(Formerly posted on the forum)

After visiting the 2007 Hamm Diesel Motorcycle Rally earlier this year I have been debating whether or not to write it up. I have finally put pen to paper and the result may seem negative to some but please do not take that view. I see the current diesel bikes as being something like the scene with British bikes in the late fifties. They are about to make some significant improvements. Despite what many will tell you about stagnant development, most British bikes went to unit construction and AC electrics during that time.

It was my first visit to the Hamm rally this year and I met up with Stuart and Jeff after they had disembarked from the ferry at Dieppe. From there we made our way across country to Mons. I had not seen a diesel bike in the flesh before and was surprised how agile Stuart’s bike was up to 45 mph. It also cruised happily at 50 to 60mph when up to speed. The only time the diesel was at a real disadvantage was when on the German autobahn; mainly because their lorry’s are not allowed to over take and from time to time we were slower than them.

At the show I spent my time looking at the various models, some such as the Sommer production bikes, and others home brewed. They can be broken into two camps, those using Enfield as a base bike and those not. The Enfield we have now is essentially the 1955 model 350 or 500 base machine. The engine fits snugly into this frame. It is of significance that the Redditch factory strengthened the frame in the early 1960’s when they fitted the more powerful parallel twin engines.

The Indian made diesel variant had a motor that was virtually the same dimension as the original single cylinder motor, consequently it was a good fit to the frame. Most of the other conversions I saw cut away significant parts of the front down tube in order to accommodate a bigger engine. I believe this weakens the frame, especially around the area of the steering head. Given that the frame is already near its limits I think this is a bad idea. Many had an alternative assembly, most with the join being held together with two bolts which looked to me as more likely to pivot than provide support for the steering head. What I think would be better would be to cut the backbone tubes and weld in a small extension and at the same time beef up the backbone and steering head. This would give all the space needed, and improve rather than weaken the frame. This would be especially useful to those using the Hatz IB40, as then you could make room for the five speed box.

Almost all the engines used were industrial engines designed to run at a fixed 3000 or 3600 rpm, in order to run generating sets. These speeds coincide with European 50 Hz electrical supplies (3000) and American 60HZ at 3600rpm. Because of this the fuel injection system is statically timed for peak efficiency at those rpms. Unfortunately this is too far advanced for quiet idling and any low rev power. This makes the power band very narrow. Sadly the Enfield four speed gearbox has a wide change of ratios between third and top, which means the engine drops to the bottom or below the power band when changing from third to top; the five speed box avoids this. This is why I advocate using it.

The other problem which needs to be fixed is the timing. Somehow we will have to get variable timing, if only a two position, slow speed and fast speed setting. Having looked at a few pumps, I have not yet thought of a solution to this problem. I had hoped they were like tractor and older car pumps, mounted radially so could be manually controlled by a handlebar mounted control lever like the old fashioned manual advance and retard of fifties bikes.

I think that for myself, I would want a bike with these attributes plus enough power to cruise at 70-75. I did not see the bike for me at the show, but many showed promise. I think the ideal diesel bike will have three cylinders. This for balance and vibration reasons. The same applies to petrol engined bikes.

If you look back to cars, I can remember some fitted with commercial diesel engines back in the fifties and sixties. They were cheap to run but no fun to drive. Diesel engined cars did not catch on until Peugeot made better, quieter engines. I think diesel bikes will not catch on in a big way until they too are much more refined. This is what we have to do and I hope I have pointed in the right direction.

I have half a mind to get an Electra with a view to converting it some years down the line when maybe the need will be greater and availability of suitable engines better.

If you compare a new Enfield Electra against a Sommer diesel. With a couple of extras, better seat and bigger tank, the Electra would come in at £4000, and the Sommer at about £5000. Assuming the Electra does 80mpg and the Sommer 160 mpg it would take 32,000 miles to break even (approximately), three to five years for me. This is fair enough, except I think the Electra would be nicer to ride (very subjective I know) This is because the Electra could match all the slow speed attributes of a Sommer, but also manage faster bits of motorway much better. I.e. it could hold 75mph maybe not where the Electra is happiest, but certainly double, not so for the Sommer.

I really think instead of chopping the frame to fit an engine, we need to build a motor to fit the frame.

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