Yoshimura's Daytona Winner
Yoshimura's Daytona Winner
Regular readers know we featured the Yoshimura Suzuki GS1000 in the May issue. At Daytona this year, Yoshimura R&D of America team, riders Ron Pierce, Wes Cooley and David Emde finished first, second and third in the Superbike Production race. Roberto Pietri, who bought a complete. racebike from Yoshimura for $10,000, ran fourth until his bike lost its drive chain.
Race machine development never stops. The motorcycles ridden by Pierce and Cóoley we .rc completely new machines, built up from stockers. with less frame bracing. more shock angle 4nd: bigger en gines (1023cc): than the bike in our May Issue. Emde mde the bike we testód, but with a new 1023cc engine. Pietri's bike had the same chassis ridden by Steve Mcifcaughlin and Ron Pierce in 1978. but with a 997cc powerplant.
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The extra 26cc displacement came from increasing the bore from 70mm to 70.9mm. while stroke remained stock at p4.8mm. Power increased along with size— the latest Yoshimura engine makes four tjnore bhp than its predecessor.
The CR carburetors fitted to the team hikes had a new idling circuit machined in by Pops Yoshimura. The extra circuit almost eliminated response hesitation at half throttle, normally a problem with CRs. fhe velocity stacks are staggered in length ^o compensate for slightly unequal intake tract lengths in the Suzuki cylinder head casting—the two outside stacks are 2.5-in. long, while the inside stacks are 2.125-in. flong.
Last year the Yoshimuras welded an "extra 35mm onto each fork tube, increasing overall length and travel. They mounted the rear shocks at a 54.4° angle; ^nd extensively braced the rear section of the frame and the bottom of the sw ingarm.
This year the new Yoshimura Superbikes have standard length fork tubes to comply with AMA rules. The upper shock mount tubes of the frame are bent forward Tind up and the upper shock mounts are positioned 13 in. from the swing arm pivot, fhe repositioning changes shock angle to 61° and yields an extra half inch of rear wheel travel for a total of 5.25 in. The swingarm itself is not braced.
At Daytona, the bikes handled as well as anything on the track in the sense that they didn’t wobble or gyrate. That seems to Support the Yoshimura contention that suspension, not extensive swingarm or Trame bracing, is the key to controllable handling. Daytona is in many ways a unique track, however. Because they are 4imited to stock length fork tubes by AMA regulations, the Yoshimuras are worried ábout available cornering clearance for races at other tracks—Daytona is fast and flat and the turns generally wide. At other tracks with tighter turns, cornering clearance may be a problem. For that reason. kthe Yoshimuras may fit the longer leadingaxle forks from a Suzuki GS1000L.
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Because Daytona is so flat, not all the available fork travel was used. The maximum travel available out of the forks currently is 5.3 in., or 0.4 in. less than the travel provided by the extended fork tubes used in 1978. But at Daytona. Yoshimura R&D’s chief mechanic and suspension specialist Suehiro Watanabe set up the new' front forks with 4.9 in. travel. Actual fork travel can be set by varying fork spring rate, size of spring preload spacer, fork fluid level and air pressure.
The Yoshimura Superbike, tested in May (and ridden by Emde at Daytona), was fitted with Lockheed calipers and stock front discs drilled for lightness and to discourage warping. At Daytona, Cooley’s bike sported a set of RG500 calipers and discs up front. Pierce’s bike used undrilled, stock KZ1000 Kawasaki discs with handmade disc carriers and Lockheed calipers. Before the final event Pierce was concerned because his bike’s front brakes dragged while Cooley’s didn’t. Cooley praised the RG500 units not only because they didn't drag but because they were, in his opinion, more powerful and controllable than the Lockheed set-up. Unfortunately. the pads installed in Cooley’s bike were early-model RG. which had the brake material bonded to an aluminum backing. During the race Cooley’s bike’s brakes faded seriously. After cooling down after the race they w'ere fine. Watanabe thinks that the aluminum pad backings overheated and became spongy, distorting under pressure from the caliper pistons and preventing effective braking. Pierce and Emde had no brake problems. For the next Superbike race Cooley’s bike will be fitted with late-model RG pads with steel backings.
All three Yoshimura team bikes used RG500 rear discs and calipers.
During the final race, Pierce got side-> •ways in one turn and came so close to crashing that he thought his race was over. *The force of the bars slamming violently to full-lock snapped off the steering damper niem-joint mount bolt at the frame, but handling did not deteriorate after the incident. according to Pierce. Cooley did the same thing when he discovered he had no front brake at the back-straight chicane entrance. Cooley’s sliding and tankslapping trip into the chicane run-off was so sadden and fierce that the entire steering damper bracket was ripped off the frame at "ihe weld. Like Pierce. Cooley said he didn’t notice a handling change afterwards. Emde had no problems during the race, ¿ind his bike finished unscathed.
All three Yoshimura team bikes ran 'Goodyear tires for the races after earlyweek experiments with Michelins. Each of the machines had 3.25-19 D1950 slicks up front. Cooley and Emde ran 3.75-18 D1705 slicks in the rear, while Pierce selected a ^ofter 3.75-18 D1829. After the 100-miles, it was impossible to detect a difference in wear between the 1705 on Cooley’s bike and the 1829 on Pierce's. Both looked •ready for another 100 miles. It would appear that the AMA’s fears that Superbikes are harder on tires because they weigh more than GP bikes is unfounded. k The AMA ruled that Superbikes must l^ave stock seat profiles in 1979. The Yoshimura mechanics were ingenious about satisfying the rules and yet allowing for rider safety (the stepped seats favored |in 1978 helped riders hang on). On Pierce’s and Emde’s bikes, foam was gouged out of fihe center of the seat and left standing on the edges—the whole thing collapsed when fhe rider sat down. Cooley wanted a more Pronounced bucket, so a friendly upholsterer fabricated a metal loop at the front .edge of the seat (adjacent to the gas tank), hollowed out the foam from the front ►section of the seat, rigged bungee cords from the metal loop to the rear of the seat Section, and re-installed the cover. Without rider, the bungee cords held the seat sides up—stock profile. With rider, the front section of the seat collapsed—step seat.
But for gas cap and refueling-stop trickery, Pierce’s bike took the cake. Mechanic Jyo Bito cut off the top of the key needed to epen the Suzuki gas cap and brazed on a large clutch nut. drilled the nut for tightness, and safety-wired the whole thing to the handlebars. Instead of inserting a key lit Pierce’s gas stop, Bito only had to turn the nut and pull, saving a few precious seconds in the dump-can refueling.
► The result of the changes was victory. The new bikes weigh about the same as the 1*978 versions, yet make more power. In practice before the 1978 Daytona race. Steve McLaughlin was clocked on his Yoshimura GS 1000 at 159 mph. This year, Cooley was caught by the radar at 167 ¿nph. Judging by Daytona, this should be Pops Yoshimura’s year. ®
