Designing and Building Tandems
Mon, May 23, 2022 
The traditional bicycle frame is a simple but inherently strong in design and structure, it is made up of two triangles.
Admitted the front triangle is a triangle with one point cut off to form the head tube, but never-the-less, still strong. (Picture left.)
Now let’s imagine we separate the front and rear triangle and extend the frame to build a tandem to accommodate two riders. We have now weakened the structure considerably, as we now have two triangles and a parallelogram in the center, which would most likely collapse under the weight of the two riders. (Picture above.)
As a simple way to add reinforcement to my tandem frames I liked to add a tube through the center of the frame along a line from the center of the head tube to just above the rear drop out. (Picture above.)
Now we have a structure made up of six triangles. If you ignore the front fork for a moment and treat that as a separate structure. The main frame is like a bridge spanning two points from the head tube to the center or hub of the rear wheel.
This extra tube was made up of an inch and one eighth tube, (The same diameter as the seat and down tubes.) running from the head tube to the front seat tube. Another ran from the front to the rear seat tube, then two separate tubes would run from the rear seat tube to a point just above the rear dropout.
I did this so the right-hand side of these twin tubes would be clear of the chain and sprockets, but apart from that these tubes were added along a straight line from the head tube to the rear hub. Sometimes I would leave out these extra twin stays from the rear seat tube to the hub, but still placed the main center tubes along this same center line.
Paul Swinnerton (Front.) Trevor Gadd (Stoker.) on a track tandem I built, competed in the 1978 World Championships.
It depended on the size of the frame and the combined weight of the two riders. Also considered was the intended use of the tandem. At the bottom of the frame, I placed twin 1 1/8” tubes, side by side between the front and rear bottom brackets. I always felt this option was the lightest and most pleasing to look at, as well as being strong.
Twin bottom tubes
The diagonal center tube was under tension from the weight of the two riders, and by running along a line between the two points of support, it also meant if there was any tendency for the frame to twist, this tube would also have to twist. You can probably appreciate, a tube has a strong resistance to twisting, especially if the twisting forces are applied along its exact length.
However, having said this was the lighter, stronger, better-looking option. It was also the most labor-intensive option, and therefore the more costly option.
Going back to the second line drawing at the top of this page, getting to this initial stage of the frame without the added reinforcement, is relatively easy as many of the standard frame lugs can be used.
But, fitting this extra center tube called for some precise mitering, careful fitting, and fillet brazing, because lugs were not available. A perfectly adequate and practical option was to place smaller diameter lateral tubes, either one pair from the top head-lug to just above the rear dropout.
Or, two set of twin tubes. One from the top head-lug to the rear BB, and another pair from the front seat lug to just above the rear dropout. Again, which option was chosen, depended on the size of the frame and the weight of the riders. This option although heavier, took a lot less time, and therefore brought the cost down.
Twin extensions to the center tube were left off this frame to accommodate fenders (Mudguards.) This was possible becuse it is a smaller frame designed for light touring.
I would like to thank Enrique Asturias who has quite a stable of bikes I built in England, Paris Sport, and California. He owns the green tandem (Above.) built in the UK 1976. He asked why the different configurations and supplied the other pictures to illustrate the variations.
It all came down to cost and what the customer was prepared to pay. They were all adequate and the fact that so many are still around more than forty years since they were built, proves that.
Reader Comments (3)
Thanks very much for this write-up, Dave.
My wife and I enjoy riding our tandem. We were fortunate to try a number of brands before purchasing and could tell when builders didn't have your knowledge and skill. Many tandems would flex when cornering as if they were articulated like a bendy bus. This was under 2 riders that weren't very heavy or strong.
When built properly, like we found with our Co-motion, I feel like I'm almost riding a non-tandem bike.
I know it's been described as a 'divorce machine' but for many, like my wife and I, it allows a non-regular cyclist to enjoy bicycling with their partner.
Paul.
(Posted by Dave M on Paul's behalf.)
With mainstream single frames all looking so similar tandems with their many variations have always been very interesting.
I have ridden a number of different ones over the years and the ride variation can be extreme.
Your approach is very sound and provides a spectrum of solutions depending on the problems at hand.
I’m wondering if I have one of your tandem bikes! It has a twin bottom tube design, but I cannot figure out who made it. There are no surviving decals or markings that I’m able to find.