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/125972
e new • as 01 • sus enslon oil Honda gas/oil system with remote reservoir #2 Some are available to make your bike -handle By Jim Gianatsis A long rear #1 Standard inherent prob lem s of the tw in t ub e des ign. With more oil capacity , air fo aming isn't as cri t ical as it was and ov er heating is greatly red uce d . S t ill , th ese prob lem s exist to a de gree that even wi th the larg e capacity al umin um bodies , shock per formance is far from perfect for hard use o n a LT R system. The Hond a gas/oi l system t h e g r o .u n d wor k as o t her m anu factu rers quickl y followed their examples or came up with ideas of their own . For the 1974 8 Grand Prix motocross season all th e manufacturers in competition for the World Championship had so m e type of LTR system on their team bikes . No longe r c o u ld a .factory rider stay competit ive on the Grand Prix circuit withou t LTR. Now th at various manufa cturers like Maico , Bult aco, Husky and Yam aha have in tr od uced LTR systems on their production motocross b ikes , it can be acc epted t hat LTR does work and enables riders with ability to go fast er . Yamaha probably offers the most technicall y advanced design with th eir Mo nocross sh ock, but it still needs some d etail refinem ent , For those wh o ha ve modi fed or are co nsid eri ng to m odify a standard suspe nsioned b ike to LT R, or pe rhaps have alrea d y purchased one ofth e new Bultacos or Maicos w ith LTR suspension, -o ne qu estion remains. \Vhat kin d o f rear sh ocks w ill wo rk bes t with my b ike ? If you wa nt your b ike to be as clo se to Grand Prix spe cs as possib le , yo u' ve probably noticed t hat all the facto rie s are now using some form of gas/oil sh ock s on their te am b ikes. The aluminu m bodied oil shocks of las t season are already o ut dated. This is bec ause co nven tio nal rear sh o cks just can't handle the increased work th at th e new LTR suspensi o n systems impose o n them. Perhaps in local competition where short motas are run, but not in pounding 45 min ute Grand Prix races. Th e increased w ork o n oil shocks by LTR can cause blown seal s in the shocks , overheating and oil foaming which leads to decreased damping action and eventually shock failure. For an understanding of why con ventional o il shocks are no t adequate for LTR, we only need to look at how they are constructed (Diagr am No. I ) . Most shocks, like Koni and Girling, have what is referred to as a twin tube design. This is b ecause all oil filled sh ocks must have an air space inside of them , which is displ aced by the dam p ing rod wh en th e shock is co mp ressed . The tw in tub e design keep s • inside air away fr om th e d a m ping pi ston and the inner tube in wh ich th e damping p ist on travels up an d d own in . Under normal use o n a short travel rear suspe nsio n system a twin tube o il shock wiII work fine . It 's bnly wh en a t win tube design rear shock is used with LTR t hat it show s it s lim itations. First, eve n w ith the tw in t ube des ign , air inside the shock wiII n ow foam up inside the oil . ins tead of rem ain ing sep arat e as it should, d ue to the inc reased action and fo rce of the d am p in g piston, Once air ha s foam ed up insi de th e oil , t he oil's viscosity is less as it flo ws through th e damp en ingholes in the piston, resulting in a loss of much of the d am p in g action. This is th e main reason why shocks fad e o ut d uri n g a ra ce ,- and not because of o verh eating as had bee n generally tho ugh t. At least, if a good q uali ty . low viscosity hydraulic oil is used . No rmal overheating won't affect the viscos ity of the right type of oil, b ut it can change wo r king tolerances in side a sho ck whic h will alt e r d a m pin g and reduce t he life of th e sho ck. Seals mav overheat o r bl ow . and w or king part s like thc sha ft and piston can seize . The twin tube design o r- the sho ck is its worst enemy be cause th e twin wa lls ac t as an insulater in the dissipation of hea t . Also , the inner tube takes up spa ce th at co uld be filled by more oil which would allow faster heat dissipation . And even with the s m a ll e r d am pi n g p is t on. necessitated by the_ twin inner t ube , heat builds up more rapidl y wi th out ,a larger diameter wall to work again st. For most racing applica t io ns th e aluminum finned body kits for shocks like Konis do much to correct th e •• o •• • oil The easiest way to correc t th e fa ults . of th e twin tube oil ' shocks is to do away with th e in ner tube while st ill , so me ho w, keeping th e air o r gas in the shock separated from the oil. Honda is c urren tly using a sys te m o n some of th eir team bikes (Diagra m No .2) that acco mp lishes th is and wor ks very well . Actually it is quite simil ar to the idea used by Yamaha in their Monocross design , and KTM also used someth ing lik e it on their b ike wh ich won the 1974~ 250cc World Cha mpionship . - The svstem inco rpo rates a rem o te gas /oil ' reservo ir which is conn ected to b o th rear shock un it s b y hyd rau lic lines . Th e reservoir is lo cated unde r the sea t of the bike and is an al umin um chamb er which has the oil and gas insi de of it se parate d b y a flex ible rubber m emb ran e. As t he rear shocks co mpress, the membra ne flexes into the nitrogen ch a mber to make room for the oil w h i c h has been dis pl aced by th e d a m p in g sh afts o f the shock units. As oil pu mps its wa y ar o und the system it also keeps itself co ol. While Ho nda's system provides a ba lan ced set of rear sh ocks and d oes aw ay wit h th e inheren t problems of co n ven t io na l twin tube o il sh ocks, it also has the d isad van tage of being fairly ex pe ns ive to build. and so mewhat heavy and bulky . If it wa s sold as a bolt -up kit for o th er bikes, you 'd have to find spa ce so mewhere o n yo ur bike to ins ta ll the rem o te reser vo ir and the hydrau lic lines. ' What we need th en . is an even simpl er and less ex pe nsive shock sys te m . The Bilstein type gas/oil shock Bilstein was th e first to re cognize the deficencies o f th e tw in tube de sign for even o ld styl e , sho rt rear suspe nsio ns a n d loo ked to the single tube, or monotub e as the answer. When Team Husqvarna needed a sh oc k which would work o n t hei r GP b ike s, modified for LTR mid -way through the 1973 season , Bilstein alread y had it . To make th e monotube work , Bilstein had to devise