ZipZap Monster Truck Gearbox

draft version, July 19, 2005

Introduction

This article provides beginning instructions for modifying a ZipZap Monster Truck gearbox.  At the risk of stating the obvious, a standard MT comes with three gears--the motor pinion, a transfer, and an axle gear.  This tutorial describes two gearbox modifications by adding one and two gears.  These modifications allow for various gear ratios to provide a desired balance between speed and torque.        The interest in this modification was sparked by a monster truck articulation thread in a ZipZap on-line forum posted by "diets".  Several of us--Steve and "zzmods"--all simultaneously took interest in the gearbox modification.  Those two added the second gear in their gearboxes, as did I.  The following tutorial describes both of the modifications.        Tables for specific gear arrangement are included to assist in determining the resulting final gear ratios.

Changing the Pinion Gear
Perhaps the easiest modification to the MT gear box is to change the MT motor's white pinion gear to a black pinion gear from a ZipZap/ZipZap SE motor.  While not a necessary modification, the change is from a 7-tooth pinion to a 6-tooth pinion, altering the final gear ratio to 28:1.  The change requires no chassis modification.  The black 6-tooth pinion's hole should be enlarged to 0.038-in dia to accomodate the MT motor shaft.  I used a no. 62 drill to accomplish the task.  Other "drills" such as straight pins and paper clips have been used to adequately enlarge the mounting hole.      When the hole is enlarged, simply press fit the pinion gear onto the motor shaft.	Gear Ratios for a Stock MT   gear chart for this modification original 7-tooth pinion gear ZZ/ZZ SE 6-tooth pinion gear first gear set ratio 32 / 7 = 4.57 32 / 6 = 5.33 second gear set ratio 42 / 8 = 5.25 42 / 8 = 5.25 final gear set ratio 4.57 x 5.25 = 23.99:1 5.33 x 5.25 = 27.98:1
How We Did It        "The pinion was drilled out with a paper clip."  -- jakhammer, April 29, 2005

A Four-Gear Modification
The first step in adding the transfer gear is to raise the motor up to ensure its pinion gear clears the existing MT transfer gear.  Simply remove the motor clip and extract the motor from the motor well.  Insert into the motor well a small non-metallic spacer to lift the motor vertically.  I used a piece of styrene, but other materials, such as cardboard, should work just as well--it just needs to be an insulating spacer.  Note that you'll likely need to enlarge the hole on the clear PCB cover to accept the motor-clip tab now that the motor is positioned higher.  Cut out about as much as the thickness of the spacer used to lift the motor.  Then turn the motor over so that the "up" label is facing down and replace the motor in the motor well--we're adding one additional gear so we'll have a change in the rotation direction.  Fasten it down with the motor clip, making sure the pinion gear does not engage the MT's transfer gear.        The next step is to position and fasten the new transfer gear to the chassis.  Pick a ZipZap/ZipZap SE transfer gear, any gear (they're all the same diameter). With the motor fastened in and the MT's original transfer gear in place, flip over the ZZ/ZZ SE transfer gear so the large diameter part is facing out, and lay it into place.  When you're satisfied the gears mesh properly, drill a small hole in the chassis, using the ZZ/ZZ SE transfer gear as your drilling guide.  I've heard some have secured the new gear to the chassis with a small pin or nail, press-fit into the chassis.  I tapped mine for a 00-90 machine screw (0.038-in. dia., no. 62 drill) and used a 0.250-in long hex-head screw.  Select the desired ZZ/ZZ SE gear for your gearbox (see the table on the right) and attach it to the chassis with the method you've chosen.        Once I was sure gears meshed and worked properly, I removed the motor and placed a very small dab of Loctite Threadlocker Blue on the end of the machine screw which protruded into to motor well.  The Loctite formed a small, but breakable flange on the thread, preventing it from unscrewing as the gear turns.  Reassemble your MT and you should be ready to crawl.	Gear Ratios for Four Gears (two transfer gears)   gear chart for this modification original 7-tooth pinion gear ZZ/ZZ SE 6-tooth pinion gear blue gear from ZZ/ZZ SE first gear set ratio 18 / 7 = 2.57 18 / 6 = 3.00 second gear set ratio 32 / 6 = 5.33 third gear set ratio 42 / 8 = 5.25 final gear set ratio 2.57 x 5.33 x 5.25 = 71.92:1 3.00 x 5.33 x 5.25 = 83.95:1 green gear from ZZ/ZZ SE first gear set ratio 18 / 7 = 2.57 18 / 6 = 3.00 second gear set ratio 32 / 7 = 4.57 third gear set ratio 42 / 8 = 5.25 final gear set ratio 2.57 x 4.57 x 5.25 = 61.66:1 3.00 x 4.57 x 5.25 = 71.98:1 red gear from ZZ/ZZ SE first gear set ratio 18 / 7 = 2.57 18 / 6 = 3.00 second gear set ratio 32 / 8 = 4.00 third gear set ratio 42 / 8 = 5.25 final gear set ratio 2.57 x 4.00 x 5.25 = 53.97:1 3.00 x 4.00 x 5.25 = 63.00:1
How We Did It -- Adding the Blue Gear        "I used an 18-gauge finish nail . . . to drill out the blue transfer gear from the SE.  Once I got the SE gear onto the nail, I ran the gear to the head of the nail and spun it for quite a while.   I did this to take extra plastic out of the gear hole.  The head of the nail has ridges to help hold it into your wall, board, etc.  Once the gear spun nice and free I went to the truck."  -- jakhammer        "I'd like to mention that the small screw used to hold the LED lighting board to the body . . . is a wonderful size for mounting an idler when doing the crawler mod; ream the idler's hole slightly, and countersink it just a hair."  -- frizzen, July 17, 2005        "Once I had everything set and was happy, I put a dab of CA/super glue on the nail and slid it in.  Once it dried the nail was not going anywhere unless I use CA dissolver on it."  -- jakhammer
How We Did It -- The Motor Spacer        "For the motor spacer, I just used an old post it note.  I tore a small section off and folded it up to make it thick.  I put that into the bottom of the motor well.  I put the motor in and saw I needed it to move backwards in the well some.  I folded up (more of) the paper till I had the motor where it needed to be for good mesh."  -- jakhammer
How We Did It -- The Motor Clip        ". . . I tested the motor clip.  It fit in the stock location.  I cannot tighten it all the way since the motor is up and back, but it works fine in the stock location for me."  -- jakhammer

A Five-Gear Modification
This part of the tutorial describes a five-gear modification to a ZZ MT.  Using a black pinion gear and a blue transfer gear from a ZZ car, and two green transfer gears and a green axle gear from a ZZ MT, the new gearbox has a 336:1 ratio.  Computations for both 6- and 7-tooth pinion gears are shown in the table on the right.        Construction in this modification begins with adding, in order, the second transfer gear then the first transfer gear.  Finally, the motor connections are modified, the motor well given a "soft" spacer, and a new motor clip/retainer built.	Gear Ratios for Five Gears (three transfer gears)   gear chart for this modification original 7-tooth pinion gear ZZ/ZZ SE 6-tooth pinion gear first gear set ratio 32 / 7 = 4.57 32 / 6 = 5.33 second gear set ratio 18 / 8 = 2.25 third gear set ratio 32 / 6 = 5.33 fourth gear set ratio 42 / 8 = 5.25 final gear set ratio 4.57 x 2.25 x 5.33 x 5.25 = 288:1 5.33 x 2.25 x 5.33 x 5.25 = 366:1
How We Did It --        "You need to add an extra idler gear (small) and another big drive gear.  You'll have to add two more little gear shafts to support those, and you have to move the motor up so it will make contact with the new drive gear.  The purpose of the little gear is to separate the two big gears.      ". . . I put some thick plastic pieces in the space where the motor usually goes, to thicken the wall that the gears will hang on.  That gives you something to drill into and a place to put the added gear shafts."  -- Steve, April 28, 2005
Adding the Second Transfer Gear
For the second transfer gear I used a ZZ/ZZ SE blue gear.  The gear shaft is one from a scrapped ZZ car.        With the original green transfer gear in place, put the blue gear on the shaft with the 6-tooth side facing the chassis.  Mesh it with the original green transfer gear and mark the axle's location.  Hold the blue gear in place, using it as a drilling guide, and drill a 0.038-in. dia. hole in the chassis.  Remove the gear and insert the gear shaft from the outside, pressing it into the hole until just emerges at the opposite side in the motor well.  Holding the gear shaft perpendicular to the chassis, place a drop of CA adhesive on the end of the shaft inside the motor well.  When the adhesive has set, place the blue gear on its shaft and cut the shaft to length.  Remove the gear and then remove the burrs from the cut end with a file.        Place the blue gear on the shaft 6-tooth-side down.  The gear will be held in place by the first transfer gear, so there is no need to hold it captive with a screw.
How We Did It --
Adding the First Transfer Gear
For the first transfer gear another MT green transfer gear is used to mesh with the motor pinion and blue second transfer gears.        Finding the location for the first transfer gear is much the same as was for the second transfer gear.  With the green gear meshed with the blue gear, I placed an 0-80 hex-head machine screw in a hex driver, inserted it into the gear and marked the location for the shaft hole.  Drill (no. 55 drill) and tap for an 0-80 thread.        Place a no. 2 flat washer as a spacer over the tapped hole, add a MT green gear, and with an 0-80 x 0.025-in. machine screw, fasten the transfer gear to the chassis.  Spin the gear to test the gear mesh throughout the entire gear box while tightening the green gear's screw so that the best mesh is obtained.  Once the motor is in place, further adjustment might be necessary.
To keep the green transfer gear from spinning loose during rotation, I have added a small dab of Loctite ThreadLocker Blue to the end of the 0-80 machine screw which protrudes into the motor well.  The Loctite acts as a nut on the machine screw, permitting some adjustment while still holding the thread securely.
How We Did It --
Modifying the Motor
Using a green MT motor, I adapted the electrical connections such that the motor terminals would contact the existing chassis contacts.  Cutting two roughly equal-sized pieces of brass stock, I soldered to each a short piece of small-diameter copper wire, tinning both the wire and the small brass piece before soldering them together.  Note the direction in which the other end of the wires were soldered to the motor, allowing them to be bent over to the necessary length.  Carefully tin the motor contacts and the ends of the wires, then, with the "up" side of the motor facing you, place the wires so the brass contacts are also facing you and quickly solder the wires to the motor contacts.  Bend the wires over as shown so the brass contacts will press against the silver contacts in the chassis.        Note that the motor must be in its "up" position for this gear arrangement.
How We Did It --        "Oh, and you'll have to hard-wire the motor to the contacts.  You can give yourself plenty of wire length to make soldering easier, and just hide the excess in the motor cavity."  -- Steve
Motor Well "Soft" Spacer
Because the green, first transfer gear sits so high, the motor must be raised upward so its pinion gear properly meshes with the transfer gear.  While other spacer methods could be more simple, effective, or available, I chose a piece of foam cut to the size of the motor well.        Press the spacer into the motor well, then lay the motor atop the spacer, carefully pressing the motor's electrical contacts down and against the silver contacts in the motor well.
How We Did It -- Filling the Motor Well        "As for mounting the motor in it's new location, I filled the cavity with foam pieces . . . ."  -- Steve
Motor Retainer
By trial-and-error, I constructed a new motor clip/retainer from 0.5-in. wide by 0.032-in. thick styrene.  The shape of the retainer has a slight acute angle, making the retainer look somewhat like the number seven.  I found it necessary to move the motor forward as far as it would go so that a good gear mesh could be accomplished.        The retainer inserts between the chassis and spacer in the back and fastens to the clear PCB cover on the top.  Two 0-80 machine screws hold the retainer in place.        With the motor and retainer in place and making sure there is good gear mesh, drill an 0.052-in. dia. (no. 55 drill) hole through the back of the chassis just below the body mounting tab and through the retainer.  Tap the two holes for an 0-80 thread then attach the back of the retainer inside the chassis with an 0-80 x 0.250-in. machine screw.  Do not tighten the screw completely.        Again making sure there is good gear mesh, drill an 0.052-in. dia. (no. 55 drill) hole through the top of the retainer and into the clear PCB cover.  Be careful not to drill into the PCB or any of the electronic components!  Tap the two holes for an 0-80 thread then attach the top of the retainer to the chassis with an 0-80 x 0.025-in. machine screw.  Use this screw to adjust the degree of mesh between the motor pinion gear and the first green transfer gear.  Once a good mesh has been made, tighten the screw holding the back of the retainer in the motor well.
How We Did It -- The Motor Retainer        "I . . . hot-glued the motor on top of (the foam pieces)."  -- Steve
Finally, test the gear mesh with the MT running.  Charge the battery, remove the MT from the charger, and while holding the MT, run it in both directions, looking and listening for poor gear mesh.  The screw in the first (green) transfer gear might need adjustment, as might the motor retainer screws.  In the picture above left, note that the first transfer gear appears "warped" and needs to be adjusted outward, perhaps requiring another washer/spacer, so that the gears will spin freely and not bind.
Fitting the Body
Because of the size and location of the first transfer gear, the body might not fit correctly.  I have filed a small notch on the inside of the rear bumper to clear the gear.  Other bodies might require more-drastic methods.
How We Did It --        My thanks to all others who, through forum threads, have contributed their ideas for this modification.  If you have helpful suggestions in regard to this modification, please send an e-mail.