Hirobo 44Bs and Zerda :-)

[yogi-bear 2605]

given how basic the tools are, that's pretty good and will still do the job.

[silvertriple 4137]

Happy New Year and best wishes to everyone!

This morning I had my first success of 2025, I hope there will be more

The build of this Zerda starts by the differentials. Not a single original part is present on those pictures and was used. Differential internals are optima rerelease diff internals and custom made shafts. I had to drill and cut a new shaft as the second one I did had a slightly off center hole, and the pin was not fitting in the bevel gear. There is ball bearing cages added on my modded diff case.

Good news : they work perfectly. Bad news, I do not have enough 9x17x5 ball bearings to assemble the transmission fully today. Ordered some. They should arrive Friday or Saturday.

I printed the rollers used to make the belt path and the tension in TPU.

Next I assembled the carriers... Again, no original parts. I'll use some HBX wheel shafts, and double bearings to reduce the play.

Again, perfect fit... I'll continue the build soon, but I need the diff ball bearings first...

[silvertriple 4137]

Slowly but surely, the Zerda project is advancing...

While the 670-2GT-6 belt was too short, the 686-2GT-6 is too long... The proper length is probably between 676-2GT-6 and 680-2GT-6 (but none were available when I ordered, and I did not want to wait). I've modified the rollers to increase their diameter (2mm) to get this working properly.

The belt cover is not fixed yet, I'll do this later for some reason, but it fits like a glove...

The some reason is that I want to add a Kydex piece under the chassis to add rigidity (to address the weak point of the chassis). I designed the part, add a bit of room for the front holes as I will need to bend it (using some heat). And I went to the local fablab to get some Kydex sheets laser cut in 3 mm thick  (and yes, while I was there, I also got a top deck plate cut (in 2 and 3mm thickness) and as well some other part for the other ongoing project).

This mean I should be able to move this project forward and adjust the last details...
 

[silvertriple 4137]

Rather than calling this an underguard, I should call this a rigidifier. 3mm Kydex sheet, laser cut. I made a projection of the front on the plan, and added 1mm in length at the front extended all holes by 1mm to the front, to take provision for the bent.

To bent it :
- a piece of wood on the rear/long part
- a sleeve made of thick cardboard on the front area
- 1mm exposed to the heat gun
- I used the sleeve to bend the part and maintained it this way until it cooled.

Result : the fit is perfect. all holes are matching.

I added the chamfer manually, using a screw driver handle with the tool I ould use normally on the drill to avoid heat issues. Worked perfectly, although it took a bit (too much) of time.

The gain in rigidity is clear : although on the original Zerda in its original configuration, pushing the tightener roll would push the front up, it's no longer the case here.

The build will continue : I still have to fine tune the front shock tower  (the idea is to use it as servo support) and the upper deck support, and this might require a few adjustments...

[silvertriple 4137]

One part I needed to adjust was the front shock tower. I wanted to keep the chassis geometry for the shocks. But I wanted to center the steering servo head. I could have thought about adding more complexity using the two holes located at the rear end of the front shock tower walls, but I thought there was an easier way...

A standard servo is too large to have its head centered. And without adding a link it can't be installed at the front. I checked the servo I'm generally using on the 1/12 cars : it is smaller, it is quick, and it is a 3KG servo, which means it should be enough for the Zerda, which remain a quite light car (I know we tend to put more powerfull servo nowadays, but back in eighties, 3kg servo was the go to servo)... It's smaller but it will allow to center the servo head, and therefore to have a symmetrical direction.

I planed for 2mm and 3mm upper deck, but I believe that with the part added under the chassis, 3mm for the upper deck won't be needed to get a proper rigidity. But the shock tower will allow it.

I had a small mistake in the design of the C-hub (I suspect something happened at somepoint which resulted in one extrusion skipped due to (con)Fusion360), I tried to catch it back by grinding it manually, but it was not enough, and I broke one of the C-hubs while assembling them with the arms (yes, I do mistakes, sometimes). I reprinted some in PETG temporarily to complete the build, and I'll reorder SLS print later on (The 44B ran quite a lot with full PETG printed parts, so it should be fine on the Zerda).

[silvertriple 4137]

This morning, I adjusted shocks for this Zerda. Original shocks were 80 and and 90mm, but I find the rear should be shorten by 5mm. I started with some 85mm shocks. Rear are unchanged while front course is shorten by 5mm by insertion of a printed bearing on the piston shaft.

The shocks are larger than the original ones and it may require some adjustment on top end fixation... We'll see this later., but I do think it will be the same as on the 44B chassis.

The dog bones are 61mm generic one found on internet. I have 8 sitting there and 5 of them are not exactly perfect. There is one end which do have an issue on one of the side, and it requires this side to be on the wheel side, as on the diff cup side, it won't work properly. I marked them to ease the use, but they will do the job.

I printed my front shock tower, installed it on the chassis with the servo and a Tamiya servo saver (with a original Hirobo servo saver, you can not use pivot ball here). The steering will be symmetric. If I trust what I experienced with the Hilux chassis, that would be enough without adding too much complexity.

Next come the arms. Everything is ready to be positionned.

Some fine tuning might be still required for the steering rods, but beside this we have a full front end. Note the top deck in place. I used the 2mm but the 3mm would work as well (i tested). My chassis is rigid enough with the 2mm top deck thanks to the 3mm under chassis rigidifier. I added 2 toric joints in the front diff cups to avoid the dogbones to snap out at the wheel end...

The rear end was even easier... I may replace the shock tower by one a bit stronger, though.

I positionned the roll cage already to check the fitting. Some adjustment will be required for the 3 transversal bars linking the two side as they are far too easy to snap out... 

At the end it will work. I still need to put a motor inside this beast, manage the few adjustments of the roll cage, but it is already a very good progress...

[silvertriple 4137]

I modified and reprint the Bumper in TPU to take into account the 3mm thickness added to the chassis, as  before there was a gap at the front between the Bumper and the gearbox

I adapted Rims to put some Kyosho Sand Super Javelin/Optima tires. No doubt, once finished it will look like a Zerda

There was an issue as well for the belt at the rear as what I've ordered was too short initially. I ordered a range of different length to make sure I have one working. Here is a 190-2GT-6 which is too long with smaller pinions. Here it should be a 16T. I replaced it by a 188-2GT-6 and a 186-2GT-6 would be perfect for smaller pinions.

The transmission works perfectly.

For the driver I opted for an Optima re-release driver, and I designed a part to support it.

I felt I have to show somewhere thet this was not a real Hirobo... I decided to do a custom Motor Cap. "F'" is for False

I printed and installed it, as well the pulley belt cover, and it was time to fix the roll cage...

I still have to put a RX and an ESC in the car, and it will be time for paintjob and stickers...

[silvertriple 4137]

A bit of comparisons while the car is not finished...

Reminder : The original Zerda has printed rims and Tamiya Thundershot tires.

I really look forward to finish the few details : electronics (and I need to find a solution for the motor cables : they don't look accorded to the rest of the car ), driver paintjob, stickers...