly narrow range of developments for a single bicycle by todays standards even on moderate terrain, is very limited on a tandem, and even more so as soon as there is any climbing. The comfortable cadence range for a team is narrower than for a single rider, in particular to the bottom end where its not as easy to ride out of the saddle. On the other side, a tandem easily reaches high speeds already on gentle inclines due to its aerodynamic advantage.

I thus was looking for a solution, especially to extend the bottom end:
  • There are some 5 speed thread-on wide-range freewheels. But they are not very common, the gear steps feel already slightly large and finding a well preserved specimen on a tandem freewheel with french threading appeared close to hopeless.
  • Another option would be to use at least the smallest inner ring available for the Stronglight five-star crankset with a bolt circle diameter of 122 mm. It appears to be 38 teeth (source), but again I couldn't find any and it wouldn't be a massive gain.
  • Converting the Stronglight 49 D crankset to a triple, which is again rare, or using Specialites T.A. chainrings (or one of the modern copies) to build a compact or triple setup turns out expensive. And the original 49D double with the trapezoidal cutouts in the outer 5-star chainring is particularly beautiful and its easy to find spares.
In order to get a significant extension, I went for both a homebuild tripelizer and a modification of the Atom tandem freewheel with an added cog:

Tripelizing the Stronglight 49 D crankset 

Adapter plate to tripelize the Stronglight 49 D crankset with 5-star chainrings.
The inner bolt circle adapts to the 50.4 BCD crank, the outer bolt circle mounts
a modern standard 74 BCD inner ring.

I finally decided to make my own tripelizer, but instead of replacing the inner 122 BCD chainring, I made a design which directly mounts to the 50.4 BCD of the crank and accepts cheap standard 74 BCD inner rings with standard chainring bolts on the other side. Laser machining of the simple part from 4 mm high strength aluminium turned out to be affordable. Getting the inner ring to clear the chainstay requires an external spacer on the drive side. On the TA Axix crankset which the previous owner had installed in replacement of the original, this was possible without changing the spindle length or even entire bottom bracket: TA provides internal spacers to adapt the Axix to larger bottom  bracket shells. Use of a single spacer on the left (crossover) side made it possible to properly install the bearing preload with enough threads remaining for the lock-ring.

tripelized Stronglight 49 D tandem crankset
Tripelized Stronglight 49D crankset. Mounting the third chainring directly to
the 50.4 BCD of the crank keeps the light appearance of the trapezoidal cut-
outs in the 5-star chainring which according to some makes the 49D double
one of the most beautiful cranksets.
Shifting this wide range triple required a new front derailleur which I anyway wanted to change, as the original one had the plastic (delrin) clamp which has a tendency to crack. As we anyway had decided to modify the frame, I added a boss for a Simplex SJ A 223 direct mount triplederailleur.

Modifying the Atom Tandem Freewheel

As there was quite a large gap between the 28 tooth cog of the freewheel and the Atom hub brake on the one hand, and as I had a Specialites T.A. 32 tooth inner chainring left over from another project, I decided to add it as a sixth cog to the freewheel.
Sprocket adapter.

All that was required is a (laser cut) sprocket adapter and a few holes in the 28 cog of the original freewheel.
Atom tandem Freewheel
Additional 32 tooth sproket, adapter plate, Atom tandem freewheel with the last
sprocket drilled to mount the sprocket adapter.
Atom tandem Freewheel
Left, adapter placed on last sprocket, right additional 32 tooth sprocket.
Atom tandem Freewheel
Atom tandem freewheel before and after the modification.
Atom tandem Freewheel
Atom tandem freewheel before and after the modification.

Vintage shifting

Simplex derailleur and retrofriction lever
Vintage shifting. Simplex rear derailleur (modified SX410/SJ810) (6-th cog not
yet installed), Simplex SJ A223 front derailleur, Simplex SLJ retrofriction levers.

There are some negative opinions on vintage shifting out here:

"Touring with terror" ? Would you use a SJ 810 on a Peugeot Tandem (source)?
and there:
SX 410 (source).

that I enjoyed reading with delight - before owning this tandem. Fortunately our experience is a little different:
  • The combination of exactly these two 'dogs' - a Peugeot SX 410 parallelogram and knuckles (Disraeli Gears) with a SJ810 pulley cage (Disraeli Gears) does its job with period correct look and feel, and shifts even under load. The (too narrow modern) chain may occasionally skate on the smaller cog, but this is not the fault of the derailleur. The pull of the Simplex SLJ retrofriction lever is just enough to shift this contraption over 6 cogs in original 5-speed spacing. Cross chaining into big-big is not possible - maybe one day I will take out the jigsaw and make a custom pulley cage.
  • The Simplex SJ A223 (velobase) front derailleur shifts (a modern 6 speed chain) extremely well over our 52-42-30 tooth chainring combination.
Whatever this is worth. But then we rode this combo over the alps on a loaded tandem without issues.

Gear range and development

The addition of a 3rd chainwheel and a 6-th sprocket lowered the minimum development from 3.3 m to 2.0 m. What this means becomes clear in a simple example: Riding the loaded tandem up steep grade at a speed of 5 km/h, this increases the cadence from a  40.4 to 66.6.

Development in meters for a 35-622 tire. Tripelizing the Stronglight 49 D
crankset plus the additional cog lowered the minimum development from
3.3 m to 2 m.
For many passes in the Alps but even for some of the short but steep climbs in Blankenese close to our home this is just the difference between walking or riding the climb.

No comments:

Post a Comment