Vintage mid-motor Hirobo 44B/Zerda survivor (part 1)

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I got this mid motor Hirobo 44B/Zerda disguised as a Kyosho Turbo Optima!

I have never owned a Hirobo, but I briefly heard about the 44B back in the early 80’s. Back then the only source of information I had was the magazine of the Norwegian radio control car federation (NrcBF), where I saw the 44B mentioned in mid 1983. Doing a quick search through my Japanese magazines, I find it first mentioned a couple of months earlier, and from what I can determine from Japanese magazine ads, it was released just a month or two before the Yokomo YZ-834B Dog Fighter, thus making it the first commercially available 4WD 1/10 scale electric racing buggy. Yes I know the Tamiya Toyota 4×4 Pick Up was released earlier, and some even raced them on tracks, but it’s a completely different animal that don’t stand a chance against real racing buggies on a dirt track.

Early Japanese ad for the 44B.

The 44B came in four different flavours, the Rock’n City with a Honda City Turbo body, the Rock’n Vega with a more buggy-like body, the Lancia Rally with (you guessed it) a Lancia Rally 037 body, and the Toyota Hilux, with a slightly shorter wheel base. The Zerda was released about a year and a half later, used the same drive train as the 44B, and was slightly upgraded in certain areas. After studying the design of the 44B/Zerda I have concluded that this was a design way ahead of it’s time.

The SP-E4, Zerda prototype.

The double-hinged double-wishbone suspension, with adjustable upper control links was completely different to the various trailing arm and single-hinged suspensions of the (then) current racing buggies, like the Kyosho Scorpion and the Tamiya SRB’s. This is basically the same suspension system used on buggies today. The enclosed bevel-gear diff units looked much more refined than the open diff systems seen on other cars at the time, and is also an idea still in use today. Then of course the belt drive…. The Yokomo buggy developed at the same time as the 44B used a chain, and it would take some years until the belt idea would catch on, but here we are today, with belt being the preferred solution on 4WD buggies. So all in all I see the 44B as one of the most innovative buggies of all time, incorporating ideas still used in the current state of the art buggies. Not bad for a car designed in 1982/83!

This is how I got the car.

I got this car a while ago, mainly for the front Kyosho 1886 shocks, but also to finally get some experience with one of the early Hirobo buggies. It’s actually hard to tell what this really started out as, because there are parts on it unique to both the Hirobo Zerda and to the 44B series. The front bulkhead is from a 44B, while the wheels and the chassis are from a Zerda. The chassis has been cut to be narrow, like on the 44B, but it’s definitely a Zerda chassis, as it has a front kick up, something the 44B did not have. There are also some home made aluminium profile rails on each side of the chassis, but the cut outs for the suspension arms weakens them a lot in those places, to a point that they seem rather useless. They do however provide stiffness to the middle part of the chassis, and they look nice. I believe the front arms are the slightly longer Zerda arms, since the drive shafts are the same length as the rear drive shafts. The 44B had shorter arms and driveshafts at the front. The rear arms are the same on the 44B and the Zerda.

The front 44B bulk head, the longer Zerda arms, the kick-up and the aluminium profile rails.

The front shocks are, as I said before, the very rare Kyosho 1886 shocks that are shorter versions of the more common longer shocks from the CB-88 and CB-89 shock upgrade kits for the 1/8 scale buggies. At the rear there are some other rare shocks, the Tech Racing equalizer shocks, with a tube connecting the two shocks to somehow “emulate” the ideas behind the earlier mono shock concept. This was something many experimented with in the early to mid 80’s, and the idea has been tried by various hop-up brands, including Parma. I have found these shocks on a couple of other cars as well, but none complete with the tube and connector, like on this car. But the most striking feature on this car is the home made mid-motor conversion. By basically just turning the rear “gear box” 180 degrees and making some slight modifications to the tension spools (actually just filling them with layers of adhesive tape) and a few other parts, the 44B/Zerda is converted to a mid-motor buggy. And it works!

The rear “gear box” has been turned 180 deg. making it a “mid”.

While there were a couple of commercially available mid-motor kits for the 44B/Zerda, most noticeably the kit from Hong-Kong based GPM, this was so early in the electric off-road racing era, that nothing was carved in stone, as to what was the ideal set-up or design. People would experiment and do innovative things like this, to try to find the thing that would give them an advantage in races. Today every 1/10 off road buggy looks the same, and people go crazy and hail it as a stroke of genius when a manufacturer, after 2 years of research, decides to move the upper suspension arm mount 0.3 mm inwards.

Well as I said before, I mainly got this car for the front shocks, but after studying the car a bit closer, I decided that it deserved a rebuild. I didn’t want to restore it with lots of new parts, but rather take it apart, clean it a bit, change the rusty screws, and replace parts that are beyond repair. Interestingly there were actually few broken parts, most noticeably one of the front shocks (the reason I bought the car for in the first place, remember?) where the piston rod had snapped just above the threads for the ball joint. I believe I can replace it with a piston rod from the front shocks of the re-released Kyosho Optima. I haven’t decided which shocks to use on the re-build, but I might keep the Tech Racing rear shocks, and add another set I have of those to the front. Another option is to go full Kyosho on the shocks, and use the 1886 shocks together with a pair of CB’s at the rear. I could also use a set of either red or grey Scorpion shocks. I have lots of options, and I’ll see what would look best in the end. I will obviously replace the body with something more proper for a Hirobo. The Zerda wheels have been painted pink, so those will have to go, unless I can manage to strip off the paint. I do have a nib set of four 44B wheels, but I will have to find some tires for them. I had a vision that Kyosho Scorpion tires would fit as the diameter is the same. But when I tried fitting them I realised the Scorpion tires were about half a centimeter too wide, so that was a bummer. Old RC10 or Holiday Buggy tires should fit, so I’ll have to look out for some of those. The car was fully equipped with ball bearings, and I’ll re-use those after a quick bath in WD40 and then some oil. I got a few hop ups from my friend Mike Mills, like a red kydex JG bumper, a JG rear under guard and a JG fiber glass receiver plate. Of those only the bumper seems to fit the buggy after it was converted to mid-motor, so I’ll see what I’ll do. I might have to make some arrangements to fit a battery, receiver and a speedo, so a custom fiber glass plate mounted low on the chassis might be the solution, or maybe a full upper plate, like on the Zerda. It would both be proper solutions for a home modded buggy like this. But I’ll worry about that when I have rebuilt what I have.

A pair of 44B wheels and a JG bumper that I got from my friend Mike. Photo: Mike Mills.

So first thing I did was to take it completely apart. Luckily I remembered to take some photos of some of the custom solutions, so I would have a fair chance of getting it all back together. The manual is of no use due to the many mods. Unfortunately I only remembered this halfway through the teardown, so I might have to rely on some low res pictures I have of the car from before I got it, to get it all together again. The upside of not having a “manual” is that I get a better understanding of the reasoning behind how different things were done, and after rebuilding the drive train, I can see that this is an ingenious modification, that for all intents and purposes works very well, even the adhesive tape on the tension spools! The Hirobo manuals are crap anyway, nothing like a Kyosho or Tamiya manual.

Notice the adhesive tape layered around the spools to make them bigger and provide the tension needed to keep the belt tight after the conversion.

After leaving the metal parts in a WD40 bath, and the plastic parts in green soap, I gave the plastic parts the toothbrush treatment. I didn’t use the ultrasonic cleaner, as that method does not work well with this kind of dirt, from my experience. I did not polish any parts or anything, as I want this car to be what it is, an old and used race car.

The differentials were the first to be rebuilt. Having noticed the striking similarity with the Kyosho Optima differentials while dismantling the car, I was curious to actually compare them. While obviously the diff casings were different, the gears inside looked very familiar. I compared the bevel gears of both cars and to my surprise they actually looked identical! So just to confirm my suspicion, I put the free floating bevel gears of the Optima into the Hirobo diff case, using the outdrive bevel gears of the Hirobo (the axles are longer than on the Kyosho diff), screwed it all together, and the diff was smooth as silk! The bevel gears inside the Hirobo and the Optima diffs are truly identical, and Optima gears can be used as replacements for the Hirobo gears! Akira Kogawa….. I think I’ll have to have a word with you about this, lol 😉 Ok, so the outdrive axle is longer on the Hirobo, but as both are press fitted, it should be possible to do a switch. Actually one of the Hirobo outdrive axles accidentally came off the bevel gear when I dismantled one of the diffs, so it shouldn’t be that hard to do.

Hirobo diff gears to the left and Kyosho to the right. Besides the length of the axles, they are identical.

After building and greasing the diffs it was time to rebuild the drive train. By using the pictures I had taken, the rebuild haven’t actually posed any problems, so far. The only thing was that the belt seemed really loose, until I realised I had been so focused on getting the rear gear box right, that I had forgotten to mount the tension spool at the front. After doing that, and mounting the top part of the chassis, that doubles as a belt cover, everything were nice and tight, with no slipping of the fine pitched belt at all.

The chassis and drive train put back together again.

Well, that’s how far I have come today. Next will be mounting the suspension. More later.


About TomEG 74 Articles
I have been into RC cars since the beginning of the 80's, but have really never been much of a racer. I competed in some local races and some "Tamiya Cups", and actually came 6th in the Tamiya Cup Norwegian Nationals in 1987. My main interest has always been building and modifying, and that's where I still am today. Through the years I have been able to build a solid network of contacts around the world, consisting of both former and current members of the industry, as well as quite a few of the former top drivers. I have a genuine interest for the history of the 1/10 scale off-road buggies from the 80's.


  1. I have a Zerda too but I didn’t take it apart yet.
    It’s on my to-do list for years.
    Yet shed under the light of your explanations, I must say I’m very curious now.

    • A very special model indeed. Way ahead of it’s time. With a little more efficient drive line (like the Yokomo Dog Fighter or Kyosho Optima), this could have been a winner.

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