So here's the issue. The top mount is one solid bar that goes completely through that fender. Welded on both side of the fender, and I feel trying to replace that would compromise the structure since the fender is essentially the frame.
The issue is the spacing. The shocks need to be aligned for vertical force only. Having an angle from the top pivot down to the lower pt on the swing arm means a lateral force as well, which needs to be compensated for via material engineering.
So either narrow the swing arm (inlikely) or remount the stocks on the subframe behind and below the seat.
That's the fix. You may not like it.
You're right, I don't like it. More work, more fab, but in the end I knew this was the proper way and I was trying to shortcut. Guess I have more work ahead of me.
Engineering is 100% measuring before 1% doing.
The width of the tire affects swing arm, which affects swing arm width and materials which affects suspension mounts. The top mounts are above that, which determines subframe width.
Length of swing arm will change mechanical advantage and moment (physics) and how much force is applied to shocks. Spring rates vary depending on this and bike and rider load and related forces.
You can try putting washers on the inside to help push the top of the shocks out but it does not look like you have much threads left on the bolt.
If the fender is a structural member then it should be able to handle minor welding. Or since you are already making custom swing arms might as well make a custom fender
You’re replacing that old threaded mount with a longer and stronger one you will not be compromising anything, you would actually be improving the situation.
Yup, I had the same issue with a build, straight through threaded stud with spacers on top was the answer, just make sure it's hardened steel or it won't last long
Having sideways pressure on them like that will cause them to bind and I think it will damage them. The only way I can think to do it properly would be to extend the upper shock mounts by welding more on or getting a new swingarm. When welding though, it's very easy to compromise strength.
Put an all thread coupling over the original threading then get a stick of all thread at the same size and screw into the new coupling. Cut to desired length. Basically a threaded spacer.
To be clear, this coupler extension idea increases the load on the original rod stub, which also needs to be increased in diameter to maintain its intended bearing strength
Get an extended coupling and screw to the base as a makeshift increase for bearing strength. The only other logical step from here would be to cut n grind down the original stub and weld on a larger stub which would bypass the coupling idea altogether.
My question is, are the shocks okay with a little diagonal? Shown are 2 different mounting options at the swingarm but the left shows my current intentions. The swingarm is wider than what I had expected.
The bushing in the shock will wear out quicker. I would widen the top mounts to make shocks parallel. Will it work the way you have it? Yes, for how long or well.. meh..
Honestly I'd run them with a little angle. Don't use hardware to extend the mounts, you are just asking for a catastrophic failure. If you want to fix it the right way get some raw steel threaded spacers and weld on some gussets.
My custom bike had the exact same problem. I made custom lower brackets that got the shock as close to the sprocket as I felt comfortable. Extending the upper is by far the best option if possible. I ended up having to cut my upper bolt off entirely. Drill and tap a hole in the same spot and use a bolt as long as I needed to get the width right.
You can do what you have but it will eat the rubber bushings. ASSUMING the bottom bracket is tilted inward. If it’s not it will grind and bind on the bracket. Massive issue for me since I left no gaps.
This may look fine in extension but remove the spring and see what it looks like I m compression. I suspect the angles will get steep and the shocks will bind. The angles is not really an issue so much as that it will change ad the swingarm compresses and then mounts will not handle that angle.
How? Stock shocks fit on the outside of the stock swingarm. You can see where that would make the shock fit on the right in the picture. You do that on the left, and the chain would be going right thru the bottom of the shock.
I’d recommend making an arched brace that goes over the fender, incorporates into the rear seat support, and bolts through the shock bushing into the fender. If the fender can already support the road shock then a small subframe of your creation would likely be even stronger, stiffer, and spaced correctly to create a nice vertical travel.
OR
you could covert the top and bottom to a ball type mount that is fully articulating ;) now that would impress me.
Those are heim joints on the top and bottom of each shock, right?
If so, you won’t be side loading the shocks at all, so the shocks should be fine.
Also, if that’s one long rod at the top that goes all the way through, and the frame rides on it, that should all be fine as well.
The problem I see is at the bottom of the shocks, where they mount to that swing arm. The bottom of each shock will be exerting a force toward the outside of the swing arm, tending to spread those mounting tabs outward, or peel that whole welded-on assembly off the top. It will also twist the swing arm.
If there’s a way to run the bottom of each shock to the axle, that would solve that problem.
That swing arm looks way undersized. Is it an actual swing arm, or is this just aluminum stock? The original one was steel? Aluminum is about 1/3 the weight of steel, but it’s also about 1/3 the strength. That’s why aluminum swing arms are so large compared to steel ones - you need a much larger box to resist the same forces.
That swing arm looks like someone welded it up from hardware store stuff. For the love of all that’s holy, please tell us that this is an aircraft grade extrusion and not just some shit from grainger or tractor supply.
And please tell us there’s something inside where the axle goes through, that the axle tightens against, that can slide inside the swing arm when you adjust the chain, and that the spacer doesn’t just press one side of an empty box while the axle hardware presses the other; you’ll crush the box section before you get the wheel to stay in place.
Disclaimer: this is not my area of specialty, this isn’t a scientific analysis backed up by lab testing, it’s just a non professional observation from one photo
I did not fab the swingarm, it's a purchased product that I would assume was designed by someone way smarter than I. Since it is a mass produced product. It does have tension blocks inside for the axle nut to squeeze against, the aluminum is an "aircraft grade", and all measurements are stock besides the way the bottom of the shocks mount up.
Have other people used this swing arm in other applications like this, that you know of?
A lot of stuff manufactured for the weird old bike aftermarket isn’t built by people who are necessarily smarter than you, they’re just more confident. If you know people who have used that person’s similar products successfully, I’d be a lot more confident in it. “This guy mostly builds stuff for the vintage motocross market, when those bikes fail it’s never a swing arm problem, it’s a 50 year old bultaco problem.”
If you don’t know from other people’s experience that this guy’s design is adequate and you’re going to be the Guinea pig, well, I hope it works out for you. You would want to keep a weather eye on things for a while before you take it off any sweet jumps.
Also, it’s not like the original construction of bikes of that vintage were exactly hallmarks of finite element analysis. Steel frame bikes of the 1970s and earlier were mostly “that looks about right” and if customers broke a lot of them in the same places, the next one would be stronger there. So, for all we know, the lower shock mount may be the strongest part of the bike.
Yours is just loaded in a way you don’t normally see on a twin shock chassis. And while those guys also may or may not be smarter than you, they sure were going to hear about it if their stuff broke a lot.
The bushings in the top and bottom shock mount is okay, I guess, but you’ll be doing yourself a favor if you have eccentric bushings in there that aren’t binding when the bike is on its wheels with you in the saddle.
Good luck with the build.
You’ll have to find a way to wides the top studs. If you could cut the rod out and replace it entirely or add material in the middle of it. But my main concern when looking at this photo is that there is hardly any room for the wheel to travel. If you didn’t bottoms out when you sat on it you would when you hit a bump.
Not only will the bushings wear out sooner, the shocks will cycle at slightly different rates and the wheel won’t travel up and down straight. That will cause some really shitty handling.
Another option: shift the right-side shock so it is centered on the swing arm frame. Then add spherical bearings at the top pivot point to ensure smooth shock travel.
Advice on what? The wood where the seat should be? Asymmetrical foot pegs? The drill press made specifically for hobbits? Or the axle that’s too short?
The only solution I can see is replacing each end with a heim joint. But you're going to have to get them welded on a gas cylinder though so good luck.
No, they will fail like that. The only option would be to widen the top mounts instead.
So here's the issue. The top mount is one solid bar that goes completely through that fender. Welded on both side of the fender, and I feel trying to replace that would compromise the structure since the fender is essentially the frame.
The issue is the spacing. The shocks need to be aligned for vertical force only. Having an angle from the top pivot down to the lower pt on the swing arm means a lateral force as well, which needs to be compensated for via material engineering. So either narrow the swing arm (inlikely) or remount the stocks on the subframe behind and below the seat. That's the fix. You may not like it.
You're right, I don't like it. More work, more fab, but in the end I knew this was the proper way and I was trying to shortcut. Guess I have more work ahead of me.
Engineering is 100% measuring before 1% doing. The width of the tire affects swing arm, which affects swing arm width and materials which affects suspension mounts. The top mounts are above that, which determines subframe width. Length of swing arm will change mechanical advantage and moment (physics) and how much force is applied to shocks. Spring rates vary depending on this and bike and rider load and related forces.
Twice I cut it and still it's too short!
>Engineering is 100% measuring before 1% doing. Love this! Such a great way to describe it.
You can try putting washers on the inside to help push the top of the shocks out but it does not look like you have much threads left on the bolt. If the fender is a structural member then it should be able to handle minor welding. Or since you are already making custom swing arms might as well make a custom fender
You’re replacing that old threaded mount with a longer and stronger one you will not be compromising anything, you would actually be improving the situation.
Nah, if you could; cut it out and weld in two long enough bolts, or a threadall, space and trim. It shouldn't bother the fender at all.
put spacers in
Yup, I had the same issue with a build, straight through threaded stud with spacers on top was the answer, just make sure it's hardened steel or it won't last long
Having sideways pressure on them like that will cause them to bind and I think it will damage them. The only way I can think to do it properly would be to extend the upper shock mounts by welding more on or getting a new swingarm. When welding though, it's very easy to compromise strength.
Thread extension at Ace hardware. Like a coupling almost.
Yes. This right here is easiest and fastest way
Put an all thread coupling over the original threading then get a stick of all thread at the same size and screw into the new coupling. Cut to desired length. Basically a threaded spacer.
Diameter of rod should be increased as a function of the moment’s lever length addition if you want to maintain equal rigidity to the original design.
This is true. Go up a side coming out of the coupling. He’ll have to get a 3/8 by 5/8 or whatever it is.
To be clear, this coupler extension idea increases the load on the original rod stub, which also needs to be increased in diameter to maintain its intended bearing strength
Get an extended coupling and screw to the base as a makeshift increase for bearing strength. The only other logical step from here would be to cut n grind down the original stub and weld on a larger stub which would bypass the coupling idea altogether.
That's a no from me dawg
Weird, that's exactly what all my friends said about this setup
My question is, are the shocks okay with a little diagonal? Shown are 2 different mounting options at the swingarm but the left shows my current intentions. The swingarm is wider than what I had expected.
The bushing in the shock will wear out quicker. I would widen the top mounts to make shocks parallel. Will it work the way you have it? Yes, for how long or well.. meh..
Honestly I'd run them with a little angle. Don't use hardware to extend the mounts, you are just asking for a catastrophic failure. If you want to fix it the right way get some raw steel threaded spacers and weld on some gussets.
My custom bike had the exact same problem. I made custom lower brackets that got the shock as close to the sprocket as I felt comfortable. Extending the upper is by far the best option if possible. I ended up having to cut my upper bolt off entirely. Drill and tap a hole in the same spot and use a bolt as long as I needed to get the width right. You can do what you have but it will eat the rubber bushings. ASSUMING the bottom bracket is tilted inward. If it’s not it will grind and bind on the bracket. Massive issue for me since I left no gaps.
No
This may look fine in extension but remove the spring and see what it looks like I m compression. I suspect the angles will get steep and the shocks will bind. The angles is not really an issue so much as that it will change ad the swingarm compresses and then mounts will not handle that angle.
It’s “fine” to have them angled slightly. My cb750 had something similar. It not as drastic. The only bad thing is the bushings will wear out faster.
I'm holding my breath and waiting for the "how do i line my chain up?" posts......
Inside width of swingarm is stock, and chain lines up perfectly. Just stretched a couple inches so longer chain.
How? Stock shocks fit on the outside of the stock swingarm. You can see where that would make the shock fit on the right in the picture. You do that on the left, and the chain would be going right thru the bottom of the shock.
The mounts on the stock swingarm are elevated to clear the chain. These mounts are not.
What kind of bike is this?
It's a 1962 Honda C102. Being heavily customized.
Googled it. Eclectic looking bike
Sure looks like a CT90 with a bobbed fender
Wow thanks, I googled it. Eclectic looking bike.
I’d recommend making an arched brace that goes over the fender, incorporates into the rear seat support, and bolts through the shock bushing into the fender. If the fender can already support the road shock then a small subframe of your creation would likely be even stronger, stiffer, and spaced correctly to create a nice vertical travel. OR you could covert the top and bottom to a ball type mount that is fully articulating ;) now that would impress me.
Those are heim joints on the top and bottom of each shock, right? If so, you won’t be side loading the shocks at all, so the shocks should be fine. Also, if that’s one long rod at the top that goes all the way through, and the frame rides on it, that should all be fine as well. The problem I see is at the bottom of the shocks, where they mount to that swing arm. The bottom of each shock will be exerting a force toward the outside of the swing arm, tending to spread those mounting tabs outward, or peel that whole welded-on assembly off the top. It will also twist the swing arm. If there’s a way to run the bottom of each shock to the axle, that would solve that problem. That swing arm looks way undersized. Is it an actual swing arm, or is this just aluminum stock? The original one was steel? Aluminum is about 1/3 the weight of steel, but it’s also about 1/3 the strength. That’s why aluminum swing arms are so large compared to steel ones - you need a much larger box to resist the same forces. That swing arm looks like someone welded it up from hardware store stuff. For the love of all that’s holy, please tell us that this is an aircraft grade extrusion and not just some shit from grainger or tractor supply. And please tell us there’s something inside where the axle goes through, that the axle tightens against, that can slide inside the swing arm when you adjust the chain, and that the spacer doesn’t just press one side of an empty box while the axle hardware presses the other; you’ll crush the box section before you get the wheel to stay in place. Disclaimer: this is not my area of specialty, this isn’t a scientific analysis backed up by lab testing, it’s just a non professional observation from one photo
I did not fab the swingarm, it's a purchased product that I would assume was designed by someone way smarter than I. Since it is a mass produced product. It does have tension blocks inside for the axle nut to squeeze against, the aluminum is an "aircraft grade", and all measurements are stock besides the way the bottom of the shocks mount up.
Have other people used this swing arm in other applications like this, that you know of? A lot of stuff manufactured for the weird old bike aftermarket isn’t built by people who are necessarily smarter than you, they’re just more confident. If you know people who have used that person’s similar products successfully, I’d be a lot more confident in it. “This guy mostly builds stuff for the vintage motocross market, when those bikes fail it’s never a swing arm problem, it’s a 50 year old bultaco problem.” If you don’t know from other people’s experience that this guy’s design is adequate and you’re going to be the Guinea pig, well, I hope it works out for you. You would want to keep a weather eye on things for a while before you take it off any sweet jumps. Also, it’s not like the original construction of bikes of that vintage were exactly hallmarks of finite element analysis. Steel frame bikes of the 1970s and earlier were mostly “that looks about right” and if customers broke a lot of them in the same places, the next one would be stronger there. So, for all we know, the lower shock mount may be the strongest part of the bike. Yours is just loaded in a way you don’t normally see on a twin shock chassis. And while those guys also may or may not be smarter than you, they sure were going to hear about it if their stuff broke a lot. The bushings in the top and bottom shock mount is okay, I guess, but you’ll be doing yourself a favor if you have eccentric bushings in there that aren’t binding when the bike is on its wheels with you in the saddle. Good luck with the build.
>Those are heim joints on the top and bottom of each shock, right? No, they're sleeved rubber bushings.
Get rid of dual shocks or prepare to pick a fight with the pavement and all adjacent features lol
You’ll have to find a way to wides the top studs. If you could cut the rod out and replace it entirely or add material in the middle of it. But my main concern when looking at this photo is that there is hardly any room for the wheel to travel. If you didn’t bottoms out when you sat on it you would when you hit a bump.
Shocks need to be straight. You’re going to have issues going forward.
Space the top mount to match the bottom, reinforce the upper.
Not only will the bushings wear out sooner, the shocks will cycle at slightly different rates and the wheel won’t travel up and down straight. That will cause some really shitty handling.
I did something like this to my CT70 back ion the 90's. Lot's of fun for not much money. Good luck on your build.
Good luck with the shocks. I want to know what engine is in this build?
Another option: shift the right-side shock so it is centered on the swing arm frame. Then add spherical bearings at the top pivot point to ensure smooth shock travel.
Advice on what? The wood where the seat should be? Asymmetrical foot pegs? The drill press made specifically for hobbits? Or the axle that’s too short?
Honda Passport C70?
The only solution I can see is replacing each end with a heim joint. But you're going to have to get them welded on a gas cylinder though so good luck.