Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/125986
• Tech analysis Designed to compete By Lane Campbe ll U.S. Suzuki held b ack few secrets In introducing their new RM 125, which is a calculated effort to leapfrog the competition during the '75 season . Key features o f the new pack age are a totally new top end, port ed expressly for a wid e powerb and; and a 36 degr ee lay down rear suspensio n bu ilt around a ( f or Su zuki) and n ew interestingly-eonfigured gas /oil shock . Suzu ki's da mper featu res positi ve separatio n of t he workin g gas (nitrogen a t j ust over 200 p si) fro m the oil by mean s of a free- floating piston that rides in t he shock body totally in de pendent of t he working p isto n . This arrangem ent clai ms severa l advantages: I) pos itiv e separation prevents the working gas fro m frothing th e o il, 2) The 200 p si gas pressu re, like the cap on a so ft drink bottle, holds a significant amount of air dissolved in the oil, enhancing the heat-transfer capabilities of the unit, 3) the nitr ogen gas and fre e pisto n fully compensate for change-of-volu me as the unit strokes. Taken together, t he gas/oil unit is designed to respond with a minimum of sti ction to small chatterbumps whil e retaining full damping power for high impulse loads. Since the units ar e scaled and non-rebuildable, seal life will be critical. The factory (somewhat tongue-in-cheek] claims seal life eq ual to or better than standard shocks and points out that the inverted position and full me tal shielding should keep the worst of the cru d aw ay from the vulnerable rod seal . At t he sam e ti me they co ver their bets by designing the shock for simplicity and (re latively) inexpensive replaceme nt ($37 each) . Forks are co mple tely re de signe d in ternally , but set up to retrofit to the Tl\1125. Notable also is t he fact that th e RM 125 swingarm has a p ivot designed to interface with th e earlier model as we ll. When asked , Su zuki 's factory people said there was a shock lay d o wn kit in the works for This. In the engine de partment, Suzuki engineers have ma naged t o "close the circle" by wrapp ing the transfers comp letely around t he intake side. Note that each of the three po rts on a side is a different sh ap e an d eac h tran sfer p assage is a sepa ra te trac k that hits full vol umetric efficiency in a differen t RPM band. The result, wh en co mbined wi th the proper p ipe and intake tract, is a broad powerb an d with h igh vo lume flow. The diffe rence in flow is enough to go fro m a 26 rn m to a 28 mm venturi. The rest of the bi ke is just good state-of-the art: large airbox with wet foam filter and backward-louvered air inlet just under the seat, ro ller ch ain guide and wire cable guides to accom modate the extra travel at both ends, sol id-mount rear sprocket, straight-eut gears and a claim ed dry weight of 190 pounds. The factory has heaved the oil injection but retained a five-speed gearbox. The decision to retain a five speed tranny is probably the prime motivation for the broad powerband engine design. The RM 125 is a blatant technical leapfrog attempt, and you'll be ab le to measure its success by how quickly th e competition (read Honda ) responds with a similar package. • Specifications Th e RM1 2S 's backbone and hard-uiorking legs. While rear travel is 7.67 in. at th e axle , stro ke of th e angled shock is on ly 3.93 in., whic h serves to h old down piston speed (whe re have we heard that bef ore) and redu ce t he chance of cavitation (sudden fo rmat ion of air bubbles behind th e m oving piston) in th e pressu rized sho ck fluid. Th e Suzuki sho ck in d etail. It 's a sealed unit, with th e nitrogen at high pressure no rebu ilds. Suzuk i chose a fr ee piston rath er than a diaphragm design to separate th e nitrogen and oil, saying th e required diaphragm would have been too big and too costl y to do the same job at th e sam e wo rking pressure. TOP Pro tec to r 12 3cc 7.4 :1 (corrected I Carburet ion . .. •. •. . . . . . . • . . . . .VM28SC Suggested reta il price • . .• . .. . . . . . . . .5925 . ~~;::/,~~~~k;=o~,-..- Nitrogen gas c~~=,>,--- Pisto n :;::;!===~-- Spring _--..::~ Footpeg Height •••. •. •• . • • . .•• . .13.8 in . Ground Clearance 9. 65 in. Wheelbase • .• • • . . . . . .• ..... . . . . .53.5 in. Seat Height . • . . . . . . •• • . . . . . . . • . .35 .5 in. Dry Weigh t . . . . . . • . . . . . • • . . . . •. 190 Ibs. Fro nt T ir e • .. • .. . . . . . . .. . .. 3.0 0 -21-4PR Rear Ti re 3.50 -18 -4PR Horsepower {clai med) •• • • 2 3 hp / l 0 ,S00 rpm To rq ue (claimed) ... . 12 .3 ft-l bs. 9,5 00 rpm Bo re and St ro ke •.•. . . .. .•• • .. . . 56 x5Occ Displacement Compression ratio • Piston rod Rubbe r cushi o n Cy linder '\1""'=~-- Cover