Friday, December 9, 2011

From The Lost Files - Conduit Tandem Trike

What do you do when you have a weekend camping trip planned for the next morning and decide that a tandem tadpole recumbent trike would be fun to bring along? Well, if you don’t have a tandem trike, you build one out of junk the day before of course! Building a working tandem trike in a day may seem like a monumental task for even the most hardcore garage hacker, but I figured by leaving out non-essential parts such as front brakes, seat upholstery, and paint that I could indeed pull off the task.

A simple tandem trike built in a day.

I dug into my scrap metal pile and found a few lengths of 2 inch conduit and a bunch of 1 inch elbows. This was a good start. The main frame will be made of 2 inch diameter thin walled conduit (EMT) and the smaller frame members will be made of 1.5 inch conduit.

Starting on the rear forks using conduit

 The design is very simple, having a main 2 inch diameter boom and a 1.5 inch diameter truss to give the frame strength. When I built this frame, electrical conduit was less expensive than similar wall tubing from a steel supplier, but this is no longer the case. I now prefer to work with 1.5 inch square steel tubing due to its superior strength and ease of welding.

The rear of the frame completed

The frame was built outdoors “on-the-fly”, placing the tubing where I wanted it and welding it right there on the spot. I used the rear wheel to determine the overall height of the main boom and then just began welding in the other frame tubes, working from the rear to the front. The rear dropouts were cut from an old bicycle frame, and they are the only real bicycle components in the entire rear part of the trike with the exception of the rear wheel. I chose a 26 inch rear wheel for the rear and a set of 20 inch wheels for the front of the trike.

The main frame completed

The front of the frame carries the pilot’s bottom bracket, which in this case is welded in a fixed position. I did not have time to worry about a proper adjustable bottom bracket in this design since I only had a few hours left to complete the trike. The 1.5 inch truss (lower tube) forms a triangle from front to rear, creating a very stiff and strong frame that can easily carry two riders. The angle of the seat tubes was about 45 degrees, or whatever looked good at the time.

Adding the front cross boom

A tadpole style trike has one rear wheel and two front wheels, so another 2 inch tube must be placed at 90 degrees near the front of the frame in order to carry the two front wheels. A fishmouth cut was made in the top boom in between the front cranks and pilot’s seat so that there would be no leg or heel interference with this new tube when riding the trike. This is certainly not the optimal frame design, but I was just winging it without a plan, and hardly even taking measurements!

Adding the stoker’s bottom bracket

On a tandem bike or trike, the person steering is called the “pilot”, and the passenger is called the “stoker”. Here you can see the stoker’s bottom bracket, which is simply welded in a fixed position into a fishmouth cut made in the 2 inch main boom. I knew about inseam measurements, so a sliding bottom bracket like the one used in our plans was not necessary here. Another simplicity feature about this trike is that there will be only rear gearing, so the front chain rings will have no derailleurs.

Creating the front steering system

The steering on a tadpole trike is certainly more complex than on a delta trike, but then the rear drive system is much more complex on a delta trike, so it’s a fair trade. Here, I used a set of similar sized fork sets and head tubes to create the steering components for the front of this trike. The fork legs are ground away, leaving a large crown area that will have a round tube installed as to mount the 14mm axle BMX wheels. The odd angle of the head tubes is to account for center point steering.

Installing the front wheels

To fasten the front wheels to the forks, a small 1.5 inch section of tube with a 14mm inside diameter was welded to the end of the fork crown. Two 14mm axle BMX wheels are used because a standard 10mm or 12mm bicycle axle would not hold up to this kind of use and bend around the first corner. The odd angle of the wheel on the forks and the head tube on the main boom create the center point steering. “Center point” means that when the wheel is steering, the tire spins on the center of its axis. All vehicles with two front wheels use this type of steering.

The rolling tandem trike fame

It only took half a day to get the tandem trike up on all three wheels, but there was still a goodly amount to complete such as the steering linkage, seating and the transmission system. At this point I figured I had about three more hours of work, so things were coming along nicely.

Adding the steering and transmission

 The steering was to be a very simple dual handlebar system with an under frame control rod linkage. The 1.5 inch long control arms were welded to the fork crowns, angled to the rear of the trike on order to compensate for Ackerman steering, which allows the turning wheel on the inside of a corner to make a much tighter arc.

Hey, even a junk trike needs to have Ackerman steering, with center point geometry and proper caster angle in order to handle nicely! The transmission was also very simple. A chain ran from the front chainring on the pilot’s crankset to the same sized ring on the stoker’s crankset and then a second chain ran from the other ring on the stoker’s crankset to the rear wheel and derailleur. This gave the trike a synchronized pedaling system with seven speeds. The seats were made of plywood and foam and then fastened to the frame with a few metal tabs.

The trike made it out to Wolf River campground the following day and performed perfectly on the rough gravel roads and out on the paved roads. Sure, the trike was no beauty to look at, but for a single day hack, it was certainly good enough! The trike made it to several camping expeditions that year and never had any problems besides the welded joints rusting.

I guess you could call this a Viking Tandem Trike prototype, even though the Viking is so much more refined and includes such things as an independent transmission, dual adjustable bottom brackets and disk brakes. Well there you have it - when you have only a day to build something, you can usually get it done if you skip the extras and can live with no paint job!