Landracing Forum
Tech Information => Steering - Suspension - Rear End => Topic started by: Koncretekid on November 29, 2017, 09:09:50 AM
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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.
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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.
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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.
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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.
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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.
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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.
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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 .
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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
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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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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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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 won’t 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 don’t want the rear end to change direction whenever you shift...
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Tom, speed sensitive compression damping helps.
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Hi Tom looking great! Could you form the swing arm rail out of one piece instead of welding 3 pieces per side?
(http://www.landracing.com/forum/index.php?action=dlattach;topic=16962.0;attach=58076;image)
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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 won’t 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 don’t 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?
(http://www.landracing.com/forum/index.php?action=dlattach;topic=16962.0;attach=58076;image)
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
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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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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:
Ed,
I thought as you did about adding reinforcing to the top and bottom of the swingarm over the welded connections, but after an analysis of the stresses, they do not appear to exceed 12,600 psi even at the maximum force generated by the spring-shock. That's less than 25% of the yield value of mild steel, so I shouldn't have to worry - - unless my welding is faulty! Under normal conditions, the forces will only be about 3/4 of that value. But since Pete says my welds look good and he wouldn't hesitate to ride my bike, I think I should let him have a go at it. Luckily, the chain force stress occurs in the horizontal plane while the shock action is in the vertical direction so they are not directly additive (it gets complicated). The other comforting thing is that with the suspension, the impact loading is nearly eliminated resulting is less stress on other members (hopefully including me.)
Tom
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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:
Ed,
I thought as you did about adding reinforcing to the top and bottom of the swingarm over the welded connections, but after an analysis of the stresses, they do not appear to exceed 12,600 psi even at the maximum force generated by the spring-shock. That's less than 25% of the yield value of mild steel, so I shouldn't have to worry - - unless my welding is faulty! Under normal conditions, the forces will only be about 3/4 of that value. But since Pete says my welds look good and he wouldn't hesitate to ride my bike, I think I should let him have a go at it. Luckily, the chain force stress occurs in the horizontal plane while the shock action is in the vertical direction so they are not directly additive (it gets complicated). The other comforting thing is that with the suspension, the impact loading is nearly eliminated resulting is less stress on other members (hopefully including me.)
Tom
I was mainly thinking about it from the perspective of rigidity. I'm sure that your structural yield strength is good, but the gussets would have the effect of eliminating a lot of potential flex by passing the stresses through the joggled sections.
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Here is a new drawing with revised chain pull and stress calculations. I have also shown some small gussets in the area of the welded joints to improve stiffness as well as reinforcing my welded connections.
Feel free to comment. I have yet to add another crossmember just ahead of the swingarm and another tube on each side to stiffen up the fairing tailpiece connection.
Tom
I just noticed that I typed in "/1.2" which should read /2.1" on my drawing in the calculations area and I can't seem to post the corrected drawing.
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Tom, if those gussets on the zig zag are pieces of 1" x 2" tubing you're going to create a sealed chamber when you weld them up. Drill a small hole in the swing arm so the space you've formed vents. That will prevent the expanding air from blowing the weld metal back onto the tungsten. You don't need the extra practice resharpening the tungsten. :-D :-D :-D
Pete
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Tom,
What I'm looking at is the vertical stresses from hitting bumps as speed causing the sides of the swing arm to flex. With the left side having 1/2 inch more offset than the right, it looks like it will have more flex. Experience from riding choppers with worn out (and poorly designed to begin with) plunger suspensions tells me that this is not good if it happens, even in a straight line.
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Pete,
Thanks for the reminder - - I would have forgotten to drill them.
Ed,
Your point is well taken. The entire swingarm has to twist in order for that to happen. But there is only so much that can be done to stiffen up a long swing arm. The rear axle passes thru holes in the internal solid blocks of 3/4" aluminum which makes what are called "moment" connections. This ensures that one side cannot twist without twisting the other side (as long as the connections are tight). If you look at older swing arms, usually round, they terminate in flat plates that have less resistance to twisting than the rectangular tubing with the internal solid pieces, especially if they are not extremely tight. The only way I know of checking this is to physically grab the rear wheel, top and bottom and try to twist it. On this design, when I try this even with the front end tied down, the whole bike frame twists simultaneously, so I feel that it is very good. Luckily, I'm not planning to run any more slalom courses at Bonneville, at least not intentionally!
Tom
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Tom, if in doubt, using double shocks like a normal bike will take a lot of load off of the swing arm. Another advantage is the dual shocks act like struts if they are bottomed out and this takes huge loads away from the swing arm.
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Tom, if in doubt, using double shocks like a normal bike will take a lot of load off of the swing arm. Another advantage is the dual shocks act like struts if they are bottomed out and this takes huge loads away from the swing arm.
Bo,
I'm probably not normal and neither is my bike! And I don't plan on running the Baja 1000, so the possibility of the shocks "bottoming out" is hopefully not going to happen. Besides that, who wants to read a topic about adding normal twin shocks to a normal swingarm?
Tom
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How would shocks bottom out on the salt?
Back about ten years ago (when the salt was a tad smoother than today) we hadn't properly set the compression rebound on the shocks on the ZX12. It was too slow - so the bumps kept slowly compressing the shocks more and more -- and pretty soon we would get handling issues as the suspension geometry changed/shocks got "shorter".
Don't say it can't happen!! :evil:
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How would shocks bottom out on the salt?
Back about ten years ago (when the salt was a tad smoother than today) we hadn't properly set the compression rebound on the shocks on the ZX12. It was too slow - so the bumps kept slowly compressing the shocks more and more -- and pretty soon we would get handling issues as the suspension geometry changed/shocks got "shorter".
Don't say it can't happen!! :evil:
I certainly hope not. But the dampening could be an issue. You state that you had to ease off the rebound adjustment on the shocks to prevent their jacking down or whatever it is called. These Chinese pit bike shocks don't have any dampening adjustment, but they don't seem to have much, if any, anyway.
And Bo, I hope I didn't offend you with my offhand comments about the twin shocks, but truly I'm surprised nobody came up with "Why didn't you just add twin shocks?" earlier. The reason is that I was afraid that they might interfere with the fairing, and the subframe on my bike wasn't designed to handle the load at the rear extremity. And I guess I wanted to figure out how the single shock would work and now I've learned something. We get too soon oldt, and too late schmart!
Tom
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Tom,
Love what you have done to your bike! One comment, that goes along with several others, is that the fit between the swing arm bushings and the shaft need to be very close especially as the distance between the bushings is somewhat narrower than normal. I would also suggest using 1144 ground and polished rod for the swing arm bolt. It is strong, over 100,000 psi tensile, very easy to machine and the tolerance on the ground and polished items is +0 -.001. One other thing that I have found works of resisting corrosion is to soak the item in Gibbs oil after you are done. Let is set for a couple of weeks, what ever it does it seems to work.
Nice job!
Rex
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The spring/shock system absorbs and dissipates impact loads. It needs to allow controlled wheel movement so it can do its job. The front and rear spring rates are set so the suspension bottoms out occasionally on a rough track. So, adequate strength in the swing arm assembly during botttom-out is a concern.
Last year the suspension travel was used up a few times during the pass I made. The track was real rough and the suspension was working very well. Lots of impact was dispersed and dissipated before it upset the chassis.
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The spring/shock system absorbs and dissipates impact loads. It needs to allow controlled wheel movement so it can do its job. The front and rear spring rates are set so the suspension bottoms out occasionally on a rough track. So, adequate strength in the swing arm assembly during botttom-out is a concern.
Last year the suspension travel was used up a few times during the pass I made. The track was real rough and the suspension was working very well. Lots of impact was dispersed and dissipated before it upset the chassis.
Bo,
I didn't realize that you experienced full compression in your suspension last year. What is the load on your rear axle at the shocks at rest with you aboard? And what is the maximum travel of the shocks? Do you know the spring rates and especially the spring force at full compression? Knowing those numbers would help me in setting up my suspension as I can adjust the rate/travel and the initial sag to compensate. As you know, we can't determine the load on the swingarm when it bottoms out because it is an unknown impact load, and that is what will bend or break a swingarm with the single shock at the front.
Love what you have done to your bike! One comment, that goes along with several others, is that the fit between the swing arm bushings and the shaft need to be very close especially as the distance between the bushings is somewhat narrower than normal. I would also suggest using 1144 ground and polished rod for the swing arm bolt. It is strong, over 100,000 psi tensile, very easy to machine and the tolerance on the ground and polished items is +0 -.001. One other thing that I have found works of resisting corrosion is to soak the item in Gibbs oil after you are done. Let is set for a couple of weeks, what ever it does it seems to work.
Nice job!
Rex
Thanks Rex, and I will look for such a shaft and the important align boring necessary to ensure a good fit next time I'm in Colorado. For the time being, I've had a shaft made of cold rolled. Unfortunately, reaming the bronze bushings is only as good as the machinist doing the job, and when you don't have the tools to align bore/ream the bushings, they end up with too much slack. I'm guessing something like .005", but the machinists around here, both of them, normally only work on tree farmers, bulldozers, and fishing boats which apparently live well with that kind of tolerance.
Tom
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The spring/shock system absorbs and dissipates impact loads. It needs to allow controlled wheel movement so it can do its job. The front and rear spring rates are set so the suspension bottoms out occasionally on a rough track. So, adequate strength in the swing arm assembly during botttom-out is a concern.
Last year the suspension travel was used up a few times during the pass I made. The track was real rough and the suspension was working very well. Lots of impact was dispersed and dissipated before it upset the chassis.
Bo,
I didn't realize that you experienced full compression in your suspension last year. What is the load on your rear axle at the shocks at rest with you aboard? And what is the maximum travel of the shocks? Do you know the spring rates and especially the spring force at full compression? Knowing those numbers would help me in setting up my suspension as I can adjust the rate/travel and the initial sag to compensate. As you know, we can't determine the load on the swingarm when it bottoms out because it is an unknown impact load, and that is what will bend or break a swingarm with the single shock at the front.
Love what you have done to your bike! One comment, that goes along with several others, is that the fit between the swing arm bushings and the shaft need to be very close especially as the distance between the bushings is somewhat narrower than normal. I would also suggest using 1144 ground and polished rod for the swing arm bolt. It is strong, over 100,000 psi tensile, very easy to machine and the tolerance on the ground and polished items is +0 -.001. One other thing that I have found works of resisting corrosion is to soak the item in Gibbs oil after you are done. Let is set for a couple of weeks, what ever it does it seems to work.
Nice job!
Rex
Thanks Rex, and I will look for such a shaft and the important align boring necessary to ensure a good fit next time I'm in Colorado. For the time being, I've had a shaft made of cold rolled. Unfortunately, reaming the bronze bushings is only as good as the machinist doing the job, and when you don't have the tools to align bore/ream the bushings, they end up with too much slack. I'm guessing something like .005", but the machinists around here, both of them, normally only work on tree farmers, bulldozers, and fishing boats which apparently live well with that kind of tolerance.
Tom
Once upon a time, in a land far away, I was building a custom off road machine and fabricating the suspension system myself. I lacked the means of creating a close tolerance fit at the pivot points, much like you now. At the time there was a a product on the market that was sold to "repair" worn out ball joints. It was an epoxy material that was mixed and injected into the joint through the grease fitting. (Was not anything like a permanent repair and used primarily by shady used car salesmen.) I found this perfect for my needs. I put a grease zerk on the joint, smeared grease on the pivot bolts, and injected the material into the joint and let it cure. Like I said, it didn't last forever, but was perfect for the limited use I needed, and would work well for your purposes here too. I don't know if it's actually still available anywhere and don't remember the brand name, but you could probably come up with something to work the same way.
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PM sent
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Updates to show reinforced swingarm, side struts, and rear fairing mounting stalks. The added reinforcements to the swingarm are made from sections of the 1" x 2" tubing. The stiffeners at the front have to clear the links and the pivot (rocker arm?). The side struts are added to ensure the subframe is strong enough to resist the spring forces and support the rear fairing. There are two "horns" sticking out the side which will support the rear fairing mounts.
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Trouble with my snail internet. I'll try some more photos.
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I guess it's just one at a time!
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Here you can see the stalks at the side of the new struts that will support the rear fairing. The top view shows the set screw collars which I had to make as they aren't available locally. They are welded to the swingarm as well as to the stiffeners at the front. Everything is now tacked in place. The next step is to strip down the frame, finish the welding, sandblast, and repaint when the weather permits.
Tom
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Looking great Tom!! You are among several people I know that are adding rear suspension to their Bonneville racers before next year. You have started a trend!
rex
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Nice job Tom. Looking at those welds I'd continue to feel that I'd be comfortable riding your machine. :cheers: :cheers: :cheers:
Pete
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Nice job Tom. Looking at those welds I'd continue to feel that I'd be comfortable riding your machine. :cheers: :cheers: :cheers:
Pete
That is quite an endorsement .... coming from Pete
Joe
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Tom, did you get the welding finished? It was warm enough to blast up our way!
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Frank,
Funny you should ask. I finished welding a couple of days ago, cleaned off as much of the old paint as possible with paint remover, and just got done sandblasting. Not much fun in freezing weather with a Princess Auto (think Harbor Freight) sandblaster and cheap hood, but it does the job.
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I really wanted to see this mod in person; maybe in a couple of months... :)
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Painted and reassembled but I ran out of black so --- had to use some leftover orange from my first road racer for the swingarm parts because I couldn't afford gold plating. Off to Colorado for Christmas with the grandkids, so final reassembly with motor, and adding a new battery box will have to wait. Here are a few photos:
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And with the rear wheel in place and top view:
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(http://www.landracing.com/forum/index.php?action=dlattach;topic=16962.0;attach=58180;image)
Good work :cheers: :cheers: I like it,
Sumner
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I have now completed the fitting of the fairings, but no without a lot of trimming of the additional frame members that I managed to get about an eighth inch too high. I had to actually cut off the tops of both new struts and weld in patches to get the rear fairing fitted properly. For some reason, four of the rear fairing mounts had to be re-positioned in order to be able to get the Dzus fasteners to engage easily (more-or-less!)
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Rear view showing rear wheel clearance to tail. I'll be able to extend the fairing by 3-4" to stay within the 10" rule.
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It looks good, Tom.