Suzuki Rm400
SUZUKI RM400
CYCLE WORLD TEST
A 402cc Rocket
Suzuki dropped a bomb on the competition when it introduced the RM series of motocross bikes a short three years ago. The RMs were just what the weekend racers wanted. There were 125. 250 and
370cc versions and they soon started winning races.
The trend in the Open class at the time was toward smaller (360or 370cc) engines. It was argued that the smaller engine could be built from existing pieces and would be lighter while producing all the pow'er the riders and suspensions could
use.
Suzuki went halfway. The Open class RM had a 370 engine, built with its own center case. It was powerful for its size and weight and the 370 won its share of motos. It was different in 1977, though, because Yamaha's YZ400 monoshock had more power and the power could be used.
Suzuki got the message and the 1978 Open class RM engine displaces a full 402cc. It wasn’t difficult. The 370 had a long stroke, 80mm. so by keeping that and increasing the bore to 80mm. the factory came up with a larger motor that can use most of the older components.
The engine isn't the only news.
A large section curved aluminum swing arm is at the rear. The tank is a newly designed plastic unit. Forks and shocks are new. The 400 is more impressive than the first glance indicates. The fork stanchion tubes protrude through the new top triple clamps. Fork damping is almost perfect. They use springs and air to adjust preload like before but the top caps now have rubber covers like Yamaha uses. Making the stanchion tubes longer has several benefits. It’s sort of like the extended sliders which brought about the leading axle fork. The longer the various pieces are. the more engagement you can have and the more travel, because the springs have more room to compress, etc. Another gain is that when the factory designs the triple clamps and handlebar mounts to accept stanchion tubes extending upward, the rider can slide the tubes up and down. This changes ride height and effective steering rake and that lets the rider tune steering response to his style and to track conditions.
The top fork crown also has rubber mounted, rear set, handlebar mounting pedestals that help isolate engine vibration and suspension shock.
The shocks are gas charged KYB remote reservoir models that look like 1977’s. Not so. The damping is now adjustable tw'o ways. One firm setting and one softer setting. Even the firmest setting is softer than before and the rear wheel no longer kicks when encountering square holes, etc.
Rear wheel travel is up to 9.6 in. and balances well with the forks’ 9.8 in. of travel. To counter the added travel at both ends Suzuki has made the seat 3A in. thinner than last year. This allows a shorter rider to touch the ground. Our shorter riders thought it a great idea but the taller ones complained about the reduced height messing up the seat-to-peg relationship. Taller riders will probably want to use 1977’s seat or add an inch or two of foam to the new one.
The aluminum tank of old has been scrapped and a 2.2-gal. plastic one takes its place. The styling is more square, a lot like Yamaha’s YZ tank. A large filler hole has finally been utilized. The filler hole is the same size as the PE enduro’s and uses a plastic gas cap with the vent spout recessed to prevent breakage. It also incorporates a safety strap to keep the pit crew from dropping it in the dirt. Nice. Along with the new' tank material and design a new mounting system is used. Small metal brackets bolt the low'er front of the tank to the frame. They look fragile and are. Both brackets on the test bike broke by the second ride. We preferred the rubber strap system. It was a simple and effective method that didn’t break.
Both front and rear fenders have been redesigned. They are still plastic but have stiffening ribs to keep them from dragging on the tires when laden with mud. The front one is long and is wider at the front.
The front number plate is also new. It has cable guards that attach to the handlebar’s cross brace.
A conical front hub is used. It is the same one that is utilized on the 250 and does a superb job. The same can’t be said of the rear brake. It has been changed to a full floating unit that eliminates most chatter and hop on rough dow nhill stops. Only problem is the unit doesn't brake strongly enough. It is the same size the old 370 used. It was marginal on the 370 and has to be rated as poor on the potent 400.
The 400 is capable of very high speeds (even with the MX gearing) and the rear brake can’t effectively handle the job.
We tightened the spokes before riding the 400. The front ones bedded and required retightening only once more. The rear ones needed to be tightened after each outing. The rear sprocket bolts have been changed. They no longer thread through the hub. Bolts w ith longer shoulders combine with self-locking nuts to hold the sprocket to the hub. In theory this should be stronger than when the bolts threaded through the casting. The bolts seemed impossible to keep tight as you may have guessed, and on the third outing the sprocket tore the bolts out of the hub. This destroyed the hub, sprocket, chain and badly gouged the aluminum swing arm.
Test rider Steve Bauer wanted to race the 400 in a pro event the next day so we put a spare PE rear w heel on. The PE wheel had been run at the 1976 Baja 1000 without problem. The spokes had bedded and stopped loosening (on the PE). Within 15 minutes the spokes in it were loose.
The 400 has a tremendous amount of power and the spokes simply aren't large enough. They required constant tightening.
Steve raced his own RM250 and the RM400 at a CMC Golden State Series race at Carlsbad Raceway. The bike threw its chain in the first qualifier heat. In the second qualifier he got a 3rd. In the first moto he and another stock RM400 got the hole shot against several factory teams with their factory bikes. He finished 3rd in this moto.
Unfortunately he crashed heavily in the next 250 moto and couldn't finish the day.
The 400's power delivery is extremely smooth and deceiving. This complete lack of power peak allows the operator to go much faster than he realizes. Any stock bike that is capable of out-dragging factory bikes is fast. Even so, the engine can be run slowly without complaint. The reed induction allows the engine to run cleanly at any rpm range.
The transmission ratios are perfectly spaced. Shifting is smooth and positive. No one missed a shift or had any complaint about the transmission’s ratios. Its reluctance to shift under full power without the clutch seems to be its only flaw.
Clutch pull is smooth and easy. When pulled it completely disengages and doesn’t get hot when held in for extended periods. Several hefty kicks are required to start the 400. The part of the kick lever that your foot contacts has been shortened to clear the increased engine finning. This and a slippery unribbed surface can make starting the big RM a problem. Add mud to the smooth sole of your MX boots and the problem is compounded. A ribbed surface like the Yamaha IT's is needed.
Handling on the 400 is great. It is as quick and agile in corners as a 250. Gone is the sluggish, slow turning feel. The new fork crowns contribute to this quick, light feel. By moving the forks back a few mms in the clamps the trail has been reduced, so the bike turns more easily. The same thing could have been accomplished by reducing the rake, but high-speed stability would have been affected more.
The front forks work flawlessly. A smooth ride that follows irregularities with ease. Rider shock from the front is almost non-existent. Suzuki claims the damping is the same as that used in the '77 models. The main difference is the spring rate. Anyway they are completely adjustable by altering oil level, oil weight, and air pressure. The new shocks with their two-way adjustable rebound damping are much better than 1977’s. They aren’t stiff or harsh. Average riders will be able to use them until they wear out. Pros and fast Expert riders will find they get very hot and fade badly before a 35-minute moto is over. As they get hot they also begin bottoming.
In shorter motos their action is good. The back end of the motorcycle stays on the ground well. It tracks straight over whoops and severe bumps. Bumpy turns can be negotiated at high speeds with confidence.
The chrome-moly frame is the same as that used on the 370s. It worked well on the 370 and works equally well on the 400. It doesn’t feel as solid as a Maico or Hercules but it isn’t a flexie-flier either.
The new aluminum sw ing arm has ugly welds and looks a little fat. On the plus side it is a strong, flex-free unit that works well.
The IRC tires were skittery on our hard surface Southern California tracks until we dropped the air pressure to 8 psi front and rear. With the dropped pressure they work OK, better than most tires but still not equal to Metzelers.
SUZUKI RM400
$1698
FRONT FORKS
Forks on the RM400 are similar to those on the 125B and 250C, but an increase in length yields more travel and engagement. Damping rates are nearly the same, but spring rate has been increased by 3 lb. over earlier units. With about 10-15 psi air pressure and 20 wt. oil, these forks worked perfectly for us.
REAR SHOCKS
These KYB units look conventional, but have internal valving which allows a choice of two rebound damping rates. The difference between the settings is small, but the overall action of the shock is quite good. Primary spring rate is slightly soft for the pro, but should be perfect for everyone else.
Tests performed at Number 1 Products
Engine vibration is much less than the 370 but some still exists. Like most RMs the engine bolts are smaller in diameter than the hole they go through. Almost all vibration can be eliminated by drilling the engine mounting holes larger and using
American bolts that fit snugly.
We rode the RM at various MX tracks and took a couple trips to the desert with it. Stock gearing seems to be fine for either use. Top speed is onlv a couple of mph slower than a Husqvarna 390 OR (85-90 mph).
Everyone enjoyed the wide powerband and small bike handling characteristics.
Smooth working suspension components and blinding speed combined to allow' many riders to go faster than they thought they were capable of.
The RM400 will undoubtedly be cherished by many open motocrossers. With its excellent power delivery, the cross country and trail riders will also find it a suitable mount.
