(MBBO#6 Class 2) Twist of Lemon

tjwilson

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Jan 15, 2016
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Seat Area

My thought for now is to eventually add sheet metal to make the inner seat pan integral to the frame.

Struts for added strength.

seatStrutAdds.jpg


Additional pieces to complete rear of seat.

seatEndPieces.jpg


Trimming, fitting and welding seat end.

seatEnd01.jpg


seatEnd02.jpg


seatEnd03.jpg


Seat Corners

I capped the end of the rear horizontal tube with solid bar. This gave me enough material to go back and grind / shape the corners. Similar to how I rounded the front of the seat cantilever tubes.

seatEnd04.jpg


Seat Top Side Tubes

seatEnd05.jpg
 

tjwilson

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Jan 15, 2016
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Frame and Mock-up

So I've made it to the point where I can finally test the strength of the frame's design. And... result is not quite what I had hoped for. The seat area seems more than strong enough. Problem looks to be flexing in the lower front part of the frame. This area basically forms a rectangle verses a typical triangle so is not as strong. The leverage created because of the top tube / seat connection point location relative to the lower frame is creating enough force to pull the head tube up and distort that rectangle. Not a terrible amount of movement but would make for a very squishy ride feel, not to mention the fatigue to the frame over time. Plan "B" is to add a second curved sweep piece to each side between the seat and cantilever tubes. That should reduce the leverage and hopefully tighten everything up (fingers crossed).

While trimming and fitting the frame pieces I held up a second curved piece in that area (not trusting the design and in anticipation of having to make design modifications). Actually looked pretty good!

Frame Overview

That odd tube coming out of the bottom of the head tube probably needs a little explaining. I have the headset installed upside down so that I can insert a tube where the handle bar stem will eventually go. This slips into the fixture to hold the trail angle.

frameOverview.jpg


Mock-up

The clearance between the rear tire and frame is real tight. The drop out slots will require some lengthening.

mockup01.jpg


mockup02.jpg
 
Sep 14, 2013
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Bradley Illinoiz
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I love the LOOK of the lower point in front of the BB, but will it be too close to the ground? I'm wondering about a subtle bend up in the lower tube in front of the BB (mirroring the subtle bend in the mid tube) and taking an inch or two out of the front curved down tube to the point... I can see cruising and turning sharp with the point dipping down and impaling the ground. There's gotta be close to 3" of clearance now, but is that enough?

Carl.
 
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Sep 14, 2013
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Bradley Illinoiz
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Would small gussets help eliminate the undesirable flex?
I have noticed frames often spring apart when cut. Makes me believe they are assembled under pressure. Similar to the roof trusses in your house, which are forced into place in a jig and then fastened together. Makes them stronger somehow. This could work on a bike frame too.

Carl.
 

tjwilson

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Jan 15, 2016
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I love the LOOK of the lower point in front of the BB, but will it be too close to the ground? I'm wondering about a subtle bend up in the lower tube in front of the BB (mirroring the subtle bend in the mid tube) and taking an inch or two out of the front curved down tube to the point... I can see cruising and turning sharp with the point dipping down and impaling the ground. There's gotta be close to 3" of clearance now, but is that enough?

Carl.
Thanks. Clearance is just over that, somewhere between 3" to 3 1/2". Should be able to catapult pretty far if I time hitting the lower point and front disc brake just right! But yeah, could be an issue. Not sure how far the front will dip with the springer fork. I might have to limit the amount of travel. I can probably round the very front of the point and gain another half inch without changing the overall look too much if I need to. The cruiser I built for the WBO has about the same clearance. I have quite a few miles on that and haven't scraped the bottom tube... yet. Guess because I know I'm going to be the primary, or only, rider and that I'm aware of just how low it is, I'm willing to gamble that it won't be a problem. Definitely something I'll pay extra attention to on the initial test ride.
 

tjwilson

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The Fork

With nearly as many pieces as the entire frame, fabricating the fork was pretty time consuming.

Tubing for fork polished, filled with sand, capped and put through the roller.

forkTubing.jpg


Fork Dropouts

frontDropouts.jpg


The Fixture

The fork fixture with dropouts and pivot tubes held in location.

fixture01.jpg


Fitting and tack welding fork tubes.

fixture02.jpg


fixture03.jpg


fixture04.jpg


fixture05.jpg


Dropout gusset template and cut pieces.

fixture06.jpg


Welded Forks

forksWelded.jpg
 

tjwilson

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Mock-up with Fork

mockupWithFork01.jpg


mockupWithFork02.jpg


mockupWithFork03.jpg


While fitting the tubes that formed the fork I checked and adjusted the tubing positions against the frame. This helped make sure that the tubes would flow the way I wanted them to. In the process of these tweaks I failed to notice that the location of the front cross tube ended up low enough that it now interfered with the front tire! Luckily not by much. I should be able to dish out the bottom of the cross tube to create enough clearance.

lowerBarInterference.jpg
 

tjwilson

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Fork Spring

Didn't know exactly what I was going to do for the fork spring and mounting hardware. I orded a springer head kit thinking I may end up only using the spring. Looks like I'll be able to use most of it. I put the spring perch in a hydraulic pipe bender and bent it down to the angle I needed. The springer yoke comes up short even when flipped but I should be able to make something up that will work. Not sure if I've created a situation where there is now too much leverage, hopefully not. At a minimum I'll be using a grade eight replacement for the spring bolt.

spring01.jpg


spring02.jpg
 
May 20, 2009
6,181
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Isle of Hope, GA
First off, love the design of this bike.
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?
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.



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