Cycle News - Archive Issues - 1970's

£; Inside the Di:I ; Mylhi~al Marzocchi gas shock t >n rC) By J o hn Huerter ~ Marzocchi gas-oil dampers for ~ motorcycles are not really a myth. .=; They're just a little hard to find in this country. The importer, Cosmopo litan Motors, is hopefully changing that situation b ut it will (II b e time before they are S1 e som . . as common on this SIde of the Altantic as they are in Europe . "Over there " everybody is using them, mainly on motocross bikes but also for Two-Day Trial, street, and ro ad racin g. It seems like three out of four privateers at any European In ternational motocross have the distinctive finned body Marzocchis bolted up to the rear of their scooter. Some fa ctory riders also favo r t he m . There are three very good reasons for their pop ularity. Most experience has shown that, of all the co m m ercially available rear suspension units, the Marzo cchi works, las ts , and seldom breaks. It m ay be the only non-fa ctory gas damper . that incorporates bo th of the m ajo r Screws in finned body and reservoi r come out for draining . cleaning and refi lling the shock. Multiple screws ensure access to all the fl uid cav ities and valved passages in the damper. Plastic-protected top comes off with a wrench . 30 approaches to da m per de sign : in creased fluid ca pacity and separation of fluid and co mpre ssib le gas. Mar zo cchi s are also m ad e for an incredibly w id e variety o f racing m achin ery with a factory guide as to what da mper to use if moun te d in a conven tional o r lo ng travel rear sus pensio n . The Marzocchi e ngi neers have do ne their re search before build ing th is d am pe r an d the factory quality contro l is im pressive, possib ly because Marzocchi builds only m o tor cycle suspensi o ns . Most sus pension-builders hand le eve rything from m oped t o eight-wheel truck shocks wi th emphasis o n the fo ur-wheel mar ket - where the m one y is. Marzocchi, so far, is sticking with bikes. Th eir 22 0mm m agn esiu m forks for MX are im pressive un it s, too . The da mpers the mselves incorporate a ni tr o gen-ch arged rubber b ladder at th e top (or bottom, if they're m o un ted correctly: in ve rted ) of th e re mote re se rvoir which is attached to th e main shock bo dy . Depending , on the eye-to:eye length of the .d am per , the body IS co m p le tely fill.ed. with 125, 1 ~ O , o r 135cc ~f. Castro l:Glrhn~ shock fluid, The ca pacrties are ImpresSive when you note that most good-pe rforming dampers contain 70 -100cc of fluid . The body cavity and re servoir are co mpletely filled eliminating all air space. Then, the shock is sealed and the gas co mp art m e n t is charged with two atmospheres pressure. That's about 2 9.4 psi . It is the compression and return pressure of this rubberized gas b u bb le that allow for the fluid d isplacement when the damper is compressed and then forces the contro lled return of th e piston and rod. The fluid should be available now as "Castro l-Mar zocchi 11 in this co untry, reformulated to Marzocchi sp eci fic at ion s. This design differs from the other two primary design approach es. The Girling gas shock, as you'll probab ly remember, has n itrogen gas bubbles char ged d ire ctly into the damping fluid. The Bilstein, as another example , has a nitrogen gas section isol ated from the fl uid co m p ar t m ent by a flo ating seal. The Marzo cchi ap p roach is closest, in pri nciple, to the Yamaha monoshock with the fle xible membrane sep a rating the gas and fluid. The Marzocchi ap p ro ac h elimi n at es the o ccasional problem of fai lure o f the floating seal enco un tered wi th that d esign , an d ca n rely on a m ore extensive - body of knowledge co nce rn ing valving , fo r fluids, as o pposed t o gas-charged ' fluids . The ide a, in every case, is to keep the shoc k fluid from ae rea tin g and lo sin g its d amping properties . Pe rh aps th e re ason for the Mar zo cchis fu n ctional lon gevi ty , however, is t he internal valving d esign that actually force s a circulation of the flu id in to and o u t of the reservoir. 1\!05t reservoir, o r added cap aci ty , d am pers that we can b uy rely on faith and hope t hat the fluid is actually moving from the m ain shock body in to the reservoir to be co oled and then, when co o led off, forced ba ck into the pi ston operating area. Marzocchis have a sy stem of three · one-way valves b uilt into the body and body-reservoir connecting passages tha t allow a measured amount of fluid to be pumped into the rese rvo ir with each downstroke an d a supposedly equal am o un t of radiato r-cooled fluid to be , drawn back into the m ain shock body with each upstroke. That's really the only fluid re servo ir sys te m that makes any sense: one that assures an . act ual recircu lating cooling cycle. ' Mean while, th e separate nitroge n bladder is being compressed an d ex pande d but, since being separa te , it is also preventing ae reatio n of the ap proximately 13 0 cc of flui d. All of this is happe ning with co m ponen t s t hat m ay be the bes t ever seen o n an Italian-b uilt product. The shock shaft has been thickened and the m ate rial used upgraded to "the " best available sin ce the original design cam e out a year ago . TI,e bushings have been a major p roblem on most fo rward-mounted shocks. Marzocchi has designed, patented, and built so me truly impress ive b us hings. They are ca lled "Silen tblok'.' for patent purposes and the raw materials for the co nstruction of th e b us hi ngs cost as much as those for t he e ntire re mainder of the damper. It sounds ni t-picking, but those Spring pre-load cam is heavy-duty ; provides f ive pre-load positions bu t requ ires a tool to work it against high rate 135 Ib./in. sp ring. Decal lists recommended gas pressure in Kgfcm 2 • That's about 29.4 ps i. Number in the white circle (1. 2. o r 31 ind icate s p rogressive firmness of the damping. Rubber-eovered valve cap keeps grunge out of the valve . Gas chambers can be charged w ith ni t ro gen or air. deform-resistant bushings are one of the firs t t hings that riders with m uch LTR experience notice. They know it's been a p roblem. All Mar zocchis come with a little a: Ul ff- Ul ::> r z r o , >Cl

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