10/11/13- I almost forgot about this because it's one of those ongoing things... as in, an ongoing source of dissatisfaction! The drive system worked well enough to test stuff, play, and shoot videos, but I wasn't satisfied with the way the tank drove. That was true out of the box as a Heng Long King Tiger, and it didn't change much by the time I'd put in the Benedini board. Basically, it drove noticeably poorer than any of my other tanks. The tracks made too much noise at the sprockets and idlers, occasionally hicupped as it jammed at the idlers, and it didn't turn as well as I thought it should. At least the tracks didn't come off, but I did have to adjust them pretty tight to keep the sprocket teeth biting into the tracks.
Maybe I've just been lucky or have a bad memory, but I don't recall ever having had as many problems with this before. It's been a thorny aggravation with so many possible culprits: The alignment and side-to-side positioning of the idler wheels, road wheels and sprockets. It could be the quality of the components themselves: I wondered whether the tracks and sprockets were well-matched for a good mesh. I found flash in some sprocket holes in the tracks, which probably caused some of the problems, but fixing this didn't magically fix everything like I'd hoped it would.
Henntec Idler System: This was the first improvement I threw at the problem. The Heng Long idler wheels weren't adjustable and were simply screwed into the hull. The Henntec idler system connects both sides with a brass bar across the hull, which helps keep them aligned parallel with the hull, eliminating the "knock-kneed" problem.
The Henntec idler arms are adjustable in that they can be tightened in place at any angle/position by grub screws. Although they aren't spring-loaded, this lets you fine-tune the track tension after you've settled on the length of your tracks: Push 'em into position and tighten 'em down. I replaced the grub screws with shortened hex-head screws (included) so I could torque them tighter (they slipped loose fairly regularly). Unfortunately, the tracks get in the way of easy access to the screws.
The idler wheels were also attached with grub screws, but the problem was that the Heng Long axles were screw-thread ended. This made it difficult to set the axles perpendicular with any confidence since it depended on where the grub screw bit into the coarse screw thread. Before tightening the grub screw, there was a considerable amount of play in the fit of the idler axle in the idler arm. Ideally, a well-fitting solid axle should have been provided, but of course you can't easily find stuff like that nowadays.
The Henntec idler system did improve the performance; there was less noise when driving forward, but I still had occasional misalignments of the track tabs when driving in reverse. I tried adding washers, but thought it made the problem worse.
Mato 3:1 Gearboxes: These were an unlikely suspect in the search for a culprit because they installed so effortlessly: All the screws and hull mounting nubs mated perfectly with the steel mounting plate. The gearboxes mounted perfectly on the plate, and the axles seemed to align right where they should have. The only thing I wondered about was whether the steel plate might have allowed flex between the gearboxes since they weren't supported across the top. I put a brass tube across the top to eliminate that possibility.
Despite the brass gears, the Mato gearboxes look pretty beefy, and grub screws in some gears hint that they may have been designed for some customization (although I've never seen alternate gears being sold for them). Likewise, the motor's (large) pinion is secured with a grub screw, which suggests potential for customization... although the motor mount isn't cut with slots for adjusting the distance to the gear. Also, I've read that the Mato gears have an unusual pitch that wouldn't work well with just any old pinion. Likewise, I've never seen a set of Mato replacement pinions for sale anywhere.
The sprocket drive axle gears are mounted with grub screws that seemed to loosen too easily (even with lock-tite), which caused a bit of play between the sprocket and gears engaging. The grub screws poorly fit my allen wrenches, both SAE and metric: It wasn't due to ruining them by using a wrong-sized or worn wrench. I believe they're just poorly manufactured. Surprisingly, this isn't all that uncommon: When removing the hex screws from the Heng Long road wheels, some screws fit perfectly in my allen wrench, but a few fit very badly. There's no good reason why that should happen if parts are manufactured to uniform specifications. Furthermore, it's hard to believe that they used such shoddy parts in such an important area: If the screws can't tighten the gear to the axle, then the gearboxes and tank become a paperweight. Fortunately, I got them out and found some Kyosho grub screws that fit the gears and an allen wrench perfectly. Nevertheless, it does raise a red flag about the construction quality. The gearboxes look beefy and well-constructed, but I think it's just skin deep.
Although advertised as 3:1 gear reduction, they're still rather speedy for a Jagdtiger. For folks who prefer to sprint, this may be a good thing; I prefer less speed, more torque. I eventually figured out that this was why the tank couldn't make as sharp turns on carpet or in grass at a slow speed.
The Quest for Sharp Turns: On a smooth surface with one track moving and the other track stopped, turns are sharp and easy. On carpet or in grass, it's a different story. This is true for my skinny-tracked Pzkpfw IV as well, but the effect is more pronounced with the Jagdtiger's wide, grippy tracks. Tanks turn by slowing, stopping, or reversing one of the tracks; reversing the direction produces the sharpest turn and lets the tank turn in place ("neutral turn" or "super spin"). With this tank set up for no super spin, the track that's supposed to be stopped doesn't have enough resistance in the geartrain to resist the force of movement from the other track, so the rough surface pushes the grippy track, turning the sprocket back through the gear train, forcing the motor to spin without any power applied. Consequently, both tracks move, resulting in wider turns. This might be remedied by using motors with more torque/heavier magnets, or using a smaller pinion.
Ideally, one track would be braked when turning sharply but the Clark board doesn't do this in "sharp turn" mode: It cuts power to the motor which slows the motor, then stops it. As an alternative to changing motors, pinions, or replacing the gearboxes, you can turn on "super spin". This is enabled by default in the Clark board, but is pretty hard on the drivetrain so I prefer to turn it off. At sharp left and right turns, this runs the motors in opposite directions, making the tank turn in place very quickly. When driving along, straying into this zone can make your tank change directions very abruptly. It looks bad, and like I said, the sudden stress is bad for your drivetrain.
In theory, at the switchover point, the motor power goes to zero and polarity changes, so there should be a zone where only a small amount of reverse voltage is applied-- effectively, a brake. In practice, it's very hard to modulate at this point, and the problem reverses: There's likely too much resistance applied too abruptly. Both motors are receiving power as dictated by the joystick, so it's difficult to control the tank so that it turns as smoothly as it would if the motor were unpowered and its torque were providing resistance. It's complicated by the fact that the motor speed is controlled by the up/down coordinates of the joystick so how fast you're going interacts with how much left/right stick you're applying. Consequently, it's very hard to get smooth, sharp turns this way. You can get jittery sharp turns at slow speed, but not smooth sharp turns.
A programmable transmitter with custom curves helps make this work a little better by lowering the top end of full super-spin and widening the band before it kicks in. Basically, this turns off the full super spin feature, but leaves a slight bit of it to enable sharp turns on carpet and grass. This makes it slightly easier to engage the lower end of super-spin at a slower speed (just before the track goes in reverse so you can turn more sharply), and lops off the full speed direction reversal (super-spin) when the stick is full left/right (so it's less likely to turn abruptly at full speed). It's hard to find just the right curve to make it operate proportionally for all surfaces: I set up a 9-point curve on my Turnigy 9x (-78 -71 -66 -50 0 50 66 71 74). Outdoors, this produces mild proportional control in the lower end of the curve, but produces a fairly abrupt change in turn sharpness between 60 and 73 (I think). At the top end, the reverse is barely engaged, so I suspect that the sharp turning begins as the motor voltage changes from zero to slightly negative-- effectively braking the track. Indoors on tile, this curve makes the tank almost too responsive to right/left joystick motions, so you have to learn how to drive on different surfaces.
Metal vs. Plastic Tracks: Normally, I wouldn't even bother with this question. Metal tracks look better, wear better, and give the tank weight and heft which just feels better. That's purely a psychological effect, but real enough. How the tracks work is an entirely different matter. I'm inclined to be prejudiced in believing that metal tracks work better because they operate more realistically in the way they droop and hang. However, logic says that the drive components (motor, gearbox, etc.) have an easier time moving a lightweight plastic track than a heavier metal one: That's what actually moves the tank, and it's a different thing than how it looks when it moves. If the tank moves well and looks good with metal tracks, then end of story. That's a no-brainer.
I wish that were the case with this tank. Unfortunately, even with the Henntec idler system, I couldn't get it to drive as well as I thought it should. I next suspected that the tracks and sprockets were just not a good match for each other. Naturally, I blamed it on its Heng Longishness. Unfortunately, metal tracks and sprockets aren't cheap, so it's not a theory that I could easily test with a comparison to other brands.
However, I could test the plastic Tamiya tracks that I'd replaced with Impact's metal tracks on my Tamiya King Tiger. In that case, the metal tracks worked fine. In this case, the Tamiya plastic tracks worked noticeably better than the Heng Long metal tracks: Much better turning and drive performance. The sprocket and idler noise was still there but not as noticeable due to the plastic-- I think the slickness of the plastic helped with the track tab/idler wheel alignment problem. The downsides were that the pins are only secured at the ends (FWIW, the Heng Long metal tracks don't have fully enclosed pins), and they do need to be painted. They'll never wear down to bare metal from use, which is one of the cool cosmetic benefits of metal tracks. The tank also shed a lot of weight, which is one of those preference things.
Ultimately, I couldn't use them because the tracks are thicker than the Heng Long metal tracks. It's only a couple of milimeters, but it's enough to cause them to rub against the fenders.
The Torro Lower Hull: While scoping out alternative sprockets and metal tracks (convinced that this was the root of the noisy track problem), I stumbled across the Torro lower hull for their King Tiger. It had an aluminum inner hull, torsion bars, metal road wheel suspension arms, and a nifty adjustable idler system, among many other cool features. In other words, it was a cool upgrade that was hard to resist, and it might (cross fingers) solve the frustrating drive problem. Even though it didn't address the sprockets and tracks problem that I suspected, I was willing to be distracted by its coolness and engage in wishful thinking. Ka-ching!
Indeed, it was as cool as I'd thought it would be. The torsion bars are stiffer and give a much higher stance (although at the cost of the idler being closer to the rear road wheels), which makes it look more imposing. The Heng Long road wheels attached easily to the suspension arms via grub screws-- fortunately, they all fit my allen wrench perfectly, so there were no problems doing the lock-tite & tightening routine. The idler system is spiffier since the idler arm's angles can be adjusted externally from the underside by turning screws: They can't slip out of adjustment.
It came with some other cool stuff that I decided not to use because of the work I'd already done: I kept the Matorro back plate because it fit better with the upper hull, had molded zimmerit, and I'd already fitted it with a light and mudflaps. (The Torro mudflaps plug in via tabs and aren't hinged.) I'm considering transplanting the metal exhausts, armored covers, and jack mounts because they're better detailed. Also, maybe moving the power switch to the rear since that would be easier to access than under a hatch.
The Torro hull came with a nifty quick-release mechanism that's actuated by a small pin underneath the hull (more convenient than the difficult-to-access side latch on my Asiatam Tiger hull). Unfortunately, it takes up more space than my magnetic mount-- space that's needed for my speaker --so the magnetic mount stays.
The gearbox mounting plate is drilled to fit stock Heng Long gearboxes, but not the Mato gearboxes. Oddly enough, both mounting plates fit Heng Long stuff, but don't fit each other. It was a relatively easy fix though: The Mato plate was drilled with two extra holes to mount to the Torro hull. It does make me wonder about the relationship between Matorro, Mato, and Torro...(?)
The front end is fitted with metal sprocket drive supports. I left them, but I'm not sold on their necessity or superiority. There's a part of me that believes that metal is better, but the analytical part of me questions whether it's appropriate where it's used. If strength isn't necessary, then plastic works just as well, is usually better detailed, easier to work with, and holds paint better. Although paint wears off metal, it doesn't necessarily do it in a scaled, controlled way. The result can look like a toy part that has suffered from inadvertent paint wear rather than simulated, properly scaled wear from weathering. In my opinion, that seems not to be an issue with tracks, which look better when certain areas are worn down to bare metal.
Another round of zimmerit for the front.
I kept the original armored exhaust covers to avoid messing with fitting the metal ones (they're smaller), but stuck lifting nails in the sides. (The backs should have a rectangular plate.) The mudflaps were trimmed with the angle cut at the edges, and the missing latch was added.
So... Did it Solve the Drive Problem? Uhhhhh... no, not exactly! (And boy was I shocked!) However, it did spur me on to tackle those damned idler wheels again. The first thing I did was grind down flat spots on the idler axles so that the grub screws held them without wobbling. Then I ground down a bit of the flange that protruded from the inside of the idler wheel so it could be mounted closer to the arm. That helped on one side. Inexplicably, the other side needed washers. By the time I'd finished, I'd eliminated the idler noise driving forwards and backwards. The sprockets still seem to click every so often, usually following a sharp turn, but not enough to spur me into buying another set of tracks and sprockets or another hull.
I realize that I didn't need to buy the Torro lower hull, but I'm glad I did. I wish I'd known about and bought it in the first place, along with some (high quality) metal tracks, sprockets, and idlers. As it turns out, the only things I'm using from the Heng Long King Tiger are the tracks, sprockets, idler wheels, road wheels, and a few other minor parts. (The leftovers are good fodder for a fantasy WWII tank...)
That's one of the pitfalls of doing a quasi-budget build: You have to know where not to shave costs or you may end up wasting a lot of time and money. Where to focus your spending depends on what you value in a build or mod. You can make a budget tank look great by making small purchases incrementally over time, with your investment of time and effort. It's the basic foundational stuff that's a gamble when you take the budget route. Bearings make wheels turn more smoothly, with less friction. You can't easily fix or mod cheap metal tracks to work better. They may work acceptably, but if you value performance, you may be better off buying the proven, high-quality stuff at the start rather than buying budget ones and then later replacing them. This is a lesson I should have learned with my Tiger and Pzkpfw IV tank projects.
The kicker is, how do you know what you'll be satisfied with? You can't rely on reviews of stuff because everyone has a different idea of what's great and good enough: Not surprisingly, many people view whatever they've bought in a positive light, unless it's clearly a stinker. Some people are negative about everything. It's basically a question that only you can answer for yourself, and over time, even your own assessments and standards evolve. With the passage of time, I might bite the bullet and buy some quality drive components for this tank. Hell, I may even chop it in half to add the missing 1.6 centimeters! In the meantime, the money and effort that I've already spent makes it pretty easy to convince myself that what I've got is good enough!
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