Thanks, not sure yet if I will be able to get away with no tensioner on the hub to crankshaft chain, seems possible it could end up being just the right distance with hardly any slack but I haven’t measured yet
You may also need these once you break the chain.
It makes it easier to put the chain together.
The thickness is specific to 8/9/10 speed chains.
Impressively just by accident the frame lines up perfectly with the width of the rear of the bike (width was determined by lengths I machined individual crankshaft pieces down to and the 3/4" tube size that home depot had, I neglected to measure width of the bike frame previously). Just need to 3d print a spacer to move the drive sprocket out about 1/4" and everything should line up.
This thing is going to look crazy, current goal is assembly and test driving this weekend at DMS
Final piece to make is how the forward part of the frame connects to the bike, I’m thinking some vertical steel flat bar with a vertical slot sticking forward, and then it will bolt to a piece of steel clamped to the bike with a horizontal slot, that will allow adjusting the angle of the entire frame / leg assembly to try and get the foot motion in line with the front wheel, the clamping force of a 5/16 or 3/8 bolt should make a rigid enough attachment. The frame attaches to bike in 3 points, the bottom 2 being onto the bike axle, so it can pivot around and movement wont affect the chain
(2x4s are just to hold everything up for mockup purposes)
I successfully found the weak point in the initial design
The foot snapped as soon as I put my weight on the bike, I likely underestimated the dynamic load of my weight jumping up / down on the crank to start out, was probably 400+ pounds of force. Will just need to cut out a replacement foot design with a more reasonable traingular design instead of a peg leg. There were no clearance issues that force the peg leg design, I just thought it looked interesting
First half of vertical supports being welded onto frame
Then using some 2x4 spacers I welded the vertical support onto some 1" flat bar that I drilled and mounted on the main rear axle. I went with a 3d printed abs part to connect the forward edge of the frame to the bike. All of the propulsive force and torque generated by the foot touching down behind the axle will be going through that 3d printed part
Welding the crankshaft pieces together on the frame worked well, I had just moved the welding clamp here and forgot to move it back before tacing the last piece, luckily I didn’t destory the ball bearings by sending the welding current through them.
Ended up not getting lucky on chain length and had to put in a fixed tensioner for now, I had this spare sprocket from when I was still figuring out the drivetrain
I was lucky in that everything cleared and there was no fundamental flaw in the frame or assembly process, I just had to switch from a cap head to button head screw and the tensioner retaining bolt cleared the strut here
One of the design goals was not physically altering the bike, so the mechanism can be unbolted from the bike easily after removing the chain
It’s pretty satisfying watching it come together. Thank you for keeping up with the thread.
Will be at DMS late tonight to cut out the foot v2 pieces. Planning on using some of the extra 1.25" tube to make an 8" wide foot (with some rubber tread on the bottom of it) for each of the feet to help improve tipping / balance
Got the new feet cut out, a lack of any sand paper in woodshop slowed down prepping them for glue
Was able to remove the bushings from the v1 feet, they will be reused
Got some more use out of annular cutter making plates that will attach the tube feet to the plywood, puts other hole cutting tools to shame
Have been gluing up the new legs, here you can see the difference in design
Will use some 1.25" tube as the bottom of the new foot design, used the now scrap old legs as spacers while welding the tube onto some 3/16" flat bar that will bolt to the plywood, I’m assuming they will be strong enough without gussets
Wrapped the tube in some rubber stair tread material to give it some traction
Next will reassemble and try a test ride again, but it may be next week before I can get to that.
WOW!!! Looks Killer!!!
Got the new feet on
I didn’t analyze the 3d model as closely as I did the first iteration and so I missed that the new leg design collided with the strut at one point in the motion
Would have been obvious if I looked
It just required a bit of clearancing to fix
Will do some more test riding in the daytime, at least mechanically everything seems to work well as designed but preliminary low speed trials indicate that the 3 leg approach is not a good idea.
The initial hypothesis was that since in the motion the foot is only at its lowest point for 1/3 of a crank revolution should be able to have 3 instead of the typical 6 legs (removing static stability but hopefully increasing maneuverability by making it easy to turn vs the 2 points of contact with more than 3 legs)
What I didn’t consider is that for 3 legs to operate smoothly requires the bike to remain perfectly vertical, which is not possible (at least at slow speeds). As you pedal and the contact point moves the bike naturally leans left or right, when you transition from the left foot to the right foot being on the ground with the bike leaning to the right there is a jolt as the right foot is now lifting the bike vertically the lean amount.
I forgot to finish the welds on the crankshaft and the 4 tacs were not enough to hold the torque, broke while starting so nothing else was damaged, just need to reweld it
Also threw some pins in the main axle as I noticed it had started to walk it’s way out previously, need to pick up some cotter pins to replace the rod + painters tape
While testing the middle upper leg snapped
A lack of attention to needed clearance on the feet that I added plus no safety factor in the low grade home depot plywood lead to the failure, here you can see the 4 frames where I think the far right foot collided with the middle leg as it was going forward. Despite the bike crank moving the whole time you can see crankshaft stop moving for a frame while the leg frame bends, along with the uneven movement of the right foot
Here is the full video with those frames happening just after 07 seconds, its more obvious with the sound of it hitting. It hits again at 09 seconds which is when the plywood breaks. There was a solid 2 seconds of it working fine at the beginning…
These are the main source of the failure, making these steel tube + rubber feet ~3/4" narrower would probably have resulted in success, during static testing they clear with 1/4" inch but during normal movement but there is considerable flex in the plywood as its moving
Options now are to either recut all plywood pieces with not home depot plywood and a slightly stronger top triangle design and angle grind off some of the feet or just say project has been enough fun so far and call it complete.
Came up with a 3rd option, just splint the broken leg and try again
Sliding the main shaft out allows easy disassembly
There are two of the upper triangles here, I used a second one and some of the axles through the bushings to hold the broken one in alignment during the repair, first using some original gorilla glue and then bolting some aluminum flat bar over the break
And cut the feet down and filed down the bolt ends it was catching on
Another thing I realized is that due to the suspension on the front of the bike I am using, when I’m actually on the bike the leg track is not level, this is very bad as you can see from the foot path here
Imagine that diagram moving up a hill to the right, its possible that the foot could come into contact with the ground as its still moving forward (especially if the bike is leaning left or right as that left or right foot is coming forward), this leads to either the mechanism stalling and the foot skipping along the ground or more broken parts. Jansen linkage run in this direction really only works on very flat / level ground.
I used the adjustability designed into the front of the attachment to give the maximum angle I could vs what I was running previously, which could lead to the foot lifting the bike at the end of its travel but it will be less likely to destroy itself if I run across a small bump. There is a slot in the 3d printed Y part where the 2 bolts attach the metal frame to the Y. Since the only other attachment point is the rear bike axle that sliding lets the entire assembly pivot a few degrees. Interesting to note that it is entirely the clamping force of those 2 bolts onto the 3d printed abs that transfers forward force into the bike.
Hey Beesty Boys and Girls… Why not make the legs out of aluminum? Also, would it be practical to have 4 instead of three? (might be more stable.
Yeah… so what’s your point?
You asked why not Aluminum, .375" plate 6061, half the thickness of a sheet of .750" plywood:
I’m guessing he paid about $50 for the sheet of plywood he used. For only about $1,300 (25X) more he could upgrade. That seems like a good reason on a prototype not to use it - unless cost is no object.
I’ll check Garland steel tomorrow… got a dxf?