Make a Tadem Bike frame

[sinretel]

Hi guys. I am new here (im not speak english, but ill try to do my best).

My and my wife want to travel through the chilean patagonian and Argentina in 1year (2025), so i designed a tadem quadbike. Why not simply travel with two bikes? My wife has a slight lumbar injury that causes her pain and sudden vibrations will not do her any good, so a recumbent bike solves her problems at moment to bike.

I need some advices, please, to build a frame for two. It is my first proyect like this. My intention is to fabric a four wheeled bike as the picture below shows. It is based on cargo bike, but instead to has a rear bike seat, i want to a reclinable seat for me (last time i traveled 1250km alone, my neck suffers a lot). I think, my design has to make a couple of things:

1.- Carry us. I am thinking to build it with 5cmx5cmx2mm steels bars for the frame and X bars of C 3cmx3cmx2mm.
2.- Our geography is like this: In the north is very warm (we live here), in the central zone of chile the climate is warm and cold, but in the sourth raind a lot. The cold wind is very hard sometimes, so for my wife goes secure she will bike inside an acrylic cage with windows (A). It will open with a Gull-wing door.
3.- Under my seat (B), i wanto to build a place to store luggage and close it with padlock.
4.- I don't mind sacrificing speed for a good frame that can handle various rocky, muddy, wet, or uphill roads (I can push). I think we will travel between 8,000 and 9,000 km. I want her to be comfortable because it is her dream to ride a bike and I have been designing, buying and assembling parts for almost a year in order to make her dream come true. Thank you.
5.- For the front suspension I was able to buy a quad bike suspension for a more ridiculous price. I have the shock absorbers that I bought on aliexpress.
To direct the direction, I think of using a handlebar with brakes, as if it were a normal bicycle, but with the reclining seat for me. For the transmission, each will have a 1x10 or 1x12 speed sprocket. Finally, I don't know if it's better to use 20" wheels for all four wheels or 29" rear and 20" front or all 29" wheels. I weigh 100kg and my wife 60kg.

Diseño y medidas — Postimages

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[stormbird]

Welcome interesting project.

Call it a quad rather than a tandem especially when doing Internet searches and on here you will get more relevant results and quite a few are 2 seaters.

Emiels Quad

Definitely 20" wheels and BMX one's will have stronger rims and higher spoke counts all desirable.
You have 3 main problems.

1. connecting up steering
2. getting the drive to the wheels
3. getting brakes on the wheels

Have you seen Brads Plans ?

Maybe the Viking Tandem or the Loderunner SUV would be a better place to start , Brad has dome most of the work for you regarding the above problems ?

I think it would be a tall order for a beginner to get this right for 2015

Best of luck with your project , I am sure others will be along shortly with more advice.

Paul

 

[Popshot]

I think it would be a tall order for a beginner to get this right for 2015

The time machine alone will be a bugger to build!

That is going to be very heavy. It's the easiest thing in the world to build heavy and then find it's ridiculous to pedal it. Unless you plan on adding electric assist or riding in flat areas only I'd look to something much lighter. As Paul says definitely 20" wheels and I'd opt for 20mm hubs at the front. The rear seat is too high. You will reduce your ability to pedal with the seat higher than the pedals. The steering is going to be difficult from the rear as the steering wants the same space as your wife. It would be easier to steer from the front with under seat steering. If it has to be the rear that steers then again it'll need to be under seat with a rod passing to underneath the front seat and multiple linkages. This will be heavier, more effort, more vague and more cost than front seat steering.

I'd make a simple beam frame rather than a spaceframe to save weight, lower the rear seat and put a lockable box behind it. Using a simple beam means it's quicker, easier and cheaper but it also means your heels will not have clearance issues. As designed your feet will strike that X shaped crossmember. For reasons such as that, as Paul says, buy the Viking tandem plan and the Load Runner for the rear end and see the issues ready solved. Modify them to suit your needs which will mean seating yourself further back than than the Load Runner does so you are over the rear axle with the box behind it. It would be silly long wheelbase if you keep the Load Runner rear as designed. Placing yourself over the rear wheels is important or the main beam will struggle. Suspension is always nice but it almost always equals more weight. This will never be light and it will also carry all your gear too. Try to work out a design with and without suspension and see the extra chassis needed plus the weight of the wishbones and suspension units and ask yourself if you and your wife are willing and able to move the extra weight. Instead of actylic sheeting for the front what about some tent window PVC and some bendy tent poles to make an easily removable tunnel over your wife's lower half. This will easily stow away when not needed and save a huge amout of weight.

 

[maddox]

I suggest you look at this thread.

Or like Popshot wrote, the Viking tandem.

Now, the Viking is an old desing, but it works. And with new tech it can be updated.
Use modern 20mm shafted hubs, combined with 20" heavy duty rims and balloon tyres like the Schwalbe Big Apples and suspension becomes "easier and lighter".

And if possible, add a decent rear wheel motor and the biggest batterypack you can afford.