Mugen’s new Y2K worlds’ contender

Mugen MBX-4RR: Mugen’s new Y2K worlds’ contender

Pond, Steve

About a year and a half ago; Mugen* introduced the MBX-4-an 1/8-scale off– road competition buggy-to replace its aging Super Athlete. The MBX was a ground-up design that represented the latest technology. Flash forward to today: we’re staring down the barrel of yet another new Mugen machine-the MBX-4RR! The relatively short time between the release of the original MBX and the new RR is undoubtedly because of the upcoming IFMAR 1/8 Off-Road World Championship to be held in Las Vegas, NV, later this year. This competition is always fertile ground for new releases, and Mugen has fired the first shot with the new RR.


VEHICLE TYPE 1/8 vitro-powered

4WD off-road buggy

BEST BUYER Intermediate to advanced builders and engine tuners

KIT RATINGS (poor, satisfactory, good, very good, excellent)

Instructions Satisfactory for intended buyer

Parts fit/Finish Very good

Durability Excellent

Overall performance Excellent






Wheelbase 12.83 in. (326mm)

Width 12.1 in. (308mm)


Gross (as tested) 122.4 oz (3,497.3g)


Type 3.15mm channeled plate

Materiel 7075 aluminum


Type Shaft-drive 4WD

Primary Clutch bell/spur

Drive shafts (F/R) Universal/ dogbones

Differerrtial(s) 6-gear bevel (sealed)

Final drive ratio 10.95:1

Clutch 3-shoe centrifugal

Bearings/bushings Rubber-sealed ball bearings


Type (F/R) Double A-arm w/pivotball joins/lower A-arm w/adjustable upper link

Damping Aluminum oil-filled shocks


Type One-piece plastic


Type Pro-Line 9011M2


Engine Paris Racing/NovaRossi* BX21/SBK

Carb 7mm slide

Pipe Rex 086

Fuel Trinity Monster Horsepower 20% nitro

RADIO GEAR (not included)

Transmitter KO Propo EX-1

Steering/throttle servos KO Propo 2143FET/KO Propo 2144FET


Extras, including shock oil, diff fluid, hex wrench, fuel tubing, tubing clips and exhaust coupler, are all included.

Suspension-geometry changes are quick to make and don’t require any parts changes.

Sealed bearings-puff said.

Excellent bang-for-the-buck ratio.


Fuel tank might not provide enough fuel flow.

Sharp edges on chassis plate.

YOU’LL NEED * 2-channel radio w/two high-performance servos * 5-cell rechargeable receiver pack * 20- to 30-percent-nitro fuel * Fuel bottle (highly recommended) * Glow igniter * Polycarbonate-compatible paint * CA glue Thread-locking compound * Starter box (for use w/non-pail-start engines).

building & setup tips

The best buyer for the MBX4RR is someone who has intermediate to advanced building and engine-tuning skills. This type of car can present too many challenges to those who lack reasonable experience. This is further compounded by instructions that lack the explicit detail an inexperienced builder would need for successful assembly-and it’s a less-than-ideal English translation. The instructions do, however, provide detailed drawings and exploded-view diagrams that are sufficient to guide an experienced builder through to completion. The following steps are those 1 feel need a little more attention.

Step 1. Seal the differentials with a little silicone sealant. The provided gaskets contain diff fluid, but they can leak. Using silicone sealant in addition to gaskets provides extra insurance against having to remove the diffs to reseal them if they develop a leak. Also, be sure to “break in” the differentials by driving slowly in tight circles, especially if the diff gears feel a little tight. The diff gears can fail if they’re not properly broken in.

Step 4. Trim the front bumper as shown in the photograph to the left. The bumper comes into contact with the new reinforcing rib on the front edge of the A-arms, and that limits front suspension down-travel by about 1/4 inch. Also, before you install any hardware on the chassis plate, file down the chassis’ sharp edges and around the flywheel opening. Taking off the edge around the chassis’ perimeter could save some skin, and smoothing the edge around the flywheel opening will prevent your starter wheel from looking as though it has been used to start a potato peeler.

Step 10. Insert the dogbone section of the front drive shafts into the universal joint only until it’s flush with the inside edge of the U-joint. Installing it further in may make the drive shafts too short and might damage the U-joint when running at maximum steering angles. Use red Loctite to secure the 4mm setscrews. Step 11. Use pieces of fuel tubing to prevent the rear dogbones from sliding too deeply into the outer drive cups. The extra suspension travel may allow the outer drive cups to chew away at the dogbones when the suspension is at full extension.

Step 17. Check the fuel tank for proper fuel flow. I experienced flow problems with the fuel tank, so it would be wise to ensure that adequate flow is possible. First, blow through the included fuel line to see how much resistance is to be expected. Next, attach one end of the fuel line to the fitting for the fuel pick-up and blow through the tubing again. There should only be marginally more resistance. If it’s twice as hard to blow through the fuel line once it has been attached to the tank, you may have a problem. Mugen is already aware of this happening in a few other preproduction kits and mine. I’m confident they’re taking every step to ensure that the tanks in the production kits will function properly, but just take a second to double-check.

Step 21. Bend the brake linkage (as shown in the main photo on page 58) to improve alignment of the linkage rod with the brake arms. Failure to do this may cause enough drag on the brakes when applying throttle to hamper acceleration and top speed.

Step 25. The instructions don’t specify the number of spring preload spacers to be used initially. l found that using Tmm (a combination of 5 and 2mm spacers) on the front shocks and 5mm on the rear was a good starting point.

Step 29. Wipe the wheels and tires clean with solvent before gluing. The new rims don’t have the outer bead that can save you from a poor glue job. You may even want to roughen the gluing surface of the wheels for a little extra insurance.

Chassis. The RR’s chassis plate is made of 3.Smm channeled, die-cut 7075 aluminum plate that has very sharp edges. The chassis’ bottom features countersunk screw holes front to back, except for the engine mounts’ recessed screws which, just barely peek out from the chassis’ surface. The RR’s chassis is 6mm longer than the original MBX’s-similar to the long– wheelbase chassis previously offered as an option on the original car. Early press releases indicated a narrower chassis than before, but except for length, its dimensions are the same.

* Suspension/steering. The most obvious changes are in the suspension. The original MBX featured unusual suspension– arm mounts that held the hinge pins at an extreme angle. The rear-suspension hinge pins were angled outward toward the rear, and the front pins were angled outward toward the front. The new RR returns to a more conventional design with the hinge pins more in line but still at a slight, though much less severe, angle than the original car’s.

The RR’s new standard suspension mounts are aluminum, which was an optional feature of the previous model. Also, setscrews now secure the hinge pins in addition to E-clips at both ends; this greatly reduces the chances of your losing an E-clip and ending your race. The front suspension arms are largely the same. The insides of the lower arms now accommodate the new hinge-pin angle, and a reinforcing rib on the lower arm’s front edge improves durability. The upper arms, pivot-ball steering hubs and front anti-roll bar (2.1mm) are the same.

The rear suspension changes are the most significant: new lower suspension arms; a new rear anti-roll bar; and a new mounting location for the rear shocks and the rear toe adjusters. The suspension arms are now more symmetrical than the rear– swept arms of the previous car. This puts the wheelbase at 326mm-just lmm longer than the original car. To accommodate a new, 2.6mm, rear anti-roll bar, the toe-angle adjusters are at the front of the suspension arms. The rear shocks have also been repositioned for the same reason and are now attached to the rear of both the shock tower and the lower suspension arms.

The improved front and rear shock towers, options on the original car, are now standard. The front shock tower appears to be the same height; the rear shock tower is slightly taller. The RR now has 10 upper shock-mounting positions (four more than the previous design) that offer more ride-height and progressive-damping options. Speaking of dampers, there’s also a difference in the shocks. The rear shocks are a whopping 10 to l2mm longer to allow greater suspension travel. The front shocks are the same length with new springs at all four comers. The kit includes a sizable bottle of 350WT shock oil (about the same as 30WT in the Losi line)-enough for two or three refills of all shocks.

The RR’s steering system is a straight carryover from the previous and very effective model; it features a built-in, adjustable servo-saver.

* Drive train. We anticipated changes to the drive train’s final drive ratio but it is actually identical to the previous buggy’s except for a 14-tooth instead of a 13-tooth clutch bell. The major differences are the newly designed differential housings, which are now made of molded composite instead of aluminum. They are marginally lighter (for quicker acceleration) and easier to seal, so it is easier to prevent the diff fluid from leaking out. New to the RR kit is heavy silicone diff fluid-3000, 7000 and 1000WT-for the front, center and rear diffs, respectively.

The center diff transfers power via dogbone drive shafts to the front and rear diffs. The rear diff has a longer drive cup to account for the 6mm increase in chassis length. Power is then fed to the wheels through universal-joint drive shafts in the front and dogbones in the rear. The RR retains the l7mm hex hubs at all four corners.

Two vented steel discs-one in front and one behind the center diff-handle braking. Combined with the independently adjustable brake linkage, the brake configuration allows brake bias, if that’s what you want. Supporting the whole drive system is one of my favorite new features-sealed bearings! I have been somewhat critical of Mugen and other 1/8 buggy manufacturers who don’t use sealed bearings. The environment in which these cars run can be brutal. Dirt and debris can quickly foul the bearings, especially the wheel and outer diff bearings. The original MBX has shielded bearings, but the new RR has a full set of sealed bearings!

* Wheels and tires. The RR comes with newly designed wheels and a set of Pro-Line 9011M2 Multi-spike tires. The wheels’ bead design is new; conventional wheels have an inner and an outer lip that form a groove into which the tires are mounted. The new wheels have only the inner lip.

* Body and wing. The RR’s body resembles that of the original MBX. Because of my lack of ability in the painting department, I recruited the help of coworker Bob Hastings-our “go to” guy for last-minute paint jobs-to lay down the graphics on the new Mugen.The wing is a carryover from the MBX. It’s durable, molded and has been proven to be able to withstand a beating. It’s attached to a strong mount that I tested to the extremes of endurance.


I first tested the RR at Xtreme R/C in New Milford, CT. Its newly designed track is much more suitable for 1/8 off-road buggies. It has been extended and the lanes widened to make room for the menacing nitro machines. I used this solo shakedown run to get the RR dialed in before I headed upstate for some wheel-to-wheel competition at R/C Madness in Enfield, CT. This is where a puzzling problem revealed itself-one that took two days to sort out.

My well-broken-in NovaRossi engine ran extremely well until I applied full throttle. It had the typical signs of a lean, high-speed– mixture setting, but with a twist: the engine ran up to speed without hesitation as long as I did not apply full throttle. At full throttle, the engine exhibited the same characteristics as it would have if being run with a lean, high-speed mixture. After opening the highspeed needle about 3 turns without seeing any improvement, the only option was to head back to the workbench for a more detailed diagnosis. I’ll spare you the blow-by-blow, but it turned out to be a flaw in the fuel tank. Mugen was already aware of a problem with the tanks shipped with the new RR and had sent me a replacement but the new tank had the same problem! A partial blockage in the fuel tank’s pick-up tube allowed only enough fuel flow to satisfy the engine up to about 3/4 throttle. At full throttle, the engine ran very lean, and before I determined the source of the problem, I cooked three glow plugs and risked damaging my new NovaRossi engine. I speak of this problem in the past tense because with a call to R/C Madness, I knew that a fresh tank awaited me. The RR uses the same tank as the original MBX. The only difference is a new filler lid with a canted fitting for the pressure line. After I had changed lids and given the tank a “flow” test by blowing through the fuel line, my engine problem was solved.

The only thing that kept me from greatness at this point was six months’ worth of rust build-up (while I was away from the track). I managed to qualify third and finish third in the A-main. A problem with the antenna tube’s breaking right where it exits the body led to the bare antenna’s wrapping itself around the rear axle. Even though I was sidelined for a lap, I still managed a podium finish. I feel I certainly had the best package on the track, only I lacked the skills to exploit it.

The new MBX-4RR is a cut above the previous model. Quite a few design changes have improved its racetrack manners and make it easier than ever before to click off the fast lap times.

* Acceleration. Getting power to the ground is marginally improved compared with the already nimble MBX. Positioning the rear diff 6mm. farther back seems to have improved weight transfer slightly and gets the power to the ground more quickly. The new, 14-tooth clutch bell takes away just a touch of the bottom-end punch, but there’s plenty of that with the NovaRossi BX21/SBK. When running on relatively high-bite tracks, forward bite can be improved with slightly heavier fluid in the center diff, but the stock setup seems to offer a good balance for average conditions.

* Cornering. This is where the RR really shines; 18 4WD buggies are notorious for poor steering, but the RR is the most responsive I’ve ever seen. Adding a rear anti-roll bar brings the rear end around quicker than before, and a slight reduction in wheelbase over that of the original MBX with the optional long chassis further adds to its responsiveness. I installed both of the caster shims to the rear of the front upper arms; that provides more steering than most drivers will ever want. Regardless of your preferences, the key is that you’ll never mutter, “C’mon, … turn, turn!!” Even more impressive is that for even more steering, the anti-roll bars can be removed.

The stock setup favors flat to marginally bumpy conditions with medium traction-a very good, neutral starting point. For bumpier tracks, go with lighter shock oil (25WT Trinity or Associated*, or Losi* 30WT) and lose one or both of the antiroll bars. The standard Pro-Line 9011M2 tires offer excellent bite-enough to allow me to snip off the inner- and outermost row of pin spikes to reduce side bite.

* Over the jumps. The RR handles big bumps as well as any world-class buggy. The extra rear-suspension travel and the innate ability of nitro buggies in general to execute midair corrections combine to make the RR an excellent flier. The extra suspension travel also helps smooth out the landings.


The new RR is an excellent package for those who are interested in a competition 1/8 buggy. It doesn’t have the depth of features found on its chief competitor-the MP-6 International-but what it lacks (a full set of universal-joint drive shafts, quadruple disc brakes and hard-anodized chassis plate), it makes up for with what I call meat-and-potatoes features. It comes with sealed bearings, pivot-ball front suspension and most of the other equipment you’ll need to complete construction. A huge bonus is that the RR kit also includes shock oil, three bottles of diff fluid, a combination wrench for all the fasteners (including the wheel nuts), fuel tubing, an exhaust coupler and even fuel-tubing clips.

The MBX-4RR represents the best allaround package in 1/8 off-road racing. The MP-6 and its predecessors are multiple world and national champions with the polish and prowess of a finely tuned Porsche. I see the MBX-4RR, however, as more Viper-esque: lacking heritage and refinement, but well– featured, incredibly nimble and available for a comparatively reasonable price-certainly the best bang for the buck in the field.

*Addresses are listed alphabetically in “Featured Manufacturers” on page 216.

Copyright Air Age Publishing Jul 2000

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