Transportable Tandem II

[Mr. Gangrene Genes]

Howdy, this is my second attempt at building a tandem that will fit on the KCATA Metro bus, or a standard carrier without hanging off the ends. We had a Schwinn Twinn 5-speed before our daughter was born, just too heavy, and way too long. Wife wasn't interested in riding. Sold it for what we had into it, should have swapped out the multi-speed drum brake to build a klunker. Anyway, here is a picture of the first attempt, sort of a Cycle-Truck with a small front passenger instead of a basket.



Got the idea from the Atomic-Zombie Bicycle-Bonanza book that someone gave me at work. He was using the back half of a bike as the front wheel to build a drift trike.
http://www.amazon.com/Atomic-Zombies-Bicycle-Builders-Bonanza/dp/0071422676
Good fun book, he cuts his bikes down to smaller components than I prefer. It is full of interesting ideas.
I did not study Cycle-Truck theory very well, the basket is suspended from the frame. When I strapped a 40lb bag of dog food to the light blue part to stand in for my passenger, the result was ride-able, but not reassuring. Back to the drawing board.

 

[Mr. Gangrene Genes]

I try to look into how others have solved my preceived problems before venturing down the path. One of the packaging issues with a full size tandem is the 26" wheels. How small can an adult ride? Have always liked the Schwinn Run-About, real ones are stupid expensive, just build my own to test the theory. Started with a 1979 16" Scramber because the bottom bracket on the BMX diamond frame is higher than the same size cantilever frame, and the head tube is gusseted. It had a "lucky Seven" 32 tooth sprocket and the crank off a 20". Swapped that to a 39 tooth sprocket known as the "Scramber" sprocket, also available as the low gear off some Varsitys or Continentals. Swapped the factory crank to the one off a 24" bicycle. Had to be taller, so on went the seat and post off an Excer-cycle, and the narrow ape-hangers off a later Stingray. Added the rear fender, and made an adaptor so I could have a side-pull front brake.


Have read that Schwinn made a marketing error by promoting the Run-About to kids. Once graduated to a car they were not going to buy an over-priced clown bike. Some think they should have been sold to Sportsman or campers.
Half of all those RV's with Honda Trail 70s could have held a small bicycle for less than 1/4 of the price.

 

[Mr. Gangrene Genes]

The wanna-be Run-About is fun and adult rideable (6'02" with 34" inseam). However my 200lb, plus a kid is a lot to ask of 16" S-7 tractors. Will save it for a car show bar stool. Went looking for another 20" multi-speed Pacific to match the light blue one I got from my Dad. They're modular don't you know?
Funny thing, these "new Chinese" bikes are from 1986. Found a pink one cheap on Craigslist, am sure I looked kinda funny riding it to the bus stop to get it home. It had good 20"x2" tires instead of the 20"x1.75" of the blue one. Figured the total allowable length at about 5'-5", only had 10-1/4" to add. Cut the upper tube of the blue bike short, and the lower tube about where it might work. Used the headtube cut long ways to fit over the pink seat post after cutting the rear triangle off 1-1/2" behind the bottom bracket. Used some tubing off a jogging stroller to tie the bottom brackets together. Punching a hole into the seat tube of the blue frame instead of drilling, to leave more steel to weld to.

 

[Mr. Gangrene Genes]

Here is a mock-up with a regular version behind.

The Pacific "Pro Wheel" sprockets are 40 tooth, knocked the chainguard rivets off of one. The 39 tooth "Scrambler" has tapped holes that are very close if you drill both sprockets oversize together. Used spacers from an old mountain bike to keep the chains from rubbing. The "Scrambler" sprocket center hole fits over the crank arm without drilling, and is also narrow for the multi-speed chain. Found a non-drilled 1/8" wide one on eBay for the Run-About.

 

[Mr. Gangrene Genes]

Cut one of the chains down for a transfer between the two sprockets. Had to build a chain tensioner (modeled after a Pete Jackson idler gear drive). Took two links out of the drive chain to increase the tension to reduce bouncing. Added a banana seat for the passenger. Why no rear seat handlebar? Have you ever seen two up on a motorcycle?
If someone was so inclined, the rear pedals and seat could be removed and a decent sized basket installed, for a Yak that would still fit on a bus or car.



I think Schwinn also missed the mark on marketing the mini-Twinn. Instead of trying to get two spoiled brats to share, why didn't they have Laura Petrie riding Robbie around, or Samatha and Tabitha? A young Mom still in the prime could actually ride by herself if she chose, instead of toting a trailer bike in the back of her mini-van/SUV.
If some bicycle company takes this idea and runs with it, probably should have a fully enclosed chainguard. One of Angelina's kids loses a toe and that is bad publicity nobody wants. Think if it was purpose built, instead of a hack job the rear seat stays should have a shallower angle to allow a lower rear seat. It is still a bit tall for my small 8 year old or our friend's 5 year old. It would also reinforce the front seat tube higher so less was dangling out in space.
Rode it around a bit today, if it does not rain tomorrow, will try loading it on the bus.

 

[LukeTheJoker]

Interesting idea, let us know how it goes!

Luke.

 

[noodlenoggin]

Cool! I love engineering exercises like this one.

 

[patman73]

cranks look like both riders will hit eachothers feet !

 

[Mr. Gangrene Genes]

It does seem the feet would clash. When I first rode it, was afraid of the shopping cart/heel conflict. Had my feet well forward on the pedals, until U-turning around in the street and they hit the front wheel. I wear a size 12, from the ball of my foot to heel measures a little over 8 inches, plenty of room. Particullarly when you consider the pedals angle perpendicular to the seat post. With the banana seat the small rear rider pedal angles even more.
I did ride it to work the other day and am pleased that it couldn't have fit the bus better if I had used their rack for the welding fixture jig. The wheelbase is 44-1/2" with a total length of 65". My trusty bathroom scale shows it weighs about 43 lbs. The pretend Run-About comes in around 31 lbs, with the purple Pacific bike at 27 lbs. The rear brake works well, the front pads seem like they may be glazed. It does U-turn in the two lane street, something the Schwinn Twinn would not do comfortably.
The small tires limit the speed, and I am not a strong enough rider to climb the big hill between the bus stop and our house. Otherwise it seems to work well enough. Need to have some others ride it.

 

[Mr. Gangrene Genes]

Here is a picture that shows the clearance between the riders feet.



The front seat could be more comfortable, however the rear passenger tends to hold onto the seat frame and a sprung seat would pinch fingers. Have since cut the rear frame down so the banana seat will lower to the rear stays. Sliced the front of the frame tube and put the seat clamp under the stays.
The dual idler chain tensioner bows outward when pushing the pedals hard. The right crank arm clicks on the bottom bearing. Will add another brace to prevent.

 

[Mr. Gangrene Genes]

I reinforced the primary chain tensioner, and spray painted the hacked parts a bit. Made some rear hand grips with all-thread and a seat post clamp. My daughter and I took it on the bus downtown to the local fall parade. Worked out great, as well as could be hoped. We were able to ride the whole route and check out all the bands and units before the parade started. Many friendly comments, even from the Law Enforcement Officers at the barricades we were riding around. Planned to ride it in the local Halloween parade, she had a party to go to so it has been parked for the winter. Bought another full size drum brake tandem cheap, still don't like the ponderous length. Robbed the HD wheel set off for a little "c" clunker. May chop a foot out of it and see if it works out.