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I've been lurking here for a while but have never been satisfied with any of my MOCs enough to post them here. The nearest I came was an MLRS that worked well but looked rubbish :P

Anyway, I'm in the process of designing a Grave Digger monster truck replica (approx 1:15 ish scale).

Features:

- 4 wheel drive

- 4 wheel steering

- Steering modes: 2-wheel, 4-wheel, crab

- 4-link, very long travel suspension

- Suspension travel is also to scale. Almost.

- All motors contained in the chassis (no servo-on-axle affairs)

- SBrick control

- Tyres from a Tamiya Lunchbox

I'm designing this in Sketchup and rendering with the Twilight Render (free) plugin.

Note that I've drawn all the pieces myself, and they're all drawn to have a low polygon count so that my machine doesn't croak to a halt. Some of the pieces aren't very accurate atm (e.g. the XL motors).

So far, I think I've got the chassis designed:

chassis.jpg

And here's a couple of closeups:

drivetrain.jpg

axle.jpg

axle-bottom.jpg

Issues I can foresee so far:

- Friction from so many knob wheels

- Bump steer (but I don't want to sacrifice suspension travel by putting the servo on the axle)

- Not sure whether having the steering upside down like that will have any negative effects.

- Not sure if I can get all of the parts in green that I want

Can anyone see any other potential issues with this design (before I start putting in BL orders)?

Thanks

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Several parts for the bottom of the chassis aren't available in green. and what and how many motor did you use for these model?

Edited by KamalMYafi

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The manner in which you mount the springs to the axles will cause issues. They are likely to slide of of the pin. I would recommend putting the 'spring axle mounts' in double shear.

It also looks like you will have problems with the 9l links popping off of the balls or the ball pins popping out of their location. They should be trapped.

v/r

Andy

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Also I did do a potential second bodywork design - Bigfoot 18's trophy truck-esque body:

bf.jpg

But a) those curved panels are too rare in blue at the moment, and b) some of the other stuff in blue is rare/difficult to find at the moment, and c) the wheelbase of the chassis is actually a bit too long for that body to be in scale.

Potential improvements I may make at a later date:

- Custom 3D printed rims, which would leave much more room for...

- ...Custom 3D printed planetary hubs (real monster trucks have planetarys, and the portals mean that the axle ends up too high, making the suspension a pain to get correct. Not to mention the atrocious steering geometry)

- Pneumatically dampened suspension, like the real thing

- Gear box - going to leave this until I can assess how it performs without one, so I can see what kind of ratios I need.

Edited by AxeSlash

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Several parts for the bottom of the chassis aren't available in green. and what and how many motor did you use for these model?

Yeah I thought that might be the case. Drive is 2 x XL motor, steering is 2 x servo. There is an M motor for switching the steering mode, but I might be able to get rid of it if the SBrick profile designer will allow me to do it there instead.

The manner in which you mount the springs to the axles will cause issues. They are likely to slide of of the pin. I would recommend putting the 'spring axle mounts' in double shear.

It also looks like you will have problems with the 9l links popping off of the balls or the ball pins popping out of their location. They should be trapped.

I've used the suspension setup before and the links are easily secure enough on the ball pins themselves; but yeah I see your point about the ball pins popping out of the beam.

Can you explain what you mean by 'double shear'?

Thanks

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500px-Bolt-in-shear.svg.png

The top part is mounted in single shear while the bottom is in double shear. The spring would be the part on the right and the axle would to the left.

In this example, instead of mounting the spring onto the end of a pin, support both side of the spring mount. This will prevent the spring from slipping off of the end of the pin.

Here is a pic of what I am talking about on one of my MOCs.

hopefully this clear things up.

v/r

Andy

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My favorite IRL monster truck. Looks very promising so far, but its impossible to foreseen problems since they shows up where you dont expect it, im working on a project in green myself right now and most of the beams are actually available in green, both the 9l and 7l are, and so are the L shaped beams. it may just be some of the links you need use in other colors. Smart move to go with good ole lunchbox tires, you nailed the 1/15 scale there pretty much right on.

The shocks are probably gonna bend a bit no matter in which direction they're mounted, a good way to prevent it could be to link everything in the suspension to ball joints with using i.e a cantilever system.

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The top part is mounted in single shear while the bottom is in double shear. The spring would be the part on the right and the axle would to the left.

In this example, instead of mounting the spring onto the end of a pin, support both side of the spring mount. This will prevent the spring from slipping off of the end of the pin.

Here is a pic of what I am talking about on one of my MOCs.

hopefully this clear things up.

v/r

Andy

Yeah thought that was what you meant, I just wasn't familiar with the term 'double shear'

My favorite IRL monster truck. Looks very promising so far, but its impossible to foreseen problems since they shows up where you dont expect it, im working on a project in green myself right now and most of the beams are actually available in green, both the 9l and 7l are, and so are the L shaped beams. it may just be some of the links you need use in other colors. Smart move to go with good ole lunchbox tires, you nailed the 1/15 scale there pretty much right on.

The shocks are probably gonna bend a bit no matter in which direction they're mounted, a good way to prevent it could be to link everything in the suspension to ball joints with using i.e a cantilever system.

Lunchbox tyres aren't exactly correct scale dimensions, but they're the nearest I've found. I'd still prefer something that would fit on a smaller rim (with the same overall diameter), but it just doesn't seem to exist. The tread pattern isn't technically correct either, but again it's the best compromise that can be found at the moment without going up to something silly like Clodbuster tyres and custom parts to mount them on...which would result in an ENORMOUS MOC that would be fairly difficult/impossible to power without stripping gears etc.

I also didn't want to go cantilever because the real thing doesn't have a cantilever system. If I was doing one of the older Bigfoots then yeah, that would be cool and probably work a little better.

I guess it's always difficult to draw the line between realism and functionality...I'm trying to lean towards the former here.

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Yeah thought that was what you meant, I just wasn't familiar with the term 'double shear'

Lunchbox tyres aren't exactly correct scale dimensions, but they're the nearest I've found. I'd still prefer something that would fit on a smaller rim (with the same overall diameter), but it just doesn't seem to exist. The tread pattern isn't technically correct either, but again it's the best compromise that can be found at the moment without going up to something silly like Clodbuster tyres and custom parts to mount them on...which would result in an ENORMOUS MOC that would be fairly difficult/impossible to power without stripping gears etc.

I also didn't want to go cantilever because the real thing doesn't have a cantilever system. If I was doing one of the older Bigfoots then yeah, that would be cool and probably work a little better.

I guess it's always difficult to draw the line between realism and functionality...I'm trying to lean towards the former here.

Oh yes but they are, they are so close in size that they are only a couple of mm off if you doing the calculation comparing to the scale. The tires grave digger uses (and almost every other truck too) is a slightly cutted down version of the classic 66 x 43 size tundra tires, originally used in harvester and farmer equipment, as well in 4wd 2 axle skidders.

Lunchbox tires are almost exacly 110 x 71 mm, now take that x 15 times and we have 165 x 106.5 cm. The 66x43 tires the monstertrucks using are originally 167 to 172 cm in diameter and 105 to 109 wide depending on the manufacter, and after the tread pattern are being cutted down a bit to make them lighter, they pretty much ends up as exacly in scale to the lunchbox tires. Well down :thumbup:

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Oh yes but they are, they are so close in size that they are only a couple of mm off if you doing the calculation comparing to the scale. The tires grave digger uses (and almost every other truck too) is a slightly cutted down version of the classic 66 x 43 size tundra tires, originally used in harvester and farmer equipment, as well in 4wd 2 axle skidders.

Lunchbox tires are almost exacly 110 x 71 mm, now take that x 15 times and we have 165 x 106.5 cm. The 66x43 tires the monstertrucks using are originally 167 to 172 cm in diameter and 105 to 109 wide depending on the manufacter, and after the tread pattern are being cutted down a bit to make them lighter, they pretty much ends up as exacly in scale to the lunchbox tires. Well down :thumbup:

Yeah the OD and width is correct, but I was talking about the RIM diameter - lunchbox rims are too big. There are some older monster trucks that had rims of a similar size, but most modern ones are much smaller.

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Build progress:

20160425_212044_hdr.jpg

You can see the triangulated 4-link suspension arms there (and the horrific steering geometry). Lunchbox tyre + 44772 just to show that they fit together, in case anyone was dubious.

You can actually fit two 44772's inside a Lunchbox tyre, but this happens:

imag0265.jpg

You get an unnatural looking bulge in the sidewall because the bead is being pushed outwards (the rims are slightly too wide). That's kinda the opposite of the effect most monster trucks have - they usually use thinner rims than recommended for their tyres so that they get a more 'balloonish' effect on the sidewalls. So I'm opting to just put one rim in there, and leave the other bead entirely unsupported. I reckon the tyres can handle it. If not I'll make some foam inserts for them.

So far I haven't had to modify the original design at all (apart from having to use a grey worm gear - one of the black ones I own turned out to have too much friction inside), and everything feels fairly sturdy...the only minor problem so far is that at full suspension compression and extension, the worm gears on the driveshaft and steering shaft foul on each other. That said, I suspect that once the springs are installed that they will limit the travel before it gets to that point - if my calculations are right I should get around 6L of axle travel with the springs in place, which is less than the approx 8L of axle movement at the moment (with no springs)....we shall see.

I may need to give the racks more guidance as well, there may too much play in them at the moment. I'll deal with that once the chassis is built and I can see whether it causes an issue or not.

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Looks good but yea that steering geometry is HORRIBLE! have you considered a kingpin incline like in my crawlers?

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Built some more on this tonight:

20160426_213004.jpg

The steering is a joke - way too much play. Once I've added the bodywork, the steering at the axle will be in for a redesign (when I know what parts I have left to play with). Probably going to use the longer racks and cut out the two 3L beams completely.

It's also a bit slow. There's plenty of torque available so I think it's time to gear the XLs up a bit.

The suspension has no sag at the moment, but hopefully the weight of the bodywork will help with that. Other than that the suspension works quite well at the moment, although I had to flip the saddles at the top of the shock assembly around to make the shock assembly longer so that the chassis did not bottom out when fully compressed. Body roll might be a problem when the rest of the weight is added, but I'll cross that bridge when I come to it. Might need to add a sway bar system.

The lunchbox tyres can hold the weight easily so far - no sign of deformation yet. We'll see how they hold up when the bodywork is on :P

The axles might need some lateral strengthening - I get a slight (unintentional) kingpin incline at the moment ;) Slapping a 7L beam across the top of the diff will probably do the trick, although that may limit the suspension travel a bit.

Edited by AxeSlash

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The truck in Fury Road is not a Bigfoot.

In fact, there were two trucks used in the film, both of which were Australian...I'll try and find the source on that info.

I toyed with doing that as well (Fury Road is pretty much my favourite film ever), but the curves would be difficult/impossible to make look good without going with a full stud body...which my bank balance can't cope with right now :P

EDIT: Ah I see, it's called 'The Bigfoot' in the film, although in real life it is not a Bigfoot 4x4 Inc. truck (pretty sure Bob Chandler's lawyers are itching right now):

http://monstertruck.wikia.com/wiki/Shattered_Silence

Edited by AxeSlash

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I toyed with doing that as well (Fury Road is pretty much my favourite film ever), but the curves would be difficult/impossible to make look good without going with a full stud body...which my bank balance can't cope with right now :P

http://monstertruck....attered_Silence

I need to get the shock absorbers, hubs and wheels, otherwise I have plenty of light blueish gray for a studded body.

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More progress.

  • Changed some bits at the very front of the axle to reduce rack twist.
  • Strengthened the bottom shock mounts a bit (they were prone to coming apart)
  • Swapped out some UJs for CVs in an attempt to reduce backlash in the steering. It's a lot better, but still not very good...it's as good as it will get without redesigning the entire centre chassis section and/or doing away with the option of crab steering. Whether the CVs can cope with the strain of moving those big tyres remains to be seen.
  • Added a beam across the bottom of the portal pivots to strengthen. Haven't checked if it's helped yet - ran out of time to do the other side.
  • Added some retro axle safety chains, 80's monster truck style :P
  • Bought a PS4 controller so I can use the SBrickController app (never could get the Xbox One controller to work with my M8)

20160428_213705.jpg

20160428_213746.jpg

20160428_213818.jpg

The two tan 20T gears visible in the last shot are the main culprits in the steering backlash problem. The axle on the right is now in it's finished state. The axle on the left still needs some bits changing to match the other one.

Next up: battery and SBrick supports, then bodywork (during which I will almost inevitably change something and as a result run out of black parts, then have to wait for a Brick Owl order to arrive...)

Edited by AxeSlash

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Impressive chassis Liftarm setup/geometry...really cool! :wub::thumbup:

That probably comes from the lack of limitations that Sketchup imposes :P I suspect that something LDD might not let one achieve some of those not-quite-pythagorean triangles :P The green stuff is loosely based on the real thing, as far as lego dimensions allow.

Edited by AxeSlash

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For anyone that's interested, here's the drivetrain and steering, although the UJs on the steering shafts are now CV joints:

drivetrain_and_steering.png

Edited by AxeSlash

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Just done some more work on this - the quest to reduce steering play continues!

The result...a more primitive rack & pinion setup:

20160429_232606.jpg

Note the rack is now half a stud further away from the chassis. so that the red parts clear the diff (you can see how close it is to fouling).

The play has been reduced by getting rid of those wobbly links between rack and portal. There is now only one frictionless pin on each side instead of two, There is still too much play, but at least now the wheels are both pointing in the same direction :P Ironically, for some reason I seem to get more lock out of this setup as well. So much so that I'm a little worried that the tyres might rub on the suspension links.

And as for the return of the UJs...the CVs couldn't handle the torque of turning those big tyres. I'm going to have to look into reducing the backlash in the gearing in the centre of the chassis instead, which will be difficult.

In other news, the motors are now geared up 3:1 in the chassis, then 1:3 at the portals. Accounting for the diffs as well, that gives a final ratio of 1:1.4. It's definitely not a 'fast' model, but it's not a crawler any more. It feels like there's some torque to spare. Depending on how it copes once the bodywork is on, I might see if I can get away with going up to 5:1 in the chassis ( a 40T driving an 8T instead of the 36T driving a 12T that it now has). That would give a final ratio of 1:1.2.

The area where the suspension arms mount to the axle isn't strong enough either, needs some reinforcement.

Edited by AxeSlash

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I think the main problem of your bump steer is how FAR away you steering pivot is away from the wheel (as you said earlier) so why did you make it farther away by putting pulley wheels on the portal hubs? I don't see the wheels rubbing on anything.(self promotion here) Why dont you try using kingpin like my Super crawler and 9398 MOD to get the steering pivot near the center of the wheels?

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Sometimes the solution cannot be nailed as perfect as the RC manufacters (they can just make the pieces they want) but here we need stick to whats available in lego. But its a really smart move to use worm gears for telescopic axles ;) :thumbup:

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