Cycle News - Archive Issues - 2000's

DON'T KILL IT! Four-stroke riders need to remember one thing if they run off the track. They'd better keep the engine running. It's not impossible to bump·start the new-generation 990cc prototypes., but it's far from easy, and very different from the two-strokes, which fired up after a couple of paddle steps. It is only in recent weeks that the Suzuki has been modified so that it can be bumpstarted at all - with the earlier version of the youngest of the Japanese four-strokes, it was simply impossible. The slipper clutch wouldn't allow the back wheel to turn the motor over. Nobody has revealed what compression ratios the new motors are running, but it's clearly high enough to make turning over a dead motor something of a chore. Firing up the four· strokes varies from one pit to the next. All but Aprilia use power rollers on the rear wheel, with the fuel· injected Honda and Suzuki catching almost instantly, but the carbureted Yamaha producing a fair bit of popping and banging before bursting into life. An, say technicians, can be push-started, once warm, and the Yamaha crew proved it in practice at Suzuka. That's with someone else pushing, however, and the rider already in place. The rollers are pretty fast - spinning the wheel at speeds up to or even more than 100 mph. Stand well clear, if anything looks like it's going to fall off the stand. For Suzuki, this means a special plan to disable the slipper-clutch. Their first solution, seen at IRTA tests, was to fit extra clutch springs for start-up, hastily removed before the rider could move off. By Suzuka, they had evolved a QD tool inserted into the clutch that disables the centrifugal slipper-clutch mechanism. This also is quickly whipped out before the bike leaves the pit. Aprilia's Cosworth-developed motor uses a different system, derived also from Fl with a direct crankshaft starter. They also have a different slipper-clutch system, operated not centrifugally but by engine vacuum. Close the throttle at speed and the vacuum releases the clutch enough to stop the rear wheel locking. With a dead engine, of course, there is no vacuum... and no slip. .nt tl!lles five com..... workers to help caplrostll ...f' ... his N8R500, how . IIIlIIIW do I/OU tIIInk It will tlIke to bump-Rart • 99Occ, fou....stroke MotoGP bl.ket torquey twin-cylinder Superbike takes a jump on a four-cylinder. Once they get wound up, their performance is closer, but the two-stroke never really gets a chance to get that jump back again. The difference at Suzuka came mainly at two places: Acceleration out of medium-speed turns taken in third or fourth gear. Here the fourstroke's power really counts. Direct comparison of one rider's fastest laps last year and this showed a difference of 1.4 seconds in favor of the fourstroke. Five tenths of that came from an alteration to one corner, Dunlop Curve. This had been tightened, but with a 128.7-ft reduction in track length giving back more than had been taken away. Almost all of the rest came at those two points of midrange acceleration, where the fourstroke just grabbed a chunk of time in one gear. From then on, the twostroke speed trace matched the fourstroke very closely. Just a few mph slower. Our sample came from Team Telefonica MoviStar Suzuki and Kenny Roberts. The two-stroke lap 'fIMe . 'fa 'ffllJVeJ( u Kenny Roberts' impressions confirm the read-out. "There's a huge difference between a 500 and the four-stroke. On the 500, you feel as if you're doing a million miles an hour. On the four-stroke, everything happens slower and you have more time to think about it. But you do the same lap time. Or faster. "It makes it much easier for an average rider to be competitive. Uke Superbikes. The bike feels slower and looser, and it's easier to apply the power. "The similar corner speeds will change - the four-strokes will get a higher comer speed, because it is much easier to open the throttle in the comer. "For me, the transition seemed natural - no difficulty. Riding the four-stroke was nothing like the challenge the flIst time you jump on a 500. There's nothing that has a powerto-weight ratio like a 500. "The corner entry is different, but that seems to be getting better with development to the slipper clutch. But it's still definitely not like a two-stroke. Doohan used to declutch on comer entry on a four-stroke, but then you have to engage the dutch again when you're leaned over. Anyway, r try not to do anything the way Mick did. "In spite of the extra weight, the four-stroke tums better than the two-stroke, especially in slower comers. But we have a way to go with our bike. We're really limited by tires and grip at this stage. In testing, I could set the bike so it could slide like crazy - the way I like to ride. For racing, r have to ride with it more in-line. At the moment." was traveling faster at the end of the straights. "The four-strokes have a lower center of mass, and that helps braking performance," explained Willing. At almost every corner apex, the two-stroke was faster. But there is another factor. Last year's two-stroke Suzuki RGv500 had a certain weakness in bottom-end and mid-range power. As a result, Roberts had to ride it more like a 250, carrying more corner speed rather than squaring the corners off as the Honda riders could, for a slower apex but an earlier throttle opening point and a faster sliding exit. The torquier four-stroke has given him back much of this ability. This accounts for at least some of the differences in speed at the apex points. eN was his best last year, in morning warm-up for the race, at 2:06.6. This year's four-stroke lap was from morning practice, in the best conditions so far, at 2:05.2. Our guide was Suzuki technical guru Warren Willing, and the amount of data he could summon on the computer was awesome. We tried to trim it back to make it easier to understand. The first thing to do was to eliminate the changed corner from the comparison. Apart from that, suspension traces could match the two laps, bump for bump. The real surprise was just how closely the two traces were matched the throttle opened at almost exactly the same point, and even the braking points were almost identical. This was in spite of the fact that the four-stroke WHERE AND WHY The extra power of the four-stroke is only worthwhile where it can be used. "There are definitely some circuits where the four-stroke's advantage is narrower. You have a much wider powerband, but you need to be using full throttie to take advantage of it," said WilJing. "Also, at circuits where there is a lot of braking into corners, the two-stroke has an advantage, because of the problems caused by a four-stroke's engine braking.· Track by track, this is his assessment. Suzuka - Looks like four-stroke territory, all things being equal. Welkom - Tight and slow: two-strokes should be good. Jerez - ditto Le Mans - A series of short straights linking mainly medium-speed comers. Four-strokes will be strong there. Mugello - Another track with some speed: four-strokes should prevail. Catalunya - We've already seen at I.RTA tests that the four-strokes are much faster there. Roberts Jr. In pre-season testing on the Suzuki XREO, 60-degree V-four. Assen - Very interesting. Corners run together, and there's not much full-throttle work, although it's fast. The two-strokes 6.2-lb. weight advantage could give them the edge. Donington Park - The same thing as Assen, but a slower track. Again, the two-strokes might be better, because of their power-to-weight advantage. Sachsenring - Like Welkom, there's not much acceleration in the range where four· strokes are strong. Slow comers, where two-strokes are better. Brno - Tests have shown that the four-strokes can use their power well there. Estoril - Another slow track. The two-strokes were faster there at IRTA tests. Jacarepagu. - Wide with fast comers. Looks like a four-stroke track. Twin Ring Motegi - More drag strips like Le Mans, should favor four-strokes. Sepang - Again, we already know that four-strokes have smashed all the records there. Phillip Island - Like Assen, it's not a horsepower track, which is why you get close racing. There's high speed, but you can't use fun throttle. It will be close between the two types. Valencia - Two-strokes won the IRTA tests at this slow and twisty track also. ... y c I e n e "" s MAY 1 ,2002 49

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