Yokomo Challenger Crawler review in RRCi - CML Distribution

1/12th ‘Challenger’ Yokomo 1.9" Rock CrawlerBy Peter ‘Mad Pete’ GrayCan YouDig It?1/12th ‘Challenger’ Yokomo 1.9” Rock CrawlerRock crawling is the new R/Ccraze that is setting theindustry alight. Easy to doanywhere and anytime it opens uplimitless possibilities and hours of funfor little expenditure. Yokomo haverecognised this and produced the 1.9”Challenger, a 1/12th scale dual motordesign that is both great value formoney and the perfect introductioninto the world of rock crawlingwhether you are tackling ruggedterrain or the living room furniture!This 1.9” crawler is factory assembledbut not ready to run, though it is wellequipped with a dedicated speedcontroller capable of supplying bothdrive motors with sufficient currentflow to climb the steepest ledge,responding to the helm with minimumdelay to ultimate control to get youout of the scrape you just dared to getyourself into.1.9" WHAT?The 1.9” diameter wheel rims set the 1/12thscale for this class, smaller than the 2.2" ofregular 1/10th scale competition. 1.9" is arelatively new crawling class, with their ownrules and regulations being developed to keepit fair for all.Inspecting the Challenger as it rolled outof the box we found a ready painted shell,trimmed and fitted to a twin plate chassis,using metal ball joints in plastic tie rodsas suspension linkages to control the twingearboxes, which are driven completelyindividually. The rear steering linkage is fixedto a rear servo mount to keep the rear wheelspointing dead ahead but it looks identicalto the front system which is all set-up andjust awaiting a servo installed on the readymounted posts and connected to the servosaver supplied. 4WS (Four Wheel Steering)could not be more obviously intended, oreasier to rig up than on the 1.9" YokomoChallenger!With kinked lower links to improve clearanceover ledges and cliffs, the upper pair of linksare triangulated to provide axle alignment butare virtually in line with the lower arms withonly 8 mm offset to separate them and controlthe axles torque reaction.Wheelbase alters from 232 mm at full rideheight when there is 78 mm of clearancebelow the central skid plate, to 248 mm at fullcompression when there is still 33 mm groundclearance below the chassis. It’s not thechassis that is going to get hung up then, butthe gearbox lower casing and bumper mountshave very square bosses that protrude waybeyond the profile of the lower linkages, sothere are bound to get hung up unless youchamfer them off. The bumper mountingscrews sit right at the bottom of the recessedbosses so quite a lot of material can beremoved safely for an improved, chamferedlead in to rocks and other protrusions.The chassis articulation is simply amazing,the twin motor set-up allows the chassis totwist until the front and rear axles are 90degrees to each other without the constraintsof propshafts. As the motors stick so far outsideways, the gearboxes are forced to belocated offset from centre, so the primarydrive hatch cover will just touch the shockabsorber on full chassis twist and limitarticulation slightly, but under normal drivingconditions I have yet to see the axle twist thatfar around and it shouldn’t present a problem.92 WWW.RADIORACECAR.COM 01/09

ci featureWith no central gearbox or propshafts in the way, the drivebattery can be under slung to dramatically drop the centre ofgravity, so the top deck accommodates just the lightweightelectronics like the receiver and speed controller and you can wireit all in neatly with little time and effort. I found that I could fit theelectronics to the underside of the top plate and keep the weightnice and low. The battery retainer uses two flexible hooks to keepthe drive cells located, so you just bend one down to slide thebattery out for replacement, though most people will leave themfitted forever and just recharge as required between runs.The major effect of a twin motor chassis is that no propshaftsmeans there is the zero torque twist induced as the truck drivesup an incline or squirms to get out of a crevice. The motors andgearboxes are in line with the axles, so there are no bevel boxes totry and twist the chassis sideways, you could literally run with thesoftest springs fitted to the shocks and it would still crawl, all beit with a dramatically reduced ride height, like an ultimate droopset-up.TRY YOUR NEW BOOTSThe first job we did was to give the tyres the tug test, sureenough with little effort we could displace the tyres from theplastic bead lock rims, so the first thing to do was strip each wheel,degrease the tyre beads with lighter fluid and a cotton bud, thenreassemble with a thin bead of CA adhesive run around the tyrelip. Screwing the front plate back on as the glue starts to go offyou know straight away that the tyres are going to stay put, nomatter how rough the terrain gets or how tight you turn underload. There’s nothing worse than shedding your boots when thegoing gets rough!The bead lock rims are a novel approach, with just three piecesand only one set of screws on the front plate as the centre portionpulls through an internal clamp ring to pinch both beads at once. Aneat idea but due to the super soft rubber compound of the tyresand the soft flexible material of the wheel rims you are asking toomuch if you expect the assembly to hold together under tension.CA to the rescue! Don’t worry about gluing your new tyres to therims, as they will last so long that you may never need anotherset of tyres as long as you crawl. If you do decide to upgrade thetyres at a later date you would probably go for some of the newoptional Yokomo bead lock rims, which look like they will do thejob properly.Despite having a vented tyre (the middle ‘O’ of Yokomo cleverlydrilled to let the tyre breath as well as a hole in the rim) the tyresdo not deform under the weight of the truck, which weighs in atjust 1400 g complete and ready to roll when installed with radiogear, a drive battery and steering servo! The set of four wheelsand tyres weighed only 300 g, but by adding stick on lead weightsall around the inner rim face we can double the weight of eachwheel, from 75 g each to 150 g exactly! From our experience incrawling so far I’d suggest only weighting the front rims and allowthe lightweight rears to pull up and over a face lip once the frontsget a grip. Weight will transfer rearwards under load anyway sothe rear wheels never need so much ballast as the fronts.PUSH ME, PULL YOUA twin motor set-up will usually ‘over speed’ the front to preventthe rear catching up and pushing it along, ensuring the fronts arealways keen to be first over any obstacle and it’s up to the rearsto keep up if they can!The front motor is tagged with a red badge, while the rear hasa blue badge, so we just had to measure their revs and see if howmuch ‘overspeed’ the front gearbox would have to offer, if indeedthe rumours were correct. As the gearboxes are identical but therear faces the opposite direction to the front, the rear motor hasto rotate the opposite direction, so they are wired differentlyhence I think they are tagged with the colours simply for easyassembly identification.The red tagged front motor made 7680 rpm counter clockwise,while the blue tagged rear motor made 7800 rpm clockwise. So ifanything the rear is over speed and pushing the front along, andbesides it’s hardly a lot of difference and nothing I wouldn’t expectfrom standard manufacturing variation, so I think we can confirmthey are indeed the same motor, just one is wired backwards.Both motors wear 13T 48DP pinions on their 2.3 mm (3/32")diameter shafts and drive identical reduction gearboxes. Neithermotor has a flat on their shaft to offer the pinion grub screw asecure footing, so I was a little paranoid that all drive could belost due to this. The motors come out easy enough for sure, sopinion access is not at all difficult, but if I’m driving a long wayLeft: Twin 370 motors rotate the opposite wayto each other, hence they need colour codingRight: Included Yokomo ‘SCR CL’ ESC has twinmotor output and simple one button set-upBelow: High ground clearance and no centralgearbox are key to the design01/09 WWW.RADIORACECAR.COM 93

1/12th ‘Challenger’ Yokomo 1.9" Rock Crawlerto a special crawling venue I expect to be there to enjoymyself and not have to work on the truck during the day.A stitch in time saves nine, so it was out with the Dremeland I ground a small flat area on each motor shaft as asecure footing for the pinion grub screws. It might neverslip in operation, but why take that chance.The primary drive mesh is fixed by the motor locationscrews, so removing a motor for inspection or cleaning isnever a worry, it goes back into perfect mesh every time.I measured the overall drive ratio at 30:1 by just rotatingthe wheel once and counting the number of motorrotations in the process. That’s a crawling ratio, and overour usual ‘flying’ 20 metre top speed course with a freshlycycled 1400 mAh FTX ‘Voltz’ battery pack, the littleChallenger measured a sultry 13.8 seconds, a speed of 1.4m/sec (3.2 mph).Above: The rear end is tied down butvery simple to adapt to 4WS with asecond servoLeft: The 30:1 ratio gearboxesare fully ball raced, and are keyto the overall performanceAbove: The bumper mounts and gearbox mouldings presentobvious problems below the centre line…Below: …but they can soon be removed with a Dremel,for a smoother passage over obstructionsDOUBLE WIREDThe included Yokomo ‘SCR-CL’ speed controller hasdouble output wires specifically for twin motor set-upsjust like this. Fitted with a Tamiya power plug I couldn’tsee how I would connect it to the little square plug on thecells, until I spotted the adaptor lead curled up in the ESCpackaging, they really have thought of everything. Themassive heat sink suggests it is built for heavier dutiesthan these little 370 motors, and by removing the heatsink I found I could install both the ESC and the receiverunder the top plate, either side of the battery pack fora real tidy set-up, with the centre of gravity as low aspossible.The motor leads are terminated in regular bulletconnectors and could be plugged straight in, but for thebattery connection I prefer to chop the Tamiya and microplug right off and rather than use an adaptor lead I justsoldered myself a pair of 3.5 mm FTX gold tubes, witha male and female on the battery and correspondinghalves on the speed controller so I could never get themplugged in backwards in a hurry.The kit Yokomo ESC is rated to 60 A continuous so thegold tubes are more than adequate for the job in hand,and even these twin motors pulling together I doubt we’llsee enough current draw to warrant the heat sink, even ata stall on a hill, with maybe just an occasional spike whenwe jab the throttle to get out of sticky situations. Withonly a 5 V BEC regulator built in the servo speed will suffera little, but even at 5 V the Prolux high torque, all metalgeared, digital servos we have fitted for this test offer 8kg/cm of torque and a transition time of 0.27 secondsfor 60 degrees rotation. They should be plenty fastenough to correct a mistake and strong enough to holdany steering angle despite any sticky situation you findyourself in. Adding a second servo to the rear gearboxlocation and simply extend the steering tie rod to reachthe servo arm and you have 4 wheel steering! It couldn’tbe easier! The 4WS installation literally halved the turningcircle, from 6 to just 3 feet and you could tighten thatfurther still by using full power as the rear literally pushedthe fronts around in an ever-tighter circle.As we installed the computerised radio found on theVenom GPV-1 bike, we don’t even need a ‘Y’ lead, asthe 4-channel receiver has plenty of extra sockets. TheVenom VR3T is a computerised steerwheel radio with 15model memories so I can also use it for several other usestoo, how handy is that! This three channel transmitteroffers channel mixing so I can dial the rear steer servointo perfection and reduce its rate a little compared tothe front so that it just assists the front steering actionrather than matching it. The throttle exponential curvescome in handy too, being able to set a very sensitive lowspeed curve in, and yet still have max power on tap whenI need it.94 WWW.RADIORACECAR.COM 01/09

ci featureReady built and motors installed;just add radio and install thespeed controller suppliedThe only setting missing from a specific crawler speed controlleris drag brakes, but the 30:1 ratio of the ‘Challenger’ gearboxes is sohigh that the brushed motors hold the truck stationary even at someincredible angles with no throttle or brake applied at all. The ESC has noreverse lag, so you can change direction in the blink of an eye and catchrollovers before they get terminal. Any reverse delay would kill its rockcrawling potential straight off. The change of direction is so abrupt thatyou can pull short wheelies by jabbing forwards and back repeatedly.Childish mechanical abuse for sure, but entertaining nontheless and alltaken in its stride, proving a robust driveline and an adequately specifiedcrawler ESC.CLIMB EVERY MOUNTAINWe found the ‘Challenger’ would climb an amazing 60 degree incline,front wheels spinning away merrily as they unloaded, while the rears dugin and slowly pushed the truck forward without even a hint of rolling overbackwards, very stable indeed. Placing the truck on the inclined surfacewith the throttle at neutral produced a very slow controlled roll as if ondrag brakes, retaining grip and control all the way down.In side slip we measured an incline of 64 degrees before the truck wasin danger of rolling over rather than traverse the angle properly, againmightily impressive for such a small truck. The kit Yokomo tyres wereworking on many surfaces despite lacking an insert of any kind, and stillhardly deforming at all.We tested the theory that the Yokomo ‘SCR-CL’ speed controller israted to handle 12 V by plugging in an 11.1 V 3s LiPo and to our delightit performed brilliantly, even better than we could have hoped for, evenwith the heat sinks removed remember. The extra top speed providedjust enough ‘POP’ to get us up and over obstacles that would otherwisehave defeated us. The top speed measured 10.5 seconds for the 20metre ‘dash’, equating to a more usable 1.9 metres per second (4.3 mph)top speed.With no centre gearbox or propshafts, thebattery can sit real low in the middleA quick test in the street showed that going up and down kerb stoneswas water off a duck’s back, and not a hint of chassis twist was evidenteven when cornering hard on tarmac or carpet alike, the bodyshellremaining flat and level at all times, perfect! Attacking a 10 cmkerbstone was not such a problem so long as one tyre got to grips withthe vertical face before the bumper held the truck at bay. Such a steepclimb could be taken smoothly in typical rock crawling style and the rearwould follow up and over without hesitation by virtue of the rounded topedge to typical street side kerb stones.Attacking a sharp edged 10 cm thick breeze block was a different story.Again you had to get one front tyre to star climbing before the obtrusive01/09 WWW.RADIORACECAR.COM 95

1/12th ‘Challenger’ Yokomo 1.9" Rock Crawlerfront bumper got hung up, but once you were on the way and the frontswere over the lip, the rear would not follow because the sharp edge ofthe breeze block was catching on the rear lower arms. Now, you couldjust go at it on full throttle and watch the truck hop and skip about,eventually getting over the obstacle regardless, but in an undignifiedway, but I prefer mastering the correct technique of approaching anymajor obstacle at a very slight angle rather than square on, so you couldtraverse even the sharp edged high climbs with style, by getting onefront tyre and then one rear tyre to make contact with the vertical faceand begin lifting that axle over clean and smooth.TO DIG OR 4WS?As much fun as four wheel steering is, 4WS was banned for the 2.2"class so it stands to reason it probably wouldn’t be allowed in any newrules formulated for the 1.9" trucks either. The beauty of the twin motordesign of the Yokomo ‘Challenger’ is that an electronic ‘Dig’ unit couldeasily be rigged up to interrupt the power to the rear axle and thenshort the motor wires together to form an electronic brake. Effectivelylocking that axle and allowing the front axle to carry on powering rounda bend while the rear end holds fast, like a hand brake turn. A proper‘Dig’ unit would lock the rear prop shaft to hold the axle still and requirea separate servo, an specifically designed two piece rear propshaft witha separating spline drive and the complicated set-upthat entails. The Yokomo could be quickly convertedto an electric dig unit with nothing more than a microswitch controlled by a tiny servo plugged into the thirdchannel socket. Hmm there’s food for thought. We’llhave to see how the 1.9" competition rules work outand take it from there if we need to, but for now4WS is the simplest and most enjoyablesolution.ENTER THE CHALLENGERWhen the bug bites, it bites really hard, and the main joy of crawling isthe ability to do it almost anywhere and that’s what gets you hooked! It’sgood to keep a crawler ready to go in the boot of the car, so whereveryou find that perfect spot you can take some time out and challenge bothyourself and your rig to get to those places you never imagined werepossible. It’s a great way to relax and unwind, and even get some exercisethrown in too, because wherever the crawler goes you normally have tofollow close by.This twin motor rock crawler will introduce you to the world of RCRock Crawling, and with a little modification to remove the undersideprotrusions, having glued the tyres on, installed a 3s LiPo for a little extrapower and set-up another servo for rear wheel steering, we found thelittle Yokomo ‘Challenger’ was great entertainment, indoor or outdoors, sono matter what the weather your truck trialling fun never has to end. RRCiBelow: The threepiecewheel rimsmean only one beadlock ring is requiredFar below: Thewheel assembly wasjust too soft to stopthe tyre being rippedright off, so CA tyreglue was required.Note the Yok’O’movented logo to let thetyre breathAbove: With the ESC heat sinkremoved I installed all theelectronics below the top deck,for a lower centre of gravityBelow: Solid centre drive and ballraced axles keep all the powerflowing to each wheel96 WWW.RADIORACECAR.COM 01/09

ci featureFar right: Fitting a Venom three channel computerised radiomeant we could dial it into perfection. Prolux servos and FTXVoltz batteries completed the buildMain pic: Even climbing 60 degrees up a high grip surfacethere was no torque reaction, as there are no propshafts!QUICK SPECClass: 1/12th 4WD 1.9" Rock CrawlerType: Ready for Radio, includes ESCManufacturer: YokomoPrice: £139.99 RRPRequired to CompleteRadio: Venom VR3T incl. 4-channel RxRadio Batteries: 8 x GP2700 NiMH AASteering Servo: Prolux Digital 5009FBattery: Either….7.2 V FTX Voltz 1400 mAh NiMHor 11.1 V 3-cell 1500 mAh LiPoCharger: ProluxTyre CA adhesiveDislikesBeadlock rims needed glue tooLikesFactory assembled kitFully ballraced drivelineNo propshafts = zero torque twistEasy to convert to 4WSEasy electric ‘Dig’ conversionCONTACTCML Distribution, Tel: 01527 575349or visit www.cmldistribution.co.uk01/09 WWW.RADIORACECAR.COM 97