MST CFX-W Project

Page 1: Chassis Construction

The box is not particularly large since it is only a chassis divided up into small parts. I thought this price was pretty good for what you get. Let's see if I was right.

Inside the box you can see individually numbered bags up to 14. 13, not shown, is the wheels and tires. Dividing up the build like this makes it very easy to follow everything. The chassis rails are bagged on their own. The selection of tools that comes with the set is a little unusual. There is a turnbuckle wrench, but the cross wrench uses a cross bar instead of being one cast part. There is also some black grease and some mineral oil for shock oil. This is about as cheap as you could go for shock oil, but then the shocks aren't very good anyway. There is also some servo tape and some foam strips.

You would think that each bag would go with a step, or perhaps there would be multiple steps per bag. Actually, it is the other way around. Step 1 requires you to open bags 1-3. Those parts are shown in the left hand image. Feast your eyes on all those metal gears! The kit also includes a full set of ball bearings. The build starts with the transmission, and the transmission starts with the sun gear shaft for the planetary gear set. The other end of this shaft will be the motor spur gear. The sun gear has 14 teeth.

Next comes the 42 tooth metal ring gear which sits in a mostly triangular slot in a plastic housing. The right hand image shows the planet carrier. Each planet gear has 14 teeth. The output gear is attached to the carrier and has 24 teeth. The reduction of the planetary gear set is therefore 4:1 in a single stage.

Planetary Ratio = 1+(42/14) = 4:1

The left hand image shows the installed carrier which hides the planetary gear set behind it. We're not done yet though. There is one more gear shaft. This gear has 24 teeth just like the planetary output so there is no reduction here. It serves to reverse the direction of rotation and also to put the final output below the main axis.

Time to button up the transmission. You wouldn't know all that metal goodness was inside, but this is a pretty impressive transmission as well as nearly bulletproof. It should handle any torque you can throw at it. With that in mind, we're going to need a slipper clutch. The clutch is built into the spur gear as shown. It has friction pads on both sides.

The slipper/spur setup is nestled into the transmission assembly and then covered with the motor mounting plate. The spring to adjust the slip torque is on the outside of this. Notice how little space the entire transmission takes.

I wanted a good quality brushed motor for this model so I went with a Tekin 35 turn which has been excellent. The kit actually did not come with a pinion gear, just a chart telling you the different gear ratios you get using from 17 to 25 teeth. Luckily I've got a pretty good collection of spares.

Once the gear mesh is set, the motor can be tightened and the gear cover installed. Now the transmission is complete. It is remarkably compact unit for how much is going on inside. The final bits shown in the right hand image are the mounting arms which will attach to the chassis rails. This was all part of Step 1 which had 11 substeps.

Step 2 is much quicker: building the transfer case. The t-case reverses the direction of rotation, drops the output down to a lower level, and performs a further 32:24 = 1.33:1 reduction. Again, all the gears are metal and every shaft is supported with ball bearings.

Here's another instructions oddity. Having finished with bags 1, 2, and 3 in Steps 1 and 2, now Step 3 uses bags 4 and 14. Why skip all the way to 14? I don't know. This step involves building the ladder frame. There are front and rear plastic cross members, a center skid, and a mid aft cross member which serves as an electronics tray. The gearbox serves as a mid forward cross member. With 5 cross members this thing is very stiff. The shock hoops are also plastic and are installed at this time along with the slider supports. The transfer case attaches to the skid and is connected to the gearbox with a short straight drive shaft. You can see all the webbing in that skid making it extra sturdy.

Step 4 uses parts bag 5 and builds all the links. 8 are for the suspension, then there is a pair of steering tie rods. These are nice metal rods using plastic rod ends. Forward and rear links are the same length so you can't screw it up (unless you mix up upper and lower). Those little diagonal links you see in the left hand image are a mystery for now. We'll find out more about them later.

Step 5 builds, but does not install, the shocks using bag 6. These are fully plastic oil filled shocks. Nothing special, but adequate.

Step 6 builds the axles and requires the parts from bags 7, 8, 9, and 10. There are a LOT of parts there for just axles. Look at all those bearings and gears! The pile of parts you see there are just for a single axle, but the front and rear axle are exactly the same so we get to build it twice.

We start with the differential which isn't really a differential at all since it is locked. The ring and pinion are straight cut bevel gears of 36 and 15 teeth, respectively. This results in a 2.4:1 reduction.

Now the nice CVD axles are built and inserted. Here's something that shows the designers were using their heads. The CVDs are built left and right. The difference is the direction you install the spiral clips which retain the cross pins. You want the axle to rotate in the direction that tends to keep the clip in place, not the direction that would tend to pop them off so they are build as mirror images. Just don't back up too much! The C-hubs can be installed with various kingpin inclinations. The stock setting is 10 degrees, but you can reduce it to 0 or increase it to 20. The inclination on the rear axle is meaningless since it does not steer.

A typical portal axle has only two gears: a driver and a follower. This usually results in some significant gear reduction in the hub. In the CFX-W there is very little reduction: just 16:14 = 1.14:1. It includes a set of idler gears to generate an even larger offset between the wheel axle and the drive axle.

Time to install the steering servo which is mounted to the axle. I decided to try out an Xpert servo. I'd never heard of them, but the specs were impressive and the price was right. This is a high torque, waterproof, metal gear servo. So far it has performed very well indeed and I've used it on more than one vehicle since. The right hand image shows the servo installed along with the tie rods and shocks.

Here are the completed front and rear axles. Those little diagonal braces from earlier are used to lock the steering angle on the rear axle. If you wanted to, nothing is stopping you from making this 4-wheel steering. Step 7 uses parts bag 11 to build the drive shafts which are splined plastic telescoping units with universal joints. Now the axles can be attached to the links, the shocks to the shock hoops, and we have a chassis! I've also installed the bumpers from Step 8, although I will be removing them later when I put on the body. You can also see the battery tray behind the transfer case.

Step 9 builds the wheels and tires. These are plastic wheels which are not beadlocks. I knew I was going to change them so I didn't bother gluing them. The tires are actually really nice. This final image shows the installation of the electronics and factory body mounts. I used a Hobbywing crawler controller which I really like. There are also some upgrades visible here which I'll talk about on the next page.

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