Isdt Bmw 750
ISDT BMW 750
Cycle World Impression
If a German walks up to you someday and babbles something about weight, followed by, "wanna wager," don't even consider it.
Bob Atkinson
WHAT WOULD you do if some guy walked up and told you point blank that his 750 dirt bike weighed less than your 350 Honda Single? Granted, the Honda is no feather, but you'd no doubt put your paycheck on the line and relieve this guy of some money.
Most of the time you'd win. But not every time! if the guy in question is Volker Beer and the machine an ISDT BMW, you can kiss your paycheck goodbye, because you’ve just lost by close to 20 pounds.
Think about it. Residing in Germany are a very few factory machines that have got more horsepower, more torque, more speed, and less weight than the majority of 350 class dual-purpose machines sold in this country. That's four major advantages to offset a singular disadvantage: excessive width caused by the opposed configuration of the engine. Think about it. Residing in Germany are a very few factory machines that have got more horsepower, more torque, more speed, and less weight than the majority of 350 class dual-purpose machines sold in this country. That’s four major advantages to offset a singular disadvantage: excessive width caused by the opposed configuration of the engine.
I first saw the machines at the International Six Days Trial in the Berkshires earlier this year. And, after watching Herbert Schek power his way to a gold medal, I was impressed. Really impressed. So I asked for the loan of one of the machines. Volker Beer (flashing a crafty smile) and BMW came through.
Berkshire mornings are cold and damp. And upon my arrival at the impound area, I found exactly those conditions. The Bee Em was there; mud, number plates and all, and so was Volker Beer. Volker didn’t have a whole lot to say, except, “Tickle the carbs and have fun.” But that smile of his was radiant, and I knew there was a catch.
Starting a cold Bee Em is that catch. The ISDT riders had been plagued with hard starting in the morning, and so was I. I kicked. Kicked some more. Kicked harder. Gave up. Kicked again. And then the thing started.
Relieved, I stepped down on the tiny gearshift lever to engage low, and pulled into the woods. The ISDT has a suramount of torque, and when you > let the clutch out, the rear of the machine lurches upward, just like the street version’s does. It’s really a strange sensation off-road!
But that’s not all that’s strange. Wi^ all of that torque, the broad power band, and long wheelbase, I just knew the Bee Em was a slider. It is all right, but there’s a problem. When you go to put your foot down, it lands right on top of the valve cover, instead of on the ground. Result: slide out instead of slide. The trick is to shift your body position on the seat, and to slide conservatively.
Another inconvenience is carburetor/footpeg clearance. It isn’t as severe as it is on the roadster, because the vacuum-type Bing carburetors have been replaced by smaller slide-type units for ISDT machines. But the problem still exists, especially when you’re in the standing position.
Standing is something that you have to do on all dirt bikes, but on the Bee Em, I very seldom found myself up on the pegs. Outstanding suspension is the reason. Damping action is faultless up front; and those famed BMW forks have Bl9 in. of usable travel! Rear shock absorbers are impressive, as well, with good damping and 4.92 in. of travel.
After an hour or so, I was familiar enough with the machine to push it a bit and evaluate the handling. Power doesn’t come on with a rush, and acceleration isn’t all that rapid, but there’s enough torque to keep the wheel spinning—especially in the lower gears.
Because the bike has a low center of gravity (another benefit of an opposed engine), you can pitch it into a turn. Roll on the throttle and the rear end will ease to the outside slightly, then stop. Just the way it’s supposed to. If you can remember to miss the cylinder with your foot, everything works out fine, even at a high rate of speed.
I was concerned initially with dragging a cylinder, but this never occurred. The lean angle that is possible is greater than that on a stocker, because of a narrower-than-production engine. More on this later. As long as there aren’t any big rocks in sight, it’s best to forget about dragging things. If you dwell on it, you unnecessarily lose faith in the machinery. And, without a proper frame of mind, you just can’t get it on.
Flying over bumps isn’t really a good idea on the Bee Em; and rocks should be avoided if possible, because the wheel rims really take a beating. 298 lb. is light for a 750, but it’s still too heavy for acrobatics. Also, it is a good idea to be cautious in blind corners. Deep ruts will get the cylinders too close to the ground and abnormally tight corners require a little forethought. The reason for this is that the combination of a long wheelbase and a conservative fork lock make for a rather generous turning radius.
Wheelies are more the big Bee Em’s forte. Either first or second gear will work. Just sit back, nail the throttle, and jerk up on the bars. I found it best to leave the power on and control the height of the front wheel with my body weight, it sounds impossible, but it really is fairly easy. And those wheelies last forever.
By now you’ve no doubt realized that the ISDT BMW is a bit of a curiosity. Because of that fact, two questions immediately come to mind. First, why did the factory build it? And second, since they did build it, how did they go about it?
To answer the first question, BMW built this and a few other bikes like it to prove the reliability of their street bikes. Their feeling is that if you can take a machine out of the element it was designed to cope with, subject it to a good deal of mechanical abuse in a foreign environment, and come out looking good; you have a winner in every sense of the word.
Now for how. BMW personnel began with the engine and drive train of a standard 750, then concentrated on weight and compactness. To aid in weight reduction, the electric starter and complex air intake system were discarded. Likewise, the ignition system was modified to run without a battery, and lighter slide-type carburetors were fitted.
To make the unit more compact, the connecting rods were shortened. Cylinder barrels were abbreviated an equivalent distance at the base end, which is a clever way to keep bore/stroke dimensions the same as stock.
Because the ISDT Bee Ems do not need to rev very high all that often, different pistons are used. Much like those on a two-stroke, they have only one ring. Single-ring pistons are commonplace in racing two-strokes, but to my knowledge, their use in this bike is an unusual first in four-stroke engine design.
Exhaust gases from the narrowed engine pass through upswept pipes into a single muffler on the right side of the machine. The exhaust note is typically BMW...quiet.
A single-plate, wet clutch is used, as is a four-speed transmission that issues the characteristic BMW clunk when shifted. Current year street versions have a smoother-shifting five-speed transmission, but the extra gear is not needed off-road. In fact, it would be a drawback, as the extra components would increase the weight of the machine.
The frame is painted black and is special...just like the engine. The rear subframe on the ISDT model can be unbolted and replaced in a matter of minutes if it is broken. This is a good idea for the Six Days event; and it is perfectly legal, since only sections permanently attached to the main frame are considered part of that unit. Frames, of course, are one of the components that are marked, and that must go the entire distance.
Fuel tanks on the ISDT bikes are al^ minum. Fenders are flexible plastic. And lighting is the absolute minimum required for legality in Massachusetts. BMW, incidentally, got an award for the smallest taillight in the Six Days Trial. And let me tell you, that dubious honor was well-deserved! A squeeze-bulb horn and a small rear view mirror complete the legal components package.
The ISDT BMW certainly is not everyone’s idea of a dirt bike. It even stretches my idea of what an off-roader should be, and I’m about as openminded about such things as you can get. But I’ve got to admit, the bike is something else. It’ll do things I never dreamed it could. And it is, without doubt, the greatest, conversation piece I’ve ever ridden.
