The JCM trials machines were the result of the efforts of their visionary creator Jöel Corroy, who is still active in trials and runs the very successful Trail 70 concern together with many trials in eastern France. The star burned bright for a few glorio
Ihave known Jöel Corroy for a number of years and today he is still as passionate about the sport of motorcycle trials as he ever was. His shop, named Trail 70, in Vesoul, is a multi-franchise dealership selling all manner of machines. Upstairs there is an excellent museum covering off-road machines and a section dedicated to JCM which includes some prototypes and production moulds. If you look at period photos of Mick Andrews you will often see the shop’s name displayed on his riding gear.
During the 1970s and 80s Trail 70 became the reference point for the development and improvement of many Spanish marques, particularly the Ossa. With this wealth of experience he decided to take development work to its ultimate and logical conclusion and create his own machine. The first prototype was built by 1st March 1982 and was the first modern trials machine to be produced with a monoshock for the rear suspension.
The JCM Company was created at the beginning of 1983 and, along with Charles Coutard, conceived a very compact frame with an advanced single shock, using an air-spring rear suspension unit. The powerful motor came from the Italian Tau factory. The prototype was very successful and production machines were being produced by November. An excellent publicity exercise on the 26th October had Charles Coutard and Joel Descuns riding up and down the two first levels of the Eiffel Tower! If you have ever walked up it you will know how tight the turns are, up the never-ending staircases.
The model we tested is actually a 1984 model. In total some 420 examples of this second generation JCM were made in 1984 and 85. These were the production version of the models ridden by Charles and Joel. The JCM has always been a very innovative machine and they introduced the first monoshock trials machine, which was powered by the Tau engine mounted in a fabricated steel frame. The footrests featured a substantial bolt-on mounting plate which enabled easy replacement. Fuel was held in the conventional position under the tank cover. There is also a strong wide, flat sump plate acting as a stressed frame member. Forks are the industry standard Marzocchi magnesium slider units fitted to many other machines of the time.
The Tau engine is different in the way the crankcases split horizontally — like today’s Vertigo — rather than by the more common vertical arrangement. It is a conventional piston ported two-stroke. It is well over-square with an
82mm chrome plated bore combined with a 60mm stroke. The capacity is 317cc and the claimed power output is 19bhp @ 6000rpm. It has a sixspeed gearbox fitted. The clutch has a bolt-on alloy actuating mechanism operated by a mechanical lever. which also serves as the mounting point for the chain derailment protection. It would have been easy to upgrade to a hydraulic unit at a later stage if required.
The production model featured an air intake under the steering head bracing; air was then drawn down through the top frame tube into a large-volume airbox, where it passed through a Twin air filter into the carburettor. Maintenance access for the filter was through a large circular hatch on the left side. This raises the air intake to the highest point on the machine away from mud and water.
It is clear, looking round the motorcycle, that a lot of thought has gone into the design of the hand built machine. High quality components abound and the lines are smooth. The exhaust is well tucked in until the rear silencer, which may be a candidate for replacement in a mega crash as the fixing does not look very substantial. The fuel tank / seat unit is held on by a front bolt and a clip at the rear, meaning it can be removed in a few seconds. The rear sub-frame can then be unbolted, the silencer and air filter connection loosened and the entire rear end removed as a single component. This means access for maintenance is un-paralleled.
The rear suspension is probably the most advanced feature that is directly mounted to the box section swing arm. The suspension unit, which is of a JCM patented design, does not have a spring but instead uses Boyle’s Law to provide progressive compression and extension of the suspension. There are, however, limitations to the system such as the high initial force required to overcome the large seal surface retaining the damping oil, which is separated from the pressurised air chamber by a bladder and the absolute reliance of the flexible rubber bladder to maintain gas pressure. In addition the gas temperature also needs to be constant or the damper characteristics change more dramatically than in a conventional sprung unit. The use of a large aluminium body, which acts as a radiator, minimises this effect.
In the UK the machines were imported by Quinn’s of Gateshead and ridden by riders such as John Reynolds and Phil Alderson. The test machine was supplied by Joel and had been lovingly restored in his workshops back to factory fresh condition. Our good friends from
The first thing you notice is that the short, high-mounted kick-start requires a fair effort to turn over the 317cc engine with an 8.5:1 compression ratio. The engine can be started with a gear engaged, and it starts immediately thanks to the electronic ignition that is fitted. The engine note is well silenced with a characteristic metallic rattle from the steel middle-box and silencer. The revs rise and fall quite slowly and the inertia seems high, as it takes a while for the revs to die away when blipping the throttle. The clutch has a nice light feel and first gear engages with a gentle clunk. Moving off, the first thing you notice is that the front end of the machine feels quite low and the forks are very softly sprung. In contrast the rear unit feels much stiffer and much more reactive; just like a modern trials machine in fact. The rear wheel travel seems much more substantial than a twin-shock. The claimed travel is 8.5 inches, which is about two inches more than normal, and it certainly felt more ‘plush’. It also has more of a marked progressive feel towards the end of its travel, exactly what you want!
One thing that is totally different is how light the front end of the machine is. Even a gentle opening of the throttle lofts the front wheel and makes floater turns so simple if you have the skills. Personally I found it more than a little disconcerting to be climbing and turning on a banking and suddenly all front tyre grip disappears! A swift dab was the inevitable result for me. Hans however, being in a different class, showed how you could flip the front end round using his body English style. The steering head angle feels modern in that it is so responsive, and tight turns are easy on the level.
Weight distribution is biased towards the rear. I found the relationship between footrest and handlebars to be very good, unlike a Yamaha TY mono-shock where the footrests feel high although the front end seems low; a set of higher handlebars should cure this sensation.
The engine itself is a corker; powerful, strong and yet smooth as you can let the revs drop right away and, only using a whiff of throttle, search for grip with the help of the ultra- compliant rear suspension, or wind it up with the clutch slipping and then drop the clutch and burn through to a solid surface below. The only obstacle to this technique is the light front wheel, and you really need to get your body in position before letting fly. Personally I liked the tractable nature of the pilot jet but I was certainly aware that there was a lot of potential waiting to escape if I was not careful in tight corners or delicate situations.
I found steps the most daunting aspect as you needed some speed to climb them rather than being able to blip the throttle at the base. Basically you need to plan ahead and execute perfectly, which is not my forte I confess, but watching Hans you realise the machine has lots of potential in the right hands. I did not enjoy riding downhill so much, as the low front end combined with the softly sprung forks and frankly less than inspiring Leleu front brake gave the impression that things could quickly get out of control as the steering became very responsive in these circumstances. I would have preferred a much stiffer set of springs in the front or perhaps slightly less pressure in the rear unit to level the machine up more. Joel later confirmed that the rear suspension was slightly over pressure. I am willing to give this issue the benefit of the doubt if we could have had more time to set up the rear shock. Happily the back brake pedal was perfectly placed and the rear drum brake worked well! You certainly need to get your weight a long way back for confidence.
So, how could you best sum up the experience? The machine itself would have been at the peak of performance for the time, if you watch video clips of riders of the time such as Charles or Bernie Schreiber on the SWM Jumbo they climb all over the machine as they negotiate a section. Watching a good rider like Hans, who also rides a Bultaco Sherpa in classic events, ride in a similar style makes it obvious that you need to be much more physical and dominant than on a modern machine. I guess my nearest description of another machine to compare it with would be a late model Bultaco Sherpa with a rear suspension that really works.
So there you have it; a machine that heralded the dawn of a new technology combined with the riding sensation of a well tried and tested formula should have resulted in much greater success for the marque. Unfortunately the Japanese Yamaha factory were to produce the equally ground breaking TY250 mono and, given the respective resources available for development and production, there was only going to be one winner. The JCM star burned very bright from the beginning and then faded as the complexity of commercialisation and securing the necessary funds to grow the business became apparent. I wonder if there is a lesson to be learnt today.
Our thanks go to the ever enthusiastic Joel for the preparation and loan of the machine and to Hans Greiner for sharing the test with Classic Trial Magazine.
Note: “Boyle’s law is a gas law which states that the pressure and volume of a gas have an inverse relationship, when temperature is held constant. If volume increases, then pressure decreases and vice versa. When the volume is halved, the pressure is doubled. It can be mathematically expressed as: Pa”
Innovation could be found everywhere, like these inboard chain adjusters
The suspension is put to the test by Hans Trialsport in Germany were also present in the form of Editor Hans Greiner, who is a former top-20 World runner, and the results are a combination of our impressions.
We were riding a few hundred yards away from the village of Arbecey, which is the home of Joel and the superb two-day classic trial so we were sure the JCM would cope with the terrain as I suspect a lot of development work was done in the surroundings.
The large motor was a snug fit in the frame and access for maintenance was very good
The rear silencer was very exposed
Hans uses ‘Body English’ to hold the line
It was quite ‘busy’ around the carburettor area
Joel Corroy with his JCM collection in his own museum
Matt turns on the style for the camera