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Author Topic: 1974 CB750 K4 Monoshock Conversion  (Read 342 times)

1974 CB750 K4 Monoshock Conversion
« on: Apr 11, 2018, 12:56:37 »
Hey guys, this is my first post on this forum. I need some assistance. I wanna do a monoshock conversion to cut off the subframe of my bike. I am attaching a photo of the bike and an option I was considering as far as mono fabrication. How horrible of an idea is it to use the stock swing arm and fabricate a shock here reinforced between the swing arm and where the tail and frame meet. This is just an idea. Iím sure thereís more to it but I was curious. (Also, the shock in the photo is upside down I believe)
« Last Edit: Apr 11, 2018, 12:58:33 by JonathanHValencia »

Offline focusinprogress

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Re: 1974 CB750 K4 Monoshock Conversion
« Reply #1 on: Apr 11, 2018, 15:56:23 »
so, I'm going to start off by speaking in theories and then say what I personally think....

the stock swingarm, pivot bolt(size and bushings and mounting points) are all designed for the inherent stability afforded by having two shocks mounted farther back to keep it from racking at all. It's also designed for the forces of suspension travel to be applied at the rear, and by moving to a mono shock (even with heavy bracing)....you're making the rear axle and any part of the swingarm behind the shock a lever with which to bend the swingarm at the point of the new shock location with respect to the fulcrum and the applied force ( a bump).

now, that's all real bad stuff in theory. based on that, your bike will be less stable and you run the risk of bending or breaking a bunch of stuff.

but, in practice....you could probably get away with making a trellis style bracing system to beef up the swingarm and get some of the rigidity back into the swingarm and make this happen....without riding the bike at the ragged edge it probably wouldn't be too large an issue to worry about.

That said, it would also be way easier and probably cost effective to swap in a swingarm from something monoshock that has a 20mm rear axle and work out the pivot bolt with bushings and then you just have to make a stout mount to the frame for the top of the shock. this also likely means using a linkage system for the bottom mount of the shock which will really make the bike perform a heck of a lot better than just squeezing a static rate shock and spring.

I'd say look into early honda Hurricane swingarms, they go for CHEAP....If memory serves that'll fit between the cb750 frame and has a 20mm axle so you could retain the stock wheel with just some spacers....not sure if pivot bolt is the same diameter though.
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Offline jpmobius

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Re: 1974 CB750 K4 Monoshock Conversion
« Reply #2 on: Apr 12, 2018, 10:38:14 »
Good points from focusinprogress.  Presumably, the intent here is to improve your motorcycle rather than simply change its appearance.  That is quite a tall order compared to what at least visually seems like a fairly straight forward job.  There are several areas to focus your attention on.  The first is the kinematics involved, which is the relationship between the moving parts as they pass through the travel of the system.  It is important to consider what the total travel of the swing arm will be.  Generally you will need to reproduce the stock travel, both in degree orientation to the frame and the range of motion.  That is due to the constraints of the chain drive - you can't change it substantially without incurring trouble, and this includes the static sag of the new suspension.  Once this is determined, the shock location and orientation needs to be determined.  This is driven by the angle it ultimately makes with a line drawn from its lower mount and the swing arm pivot point.  The swing arm itself is in no way a factor here aside from whatever mechanical attachment and structure needs to be created to accommodate the desired location.  This angle is the driver because as the suspension compresses, the angle the shock makes with the swing arm pivot changes and in doing so changes the rate at which the shock compresses which is critical in how the suspension behaves.  Generally, the rate at which the shock compresses should increase and max out at full compression.  How much, and how fast this should occur is worthy of a book, or at least a chapter or two.  The idea is that small bumps should easily move the suspension, but the increasing rate will keep large bumps from overwhelming the system.  This is easy enough to manifest, but aside from doing it all on paper (or a computer console) I can't imagine doing it without a bare frame and springless shock with which to mock up the system and run it through its range of motion to make sure it is all within reason.  Since you are starting with an existing bike not intended for this arrangement, your decisions will be driven by the existing architecture and what you are willing to delete.  Secondly, serious consideration must be made regarding the structure of the modifications, as has already been mentioned.  The existing swing arm pivot is likely ok, despite the new loads being MUCH higher than with the dual shocks.  Increased wear could be a problem, but I'd reckon there is no actual safety issue.  Whether or not the increased loads will create generally undesirable flexing in the chassis creating durability or driving issues is hard to say.  The important thing to realize is that the loads on the shock mounts are VERY high, and spreading them out into both the arm and the frame appropriately is critical to avoid localized failure.  This is not the same thing as reinforcing the frame and arm to keep flexing at bay.  You could triangulate the frame to make it plenty stiff, but still have the actual mount fail if it isn't engineered sufficiently.  Finally, the sub frame supporting the rider needs some consideration.  Where with the twin shocks, that weight was substantially transmitted directly to the ground via the shocks and wheel, now the entire load must be transmitted back into the frame and then into the rest of the chassis before reaching the ground.  Much of course depends on the riders position here, but I'm assuming this is a street bike where the rider actually sits a great deal of the time.  Again, it's good to do an actual full motion mock up to see if everything works before you carve things into stone.  Looks like your hoop will become a brake at full compression.  Designing the sub frame is simple compared to the suspension, so finalize the hard stuff first.
Mobius


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Offline focusinprogress

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Re: 1974 CB750 K4 Monoshock Conversion
« Reply #3 on: Apr 12, 2018, 11:04:20 »
jpmobius - school is in session. ha.

well written and concise. A+
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