Cycle News - Archive Issues - 1970's

Bob and Jeff Fox: Correcting a shocking situation By Jim Gianatsis Necessity is the mother of invention, it's said, and no saying rings more true in the technology of racing motorcycle long travel rear suspensions. Motocross racing forced the advent of 12 long travel rear (L TR) suspe nsion as a . means of gett ing fro m point A to point : B mu ch more quickly. The case being th e d istance from point A to point B is th e roughest mot ocross track you ca n think of - somethin g like Carlsbad th e day after th e U.S . Grand Prix . W ith LTR ca me th e discovery th a t th e ordinary shock absorber or rear suspension units everyone had been using wer en't up to the task. Peopl e started modifying ord inary motorcycle shoc k absorber uni ts to in cr ea se th eir reliability and performance for LT R, but it just .w a s ri' t e n ough . A co m ple tely new shoc k absorber des ign was needed to com b at th e dem ons of fade a nd unrelia b ilit y. To th is end th e Bilstein deCarbon -principle shock and the Girl ing gas -charge d shoc k led the way. T od ay . th e d esign prin ciples th a t Bilstein a nd Girl in g introd uced are still used , b u t other manufa ctu rers ha ve incorpora te d im provem ents to th e basic designs to mak e th em better. Because of t hese im p rove me n ts th e presently top -rated shocks for serious motocross competition ~ re th e Ka yaba and Koni rese rvoir units. and th e two typ es of Moto-X Fox shocks; th e Fox Shox and Fox AirShox . It is interesting to note that Kayaba and Koni are two of th e largest shock com p anies in th e world, and that their shock designs are ba sically th e same using the deC arbon principle with external reservoirs to help reduce heat fade. Also. th eir shocks are not readily available to th e general public, except for the Kayaba reservoir units which are available in limited lengths from Suzuki. Moto-X Fox. on th e other hand , is defi nitely a small company with some very radical id eas on what shocks should be like . Their ideas work extremely well . and th ey sell every pair they can crank out of their little shop . Almost every top p rivateer on th e National circuit uses Fox Shox and AirShox, as well as some pretty quick factory ' rid ers . Kent How erton used th e sa me pair of Fox AirShox on his factory Husqvarna the en tire 1976 racing season . winning the 500cc National championship in th e process. O th er top riders using and helping to d evelop Fox s ho~s incl ude Brad Lackey and Pat Richter. For a better insight into wh at Moto-X Fox is all about we pa id a visit to th eir rapidly growing little com pa ny, located in Campbell , California . just south of San Francisco , where we talked to th e men behind th e Mot o -X Fox name. Jeff and Bob Fox hardly look like dirt bikers , and for good reason . The two brothers hav e degrees in physics. with Jeff holding a Ph .D . from the Unive rsity of W isconsin. For four years Jeff taught physi cs at the Coll ege of S anta Clara , so me h o w g ett in g involv ed in dirt bik es as a hobby. T h is led to part ownership of Gr and Prix Cycles in Santa Clara for two yea rs, th en Jeff sold his share of th e business to begin Moto-X Fo x distributors in February . 1974 . Brother Bob meanwhil e, wa s wo rking for a n ae r osp ac e fi rm ca lled U ni te d T echnology building space rockets. From th ere Bob work ed in marketing for an electroni c's firm . Bob did a lot of hangin g a ro u nd J eff and go t hooked up on b ikes as well . Soon , th ey wer e involved in t he dis tribu ting bu siness together. "Moto-X Fox ha s been a su ccessfu l business because we co ncentrate on quality a nd good service ," says Bob . "There was a lot happening with suspensions three yea rs ago , as there still is today. Since we were engi neers, we decided to stu dy th e shock absorber problem and come up with someth ing better." The shock absorber wh ich made th e brothers famou s because it worked so well a nd proved reliable was th e Fox Shox, Isn 't it basi cally a Girling gas charged type shoc k, and is it more effecti ve th a n th e d eC arbon type wher e th e oil and gas a re sep a rated by a floating piston ? "The Fox Shox is sim ilar to a Girling in princip le ," says Jeff, " bu t the heart of a shock is the valvi ng a nd that's a ll our own design . W e ex pe rimented with both the Girl ing and d eCa rb on design s a nd found the Girl ing design to be just as effective as th e deC arbon , whil e being sim pler a nd mor e relia ble. The adva nt ages of ou r Fox Shox over th e Girl ing a re numerous. Our shocks are rebuildable . have stro nge r on e half inc h sha fts , and th e latest models ha ve an even newer damping piston system ." . "Test riding is wh at we use to determ in e ou r damping design ." adds Bob. " N ovice and Pro riders on th e AMA ci rcuit help us to test our shoc ks. and now we offer th e Fox Shox in three damping rates. W e have a shock dyno, but it's used mainly for testing new shocks to see that th ey're workin g properly aft er being assem bled." What exactly causes shock fade? Is it th e oil thinnin g ou t from heat , foaming up with air, oil leaking past the outsid e of th e piston when th e cylinder walls exp a nd from heat, or all of th ese? "O n a well d esigned shock like ours," answers Jeff "fa de ca n only be caused by a change in oil viscosity from heat. Sinc e th e oil is already gascharged, there is no ad di tion al oil foaming once the sho ck is work ing fo r more than a few strokes. so the valving is des igned to work with gas and oil mi xed . . "T h e d amping p iston will usually expand with hea t, th e same amount that the outer body of t he shock do es. so no oil ca n leak around th e pist on. Besides, on our shock we hav e a piston ring on the damping piston , sim ilar to tha t in a p iston engi ne , where oil pressure expa nds the ring to make it seal harder. This prevents almost all blow-by, "Since shoc k fade is th en caused only by oil changing its viscosity due to heat, the idea is to use th e thinnest viscosity oil possible - oil that ca n' t get much thinner then it a lready is. We've done a lot of expe rim enting with oils to find th e best. So far Bel Ray LTIOO works th e best in ou r shocks of all th e oils we've tested. LTIOO fades less th an LT 200 , which fad es less than LT300 . When some one wants more damping in our shoc ks we recommend ch anging val ving, not cha ngi ng to a . thi cker oil. Valving d esign is a lso very im po rt ant , because some designs hea t th e sa me oil up m u c h more qui ckl y th an o t her designs." "W e've used water in our shoc ks." says Bob . "mixed 8: I with wat ersolub le oil for piston lu bricati on . a nd fa de becomes non -ex istent. (Water has The ones responsible. Bo b Fox (Ieftl and Jeff Fox . a viscosity of 0 which remains the same at any temperature short of freezing or boiling) . With water a bladder system ha s to be used to separate th e ai r and water. " How does a reservoir type system help? . "A reservoir system only helps actual damping abou t five percent when a floating piston is used to separate the air from th e oil ." explains Jeff. "T he oil being used is thin enough already in our shocks ' tha t foamed air or nitrogen in the oil hardly affects the damping. W e do offer a reservoir ki t for ou r shocks. and whe re the reservoir proves useful is that it allows a larger volume of oil to be used which can stay coo ler longer. A floating piston or bladder must be used with a reservoir system though , or else when the shocks are mounted upsid e down - like on th e work s Suzuki and latest RMs - air will get int o the lin es and the shocks won 't dampen properly." Isn 't it rather clumsy and unreliable to use externally-mounted reservoirs like th e factory teams use with th eir Ka yaba , Koni , and Fox shocks? Isn 't a reservoir setup like on the Marzocchi Regular Fox Shoxs utilize the Girl, ing-type pressurized gas/oil system. S H(K'K u ::~ . ,,~ , . shocks where the reservoir is part of th e shock body a much better idea ? "Ce rta in ly," answers Jeff. " If every shock I was to build and sell had to have reservoirs with it , th en I'd build the shocks with the reservoirs welded on . But 80% of our shocks sold aren't used with reservoi rs. The customers aren't using the shocks hard enough . that they'd benefit from the slight increa se i n fade res istance the reservoirs offer. For them it isn't worth th e extra money. I think m y Fox Shox is the best there is for the price. I also think Bob's AirShox is the ultimate, but then it's not for everybody." ; Why does th e Fox AirShox seem to be the ultimate in shock absorber design? ( "T he overwhelming advantages are stre ngt h , adjustability, reliability and wear resista nce, " says Bob . "Not one of the AirShox riders in this past Trans-AMA Series had a shock fa ilure. No one has ever bent a shaft. They are easier to work on than other shocks: They are totally adjustable for damping and spring rate. so they can be set up p erfectl y for track conditions. When they are set up right they 'll never bottom out hard because of the excellent progressive effects that air offers. Both Lackey and Howerton say they lack th e bottoming-out jolts of conventional steel sp ring shocks that really ca n tire out a fast rider. " With so many advantages over conventional shocks. why isn't the AirShox th e best sho ck for all dirt bike rid ers ?

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