Saturday, September 21, 2019

Wltoys 12428b (part 25: Fixing slop in steering)

Setting less terrible toe angles seemed a bit pointless when the wheels kind just point wherever they want anyhow.  So that's the next order of business.

I needed a very thin shim, and after looking at the cost of stock at the hobby shop, i very carefully cut down an aluminum can.  I cut this little strip with a steel straight edge and 4 or 5 passes with a Stanley knife. 

0.16mm thick and ~1.89mm high.
Curiously, a can is about 0.25mm thick at the bottom and 0.12mm thick at the top.  So not only is it free, but we can make shims of different thicknesses!

The idea was to remove the play in this part of the steering mechanism.  I'm hoping a picture tells a thousand words, because i'm not sure i can explain it..

Craft knife points to the shim being inserted from the right.
It took a little fettling to get it to fit, and in the end instead of pushing the shim in, i ended up holding it in place with long nose pliers and moving the normally stationary part onto it (if that makes any sense).  

When it was inserted, I tightly folded the ends over so hopefully it stays in place.

Wltoys 12428b (part 24: Fixing front toe out)

From standard these cars come with a toe out that's quite visible.  When i measured it (I am not an engineer..) it seems about three degrees each side (with the standard fixed, equal length 61mm steering arms).  I did this by:
  1. Placing the car on a piece of paper, aligning it perpendicular.
  2. Place a straight edge along the front wheels, draw a line (the outside ones)
  3. Then rule progressive parallel lines towards the centre until they cross.
  4. At the crossing point, use a protractor to measure the angle.
  5. Halve that number to indicate the toe out for each wheel.
Such science!
Then we need to make up some adjustable steering arms so we can alter the toe.

Attempt 1: I cut the heads off some random bolts and screwed in the spare piston ends from the CVA shocks.  This would have worked, however, they're non adjustable.  Worse, I cut down the only bolts I had, and they were too short.. I really could have gone and got some more, cut them to the exact length.

Attempt 2: Buy some adjustable steering arms.  Ended up getting the Tamiya 54539 Full Turnbuckle Set. I fitted them in the following combination to yield an adjustable arm of nominal length 61mm:

Top: The new parts  Below: The standard 61mm arm.
Long story short, after a bit of mucking around I was able to produce 3 degrees toe in with equal length steering arms of 60.60mm.  That's about as good as I am willing to attempt right now, mainly because there will be wild variations in actual steering because of the play in the plastic steering mechanism.

Friday, September 20, 2019

Wltoys 12428b (part 23: RWD with locked rear diff)

So the rear wheel drive conversion has been very interesting.  Not only has it allowed me to isolate and then optimise the rear end, it's unlocked an entirely different vehicle from the AWD one.

However, one thing was immediately obvious, rear wheel traction was sorely lacking.  Hence, locking the rear diff with BlueTac :)

I kept another diff open, hence labeling.

I did this rather hastily and didn't bother taking any in-progress pictures.  However the process is pretty straight forward:
  1. Remove the wheels and rear differential cover etc.
  2. Remove the differential and axle assembly.
  3. Carefully open the differential by removing the 4 screws.
  4. Carefully note the arrangement of the spider gears.
  5. Carefully remove the spider gears onto a sheet of white paper.
  6. De-grease and clean the spider gears and inside their housing.
  7. Take a wad of BlueTac and shove it into the housing..
  8. Reinstall the spider gears, embedding them in the BlueTac.
  9. Shove more BlueTac on top of the spider gears.  Aim to fill it with only the smallest amount coming out of the screw holes upon re-assembly.
  10. Reassemble the differential, carefully tightening screws.  Don't force it.  If you have not enough then the spider gears will still move.  Too much and it won't go back together properly.  
  11. ..fettle until it works ;)
  12. Reinstall in the reverse order of disassembly.

Road Test:

OMFG.  The thing is a beast. Totally different animal..
  1. It feels more agile, nimble, responsive.  And powerful.
  2. Will readily lift a front wheel under acceleration.
  3. Brake application causes rear wheel lock, done carelessly induces spinout.
  4. Turning circle doesn't seem to be badly impacted.
  5. Can now easily do doughnuts.. ;)

Offroad Test:

Hmmmm... something isn't quite right.  It's nearly uncontrollable on full throttle.. I think it's torque steer?

  1. Whenever the car is at full throttle it will steer to the right (clockwise).
  2. From a standing start it will just do a doughnut, always the same direction.
  3. Trim the steering to the left, under full throttle it will travel straight, and then off throttle will travel to the left.  So i don't think a steering or even a toe issue.
  4. It only appeared to happen when the diff was locked.  RWD with open diff didn't have this issue.
Some interesting information.  Fixes? I'll try adjusting the front toe, that needs doing already.  Failing that I don't mind going back to AWD.  I feel like the rear is solid now which is how this all started in the first place.. ;)

UPDATE 1: Fixed the issues with toe out and steering play, but even on tarmac it still very readily wants to steer to the right.  

Monday, September 16, 2019

Wltoys 12428b (part 22: rear wheel drive conversion)

I was getting some quite serious While i was doing some maintenance

So I was halfway through installing a spare front diff from the parts car when i decided I'd had enough of front diffs and their noise and friction and various wear issues, so I decided on a whim just to pull it out, and the front drive shafts as well:

Front diff and drive shafts notable in their absence.

So although it's dark out, i went up and drove it on the road.  On a LiIon battery in storage mode (~7.4v) and with throttle trim turned right up, the thing absolutely flies.  It's a completely different car!  I'm slightly in awe.

  1. It accelerates like a demon, especially on the road (traction)
  2. It's relatively very quiet and the motor noise is much more evident.
  3. It doesn't suffer from power on under-steer.
  1. It's really fast.. ;) and i hit the curb more than once :/
  2. The brakes only work on the rear wheels..
  3. Toe out factory front wheel alignment causes tracking issues.
  4. More power through the rear wheels induces wheelspin
  5. The open rear diff servers to accentuate this issue.
  6. The tail-shaft CV joint angles could be improved (noisy).
I do have even more front diffs on order, but i don't know what to do now!

UPDATE: I took it up the hill to play on the dirt and the major limiting factor regarding acceleration is the open rear diff.  You can hear it variously spin up one wheel or the other, and it won't power slide as it lifts the inside wheel and just spins it.

I fixed the CV joint issue by lowering the rear ride height a bit.  Fwiw; for ease of maintenance etc when on the bench I have the car in a cradle (you can see one of the dowels in the above picture).  This means that there is no weight on the wheels and the extra droop causes the CV noise issue.

Possible mods, lock the rear diff and fit adjustable steering arms.

Saturday, September 14, 2019

Wltoys 12428b (part 21: rear suspension geometry changes)

While i was very happy with the CVA shock upgrades, the rear mounting (install ball mounts from standard shocks and install i) left a little to be desired, in that:
  1. Snugging up the top screw placed sideways pressure on the shock top, and i could see whiteness (plastic fatigue) where the shock top joins the cap.
  2. The spring required all the spacers I had and even then the ride height could have been higher.  Upon removal, the spring had compressed.
  3. The shock top mount was non-adjustable.
So, i decided to make up new rear top shock mounts.  Unfortunately, I'm still rather devoid of any actual trained understanding of suspension, but I worked on the following approaches:
  1. Suspension components require compliance (movement) in use.
  2. But mounting points themselves (this part) should be solid.
  3. Parts should not be under any static fatigue when installed.
  4. Parts should not be exposed to any avoidable fatigue in use.
  5. The part should allow adjustment for final positioning and tuning.
  6. Anchor points labelled consistently between designs; A and B (see pics)
  7. Incorporate only parts from the 50519 and 50520 kits, specifically use the "hex ball head connector" (ala tamiya 53968)
  8. For maximum rebound effect, the shock should be as vertical as possible.
  9. The shock should be allowed to always retain it's full travel and not be overly compressed when variously adjusted.
  10. The final assembly should not foul the chassis and/or body in any respect.
  11. Where all the above are satisfied, body roll should be decreased by lowering the center of gravity by not using overly stiff springs or via an equivalent adjustment.
So, that setup, I variously started with some drawings, and then mocked up in cardboard, test fit, then onto test parts (1.6mm aluminum sheet).  The process involved several designs:

Designs are numbered, left to right, top to bottom.
V1 through v5: These were produced on a principle of three anchor points being good and strong (A, B and the 3rd not labelled but it's visible in the part marked v4).  The idea here was to have the suspension mount to one of the line of numbered holes along the edge.  This design was abandoned because ultimately the angle precluded use of the hex ball head connector if mounted directly on the part (a stand off of some sort would have worked)

V6 through 9: Worked on the idea that i needed to be actually able to use the hex ball head connector, so it incorporated proper clearance from the start.  It also reduced the number of mounting points down to two, given that 3 was overkill here.  Adjustment in this design was incorporated by removing the mounting bolt A and rotating the part.  This in turn moved the shock top through an arc.  The precise arc being difficult to determine, hence the incorporation of a separate arm used for positioning (v8), then finally built up in aluminum.  This design was abandoned because making the adjustment was fiddly, and the optimum angle was hard to find.. i felt like i needed more science and the fun factor was waning..

and finally V10: Back to basics.  Two solid chassis mounts (A and B), a single non adjustable mount for the hex ball head connector in a good average position which didn't require spacers and allowed the shock its full travel.  The whole assembly installed using long bolts and using the brass stand off pieces (two of which are missing from this picture):

Two brackets machined together, then one reversed for install.
Finally, installed it all into the car.  In this picture it's clear how the brass standoff tubes have been used to give clearance of the bracket from the chassis, and to move the shock into a more vertical position:

The final part as installed on the car.

Verdict: On the road it feels pretty much the same as before :) but I'm happy that the shock top won't fatigue early, and also that I admire my handwork through the quarter window behind the drivers seat :)

Wednesday, September 04, 2019

Wltoys 12428b (part 20: Oil filled standard rear shocks)

So, I'm very happy with the CVA suspension upgrade.  If i could justify it (and if the parts were available) I'd upgrade the other cars too.  But i can't and they're not.  So what I'm left with is trying to make good on the stock rear suspension.

  1. It's very stiff especially compared to the horizontal front.
  2. Ride height is quite high which accentuates body roll issues
  1. I have multiple spares
  2. Trivial inspection shows all the makings of something better.
NOTE: High precision drafting skills.
I'm not the first person to suggest filling these with oil. There is even a mysterious and seemingly useless little plastic tube (2.55mm x 4.85mm) located exactly where the o-rings would go (as per my recent experience with the CVA shocks).

Option 1: use the standard plastic o ring:

Kevin tells me the oil i needed was around 800cps (which he then retracted and suggested 1000cps).  I have some 750cps, so I've used that.

There's nothing like a categorical No.
Within a heartbeat it all just bubbled out the small end..

Option 2: source proper o rings:

So off to the internet i go to find some appropriate o-rings.. Looking online they're specified using their ID (internal diameter) 2.55mm and their CS or t (cross section) which for me is ~1.1mm (~4.8 - ~2.6/2).  So check out these guys.. this is a list of o-rings of various types .. or some reading on this extensive post suggests these will work

But before i got around to purchasing them..

Option 2a: Use some crappy foam instead of proper o-rings:

..I literally found this on the lab floor, i have no idea where it came from.  It's some kind of foam rubber:

it just .. appeared

I fashioned an o-ring around the shaft of the piston (in place of the small plastic o-ring), and also another around the large end cap.  It worked well enough to see that o-rings won't help..

the second issue is that the springs are very stiff.

Option 2b(?): Cut the springs..

I thought shorter springs = softer.  But it doesn't really seem the case.  It seems they're just .. shorter.  I wish there was more i could say, but even cut they don't seem to be any softer.

Option 3: Just do it properly..

Really.. these just work.  Unless you're on a total shoestring, just do it.

Engineering works..