Mono shock and fork conversions

Hi,
I have started a build on a 78 KZ650 that was mostly beyond restoration, It was sitting in some over grown weeds for over 10 years. The motor still turns and has compression, The frame, tank and some plastics I can save. The shocks, front forks, wheels and tries are junk along with most of the wiring. I had done a lot of research before starting the work and everything I found on this topic had me concerned, I just wanted to give this bike a new life so I went at it anyways. I picked up a parts bike for cheap, A 2000 YZF 600r for $50.00 that was missing all it plastics but had everything I was looking for. Now this may sound wired but it was nothing as explained, This swap seemed to be easy and that was unexpected. I am a steel fabricator by tread and I do have a full shop to work out of but no machining tools, Hence my concerns. I was able to install the YZF front forks for the cost of new stem bearings as the stems was the correct length. I did a mono shock conversion using the YZF swing arm and shock, The YZF pivot point uses the same size bearing as the KZ and the O.D. of the pivot bolts are the same, I only had to shave a mil. off of each frame boss at the pivot point as the OE KZ swing arm was 2 mil. narrower than the YZF swing arm. At this point I fabricated mounting tabs for the rear shock and set it to the exact angle and location as it was in the YZF and tacked it in place. I no that I need an offset sprocket of about 5/8" to 3/4" and may make something out of some old sprockets to test exactly what the offset is. I had never heard of anyone using a YZF to do a swap like this and maybe I'm the first but it just seemed way to easy and that has made me nervous, I feel like something is wrong. There was no machining anywhere, I didn't even need to relocate the battery. I haven't full welded any tabs yet, I'm hoping someone might be able to either restore my confidence or point out what may be wrong with this build. Thank You for reading and I'm looking forward to any response.

I Like it! Congratz on finding a simple donor. Most conversions aren't that easy. Keep it up!

Copying the geometry of the donor shock mount was a smart move. Looks like you did a god job. The biggest issue many modern swaps face is the loss of ground clearance and a relatively small fork offset in the triple trees. Which produces a large trail number which can make the bike sluggish to turn at speed. Keep up the good work. heres a link to a trail calc if you are so inclined. www.rbracing-rsr.com/rakeandtrail.html

I always love seeing innovation like this, well done.

The engineer in me is a little worried about the amount of movement you might get in that top mount though. The shock will be applying pressure right in the middle of that cross-member, which would likely bend it up over time. And with the mount protruding from the cross-member it will provide a twisting force too. Some gussets for the cross-member should help prevent the cross-member from deforming, however the twisting force would be a bit more difficult to solve without adding another cross-member.

Irish Yobbo wrote: I always love seeing innovation like this, well done.

The engineer in me is a little worried about the amount of movement you might get in that top mount though. The shock will be applying pressure right in the middle of that cross-member, which would likely bend it up over time. And with the mount protruding from the cross-member it will provide a twisting force too. Some gussets for the cross-member should help prevent the cross-member from deforming, however the twisting force would be a bit more difficult to solve without adding another cross-member.

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I’m not a fabricator nor an engineer, just an old carpenter, but I echo Irish Yobbo’s thoughts. I’m also worried about the torsional forces applied to the spreader pipe the upper mono is mounted to. Along with the upper force bending it like IY mentioned.

Hello Mr. Yobbo,
I was planning on boxing in the top and bottom mounts, After mock up is complete I will strip the bike to full weld and cut off any un-needed components. I did want to keep this as clean as possible and only add what is needed. The mount on the donor was designed in similar way and out of a much thinner material. The OE. mounting tabs on the YZF are much shorter though, The centerline of the bolt line up with the outside face of tube. On that note would the torsional load still be a concern to you? I can, After boxing in the mount run a tube vertically and tie into the frame under the seat somewhere. Thank for you impute... Looking forwarded to more.

Thats essentially how the oil cooled GSXR frames were designed, maybe marginally bigger on the cross-member, but that portion was a cast aluminum piece. They were not not known for their rigidity but they were competitive bikes in their day. That being said i think it would be worth it to brace the frame there regardless. including by the steering neck, and upper backone. All the flex the swingarm and forks had before is now more directly translated to the frame via the new stiffer suspension components.

Was it only the first few years of the GSXR frames that tended to fracture? Ed

www.google.com/search?source=hp&ei=XNM9X...r8C2gQ4dUDCAg&uact=5

650ed, bad, bad, bad!

First of all; thats a water cooled GSXR. looks like a K6 which means its 20 years younger than what im referring to and it has almost nothing in common with what i'm talking about. i know there was a frame recall in the mid 2000s on the 1000. Regardless, that is a perimeter style frame and also it cracked near the head-stock not the upper-shock mount, so i really don't see how that is relevant at all, unless you are trying to push your "motorcycle engineering peaked in 1978" ideology.

The early oil boiler frames is what i'm talking about. 1986-1988. The frame design is similar to the tubular steel frames that preceded it so it is a good reference point when adding a mono shock.

Looks like a gusset that ties A to B

that is the lower shock mount on the swingarm. I have torn several of these bikes down, there is only the single crossmember