Monday, 7 May 2012

Cutting, Mitring, Pivot and Go!

Hi All, I haven't posted for a few months due to not really having the cash flow to get the tubing I needed and bills were top of my priority list.

Anyway here's a teaser of an update for you. All my tubing is here to make the frame Quick list-
  • 4130 Head Tube and BB Shell from and old steel frame
  •  Ceeway 'Thron' Seat Stay Tubes (4) and a set to be unused top tube
  • Reynolds 853 Seat Tube for 30.9 Dropper Post
  • Reynolds 631 Top Tube
  • 1.2mm Walled Down Tube from Sky-Craft (thick walled to handle suspension loads)
So i got the tubes and bolted the BB and HT into the jig I had previously made so I could set about cutting and mitring the tubes of the front triangle prior to welding. The joints on the tubes need to have gaps of less than 1mm and preferably uniform all round so as to avoid distortion when welding due to the heating expansion and contraction during the process (welds heat up and then contract, pulling the metal out of shape if its not held securely, imagine zipping a jacket up that's too small for you)

Head Tube Joint with Down Tube, including DT gusset

Another minor problem I encountered was that a 30.9mm seat tube, with its 1.2mm top wall thickness, meant my outer diameter on the tube was 33.3mm. Nobody makes a 33.3 seat clamp so my choces were make a seat clamp, fixed or otherwise, or make the 33.3 into a 34.9 to take a standard size QR clamp. I decided to step up the size on the seat tower so it would take an existing clamp as making a custom one may be costly. I lucked upon the fact that my top tube was .8mm wall and an outside diameter of 34.9mm- Perfect.

I cut a short length of this tube as I needed the remainder for the top tube, you have to cut from the long butted end with butted tubes so this is usually scrap anyway. Once I had cut it down I pressed it onto the 33.3OD tube and smoothed off the cut edge toline with the top of the 33.3 tube. Pretty happy with the results shown below.
Seat Tube Tower with 34.9mm adapter pressed onto it, ground a little chamfer on to tidy things up.       

With Seat Clamp attached, clamp slot to be drilled and cut out
Next up I ground up the tubing to fit around the BB area, I've always used a chain device so ISCG tabs are a must to help alignment and durability.

DT/BB Junction. tubing to be cut to allow possibility of future internal cable routing for dropper post

DT/BB/ST junction. to be marked out on down tube for further dropper post cable clearance

Thats where the frame is up to for now, other updates include a longer stroke shock so as to allow lower air pressures and less damping stress.

Also got someone to knock up some Bushings and an axle for my pivot. PTFE bushing 13mm alloy axle. Check Trevor Bennett's blog out here. Also a big thnks to his late father, Wilf, whom without his engineering prowess, trevor may not hve had a lathe. Cheers Wilf.
Pivot Kit, consisting of housing, two press fit bushings and a 2 piece Axle

Welding front triangle this week - stay tuned


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