The Big Rig

The BIG RIG has been a long-term project of mine, I used the Brooklyn machine works race link as the inspiration for the frame as I like the zero feed back from their jack shaft linkage, and I’ve always loved the look of the bike and the ethos of the company. I was working from scratch design wise as I had no bike to use as a reference so there was a lot of time spent with pencil, paper and ruler (this was before I was working with autocad and bike cad).

I spent a lot of time researching the dimensions of bearings for the main pivot and working out how to make the bearing housings. In the end the bearings for the linkage cost me in excess of $350 (They are some shit hot bearings though) the bearing housings are machined from solid 45mm 4140 tool steel billet, (it took me a solid couple of day just to turn these out).

As with most frame designs I had to get fork and shock dimensions to make sure everything was going to fit and the dimensions would work. I prefer to have the forks and shock in my hand for this so I can cross-reference and get true measurements from them, so next it was putting in an order with avalanche racing in the US. I purchased their 8″ MTN triple USD forks and a custom DHS 3″x 9.75″ shock, $3500 in total (had to sell my skydive rig to pay for that lot).

I wanted a 1.5″ head tube for the rig, so it was of to The Oracle, (the inter web) getting dimensions.I was going to use thick wall 4130 chromo steel but nothing came close to the size I needed so it was back to the lathe ripping the middle out of a solid lump of 65mm solid 4140 tool steel. I made 2 units over 2 big days of machining, these puppies aint flaring out any time soon!

For the tubing my plan was to use T45 tubing from the UK, it was developed in WW2 for use in building the spit fire fighter planes and has 1/3 more tensile strength over 4130 steel. It’s an air hardening steel and turned out to be 3x more expensive than 4130. The way I  had designed the frame there was no need for the extra strength so I went with 4130 tubing. The  design of the main frame has all the tubes piercing thru each other in the same way planes are built makes, for a super solid structure .

I had by this stage decided to build a hard tail frame using the same front triangle design and had the plans drawn up for that frame as well,so it was time to cut some tubes. Mitering went very well and I had 2 front triangles cut in a couple of hours, then clean, dressed and set up in the jig, (small note here I used my own designed eccentric BB on the frames  which use a 63mm inner diameter BB shell as this is a very large size I had to turn some custom fittings for the anvil jig) by the day’s end had both front ends welded and sitting in the soaking tank.

Next I had to make up a fixture to align the bearing housings exactly in place in the frame. took lots of setting up and double checking but I got it fitted in the end. Spent a good week dressing the welds on both frames (wore a lot of skin of the finger tips that week)

Then it was time for the pretty shiny bits, I sketched the skull head badges and the skull and back bone out on paper then transferred the images on to sticker vinyl, cut them out stuck them to 1.5mm stainless sheet. Stitch drilled around the design. Cut and filled them to shape. In all several weeks went into this process. I then made a mandrel to bend them to match the radius of the tube (this was a tense process as it’s very hard to bend the cut plate and not destroy or misaligned the badges but  it all went well). Next it was time to fit the badges and silver braze them on. This was a really big job and required me going back over and filling any small sections that I’d missed with The silver wash. I finally had them fitted, it took three days for both frames, then the real fun began getting stainless mirror finished takes a LOT of elbow grease.

First I start with a course file to relieve the high spots, then it’s a finer file to remove the marks of the first file, then emery cloth on the files to gradually finer grades till you get to wet and dry grades to 1200 grit, then its wire wool and finally polishing paste. I often had to go back a grade of  paper if I found a scratch that hadn’t been taken out enough. Three weeks of doing this and a visit to the physio as I’d given my self  tennis elbow and I was very close to just painting over the badges and leaving it at that, I persisted with it and  the results speak for themselves.

Time for the swing arm, I used 25×50 4130 box for the swing arm set each arm up in two vices on the milling machine and ran a face cutter along to relieve some weight from the box section and to give it a funky look. I designed the tips on cad and cut and peeped the 4140 billet slugs for fitting and cutting on the CNC at Cycle Underground. I machined the bearing cups for the frame end on the lathe again using 4140 solid billet. Mitered the box to house the tips and bearing cups then had to make up a jig and spacer to fit it all together on the fabrication table, after much fine tuning and re checking I layed down some tasks and it all brazed into place. A lot of filing and a couple of re-,cuts after dry fitting and finding issues with the shock clearance in spots, a redesign of the bearing pivot placement on the swing arm and it was ready. I really enjoyed building the swing arm and it was a pretty big day to see the arm fitted and running smoothly with the spring removed from the shock.

The main pivot uses a 17mm axle that has a plate  machined on one end to carry the cogs for the carrier sprocket. I use Cycle Underground 6 shooter rear cog for the double chain drive system. The axle started life as another lump of  60mm 4140, nearly two full days machining it down to size, then fitted on the CNC and had the six clearance  holes machined for the carrier sprockets .

Fitted and brazed the cable guides for gear brake and shock on the frame, there are quite a few. Next it was off to talk to Peter from Star Enamellers about masking the stainless steel badges, up he ran me thru the process another day spent masking up then cutting the badges out, was a lot harder than it sounds it was very easy to slip and scratch the stainless steel, not what I wanted to do after all the earlier work. When that was complete it was of to Star Enamallers for what has to be one of the best paint jobs I have ever seen, thanks Peter.

Now its been getting built up slowly with all the funky bits I’ve been hoarding for her over the years. There is only two more fittings to go till I can finally set this monster free down some big hills. I’m getting the swing arm linkages CNC cut, just have to wait till it’s my turn in the line up on the machine. Every day I look at it sitting in the bike stand and know it’s one day closer to the big ride .

It’s funny how you forget how much work goes into something like this, writing it all down certainly brings it all back. A huge thanks to Big John from Cycle Underground for putting up with my endless questions, his invaluable knowledge and extensive work shop with out which this or any of my projects would not be possible.

I’ll post shots when it finally rolls out the door .