Denis Curtis started to race in 1965 on a stock BSA 350 Gold Star that had been lent to him by a friend. Like many at that time, Denis often rode the bike to Cadwell Park, Lincolnshire, taped-up the lights, added number plates and went racing. As he was only 16 years old, his mother had to sign a waiver, leaving his father out of the secret. But Denis also had another secret; a plan to build his own racing bike, a Vincent special to compete in the Clubman Open class in the UK championship.
The project began in 1965, while Denis was an apprentice at Jig and Tools Design at Lincoln. His vision was not to make as a classic Norvin, but rather in a way to use the most of the original racing bike to make it more competitive. Thus was born the concept to replace the Manx engine with a Vincent V-Twin, built on the exact crank centerline of the Manx, for better weight distribution and ease of maintenance (valve clearance adjustment, etc.). Consequently the Vincent twin unit had to be chopped to remove what was not necessary – the swinging arm anchorage, the primary transmission case, dynamo mounting and gearbox in order to mate it to the Manx 4-speed gearbox/transmission.
The frame also had to be heavily re-engineered. The front down rails were removed to suspend the Vincent engine from plates attached to the steering head and boxed-in for strength (1). Completing the picture was the upgraded engine to the Black Lightning specification. Once the project completed in 1967, Denis raced the bike himself for the first season and subsequently sponsored Martin Bland for a couple of races before selling the bike to Bland. Between 1968 and 1970, Curtis made seven more Curtis-Vincent twin cylinder race and street bikes using Norton frames, but only three of those used Manx Norton frames and cycle parts (2). In all cases, the front down rails were cut out just underneath the Norton steering head.
In 1970, Denis emigrated to Canada where he started his own business in 1972 (3) producing frame kits and cast magnesium wheels for both race and street bikes. But in 1973, Denis was approached by a customer asking him to make a Curtis-Vincent specially for him as the customer had a spare, chopped twin engine for which he had no use. Curtis accepted the commission and used the same concept as previously, but made the frame and swinging-arm from scratch with 4130 Aircraft Chrome-Moly Steel tube. A total Dennis built five bikes in such style, and all were for chopped engines (4).
In 1977, Curtis sold his business in order to focus on other activities. But 30 years later, his new company CMR (5) was incorporated with the intention to resume motorcycle rolling frame manufacture. Several collectors soon learnt that Curtis was producing frames again, and asked him to consider reproducing and updating his Curtis-Vincent concept. Denis accepted the challenge and CMR is currently working on three special orders that we will have the opportunity to feature in the future as the bikes are completed. Would it be a surprise to tell you that all projects are using chopped cases? No, thought not. CMR is now ready to take on more projects, although the core activity is focused on building traditional racing frames for Yamaha 4-cylinder and 2-cylinder, 2-strokes and 15 other classic racing models, which are becoming popular in classic racing circles.
Unlike Fritz Egli, who built the frame around the Vincent power plant, Curtis took the challenge the other way around. In essence, carving the Vincent engine to fit it into a Manx Norton frame in order to deploy the cutting edge racing components, brakes and forks of the time. As a result, the racing bike was light, handled well, and was much lighter (like the Egli, and London built Parkin-Vincent) than the standard Stevenage product. Vincent purists have often derided the non-reversible alteration of the Vincent engine. It is certainly true, but at that time it was current practice for racing: Vincent twins were among the fastest power units available until the early 1970s, and were also cheap. Racers, then as now, want the maximum power for a minimum outlay. Hence the slew of specials, in road racing and sprinting, utilizing the 1000cc power plant, harnessed in more modern frames and running gear. Paradoxically, Curtis’ idea today turns out to be a great way to revive the already chopped Vincent engines that are looking for a second life from their repose on the shelves and under benches in garages around the world.
Philippe Guyony © 2014
Pictures by courtesy of Denis Curtis that I thanks here for his collaboration to this article. Special thanks as well to David Lancaster for his help in this writing.
(1) For those who are interested in the details of the frame modification, here is Denis’s description: “In all cases I cut out the front down rails from just underneath the Norton steering head to a point approx. 3 inches from the lower rear frame cross tube (where the lower rear Norton gear box plates attach), and to the rear frame lower cross tube with the complete twin engine. The idea was to place the engine on the same approximate crankshaft centre-line, for better weight distribution and ease of maintenance. The Vincent twin engine, using the earlier B head steadies, was suspended from plates attached to either side of the Norton steering head which were also attached to the frame cross tube just behind the steering head. These plates were boxed in for strength. I ran a tube from the upper rear of the steering head back to a cross tube just above the rear head steady, which I welded into the top tank frame rails. This was a triangulation, and under this cross tube was the rear cylinder head steady mount of similar design to the original Vincent. The rest is easy to see from pictures”.
(2) Denis provided a brief description of each frame from memory. Details and names of most of the purchasers were lost in a flood at one of the homes:
#1 The original Curtis Vincent Twin prototype, modified Manx Norton frame, chopped engine, race bike sold to Martin Bland of Sleaford (UK)
#2 A modified Norton wide-line frame, full Vincent Twin engine, for street, Denis’ own bike sold to Stuart Howe of Leicester (UK)
#3 A modified Norton slim-line frame, chopped Vincent twin engine, Manx gearbox, full-race, sold to S. Howe of Leicester (UK)
#4 A modified Norton wide line frame, chopped twin engine, Manx gearbox, full race, sold to someone in Perth Scotland (UK)
#5 A modified Norton Manx frame, same spec as #1, full race, sold to Terry Fielding of Oxford (UK)
#6 A modified Norton slim-line frame, full Vincent Twin engine, for street use, sold to a customer in Kent (UK)
#7 A modified Norton slim-line frame, full Vincent Twin engine, for racing use, sold to a customer in London (UK)
#8 A modified Norton Manx frame, same spec as #1, full race, sold to a customer in Bedford (UK)
(3) “Curtis Manufacturing and Service Ltd.” in Vancouver B.C.
(4) Denis also provided a brief description of rolling frame he built from his Canadian workshop:
#1 Curtis-Vincent Twin, chopped twin engine, Norton gearbox, Commando Chain-case. Sold to Jim Brokensha North Vancouver and subsequently purchased by Jay Leno who still owns it
#2 and #3 sold to a Vincent enthusiast in Winnipeg, Manitoba Canada who wished to make a race bike and a street bike. No contact info
#4 sold to a customer in Seattle Wash. USA for racing. No contact info
#5 sold to Steve Harding of Vancouver B.C. for street use. Now owned by Oliver Brown of Rhodes Machine and Restoration Vancouver, who has fitted a Norton Commando 750 Twin engine into the frame
(5) “CMR Racing Products Inc.” was incorporated in 2007, more details on: