Thread: ORGS build up
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Old 11-20-2010, 01:30 PM   #102
x3300 OP
Joined: Mar 2005
Location: San Jose, CA
Oddometer: 170
Shock Mounts

maverick, I assume everything is aligned; trans output shaft, final drive input shaft, swingarm pivot, etc. I didn't check that, but I can see my extended shaft and arm seem to have the proper alignment.

Beemerguru and others, I have received a lot of inquiries about the gussets so I created a new thread dedicated to them here:

To get the rear wheel travel I wanted with my extended swingarm I either needed to fit a custom shock to work with the original GS upper shock mount or make a new shock mount to fit a shock from some other bike.

Modern off-road suspensions have progressive damping, where the damping increases as the shock compresses. This is done either through a linkage mechanism or through shock valving. Such systems have been well received, so I figured I'd try to get some progressiveness in my design. Since the GS has no linkage I would need to fit a shock with progressive damping. Not too many bikes use such a shock and this requirement would really limit the shocks I could choose from.

As a start to get an idea of what shock length and stroke values would work for my application I made a spreadsheet that calculated the shock length and stroke needed to get my 280 mm of travel using the original GS upper shock mount. I set it up to do the geometric calculations at 2 mm increments along the swingarm.

I used the values to compare what would work with what was available from various shock catalogs. I wasn't so lucky, no stock shock would work with the original GS mount, and the only progressive shocks that seemed like they might work were from a BMW G 450X, or a few of the KTMs. I figured KTMs are pretty common compared to a BMW G 450X, so decided on fitting a KTM shock. After a bit of searching around the Internet this box arrived with an Ohlins TTX44 for a KTM 250 SX-F. The length is advertised as 410.5 mm and stroke 109 mm.

With the shock in hand I could estimate how much the stroke was decreased by the bottoming bumper and how much space was needed between the lower shock mount and the swing arm centerline. Both these need to be considered in the design.

The geometric equations that describe the set of mounting points that would work with this shock are non-linear and difficult to solve for even using numeric methods, so I decided to use a graphical method. I used a large piece of paper to layout a full scale drawing with the swingarm pivot, upper shock mount, and swingarm at the upper and lower limits of travel. I used two thick pieces of paper marked at the extended and compressed lengths of 410.5 and 306.5 (5.0 mm bumper thickness) to find a set of mount points that would work.

Here's the detail near the top mount. As the drawing shows, any lower mount from about 240 mm to 320 mm rear of the swingarm pivot won't work with this shock because the frame tube is in the way of the top mount. The points from around 320 mm on back could give the proper wheel travel but would put the upper mount in the middle of the frame tube and I thought that that would put a lot of bending load on the tube so I chose not to use those points. Around 230 mm seemed good. This would put the shock and mount up above the original shock mount, but I thought it would be OK since I needed to make a custom sub-frame and seat anyway.

Once I got some mount points I thought would work I laid those out on the frame and swingarm with some masking tape to do an initial check of the fitting.

When I was confident I didn't need the upper mount I used a cutoff wheel and angle grinder to remove it, then laid out some mount points on card stock tapped to the frame.

And checked what would give enough clearance between shock and frame.

To accurately check the fitting with the swingarm in the up and down positions I made this jig with holes drilled at 410.5 mm and 306.5 mm.

Then, after all the checking I made this paper template for the upper mount.

Here are the upper and lower mounts I made up. I thought it would be hard to do the bends a one piece upper mount would need so I decided to make it from two pieces that would weld together.

First I welded on the outer piece.

And then the inner.

Here's a view from the inside with the shock in place.

With the upper mount welded on I used the jig to mark arcs at the extremes of swingarm angle at my chosen lower mount point (230 mm from the swingarm pivot). The red shows the upper limit that would not allow the transmission yoke to contact the swingarm housing when the suspension bottomed out, and the green the lower limit that would keep the transmission output flange bolts from tearing up the rubber swingarm boot when the suspension topped out. I would have liked a little more margin for safety, but had to accept what I was faced with. The black cross shows my drill point.

The lower mount would need a spherical rod end bearing to match the shock's lower clevis mount. Some searching in the McMaster-Carr catalog showed two bearings that could work, a 12x30x16 metric bearing and a 1/2"x1"x1/2" inch bearing. I decided on the 1/2"x1" bearing since it was cheaper and would allow a lower profile mount. I made up this mount from some rectangular tubing and round stock.

Here are the internal parts of the lower mount. I bored out the round stock to allow a press fit of the bearing and turned down the edges of some aluminum stock to make two spacers that allow the joint's ball to rotate a few degrees.

Here are the pieces assembled and the joint set at its limit.

And the mount tack welded to swingarm. The shock is installed for checking. I'm thinking to make some short tube shaped seals from neoprene sheet to keep dirt and water out of the bearing.

Not much was left to do other than to finish the lower mount welds. Here's a view from the rear of the finished installation. The reservoir at 45 degrees sticks out a little, but I'll make the sub-frame go around this as needed.

And a view from the side.

I'm really happy with the result. 280 mm (11") of travel and a first class shock. The hollow lower mount gussets didn't work out as well as I thought they would. The rectangle tube wall was relatively thin and difficult to weld to the thicker swingarm. I think some gussets from 1/16" or a little thicker sheet, maybe with some drilled holes would be better.


x3300 screwed with this post 11-27-2010 at 11:26 AM Reason: Correct thread ID.
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