Traxxas Bronco Project

Page 1: Re-Assembly

The Bronco comes in a surprisingly small box packed very tightly with goodness. Traxxas does a good job of highlighting all of the main features.

I didn't even try the model out first, I went straight to tearing it all down and rebuilding from scratch. I'll admit that I only tore down the front portal box since the rear is the same so rebuilding one will give me the experience I am looking for. I also didn't disassemble the shocks or links. You can see what I ended up with after the teardown on the left. On the right you can see the formed C-channel chassis rails.

Here is the hardware somewhat sorted. Hex fasteners are lower left, bearings and shafts are upper left, gears are upper right, and miscellaneous hardware is lower right. On the right you can see the complete electronics package. The controller for the lighting package I added is also in this picture. The 3 micro servos control the transmission and differentials. This might be a cheap brushed motor, but it does a very good job and I didn't see any reason to replace it.

Let's get started on the transmission. The two gears shown on the left nest together using an interesting "flower petal" shaped spline. The result in shown on the left hand edge of the right picture. Those two hourglass shaped driving dogs will mate with those on the black drive ring at far right. The cruciform slot in the same part fits over the input shaft. There are a matching set of driving dogs on the opposite side of the black part which fit into the slots on the middle gear. Depending on whether the driving ring is pushed to the front or back, one or the other of the gears is locked to the input shaft. Note that both gears ride on bearings which allow them to spin freely on the shaft when not engaged.

Here are the plastic main parts of the gearbox housing with bearings inserted. On the right the main gear shafts have been inserted. That little silver tab sticking out of the gearbox will be used to drive the shift fork adjusting the gearbox from High to Low. I should have had a photo showing the inside of the gearbox, but somehow I forgot to take that one.

This back end of the gearbox contains the transfer case. The upper gear is the transmission output, the lower left goes the rear drive shaft, and the lower right goes to the front drive shaft. Note that this means the front and rear drive shafts rotate in opposite directions. The slipper clutch exploded view is shown on the right. It installed onto the transmission input shaft.

The installation of the slipper and the shifting servo completes the assembly of the gearbox. This is a pretty compact unit considering how much is going on inside.

The front bumper is made up of a surprisingly large number of pieces as shown on the left. On the right they are all assembled. The front skid plate and chassis cross member are integrated into the assembly as is the faux winch and D-rings.

The frame goes together very quickly. The rails are metal, but all the cross members are plastic. The shock hoops are integrated into the inner fenders which also contain the clipless body mounting system. Both the front and rear bumpers are connected directly to the frame for maximum durability.

The gearbox sits directly on the center skid plate as shown. The spur gear sits in a slot which protects the gear teeth from ingesting debris from below. Since the servo is connected to the gearbox housing and not the frame, chassis twist will not move the servo and create any input to the gearbox.

The plastic motor mount is indexed for pinion gears ranging from 9 to 16 teeth with the stock 45T spur, or from 15 to 22 teeth with the optional 39T spur. This results in motor reductions from 5:1 to 1.78:1, a huge range. The stock pinion is 11 teeth, among the lowest speed possible ratios. I'm not sure why anyone would want to make it faster. It is already ridiculous on 3s in high gear. The motor installs using a clever system that allows it to slot in from above making it very easy to remove and almost impossible to mess up the gear mesh.

Here I've installed the steering servo. Traxxas calls it "High Torque", but since that term has no definition it can mean anything. It is passable for an RTR but not great. On the right you can see that I've also installed the XL5 electronic speed control This is a competent unit with drag brake and crawler mode that operates smoothly at almost any rpm.

Here is bracket that holds the two micro servos to control the locking of the front and rear differentials. Small servo savers are integrated into the servo horns which are important since these servos could spend a lot of time stalled against an external stop. On the right you can see the very busy waterproof radio box. I almost always replace the standard radio system with my Spektrum system, but in this case I decided to keep the Traxxas system. This was partially so I could operate the cruise control from the transmitter and lighting through Bluetooth, but also so that I could trail with a friend by letting them use a separate radio.

Here is the substantial battery track. The reversible hold down lever allows firm retention of batteries of various thicknesses. On the right the battery has been installed into the chassis so I could try everything out. The battery is quite high which isn't great for CG, but the optional tiny front battery tray can be used with a smaller battery to move the CG forward and down.

The TRX-4 is the first commercially successful chassis I can think of to use remote locking differentials. The picture on the left shows the standard differential components which include 4 metal spider gears. On the right you can see the locking ring. When engaged, the metal part connects the differential housing to one of the output axles effectively locking the diff.

Here's a view of that locking mechanism installed. The metal part on the right can translate along the axle. On the right you can see the differential ready to install into the axle housing with the pinion gear already in place. This model uses straight cut gears.

This is the cable operated fork used to actuate the differential locking function. Cables are not always the strongest actuation mechanisms, but I have to admit that I've never had any problem with this particular implementation.

These pictures show the difference between a locked and unlocked differential. The picture on the left is unlocked, the right is locked.

Now the diff cover has been installed and I can start working on the portals. The picture on the left shows the outer straight portal supports installed. On the right I've installed the portal gearboxes. I didn't disassemble the rear, so see the front assembly below for the internals of the portal boxes. This completes the rear axle.

The center portion of the front axle is nearly the same as the rear, but the differential is off center and the housing has attachments for a panhard bar instead of a 4-link. The picture on the right shows the housings for the front steerable portals.

The front axle is a CVD style with a gear at the end as shown. In the right picture you can see this gear protruding into the portal housing (far upper right). This portal box does not use any idler gears; the axle directly drives a larger output gear resulting in a reversal of rotation direction. The portals have a 2.56:1 (23:9) reduction.

Here is the completed front portal box both separately and installed on the axle. Note the very large amount of space consumed by the portal box which limits the type and offset of the wheels that can be used. The front kingpin inclination is clearly visible.

Here are the shocks which I did not take apart. They are relatively small diameter (which is scale accurate) with aluminum bodies and adjustment collars. The drive shaft are plastic bodied with metal universals.

Here I've added all the links and shocks to the axles. The links are metal with plastic ends and steel balls. The front axle is shown on the left and the rear on the right.

The final step is to connect the axles to the chassis with a handful of bolts. These pictures show the completed chassis from above and below. Everything is very neat and tidy with carefully planned wire routing using hold down clips. Maintenance access in general is excellent.

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