Honda CB900 – Custom Chopper Build
It was inevitable that Purpose Built Moto would build a custom Chopper one day. All we needed was someone to trust us and see the care we put into our Cafe Racers and Scramblers would transfer really well into a Chopper format. The building skills are the same, and I love the style of bike, it’s just a different thought process.
Enter our Protagonist, John. He had rescued a wrecked CB900 10 years ago and decided to have it built into a Chopper. John is from central QLD and had the work done by a builder on the Sunshine Coast. It took years to complete, and when it arrived it never worked right. It also looked like this….
Understandably, John was disheartened after all of the trouble with this bike. The years of having it in the build, not looking right when he received it, and never really working correctly either. On pulling it down I found a nest of wires under the seat, some of which I couldn’t even make sense of. The exhaust was also a nightmare. A 4-1 collector that looked like it was pulled off a Daihatsu. One thing I will say is that the frame that was built from scratch was very well fabricated. Apart from the rearmost of the frame would be kept.
I couldn’t let this bike go back to its owner without being the envy of any red-blooded man in his hometown. I wanted it to be right, and something John would be proud to ride and show off to his mates.
Let’s get started… The first job was to strip it down, pulling the old electrics box out, and rebuilding the front wheel with a 21” rim. The rear wheel was retained with the 17” rim that we got the bike with. Both wheels are wrapped in an Avon Cobra tyre.
Next its time to rebuild the tail section in a swing arm chop style. This whole process was foreign to me. I had to consider a new set of geometry dynamics to achieve a good and solid chopper look. If I had built the bike from scratch the backbone would have more of an angle down to the rear wheel to create an arc back up over the rear wheel when building a seat. Working with what I had, the tail was rebuilt from 28mm Chromoly tube, and some new shock mount spigots turned up. This created a shorter sharper look for the tail. The rear swingarm was also shortened about 50mm to tuck the bike up more. The stance was finalised with a set of shrouded 10” shocks supplied by Ride Dynamics.
With the wheels finished, wheelbase and suspension sorted, it was time to start styling the bike out.
The huge mustang chopper tank had to go, and I landed on a Honda CB750 K2 tank for the CB900 build. I liked the shape, it fitted with my ideas for the bike, and it allowed the huge engine to protrude out from under the tank.
Moving on to the rear fender and sissy bar set up which further pushed into the chopper style I was chasing. With some crafty round bar work, I wanted to match the lipped fender, and mount the prototype of the Purpose Built Moto Orbit tail light on there. With that checked off, I moved to the seat and electrics pan.
The seat was shaped and finished in nubuck black leather with plenty of character in the linear stitching finished by Jamo at Timeless Auto Trim. With the rider’s seat taken care of, we needed to find a place for his feet. The previous pegs were far too rearward for any comfortable ride. I wanted the CB to be able to take long strolls so I set about mounting some mid-set pegs on either side of the motor. Finished by welding threaded bungs into the frame rail, and fabricating some bolt-on peg mounts to house the Tarozzi pegs, shift linkage and Brembo rear master cylinder.
It’s an odd thing to think about achieving a chopper/bobber style set, but taking into account a sissy bar and rear fender that moves with the suspension. It was hard for my mind to process, but I had a red hot go and I think we achieved the best possible outcome for this Honda swingarm chopper.
On the front end, I designed a vintage-style top triple clamp taking inspiration from the Ceirani race clamps of old, and re-purposed them with 1” bar clamps. The design was CNC cut and polished once we had it in hand. A set of Low Ape bars were narrowed by 30mm and fitted to the Honda custom chopper. Using the stock forks, I built a fork brace (which you can learn how to do here) and begun the process of mounting 2 x 4.5” LED Headlights vertically.
Fabrication on a custom chopper requires a very different mindset to building a Café Racer. I could be completely off the money here, but my personal experience was this. To me, when I’m building these racers I’m always thinking, “what’s the simplest, and cleanest way to get this job done?” for a chopper I’m thinking “What is the best looking, and most elaborate way, to get this job done?”I took that mindset to practice with this bike, particularly in the front headlight design. Stacking 2 Purpose Built Moto 4.5” Flashpoint Headlights vertically and fabricating a flowing round bar mount that stretches from the bottom triple clamp, over the handlebars and finishing in a hand-shaped ducktail to match the rear sissy bar. The bar work has been brass coated and will be left to patina over time because I like the idea of this bike aging gracefully.
I wanted to keep the handlebars vintage-looking, and as bare as I could manage. I used some PBM button switches, and a set of brass inverted cable levers finished in some leather wrap grips. The look was there, but cable levers don’t operate hydraulic brakes….. So, again, I got creative.
Using a cable operated master cylinder hidden under the tank (thanks BMW for the idea) I was able to keep it simple on the bars and retain the disc brakes on the front end.
The CB900 engine was cleaned and dressed up with a coat of black paint, some mildly polished cases will also be left to patina over time with the brass accents. The engine also received a set of Keihin CR carburetors and DNA performance filters. It really opens up the power curve of the 900 four and delivers a far more intense ride experience.
As mentioned before, the Exhaust needed some looking at, and to compliment the ‘in your face’ design the 4 cylinder screams through a set of 4-4 straight through pipes finished with some turned brass exhaust caps. Its obnoxious and loud, but so is your shitty Harley, so deal with it. This new set up was run and tuned with the guys at Dynomite moto dialing in the CB900.
Looking at this bike in bare metal, fabrication completed it was something very different from anything I’d done before. It was time to get the vision completed with some paint. Very early in the piece I had decided to run with a CB750 K model variation candy red, and gold leaf. This would tie my brass bar work in, and give it the eye candy appeal I wanted. It would also ruffle a few feathers with purists seeing a nice CB750 tank on a bastardised Honda Chopper. Just quietly I love revving the old blokes up!
Justin at Popbang Classics really nailed the paint here with a deep candy red, giving the custom chopper the coat it deserved. On the final assembly of the Honda Chopper, there were a few details missing, so with the bike sitting assembled on my table. I went to work on adding some details for nothing more than my own satisfaction and to dress Ruby Red up just a little bit more.
To add some consistency to the design, I turned up some bolt bosses for the rear shocks and tank mount bolt out of some brass round stock, and the cherry on top was the brass tie wire that I braided over the throttle cable. Some details because… choppers…
The CB900 holds a big presence on the road. It’s loud, bright and a joy to ride. It doesn’t corner like a demon, but the pegs and pipes allow enough tip angle to bring a big wide smile as you point it through and open the Keihin carbs up. Something I’d never understood until now was the allure of riding a chopper. I’ve always loved winding roads, fast-paced corners and a bike that handles great. But on this bike, it doesn’t matter where you’re riding, it’s about what you’re riding. The excitement is all in the novelty, and this strange imagining of a CB900 has yards of it.
It’s exciting to me that someone trusted Purpose Built Moto enough to step outside of what we know. Jumping into a new style was a shift, but when the owner’s face lit up when we showed him the finished product, I felt accomplished with our first ever custom chopper. We helped John finally realise his dream bike. After having a few hard knocks along the way, I’m stoked PBM could be the one to finish the job.
The bike looks great, but please don’t ride it in the rain – your tires are on backwards
You’re on backwards 😉
Get those tires on right knucklehead lol. You did a great job looks tight . I need a back shifter for my 71 K 750 can you point me in the right direction, kinda like that arrow of rotation on the tire .