Full featured steered axle for heavy trucks

[MinusZwei]

I have started designing a steered axle for my heavy truck project with the following features:

- Blocking differential (will be controlled by a PF M motor)

- Pneumatic breaks (following

Sheepo’s Peterbuilt

system)

- Portal Axle with 1:3 reduction

- Triple wishbone independent suspension

- Ackermann steering (still WIP)

Please note that it is still work in progress, and the design is not yet final… The steering is not yet fully designed, and the “casings” might also change here and there.

As this is more or less the beginning of my first real big MOC, comments are always welcome. I think there is still lots of improvements possible, and I would love to get expert technic builders advice :)

I have attached the LDD file (might not be 100% up to date) so that you can have a closer look to my mechanics, but I will anyway describe them in more details here.

1. The Wheel

Let’s start with the simplest element: The wheel itself.

I am thinking big; therefore, I take the Unimog wheel for my truck (44772/92912). Regarding the cosmetic aspect, I put a black dish to give it a 41999-ish look to the rim that also fit nice with a military truck.

The inside is far more interesting. Following Sheepo’s Peterbuilt drum breaks, I put another rim (60208), maintained in place with 3 axles. At first, I used 3L blue pins instead, but using 3L axles you spare ½ stud, and the drum is “sticks” to the rim as much as possible.

2. The portal axle + break pads

As I needed to put the pads mechanism on the portal axle, the use of the “standard” Technic portal axles (92908) was not really an option. I started to design my own ones, taking some inspiration from Atr’s Volvo FL12 8x8 Trial Truck.

I used the new 8-tooth gear and the “good old” 24 tooth one to make a 1:3 reduction (If you plan heavy trial trucking I think replacing with bevel gears would be a smart choice, but you would have 1:2 in this case).

Please note that the steering is not fully finished yet. I still have to calculate the correct angles and gear reductions to make a nice Ackermann steering. I am also planning to use the 13L gear rack (64781) for some axles, to maximize the steering angle.

3. Suspension system

The original truck that I try to reproduce in my MOC features independent suspensions with 2 wishbones. For my MOC, I have decided to add a 3^rd one to keep the driving axle a bit more solid. Apart from that, nothing really special here.

As my truck will have lots of “Model Team” stuff, I also use the Unimog extra hard shock absorber (95292c01). Currently I only put one per wheel: I don’t want to have too hard suspensions, and adding a second one may block the portal axle when steering at extreme angles.

Using the standard 6L shock absorber is not really an option here: Its amplitude is 1 stud less than the 9.5L shock absorbers. This would reduce the wishbone course too much.

4.Blocking differential

For the differential, I followed one of Sariel ideas. I needed to modify it a little, to let the driving axle continue to the other axle: I just “lifted” the locking mechanism using 16-tooth gears. Using this trick, I can also put all the driving gear on the same side and therefore motorize it far more easily.

Please also note the thin 1x5 lift arm: It is used to limit the course of the changeover catch so that it doesn’t get out of the driving ring when unlocking the diff.

Steered Axle 2.0.lxf

[jorgeopesi]

The final weight will be a nightmare if you use axles, in my opinion for this scale trucks we have to use the unimog portal pieces or even turntables but I will watch with attention this project .

[MinusZwei]

The final weight will be a nightmare if you use axles, in my opinion for this scale trucks we have to use the unimog portal pieces or even turntables but I will watch with attention this project .

I have tested with some weight on top (~1.5 kg), and I did not see any big bending on the axles.

Of course, the wheel would be better attached with the 3 pin Unimog portal piece, but in such a case, I would loose the space to place the brake system.

[nicjasno]

This is not good. The steering pivot is so far away from the wheel, that it'll require a ton of force to steer this. And i can't imagine what any weight will do to this. Also, the axles will just bend.

The brakes can be put inboard at least, on the side of the differential, so you can mount the wheels directly to the wheel hubs and get at least some rigidity. The middle wishbone is also not necessary.

[MinusZwei]

This is not good. The steering pivot is so far away from the wheel, that it'll require a ton of force to steer this. And i can't imagine what any weight will do to this. Also, the axles will just bend.

The brakes can be put inboard at least, on the side of the differential, so you can mount the wheels directly to the wheel hubs and get at least some rigidity. The middle wishbone is also not necessary.

So what you are saying is that I should remove the brakes from inside the rims and place them somewhere else, to reduce the distance between the pivot and the wheel, right ?

[jantjeuh]

Yes, that thought also occurred to me. No need to have the brakes right next to the wheels.

[nicjasno]

Yes. At least that.

Place them on the side of the differential, like this.

And you should move the pivot more to the outside, closer to the wheel. Watch my unimog video to get an idea:

You would of course use wishbones and ball joints to achieve the kingpin.

If you need help, i can prototype an axle over the weekend to show you what i mean.

When dealing with big loads, having the pivot as close as possible to the center of the wheel is of outmost importance. And you want it to be as rigidly connected to the wheel hub as possible aswell.

Edited September 19, 2013 by nicjasno

[jantjeuh]

Nice video nicjasno, makes me want to build the 8110 with your axle design :)

[nicjasno]

Nice video nicjasno, makes me want to build the 8110 with your axle design :)

Steering is light and It suddenly becomes very fun to drive.

[jantjeuh]

Yeah, I noticed the sluggish steering and bump steer in 8110 as well, but didn't have the technical know-how to put it down to the bad pivot geometry and caster angle. I guess 9398 and 41999 suffer from the same problems as well, since (if I recall correctly) they use a similar suspension/steering setup.

Edited September 19, 2013 by jantjeuh

[nicjasno]

Yeah, but lego got around that problem by using stering arms that are very long, so that the steering has a huge lever. But that is remedying the symptoms, not removing the cause. And i don't like this approach.

[Junpei]

Yes, I will never use the large ball joints for live axles, there is almost always too much negative caster. Aren't those wheels a little small?

[jantjeuh]

Yes, I will never use the large ball joints for live axles, there is almost always too much negative caster.

You mean 92910 and 92911? I guess they can still be used, you just have to compensate for the angle, like nicjasno did.

[MinusZwei]

And you should move the pivot more to the outside, closer to the wheel. Watch my unimog video to get an idea:

You would of course use wishbones and ball joints to achieve the kingpin.

Ok. Got the idea. So basically:

- I should make a kingpin and not a straight vertical pivot

- I must reduce as much as possible the distance between the pivot and the center of the wheel.

If you need help, i can prototype an axle over the weekend to show you what i mean.

Hummm, thanks for the offer but first I don't want to waste your time with this...

...and I would prefer redesigning by myself, and getting your approval afterwards

[Technyk32231]

You should make the wishbone pivots closer to the center of the axle, so you can have longer wishbones, which means longer travel and less scrub in a turn.

[Junpei]

How would you make a multilink suspension without CV joints? I once made a steered and driven multilink setup and I had to use CV joints so the axle could slide out of them when it is steered. Or am I getting the wrong picture?

[AndyCW]

HMMWV use inboard brakes with portal axles on an independent suspension. Works great. The skill evident in the built so far is promising. The design so far though... leaves a little to be desired. I am hopeful that we will see another revision and a complete built. Anxiously awaiting the next update on this project. v/r Andy

Edited September 19, 2013 by AndyCW

[Saberwing40k]

Hmm, I'm not so sure that this will perform as advertised. You're going to be snapping and twisting axles left and right, since they're long. Also, the whole assembly looks rather overbuilt, with too many parts in some areas, and too few in others. You'd be able to get rid of half the parts in this thing, and still have it work. Also, why do you have brakes? I have never seen a Lego vehicle that actually needed them, but if you want to include them, that's fine.

Also, since when were those pneumatic shunt pieces available in DBG? I thought they were available in only blue.

[jantjeuh]

Also, since when were those pneumatic shunt pieces available in DBG? I thought they were available in only blue.

That new Horizon Express train has four of them, for greebling purposes.

[Technyk32231]

How would you make a multilink suspension without CV joints? I once made a steered and driven multilink setup and I had to use CV joints so the axle could slide out of them when it is steered. Or am I getting the wrong picture?

You could use universal joints if you lined up the pivot of the universal joints with the intersection of the links, but that defeats the purpose of the multilink in the first place.

[nicjasno]

Keep it simple. A normal double wishbone will do just fine. Lets first simplify this and get some stiffness.

MinusZwei: very well. Looking forward to the next revision.

Ok. Got the idea. So basically:

- I should make a kingpin and not a straight vertical pivot

- I must reduce as much as possible the distance between the pivot and the center of the wheel.

Yep. That must always be your guide. Especially the second point, since a kingpin is not always achievable. But the pivot does need to be as close to the center of the wheel as possible. Otherwise you have a hard tome controlling all the forces acting on the wheel.