Triumph Dirt Bike by Atom Bomb Custom

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" Back in May of 2010 I just walked out to the shop one morning and started planning the frame for this bike. Beyond “build a Triumph dirt bike” there wasn’t much of a firm plan." Says Clay Rathburn from Atom Bomb Custom Motorcycles.

While I was laying the frame out, I kept running into the fact that the motor was too big, so I cut the front motor mount “triangle” off the cases allowing the engine to be slid much closer to the front of the frame. That let me design the frame and swing arm such that I got a swing arm 3.5” longer than a stock OIF Triumph with no increase in wheelbase.

Somewhere along the way, the project took a turn towards a serious all out custom build. Race Tech Suspension got involved and they supplied a set of custom rear shocks as well as new fork valving and springs. I had the Yamaha front end on hand because I use this same fork on my rigid bikes quite often (cut down of course). At that point I knew there was no turning back on this thing, and the madness took over. I got to thinking that the one thing you don’t see very often is a full on custom dirt bike, and I decided to go full bore on making this bike as functional and nice as I possibly could.

Probably the most involved aspect of this project was the cylinder head and carb layout. The stock Bonneville layout points the carbs outboard at a pretty good angle, which puts them in the way of your knees on a dirt bike. Logic would dictate that switching to a single carb head would be the way to go both for packaging and simplicity. 10 bolt Single carb heads are difficult to locate and I REALLY wanted to keep it twin carb anyway, so after a bunch of head scratching I decided to relocate the intake ports. After a very long three day weekend of welding, milling and work with the die grinder the head now has intake ports that are moved inboard about an inch and a half and the carbs point straight back. Everything now tucks nicely inside the frame, and as an added bonus moving the carbs freed up a bunch of room on the left side for the exhaust. While I was at it on the head, I machined up the bolt together flanges for the exhaust. They have stubs which press fit into the exhaust port, and have grooves cut in the flange surface for copper o-rings to seal the pipes.

Once all the important mechanical stuff was sorted, I had some great fun making the tank and side panels. The tank is loosely patterned after a stock Triumph; about three inches shorter with much deeper knee pockets and an overall rounder shape. The side panels were sketched from a idea I had in the back of my head, the raised portions were rolled by hand with a step roll and the indents for the bolts were made with a little two part press tool I made.
Since this was a personal project I wasn’t able to work on it every day, it often sat for weeks (sometimes months) without being touched; in fact we moved our home and shop 200 miles and have finished four other motorcycles while this project was ongoing. Because of all the starting and stopping I don’t really know how much time I ended up investing in the project, but it was one of the more time intensive builds I’ve done. By the time it was wrapped up in late February, it was more of an obsession than anything else. - Clay Rathburn


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