Diff'rent Strokers

[arlurt]

I've finally spotted the flaw in my plans for the rear suspension, the flaw that I'm sure you've all been thinking about for ages, it's the problem that means I have to rethink what I'm going to have to do to get the rear suspension to work...

The Suzuki swing arm shock linkage uses metal-to-metal needle roller bearings so that there's no give at all in the linkage bushes. This is so that control of rear wheel movement is absolute, no flexing, it does exactly what the shock tells it to. And because the geometry of the linkage requires quite a lot of angular movement. This means the shock has to use the same type of needle roller bearings in the mount that fastens it to the linkage.

The Honda CBR125 shock is direct acting on the swing arm, and rubber bushed to insulate the rider from dodgy road surface vibration. The direct acting shock requires very little angular movement in the bushes because of the geometry of the direct acting shock doesn't need it.

And there's the problem. The rubber bushed, low-rotation, bottom bolt on the Honda shock won't work with the high-rotation Suzuki linkage. Of course I didn't understand this until I'd made the spacers and reduced the diameter of the bolt to fit the Honda shock to the linkage, and I didn't really accept it was a real problem until I nipped up the bolt on the bottom of the shock and saw how solid the rubber bush was. Of course the rubber bush has to be pretty stiff because the loads through that bush are quite high, even on a CBR125.



So what do I do now? The Suzuki rear shock has absolutely had it, so I can happily cut it up without losing anything. I think I can cut the bottom bush off the Suzuki shock and find a way of fastening it to the bottom of the Honda shock.

At least I won't need any spacers to make the bottom of the shock fit the linkage!
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Light is right.

[arlurt]

The top and bottom "bushes of the old MX rear shock as (knackered) rose joints so I cant do anything with those to helps with my problem. I might be able to use a new rose joint instead of the metelastic bottom bush on the Honda shock, or maybe I could put a greased bronze bush in the bottom of the Honda shock.



I don't seem to be getting anywhere today, so I'm off to the pub to seek inspiration there!
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Light is right.

[Cal]

Mate,
I am confident that your shock can be rebuilt as most mx shocks can.
Do you know what bike it is off?
In the days before children BC I used to try an ride mx, the rose joints will be available from somewhere like crooks suzuki in barrow.

[arlurt]

Thanks for your input Cal, but the reason I want to change the over to the CBR125 shock is mainly because the Suzuki MX shock is simply wrong for the KH250. It’s got too much travel, the wrong sort of damping, and the spring rate is nowhere near what I need for a road bike. When I started out (a long time ago) I’d planned to have the shock rebuilt with different damping and a new spring, but the more I’ve thought about it the more I’ve decided it’s the wrong way to go. A road bike needs a road shock and spring.

The CBR shock top mount will be fine, it doesn’t need the angular movement of the bottom mount so I can just leave that alone, but the bottom mount needs a rethink.

Options being considered are:
Remove the rubber bush and fit a rose joint. I’d make an alloy insert that fits where the rubber bush goes, with a thread that the rose joint can screw in to.
Remove the rubber bush and make a bronze bearing the fits where the rubber bush was.
Remove the rubber bush and get a needle-roller bearing that goes where the rubber bush was.

I’m concerned about how long a rose joint would last. Obviously it was good enough for the ‘crosser, but the KH is a lot heavier.
I’m sure a bronze bush will work fine, but getting to it to grease it would be difficult, and it’s a bit of an old-school fix. Fine in 1935, but surely there are better ways now.
The needle-roller would be the proper solution but they don’t allow for any mis-alignment and won’t fit in the existing bush-tube on the bottom of the shock.

The favourite for me at the moment is the bronze bush option. I can make it, and for the miles the bike is going to do, and the weather it will be used in, it should be fine.

A fourth and rather lazy option exists: I can put it all together with the existing rubber bush and see what it does. If it fails it will be safe, it will just get noisy and rattle about. It’d probably last a while and it might even surprise me and be up to the job. I’ll talk it over with an engineering friend in the pub tomorrow.
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Light is right.

[Gillinghampaul]

Push the rubber out, sleeve it and get a roller bearing to suit? Could make the sleeve a nice tolerance fit in the shock and the id could be whatever it needs to be to fit the roller bearing in?
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Gagging for it

[arlurt]

Thanks Paul.

Nice idea, and a good engineering solution, but I don't think I can machine to the tolerances required for a needle roller solution. I think the material will be too hard to work on my dodgy old lathe, and I think it would need heat treating to be effective.

Scratching about on eBay tonight I've found I can get top-hat shaped sintered bronze bushes with the right outside diameter to fit the shock eye, and an 18mm internal diameter that I can put a hard steel bush through on the 12mm bolt that passes through the alloy shock linkage.

At the moment, this is my favored way forward.
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Light is right.

[arlurt]

After a beer-fuelled discussion in the pub last night I know what I’m going to do. Two bronze top-hat bushes, a 100mm length of 18mm silver steel rod and a nice new Dormer 12mm drill bit have all been bought on eBay for me to play with at the weekend. I'll make an 18mm steel bush with a 12mm hole through the middle that will bolt in to the shock linkage. The Bronze bushes replace the rubber bush in the bottom of the shock, and the whole lot is blathered with grease. That’s another £25 spent and fortunately I don't keep a log of hours spent.
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Light is right.

[arlurt]

I’m continuing on the course that SB has pointed out to be “wrong” regarding work on the KH, as it gives you all something to remind me of when I inevitably hit problems. I still haven’t lashed up a way of confirming that the spring, damper and linkage ratios will work with the KH’s weight, a small-bike Honda shock, and a Suzuki Moto-X swing arm and linkage. I get over-interested in the “how-to” part of the build and forget about the “will-it” part, so thought I’d better do something before I start welding bits to the frame.

The plan is to extend the bottom shock mount so that it has the same overall length as the Suzuki Moto-X shock, and therefore it will fit between the shock-linkage and the shock top-mount already welded to the frame.

I’ve cut a length of old exhaust pipe that fits over the bottom of the shock and bush, with a temporary bush welded to the bottom of the pipe. This makes the Honda shock the same effective length as the (dead) Suzuki item and will let me mount the Honda shock between the swing-arm linkage and the existing top mount that I added to the frame years ago.

Then I can use the Goldie Locks test to see what happens:

1. The suspension collapses under the weight of the bike alone. Too Soft.
2. With me, my wife, a large gas cylinder and a cage full of live chickens sat on the bike, the suspension doesn’t move. Too hard.
3. Under its own weight there’s just a touch of sag, and I can bounce up and down on it without bottoming anything out. Just right.

Is there anyone out there who wants to predict what the result might be?
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Light is right.

[1975-S1C]

3 obviously

[arlurt]

Thanks Cal, just what I need.

Aw, what heppend to Option 4? I was going to offer my project planning and suspension consultancy skills. I'm great at planning me y'known...

My new bits have arrived:



If everything is here then it's all just bound to work!
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Light is right.

[Cal]

The answer will be dependant on wether or not the chickens can fly?
If they can it will be problem solved.

[arlurt]

Here's my lash-up extended Honda shock.



I'll have a go and see what happens over the weekend.

Fingers crossed!
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Light is right.

[arlurt]

I made the steel bush for the rear shock last night.



Unfortunately, I've got a garden fence to repair this weekend so I don't know if I'll be able to get anything else done on the bike.
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Light is right.

[arlurt]

Finally got round to nailing the lash-up rear shock to the bike.



It's absolutely spot on! Sags less than an inch when you stand it up, and I can bounce up and down on it without anything bottoming out. The combination of direct acting shock and linkage swing arm seem to have come together to cancel out the mis-matched spring and damping rates.







The front brake is OK too, as much as you can tell pushing it round the yard anyway.

So now I've got to make up a new top mount for the rear shock and weld it in the the frame, confident in knowing it'll all be right enough when it's done. Really relieved it's all come together.
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Light is right.

[Cal]

Result!

Really pleased for you.