[MOC] Hot Trot - RC Trial Pickup w/ Diagonal AWD (42129 B model)

[Didumos69]

Hot Trot - RC Trial Pickup w/ Diagonal AWD

I would like to present to you my shot at a 42129 B model. About the name 'Hot Trot': On a very uneven surface, a vehicle typically moves its weight from one diagonal pair of wheels to the other as it proceeds from bump to bump. Hence the word 'Trot' in the name of this model. It refers to a horse proceeding at a moderate pace, lifting each diagonal pair of legs alternately.

Instructions are on Rebrickable.

Diagonal AWD

With Diagonal AWD I mean: 1) having two separate drive trains with equal torque that 2) keep a vehicle going on a very uneven surface while 3) preserving the advantage of having a distribution optimal for cornering.

We all know the downside of a classic 4WD setup with three open differentials; as soon as one wheel loses grip, all power will flow to that wheel, causing the vehicle to lose traction. One way to deal with that is to add differential locks like in the Zetros. Another approach that is commonly seen in LEGO Technic builds is to have separate drive trains for front and rear wheels like in the 42099 X-treme Offroader, in which case you need a slipping front wheel and a slipping rear wheel to completely lose traction. This approach however, has two downsides:

• There is no open distribution of torque between front and rear wheels, which is not optimal for cornering, because the front wheels are not free to average to a higher speed than the rear wheels.
• On a very uneven surface, for instance in diagonal tests, where wheels start losing contact with the surface, it's likely to have a front and (diagonally opposite) rear wheel loose traction.

But what if we would separate the drivetrains diagonally: A drivetrain with a differential between the left front (LF) wheel and the right rear (RR) wheel, and a drivetrain with a differential between the right front (RF) wheel and the left rear (LR) wheel? The resulting 'Diagonal AWD' would serve two major benefits:

• While cornering, the LF and RR wheels will average to a speed that is almost equal to the average speed of the RF and LR wheels. So not having an open distribution between two diagonally arranged drivetrains is much less of a problem than with separate drivetrains for the front and rear axles.
• On a very uneven surface, where wheels start losing contact with the surface, the wheels that still have contact are typically lined up diagonally. With Diagonal AWD, the vehicle will still have traction, even without additional differential locking. The vehicle will only lose traction when two wheels at one end or at one side will lose grip.

Drive trains

I wanted to have the motors at the same level as the rest of the drivetrain to keep the center of gravity low. I came up with a fairly flat setup with longitudinally placed motors, a differential at the heart of each drive train and bevel gears to make the 90 degree angles towards the wheels. Each motor drives a 16t clutch gear locked to the differential using a 'floating' driving ring.

After some trial and error the best option to properly brace the bevel gears that make the 90 degree angles to the wheels, turned out to be a symmetrical setup with the motors rotating in opposite directions. Because of these opposite directions, using a clutch gear to make the drivetrains cross in a completely flat layout, was not an option. The clutch gear would rotate in opposite direction compared to the axle it's attached to, which would give loads of friction, particularly in motorized models. But luckily, the Zetros has exactly the right gears to make the drivetrains cross independently.

The biggest challenge was to prevent the gears that make the 90 degree angles from slipping. I've experimented a lot, but no matter how strong I braced the bevel gears, I could always make them slip with my bare hands, using 24t gears to put force on the axles. So the goal was to make sure the gears can at least handle the power provided by the motors without slipping, even when one drivetrain gets completely blocked. I eventually succeeded, but it has cost me long hours of tinkering and trial and error.

I created completely locked up structures for the bevel gears, using the 4 available 5x7 frames and 8 of the 11 beams with alternating holes. These structures are also tailored to fit the main structure well and to provide form-locked mounting points for the suspension arms. The two assemblies are extremely hard to build, because you can only insert the gears and axles after the structure has been put together.

Suspension

I incorporated independent double wishbone suspension with the springs arranged in a pushrod style. The springs have been mounted such that they compress by approximately 33% under the vehicle's own weight. This gives a very nice road holding, because the springs can both expand and compress while taking bumps. I managed to optimize the spring setup for max suspension travel, which is about 3L, which fitted my objectives perfectly. The suspension travel shouldn't rule out the chances of having a wheel coming loose from the surface, because that's when the Diagonal AWD kicks in.

It seemed obvious to use the new longer male half of the CV-joints for the rear axles, so the rear axles use 8L suspension arms. The suspension arms of the front axles are shorter, because I had to build the gear-rack assembly around the drivetrain axles and wanted the steering links to have the same length as the suspension arms to avoid bump steering. The steering setup has an Ackermann geometry.

The steering links have been reinforced such that they never detach from the gear-rack assembly and the gear-rack assembly itself has been braced in a way that rules out unintended transversal movement. The 2 phone handsets came in handy for this purpose.

Chassis and bodywork

The main structure is as stiff as I could possibly make it. I even used the motors as structural elements to add to rigidity. I wanted to rule out flex as much as possible, because I wanted to demonstrate how responsive the suspension is and because a flexible chassis would reduce the chances of the Diagonal AWD kicking in. Apart from the extensions necessary to mount the springs, the main structure is just 5L tall. On top of this main structure I've build a strong structure for the cabin of the vehicle.

The bodywork does not have any features like openable doors or whatever. People that know me, know that I'm mainly interested in drivetrains and suspension. The bodywork is robust however and I did pay attention to the interior this time. You can lift it by the roof and I also made it very easy to remove the battery box.

Powered Up Profile

Because I don't use the medium motor, and because I have the large PU motors running in opposite directions, I could not use the Control+ app to operate this model. However a default profile in the Powered UP app could be easily tweaked to what this model needs:

Finally I want to mention that I started using Energizer Ultimate Lithium Mignon batteries for my MOCs. They are strong, long-lasting and most importantly; they are at least 20% lighter than alkaline batteries.

Photos

Spoiler

Edited November 26, 2021 by Didumos69

[Maaboo the Witch]

Cool build and one very innovative gear system! 

[GerritvdG]

 Brilliant, I learned something new: diagonal AWD. Is such a drive train used in real world cars? Is this patented? 

The video proofs that it works really well!

I only found the Diagonal Spin Test when searching for more information about this subject..

[2GodBDGlory]

Very interesting! I like the focus on good chassis and drivetrain design. The diagonal setup is quite unique and well worth experimenting with, though I expect it is too complex for widespread use on real machines, and too complex to beat a no-differential or locking differential setup on small, light RC cars. It's still a very intriguing concept, though!

[Didumos69]

1 hour ago, GerritvdG said:

 Brilliant, I learned something new: diagonal AWD. Is such a drive train used in real world cars? Is this patented? 

The video proofs that it works really well!

I only found the Diagonal Spin Test when searching for more information about this subject..

It is patented, but as I understand for different reasons (source). It states that when a vehicle accelerates while cornering, the wheels with the most grip and least grip are diagonal opposites. They also argue that it makes sense to transfer propulsion from the wheel with the least grip to the wheel with the most grip. If you wish to do so, supposedly by automatically braking the wheel with the least grip when it starts spinning, you need to pair that wheel with its diagonal opposite, using a differential. My doubt about this kind of system is that electronic systems that automatically brake a slipping wheel will only kick in when the wheel is already undoubtedly slipping. In sophisticated AWD system (Subaru, Volvo, etc.) you always first see a wheel slipping for at least a second, before the automatic braking kicks in. This doesn't seem to be something subtle enough to apply when accelerating while cornering. These kind of systems are improving rapidly though.

Personally I think my setup with two separate drive trains arranged diagonally might be well suited for electric off-road vehicles with two motors.

Edited November 11, 2021 by Didumos69

[MajklSpajkl]

Very interesting reading and great concept of this diagonal AWD + fantastic and obviously very capable B model. Great job!

[Johnny1360]

Great concept, great to see how you are working out the problems and making it work.

 

[AFOLegofan66]

Wow very nice job and great gear system!

[gyenesvi]

I have been waiting to see what you come up with from this set! Really interesting drivetrain concept, and the implementation is quite elegant, the way the differentials are placed and driven, and the way the two drivetrains bypass each other. I like the bouncy suspension as well. Great work!

As far as I can see, you did not use the 4th motor, right? So this can only be controlled with a custom PU profile? (though that’s a very simple one, and I guess the opposing direction of the two drive motors already rules out the stock profile..)

[westphald]

Nice, someone got the tires in the correct orientation!

Awesome build. I like the drivetrain setup as an alternative to the non-existent Technic limited slip (I've surveyed the options, none satisfy). I also like the idea of driving the differential from the end rather than the ring gear. I might have to use that somewhere.

That smooth body and chop top look make me think it should be dropped to the ground and geared up. I wonder how hard it would be to suck in the wheels and make it a street ride.

Given that the two drivetrains should operate at near identical RPM when there is full traction, I wonder if bridging them together at the motors makes sense. Then, it would be possible to add a third motor. Either that, or, add a differential between the two drivetrains and power it by a third motor. I don't see room for it in this model, but could be useful where more power is desired.

[Didumos69]

Thank you all for the kind replies!

13 hours ago, gyenesvi said:

As far as I can see, you did not use the 4th motor, right? So this can only be controlled with a custom PU profile? (though that’s a very simple one, and I guess the opposing direction of the two drive motors already rules out the stock profile..)

Indeed, I didn't use the 4th motor. I wanted to keep the concept clean. Also, I used the 2 driving rings for driving the differentials, so anything like a differential lock or a gearbox was kind of ruled out from the start. I used the PU app and tweaked one of the preconfigured profiles to my needs:

8 hours ago, westphald said:

That smooth body and chop top look make me think it should be dropped to the ground and geared up. I wonder how hard it would be to suck in the wheels and make it a street ride.

The mounting points of the springs can easily be moved 1 stud inward, so I suppose you could make it a street ride.

8 hours ago, westphald said:

Given that the two drivetrains should operate at near identical RPM when there is full traction, I wonder if bridging them together at the motors makes sense. Then, it would be possible to add a third motor. Either that, or, add a differential between the two drivetrains and power it by a third motor. I don't see room for it in this model, but could be useful where more power is desired.

Good question! In fact I started with a setup that had such a `bridge`. It had both motors running in the same direction and used a clutch gear to make the drive trains cross. However, it gave me 2 issues:

• The bevel gears making the 90 degree angles would not be symmetrical. In the rear the 90 degree gear mesh for one of the wheels would be at the wheel side and I couldn't fit in a proper way to brace that. Proper bracing is essential to prevent these gears from slipping.
• When one wheel loses traction, a hard `bridge` (without differential) will make all power (from both motors) flow to one drivetrain. When at the same time the vehicle hits an obstacle that blocks the vehicle, this led to slipping bevel gears. Too much power for those gear meshes.

By dropping the bridge and having the 90 degree gear meshes at the center of the vehicle, I could rule out slipping gears entirely.

Edited November 13, 2021 by Didumos69

[westphald]

7 hours ago, Didumos69 said:

Proper bracing is essential to prevent these gears from slipping. 

I hear you. I'm attempting some 2 and 3 motor drivetrains and slipping is a constant challenge.

I really like your build though. It finally gives me a reason to consider buying a Zetros.

[Didumos69]

Instructions in progress...

[MY1]

what is the point of the differentials if they are always locked

[2GodBDGlory]

11 minutes ago, MY1 said:

what is the point of the differentials if they are always locked

They're not actually locked in this model. Rather than driving the differential from the 28T ring gear as usual, that gear is left to idle while the actual input comes from the 16T gear locked to the casing by the driving ring. Think of it like the 16/24T differential: There are two ring gears, but only one is being used.

[Didumos69]

1 hour ago, 2GodBDGlory said:

They're not actually locked in this model. Rather than driving the differential from the 28T ring gear as usual, that gear is left to idle while the actual input comes from the 16T gear locked to the casing by the driving ring. Think of it like the 16/24T differential: There are two ring gears, but only one is being used.

Yes, thanks for explaining. It's also important to realize that the driving rings are actually floating. They're not mounted on a connector. There are 10L axles running all the way through the 16t clutch gears and driving rings into the differentials.

Edited November 17, 2021 by Didumos69

[schraubedrin]

Another incredible offroader from you
And as B model, that's even better.

There's just one important question i have:
Did you try it with bigger wheels?!
More than the original, i think this B model would profit from the additional ground clearance.

[amorti]

Also interested. Got some Class wheels, also some 120mm RC wheels.

 

[Didumos69]

2 hours ago, schraubedrin said:

Another incredible offroader from you
And as B model, that's even better.

There's just one important question i have:
Did you try it with bigger wheels?!
More than the original, i think this B model would profit from the additional ground clearance.

 

57 minutes ago, amorti said:

Also interested. Got some Class wheels, also some 120mm RC wheels.

 

I know the Claas wheels fit, but I haven't driven them outdoors. Once I've put the model together again I'll give it a try and let you know.

Edited November 17, 2021 by Didumos69

[Didumos69]

Quickly made some renders. Looks okay imo.

Speaking of renders. I also have this mad-max style Zetros alternate design laying around. Uses the same chassis as the pickup. In fact, it was a pre-study for this whole project.

Edited November 17, 2021 by Didumos69

[Scoar Sonander]

IMO looks better that the first one since it integrates the C+ Hub better, but that front reminds me of some wacky robot face 

[amorti]

I like the second one, too. It's even further away from the original model, which is a good trick when using the same parts.

Bit of a back to the future vibe about it.

[Daniel-99]

Very interesting chassis... I was trying to build a diagonal one as well, but failed...

It would be great to see this car with a bigger wheels (for example from Lego motorbikes)

 

[Void_S]

Maybe an offtop a little, does anyone know any real vehicle with such a diagonal drivetrain?
I tried to google it and found only a regular "longitudinal" scheme and "sided" H-drive one. The second has only central left-right sides differential and has a huge disadvantage of a driveline windup (which is even more common for plastic Lego parts).

Additionally, I spent some time on these schemes analysis and now am proud to present it:

Purple - an engine+ gearbox + distribution box + central diff; red/blue - independent drive "branches"; gray "tracks" show the wheel trajectories to compare the rotation speeds.

As you can see, the regular scheme is a pretty convenient one and performs the best in on-road conditions. Its off-road capabilities could be improved by locking the central and possibly (optional) axle differentials.

The H-Drive scheme is even simpler (and allows easy extend it up to 6x6 or even 8x8 wheels) and also needs the central diff lock. However, for the on-road use it has a slight disadvantage of fixes rotation speeds per side. So, it should be better to have all the wheels steered at the same angle to keep the same speed. Best for military slow-speeded vehicles, not so dynamically stable for fast on-road regular cars.

The hypothetical "crossed"  H-Drive scheme looks like an attempt to beat the "diagonal trouble" and improve the off-road capabilities (otherwise it will skid-steer randomly) with no need to lock the center differential but resulted in huge faulty: it basically impossible to turn this vehicle on the road without breaking the transmission. So, sorry, we don't have it.

And now it's time for the brilliant @Didumos69 scheme: it smoothly operates in on-road conditions, as has approximately the same speed for diagonal wheels in average, and works better than H-Drive for forward-only steered vehicles. As for the off-road use, the lack of the differential locks may be taken as a disadvantage but actually, it is comparable with a regular scheme with locked central differentials (but still turns better).

[2GodBDGlory]

5 hours ago, Void_S said:

Maybe an offtop a little, does anyone know any real vehicle with such a diagonal drivetrain?
I tried to google it and found only a regular "longitudinal" scheme and "sided" H-drive one. The second has only central left-right sides differential and has a huge disadvantage of a driveline windup (which is even more common for plastic Lego parts).

Additionally, I spent some time on these schemes analysis and now am proud to present it:

Purple - an engine+ gearbox + distribution box + central diff; red/blue - independent drive "branches"; gray "tracks" show the wheel trajectories to compare the rotation speeds.

As you can see, the regular scheme is a pretty convenient one and performs the best in on-road conditions. Its off-road capabilities could be improved by locking the central and possibly (optional) axle differentials.

The H-Drive scheme is even simpler (and allows easy extend it up to 6x6 or even 8x8 wheels) and also needs the central diff lock. However, for the on-road use it has a slight disadvantage of fixes rotation speeds per side. So, it should be better to have all the wheels steered at the same angle to keep the same speed. Best for military slow-speeded vehicles, not so dynamically stable for fast on-road regular cars.

The hypothetical "crossed"  H-Drive scheme looks like an attempt to beat the "diagonal trouble" and improve the off-road capabilities (otherwise it will skid-steer randomly) with no need to lock the center differential but resulted in huge faulty: it basically impossible to turn this vehicle on the road without breaking the transmission. So, sorry, we don't have it.

And now it's time for the brilliant @Didumos69 scheme: it smoothly operates in on-road conditions, as has approximately the same speed for diagonal wheels in average, and works better than H-Drive for forward-only steered vehicles. As for the off-road use, the lack of the differential locks may be taken as a disadvantage but actually, it is comparable with a regular scheme with locked central differentials (but still turns better).

Nice analysis! I'm not sure where this system would be practically applied, though, since torque-vectoring differentials and locking differentials seem to take less space and weight than this clever mechanical solution. Still, there may be some vehicles where this setup's durable but heavy extra driveshafts would be preferable to the more delicate locking/torque vectoring differentials. Either way, I really like this solution!