Yamaha Dt360a

YAMAHA DT360A

Outrageously Overweight, Such A Fine Machine Makes One Wonder How Good It Really Could Be.

■ YAMAHA HAS finally done it. Its infamous endo machines, the 250s and 360s derived from the original dual-purpose DT1, have been replaced in the ’74 line-up. Now there is a totally new series of motorcycles...motorcycles that have enough potential to be associated with the term “enduro.”

Take the DT360A, the largest of the new breed. Instead of building the bike around an unproven chassis design, Yamaha did a very smart thing. It pulled a production motocross frame off the shelf, welded some additional tabs on it for mounting a battery, turn signals, and the like, and proceeded from there.

Now, if you’re a local hotshoe, use of a stock Yamaha motocross frame will not impress you. And it shouldn’t, really. After all, the unit is heavy and not as forgiving as a Maico or a Honda CR chassis when the going gets tough. But for trail riding, or enduro work, where the pace is slower, it’s more than adequate in both handling potential and strength.

Realizing that it’s foolish to compromise a good chassis with bad suspension components, Yamaha engineers visited their parts bin one more time and returned with a set of motocross forks and rear shocks. To these they added stiffer springs to cope with the additional weight of street legal equipment, and then left well enough alone.

Next came the engine, and here again, exactly the right thing was done. Yamaha began with the motocross versions die-cast aluminum alloy crankcase assembly, decided to leave out the Omni-Phase balancer, and other than that, assembled a lower end of standard design.

The connecting rod has a needle bearing at both the big end and at the wrist pin. Two ball bearing main bearings support the built up crankshaft which drives off the right side via a helical gear primary. The drive is then taken up by a 6-spring clutch with eight steel and eight phenolic friction plates with a cork-like facing. A five-speed transmission with a ratchet type shifting mechanism completes the drivetrain.

Both cylinder barrel and head appear identical to the 360 MX, but in reality they are different. Intake, exhaust and transfer ports are all smaller and the cylinder head has been reshaped to provide a lower compression ratio.

Torque induction is featured as usual and on the DT360 the system consists of a 6-petal reed valve assembly which works in conjunction with a specially windowed piston. Besides allowing an early intake of the fuel charge through the ports in the piston, the reed prevents the fuel from backing up into the intake tract when crankcase pressure changes from negative to positive. Consequently, more radical port timing can be used without making the engine as temperamental as would normally be the case. >

Cycle World Road Test

The piston itself differs from the motocross version in that a second compression ring has been added. The bottom ring is of conventional design, but the top is an L-type Keystone ring which provides better combustion pressure sealing. M

Autolube is present as always and, like on other Yamahf Singles, the pump can be removed if the rider so desires. Mixing gas and oil, however, is a bothersome chore and the 4 lb. or so saved by removing the pump and oil tank are really not worth it if the machine is ridden frequently. Incidentally, the pump cover and other engine case castings are secured by alien head bolts which are easier to remove and retighten than the Phillips head type.

Complimenting the chassis, suspension and engine is a really sanitary exhaust system. It is not only high mounted, but also passes right down the center of the chassis, totally out of the rider’s way. Unfortunately, there is a clearance problem with the rear tire on early production models, but this isn’t as serious as it first seems. The solution is simple. Go down to your Yamaha dealer, buy a 15mm spacer, and install it under the rear exhaust mount bolt. That’ll do for tires up to a 4.00-18 knobby. For tires of a bigger size, like a 4.50, you’ll have to dent the pipe!

Major components on the DT360 are fantastic. No doubt about it. But when it came time to bolt on the small stuffy Yamaha dropped the ball-at least as far as off-road riders are concerned. Sure, there’s a high mounted, flexible plastic front fender, but at the rear there’s an overweight steel one. Sure, there’s a speedometer with resettable trip odometer, but there’s a useless tachometer mounted next to it on a bracket that’s difficult to get rid of short of hacksawing and filing it away.

And where are the cleatèd footpegs? Rubber ones are all but useless when conditions are wet or muddy. And where is that superb motocross front hub? The full width unit no doubt provides a margin of safety for street riders, but the brake is not all that strong anyway and the hub adds too much unsprung weight! And finally, where are the light, strong D.I.D. rims that are becoming commonplace with Japanese dirt bikes? Come on, Yamaha. You’re too close for this.

Now for the big question. How have these compromises affected the performance of a potentially great machine? First impressions of the DT360 are perhaps the best. The bike has a slim profile. Nothing gets in the way, not even the kickstart lever as in the past. It’s a rangy mount, too. There’s plenty o^ room to spread out and readjust body position for long periods in the saddle. Neat.

Flip out the kickstarter and kick it through. Effort is minimal, thanks to an automatic decompression valve activated by the kickstarter itself. One or two kicks will suffice unless the engine is hot, then several are the rule.

After a brief warm-up period, the Yamaha will move out smartly. Gearing is on the high side for tight trails, but for casual riding or fireroading, it’s OK as is. Clutch action is smooth and gearbox feel is typical Yamaha.... Nice.

So much for familiarization. Pick up a little speed and enter a typical trail network. As soon as you begin negotiating turns, the bike feels heavy. There’s too much weight up high. The fuel tank is steel. The brackets are steel. Everything is steel. And a good deal of the weight is too far forward. Consequently, you can’t get the front end up and that is definitely a speed limiting factor off-road. Also, the front tire is too narrow. It tracks in all the ruts. It makes the bike all but uncontrollable in a sand wash. And it transmits too much shock to the handlebars in rocky sections.

The bike does steer well, however, and as long as you’re not in a tremendous hurry, it is very comforting and stable along> mountain trails. When the terrain is rough, the suspension do^ an excellent job of soaking up the bumps, too, but the forks top out. And, on a washboard road or cobblestone surface, the ride is harsh. The reason is simple. The bike is oversprung for a 160-lb. rider, so for us a change to lighter springs and/or oil front and rear was necessary.

YAMAHA

DT360A

SPECIFICATIONS_

List price.........................$1026

ACCELERATION / ENGINE AND ROAD SPEEDS / RPM X 100

10 20 30 40 50 60

TIME IN SECONDS

Get on a smooth stretch of fireroad and, turns permitting, you can blast along at over 80 mph. Unreal. And the bike is stable at speed as long as you don’t try and slide it. Get on the brakes until the bike slows to 30 mph or so, pitch it sideways, gas it, and some interesting things happen. The power is there to hold a graceful slide, but the Yamaha doesn’t want to. Instead of carving a smooth arc, the 360 will break and regain traction repeatedly. We never got pitched over the bars, but it’s a possibility, as the Yammie picks itself up in a helluva hurry!

Hillclimbing is a bit of a blow out, too, but this isn’t really a design problem. Change the rear trials universal to a knobby, fit a 14 instead of a 15-tooth countershaft sprocket, and the DT360 will climb anything most riders have the courage to try. In fact, it’ll climb some hills two-up that the majority o| 125s won’t go up solo!

Sure we’re being hard on the 360. But it could have been an excellent off-roader instead of being a cut above average. And with an excellent off road rating, it would be THE bike because it is fantastic on the street. Vibration is acceptable for commuting. Gas mileage is around 50 mpg. It’s fast enough for expressways. There’s plenty of power to pass. It’s a safe, predictable handler with the stock tires. All this, and quiet too. When Yamaha tested the DT360 for sound, it came up with a respectable 85dbA. No more.

So, for casual trail riding and for light street duty, the DT360A is an excellent buy. It’s a lot of motorcycle for $1026. But for riding enduros or for just plain going fast, it’s not quite right...not quite right even though the basics are there.

Perplexed by this conclusion? If so, get right into the next article. We’ll show you how to make the big Yammie right. EB