Wednesday, September 23, 2009

Auto-Mini Executive

I've been interested in older folding bikes for a while and now have a few that I tinker with. I had one as a kid that I now think may have been an "Auto-Mini", an Austrian-made bike with 20" wheels and a large main tube that is rounded on the top and bottom but flat on the sides. Various sources refer to two different models (Junior and Executive), but the difference is unclear and both of the Executives I have with original decals say Junior on the "T" of the Auto-Mini logo. I recently picked up a few of these bikes in various states of disassembly with the idea of putting together a recumbent quad bike with two of the frames side by side. These things are pretty common in Toronto but I haven't been able to find much on them on the internet in any one place.

Folding of the Auto-Mini is done by releasing a simple clamp lever. A lot of older folding bicycles I have seen require loosening of a nut or bolt which takes longer and requires a tool in some cases. Recently I learned of another very similar-looking folder that uses the same type of clamp, the Leader Voyageur (misspelled in the link as "Voyager"). Close inspection of the decal on the front reveals that it was made (or at least assembled) by the Victoria Precision Works of Montreal. This bike looks almost identical to the Auto-Mini in most respects, but appears to have a one-piece crank and a different type of quick release for the stem. The Auto-Mini has a slot cut into the steering column and the quick-release clamps around that, making direct contact with the fragile threads. The Voyageur has a more normal-looking headset and stem with a quick release acting on the stem binder bolt, which I think may be a more durable design. Another "Canadian-made" folder, the Supercycle Traveler, looks to be pretty much the same as the Voyageur.

The Auto-Mini's hinge clamping mechanism and quick releases (for stem and seatpost) are marked "brev. stop" or "stop brevettato". It turns out that "brevetatto" is not a brand name but an Italian term meaning "patented". Is "stop" the brand name? In any case, one nice thing about these quick releases is the fact that they use a 17 mm hex nut. Similar quick releases I have seen on folders use a rounded nut with a minimal amount of knurling, making it difficult to get them very tight without marring them with a tool they weren't designed for.

Originally, these bikes game with a generator and headlight, but on all the bikes I've seen in person they were long gone, with only the generator mount built into the fork remaining. There's a mount on the back of the seat tube for a pump, also missing on my bikes. The original rat-trap style rear rack was made by Pletscher of Switzerland.

The Auto-Mini's original brakes are different in the front and back, which made me think that one set had been replaced at first. The front brakes are "Arai Gold" (appears to be chrome-plated steel) and the longer-reach ones in the back are unbranded, made of aluminum alloy and look a lot fancier. The alloy brake levers are pretty lightweight (pair weighs only 93 grams) and also unmarked.

The bike's crankset is a 2-piece cottered system (only the left crank is detachable from the spindle) with an unthreaded bottom bracket. Apparently this is what's known as a "Thompson" style bottom bracket. I find the original pedals a bit small but that might just be me. There is a 44 tooth chainring in the front, and both of the original rear wheels I've got have 18 tooth sprockets on different 3-speed hubs. One is a Sachs Torpedo Dreigang 415 and the other is a Sturmey-Archer AW. Martin Bergman's tables list the 415 with ratios of 0.73, 1.00, and 1.36 (total range 1.87), and the AW's ratios are 0.75, 1.00 and 1.333 (range of 1.78). According to the late Sheldon Brown, the AW line was introduced in 1936 is "far and away the most common model" of hub made by the S-A company. I think the "W" refers to the "wide" spacing of gear ratios.

Another commentator describes an Auto-Mini with a Sachs Torpedo Duomatic hub. The Duomatic is a 2-speed hub with a coaster brake that is easily mistaken for a single speed. After reading the above article I realized that I had one of these on another folding bike, an Italian-made "Amica" that I got in 2005 or so. The Duomatic is shifted by backpedaling, and a related 2-speed model, the Torpedo Automatic, shifts at around 16-18 km/h using a centrifugal clutch. The 2-speed Torpedo models' top gear has a 1.36 ratio just like the Dreigang, so the only difference apart from the coaster brake appears to be the absence of the lowest gear. With a small-wheeled bicycle the distance traveled per rotation of the wheel is much smaller than on a regular bike anyway, so second gear is usually low enough for even a steep hill.

The bigger problem on a small-wheeled bike in my experience is mediocre performance in the top end. Three possible solutions come to mind: a larger drive wheel, a smaller gear in the back, or a larger gear in front. The auto-mini's frame does have enough clearance to allow a somewhat larger drive wheel, but 22" wheels are not common and 24" would be too big. A 22" wheel with internal gearing might need to be custom built. Brakes would be another issue. Sounds like a lot of effort overall for only a 10% gain.

Replacing the cog on a the stock hub with something smaller than an 18T may be the cheapest and simplest option. A 14 tooth cog would give 28.6% more distance with every stroke, and nothing else would need to be changed on the bike.

The third option, replacing the crankset, is complicated by the weird bottom bracket but not impossible. The user "zepi" on added a custom-machined 72T chainwheel to his Auto-Mini using a 40 mm Shimano BB pressure fit to the shell. The bike's owner is identified on Mark Rehder's Auto-mini page as Martin Zeplichalt.The rear hub is a Duomatic with coaster brake, and 16 and 19T cogs are mentioned at different points in the forum discussion. The 72T gear is so big it looks almost comical, probably 2/3 the diameter of the wheel, but I bet the thing flies.