Written by Jon Roberts  |  Category: Cycles
 |  Hits: 6547  | Wednesday, 26 February 2014 15:35


The sidecar started with the need to be able to carry more on the BMW R75/5 as I used this bike as my main form of transportation for awhile.  Also I just thought they looked cool and fun!  So after a lot of research online I decided on the Steib style sidecar that most people associate with the older BMW bikes. These came in a several forms back then and I like the S500.  But I wanted the larger trunk space of the S300 style so I decided to make my own and combine the best of both.

I began by building the sidecar frame.  This required a large diameter bend (26 inches). So I capped one end of a length of 1.5 x .095 wall round tube and filled it with sand then I capped the other end with a short length of square tube.  I then found an appropriate sized tree.  I chained the square tube capped end to a nearby tree so the tube could not slide and 'kink' then I used the bucket of my tractor as a straight edge so the sides would not bow out.  Then I slipped a 10 foot lenght of 2in 1/4 wall square tube of the other end of the round tube and bent it around the tree! Easy as pie - no heat needed.
arkbender4arkbender3Now I will say it is important to find a tree that has a nice round trunk because the tube will follow any flat spots and will not make a nice bend!  I call this my 'Hillbilly bender'!

Two of these bends were necessary for the frame I had in mind.  One as the main frame and one for the grab bar.



bmwmock1bmwmock2After getting the necessary bends I did a mock-up with the R75 to get a better idea where the grab bar would mount and where to put the swing arm mount to get the sidecar axle lead that I needed.  The axle lead was to be set at 9-11 inches based on internet research. 

frame06Next I drew up some layout plans on my CAD... er uh floor. 

I put the tubes I had bent on the floor and marked and cut the ends and angles and welded them together.  I put in a vertical brace of 1 inch tube to locate the grab bar at the proper angle and to reinforce what will be the upper coilover mount.




To the left you see the raw materials usedswingarm04 to create the major components of the swingarm, some heavy wall round tube, 1 x 1/8 strap, the shock, axle, and some 1 x 2 heavy wall tube. Not shown is some 1 1/2 round stock that I machined to make an axle carrier.  I could have just machined it into an axle but I like making things replaceable for the future.  To the right you see the machined axle carrier. The head of the axle seats in the carrier and a bolt goes through it to hold it in place and allow axle nut to be tightened.
swingarm07Here is the axle in the carrier marking where to drill for the locking bolt. Next is the axle in the carrier with the locking bolt installed.
swingarm12At the left are the parts of the swingarm pivot assembly. The heavy wall tube was machined to accept roller bearings that ride on the smooth portion of the bolts. It is hard to see the rubber seals at each end of the tube. Next is a picture of the pivot assembly next to the axle.  Both of these will be welded into holes drilled in the 1 x 2 tubing shown earlier.
swingarm20Here are the parts of the swingarm.  The tubing will be trimmed and rounded of to look a little better.  On the right is the axle carrier welded in and the tube rounded ready to be capped.
Here the tubing has been capped with the 1 x 1/8 strap.This dresses it up a little but mainly increases the strength of the part.  That completes the parts for the swingarm.  The tube with the pivot bearings has yet to be installed in the arm, this allows me to build the pivot assembly without dealing with the cumbesome arm. Next I build the pivot assembly. 







The pivot assembly begins with another piece of 1 x 2 heavy wall tube that will make up the saddle. I cut a relief in the side for the frame tube to fit in and in the end for some 1/8 plate to cap it.  On the right is an example of the way it will fit to the bottom of the sidecar frame.



pivot05pivot07On the left is a picture of the capped saddle. To the right you can see the outer pivot bearing mount being made.  The end was marked then tacked together then the holes were drilled to the size needed to tap the plate for hardened metric allen bolts.  The plates were then knocked apart and the smaller piece was tapped and the larger (outer) piece had its holes enlarged to allow the bolt threads to pass through.


pivot09pivot12Left are the two plates ready to mount. They are the two bottom plates on the picture at the right.  This picture shows all the pieces that make up the pivot assembly ready to be welded.  The round tube at the top is an example piece of frame tube used to posistion the other pieces.  It is sitting in the relief of the saddle. To the left of the saddle is the pivot bearing tube with its associated mounts.  To the right of the saddle is a 1 inch round tube that will brace the end of the pivot assembly.

Left are the pivot bolt assemblies.  The inner pivot bearing mount will be fixed with the outer being adjustable.  The inner mount is at the top of this picture and will be welded to the saddle along its front edge and to the sidecar frame along its top edge.  it is the piece above the pivot bearing in the above right picture.  The large hole in the outer mount is threaded metric fine for the pivot bolt.  It will have a locking pal-nut on the bolt.






pivot17Here are pictures of the inner mount welded to the saddle.  Once it is on the sidecar frame it will be welded along the top edge along with the saddle being welded fully.  Notice that the pivot is a press fit in the mount then welded inside and plug welded on the outside.




Here is the complete pivot assembly ready to be welded to the sidecar frame.  There will be alot of welding to the frame tube so preheating and proper cooling along with some pre-loading will be necessary to prevent warping the frame.




susp1susp4Here is the complete pivot assembly welded to the frame. To the right you see the frame with the pivot assembly and wheel.  Next I will build the coil over shock mounts.






Here on the left you can see the upper shock mount area on the frame.  If you look closely you can see the pilot hole drilled through the frame for the upper mount.  On the right the hole has been enlarged enough for a bolt to pass through to aid in mock up of the shock.




The picture to the left shows the shock sitting on the swingarm with the top of the shock slipped on the bolt in the upper mount area. You can see in this picture and the one on the right how the shock is leaning in toward the frame.  With it mocked up like this I measured how long the upper mount shaft would need to be to get the top of the shock in the same plane as the bottom (more verticle). 

shock8Here is the upper mount turned out and threaded on the lathe.  There is a step turned down for the shock to slide over, which is cut to the correct length so that with the nut tightened down the shock can still move on the mount. The overall length was cut after a trial fit on the frame. 

On the right the outer hole in the frame has been enlarged enough for the mount to slide in, the inner hole was enlarged slightly for a plug weld on the back end of the shaft.

shock10Here are some pictures of the upper mount welded in.  It is perimeter welded on the outside and then proceeds through the tube and is plug welded on the inside of the frame.
shock12These are pictures of the lower shock mount.  It was made from 1.25 x 3/16 strap.  They were welded to the outside of the 1 x 2 swingarm tube.
frmheight4Then it was time to see how it worked under load. I placed a couple of boards on the frame and then put two 50 pound bags of sand on them.  Unloaded the frame height at the mount was about 14 3/4 inches.  Loaded it dropped to about 14 inches even.  This was at the lowest shock preload setting.




 Next I built the subframe that connects the sidecar to the motorcycle thusly becoming a 'Rig' generally refered to by the bike name followed by rig, therefore this will be an 'R75 rig'. I wanted the subframe designed so that it can be removed and leave no extra holes or changes to the original R75 frame.  To this end it was built in two parts the bent portion mounted to existing fasteners (upper shock mount, the passenger foot peg mount,  and the two lower engine mounts), and an inverted  "L" shaped portion that connects the double down tubes at the top and to the bent portion at the bottom.


The first step was to bend a length of 1 inch heavy square tube to the same angle as the rear tubes on the original frame. This was done on my homebuilt tube bender (shameless copy of one from the web).  Next this tube was bent again, more of a twist done by clamping the upright portion in a vise and then using two large adjustable wrenches to twist the bottom portion slightly.  This was necessary because the the original frame narrows slightly at the bottom. In order to keep the tube straight with the lower engine mounts it had to be twisted a bit.







subframe4To the left you can see the heat affected zones from welding on the tabs that pick up the mounting points listed earlier.  You can also see the slight twist that was necessary to align the mounts on the horizontal portion of the subframe.






The next component to make was the actual body of the sidecar.  Some people refer to this as the 'boat' but I prefer 'body'.  The body I had in mind was to be a hex shaped thing with a trunk and windshield.  Randy from work helped me mockup the whole body in cardboard to get an idea of the size and process to build.  The hardest part was getting the nose right (think beachball).  Each piece was a chubby triangle (flat on the bottom with other two sides curved slightly outward).







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Jon Roberts --- [email protected]



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