Arming Your Yamaha Yz With Aluminum
ARMING YOUR YAMAHA YZ WITH ALUMINUM
Make One of the Best Better with an Aftermarket Swing Arm
Len Vucci
"To consider the power, the smooth response, the handling and the ride is to declare that the Yamaha YZ400D has no peers in class." Thus concluded our March, 1977 road test of Yamaha's open class production motocrosser.
In the half-year since, no new MXers have been introduced that would oust the YZ400D from its prime position. But, as always, the competition is never far behind.
For obvious reasons, no motorcycle manufacturer can capitalize on each new technological advance without some delay between inception and production. The accessory manufacturer can reduce that delay considerably, making retrofit components available to the public virtually overnight. The release of any new model is usually followed by a deluge of aftermarket components which can sometimes make it better.
The floodgates opened wide after introduction of the D-series YZ motocrossers. And CW staff members did observe a correlation between faster YZ bikes/riders and certain basic modifications, notably, the substitution of an aftermarket swing arm. Combined with feedback from several friends who own YZ400s, this led us to believe setting up the newest YZ might be a worthwhile project.
The results? We weren’t disappointed.>
SCOPE OF MODIFICATIONS
Seldom is a production racing motorcycle actually race-worthy without first fitting the machine to the rider. The 400D is no exception. Just how comprehensively one sets up the machine is dictated largely by riding conditions, rider ability and. of course, financial resources. This last criterion is usually the limiting factor, so our approach ranged from basic nuts and bolts preparation to accessories totaling between twoand three-hundred dollars.
THE TEST BIKE
A staff member’s brother provided a 1977 YZ400D. used primarily as a play bike. The five-month-old Yamaha had recently undergone a top-end job, but was otherwise box-stock. It was your average eyeball-popper YZ. as strong, if not actually stronger than, our pre-production test bike.
In the handling department, this production YZ was much like that test bikeup to a point. Surprisingly, when pushed hard, it behaved erratically. The bike steered indecisively, its rear wheel often sought lines of its own. Unlike our original test YZ, whose handling was on par with its power, this specimen was often scary when ridden at speed.
The problem was first thought to be one of misalignment or improper adjustment. The usual remedies, including wheel and spoke maintenance, suspension tuning, and tire changing, failed to öfter even the slightest bit of relief.
REVELATION
We spoke with several other YZD owners, and were surprised to find their bikes had exhibited similar handling deficiencies. One owner had installed an aftermarket swing arm—a logical move—with good results.
The original test bike was reduced to scrap metal (a factory policy, to our chagrin) long ago, so we can only speculate as to the différences between prototype and production machines.
Evidently there were others who experienced this handling deficiency. Yamaha Motor Corporation, in the Motocross Support Group YZ Racing Information bulletin. dated June 20, 1977, stated:
“Subject: All YZ Swing Arms. Several riders have installed Pro-Fab swing arms and report good results. The 125 arm is 12mm longer and 2Vi lb. lighter. The 250/ 400 arm is 35mm longer and 2lb. lighter.”
Profab arms are also available in stock lengths. Their actual weight differs from the corresponding stock item by approximately 2'/4 lb.
Based on Yamaha’s bulletin, and Profab’s good reputation, we elected to use that swing arm in our testing.
The bare Profab arm is an impressivelooking piece of workmanship. It is constructed of 6061 aluminum, heat-treated and solution-etched to T-6 specifications. Its main members, gussetting, and reinforcing leave little doubt as to strength, yet the unit offers a one-third reduction in weight over stock. Utilization of all stock components, including pivot needle bearings and chain tensioner is maintained, and no frame or other modifications are required.
Profab arms are available for all Yamaha monoshock models, and retail for $149.95, in either stock or the longer lengths.
BASIC PREPARATION
Not wishing to rely on luck or fate during the ensuing tests, we set about prepping the YZ. Besides routine preventative maintenance, several component changes were made, typical of a privateer’s bottom-line race preparation.
The well-worn tires were first to be replaced. A 3.00-21 Metzeier MX/Enduro handled the steering. Traction was provided by a 4.60-18 Yokohama 905.
An appropriate set of handlebars was in order, and, because of the rear-set clamps, had to have shallower bends if the fit was to be right. Malcolm Smith provided a set of #3151 solid aluminum “J-Bars.” which filled the bill perfectly.
Note: Before installation, the outside edges of the handlebar clamps should be rounded slightly to relieve highly localized stress points. Without this relief, solid aluminum handlebars can fatigue and fracture, with obviously unattractive consequences.
Finally, a JT Phase II double-layer foam filter replaced the stock item. This novel design uses two separate layers of foam which have different densities. Its action is analagous to the use of different grades of sandpaper. The outer, more porous layer is for coarse work—it cuts the big particles, shirking oft' much of the debris, but does not clog easily. The inner layer of finer foam finishes the filtration process, trapping the small particles which pass through the outer layer. By employing this approach, a high degree of filtration can be maintained, yet susceptibility to clogging and consequent restriction of air flow can be reduced.
(Coincidentally, use of the JT filter was also recommended in the June 20 racing bulletin.)
THE TEST
Motorcycle, rider, support crew, and well-wishers converged on Corona Raceway, east of Los Angeles. It’s mostly a neighborhood track, with good club racers and some professionals. Just the place, in short, to test a production motocrosser against itself and its rivals.
Because the principal purpose of the racing was to test the swing arm, we planned to make the actual change in midstream, that is, between motos.
Rider Steve Bauer took the YZ out for practice. After a few exploratory laps he reported the rear was too stiff, the front too soft and the power just fine. We backed off the monoshock damping, pumped more air into the forks and sent him out again.
Better, but not perfect. The bike wasn’t coming off' the jumps as straight as he’d have liked and the back wheel was wandering, especially in traffic when the berm was occupied.
MOTO #1
Out of the gate, our entry went through the first turn in the middle of the pack. A couple of positions were quickly gained by riding ability, and several more as speed and gravity took their tolls. By mid-moto. rider positions were established w ith Steve running in the front four. The checkered flag fell, and as the bike was ridden into the pits, it was attacked by a horde of working journalists.
PIT STOP
The light of the moon is fine for many things, none of which is between-moto pit activity—of a mechanical nature, leastways. By the light of a van—highbeams— the arm transplant was undertaken.
To expedite matters, we had previously installed a set of needle bearings in the Profab arm. Normally, one would simply use the original bearings, after tapping them out of the stock arm.
Also, since we had chosen to use the longer Profab arm. a short length of drive chain was on hand to lengthen the stocker.
The actual conversion was, well, uneventful. The stock arm came oft', the chain tensioner and guard were removed and installed on the Profab arm. which was then installed on the bike. Slick. This was one bolt-on that actually was.
The only moment of hesitation occurred when the brake rod, which must be bent slightly to clear a reinforcing brace, was hooked up. Half the crew wanted to run it outside the brace; the other guy wanted it on the wheel side. Routing it inside was tried first—correctly.
Since there were no means of ascertaining suspension tuning changes which might be necessary because of the 35mm increase in swing arm length, doing nothing w'as deemed most appropriate.
Approximately one-half hour after the bike was pitted, the gas tank was topped off’ and capped.
MOTO #2
The start of the second moto was a repeat of the first. Less than a half lap later, the armed YZ was contesting the leader for 1st. The lead change occurred not long after, in typical dive-to-the-inside, roostas-you-go-by fashion. A real race was on. The two riders ran within a few bike lengths of each other for the duration of the contest. Steve maintaining the lead throughout.
Nearing the end of the moto, separated by inches, they accelerated up the steep 100yard incline, terminated by a 180-degree berm shot, and a ride down. Steve had commented on the touchy rear brake—the wheel had locked in this very corner in practice, killing the engine—and, when Our Guy failed to materialize on the downhill side, we knew why even though we could not see the corner clearly.
What we didn't know was why he didn't restart and continue, as the rest of the bikes were now passing him. A couple of laps and the moto’s end later we were still wondering, as Steve rode the bike back to the pits.
“Threw the chain.” “No!”
“Yeah, just before the turn. 1 had a real hard time getting it back on, too.”
The mangled chain was still fairly tight, even though one of the chain adjuster bolts had been torn oft' in the incident. We checked the rear wheel—it appeared to be in alignment—and dejectedly loaded the bike into the truck.
After removing the moto-mud from his body Steve commented the changes in handling were all good. The bike felt more positive in general. It steered better, and the slight softening of the rear end (because of the Profab arm’s additional length) improved the bike’s handling over jumps and stutter-bumps.
An unexpected benefit was also derived. The rear brake, originally touchy and thought to be the cause of the DNF. was operating better. Bending the brake rod to work with Profab arm had a cushioning effect, which desensitized the brake.
We had recorded lap times which were consistently two seconds or more better than the quickest first-moto lap. The operation was a success, but . . . we'd try it again next week.
SPECULATION
Daylight revealed the rear wheel was not as straight as it should have been. The drive chain, original and well used, should probably have been replaced, we surmised. The swing arm’s fault? The longer arm, which provides a slightly more shallow wheel arc, should have been easier on the chain, if anything. But we had to know for sure.
FINAL ANALYSIS
One week later, we were back at Corona. DG Performance Specialties had provided us with a Tsubaki HD chain, the rear wheel was definitely in alignment, and we were up for it.
Steve got 2nd in both motos, behind that same guy, who must have been up for it a little higher.
We would have compared lap times to the previous week, but 1) MX tracks change so fast that relatively close times would be inconclusive. 2) The track-prepping dozer had broken a not-so-buried water main, liquifying one corner and a short straight. 3) Someone (who shall remain nameless) left the stopwatch in the truck.
We were, nevertheless, elated over the evening’s course of events. To our satisfaction, the merits of an aftermarket swing arm were proven, and we're happy to pass along the results. If your stock YZ is the handler you desire, we’re happy for you,
too. If it's not, you might try a swing arm from Profab, or, if you desire, from another manufacturer. If a local dealer doesn’t stock the arm of your choice, write to the
top. We're aware of the following manufacturers. and thus provide addresses. J5J
C&J Precision Products 1151 E. Mission Fall brook, Calif. 92028
Cross-Up, Inc. 117 W. B Street Wilmington, Calif. 90744
DG Performance Specialties 1170 Van Horne Anaheim. Calif. 92806
Profab 3185C Airway Ave. Costa Mesa, Calif. 92626
Thor Racing Fabrication, Inc. 6905 Oslo Circle East Buena Park, Calif. 90621
