Speedy's GF01 Heavy Dump Truck Build Thread

[speedy_w_beans]

When I finally saw pictures of this kit it really put a big smile on my face and spoke to me on many levels. It revived some childhood memories of playing in the sandbox with construction toys during the summer months. It felt like another potential classic Tamiya in the same vein as the Lunch Box -- exploit its wheelie nature to the fullest, or customize it with small details to your heart's content. I imagined putting a hot motor in it and impressing all the little kids at the beach with it. It felt like it could be a fun homage to the original Mammoth Dump Truck, too. And the timing was perfect -- I was just finishing a much more complicated TB EVO 6 and was looking for something easy to relax with. I guess between the memories, tie-in with other models, degrees of freedom to customize it, and relaxing simplicity of the build, I really wanted one of these badly.

So my kit arrived about a week ago. Normally I take pictures of the box and all the parts trees spread out, but I was so excited to get this kit I tore into it right away. Immediately I went through the manual, dug through my leftover parts box, and placed a few followup orders to Tower to get some hardware and parts to personalize my build.

The first envelope of hardware showed up yesterday, so today I went through the stock kit and started to lay out the parts. Here's everything except the cab and bed of the truck:

More to come soon...

[berman]

Sweet! I hope to get one, one day! I love everything about this wacky thing I can't wait to see your build, even if you built it stock I would still enjoy it immensely.

Popcorn at the ready

[Dangerseeker]

Looks awesome! I thought seriously about buying one not too long ago but finally opted for a Mad Bull, because I believe the Bull is also fun to drive and might survive longer with 2 kids around...

So I am excited how your build develops!

[ChrisRx718]

I want to build one and put a Lunchbox body on it, would look ace all in yellow (in my head at least!)

Not sure the body would be worth anything on its own though...

[speedy_w_beans]

Yesterday I laid out all the original kit parts; today I swapped several of them for upgrades.

Some of the bigger changes in the parts include:

- Hex drive machine screws -- not a fan of self-tapping screws

- Ball bearings -- the standard upgrade from bushings

- Steering turnbuckles -- makes it easier to center the steering and adjust toe

- Lower hinge pin screws with locknuts -- not a fan of self-tapping screw pins

- Adjustable upper camber links -- for setting camber

- CVA dampers -- takes some of the bounciness out of the chassis

The dampers were really a great stroke of luck. I had bought a set of old stock XC01 dampers from TamiyaUSA several years ago for about $5; they've been sitting in my parts stash all this time waiting for a build. It turns out the GF01 kit shocks and the XC01/CC01 CVAs are almost exactly the same length with the same stroke, and the GF01 kit springs fit them fine.

I have some slightly different lower hinge pins coming that should fit a little better, but I think I have enough to get started with this build in the next day or two.

[speedy_w_beans]

OK, time to build some diffs... These are very straightforward. Both of them are identical.

Bevel gears, shims, and spider gear shafts lubed with anti-wear grease...

Bevel gears dropped into place and teeth lubed with more anti-wear grease...

Spider gears dropped in and diff halves screwed together for both units...

[speedy_w_beans]

Next up are all the counter gears.

This is super-easy. Just press the bearings into the gears and slide the shafts into place.

All the gears are lubed with ceramic grease. I used more than necessary, but I don't plan to open the gearbox ever again.

Bearings pressed into the chassis halves, antenna pipe holder installed, and gears/diffs dropped into place.

Chassis halves assembled and outdrives installed so I can exercise the gears and get the grease to spread. The counter gears took a few revolutions to loosen up a little more. The diffs feel pretty stiff just twisting the outdrives, but from past experience I know they'll loosen up with running as the anti-wear grease spreads inside the diffs.

That's all for now. This opened up quite a bit of space on my work mat.

[speedy_w_beans]

I had a decent block of time to work on this tonight, and I made some good progress. First up are the dampers. As mentioned earlier, this set was a TamiyaUSA closeout deal I grabbed about five years ago and had been holding. They're the same length as the stock kit spring holders.

The pistons and shafts are one-piece units; the pistons have no holes. Frankly, they remind me of roofing nails with threaded ends. The fit between the pistons and the CVA bodies is relatively loose. Just for the heck of it I thought I would try some 3000 cst oil to see if I could slow these dampers down since the kit springs are pretty stiff and the model overall is pretty light. Normally Tamiya supplies 400 cst oil with their CVA dampers in most of their kits; I don't recommend 3000 cst for general use as the dampers will definitely be too stiff for bashing around. I'm going in a different direction with this build where I want the bounciness of the chassis totally eliminated and I want the chassis to react more slowly to terrain changes.

Prelubing the shafts and O-rings so everything slips together easily without snagging...

Slipping the O-rings over the threads outside the CVA bodies... Again, this reduces the change of slicing an O-ring and getting a small leak.

[speedy_w_beans]

Bottom caps installed.

Here I've filled the CVAs with 3000 cst oil and pumped the pistons a few times to get the air out from under them. There were still quite a few small air bubbles in the oil, so I used some wheels to hold the dampers upright while giving the bubbles a chance to escape. I let them sit like this for a half hour.

With the air bubbles gone, on go the bladders...

Then on go the top caps...

[speedy_w_beans]

Lower eyelets, spring retainers, springs, and ride height collars installed; the dampers are now finished.

Next up is installing the ball connectors for the dampers.

Ball connectors installed on the front of the chassis. There's another set installed at the rear as well.

Since the other ball connectors are installed on the suspension arms, now was as good a time as any to put the arms together. The rightmost arm was assembled incorrectly with the lines on the parts not matching; this led to some binding later as I was putting the front end together. Removing the screws, flipping one half over, and reassembling fixed the issue. If you build a GF01 and find one of the uprights is binding in the suspension arm, check to make sure the arm is assembled correctly.

Arms assembled. Notice the misaligned line in the rightmost arm.

[speedy_w_beans]

Time to build up the rear end of the chassis...

The rear uprights are easy. Press in the bearings, slide in the shaft and cross pin, press on the wheel hex, and install a ball connector.

On the chassis I pressed in some flange tubes so the screw and ball connector would not stress the plastic flange. The flange tubes are from a 50520 CVA kit I bought five years ago; these were leftover parts. The same tubes are used both front and rear.

Screw and ball connector installed...

[speedy_w_beans]

The kit upper camber links are replaced with 5mm adjusters; these are pretty common in many Tamiya kits. I had enough spares along with the right set screws to make some adjustable links. To do this I had to trim 2mm from the ends of the adjusters.

Finished camber links...

All the major suspension parts are now assembled and ready to be installed on the chassis.

Rear suspension and drive shafts installed. I have some hinge pin screws temporarily installed loose while I wait for some better ones to arrive.

Rear dampers installed.

[speedy_w_beans]

Now for the front end...

The front steering knuckles are similar to the rear uprights -- press in the bearings, slide in the shafts and cross pins, press on the wheel hexes, and install ball connectors.

Kingpins and ball connectors installed in the front uprights...

The front has the same setup as the rear with respect to ball connectors and flange tubes...

[speedy_w_beans]

Here are the front camber links. I trimmed 4mm from the ends of the adjusters. If I were to do this again I think I would just trim 3mm from each adjuster.

All the major front end parts are now assembled and ready to be installed on the chassis.

Front suspension installed with temporary loose-fitting hinge pins. This is where I had to reassemble the one front suspension arm to eliminate the binding with a front upright.

Front dampers installed.

Here's the chassis where it stands as of now.

[speedy_w_beans]

After getting the chassis in the state shown above, I was eager to see if the direction I wanted to take with this build would actually work out. Plan A was to turn the Heavy Dump Truck into a Mini Mammoth; plan B was to use the kit wheels and tires and just make an overpowered beach basher.

I'm really pleased that Plan A, the Mini Mammoth, looks entirely feasible. The picture below is just a quick capture of throwing some parts on the chassis and checking fitment.

Ignore the tire tread pattern; these will be replaced with tires that more closely resemble construction equipment tires.

Ignore the white wheels; they will be painted with Camel Yellow to match the body and chassis.

Ignore the sprues still attached to the dump bed and the main cab; I just pressed the body posts into place and temporarily dropped the body parts on the posts to judge proportions.

Ignore the damper oil bottle; it's there to hold up the dump bed and keep the cab level.

Yes, there is adequate suspension travel and there is still full steering even with the short course truck wheels.

With the right tires, a little paint, some 3D printed detail parts, some scale handrails, some LED lighting, and attention to more details I think this will come together quite nicely as a Mini Mammoth.

[XV Pilot]

A Mini Mammoth sounds like a cracking idea! Looking forward to seeing how it goes together.

[speedy_w_beans]

Some new tires arrived while I was out of town, so tonight I tried a few things:

- Replaced the AKA Wishbone tires with HPI Yokohama Geolander tires for a blockier tread pattern.

- Replaced the D4 wheel hexes with B14 wheel hexes to tuck the wheels in a little more.

- Dropped the front cab and the rear bed by three positions on the body posts to reduce the gap between the tires and the body parts.

- Separated the body parts from the sprues -- I left some nubs in place to cleanly finish later.

[Mannyp1980]

This is very cool, will be following to see what else it gonna happen.

[Nobbi1977]

It looks great with those wheels, suddenly less comic and more real

[speedy_w_beans]

Not too much excitement tonight... Just a few basic things.

The bumper parts and wheelie bar won't be used, so they're going in the kit box for storage.

The steering parts, battery strap, and body posts will be used however.

The steering assembly is simple; just press in the bearings, build the links, and pop them onto the ball connectors.

The steering assembly is held in place with a single shaft; the shaft is held in place with a screw. The equal-length links pop onto the ball connectors on the knuckles. I don't see any evidence of bump steer while exercising the suspension; I'm happy about that. The battery strap just slips into place and is held down with two clips. The body posts are held in place with screws.

[speedy_w_beans]

I've been thinking about a motor to move this model around, and while the kit's torque tuned motor is fine for higher-speed bashing, it's difficult to achieve scale speed with it.

Initially I looked at adding a gear reduction unit from HPI or RC4WD, but the HPI unit interferes with the chassis and the RC4WD unit doesn't inspire confidence -- their own web page talks about how it won't last a long time with higher-powered motors, and the extra length (about 18 mm) makes the motor can stick out even farther. This is not good with the wheels and tires tucked in so close to the chassis already.

I also thought about sourcing an outrunner brushless motor as some of these are available with lower kV ratings. A stock silver can is about 2000 kV (about 14500 rpm at 7.2 V), so a brushless motor that has a rating of 500 kV or less could be a good candidate for slowing down the top speed. At the same time, I know from past experience that an outrunner's notchiness can induce some extra drivetrain noise. I searched quite a bit and couldn't find the right combination of mounting flange, shaft diameter, shaft length, kV rating, and can diameter to go any further with this thought.

The motor I'm going to try is a small brushed motor with a gear reduction drivetrain already included:

The motor is rated at 20,000 rpm at 6 V, and the gear reduction is 30:1. Max power output is 1 watt. This seems like a ridiculously small motor to move this chassis around, but I'm not trying to do speed runs or jump off ramps. Instead, I'm putting a high value on the inherent smoothness of a brushed motor and the gear reduction's ability to limit top speed and multiply torque. When I crunch the numbers (24000 rpm at 7.2 V, 60 rpm for every 1 rev/second, 30:1 gear reduction, GF01 18.03:1 internal ratio, 115 mm tire diameter) I come up with a top speed of about 267 mm/second, or about 10.5 inches/second top speed. Tamiya markets this model as a 1/24 kit, so full scale speed is about 21 feet/second, 14.3 mph, or 23 km/hour. If we're comparing the model to a full-size Caterpillar 797F, on the other hand, the scale is more like 1/41. In that case the scale speed is 36 feet/second, 24.5 mph, or 39 km/hour. Top speed for the real truck is closer to 40 mph, but typically there are speed limits in mines for safety. It's a tiny motor, but its output is divided by 541 (30*18.03), so it's worth a try. I ordered a few today and they should be here in a few days.

[Grastens]

Can you not put in a 50T+ motor to slow it down? For scale speed at that scale, even a motor with 80T might look good... I only ask because I heard about putting in high-turn motors for first RC cars, to make them slower and thus easier to handle.

[IBIFTKH]

Looks like you're having fun with this one!

Reading your comments about eliminating the bounciness from the chassis, I think you may find the kit springs too stiff. The 53163 on-road/rally spring set are about the right length (the blue springs are a little short). Even the stiffest springs are softer than the kit ones. I've used these on several of my WR-02s and GF-01s and they really make a difference, allowing the suspension to work more realistically, especially at lower speeds. You'll need two sets if you want the same spring rates at both ends.

[Mannyp1980]

I have the same GF01 chassis, I ran it with the stock 27T brushed motor and tires and it is a lot of fun. I have now put on some 1.9 wheels and crawler tires and a 55T brushed motor. The speed is nice but not too slow. You should consider the 55T or 60T motor, maybe even higher. I'm running it off the stock Tamiya ESC without any problems.

[speedy_w_beans]

Reading your comments about eliminating the bounciness from the chassis, I think you may find the kit springs too stiff. The 53163 on-road/rally spring set are about the right length (the blue springs are a little short). Even the stiffest springs are softer than the kit ones. I've used these on several of my WR-02s and GF-01s and they really make a difference, allowing the suspension to work more realistically, especially at lower speeds. You'll need two sets if you want the same spring rates at both ends.

Thanks for the tip! I looked these up and I think I have two sets of 53440 in my stash right now. 53163 has three pairs of springs (red/yellow/blue), and 53440 has four pairs of springs (red/yellow/blue/white). I think I'll do some followup work and check spring rates with these options. I was also thinking about adding a half kilogram of mass to the chassis, and the tires currently have no foams in them so the sidewalls bulge just a little bit under load.

Can you not put in a 50T+ motor to slow it down? For scale speed at that scale, even a motor with 80T might look good... I only ask because I heard about putting in high-turn motors for first RC cars, to make them slower and thus easier to handle.

I have the same GF01 chassis, I ran it with the stock 27T brushed motor and tires and it is a lot of fun. I have now put on some 1.9 wheels and crawler tires and a 55T brushed motor. The speed is nice but not too slow. You should consider the 55T or 60T motor, maybe even higher. I'm running it off the stock Tamiya ESC without any problems.

Both of you are 100% correct; I forgot to mention looking at a few 85T motors. From the specs I could find they were rated at about 8000 rpm at 7.2 V; this is about half the speed of a silver can. Another factor to consider is I want to add (fake) hydraulic cylinders to lift the bed of the truck later in the build. Ideally there would be a very flat pancake motor with low running speed and super-smooth startup to take the place of the kit motor; unfortunately I haven't found anything like that just yet.