GMade BOM - Yet Another Scale Rig

[Mad Ax 9312]

OK, so a little bit more talking after all the pictures.

You may have noticed that the bottom shock mount has moved to the lower link mount.  There wasn't really enough space to get the screw in to the shock mount.  This does change the geometry slightly, but there's so much more going on that its effect will largely be swallowed up in all of this:

The shocks came to me a few years ago attached to a Manta Ray that I got from ebay.  The reservoirs are false, they have bladders in the top but like a lot of cheap shocks it isn't possible to fit the bladder before screwing on the cap - you have to push-fit the bladder into the cap first, then screw it on.  That means you have to sort of guess how much oil you need and bleed it a few times to stop it locking up, but eventually I got both shocks working nicely.

The pistons inside the shocks didn't really fit properly, and there was a lot of play in the rod, but I found some thicker 3-hole white pistons that were a perfect fit.  I think they might be 3-Racing pistons from their M-chassis / touring shock range, but I could be wrong.  They could be TRFs, it's been a while since I've seen a set.  The holes were very small, way smaller than a regular Tamiya CVA piston hole, so they feel more like 2 or even 1-hole pistons.  I used Element oil from the Enduro kit, I would guess it's a soft oil for crawling but not sure how it compares to the 10 - 30wt standard that most other crawler rigs seem to come with (I would guess it's on the stiffer side, at least Element shocks feel stiffer than most other crawler shocks).  The shocks actually felt a little too stiff initially, but first let's skip to the springs.

The eagle-eyed among you will have noticed they are a progressive spring, and they're very stiff.  This is good.  Mounting the shocks inboard increases the leverage the axle has when rocking side-to-side, and mounting the shocks on an angle reduces the effective spring rate also, so a stiffer spring is necessary to compensate (the same applies to the BOM's standard cantilever rear end, and is why the standard shock is very short and the standard spring is incredibly stiff).  The harder shock is necessary to control the harder spring, but the effects at the wheel are reduced by the non-standard geometry.  The progressive spring rate should also counteract the progressive effect of the angled shock installation.

Still, despite all of that, my initial thought when I squidged down on the back end was that it was too stiff.  Well, no matter, I could always swap out the shocks or springs later, or switch to a different mounting hole.  It passed the bounce test, landing nicely on the workbench and absorbing the shock in the back end without the tyres coming off the ground.  In fact the front end bounced a little - I may need to investigate the front shocks.

There is one more minor issue - on full articulation the top links interfere with the springs.  This doesn't really impact it operationally, since the springs just bend, but possibly I could install some bent links left over from the Element Enduro builds to reduce this problem.

The real test was putting out on the 1:10 scale mountain.

If I had any doubts about the spring being too stiff, they were gone the moment it completely aced the opening section, and remained flat and true over the leany-bit.  The BOM usually flops over sideways and rolls off the mountain when I traverse the Valley of the Holly section, but with the new rear shocks it was way more controllable.  I did manage to roll it once, after getting the rear tyre stuck in an axle-breaker, but after recovering it back backing up to try a few different lines, I got through.  Normally "backing up to try a different line" isn't an option with a BOM, because if you can the line wrong first time, it'll go upside down.

I'm now officially in love with this truck all over again, and can't wait to get out on a proper comp course.  Roll on May, and the UK G6 (also the same weekend I head to Carlisle for the Iconic Cup and a few days holiday in the Lake District, so quite an exciting time for me!)

[Badcrumble 4203]

Lovely work on those mounts.

Nice that you're happy with the truck too - it always had that weak spot in your previous reports. 

[Mad Ax 9312]

Yet Another Update on Yet Another Scale Rig - because it's SST time soon and this truck needed a little bit of attention.

Beck's truck hasn't seen any action since the open day at the quarry back in December, so it was overdue for a quick check-over.  There's nothing like turning up at an event with a rig which is DOA (I have done that before...)

We begin here, with the rig looking in a mostly-acceptable condition.

One problem this rig has always had is the somewhat lame and floppy interior.  I'd already wasted an hour trying to modify an alloy hex to fit an SCX10 axle, which of course you know about because you read the thread already, so I didn't want to spend the rest of the day rebuilding the interior from scratch.  Instead, I had to find a way of stopping it from flopping around.

"Hey!  Who stole my magazine?"

Another problem was that I didn't have any countersunk screws when I made the battery tray, so these button heads will slowly rub away at the edge of my soft LiPos while it's wobbling around the trails.  Not ideal...

It only took a few moments to pop the battery tray out and countersink the holes

[Mad Ax 9312]

After that, I put two body posts on to the floor pans to secure the interior plate.

thus

[Mad Ax 9312]

While I was working on the truck, I noticed the front suspension was seriously floppy.  I guess the oil has either escaped, or is way too thin.  I used to think that crawlers were better with a really soft oil, but the more I crawl, the firmer I want them to be.  If they roll around too much, the truck will want to flop over at the merest hint of a sidehill (the BOM had a tendency to do that already, thanks to the weird rear end, thankfully fixed with the new rear shock setup).

Looks like a fair bit of oil around the top of these shocks.

It took me while to rebuild these, and I tried a few thicker oils before I was satisfied.  I used HPI 40wt.  I still think it's a bit soft, but at least there is now some noticeable damping, if not a lot.

I have to say, these are really hard shocks to assemble.  I find most modern scaler shocks are like this.  I guess they're aeration dampers, as they need to have some air in them to work properly.  The instructions say to fill the shock with oil, then put the top on (you can have the piston all the way in for slower rebound, or all the way out for slower compression - I guess this affects the initial preload by determining how much volume change must be compensated for by the air in the oil.)  But the only air in the shock during assembly is in a tiny little pocket in the cap.  If I assemble the shock with the piston all the way in, around half-way through screwing on the cap, the piston starts to push out on its own.  By the time it's fully down, there's very little travel in the shock because the cap has forced the oil in the piston town as it screws in.

The instructions aren't clear, but I'm guessing it's necessary to only fill the shock to the bottom of the thread, so there is an air gap to screw into.  This does seem to help, but even so, I spend a long time releasing the cap a little to bleed out excess fluid, otherwise it binds up solid.  TBH I prefer shocks with the bladder on top.

Anyhoos, with all that sorted, I put the rig back together and turned on the lights, just to make sure I hadn't broken anything while I was rummaging around.

Looking good now - I think this one's ready for the trails

[Mad Ax 9312]

Quick update on this one, after the last round of shock updates.  I took the BOM to Southern Scale Trail back in September, but it really didn't go well.  The front end was still way too soft and underdamped, and as I mentioned on that thread, it fell over a lot.

Well, if you've been following my Hillwalking thread, you'll know that I really enjoyed having the BOM as my trail companion a few weeks back, so much so that I have considered making it my #1 walking rig, and modifying it to make it a more dedicated walking truck, at the cost of being good around the tougher competition courses (which it isn't).

I figured the first step in doing this was to stiffen the front end, and lower the truck a little.  This would reduce its tendency to get a nasty roll on over bumpy terrain and hopefully make it a little more stable.  I don't have a lot of good-quality shocks around, but I do have these.

I'm pretty sure these are bargain basement eBay specials.  The rods aren't well formed (there's a noticeable vibration under compression, as if there's air in the oil) and the included springs are very stiff, but they look kinda good with that faux remote reservoir, and they are very marginally shorter than the standard shocks.  Just enough to bring the ride height down a little.  Plus, they are the same brand as the rear shocks I fitted a while back - they came on an eBay Manta Ray that I bought a couple of years ago.

The shocks don't have a lot of damping as standard, in the end I went with the thickest oil I have (a Tamiya oil, around 900, I think).  They still felt a bit soft, but what do I know?  I figured the included springs would be too hard, so I put the standard BOM springs on.

I completely forgot to take any photos, but they went on without any hassle or mods and immediately firmed up the front end.  I moved my rear shocks to a more upright position on a higher hole, to lower the back end to match the front.

I wasn't going to bother taking the rig to the UK Scale Nationals, but at the very last minute, I did.  And I'm so glad!  It was pretty gosh-darned awesome.

Surprisingly, even with the stock springs, the front doesn't want to articulate as much.  It wants to lift a wheel over obstacles now.  But that seems to hold it closer to the rocks, and stops it flopping over all the time.  Top stuff!

Photos on the trail to follow in the UK Scale Nationals thread that I'll be making later this week