HMMWV Project

Page 1: RTR Original

The box is pretty impressive and well decorated. The back includes details on all the features of the model in broken English. There are a lot of power, torque, current, and speed specs on the motor which is way more information than usual. On the other hand, basic information like turn count is not provided. Based on the other specs, I'm assuming this is roughly a 27 turn brushed 550 motor. The open differentials are all metal and can be locked via insertion of a pin. The transmission is a 2-speed unit with steel gears. Like the real Hummer, the suspension is independent with portals on all four corners. The frame is a metal ladder frame type.

The side panel of the box contains a brief history of the real M1025 HMMWV. The English is much better here suggesting this text was taken from some other source. Inside the box is a foam case for the model and the included 16-channel twin stick transmitter. This case protects the model very well and makes it easy to transport safely. The box is made from a high density foam with a hinged lid, carrying handle, and embedded magnets to hold the lid shut. The only problem is that it is very hard to remove the model from the case because there is not much exposed structure to grab on to. The roof mounted gun must be removed when using the case, but it still fits inside under the model.

The hard plastic body of the HMMWV model has a wealth of detail both inside and out. We'll start with the hood. The hood has real metal latches on the sides. Once those are released, the hood can swing forward to reveal a faux engine bay including a radiator under the hood vent. This engine is one piece which can also swing open to provide access to the electronics bay. On the real HMMWV, the front grille is fixed and does not rotate with the hood.

All four doors can be opened and use spring loaded latches. On the inside, the doors have panels and arm rests. Some Humvees have additional armor plating on the door and over the side windows, but this model does not simulate that package.

The seats have cushions that are actually cushioned. The area you see in black seem to be some kind of rubber. There's a full dash including gauges and steering wheel, and there is a radio on the center console. The black diamond plate platform you see between the black seats allows a gunner to stand and use the roof mounted armament.

The turret on the roof can swivel and uses a ratchet system to lock into position. The gun can be rotated vertically and horizontally. The armored hatch can be folded open allowing access to the gun. The gun itself is metal while the ammo can is plastic.

The rear hatch and tail gate can open to reveal the cargo area. There is a large crate inside which serves to hide the battery. There are also spare weapons back there, but they are bolted down to cannot be easily removed. The hatch is held open with spring cartridges.

The tail gate is held shut with pins hanging from real chains. The tail lights include turn signals, markers, brake lights, and reverse lights. On the front are headlights, markers, and turn signals. The extra light on the left side is apparently an infrared emitter on the real vehicle.

Additional details include windshield wipers, lift points, side mirrors, and a snorkel. The model has as much detail as you could wish for.

The body can be separated in one piece by removing only 6 screws. Out of the box, the model is entirely desert tan including the chassis. I found this a bit monotonous and, based on the reference photos I could find, the chassis should actually be black. I knew that if I painted it black while assembled I would miss a lot of things and some things would get painted that shouldn't. Even though this model is RTR, it comes with a full construction manual and therefore it is a simple matter to tear it down as far as desired to paint and rebuild. I took it substantially apart and used flat black. The first picture on the left shows the chassis as it arrived, but with all the electronics removed. On the right you can see the frame with the paint completed.

The front and rear differentials are identical. I did not open them up, but the instructions reveal that they are open planetary type. Both gears and housing are metal and full ball bearings are used throughout, and the assemblies are very heavy. There's an access port that allows insertion of a locking pin if desired. This greatly improves off road performance but limits turning radius. The picture on the right shows the differential housings installed on the chassis frame.

The next thing I worked on was the transmission. I opened it up to check for proper lubrication but otherwise did not rebuild it. I did find several loose screws stuck to the motor magnets and interfering with the rotor, so I'm glad I took this thing apart. The transmission uses a first stage planetary reducer with a driven sun gear, a fixed ring gear, and the planet carrier as output. I was surprised to see that the reducer was upstream of the shifting mechanism rather than downstream which forces the shifter to carry much more torque. The central shaft uses a single shift fork to switch between high and low gears. The transfer case is integral to the transmission with separate front and rear output shafts. The resulting transmission and motor assembly make up the heaviest part of the model. The instruction manual refers to "batholith parts" when showing the gears, but I haven't been able to come up with any useful explanation for that translation. The right hand picture shows the bottom of the chassis with the transmission and drive shafts installed. The drive shafts are heavy duty steel units. There's really no need for them to telescope since the differentials are fixed to the frame, but some of them have that capability anyway. The rear driveshaft also has a mid-span bearing support. The rear drive shaft has a rather extreme u-joint angle.

According to the manual, low gear is 28:1 while high gear is ~15:1 so there is an almost 2x speed difference between gears. The pictures above from the manual show the torque path from the motor, through the planetary reduction, into the primary shaft, and then into the forward and rear output shafts in parallel.

The exhaust is made up of real bent metal tubing. It snakes around the chassis quite convincingly. The exhaust exit is low suggesting this model does not have the deep water fording kit which uses a high exhaust stack exiting above the roof. On the other hand, this model does have a snorkel which implies it does have the deep water kit. I guess that's a consistency error.

Here's the rear suspension which uses a cross brace and double wishbones. The axles are dogbones on the outboard end and CVDs on the inboard end. The shocks are nice metal spring cartridges but they are not oil filled. The hubs are portal hubs which contain internal gearing to increase the gear ratio and raise the level of the differential housing further above the axle for more ground clearance. All of the suspension parts added in this step are metal, either cast or machined.

The front suspension is similar to the rear but with the obvious addition of steering. The steering is a dual bellcrank type with an idler arm and Pitman arm. The Pitman arm attaches directly to the 15kg-cm steering servo. The cranks are among the very few plastic mechanical parts. The steering geometry features mild Ackerman correction.

The wheels are metal beadlocks with realistic rubber tires and internal foams. The wheel cover goes over the top of the nut, meaning that you have to remove all 8 screws securing the cover before attempting to remove the wheel. It looks really good though, appearing to be 8 lug nuts. Installation of the wheels and tires completes the rolling chassis.

The electronics that come with the model aren't bad. There is a 15kg-cm metal gear waterproof steering servo and a mini shift servo. The brushed ESC is claimed to be good for 160A though I can't imagine that can possibly be true. The manual says to only use 7.4V (2s) but I didn't read that until I'd already run the model on 3s so I can confirm it works fine. The model comes with a 16 channel transmitter and receiver, but only 3 channels are in use out of the box. Even though I am not fond of twin stick transmitters, I was tempted to keep using it to make use of many channels worth of upgrades. However, the transmitter is not programmable so I couldn't set end points or exponential. In the end I used a 5 channel receiver for my usual Spektrum radio. This gives the throttle, steering, transmission, lights, and horn. (The 6-channel receiver shown in the pictures was later replaced.)

I thought the paint used on the model was a bit too glossy so I masked the windows and went over the whole thing with flat clear. I think this gives it a bit more weathered texture. I also added the decals which were quite difficult to apply over the ridged paneling on the doors.

The final model with my updated paints and stickers. It doesn't look all that different at first glance, but if nothing else I had a lot of fun rebuilding it.

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