Tuesday, June 16, 2020

Wltoys 12428b (part 27: 4600 mah LiPo upgrade ;))

Party time! :)

Can you tell I'm excited!?  What we have here is the Turnigy nano-tech Ultimate 4600mah 2S2P 90C Hardcase Lipo Short Pack, which I intend to install in the 12428.  My research indicated that should fit with some modification of the battery tray.  Most important are the dimensions, advertised and actual; the actual battery I measured was slightly smaller than advertised:

  • Length: 96mm advertised vs 95.47 measured
  • Width: 47 advertised vs 46.9 measured
  • Depth: 25.1 advertised vs 24.9 measured.
So! I intended to sit it vertically in the battery tray and a little cutting of the battery tray was in order:

Tight clearance between battery and strut towers

Note shiny new motor heatsink.

Yes, this is quite high, and very far back.  As high and as backwards as possible.. Now I'm thinking of CG and it's not really ideal.. :) However, it fits!! I don't really care if it wheel-stands too much, these are all about the fun :)

UPDATE 1: Well.. It works ;) it's extremely punchy!  my wife says it sounds beastly! Characteristics are:
  1. At 8.4v the car will easily lift a front wheel under hard acceleration.
  2. Can't speak to playtime as I only reduced it down to storage voltage today.
  3. I'm running a locked diff, so it doesn't turn well at speed.  So that probably means a reduced tendency to roll, which is a good thing.  Although it still happens and the battery impacted the ground..
Hmm.. More engineering required.

UPDATE 2:  With the intent of allowing the battery to move forwards further I removed the battery tray and removed part of the front with some side-cutters; I only removed enough so the battery would fit through.

Battery tray with half the front removed.

I also had to remove about 30mm from the driver insert so it would clear the battery.  All this allowed the battery to move about 25mm forwards where it is nestled and protected inside the rear roll bars. 

Can you see the battery terminal on the top right?

I also made an aluminum tray for the battery to slide along and with just the right amount of bend, it holds the battery nicely without the need for a retaining strap.

The battery is held captive in all dimensions

Finally I shortened up the power cables, and reduced the thickness from 10 gauge to 14 gauge:

New power cable & connectors, reused balance bits.

Combined, these changes made all the difference :) I went for a long play up the bush the other day (uphill, damp earth, rocks).  Characteristics now are:
  1. Extremely long run time :) I charged both batteries but only used one.
  2. A motor heat-sink is now a must, it cut out a couple of times regardless.
  3. Pretty good weight distribution: ~52/48 front to rear ;)

     Weight (g)
    Distribution (%)
    Front Left
     430 24.6
    Rear Left
     425 24.3
    Front Right  471   26.9
    Rear Right 422   24.1
Who'd have thought there would be so much potential for fun in these little cars.  I actually bumped into some Tamiya owners who claimed their 90's cars were brittle and tiring to own.. After a little demo I may have converted him ;)

I'm still loving this thing .. :)
UPDATE 3: Note that the standard ESC does not appear to have reverse polarity protection; so be very careful that you connect the leads to the battery the right way around etc..

Wednesday, June 10, 2020

Wltoys 12428b (part 26: Two bent drive shafts)

So we've been having a bit of fun lately, and so this morning I was doing a little proactive maintenance on my boys car; his always seems more .. agricultural .. than mine and I wanted to have a closer look.  Among the general increase in wear and tear (he's a kid) I noticed that both drive shafts (front and rear) on the right hand side of the car were bent.  And during disassembly I then broke the screw holding the pinion to the shaft (as has happened elsewhere on these cars).

Figure 1: Bent and broken

Ignoring how the bent shafts might have happened I was able to get his car working smoothly by replacing the whole rear diff assembly and also a front CV shaft from the spare parts car.  The specific replacements of the parts are below, and I ordered them from Bangood.
The rest of this article discusses how to repair the bent shafts, and replace the broken screw.  It does assume you have seen inside the rear diff of a 12428b ;)

After rolling it on a flat surface, I found the high side of the rear axle, I held the axle in a vice and tapped the high side with a light hammer.  After repeating this a few times it came good.  Or so I thought; I then put it in a drill and it's still out of round by half a mm or so.. hmm..

Harder was the repairing the broken screw which had to be drilled out to 2.5mm and tapped at 3mm (I've also had to do similar previously).

Figure 2: After drilling out the broken screw

In Figure 1 the parts are lined up screw, spider gear, axle housing and axle shaft, with the (now broken) screw fixing the spider gear to the shaft.  During reassembly I didn't have a m3 screw with a small enough head to clear the spider gears, so I had to phone my friend Randal who specializes in such things :)

And finally, the front CV shaft was also quickly fixed with a couple of gentle taps of the ball pein hammer. 

Sorted :) .. hmmm but now what .. ;)