I doubt that it’s possible to overstate the degree to which the world changed during the 1960s. Indisputably, it was a decade of innovation and change in many areas besides motorcycling. But as surely as history was being rewritten in Saigon and Woodstock, so changed the look, feel and performance of motorcycles.
According to Yamaha market research, there was vast and untapped market for dual-purpose off-road bikes. Sure, people had been removing lights and making upswept pipes and homemade skid plates for years, but now Yamaha would do it all at the factory, on a mass scale unlike the little specialty off-roaders. The Yamaha DT-1 (above) arrived in 1968, and the first run of 12,000 bikes sold out immediately. Yamaha had a runaway hit on its hands, and soon there was a full family of DTs.
It’s likely that there will never be one bike that satisfies everyone’s needs. For those going off road, weight and size are the enemy. What’s more, while a two-stroke single engine will handle most street duties, it’s not meant to go cross-country or interstate, only across town. Some contemporary British bikes were capable on both streets and highways, but they were pretty heavy. Ride to the dirt–no need for a truck to tote the bike. Why did no manufacturer before Yamaha recognize that? Beats me. They were breaking new ground when everyone else was playing catch up.
On the other end of the spectrum were trails-oriented bikes like the brilliant Bultaco Sherpa T, which made all the old English four-stroke thumpers obsolete. But you wouldn’t likely consider riding this down the road for any length. What America needed was an affordable dual-purpose bike, and Yamaha gave it to them, in spades.
I guess one could gather that Yamaha was full of bright people who knew what the American buying public wanted. That would make a good story, but in truth, Yamaha was full of bright people who knew how to ask questions. Plenty of desert (and other) racers were more than happy to give them some good answers. But before we look at the different models in the Yamaha DT lineup, let’s review what’s essential to a good dual-purpose bike.
First, dual-purpose tires: My first on- or off-road tires, Bridgestones, had a more aggressive tread than street tires. They looked a lot like this, and actually put a lot of rubber to the road surface.
I recall the time I hit an oil slick on the road that moved me across a full lane of traffic. If I’d been using dedicated off-road knobbies like these, I probably wouldn’t be writing this story. I feel lucky as it is. A number of people who fitted them for off-roading were shocked to discover that they turn pavement into something very slippery.
Obviously, an off-road bike needs more ground clearance than a street bike. On the street, the extra clearance simply increases wind resistance, which becomes a factor when you’re move out. On the other hand, if you take a street bike off road, you’ll likely be leaving (1) the exhaust pipes on a rock somewhere, or (2) your bike at the spot where it is. I’d say that this bike’s problems stemmed from its tires and weight. Of course, some terrain just isn’t bikeable. Really porky adventure bikes can literally provide you with an adventure.
I’m not an engineer, which makes explaining these engine types a bit difficult. Let’s just say that the two-stroke single is narrower and lighter than a comparable four stroke. Moreover, a single gives you better traction in the dirt; each interrupted power pulse gives the wheel a chance to dig in. Power isn’t the only reason large, multiple-cylinder engines were banned from dirt track: They tend to spin, and not dig in.
The DT-1 was a 250, with 23 hp and a wide powerband, perfect for the type of use it was expected to see: more on-road than of-road, but capable in either one.
This little jewel is identical to the bike I rode all over the Panamanian rain forest. A DT250 was certainly heavier and less powerful than such professional off-road bikes as Bultacos and Husqvarnas. It costs money to make high quality light bikes, but after you took the time to strip one of these, you had something that would do quite well. It was fairly light for the price, and powerful enough to keep up with traffic, on or off the road. With the 250, Yamaha hit a home run. I owned one and loved it, but I’m still not certain it was their best bike.
Yamaha knew that buyers in the U.S. were already comfortable with four-strokes, and that they didn’t particularly like mixing oil and gas. Their answer was automatic lubing system, called Yamalube, whose only visible component was a small glass circle at the bottom left of the oil tank. If you couldn’t see oil in it, catastrophe was imminent. Just fill it and forget about it, at least for several tankfuls of gas.
A pulsating air system gave the Yamahas a power advantage over other two-stroke engines. In a normal two-stroke system, the piston tries to push air back through the intake port. This reed valve is a one-way system that stops that problem. You are viewing it from the engine side.
The Yamaha exhaust system contained elements of an expansion-chamber system which, when combined with the reed valve, made the engine far more efficient: Exhaust gas exiting the engine is allowed to expand before being constricted just before it leaves the pipe. That sends a power pulse back to the engine that, with correct timing, pushes fresh gas and air back into the cylinder instead of letting it escape. It’s not the full expansion chamber associated with motocross bikes; after all, that would create licensing problems. In addition, these bikes were fitted with muffling and a spark arrestor to comply with local laws.
I can’t get enough of this little animated graphic of a two-stroke, so here it is again; maybe not for the last time.
Long travel suspension is absolutely necessary for going fast off-road, but is not good for street bikes for many reasons. Looking at the top bike, you can see how its fork and rear shock are extended; the travel on a modern dirt bike would be longer. Thus, Yamaha compromised. I felt very comfortable at 60 mph, and slightly less so trying to go fast in the dirt.
Early in the DT’s evolution, Yamaha decide to adapt existing motocross frames for use on the next year’s DTs. Better in the dirt. Not so on the road.
The DT-1 was a mammoth success, and soon was selling at some 50,000 per year, almost unheard of. Yamaha instantly knew it had struck gold, and a whole family of DTs soon arrived; everything from 50cc to 400cc. Yamaha’s DTs re-defined the bike market, and had an unbelievable impact on the market. Dual purpose bikes were cool, until the fad (like all) petered out.
There are still some out there if you look. Just yesterday I stumbled across a DT175, bought by a friend who is in the process of repairing it. After only 4,000 miles the spark died, so he’s busy installing a 12v system from a Honda ATV. I would be lost in there, but he isn’t–and he’s looking forward to having fun.