It’s as if Hiroyuki Nakamura was picked up in 1973 by Doc Brown in his souped-up DeLorean, transported to the present day where he thoughtfully grabbed some key components, and was then sent back to the ’70s so that he could go nuts on the new Kawasaki Z1 in his shop.
In reality, of course, the RCM-230 is a contemporary vintage custom meticulously hand crafted by AC Sanctuary, a Japanese company helmed by Nakamura, who has been building their so-called RCM (Real Complete Machine) bikes since the early 2000s, based on Japanese motorcycles from the 1970s and ’80s.
I’m already a huge fan of the original Kawasaki KZ, but seeing the original updated in this way leaves me almost at a loss for words. Almost. Nakamura and the staff at AC Sanctuary have taken what was really the first superbike and transformed it into a modern custom that appeals to my sense of aesthetics, perfor- mance and history.
As much I love the Kawasaki 900 — a couple of which I have owned — I covet the RCM-230 a whole lot more. For me, this is one of the most beautiful motorcycles I’ve ever seen, and I am now going to dedicate myself to figuring out how I can eventually own one.
“With RCM, we aim to improve its potential as a road sports motorbike, while retaining the image of the original vehicle,” Nakamura explains. “Because it is an old model, it wouldn’t be able to match with the best quality ones currently available out there. However, when we are done, its driving performance will still improve significantly.”
Starting with the Z1’s handling, AC Sanctuary’s efforts focus on the suspension, wheels and upgrades to the swingarm and frame itself. Öhlins FG424 inverted forks are used up front, while at the rear a pair of Öhlins S36 piggyback shocks are attached to an in-house custom swingarm. Race-quality Australian OZ Racing Piega forged 17-inch aluminum wheels are shod with Italian track-ready Pirelli Diablo Rosso Corsa tires.
When you have this level of rubber and suspension, only the highest-quality braking will do. The RCM-230 is fitted with twin Brembo CNC P4.30/34 billet calipers that are radially mounted, as is the Brembo radial pump on the Japanese-made Daytona brand bars. Floating 310mm Brembo discs complete the package up front. In the rear, there is a single Brembo caliper and Nissin disc. “Seventeen-inch high-performance tires provide amazing cornering performance for a motorbike of 1970s,” Nakamura points out, “and brake performance is far superior, too.”
With so much traction and braking from the improved pieces at both ends, a beefed up frame is a must. “If you ride under such tough conditions, the entire stress will be on the engine mount instead of the main pipe of the frame,” according to Nakamura. “As a result, mount bolts could get bent, or brackets which hold the engine may get cracks.”
“We use a laser to measure the curvature and other factors, and make alterations when needed,” Nakamura explains. “We then strengthen the frame with pipes or sheet metals, but it has to be strong enough to use 17-inch high-performance tires, and other top quality parts.”
Still, AC Sanctuary does not tamper with the dimensions of the original Z1. “Although the frame will become around six to eight kilograms (13 to 18 pounds) heavier once strengthened, we can make it around five kilograms (11 pounds) heavier over – all by getting rid of some unnecessary portions,” Nakamura says. “Length, width, and height of the frame will not be changed for the standard RCM range. The frame size will usually be the same as the original one.”
“What we are careful about the most as we strengthen the frame is not to have any curvature or deformation when welding the materials,” Nakamura reveals. “We will even use a special tool to weld on some difficult areas.”
To help keep the stress on the strengthened frame within reasonable boundaries — and to preserve reliability — the Z1 engine is not modified with a goal of maximum power.
“How much load there will be on the frame depends on the engine power, how you ride the motorbike, and what you use it for,” Nakamura says. “For example, if it has a high power engine and you ride with high speed for a long period, there will be a significant amount of load on the frame.”
Regardless, the Z1 still gets a 36mm Mikuni TMR carburetor and 4mm-overbore JE pistons that bump up the displacement to 1015cc, along with the spectacular looking Nitro Racing (AC Sanctuary’s in-house brand) full titanium exhaust; but much of the engine work revolves around making the original design more durable.
“We haven’t measured the engine power of RCM-230 yet,” Nakamura admits, “but it is not overly powerful, as our owner wanted to focus on building the engine to last long. It is estimated to have around 100 horsepower.” Stock, the Z1 put out around 82 horses.
The resulting bike is best compared to a vintage bike, rather than a modern motorcycle.
“Naked bikes which have been manufactured recently may appear to be similar to old motorbikes, but we consider them to be completely different,” according to Nakamura. “They are really well manufactured vehicles with new basic designs on the position and weight of the engine, as well as the shape and size of the frame.”
“On the other hand,” he continues, “you wouldn’t be able to turn vehicles of the 1970s into [modern] naked bikes no matter how much you customize, because their basic design is too old. However, old bikes have their own beauty, and we actually think it is a shame to totally take the characteristic away. That is why we don’t change the size of the frame, although we will still strengthen it.”
It is that premium blend of ideas that makes the RCM-230 so utterly intoxicating. The paint by American Dream uses modern chemical composition for a color and finish that could not have been dreamed of in the 1970s, and other details such as the radial-pump hydraulic clutch are fascinating to observe and consider.
In the end, I am caught up in thoughts of purchasing an RCM-230 for myself. Nakamura discourages that, but does offer me some hope: “We have exported a few RCMs over- seas so far, but the situation is quite complicated because of many legal restrictions. Despite that, we are receiving more and more requests from overseas to sell our products. Therefore, we are planning to establish the system to export them overseas, by opening a branch in the U.S. to deal with legal issues, and becoming a licensed manufacturer. To achieve this, we are currently looking for a business partner in the U.S.”
AC Sanctuary RCM-230 Specs:
- Engine: Kawasaki Z1 w/ 4mm-overbore JE pistons
- Clutch: Nitro Racing hydraulic kit w/ Brembo radial pump
- Carburetor: 36mm Mikuni TMR
- Exhaust: Nitro Racing hand-bend titanium
- Frame: AC Sanctuary modified stock
- Handlebars: Daytona Seperate
- Forks: Öhlins FG424
- Shocks: Öhlins S36
- Wheels: Oz Racing Piega forged aluminum
- Tires: Pirelli Diablo Rosso Corsa F: 120/70-17; R:180/55-17
- Brakes: Brembo
- Paint: American Dream
AC Sacntuary RCM-230 Photo Gallery:
Photography by Hideaki Togashi
Story from Ultimate MotorCycling magazine; for subscription services click here.