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Author Topic: Rigid rear conversion to single shock swingarm APS-PG bike  (Read 8369 times)
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Koncretekid
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« on: November 29, 2017, 09:09:50 AM »

My last run at BMST this year resulted in a 144mph speed in the mile with no speed in the kilometer due running outside the kilo timing lights and moving back in time to catch the mile lights.  The salt was there but not exactly flat which caused me to drift badly during the timed mile.  I decided that maybe it was time to add rear suspension.

I searched the internet for single shock designs realizing that twin shocks would be difficult to fit under the rear fairing.  I couldn't find a design suitable for my bike due to location of the gas tank and battery.  The battery is not an issue, as I can use a smaller one mounted just about anywhere, but the gas tank is unique.  So I came up with my own design shown here, but modified since I made this initial drawing.


* BSA LSR Swingarm drawing.jpg (159.98 KB, 1806x875 - viewed 151 times.)
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Koncretekid
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« Reply #1 on: November 29, 2017, 09:19:58 AM »

The first photo is just a side view of the rigid rear where you can see the gas tank as well as the battery box (lower left) but the battery has already been removed.

Following are the beginnings of the swingarm. The front will be used as shown, but the rear will have to be widened to accept the drive wheel and sprocket.

I should mention that the insides of the swingarm has only been mig tacked at this point, hence the ugly tacks.  After tig welding the outsides, I ground off the inside tacks and tig welded them as well.


* Narrow swingarm fully welded.jpg (181.68 KB, 1280x853 - viewed 108 times.)

* Welds on swingarm.jpg (211.67 KB, 1280x853 - viewed 113 times.)
« Last Edit: November 29, 2017, 10:12:47 AM by Koncretekid » Logged

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Koncretekid
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« Reply #2 on: November 29, 2017, 09:22:21 AM »

Swingarm and pivot shaft supports to be welded into frame, and trial mounting to locate the shafts.  The bike is mounted on the same jig I used to build it, so the rear axle supports are used to support the rear axle in its new position, about 3" further back.


* Swingarm and pivot axle supports.jpg (153.55 KB, 1280x853 - viewed 100 times.)

* Swingarm and pivot trial mounting.jpg (206.24 KB, 1280x853 - viewed 126 times.)
« Last Edit: November 29, 2017, 10:15:09 AM by Koncretekid » Logged

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Koncretekid
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« Reply #3 on: November 29, 2017, 09:36:31 AM »

Swingarm and pivot shaft mounts welded to frame and upper shock mounts.  The shock unit I have chosen is sold for 100cc pit bikes with a maximum spring force of 900 lbs. (indicated on spring).  I could not fully check this, but initial preload seems to be about 300 lbs.  The upper shock mount is infinitely adjustable so initial sag can be adjusted.


* Swingarm with welded supports copy 2.jpg (238.85 KB, 1280x853 - viewed 118 times.)

* Swingarm mounted with upper shock mounts copy.jpg (242.54 KB, 1280x853 - viewed 117 times.)
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Koncretekid
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« Reply #4 on: November 29, 2017, 09:43:04 AM »

Too late to turn back now! Somebody cut off my frame!  Also shown are the swingarm and pivot parts on the bench.  Swingarm is obviously too narrow to accommodate the drive wheel, but I had a front wheel I could mount in it with sprocket just laid in against it to check chain clearances.


* Frame cut off.jpg (228.31 KB, 1280x853 - viewed 117 times.)

* All swingarm parts - narrow.jpg (176.06 KB, 1280x853 - viewed 108 times.)
« Last Edit: November 29, 2017, 10:16:40 AM by Koncretekid » Logged

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Koncretekid
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« Reply #5 on: November 29, 2017, 09:46:07 AM »

The wheel I was using for the trial fittings is actually a CB350 front wheel with a 17" rim and 110/70 tire which fits but is 7mm too small on the radius as well as 10mm narrower, but fit in the swingarm.  The next step was to widen the swingarm for the proper rear wheel on which I run a 120/70 tire.


* Narrow swingarm with front wheel.jpg (236.16 KB, 1280x853 - viewed 97 times.)

* Swingarm widening on jig.jpg (178.07 KB, 1280x853 - viewed 122 times.)
« Last Edit: November 29, 2017, 09:49:33 AM by Koncretekid » Logged

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Koncretekid
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« Reply #6 on: November 29, 2017, 09:56:16 AM »

The  widened swingarm fit almost perfectly - - except that the wheel was 3/8" too far to the right for chain alignment.  I had neglected to allow extra width because the original rigid rear had only a flat plate at the rear which allowed the wheel to be set closer to the left.  So back to the cutting table!  But all went well and the second try fits and works.

I had to remake the pivot to change holes and hence the leverage, but now I have about 3/4" of sag with an overall swingarm to spring ratio of about 2 to 1.  The spring has 2" total movement available (theoretically) so the swing could move 4".  So far, I haven't been able to move it more than 2", but that should be plenty .


* Rear wheel offset on swingarm.jpg (190.39 KB, 853x1280 - viewed 113 times.)

* widened swingarm mounted with wheel.jpg (237.45 KB, 1280x853 - viewed 365 times.)
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We get too soon oldt, and too late schmart!
Life's uncertain - eat dessert first!
Koncretekid
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« Reply #7 on: November 29, 2017, 10:06:15 AM »

Brake side is fine as well.  The rear fairing piece fits but I need to add 2 more mounting points for the Dzus fasteners.  The additional benefit of this change is that I've moved the wheel rearward by 3-4" which will allow me to extend the tail piece in the future for improved streamlining.

Now for some questions.  I need to get shafts made for the swingarm and for the pivot.  They will extend thru the frame supports, probably with castle nuts and cotter pins.  I installed brass bushings in the swingarm which need to be reamed to fit a 3/4" shaft.  Can I use cold rolled round for the shafts or do I need something better?

Anybody else out there want to try out this bike to check my welding skills?

Tom


* Swingarm brake side.jpg (237.95 KB, 1280x853 - viewed 131 times.)

* New swingarm with rear fairing.jpg (149.01 KB, 1280x853 - viewed 104 times.)
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Peter Jack
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« Reply #8 on: November 29, 2017, 12:01:04 PM »

Tom, I looked closely at the welds you put on the bushings and cross tube. I'd have no fear getting on that bike. Well done!  cheers cheers cheers

Pete
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« Reply #9 on: November 29, 2017, 12:25:36 PM »

Hey Tom
     You have been busy---  nearly ready for '18

    If you haven't reamed the pivots to size yet you might be able to find a 19mm axle or
     a swingarm pivot from a stock Japanese bike which you can use or modify.
    19 is an odd size for a metric bearing though.
    I think the cold rolled will be ok for the shaft but something a bit harder would make a better
    bearing surface
    You may be able to find some 3/4 ground bar for the axle( save a bunch of machining)
    
  
    Maybe 3-4" is unnecessary for the wheel travel. My forks have under 2" and I feel 90% of
     the vibration this year came from the rear. I think 2" is enough
    My bike ran  the same at Bonneville-205 on a bad track and at Gairdner also 205 on a good track but
    but this year at Bonneville I was having the $hit shaken out of me.
  
    I think it is the vibration that we are trying to eliminate.

    Just my 2c worth
    cheers    Bones
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« Reply #10 on: November 29, 2017, 07:06:18 PM »

Koncrete,
Now that you have kinks in the swingarms, it might be a good idea to think about just how rigid the arrangement will be when subjected to the chain load and tractive load.  It wonít take much flexibility in the rear end to make for an anxious ride.
You might want to scab on a half-piece of tube gusset on the outside where the kink goes narrow if there is room--anything will help.  Also, a piece across from arm to arm in front of the tire would be beneficial.
Might tension up the chain and then spread the two runs and see what happens.  That would test the stiffness of the pivot arrangement too, which is kind of narrow.
You probably donít want the rear end to change direction whenever you shift...
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wobblywalrus
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« Reply #11 on: November 29, 2017, 11:14:04 PM »

Tom, speed sensitive compression damping helps. 
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« Reply #12 on: November 30, 2017, 03:53:26 AM »


Hi Tom looking great! Could you form the swing arm rail out of one piece instead of welding 3 pieces per side?

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Koncretekid
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« Reply #13 on: December 02, 2017, 07:22:53 PM »

Tom, I looked closely at the welds you put on the bushings and cross tube. I'd have no fear getting on that bike. Well done!  cheers cheers cheers

Pete
Thanks Pete, but you only saw the easy parts.  It's one thing to weld in a ferrule that is a close fit with no filler rod required while it is clamped in a vise and I'm sitting on a stool resting my arm on the vise.  It's quite another to do the vertical welds I needed to do while the swingarm was clamped to the jig that the bike was sitting on to prevent it from warping.  My vertical welding skills still need some improvement.
Hey Tom
     You have been busy---  nearly ready for '18

    
    Just my 2c worth
    cheers    Bones
Yes, almost ready, but aren't we always almost ready.  I'm thinking that 2" of travel is enough as well, but I'm worried that the quality of the shock may be questionable, as I can't feel any sort of dampening.  As you can see below, I got a local machine shop to make me some shafts from cold rolled and ream out my bronze bushings.  I also remade the "links" that connect the link shaft to the link so there is almost no slack.  When the mechanical advantage of the rear movement to the link shaft movement is 7 to 1,  a small amount of clearance, say .010" on each end, get multiplied into .140" movement at the rear.  So without proper dampening, the first 1/8" or so is just going to vibrate.

Koncrete,
Now that you have kinks in the swingarms, it might be a good idea to think about just how rigid the arrangement will be when subjected to the chain load and tractive load.  It wonít take much flexibility in the rear end to make for an anxious ride.
You might want to scab on a half-piece of tube gusset on the outside where the kink goes narrow if there is room--anything will help.  Also, a piece across from arm to arm in front of the tire would be beneficial.
Might tension up the chain and then spread the two runs and see what happens.  That would test the stiffness of the pivot arrangement too, which is kind of narrow.
You probably donít want the rear end to change direction whenever you shift...

Thanks for that observation.  It's been a long time since I've done of those kinds of calculations, but I've included another drawing here that shows the chain pull, the offset, the cross section of the tubing, and my calculations.  They're not exact, but the 19,000 psi stress caused by the chain pull is more than it will see, as the chain pull is actually about an inch to the right of the left arm so some of the load will actually be shared by the right arm.  Also, the connections of the rear axle form a rigid moment connection, so I think that also spreads the load.  In any case, the maximum bending moment occurs at the kink in the arm which is just where it is hardest to reinforce.  I would be happy if you would check my calcs.

Correction on my drawing - -chain pull would be closer to 1100 lbsf, so stress would be significantly less, based on 50 ft-lbs torque, 1.86 primary ratio, 2 to 1 transmission ratio, and 21 tooth front drive sprocket.


Hi Tom looking great! Could you form the swing arm rail out of one piece instead of welding 3 pieces per side?



Alp,
That may be possible, but I had to bump out that arm in a very short distance to avoid having to change my footpeg location and avoid the rear tire.  Besides that, I didn't have the tools to bend rectangular tubing, but I did have a chopsaw and a welder.

Tom




* Swingarm top view drawing.jpg (202.92 KB, 1503x872 - viewed 109 times.)

* Swingarm with new shafts.jpg (73.71 KB, 640x427 - viewed 106 times.)
« Last Edit: December 02, 2017, 09:42:40 PM by Koncretekid » Logged

We get too soon oldt, and too late schmart!
Life's uncertain - eat dessert first!
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« Reply #14 on: December 02, 2017, 07:42:11 PM »

Good work Tom! I think if I were building that I would incorporate plate gussets top and bottom on both sides. Twice the wall thickness of the tube, with tapered width on front and rear to carry the stress through the butt welded joints better. Maybe a  better description is they would look like lightening bolts.  cheers
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