Schumacher CAT XLS Project

Page 1: Assembly

Here is the modestly sized box for the CAT XLS. Inside everything is bagged in sequenced groups. Each bag contains all the parts and hardware for a certain number of steps. There are smaller bags within: one for each step. This makes it very easy to find parts and proceed with the build.

There are also some tools and supplies in the kit. Included are allen keys, a 3mm drill bit, thread lock, two greases,and two shock oil viscosities.

The build begins with the fiberglass lower chassis pan. At this point a decision has to made as to what kind of battery to use. You can install rounded straps for a NiMH style pack, or you can install clamps for a shorty Li-Po pack as I have done. There is also an antenna bracket installed in this first step. Next we route the main belt under the chassis pan and then protect the whole thing with an undertray. The tray gets screwed to the pan and is really locked in at this point so you can't go back and paint it later. I left mine clear. The main transmission housing and metal motor plate are also attached at this time.

These bearing supports are then attached to the motor mount. The holes are not tapped; these are very short sheet metal screws. The lower support is a cam and can be rotated to adjust belt tension.

Time to build the first of many ball differentials. This design was apparently invented by Schumacher. The small ball diff goes on the upper shaft which will connect to the spur gear at the other end. At this end it drives two pulleys which will connect to the rear left and right wheel pulleys.

This second ball diff looks much bigger, but the balls are actually the same size. Only the outer gear diameter is bigger. This is the 48p spur gear. The raceway washers have to be manually glued to the plastic parts. This is a bit tricky to do without getting glue anywhere it shouldn't be. This is both a slipper clutch and also a torque splitter. Each side of the slipper is connected to a different side of the first ball diff.

Now for the rear ball diff assembly which the instructions call the "Integrator". This serves as a combination rear differential and center differential. On the right you can see the tiny balls in the thrust bearing assembly at the base of the long bolt.

The final integrator has three pulleys and it is not immediately obvious what they are for until the unit is installed in the chassis. On the right you can see the two small belts which connect the spur gear differential to the integrator. This allows the motor to drive the rear wheels even if one is slipping. The center pulley connects to the long belt which will go to the front wheels. Because it averages the left and right pulleys, it will always be proportional to the motor. The purpose of driving the front wheels from the lower pulley instead of using the upper shaft is to take advantage of the reduction first.

Now a cover has been installed over the integrator assembly to protect the belts. Then the upper chassis plate has been installed along with a protective clear belt cover.

Here's an unusual assembly step. The ball joints on the sway bar are attached by soldering rather than using a more standard set screw. No big deal if you have a soldering iron, but might be a problem for newer builders. The sway bar attaches to the upper chassis plate with small plastic brackets acting as hinges.

The front differential is the simplest ball diff we'll build. It is pretty standard and contains a pulley for the long belt in the center. It is then installed between the upper and lower chassis plates.

Now we'll do a bit of work on the front suspension. The universal joints need to be manually assembled which is tricky. You need to use a special tool to pry open the yoke and insert the cross. Then the front bumper is installed with long bolts that go all the way through the differential housing.

The rear suspension follows next. The rear drive shaft universal joints are built in the same way as the front. Next the lower control arms are installed with a long wire as a pivot axis. There is a lot of cross bracing in the back to keep everything nice and rigid.

The front lower arms are installed much like the rear. They are very long one-piece arms. I've also installed the shock towers. The 3 big black rubber bands you see right behind the bumper are the crash relief system. This allows the wheels to splay outward if a front tire hits an obstacle. The bands are stiff enough that they won't effect normal driving.

All four upper arms go in next and then the hubs are installed. These are not your typical C-hub design. The hubs are large discs the size of the inside diameter of the wheels. All of the ball joint sockets point down. This ensures that nothing can pop out when you get big up forces from jumping, but it also makes things a bit difficult to assemble. The second half of the drive shaft is attached to the hub and slides over the splines on the inner shaft.

Time for the beautiful shocks. Each machined aluminum cylinder gets a whole series of o-rings. The first and largest o-ring sits ahead of the brass gland. The gland provides lateral support to the rod. Then there are two smaller red o-rings with a black spacer in between. Finally there is a Delrin scraper. There are no threads (which consume precious stroke) here, instead the whole thing is retained with a tiny C-clip. Good luck installing this. I don't have any C-clip pliers remotely small enough so I had to grind the tips on a pliers to make something work. Then the piston heads are installed with E-clips as shown on the right.

Once the rods are installed the rod end is threaded on and then the shock is filled with oil. The springs and spring perches are last. The front and rear shocks are different lengths and also use different oil weights by default.

With the shocks installed this looks almost like a rolling chassis. However, the steering has still not been built.

The dual bellcranks for the steering come next, and installing them is the hardest step in the whole model. The screws must go in from below and there is simply no access. The instructions should be changed to perform this step much sooner. I also had to trim away quite a bit of the undertray to make room for the steering system motion.

With the steering cranks finally installed it is time to attach the tie rods and the push rods for the sway bar. These all use turnbuckles. Once that is done, the rolling chassis is finally complete. The last step is to trim and attach the wing.

Now it is time to install the electronics. At the time of my initial build on the left I did not have a motor yet so I just installed a 540 silver can as a placeholder. I later installed the proper Hobbywing 17.5T brushless and trimmed all the wires to fit as shown. Brushless did not exist when this model was new, of course, so the solder tabs on the motor interfere slightly with the body.

The final step is paint. There are no real instructions for this so I masked to try to duplicate the box art and I think I did a reasonable job. I thought the colors on the wing were decals so I painted the whole thing black but then discovered my error and had to go back and paint the yellow and red on the outside. It came out fine and I don't think anyone will know the difference. There is no driver for this model so I tinted the windows with smoke paint.

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