Durandal: Custom 2.2 Monster

[Mad Ax 9329]

I also took advantage of some warm, dry weather to paint the body.  Not a perfect job (I really must stop using cheap masking tape to cover the patches left between Tamiya masking tape - I always get a little airhole open up that lets some dust in that I don't notice until after I've added the next colour).

It took me a couple of days to come up with a name.  First I thought Emperor Blackfoot (since it's better than a King Blackfoot), then I just thought Emperor; I toyed with going all romantic and calling it Pied Noir, and eventually decided on a mythical sword name.  Excalibur has been done to death, and I didn't want to dredge up one of the hero swords from my fantasy novels, so I went with Durandal.  Sharpest of all the mythical swords, forged by Wayland the Smith for Charlemagne and passed on to his paladin Roland.

My new laser printer came in handy to print the truck names; the SCRAPSpeed logos were left-overs from a set printed for me by a friend for FiftyFifty back in the Before Times.  The rest of the decals are a mixture of JConcepts, MCI and various cheap sticker sets I had lying around (including some from a Max Power magazine that I bought in a newsagent way back in 1999).

I left space on the bed and along the black side panels to add some silver Ford decals from MCI's Ford sponsor decal sheets, but I've probably spent way too much money already and should back off buying yet more decals for a while.

[Mad Ax 9329]

So - what's next?

Well, the shocks are way too stiff, so I'll probably do what I always do and buy some CVAs instead.  They're just blatantly better than any unbranded alloy stuff.  Then I can tune the springs and damping myself.

I'll probably need some sway bars to deal with torque twist.

I need to look at the alignment.  I think some of my screw holes may be a little bit out, causing my body to not sit entirely flat.  This probably means my shocks aren't entirely level either, which partly explains why the truck veers off to one side on hard acceleration.

The geometry is pretty bad also.  The top links are too short, so the axle rotates a lot under compression, upsetting the caster angle.  I'll probably have to come up with a new link arrangement for the front, maybe some kind of 3-link or reversed 4-link.

Also I need to make a battery retainer solution.  Cutting some slots in the top and feeding through some straps should do the trick.  I considered an underslung battery but there isn't really enough clearance on this rig - the transmission takes up too much space.

[ThunderDragonCy 7884]

@Mad Ax That looks fantastic (as usual). Shame the meet this saturday is off now. Was looking forward ro catching up. These trucks of yours just look so cool. 

[Mad Ax 9329]

@ThunderDragonCy thanks

I was looking forward to seeing you guys and getting one last run of all the crazy stuff before the winter sets in.  I just about managed to get this one ready to play with, too - would have been nice to see how it went around the indoor track.

All my monster projects look cool enough but sadly fail to deliver where it counts - the Mod Clod kept breaking lower link rod ends (crawler geometry was probably as much to blame as weak components) and FiftyFifty was too heavy for its own good, but I thought I might at least get a few laps in with this one before something important fell off it.  I still have plans to make FiftyFifty lighter and reduce the twisting force on the bottom arms, and to rebuild the Mod Clod with a more race-oriented chassis, so maybe I'll actually have some half-decent runners to play with when things re-open again.

I've been hoping to set up some kind of monster jam event for a while now, but I think it makes sense to see what 2021 brings us before making too many big plans... 

[ThunderDragonCy 7884]

7 minutes ago, Mad Ax said:

@ThunderDragonCy thanks

I was looking forward to seeing you guys and getting one last run of all the crazy stuff before the winter sets in.  I just about managed to get this one ready to play with, too - would have been nice to see how it went around the indoor track.

All my monster projects look cool enough but sadly fail to deliver where it counts - the Mod Clod kept breaking lower link rod ends (crawler geometry was probably as much to blame as weak components) and FiftyFifty was too heavy for its own good, but I thought I might at least get a few laps in with this one before something important fell off it.  I still have plans to make FiftyFifty lighter and reduce the twisting force on the bottom arms, and to rebuild the Mod Clod with a more race-oriented chassis, so maybe I'll actually have some half-decent runners to play with when things re-open again.

I've been hoping to set up some kind of monster jam event for a while now, but I think it makes sense to see what 2021 brings us before making too many big plans... 

Reminds me. I had your Fifty50 plates packed to bring at the weekend. I need to post them back. Drop me a PM with your address. 

[Mad Ax 9329]

7 minutes ago, ThunderDragonCy said:

Reminds me. I had your Fifty50 plates packed to bring at the weekend. I need to post them back. Drop me a PM with your address. 

Oddly enough, I was going to reassemble it the other week.  I was 100% sure you had already posted them back to me, but they weren't in the FiftyFifty box with the rest of the bits.  In all seriousness, I was planning to go hunting around for a jiffy envelope hidden among my storage on Sunday.  If you hadn't posted this I'd have probably started cursing myself for never putting things away properly.

Will send PM

[Aerobert 2148]

The truck looks great. Love the paint theme, espeacially the Durandal logo. Great work. 

[Mad Ax 9329]

It was 4pm on Sunday and I still had 90 minutes to kill, so I was rushing around looking for something I could do that would keep me busy and not involve starting something new or having to order hundreds of pounds worth of stuff.  I had a note on my To Do list to make a battery tray for Durandal so it would officially be a proper runner and not just a truck that sort of runs very roughly if you secure a battery in it with tape.

Once it was on the bench I gave it a more critical eye and realised the wheel base was actually wrong for the body, and the geometry is horrible under compression.  The top links are mounted way forward on the chassis and way back on the TLT servo mounts, making them very short.  As such, under compression, the axle rolls forward and adds negative caster, making the truck very unstable and tippy under compression.  As a heavy-wheeled monster it's tippy enough as it is (it took about 30 mins to get a single effective shot of the truck launching over a ramp on a hard flat surface for a video).

So - I quickly whipped off the side plates and made some new holes as far inboard as I could.  There will be a bit of clearance fun with the motor later but we'll cross that bridge when we come to it.

Here's the chassis extension plates - original holes were based on the Axial chassis plates and are visible left and right of the bottom of the chassis extensions.  New holes are close together in the centre.

Then I dug out my trusty chassis jig and re-drilled it for the 10.25" wheelbase.  It was at this point that my wife politely pointed out it was half past five and I was supposed to be coming in for dinner, so I left it all out on the workbench for another day.

Watch this space - further updates pending...

[Mad Ax 9329]

I had thought I might get some time last week to work on this, but with all the things going on in the world, at work and at home, it ended up staying on the workbench until my day off on Sunday.  And then with some family issues to contend with I didn't have the headspace I wanted to really get into this project, but I did get a few hours in the workshop, so a little progress was made.

First thing was to get the jig set up properly.  This jig has been re-purposed for a few projects this year, every time I learn a little more and put a little more effort into accuracy, so I hoped this time I would have a really solid and reliable jig to get my dimensions spot on.

Wheelbase as made and wheelbase as measured were further out than they looked, so there was a lot of meat to take off these links.  Around 15mm from each lower link - that's a total 30mm shorter.  It's surprising it didn't look worse...  More on this later.

Here I decided to go all-out, measuring everything accurately to try to get a perfectly square jig.  In the past I had done everything from measuring by eye to doing some basic triangulation, but this time it was properly marked measured.

The cross brace stops the jig from moving

And the plates screwed to the cross brace hold the chassis plate in the position it will need to be on the final rig, under preload

[Mad Ax 9329]

To get the chassis base in exactly the right spot, I needed to add the plates, body posts and body, so I could measure where the axles should be in relation to the arches.

[Mad Ax 9329]

Plates and transmission removed, chassis bottom screwed into jig

I chiselled in some cut-outs so I could get my tools onto the link mounts

Front lower links adjusted, new upper links in process of being adjusted

Pretty soon it became apparent there was an inaccuracy somewhere in my jig.  I suspect the chassis bottom mount.  The links were visibly skewed and couldn't all be fastened at the same time, so I popped it all off the jig for final assembly.  All links were measured against each other before installing, so if the chassis was out-of-line once assembled then it simply had to be that the axle mounts weren't square, and I didn't make those, so I couldn't be to blame.

[Mad Ax 9329]

Anyway, everything looked better once it was assembled, wheelbase was correct (as measured) and the links looked square and even.

I had measured the front caster angle to be around 8 degrees under compression, but with the stiff shocks re-fitted it was more like 20 due to the geometry change.  Even with significantly longer top links, there's still quite a lot of change in the steering geometry as the links move through their arc.  A possible solution here is to switch to a low-profile servo and re-make the axle upper link mounts, or to switch to a chassis-mounted servo and use a totally different link mount.

I had to re-fit the axial transmission, as the can interfered with my new upper links.  At one point I thought I'd wasted a lot of time and effort, but with some careful repositioning it was possible to get everything in place.

[Mad Ax 9329]

Bizarrely, at this point I seem to have run out of photos, although I did reassemble the entire truck.  The shocks are still painfully stiff and totally screwing with the geometry, so I have ordered some Tamiya CVAs.  Hopefully these will allow some static sag.

Also, although the wheelbase is now correct as measured, it visibly looks a bit short.  This is possibly because the stiff shocks cause the suspension always to be at full extension, so there's a little longitudinal movement on the axles, and also because the truck looks so tall it appears a little weedy.  Better-adjusted shocks should solve this problem, but if not I have got room to extend the wheelbase by around 10mm.

The rear driveshaft (which is an SCX10 item) is binding on full extension - I've angled the rear axle upwards as much as I can, but the shorter wheelbase combined with relocated transmission means it's the inner joint that is struggling.  It probably won't be a problem when running because the compression will keep the axle within the maximum angle of the CV joint, but on full extension (e.g. over jumps) it might send a vibration through the chassis or even damage the CV joint.  Long-term I should think about putting another Cross RC shaft in there like I have on the front, as these slide much freerer and have a wider useable angle.

I've got another workshop day planned on Sunday, so maybe I'll get the new shocks fitted then if they arrive.  After that I can decide what the next steps will be for this rig.  It will almost certainly need some sway bars to control torque twist, but I don't know how easy it will be to make some.  I guess I'll need some sprung steel.  I still have the standard sway bars off the TXT-1, which could be repurposed, maybe...

[Mad Ax 9329]

Tamiya CVAs arrived this week, so I built them yesterday afternoon while I was watching Disney's Robin Hood with my daughter.

Today I had a bit of time after fitting the battery strap on Fifty Fifty, so I pulled Durandal off the shelf and fitted the new shocks.

They went on without much hassle, I have plenty of ball studs which fitted easily in the top mounts.  For the lower mounts it would have been better to use ball nuts but I only had one left - I'll have to order some more.  Ball studs used temporarily.

I went with a more upright shock position as the new CVAs are very soft (I used the soft springs from the kit).  I've got quite a lot of tuning options now (harder springs, different mount points) to tune the truck as and when I get onto a track, however reducing the wheelbase has meant two of the rear shock mounts aren't really usable as they put the shock over the wrong way.  OK, it will still work but it looks all wrong and the progressive springing rate will alter as the axle moves inboard.

[Mad Ax 9329]

With all 4 shocks fitted, I put the ESC and receiver back in and started thinking about a battery tray.

I glued on some pieces of styrene square section to act as battery stoppers.  I'm not sure this plastic is ideal for gluing as one of them came off while I was fiddling with something else.  I might have to screw them on instead.

I cut a slot in the plastic to accept a velcro strap

I still need to make something to secure the front of the battery, as it will slide forwards under heavy braking, but otherwise it seems like a good installation.

I took the truck out for a quick blast in the garden and on the back lane, and it's now way better than it was before.  Suspension is soft but manageable and the torque twist isn't as bad as I thought it would be.  I took off the spring collars to drop the ride height a little, which reduced the roll.  It's now just about as much fun as it was when I started this project, although it felt quite slow - I was using a LiPo at storage charge, so it might have gone into cutoff mode.  I didn't see the point in charging up a pack as I wasn't planning on driving around on my own.  Anyway, I had a lot of fun doing wheelies and driving on two wheels - for such a small truck it's remarkably drivable on the edge.  A few times it threatened to roll and I just turned into it and kept it on two wheels for a foot or two.  Probably won't win any freestyle events but it's better than having to flip it over every 30 seconds.  Actually it only got stuck upside down once, when I braked a bit hard on tarmac.

Ultimately I'll want to put some sway bars on, although I'm still not sure what I'll use.  I don't know if it's possible to buy various strengths of sprung steel and bend it into shape, or if I'll have to repurpose something from somewhere else...

[Mad Ax 9329]

New wheelbase looks about right with the proper shocks installed

[Mad Ax 9329]

Sunday morning was my first official day off of 2021, so after I'd wrapped up warm and got the log burner going, I started looking around the workshop for Things To Do.  I had previously stripped down a Hotshot re-re to build a custom chassis, but then I remembered I still haven't made the battery tray for Durandal.  That then led to the following sequence of events, which lasted the entire day and left me with a box full of bits.

Here is Durandal, with the electrics tray modified from the original Maverick Scout item

There's nothing to stop the battery coming out front or back, or twisting sideways.  I had tried to glue on some ABS pieces to assist, but the plastic tray is the wrong sort of plastic for cement, and they fell off.

While I had the truck upside down, I noticed more problems.  The rig is slightly twisted.  At first I thought this was because I hadn't drilled my chassis plates with enough accuracy, but actually it was worse than that.  The delrin base plate (which I made years ago) isn't entirely square.

The twist is most obvious in this photo

So, what to do?  I could make another base plate from Delrin, but my new bandsaw has already proved itself quite bad at cutting straight lines, and my new mitre block still hadn't arrived to help me cut accurately by hand.  However I do have a few sheets of 2mm and 3mm FR4, which I figured would be lighter and easier to cut, and allow me to use pre-formed right-angle material to get perfectly vertical chassis plates.

I started by cutting some brackets from 15x15 alu right-angle.  As you can see, my hacksaw cut is not entirely square.  As these cuts need to be very accurate, I decided to cut them deliberately wide and finish with a file.

[Mad Ax 9329]

Cutting the second bracket is easier.  Cut it big, line it up with the first, then file down until it matches.

I then added some screw holes to mount to the vertical plates.  The holes were positioned by placing both parts together and marking - serendipitously the hole spacing is perfect for the nut to be captivated by the right angle.

To drill the opposite bracket to the exact same dimensions, I lined up both brackets and clamped them, then put both in the drill press, using the existing hole to locate the drill bit

[Mad Ax 9329]

Then I turned my attention to the FR4 sheet.  Here I was making the bottom chassis plate, so I used 3mm for strength.  I cut this on the bandsaw - as you can see, the line isn't entirely accurate.  I had to finish it with a file.

It was at this point that I went to drill some more holes in the right-angle brackets to mount the horizontal plate.  I was using the drill press, which is usually a pretty safe way to drill, but somehow a huge lump of swarf got flung off the bit on a direct trajectory into my right eye.  I'm usually pretty strict with PPE (in fact I'm usually the one remining my wife to wear gloves / goggles / hairbands / tie her apron strings back when working on the wood lathe) but I've got complacent with the drill press.  I once spent an hour lying on my back while a nurse used the tip of a needle to scrape a burned off lump of steel grinding swarf that had burnt itself into my eyeball, and it's not an experience I want to repeat.  (I was wearing safety glasses back then, but I was in an enclosed space with me head up inside an Austin Mini A-pillar and some swarf managed to blow back off the firewall into my eye, such is the risk of using a grinder with a vented fan on the motor).

So at this point I downed tools, rushed inside without daring to blink, looked in the mirror to see a jagged razor of aluminium resting on my lower lid.  I figured I'll pull my lower lid down to hopefully tilt it away and drop it into the sink, but it had caught on the soft flesh of my eyeball, and instead of rolling out it rolled deeper in.  Now I daren't let go of the lower lid, as it would close over the caltrop and lacerate both eyeball and lid and probably involve a very embarrassing journey and a very long and painful queue and a very unpleasant procedure in a very over-subscribed hospital.  So at this point I sort of gritted my teeth, prayed, and stuck my fingertips in to tease it out, thankfully without breaking any skin.

So please - learn from my mistakes - WEAR YOUR PPE!!

[Mad Ax 9329]

Anyway, disaster averted, I went back to the job at hand.  I realised I had drilled the lower horizontal plate and the right-angles but I hadn't measured the transmission location.  This means I now have extra holes that I don't need, which might weaken the plate, but there you go - one lives and learns.

OK - now for assembly:

Axial transmission mounts double up as securing screws for the plate to the right-angle.  Hopefully this will be OK.

View across the top now reveals a much more square chassis.  Here it shows very slightly out as the plates are very slightly splayed (I think the right-angle alu stock is not entirely right-angle), but it's way better than it was and it's about as accurate as I can manage without precision equipment.

[Mad Ax 9329]

Next plan - battery tray.  Here I have cut and mounted one right-angle bracket.

Plate mounting holes drilled

contours added so it doesn't appear behind the chassis plates

2mm FR4 sheet cut and filed to make the tray

[Mad Ax 9329]

Mounting in progress

Mockup of electronics positions

Battery stays made from more right-angle section.  These should be symmetrical but I had to file each side by eye, as the right-angle makes it impossible to flip-and-file to get the same profile both sides.

Mounted.  Annoyingly I measured this to make sure they were central, but somehow they have some out off-centre.  I'm not entirely sure what went wrong here but I expect it was a mistake in finding the centre of the brackets (due to the assymetrical rounding on each end).

Finally I drilled holes for the battery strap.  This is where things unfortunately get messy - I do have a router, but it's too big and clunky for this kind of work.  FR4 files quite nicely but it took a while to tidy this up, and I was at the end of my workshop day before I was done.  So, swept everything off the shelf and into the box for another day.

So - we're very nearly done with the battery tray, using a smaller shorty pack for better packaging, and with more accurate geometry and more rigidity for more consistent handling.  I still don't have any piano wire to make some sway bars but I'll probably leave that until another day anyway.

More to follow when I have time to finish the battery tray and reassemble the truck for a test-drive.

[Dakratfink 967]

Great progress on an awesome build. Glad your eyeball is ok.

 

[Mad Ax 9329]

8 hours ago, Dakratfink said:

Glad your eyeball is ok.

So am I.  There's probably a 6 week wait just to get into A&E at the moment, although maybe once I'm in they can pull the bad tooth that's been bothering me since October  

[Mad Ax 9329]

I started Sunday morning bright and early with a roaring fire (courtesy of some freshly-delivered heat logs) that soon had the workshop feeling, if not exactly toasty, then at least bearable.  In fact by early afternoon the temperature in the paint booth was over 11 degrees C, which is pretty good going since it's a few metres away and behind a curtain.  At times it was too hot to stand at the end of the workbench where the vice is mounted.

First thing I wanted to do was tidy up my strap slots.  I wasn't happy with how rough my drilling had been so I found a grinding wheel for the Dremel and tidied it up.

That's better - a square chassis