[WIP] Grave Digger

[Lipko]

For reducing steering slack you could try double steering racks. Dunno how that would work with your Ackermann setup, but maybe it's worth a try.

[AxeSlash]

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?

It's not bump steer I'm concerned about. It's actual slack / backlash / play in the steering - with servo centred, you can still manually move the wheels up to somethign like 30 degrees EACH WAY. That's down to the backlash from the UJs and gearing in the centre; it's nothing to do with the pivot point.

And yes, the wheels DO rub on the 4 suspension links at full lock, hence the belt wheels to space the wheels out further. I could potentially lose the belt wheels if I sacrifice (and limit) steering lock instead. I am aware that the pivot point is in a stupid place, but the portal hubs limit my options for the moment.

As I said in a previous post, the axles will be in for a complete redesign anyway, so stay tuned for that - I will be dealing with this problem then.

In the meantime, everything else is now complete; there were some slight changes to the bodywork due to me failing to order a few parts from Brick Owl, and the bonnet (hood) needed strengthening.

The entire body is removable by removing 4 quick release pins (the grey bush pins at the bottom of each side of the main bodywork. The battery can be turned on and off by a pulling/pushing a beam via at the rear (forgot to take a pic). The battery box is removable by 2 quick release pins as well.

Hopefully performance testing and a video will follow tomorrow.

After that I will attack improvements to the steering and assess whether I can get away with more speed and less torque. I suspect that I will need an anti-roll system as well - it's fairly unstable now that the extra weight of the bodywork is on. Also there still isn't enough sag in the suspension for my liking, and the whole thing is riding too high right now. I might have to order 4 more soft shocks and replace the yellow ones with them. That's gonna make the body roll horrific...

For reducing steering slack you could try double steering racks. Dunno how that would work with your Ackermann setup, but maybe it's worth a try.

It's not the slack in the rack/pinion that's the problem - it's the transmission from the servo to the pinion that has the slack.

Edited April 30, 2016 by AxeSlash

[AndyCW]

Looks like your're making progress and learning as you go. We patiently await a video of the performance and ruggedness of the design.

v/r

Andy

[AxeSlash]

Just tried driving this outdoors...it's rubbish.

a) it's too slow, b) it has nowhere near enough torque to get over any decent sized obstacles (i.e. it needs more motor power if it's going to maintain a good speed and have enough torque to do anything sensible), c) the steering is absolutely atrocious. It's almost undrivable, and d) the body roll is a major problem. It definitely needs sway bars.

I won't be posting a video until I've fixed at least the steering and roll issues. I haven't got any more XL motors to put in it so that issue isn't going away until my bank balance looks a bit better. I might try a LiPo in it instead of the 9V box that's in there at the moment.

[Saberwing40k]

I think you'd be better off using some gear reduction, as I think both your speed and lack of climbing ability are caused by a lack of torque, and the ineffiencey of gearing up. I'd try using 2 24z gears in the drivetrain, instead of the 36z-12z combo you have now. It may sound counterintuitive, but sometimes lower gearing makes a model faster, if it was slow due to lack of torque. You can also use 12-20z gear combinations in the portal axles, if you still need more speed, as that would eliminate the need for gearing up. Also, keep your expecations in line. This is Lego, and although some builders achieve high performance, it often times is an unrealistic solution, like mounting motors on axles. Don't expect performance on the level of an R/C car, and while it is good to set a challenge, I'd aim a bit lower for your first MOC. Monster truck performance is really hard to replicate, so you might have to settle for a crawler. But keep it up, it looks as if you have a good design, and only a few hiccups.

[2LegoOrNot2Lego...]

Getting closer and closer...so cool! What about adding stickers? A monster jam truck this is hardly imaginable without any stickers...

[agrof]

Insane truck, You delivered the look!

I hope You can bring it to life, I follow the next steps.

[AxeSlash]

Thanks for the encouragement guys.

If the speed problem was down to a lack of torque, I'd be expecting it to accelerate much more slowly, but it's actually pretty quick to accelerate to top speed. I already tried it with a 12 -> 20 -> 12 tooth combination in place of the 12 -> 36 combination, and it was VERY slow - it had plenty of torque, but was basically a crawler, which isn't what I wanted for this model. I have a crawler design that I'm working on already that will be a torque beast, but for this model I wanted to aim a bit more for speed. I'm aware of the limitations of lego - I just want to achieve the best that I can.

It's quick enough for my liking on a flat, smooth surface (a wooden floor), but outside on a slightly uneven surface (crazy paving), it really struggles. Maybe some slight reduction is needed...but I'm in the middle of redesigning the axle completely at the moment anyway (I'm completely removing the portals, and moving the reduction to the other side of the UJ...hopefully the UJs can handle the torque with snapping :S This should move the steering pivot outwards quite a lot). The steering transmission is the bit that really needs work though...but that's a big job :P

As for stickers...I guess I'd have to have some printed. I tried to find a 1:15 Digger sticker set, but couldn't find anything. I don't own a colour printer, so that idea is out of the window. Does anyone know of any (cheap) sticker printers that do one-off stuff (based in the UK)?

Thanks

[AxeSlash]

So I've had a go at redesigning the axles a bit.

Kingpin incline isn't possible until I can afford another Brick Owl order, so for now this looks a bit better...although it's physically much bigger. Some ground clearance has been sacrificed. The pivot point is much further out now, which should mean less stress for the servos and hopefully a bit more steering lock, but I've still maintained the 1:3 gearing at the axles. It should be a bit stronger as well, I was always dubious about the strength of the 0.5L thick parts used near the shock mounts on the previous axle design.

The problem I'm having now is the steering transmission. It seems to be almost impossible to do without having the servo stick out somewhere awkward and/or making the wheelbase much longer. At this rate I'll end up with a motor on an axle, which I was hoping to avoid.

Edited May 2, 2016 by AxeSlash

[GoldVillage]

Just tried driving this outdoors...it's rubbish.

a) it's too slow, b) it has nowhere near enough torque to get over any decent sized obstacles (i.e. it needs more motor power if it's going to maintain a good speed and have enough torque to do anything sensible), c) the steering is absolutely atrocious. It's almost undrivable, and d) the body roll is a major problem. It definitely needs sway bars.

I won't be posting a video until I've fixed at least the steering and roll issues. I haven't got any more XL motors to put in it so that issue isn't going away until my bank balance looks a bit better. I might try a LiPo in it instead of the 9V box that's in there at the moment.

Are you familiar with the Tamiya Clod Buster steering? a servo in the chassi center and then linked to axles with rods. This could work if your suspension travel doesnt have too much of a play, if it has maybe the best solution is to have 1 servo mounted / per axle, but then you need redesign them again :/

[AxeSlash]

I'm not familiar with the clod steering system, and google just seems to turn up images of a servo on each axle. However, I think I understand what you mean - solid rods coming from the centre of the chassis instead of an axle; i.e. it's a linkage instead of a gearing system. I like the idea, but that's a pretty major redesign to fit it in. Plus it would necessitate more ball links than I currently own. Definitely something I will investigate in the future though :)

I've redesigned the steering transmission now, and there is only one set of gears involved now. Tomorrow I will test how much backlash there is between two knob wheels and between two 12z gears to determine which will be best to use. I've basically split the drive transmission in two, and fed the steering transmission down the middle of it (i.e. the opposite of what I was doing previously). So the drive will have more gearing (and thus more friction) in it, but the steering should be much better. It does, however, mean that I'm only using one servo, so I'm hoping it will be strong enough to turn the wheels. If not I think I've got room to chain two servos together without any extra gearing.