Cycle News - Archive Issues - 1970's

wheel travel for tight, slow tracks, and after the first few races, rear wheel travel was decreased further by bolting on shorter shocks to give a much more balanced 12 inches of travel at both ends of the bikes. This is an approximate figure for suspension travel since different Kayaba air/spring fork 'units may be in use by different team riders, as well as different rear suspension units. The usual rear suspension components used by the Suzuki team 'riders on ' their works bikes are Kayaba factory gas /oil reservoir shocks ; Working on the deCarbon principle, these shocks are available to the team with steel or aluminum bodies. different damping designs, and a gas chamber which can be refilled or adjusted. The nicest thing about the works Kayaba shocks is that the team has available a complete selection of different rate two-stage springs in. both straight wound and progressive designs. This allows each rider to dial in his rear suspension exactly. Front forks. of urse, are tuned with different oil weights and volume, and different air pressures. Other rear suspenion units have been used by the team this year . Gerrit Wolsink has used a European gas /oil reservoir shock called White Power. ~hile Roger DeCoster is presently using Ohlins. The American team riders. like LaPorte and Masuda. have been testing a Kayaba air shock similar in looks to the Fox AirShox, but the Kayaba only has a single rate air spring and probably uses an internal negative spring to gain the more desirable spring action and progression of a dual stage air spring. For some t a cks the team riders like the Kayaba air shocks better than the spring stiocks. They won't be available from Suzuki, but Kayaba may sell these air shocks themselves to the public sometime in 1979. Introduced in the 1977 Trans-AMA Series was a nearly all-new engine design for the works bikes . The basic changes included smaller and lighter . magnesium engine cases with the countershaft of the transmission located closer to the rear of the cases, and hence swingarm pivot. to provide more cbnstant chain tension . The clutch actuating mechanism was also moved ~ff the clutch case on the-right side of the engine over to the left side where it now operates a more conventional pushrod running through' the transmission layshaft to disengage the clutch. This helped to narrow the engine width slightly, make the clutch actuator arm less vunerable to damage, and should provide a little' better clutch feel, since the older design caused the clutch cover to flex and absorb some of the force needed for positive clutch disengagement. And with more positive disengagement, the clutch sh'ouldn 't be as susceptible to drag and thus overheating and wear. I The new engine design is used with both the RH-250 and RN-400 this ~ason. Suzuki wants to keep calling their Open class bikes 400's for the .oo vious sales advantage of keeping the works bikes as similar to the production bikes as possible as well . as having new production bikes with 'different displacement engines not severelycut into sales of yet unsold RM's still sitting on dealers' showroom floors. It was at the opening race of the T rans-AMA Series that I discovered Team Suzuki had secretly been using 420's (actual displacement 417cc) in the Open class throughout the 1978 season , and the 1979 model RM -400 N production bike will have the same displacement and 80.0mm x 8~.Omm bore and stroke. Carburetors are also the same, ~6mm Mikunis; and so should be cylinder porting. . The Open class RN 's have been available to the factory riders in various dis placements for years. Hopefully the new case design hasn't outdated the older cylinders which would give RM owners a nea rly complete selection of engine displacements; ~70, 400. and 420cc , to suit their local track conditions and riding styles. But then , one ride on a 420 is all the convincing you need to figure out which is the best . . Once again Roger DeCoster is using the Trans-AMA Series to test new designs on his works bike for possible use on all the factory bikes next season . Suzuki doesn 't exactly relish lett ing it be known what these design changes are for obvious reasons, but prying eyes in the pits and a few wellworded questions in Roger's ears can usually unveil some of the secrecy . At Lake Sugar Tree there appeared a reed valve inducted cylinder on Roger's RN-420 built by Ivan Boysen. The reed valve assembly itself just might be a typical V-block, but with twelve single stage reed petals without opener limiting guides, six petals per side. What is really unique is that the cylinder reed induction works in conjunction with Suzuki's own case reed induction system. The advantage of two different reed valve systems is that the porting can be reworked to provide more top-end and low-end torque, effectively broadening the powerband and making it more powerful WIthout making sacrifices at one end of the powerband or the other. Other than, "a little smoother. " when compared to the other case induction only RN-420's: Roger didn't want to be very commital in expression how this new inovation performed. I asked Mark Blackwell how Suzuki actually goes about developing their works bikes , and he said the factory engineers come up' with continuous new developments based on suggestions from the race team. More than likely most of these suggestions come from Roger, which puts a heavy burden on him. since there are no development riders for the works bikes at the factory , and Roger is involved much of the year himself in actual racing. All-new works prototypes are usually brought out by the Suzuki factory in December/January of each year when the senior team riders have time to thoroughly test and evaluate them . Most major changes like frame and engine designs are kept to the end of the year, but some innovations or modifications can take place during jhe year if a problem develops or the bikes don't prove competitive enough against the competition. Probably the only real fault in the whole Suzuki development system is that the factory engineers aren't too responsive to the American racing team . The riders have no major development or technical training, and the mechanics are hired to be reliable parts changers and nothing more. So when the usually ultra-reliable works bikes began experiencing seemingly easy to correct problems that dropped their finish ing record down towards only half the races entered, it was obvious the factory wasn't responding as quickly as it should have. The best exanible of this is the chain problems the factory bikes have been having all year long. yet it wasn't until the Lake 0' Sugar Tree Trans-AMA that the mechanics were given permission to go ahead and build heavy-duty aluminum plate chain guides to replace the lightweight stamped steel tensioners which were failing. While on the other side of the coin. over at Honda. the riders are pretty free to select certain components for their bikes, and the mechanics can make modifications when necessary to improve performance and reliability. . Trying to relate riding impressions of the RN-420· 78 to you is quite difficult because it has the heritage of being the world's most successful motocross bike. In this respect it has the ability to do everything.. .handle, accelerate. brake, turn, and in every essence of the word , "perform" .. at least as well, if not better than any works motocrosser ever made. And without any even insignificant faults. it is more difficult to highlight the strong points. It is the ultimate bike that does everything to perfection. Danny's 420 started much easier than I expected, usually on the first kick. once I got over the fear of the heavy compression and big displacement and followed through on the kickstarter lever With a strong plunge. Once under way you notice immediately how light the bike is, just ounces over the FlM weight at 218-221 pounds without gas. Once you kick your way up through the 5-speed gearbox and start flying, you notice the bike is very neutrally balanced between the front and rear wheels. It tracks perfectly, taking precise inside lines on the inside of off- camber turns like there was an imaginary berm there to guide it , The bent-style aluminum swingarm is extremely long and aids the bike's tracking ability on rough ground. The rear end will not get out of shape. Shift your weight or play with the throttle and you can instantly load and unload any end of the bike you want to meet the situation. In the tight CN CN M ~ ..c ~ o Z 1. • .1 PnooUers The experimental cylinder on Roger DeCoster's 417cc RN-420 uses cylinder reed valve induction together with case reed Induction. stuff you can make it behave like a. Maico, and on the rough high-speed straights. it thinks it's a Husky. . I didn't like what Danny had chosen for his rear shock springs on his Kayabas at Lake Sugar Tree. The. soft initial springs were ideal, but his second stage heavy springs were too hard. Coming off jumps and touching rear wheel first , too much shock was transmitted in pitching the front end foward and snapping the handlebars from my hands rather than having the rear end absorb the force . Even though Danny is a much faster rider than I'll ever, be, I think for track condidions that day he would have benefited from softer rear springs. Front fork operation, on the other hand. was perfect. . The powerplant of the RN-420. like I've been discovering on the other factory bikes I've been riding, is nothing short of perfect. Only a fifth gear drag race can be the judge of ultimate horsepower, but for rough track motocross conditions. the powerband was perfect. .Onceagain the powerband is totally usuable right off the bottom end around 2,500 rpm. I didn't want to hurt the engine and short shifted it while riding. but once I got on it full throttle hard in third gear and let it eat somewhere up to around 7.000. It was a steep uphill straight. and the works Suzuki catapulted itself 100 yards up to the next turn faster than I could believe possible. The power is such that you can break the rear wheel loose easy with a quick throttle, but exercise Roger's precise control and the 5.00 rear knob will loft the front end smoothly in any gear; including first without traction, and walk its way through the.worst high speed whoops or out of the bumpiest tight section. You might even swear it was a trials bike if you never got the throttle past one-eighth turn. . Despite the RN ·420's huge brakes. particularly the giant full -hub monster up front , there are other bikes which will stop quicker with less effort . Where the Suzuki's brakes excel is in feel and response. They're very smooth and very controlable all the way. The huge magnesium hubs make them less prone to heat fade as well if that should be a factor as the bikes become faster each season. All told . it was an exciting experience to be able to ride the RN -420-78. ·1 would have to judge it as possibly the finest. most well-balanced bike in motocross today. I hope I've been able to give you a little feeling for what it's like. because in no way can it be compared to one of the present RM400 C production bikes . The question remains how similar to the works bike will the all-new 1979 model RM-400 N bike be in performance and handling. Be here next week and you will,know. • 13

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