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Mad Ax

Winch Bumper for Axial SCX10 G6 Rubicon

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OK, this has been a bit of a labour of love for a few weeks now.  Still in the prototype stage as I type, but I have a spare evening so I figure it's time for some admin and an update on progress.

So, wossis then?

Well, it's a winch mount / tough bumper for the Axial SCX10 G6 Rubicon that has been my go-to rig for trail duties for a few years now.

For why?

Various reasons.

1) The stock bumper isn't perfectly flat, so it's not easy to mount the winch on it.  The 3-Racing winch has a big metal base plate with built-in fairlead, and it doesn't sit right on the G6 bumper.

2) The stock bumper (and mount) are flexible ABS.  Good for avoiding breakages on the trails, bad for winch duties.  In fact with the ground anchor hooked over the roll cage for stowage, the entire bumper twists and looks ugly.

3) I have Proline Flat Iron XL tyres with a huge 120mm diameter, which catches on the corners of the stock bumper.

So, to begin with, I found this thing: https://www.thingiverse.com/thing:3489699

To start with, I printed it out in grey resin and offered it up.  It looks cool enough but a) the mounting posts were too close together as printed and 2) the bumper sits a little too low for my lifted body.  I figured the whole thing would look better if I lifted and deepened it, as well as adding a direct-to-chassis mount in place of the post-style mounting to reduce flex during winching and stowage.


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The above was printed with the flat top direct onto the build plate.  The curved edges of the shackle mounts didn't quite form right as there wasn't any space for support material.

So - I figured I'd have to edit the thing (or start from scratch).  As the thing looked cool, I chose to edit.  I've never edited an STL file before and, well, it was a bit of work.  Lots of crazy faces that I couldn't get rid of and lots of bits that almost-but-not-quite lined up.  Also I think the thing originally came from a larger version of the same thing and had been scaled down, hence nothing was a nice round measurement (in mm or inches).  So it took an absolute age to line up my new mounts properly, quite a lot of math and a fair bit of cursing.

My next design was printed with a fixed chassis mount and the holes for the 3racing winch already marked.




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Again, this was printed with the flat top on the build plate and again, the shackle eyes didn't form properly.  Also there was a bit of distortion on the chassis mount where it wasn't adequately supported.  I would have continual issues with supports and distortion in subsequent prints.

Anyway, I didn't want my ugly 3racing mounting plate visible, so I figured my next prototype should attempt to hide the winch inside the bumper, do away with the mounting plate and use an aftermarket fairlead.  I went with this beautiful bit of turned aluminium from rcBitz:


Yer tiz hanging off the build plate, post-build.  Note the right-hand side, where the supports have failed and the print has sagged.  This was the kind of problem I had until I found some "large model support settings" for the Elegoo Mars posted online.  However, what you can see in this photo is the epic resolution of the Mars and the lovely crisp details where I had made the cutout for the fairlead.


And this shot shows some warping in the chassis mount.  I think this is a result of having a mostly-solid model around the bulk of the bumper but with a lot of air around the mount itself.


However the fairlead fit perfectly (a cheap vernier caliper was a must for this job) and looks awesome


I sometimes get some Z axis lines on my printer.  I'm not sure if maybe there is something on the Z axis thread that's causing a problem.  It doesn't really matter for this project (in fact it looks cool) but it might be annoying on a driver figure or similar.


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Anyway, that aside, the fit (after filing back the chassis mount) was very good.




Now you may spot the accidental mistake here.  I had trimmed the bumper right back in order to fit the fairlead recess, and I'd completely forgotten to add the shackle eyes again before printing.  D'oh!  So what might have been the first usable bumper was consigned to the prototypes bin while I printed another.

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At this point I felt like I went through a hundred and one attempted prints to get a good result.  I kept having issues with the supports breaking away.  I tried to redesign the model in two pieces so I could print from a flat side down, but even that had issues.  The bucked shape of the bumper means there's no good flat side to start from.

Anyway, I did eventually get a couple of good prints.  This was the first successful one.



Unfortunately - this is all that's left of it:


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So, what went wrong?

Well, when I was trying to convert to a two-part print, I took a lot of meat out the back of the bumper in order to make a flat surface to grow from.  I had also had issues with the resin not taking a machine thread, so I had narrowed the mounting points so I could get a nut on the back of the bolts.  All that added up to a major weak spot.  One fast drop down a ramp directly into a rock at this weekend's UK Scaler Nationals, one great big crack and the bumper was in multiple pieces.

I didn't really expect it to last all weekend and it suffered a lot of smaller impacts without any marks at all.  It's clear to see where it went wrong and should be easy to add material and remove sharp edges to reduce stress points.  I tend to add fillets around all my non-visible sharp edges and corners (and my visible ones too, if they don't need to be sharp for aesthetics) as any sharp edge will focus stress and be a weak point but I was rushing this job for the Nationals and forgot to tidy up my model.

I'm looking at it again and wondering if I need to pull the bumper back a little.  I could easily lose a couple of mm and gain a higher attack angle into the bargain.  Behind-the-grille winches seem to be a new thing and I could probably pull it right back so the winch is behind the body.

Overall though, I'm really pleased.  I might look into a tougher resin as I've had issues with the Elegoo black splitting when drilled or tapped, but otherwise it's been a fun project.

I still have one intact unused prototype, so if another scale event comes up before I have time to rework this model then at least I'm not completely without a truck.

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