Lars BMW rear suspension...

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Prof
Founder, Choppers Australia
Posts: 5690
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
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Lars BMW rear suspension...

Post by Prof » Tue Mar 13, 2018 10:16 pm

lars bought an R100/7 BMW that had been converted to rigid. But being in the country meant a sore back pretty quickly, especially with the original 4.00 rear tyre. So he asked me to create 1960's R60 suspension and get it a bit more ground clearance...
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My workshop is getting full of projects, so to work on his bike I have to roll out three choppers each morning. Missing in the foreground is Stu's KZ1100 which I hope he will get to work on again soon, red rigid frame is recently purchased shovel frame for which I have everything to make a righteous chopper, next is Rob's shovel roller (I'm waiting for him to have some time to get down to continue on it).

Next is rather interesting... a BSA frame with agrafted in HD hardtail, I converted the steering head to accept HD bearings and it now has an HD narrow glide. Motor for it is under the rags; an XS500!

Next project in the lineup is another Rob's iron head that again is waiting its owner to have some time to work on it with me. Next is my CB750 project that has gone nowhere for a couple of years but I live in hope!

Chopper on stand is for sale ($6000) as soon as it has had a tuneup. Is up there to check points etc...
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Here's the BM as Lars rode it in. Hard tail is bolt on. Quite nicely made, but too flimsy for my liking with a couple of basic engineering mistakes. Anyway, it is going and we'll see what we can cook up...
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First step is to chase up a photo of an R75/5 series on the net for stock bike angle. This was traced (small piece of paper) and transfered to a larger "angleometer" via a home made enlarged protractor...
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Bike is then put on a bike jack and blocked up to get the desired angle...
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Next is ground clearance in this case 6" (150mm). We have to allow for sag when the bike's weight is on the wheels. I allow an inch at the rear and two at the front...
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Next measure up for a stand...
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Measurements onto paper...
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1" angle is to be used and the pieces cut to length. Hole centres are marked with a marking guage, but a tape measure will work fine. Note that upright being measured is set on the base to give an accurate measurement... obvious but easily forgotten...
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Holes are slotted at the top so stand can be removed once welded up. Here the rear one is bolted up and ready to weld...
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Finished stand. Red arrow points to a temporary bracket welded to hard tail so legs don't collapse until all is welded up. Green arrow points to connecting bars to make the stand a stable unit...
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Bike on stand. I had made a texta mark on the fork legs with the bike on its wheels and when I lifted it up, found that the forks actually collapse 80mm on a BMW about half their full travel. So not seen here are 20mm boards I place under the stand to get the extra ground clearance we will need plus and extra 10mm under the front of the stand...
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Today, I took out the drive shaft housing and this afternoon, took the new swing arm and old shaft housing to Lonsdale (Lee McPeake Engineering) to have the shaft pressed out and put into the new swingarm.

Next job is to assemble swing arm to bike and check the swing arm angle against requirements to see if my measurements to date are correct.

Stay tuned...
Chopit'nrideit... Prof

Prof
Founder, Choppers Australia
Posts: 5690
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
Contact:

Re: Lars BMW rear suspension...

Post by Prof » Wed Mar 14, 2018 4:35 pm

Swing arm has grease, grit, a dent and lots of pits from rust, so gets a good clean with wire brush and flap disc. Decided to not fill dent as Lars is not super financial... is on top so can be touched up any time. Also some other extra jobs he may need done as in some new exhaust brackets as the originals were terrible and battery hold down is almost inaccessible. Seat front mounting bolt is not properly retained so these may add a bit of $$ if he wants them fixed...
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Primed and painted...
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Hard tail now removed and engine mount needs re spacering with the hard tail tab now gone. Spacer that was on left fits rhs perfectly, so just one to be machined up for the left side..
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Measured and done...
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Just for your interest, hard tail had a number of deficiencies which I thought worth pointing out to help would be builders. Front end of red line shows were hard tail was bolted to the rear engine mount. Ideally another support should have been set up at end of red line to provide sufficient rigidity. Secondly green arrow shows rear axle mount. This was simply a hole in the axle plate that the axle slid through, but with no retaining bolt. Ie only the right hand side of the frame resisted lateral movement. Eventually something would have broken...
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top brackets are cut off (blue arrow) and paint is linished off the frame (red and blue) where the new sub frame will be welded on...
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Footbrake lever had to be cut off too, to allow the frame to be attached. It will be welded on again once frame is complete and be a part of the gusseting...
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With the wheel in place we can measure up the overall stance of the bike and angles of the swing arm, which should be down a little of the line of the motor...
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An extra 15mm height will make it pretty close to what is needed...
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Some pieces of 15mm timber added in...
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Shocker on R60 is near vertical so we set these up the same. Will actually tilt the shockers back another 3 degrees to compensate for the front end dropping 80mm vs the rear 30mm...
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Top 2/3rds of shocker should be covered with the mounting tube. I am waiting on some hydraulic tube that will be strong enough to mount the shocker. Here a bit of thin stock to test out set up which I will show later...
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I purchased some lengths of 1" lead pipe many years ago and have about 3 four foot lengths left. Here bent to the approximate curve (needs to be a bit wider towards the bottom) so I can start rolling the 1" DOM tube we are using for the job...
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More pics soon...
Chopit'nrideit... Prof

Youngblood
Posts: 366
Joined: Wed Oct 24, 2007 1:48 pm
Location: North-Eastern suburbs- Adelaide

Re: Lars BMW rear suspension...

Post by Youngblood » Sat Mar 17, 2018 4:24 pm

8) 8) 8)
Youngblood

Prof
Founder, Choppers Australia
Posts: 5690
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
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Re: Lars BMW rear suspension...

Post by Prof » Wed Mar 21, 2018 9:42 am

To set up angle of subframe I mount a ruler parallel to the ground...
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Frame tube is 25.4mm x 3mm DOM tube and it is tough. My roller does not like it one bit and the job is tediusly slow compared to normal 1" tube...
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Tube needs final bending with heat to get the final shape. It is then cut at each end a bit oversize, the angles cut approximately and then trimmed les than 5mm at a time until we have the right clearance between subframe and shocker...
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A piece of cardboard is folded a few times and taped onto the swingarm to get the right clearance...
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Trimming back the ends must be done with restraint; a little bit at a time. Final shape of ends need to be a close fit the the main frame for welding. This takes patience as not only does the end have to conform to the curve, but it also must be shaped so that the smaller diameter rear subframe lines up nicely with the outside of the mainfame... makes for an interesting cut. Close fit is important to reduces stresses when welding as well as preventing the tube from pulling out of alignment...
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Top cut here seen just before the final shape is achieved; you can see the slight gap at the outside front...
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Wednesday should see the second subframe tube bent and fitted. The extra challenge is for it to conform in all ways to the first. After that we can tack the two tubes and then add in cross bracing and guard, seat and exhaust mounts.
Chopit'nrideit... Prof

Prof
Founder, Choppers Australia
Posts: 5690
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
Contact:

Re: Lars BMW rear suspension...

Post by Prof » Thu Mar 22, 2018 11:25 pm

Forgot to show setting up the swing arm. It is centred using a verniers or engineers steel rule and the pivot pins screwed in to a preload of 15psi (which I guess). Then locknut needs to be tightened up...
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Have to turn a 27mm socket down on the lathe to fit...
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Has to be thin, but does the trick and doesn't crack...
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Ok. Now onto the second sub frame tube. This time done with heat. A couple of lumps of steel plate go into the vice with one end of the tube to be bent. Our pattern (the left hand side) slides into the plates as out guide. Usually tube is bent by heating the outside of the bend to prevent buckling, but this tube has a 3mm wall and I don't want the outer side to flatten as we bend, so I heat the inside of the bend which will actually result in the tube being slightly more that 3mm on inside of the bend... and no flattening...
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Takes 25 minutes. Less than half the time of rolling, so I will only charge Lars the this amount for the first side...
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To cut and fit the rhs tube takes a lot of care and measuring. It needs to start at the same point on the back of the main frame. be at the same angle and be the same distance from the shocker. Time and patience of of the essence. This can't be rushed...
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Measuring up the distance from the centre line reveals a couple of things. The rim is offset to the left by about 5mm.
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The right shocker is 12mm closer than the left..
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First job then is to machine up a spacer to set out the shocker or the whole kit and kaboodle will look lopsided with the style of seat and guard we are using...
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Spacer is made to rest on a rodge machined in the hub, which will add some support to the bolt securing the shocker...
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Centre line runs from centre of back bone down the centre of the tyre. We use a square to mark the beginning point of the right hand side tube...
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Lots of trimming and trying finally produces two matching frame sides...
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We need a cross piece to reinforce the tubes, carry the rear of the solo seat and for the forward mounting of the guard. Shocker travel is first measured to determine how far up and forwards the wheel may travel...
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The wheel is then blocked up this amount plus 3mm to allow for the extra travel of the wheel (the axle is behind the shocker and will travel slightly more)...
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We allow 15mm for clearance and add a mark on the top of the left tube...
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Measure this from a known point and mark the other side the same distance back...
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Guard which came off the rigid, needs to be reshaped to allow for wheel travel. The sides are pushed out progressively across the whole guard by adding some downwards pressure over a round surface, the horm of the anvil in this instance...
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Guard on wheel at wheel's highest point. A couple of small pieces of 15mm tinber are under each end to give the needed clearance...
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Cross piece is cut to length allowing 3mm overlap each end and then shaped on the grinder to fit over a piece of scrap 1" tube. I rarely dress this grinding wheel as I like the rounded shape for doing these jobs...
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Front guard mount is a piece of 30 x3. It is drilled before curving to the shape of the guard. Because the guard is aluminium, I will fix it with three M8 bolts so the alloy is less likely to crack; remembering Lars rides over some pretty rough country roads and aluminium is not strong...
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Bent on the edge of the anvil bed...
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FCross piece in place. Testing bracket which will be welded underneath. I actually set it in position under the cross piece and tack it very gently with the MIG...
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Bracket and cross piece then taken to the welding bench and bronzed... does a nice job with multiple curves...
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Now for the seat. Lars is on a budget, but the pivot is awful. Original is very floppy which is not conducive to comfy cornering. Original seat tab has been redrilled (crooked and weakened) for an even smaller bolt with no locking set up. A better set up is essential...
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I use 10mm shoulder screws which have a machined and hardened shaft and machined up a matching pivot piece with a 10mm hole...
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This is welded to an extension made from angle iron with a brace. The angle allows for a decent weld to the back of the backbone...
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More measuring. It is lined up with a machined join on the gearbox (blue dotted line)...
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... and levelled...
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The pivot needs to be high enough on the frame so the shoulder screw doesn't hit the frame when the seat is taken on and off. So we will lower the seat by cutting and bending its bracket. Something also needs to be done with the seat bracket because of the holes that had been drilled in it. By removing the holes we solve both problems...
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Measure up cuts with verniers so it doesn't end up cockeyed...
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Bracket welded into place and seat mounted...
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Last job on the seat is to machine some bungs for the springs adn weld them in place. Slots are cut into the front to retain the springs and do away with bolts and nuts which are an absolute and time consuming pain on sprung seats. Bungs are 30mm high so the seat does not slope back when the rider is on it...
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Seat mounted...
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Now looking more like it!
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Have had a glitch with supply of tube that will cover and mount the shockers... was supposed to be here Monday, but I found out finally this afternoon that the supplier had forgotten to notify the driver. Lars will be here in the morning and I was expecting we would make up these shocker mounts... so we'll just have to see what else we can do until the steel arrives hopefully in the early afternoon.
Chopit'nrideit... Prof

Prof
Founder, Choppers Australia
Posts: 5690
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
Contact:

Re: Lars BMW rear suspension...

Post by Prof » Tue Mar 27, 2018 10:58 pm

Got the steel (hydraulic tube) but it turned out too bulky and very heavy. Back to drawing board and we will use 1.6mm tube, beefed up in the appropriate areas. Would prefer thicker, but not available and this will be strong enough. The tube I have chosen allows about 6mm clearance, necessary because the swing arm moves in an arc and we don't want shocker rubbing on the cylinder...

So, here's the plan... the top shocker eyelet will fit into a clevis which bolts to the top of the shocker mounting cylinder allowing any rocking movement. Mitre 10 supply me with a couple of 50mm post tops that will fit the bill nicely for the top and have some nice shape to them...
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Top of mounting cylinder needs to be thicker as it will carry the whole weight of the rear of the bike. Simple way to do this is sleeve it and do a multitude of plug welds. 8mm holes are drilled with a step drill and the burrs left on the inside are removed with a grinding wheel on the pedestal drill and run at top speed...
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Sleeves cut and slit and inserted ready for plug welding. They protrude 10mm so the post caps can slide over and locate prior to welding...
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Clevis's are cut from 3mm wall 50mm x25mm rectangular tube. Here marked for cutting. Then centres of holes measured, marked and drilled. Tube is then cut to create a clevis...
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Parts laid out. M10 nut has been welded over one hole. An M10 bolt is welded to top of clevis. Caps are drilled and mounting cylinders now ready for welding...
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Left shocker assembly bolted up...
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Assemblies mounted on swing arm. Tomorrow we'll make the bracketry to weld them to frame... and then the bike can settle down on its wheels and we'll see if my measurements were correct!!...
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Chopit'nrideit... Prof

Prof
Founder, Choppers Australia
Posts: 5690
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
Contact:

Re: Lars BMW rear suspension...

Post by Prof » Wed Mar 28, 2018 9:22 pm

Today we've completed the shocker mounts. First job is to make some reinforcing bars on shocker towers. Tubing is strong enough, but might tear at the welds being only 1.6mm. So we shape up some 30x3 flat bar...
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Get both the same by clamping together and linishing...
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Run a slight bend down the length of each bar using this cool little bender that fits in the vice... you do need a substantial vice to do this length...
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8mm holes are drilled to be plug welded...
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Then edges are bronze welded. The bars are under cut and plenty of heat applied to make for a strong weld...
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Done and sitting back on the shockers...
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Shocker towers will be joined using some heavy curved tube plus gussets below. ..
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And what better to use than one of those horrible western bars. Tube is 3mm and saves bending. Two bends to be used are the two small pieces in the middle of the photo...
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Before attaching we have to make sure everything is square. Sight across the rear axle as well as a double check with measuring tape from rear of fuel tank...
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With bike level check for vertical. Wooden blocks and wedge holds it right...
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Both towers welded with mig. This would probably be sufficient, but to be safe will add the gussets as well...
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Patterns made and 30x3 bar cut and shaped...
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Gusset each side of curved tube welded in. Should be a strong set up that looks light and nice as well...
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Now the big test! you may remember the forks compress a lot when the bike is off the stand and there was a lot of measuring and a bit of guesswork about how high to start off. Well we wanted 5" ground clearance... and guess what?
We achieved it. Rear shockers are on softest setting and are fairly soft anyway so Lars may wish to preload them a bit. But we got what we aimed for, which is encouraging...
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Looks good...
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A simple rear guard bracket incorporating the tail light will come next. Rear brake lever has to be set up as a part of the gusset that will go down the bottom at the rear of the frame join. Side stand also needs lengthening and we'll have to add a couple of threaded bungs to mount the exhausts.

Need to get on with another bike for the moment and come back to this one in a couple of weeks.
Chopit'nrideit... Prof

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