(MBBO#6 Class 2) Twist of Lemon

Sep 7, 2014
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I see what you mean about leverage, the pivot action may force the spring down instead of back....
Ah yeah I see that now...:39:...needs to be more square with the arc of that pivot point...what about lowering the mounting point of the spring so the spring is in a nice line with the top tube?..:cool2:...+ maybe a longer spring?
 
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tjwilson

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First off, love the design of this bike.
Thanks!

I see what you mean about leverage, the pivot action may force the spring down instead of back.... I guess if the front ‘legs’ of the fork were longer, it would bring the spring into a more parallel position and possibly enable use of the stock yoke?
Yeah, exactly... I hadn't really looked at how the original springer actually worked before designing the forks. Seems to be a recurring theme, making design decisions solely based on aesthetics! I had assumed it worked like a shock and the spring action was guided by the bolt. Taking a closer look the yoke/crown dosen't even touch the bolt in the original intended application. The hole the bolt runs through is 1/4" larger in diameter than the bolt. And, the fork pivot point is behind the spring front and pulls back on it. I've placed the fork pivot point in front of the spring and now I'm trying to push it.

Since this is such a radical design, have you tried placing the spring perch under the bottom bearing race? At least then the fork legs would be moving in a downward direction and so would the spring...I’m no engineer, just play one....in my head.
Really looking forward to the rest of this build.
Hmmmm.... Sounds like an interesting possibility that I hadn't considered.
 

tjwilson

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Ah yeah I see that now...:39:...needs to be more square with the arc of that pivot point...what about lowering the mounting point of the spring so the spring is in a nice line with the top tube?..:cool2:...+ maybe a longer spring?
Lowering the mounting point and maybe swapping the spring out for a longer spring, or mountain bike shock, might be the solution.
 

tjwilson

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Maybe bend the perch to 90 degrees and point it down? I’m sure with your design skills, you’ll figure out a much more elegant solution than I could.....


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That would be really interesting combined with your suggestion of mounting the spring below the bottom bearings. Imagine a tie rod from the forks running to a lever that has a pivot point below the bottom bearings. The lever action then pulls down on a spring that runs parallel to the head tube. Maybe a little too Rube Goldberg for this bike, but I can definitely imagine an entire bike designed around that idea!
 

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Spent a couple hours making up a rough crown/yoke to test if the fork spring would work and try to determine what direction I may need to go in with a potential re-design. Surprise, it worked as originally intended! I wasn't able to see any movement lateral to the direction of compression and the spring bolt cleared the holes without interference. Now I just need to make something that looks a little less "utilitarian".

I included a small section of tube that the spring slips over in the design but it didn't behave like that was even necessary. I got curious enough to do a little searching online because it seemed so counter intuitive that this wouldn't try to buckle when compressed. Turns out the answer is in plain sight. Apparently the barrel shape of the spring keeps forces linear to the direction of compression minimizing any side forces. Amazing how something that seems so minor can have such an effect on how things act mechanically.

Test Crown/Yoke

testCrownYoke.jpg


The "at rest" vs "compressed" differences are a little hard to see in the images. There's probably a half inch spring travel back and about three quarters of an inch dip at the front point of the frame.

At Rest / Compressed Side View

sideAtRestCompressed.jpg


At Rest / Compressed Front Three Quarter View

frnt3qtrAtRestCompressed.jpg
 

tjwilson

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Been busy working on finishing up final frame and fork details.

Rear Brakes

Aligning and tack welding cantilever brake posts.

rearBrakePosts.jpg


A couple shots with the brake levers. I've read that center pull cantilever brakes aren't the best because of the angle that the cable pulls from. More up and away then pulling the pads in towards the rim. I've kind of made the situation worse by setting up the levers to reach in, past the tire width, to hit the rim with the pads. Haven't tested them yet but I suspect they may only offer a suggestion of slowing down. I think I'll be glad I'm using a disc up front.

rearBrakeLevers.jpg


Rear Brake Cable Stop

rearBrakeCableStop.jpg


Rear Cross Brace

rearCrossBrace.jpg
 

tjwilson

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Kickstand Mount

Drilled a hole in the bottom of the frame, welded in a nut, cut off a mount from a donor, and welded that in place. Side benefit is that the mount plate should help strengthen the chain stay / frame connection.

kickStandMount.jpg


Side Frame Additions

I needed to stiffen the frame, there was just too much flex even after completing all tack welds. I had been putting this off because I wasn't exactly sure there wasn't a better solution. I even tried tack welding a temp straight bar across the bottom of the frame that tied into the bottom bracket. That didn't work, the frame really needed the second side curves. In the end I'm pretty happy with the look of the added tubes. And, they've eliminated the excessive flex.

frameSideAdds01.jpg


frameSideAdds02.jpg


frameSideAdds03.jpg
 

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Front Tire Clearance

The front cross bar on the forks ended up too low and required a clearance notch. I ground clearance into the tube and welded in a curved piece to fill the resulting hole.

frntTireClearance01.jpg


frntTireClearance02.jpg


Front Brake Caliper Mount

I used the pad adjustment screw to clamp the caliper to the disc. I then created a template and cut a mount out of 3/16" plate.

frntCaliperMount01.jpg


frntCaliperMount02.jpg


frntCaliperMount03.jpg
 
Sep 7, 2014
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This is really coming along nicely. Front brake should stop you well if the rear doesn't but you could move the bosses in or use a wider canti to get more power. Hopefully it's like the springer and not as much of an issue as it appears to be....but it's an amazing piece of art even if it doesn't do a skid.
 
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tjwilson

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This is really coming along nicely. Front brake should stop you well if the rear doesn't but you could move the bosses in or use a wider canti to get more power. Hopefully it's like the springer and not as much of an issue as it appears to be....but it's an amazing piece of art even if it doesn't do a skid.
Wider levers would be an easy switch if it's a problem, hadn't considered that. No skids would save on tire replacement costs though!
 
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Wider levers would be an easy switch if it's a problem, hadn't considered that. No skids would save on tire replacement costs though!
AHH, but are those handlebars wide enough for you to fly through them when the front wheel locks up?
All kidding aside, discs on the back would really be ideal but a costly change at this point in the build.
Looks friggin' awesome no matter what you decide.

Carl.
 
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tjwilson

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AHH, but are those handlebars wide enough for you to fly through them when the front wheel locks up?
All kidding aside, discs on the back would really be ideal but a costly change at this point in the build.
Looks friggin' awesome no matter what you decide.

Carl.
Thanks. Yeah, hopefully I won't regret not putting a disc on the back. No room to make it through the bars so I should probably practice my forward summersault dismount technique.
 
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Just wow! I've been so busy I haven't checked this in a while. I'm going to come back and read it all very closely as soon as I get my bike done. Great work!!
 
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