KiwiBikez

How to build a great Tandem

and save yourself a fortune

Although this is an upright it would rate as one of the best bikes I have ever built… Not because it is anything special but it is the one bike my non-cycling wife will ride and my 3 daughters love getting out for a ride.

The frame is built out of a couple of old low end mountain bikes (probably department store bikes).

PLEASE… DON’T TAKE RISKS

Make sure the welding is done by someone with the necessary skills

Before you begin you need to work out who is going to be riding the thing with you… It would be silly to use a frame that was the wrong size.  Note: sometimes you can alter the size of the frame. 

Get the donor bikes that you will be using… strip them down to bare frames. Remove the forks from both frames. If you are using two different sized frames the larger should be used for the front.

Cut one frame until you are left with the rear part. seat post, and rear assembly.

Cut the other frame removing everything behind the seat post and remove the top bar (you can leave the top bar but if you are not using frames that are exactly the same size with perfectly horizontal top tubes the angle will amost certainly be wrong)

The red lines show the new tubes that will be added to make the tandem frame. 

You can see that the frame has a similar style to an expensive commercially produced tandem. You may have to fiddle around a bit to get the look you want.

 There are a few important things to note.

The length of the rear top tube needs to be the length of a standard bike plus about 150mm… the 150mm is to allow for the handle bar mounting. (I used 150mm this could vary a little depending on the frame you are using) ie… measure the distance between the seat post and the head tube and add 150mm….The rear brace will need to be made to fit after the top tube is set up. You will be able to work out the length of the bottom tube once the top tube is set up. The reason for the addition of 150mm is that it will give the rear rider (stoker) pretty much a normal riding position. IMPORTANT… You will need to shape the ends of the tube so they fit very neatly to the tubes they connect to. To get tube long enough for the rear sections I had to join some tube… You could always go and buy tube… I’m thinking that if or when I build another tandem I might use some oversized tube to give it that modern mountain bike look. Once you have all the tubes fitting neatly clamp the whole thing together… it is important to ensure that the frames are straight… Many articles talk about brazing the frame together I chose to have the thing welded (mainly because a mate has a mig welder) this gave us good strength and after a touch up with an angle grinder and a file it all looked pretty good. Another mate who is a car painter sprayed the frame for me.

You will also need to cut the cable guides for the rear brake cable… I just cut them from the existing tube and welded them onto the new top tubes.

Now to get the thing to work…

I had 3 crank-sets from cheap bikes, these had one piece cranks with the chainrings held on with bolts… To avoid having to purchase expensive parts I chose to have the sync chain on the same side as the drive chain. This also has the advantage of allowing the use of standard pedals. For the front set up I cut the large chainring off leaving the centre… to this I bolted the second largest chainring (approx 40 teeth)

Front Crank  

Looking down on the front crank set… the red shows the location of the chainring.

Note: the chainring is mounted on the outside of the centre and I had to put a couple of washers between the centre and the chainring.

 Rear Crank 

The rear (driving) crank set… Simply add a chainring that matches the front one on the outside of the large chainring… You will have to space it with a couple of washers as well.

The washers are used to get clearance for the two chains when the drive chain is on the large chainring… You will have to experiment until you get enough clearance for the two chains to operate without catching on each other… you may have to use slightly longer bolts.

Chain Tensioner  

You will almost certainly have to use some kind of chain tensioner… All I did was rear dropout from the left over rear frame (the part the wheel fits on to)… this was trimmed and welded on the bottom tube about the centre and I then used a rear derailleur to tension the lower section of the front chain.

By removing the adjustment screws and adding a longer screw you can get it to work perfectly for aligning the derailleur with the chain.

Rear Handlebar Mount

You will need to make a mounting for the rear handlebars…  I used a section of top bar with the seat mounting bracket left attached… Measure the length of the handle bar stem to work how much top bar to leave (add 1-2 cm to the stem length). This turned upside down and fitted to the front seatpost gives an adjustable mount… the older style top tube is the right size for the handle bar stem to fit inside.

You can use an extra clamp to hold the bars tight but I have found that the wedge bolt holds well enough.

This is a more recent stem for the rear handlebars, this was made by welding the handlebar clamp directly to the stem (the old top tube section).

This looks a lot neater and is less prone to twisting if the stoker leans on the bars.