Yokomo MX-4 4WD buggy: From prototype–to production!
The Super Dog Fighter is back. Four-wheelin’ fanatics everywhere should remember Yokomo’s* venerable Dog Fighter cars; this series chalked up several maJor wins, thanks to its then cutting-edge, high-performance designs. Yokomo also threw in the YZ-10 Worlds car, a replica of Mark Pavidis’s IFMAR world-champion buggy, which I had the pleasure of testing back in 1996. Yokomo won another world championship in 1997 with a new Super Dog Fighter prototype: the MX-4. More recently, Barry Baker took the Florida Winterchamps with an MX-4. Since I’m the one who reviewed the last world champ Yoke, it only made sense for me to test the latest, too; at least, that’s what told the G-Man … and he bought it!
Yokomo’s impressive kit quality starts with the packaging. When you open the kit, you will find the usual numbered bags, but within each one the parts are grouped in smaller bags to correspond with each step, Of all the kits I’ve built (hundreds!), the Yoke was the most organized build I’ve experienced.
Chassis. Yokomo still uses the finest materials. The 3mm carbonfiber chassis showed no fraying and is finished with a high-gloss shine. However, some experience is needed to build the kit, and the chassis is a prime example. The battery slots must be filed to prevent the batteries’ shrink-wrap from being cut by sharp edges, and you must grind a recess into the nose of the chassis to allow belt clearance. An addendum sheet was with the manual; it indicated where material must be removed in the rear of the chassis to prevent the suspension from binding.
Precision-machined aluminum posts support the upper deck and battery hold-downs. The batteries are held by slotted fiberglass plates that accept a variety of battery configurations-even stick packs. The upper deck has several fiberglass pieces. The MX-4 uses a stiff spring “shock” with an adjustable collar to absorb the energy of an impact and allow a bit of chassis flex.
Suspension. The MX4 uses all-new suspension components. The nose plate/suspension pivot block gives the car kick-up and a rugged base for the suspension arms. The arms are molded of a rigid composite material and include several holes for optional shock positions. Up front, the adjustable upper tie rods are attached to the stout hub carriers. The hub carriers keep the included MIP CVDs well shielded from dirt, and the walls of the pieces are plenty thick-thumbs up for Yokomo; I can’t tell you how often I’ve seen these carriers break on older cars. The steering knuckles are also chunky pieces, and the threaded kingpins ride on metal sleeves to prevent wear on the hub carriers. The rear hub carriers have three optional, lower pivot-mounting locations for tuning. The top hole is for bumpy tracks; the lower holes are for smoother surfaces.
The MX-4’s unique shock towers are attached to the upper composite bulkhead caps. The front, “Y”-shaped tower uses three aluminum standoffs to support the fiberglass camber mount. The rear shock tower offers as many camber and shock locations as the front.
Drive train. The MX-4’s new belt drive is extremely innovative. To increase the stability of the car, the differentials have been narrowed as much as possible to keep the drive-train parts centered. The rear diff is dead center in the bulkhead and uses a wide, coarse-tooth pulley and belt to minimize the chances of skipping. Two aluminum pulleys drive the rear and front belts. The front pulley incorporates a one-way bearing, and this gives the car better straightaway speeds. The spur gear is recessed to house the slipper clutch and keep everything tight. The slipper is adjusted by a large, knurled nut that doubles as a thrust washer; the unit contains 24 hard steel balls and is covered with a plate before it is assembled with a large spring washer to maintain tension on the slipper.
The entire drive train rides on smooth ball bearings with efficient MIP* CVDs driving the wheels. New, large hex hubs are bolted onto the CVDs, and each wheel is attached by three flat-head screws.
Body and tires. The MX-4 comes with mini-pin racing tires with foam liners and cool multi-spoke rims. The low-profile body has a boxy shape that won’t be mistaken for anything else on the track, and a Lexan undertray keeps out dirt. The two are attached by strips of Velcro-brand fastener.
I pulled out some suitably high-performance electronics for the MX-4. A KO Propo* Mars transmitter and receiver combination handled my commands and was matched to a KO FET-boosted 2000 servo for ultra-quick, high-torque steering response. A Maxtec* ShockWave motor and matched V-Tech 2000mAh battery supplied the car with plenty of power, as regulated by an LRP* V6 speedo. This gear was good enough for Barry Baker’s A-main win at the Florida Winterchamps, so it’s good enough for me.
2-channel radio (preferably with a competition servo and micro receiver).
Tools for assembly (file, screwdriver, pliers, etc.).
CA glue for tires.
Competition vehicles get competition-tested here at RIC Car Action, so I headed to the Northeast State Championships and entered the MX-4 in the stock event. I slipped the handout stock motor into the car and tried to put on a 22-tooth pinion gear that would allow me to finish the 5-minute races. I say “tried to,” because the small holes in the motor plate were evidently for modified motors only; they didn’t allow the larger gear to mesh properly. After slightly elongating the mounting holes, I managed to get a little play in the gears. Next, I changed all my packs to the preferred 4×2 configuration and rebent the solder tabs to go out and over the battery hold-down plates. Some racers file the slots to allow the battery bar to slide up and through, while others solder the wires directly to the battery. Once those minor setup details had been sorted out, it was time to hit the track.
For a stock-out-of-the-box car (with the exception of Lunsford* titanium tie rods), this car was dialed. With a stock motor, the car’s acceleration and straightaway speed were a lot quicker than other 4-wheel cars on the track. Since we were all running handouts, this was a good testimonial to the car’s efficiency.
The MX-4’s steering is very responsive when exiting the turns. The car exhibits a bit of push (understeer) when entering a turn on power but is easily “tightened up” by backing off on the gas. At the midpoint of a turn, I could get back on the throttle, kick out the rear end of the car and slingshot out of the turn with authority. Impressive stuff, but the loads imposed on the servo by such aggressive driving caused its mounting tape to loosen. I reattached the servo with new tape and added a bead of Zap* Goo around it for extra security.
The MX-4 jumps more smoothly than older Yokomo buggies. The classic Yokomo rear chassis “slap” when launching off a jump has been almost totally eradicated. The car landed well off the large jumps and touched down with a slight nose-down attitude. This jumping posture allowed me to quickly duck into the turns following the jumps.
When the race day was over (fourth in the A), I slipped in the previously mentioned Maxtec 10 double motor to see how the MX-4 would handle with a mod. Again, the buggy quickly shot off the line, but this time, it was accompanied by the whine of the slipper clutch. As one might expect, the slipper definitely needs to be set tighter when using a mod, but be sure to leave enough slip to protect the drive train. Properly set, the slipper will prevent the belt from slipping, improve the car’s handling over bumps and extend the time between diff rebuilds.
With the increase in power, the stock tires didn’t hook up as well as I would have liked, so I slipped on some Losi* IFMAR pin tires for better hook-up on the loamy surface. The MX-4 becomes more responsive to throttle commands with a faster motor. Blips of throttle in the air can change the car’s attitude off jumps; turns remained the same, with a slight push in the entry, but the car exhibited fast exiting abilities. I will have to continue playing with the shocks’ down-travel to see whether I can enhance the car’s cornering ability.
The new Yokomo MX4’s handling is far superior to that of any of its predecessors, and the durability of the buggy and its parts is topnotch. I drilled the car into a stationary barrier at full speed (accidentally; what did you think?) and drove away with only a bent bumper. The car’s drivability was excellent in pure stock form, and its numerous tuning options should make it easy to dial in for almost any surface. If this car could win the Worlds and the Florida Winterchamps in prototype form, just imagine what the refined production model could do at your hometown racetrack!
* Addresses are listed alphabetically in the Index of Manufacturers on page 209.
Copyright Air Age Publishing Sep 1998
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