In hindsight, the FMC (Food Machinery Corporation, interestingly enough) XR311 seems like a very strange choice for an RC model.

EA 58004 57In fact, unless you happen to be a Tamiya fan, you've probably never heard of it. A Google search returns much more information about the model than the real thing. That's because the XR311 was a prototype vehicle (hence the "X") slated to replace the venerable Jeep but never went into production. By 1977, when Tamiya released this model, the project had already been scrapped. But if we assume that the model was in development for a year or two before that, Tamiya was poised to have a working scale model of a brand new military vehicle. Now, 40+ years later, everyone has forgotten about it. The Desert Patrol Vehicle came a bit later to fill the off road speed role and became the 58046 Fast Attack Vehicle. The real Jeep replacement ended up being the High Mobility Multipurpose Wheeled Vehicle and became the 58154 Hummer, so Tamiya got their Jeep replacements in the end. The Jeep itself became the 58033 Wild Willy.

The real XR311 prototype vehicle was intended to replace the aging original Jeep and maintain the legendary off road ability but add the capability for much higher speed with the option to add a wide array of military weaponry. FMC created an interesting design using mostly Chrysler components. Like most large Chrylsers of the time, it used torsion bar suspension and the A727 Torqueflite automatic transmission. It also used a Mopar 318cid V8, but unlike any Chrysler it put the engine in the back. It had full time 4WD and 11 inches
of ground clearance. I've rebuilt a version of both that engine and transmission before so maybe I'd actually
be able to work on a real XR311!

EA 58004 56The 58004 FMC XR311 was released in 1977as the 4th Tamiya RC car ever and also represented a lot of firsts. It was the first off road vehicle (not counting tanks), the first to have four wheel independent suspension, and the first to have torsion bars. It may have been the first to use a servo saver, but I don't know enough about the first three to say for certain. The suspension layout (double wishbone) and spring style (torsion bar) does a good job of representing the real thing, and of course the body is spot on. The motor is in the back, just like the real thing. The weak point in terms of realism is that it is not 4WD. The chassis is a flat metal base plate with suspension modules attached. The body locks in place from beneath with an interesting cam mechanism which means no body posts are spoiling the appearance. This model was resurrected by Tamiya in 2000 with the same model number and released yet again in 2011 which is the version I have. Very little changed in all that time. Apart from the obvious removal of the mechanical speed controller and associated servo, the main difference is the much needed replacement of the fragile plastic torsion bars with metal bars. As a builder, the other important change is from slotted screws to JIS. Yes, models way back then actually used slotted screws. Good riddance. Although the other 3 original models have also been re-released a number of times, they have used newer chassis that bear little resemblance to the original. Of all the modern re-releases, this one probably represents the oldest near original build.

Although a model of a serious off road vehicle, the model is not suited for heavy off road use. The body is fairly fragile and the suspension will bottom out over large bumps. The model is most at home on uneven pavement or gravel, and it really shines there. The suspension stiffness and movement is very realistic. The model comes with 3 sets of pinion/spur combinations to vary the top speed and torque. The model is quite quick with the high speed gears. It is slow enough to crawl with the low speed gears, but doesn't have the traction to make a real go of it. It does have a solid real axle with no differential so that helps a bit with traction.

 

Building the Chassis

 

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The XR311 comes in a large box with lovely artwork. Note that this kit bears the same model number as the original so a special sticker has been added to indicate that this is a re-release. Inside the sections are nicely divided and the metal parts are segregated into a smaller inner box with dividers to keep them safe.

 

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 The left hand image shows the contents of the inner box for the metal parts. This includes the chassis tray, the electronics sponsons and the gearbox housing. The right hand image shows the fairly small number of parts for this vehicle. At the time it was made, the chassis and suspension were very complex but it seems somewhat simple in hindsight.

 

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The first step assembles a bunch of metal components which comprise the bulk of the chassis frame. This picture shows the bottom of the frame. The bracket you see toward the front with the cam shaped slots is used to lock on the body. The black plastic parts are the splined supports for the torsion bars. The rear metal parts are electronic trays. Much of this is the same as the original, but the shape of the electronic trays has been updated. The original were flat (and therefore weak). New electronics are smaller so we can get away with tubbed trays. The first step assembles a bunch of metal components which comprise the bulk of the chassis frame. This picture shows the bottom of the frame. The bracket you see toward the front with the cam shaped slots is used to lock on the body. The black plastic parts are the splined supports for the torsion bars. The rear metal parts are electronic trays. Much of this is the same as the original, but the shape of the electronic trays has been updated. The original were flat (and therefore weak). New electronics are smaller so we can get away with tubbed trays.

 

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The suspension build starts at the front. The kit comes with metal bushings throughout. Although I always like to replace these with ball bearings and I bought a bearing kit from Fast Eddy for this purpose, the bearing kit did not include flanged bearings so I still needed to use the kit provided bushings for the front axle. Normally the axle is fixed and the bearings rotate with the wheel, but in this case the whole front axle rotates because it bolts directly to the wheel. The front C-hubs are actually built up from 3 parts as shown. There is an upper and lower arm with a standoff between. These clamp around the plastic kingpin of the steering knuckle. This is identical to how it was done in the original model 40 years ago.

 

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Each upper and lower suspension arm comes in two pieces and clamps around the C-hub as shown. This setup makes for a lot of little fiddly parts, especially given that this is a 1/12th scale model. The steering method is fascinating. What you see here is the aft bulkhead of the front suspension which carries a pair of tiny bellcranks. The cross bar you see between them has a small stud protruding from the top. That stud will push on a slot in a C-shaped clip to move the steering. The flexibility of the clip acts as a servo saver.

 

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The left hand image shows the front suspension assembly viewed from the front. The steering tie rods are just wires with shepherd hooks on the ends as shown. There is therefore no toe adjustability. The right hand image shows the same assembly from the top. You can see the two tiny bellcranks and the steering cross bar. The cross wrench has been added to the picture for scale to show you how small this suspension is.

 

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After the addition of a few metal standoffs and a front and rear support bracket, the front suspension assembly is complete. It is actually quite complicated.

 

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 The second image shows the vertical crank used for steering input. Now the steel plate torsion bars can be installed. These replace the notoriously fragile plastic bars of the original. The torsion bars slide into spring loaded supports on the chassis, and into slots on the lower suspension arms at the other end. As the suspension compresses, the lower arms twists the bar and that twist is resisted at the other end. By compressing the spring you can pull the restraint out of the splined socket and rotate the restraint by one tooth to add or subtract preload. This allows you to tune the suspension to vehicle weight. It works really well and allows you to get a very realistic suspension motion and stiffness.

 

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The rear suspension is a bit simpler. The vertical hub is one part with an axle inserted. I was able to use ball bearings here. Like the front, the upper and lower suspension arms are two pieces each. The wishbones are very wide and therefore provide a lot of support for off road use.

 

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There is no differential on this vehicle. The ring gear is solidly connected to a drive cup at either side and fit into a cast gearbox housing. Both ring and pinion are metal. In fact, everything in these pictures is metal. The original used plastic drive cups.

 

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The pinion bevel gear is inserted though the rear gearbox housing because the motor will drive from the back. This is then bolted to the main metal housing as shown. You can see another tiny flanged bushing here that I wasn't able to replace with a bearing.

 

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The suspension arms are sandwiched between the front and rear covers of the gearbox housing. Before the front cover is installed, you can see the ring gear inside as shown. The bolts used to connect the whole thing together are the longest 3mm screws I've seen so far (55mm).

 

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The motor is cradled directly above the rear axle and faces aft. The kit comes with 3 spur gears which allow 3 different speeds. They are labeled A, B, and C which are slow to fast, respectively. Note that the number of teeth on the outside of the spur are not the only thing to change. The integral drive gear at the center also varies. This means that both gear stages are variable which drives a pretty big difference in available ratios. Let's take a look assuming a 15 tooth motor pinion and a 40 tooth output drive gear. The number of teeth is never shown in the instructions so I had to count them manually. Hopefully I got it right.
Gear Set A: 63:15 x 40:15 = 11.2:1
Gear Set B: 45:15 x 40:15 = 8:1
Gear Set C: 35:15 x 40:20 = 4.67:1
So this shows you that there is a 2.4x difference between the high and low gearing options. The mesh is not adjustable. Instead, each gear size fits into a different axle support hole in the gearbox housing. I'm using a standard silver can 540 motor which is more than adequate for a model of this size. You wouldn't want anything more powerful if you want to keep the model intact.

 

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I started by assembling the gearbox with the A gear which is the slowest speed (highest gear reduction). The motor pinion is aluminum but the A gear and the drive gear are plastic. As previously mentioned, the ring and pinion of the locked axle are metal. Once the plastic cover is installed as shown on the right, you can see the 3 round blisters which represent the bearing supports for the 3 different gear sets. They are arranged in an arc with the motor pinion so they will all mesh without any adjustment. It is a clever and reasonably fool proof system.

 

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The rear torsion bars are installed just like the front but must be preloaded quite a bit more because of the weight of the motor in the back. Don't forget to install the battery before adjusting the preload because it is a huge component of the overall weight. The wheels are a 3 piece design which must be assembled directly to the axle. There is no way to remove the wheel without also taking it apart. It is not exactly a beadlock since it doesn't clamp tight on the bead, but it does provide good support and, most importantly, eliminates the need for tire glue. I love the profile and tread pattern on these tires.

 

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Here's the completed rolling chassis. Note that everything between the axles is metal. The right hand image shows the installation of the durable front bumper. There's really not that much going on here mechanically or structurally, but the model is very sturdy. The same can't be said for the body.

 

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The original needed to make room for both a steering and a throttle servo as well as the associated MSC, but we have more space to play with here. The steering servo attaches up front with servo tape and is connected to the steering crank with a short rod. The rest of the main tub leaves room for a standard 6-cell battery pack. The trays to either side have plenty of room for an ESC and a receiver. The last image shows the completed chassis from below. Barely visible is the clear under tray which protects the torsion bars from obstacle contact.

 

Building the Body

 

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Building the body is a whole project in itself, and based on the photo at left I'd say it probably has just as many parts as the chassis. The photo at right shows a couple of brass hourglass buttons which slip into slots on the chassis above the motor to lock the rear of the body. The front uses similar buttons which lock to the cam system described earlier. Note the copyright date on the body showing both 1977 and 2000.

 

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Here are some of the details of the body. The jerry cans are mode of four individual parts cemented together: two sides, a handle, and a cap. These are attached to the rear of the body. In the right hand image you can also see the mesh which is used for the grille. Remember that the XR311 has a rear engine so this would be the radiator. You can also see the pintle hitch. Don't tow anything with it though; it is just glued on. The rails around the engine cover are another detail made of tiny parts.

 

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I started by painting the whole thing in olive green, but there is still quite a bit of detailing to do. The fenders, for example, need to be flat black. So does the area just behind the front bumper. I did all of that with masking and black spray.

 

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The realistic driver has to be painted as do the dash board and the seats. Note that this vehicle seats 3 across. I didn't have any flat green paint so the driver's clothes ended up shinier than I would like. I solved this by going over everything with flat clear at the end.

 

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The final steps are to install the handful of stickers (I only used the stars) and the soft top roof. It only looks like canvas; it is actually hard plastic. It screws in to the back and is attached to the windshield frame with double sided tape. Note that the front window is not just open, there is a windshield in there. There are wipers and mirrors as well. The mirrors are probably the most fragile part of the installation because it is hard to glue metal to plastic securely. The guards around the headlights are also very difficult to install because there are no tabs for them to slot into so they just mount to the surface.

 

Upgrades!

 

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The biggest weakness of the XR311 is that it has no external damping for the suspension. The torsion bars and joints do a reasonable job of slowing the rebound, but nothing like a real damping system would do. It happened by coincidence that RC Channel released a suspension upgrade set (for an 8 year old model) the same week that I was already planning to build my XR311. This was obviously meant to be. I bought the suspension kit, the oil shocks, and the metal under tray and commenced upgrading.

 

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Here's the front suspension before and after the upgrade. The suspension arms and steering knuckles have been replaced with metal parts and the flanged bushings have been replaced with ball bearings. A metal shock tower has been added as well as a set of small oil dampers. The front and rear metal brackets are original as is the steering linkage and servo saver (hidden down inside the assembly). I think you'll agree that it is a thing of beauty.

 

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A similar process is performed on the rear suspension. Besides the addition of shocks, the rear end upgrade also replaces the gearbox cover with a metal unit making for a full field of view of bling. All mechanical parts of the gearbox are still original including the dog bones. Original steel JIS hardware is replaced with stainless hex hardware.

 

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Although not part of the same upgrade kit, I also replaced the clear under tray with a sheet aluminum version. This probably offers no actual benefit, but it ties the whole metal chassis look together. The right hand image shows the completed upgraded chassis which looks really, really good.

 

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The weak point of the upgrade kit is the ball socket design. The balls are captured in a one-piece plastic socket and the instructions warn you not to try to remove the balls from the socket. One of mine suffered a failure of the socket as shown before I even installed it. Thankfully, the rod end for the servo link was not really necessary so I was able to steal that one to complete the suspension. The right hand image shows everything that was replaced. Virtually all of the original plastic parts have been removed. The only exception is the steering linkage.

The driveability of the model is much improved after the upgrade. It probably doesn't matter whether the arms are plastic or metal, but the addition of dampers really helps. Plus it looks awesome.

 

Final Photos

I was really surprised by how good this looks given the age of the design. The tires and torsion bar suspension produce just the right stance and the driver looks perfect. Though it may look like a simple one color paint scheme, there is actually a lot of detail work here. I even like the way it moves over moderate terrain with reasonable suspension stiffness and speed. The drivetrain is simple, yet effective.

 

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Written by Eric Albrecht

Visit Eric's personal site with lots of build threads.

  

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