Estimated reading time: 9 minutes
English racer Chris Mehew, took an interest in the ‘Reverse Cylinder‘ idea during the 1970s. His involvement in motorcycles came to fruition at just 13 years old – a passion that has persisted for the past six decades. He raced short circuits whilst spending many pleasurable hours building a variety of specials. In a mechanical capacity, he worked for most of the official factory race teams plus many privately funded teams. His projects have included imaginative racing motorcycles such as the concept ELF and Rotary Norton. Chris started CM Engineering in 1984 which grew to become CM Engineering (Lincs) Ltd and is still going today.
Basically, everything that I built back in the day, including trials bikes, were made into cantilever. In my opinion, single shock seemed to work but as the tyres progressed and improved, the ground clearance became a major issue. I was making exhaust pipes and got really fed up with seeing people bashing flats in the sides of the all-important expansion chambers. I developed an interest in expansion chambers after reading books by Gordon Gennings, Phil Irving and following the work of Walter Kaaden. Learning and understanding how to change the shape and material to achieve required characteristics, original ones would need constant repair and alterations to increase the ground clearance.
Therefore in 1977, it got me thinking about how to get around this issue and set about designing and building a reverse cylinder Yamaha 250 that had ground clearance. In order to achieve this, I decided that I would need to put the lightweight great big expansion chambers at the top of the bike and the heavy shock absorber at the bottom. This of course led to some confusion and head-scratching about how I was going to achieve this. To put the shock at the top was a major issue.
Why does a shock absorber have to push?
I started discussing it with friends and I didn’t want to go back to twin shock as everything I was making then was all cantilevered. But for a short moment, it seemed to be the only option because obviously with a cantilever you couldn’t have the shock absorber where the exhaust pipes were. Then one day just as a jovial comment somebody said, and I can’t remember who it was now, “Why does a shock absorber have to push?” and it just set me thinking… well, why does a shock absorber have to push?
The suspension unit I had was an Armstrong Formula 1 unit with a demand valve in the top. I had problems with it leaking but it was the best available to me at the time. I then got a TZ350-C cantilever swing arm, turned it upside down and made all the rose joints and everything to pull on the shock absorber and mounted it. I located it behind the engine.
Evolution becomes revolution
At this moment the underslung suspension was born. It was quite a revolution in its time. I believe that it was the only underslung unit on a reverse cylinder TZ at that time. As for the chassis, I took a YDS7 Yamaha frame and just retained the steering head and the top tubes including the swingarm pivot then reconfigured the rest of the chassis completely using different tubing. All tubing used during this project on the chassis was T45.
Whenever I made a special frame, I found it simpler to use the YDS7 as a donor just like I did with the reverse cylinder. I always kept the spine and then completely remade the rest of the chassis (using T45) to fit the engine of choice and also the choice of suspension set up. The rest of the bike was then complimented with TD/TZ parts of which were in expensive and readily available at that time.
I never used a jig as such whilst making the one off chassis. Once the cradle etc was tacked into place a three inch box section bracing was bolted from the steering head to the swingarm pivot to keep the frame true whilst welding the rest of the tubes in place. That way no complicated frame jigs were necessary.
My first frame featured a Kawasaki A1R engine, it was cantilevered and finished with many TD/TZ components. This was the beginning, and also the beginning of the making pattern expansion chambers.
The reverse cylinder machine used RD forks with TZ dampers, the same as Maxton employed on their own early racers. The calipers I used were Lockheed. The engine was a TZ250 engine and all other parts were TZ apart from the exhaust pipes. When the bike was first built it had my own made exhaust pipes. I later went on to using Tommy Crawford pipes.
A meeting of minds
I met Tommy Crawford when he came to England with Dudley Cramond, ironically with a reverse cylinder TZ250, (coincidently Jacques Bolle also in the same year). Tommy made very good expansion chambers so I purchased a set of cones from him, they worked very well. I did find that the straight pipes made the engine very peaky and the next move was to make some kind of guillotine affair to go in the header pipe but I never got that far.
I can’t remember whether it was 1977 or 1978, Derek Chatterton was short of a 250. Derek said that if the 250 reverse was ready could he ride it at the Mallory Park British Championship event because Mitsui who supported Derek at that time couldn’t supply a 250. Although I had built it for my own pleasure and interest, I agreed that he could use it.
So, we started to prepare the bike and we finished it, I remember, around 4 o’clock on a Thursday. I phoned Charlie Wilkinson at Cadwell Park and he said you can come up here to try it but I can only promise you 5 or 10 minutes max because the ambulance men want to go home.
We did that then we found some problems. These were sorted out and we arrived at Mallory Park on the Sunday. Derek won the first race. Unfortunately, he was knocked off in the 2nd race at Gerrards. Then he rode it at Cadwell at the May International where I think he finished 2nd to Paul Smart. Mitsui then supplied the 250 so the reverse cylinder bike was surplus from Dereks point of view.
Mick Chatterton then rode it at the North West 200 and Tony Rogers, who was riding Maxtons at the time, showed some interest. He was the man of the moment.
↑ Tony Rogers on the Chris Mehew built 250cc reverse cylinder. Tommy Crawford built these longer exhausts.
He rode it at Donington. He was fastest by quite a bit but unfortunately, the pin that locates the piston ring came out and that ended our day’s racing. There was talk of perhaps continuing for the following season and developing the bike further but unfortunately, that did not materialise. After Tony, it was ridden by a lad called John Keasley who was an incredible rider but unfortunately, he didn’t know his own limits.
Thwarted by funding
What started life initially for pleasure soon proved to become quite successful. The idea of my reverse cylinder wasn’t really for performance although it developed its advantages. When it started getting successful it became serious so I started throwing more money at it. Sadly, in the end my pockets were just not deep enough for my enthusiasm. I never got any further because I ran out of money basically. I was hoping for funding to further develop it and to buy equipment but again that never happened. There were just no funds and nobody to share the ideas with.
I didn’t have the equipment to manufacture things with a proper style it was just cut, shut and bronze welded. As with most things if you want to develop you need financial backing. So unfortunately, the bottom line was, there wasn’t any. In the end, somebody wanted to buy a TZ 250 engine, I needed the money so that was that. The bike was taken apart and sold separately. I really wish I didn’t have to do that. It could and should have been a lot better had time and funding been on my side.
The bike back in the day never got the recognition that it deserved really. It just became a special that people just wanted to have a go on, if you know what I mean. To me, it looked really smart and went really well.
Getting things back-to-front
Nearly everything on the bike ended up back-to-front. In fact, it was nicknamed ‘The Back to Fronter’. Everybody loved the ‘back to fronter’ and the name came from Eric Hayes’s wife actually.
“Bits that should come out of the front are coming out of the back, and the bits out the back are coming out of the front, swing arms upside down, shock pulling instead of pushing, everything’s back-to-front”
Pam Hayes
Going forward through the years and now finding myself in semi-retirement or just wanting to relive my youth, (whichever it may be), I have now built a replica of that bike which I am quite proud of! After all it was my passion and still is.
↑ Left image: Chris Mehew and Derek Chatterton with the 250cc reverse cylinder Yamaha
↑ Right image: Derek Chatterton, race winner on the British-designed Back-to-Fronter.
On display at British Superbikes
Chris Mehew kindly brought along his reverse cylinder Yamaha to put on display at our final round of 2021 at Cadwell Park, hosted by British Superbikes.
