[WIP] RC Off-roader w/ Dual Diagonal Drive

[Didumos69]

RC Off-roader with Dual Diagonal Drive

 

I think I'm onto something that will get me through the winter.

 

 

Summary

Dual diagonal drive means: 1) having two separate drive trains with equal torque while 2) preserving the advantage of having an open distribution for cornering and 3) passing diagonal tests without using differential locks.

 

Background

I have been playing with this idea for a while already, especially after seeing @KevinMoo's dual drive models (Mitsubishi Pajero and Dual-Driveshaft Pickup). @KevinMoo rightfully addressed the vulnerability of LEGO parts in RC models and the fact that using independent drive trains for the left and right sides, loses the benefit of differentials while cornering. This got me thinking. Using independent drive trains for left and right in a 4WD model does indeed drop the benefit of differentials while cornering, but what if we would pair the wheels diagonally, so pair the left front (LF) wheel with the right rear (RR) wheel, and pair the right front (RF) wheel with the left rear (LR) wheel? The resulting 'dual diagonal drive' (I borrowed the term from the electric skateboard scene) would serve two major benefits:

• While cornering, the LF and RR wheels will average to a speed that is very close to the average speed of the RF and LR wheels. So not having an open distribution by means of a differential between the two drive trains is much less of a problem as with separate drive trains for the left and right side wheels.
• On a very uneven surface, where one or two wheels may lose contact with the ground, the wheels that do have contact are typically lined up diagonally, see image. With dual diagonal drive, the vehicle would still have traction, even without locking any differentials. Only on slippery surfaces, there are chances of spinning wheels.

So this is what I'm thinking of. We start with the basic dual diagonal drive setup: Two separate drive trains, one for the LF and RR wheels and one for the RF and LR wheels. The drive trains cross using two 24t gears and an auxiliary 16t gear that sits right underneath the auxiliary axle for the other drive train. So no clutch gears are involved in this crossing. I inserted a 1L Technic liftarm inside each differential - idea from @Madoca 1977's Toyota Land Cruiser 80 - to prevent the bevel gears from popping out.

 

 

Next we add a manual locking feature, which closes the differentials with a single lever. This locking feature will force each pair of wheels involved in one of the drive trains to have equal speed.

 

 

Now we connect each XL-motor to one of the differentials, using a small 4-speed gearbox. That means; two separate 4-speed gearboxes. This may be a bit ambitious, we'll have to see in real-life whether this is feasible or not. I might fall back to two 2-speed gearboxes. I did pay attention to the amount of torque in the transmission though. I geared up the XL-motor outputs and geared down the transmission output. That makes the transmission spin faster with less torque.

 

 

The gearboxes are operated synchronously using a 90-degree stepper, which is controlled by a Servo-motor. Each gear shift axle has its own 90-degree limiter.

 

 

And finally the outputs of the XL-motors are transferred to a fake V8-engine via a normal differential. The sole purpose of this differential is to combine the XL-motor outputs for the fake engine.

 

 

For the steering I'm thinking of using a servo motor. I don't really like the directness of steering with a servo-motor, but the steering link attachment points are moved one stud backwards, which confines the steering angle. This adds to better handling and protects the CV-joints in the wheel hubs.

 

 

I don't know where this is going to end. I'm not even sure about the exact kind of car I will be targeting, but it sure needs to be some kind of all-rounder. Comments and suggestions are welcome.

Edited November 7, 2021 by Didumos69

[Rudivdk]

This looks like a real challenge. Concept seems sound enough, but I wonder if you can get everything formlocked and without slack to prevent misallignment. This will keep you busy allright

[pagicence]

This is some mighty engineering. I was hoping that for your next project you will pick something that hasn't been done before, and you did. Kudos. As for the what kind of vehicle it would be may I suggest let your imagination run wild and see what happens, or just ask your kids.

[AFOLegofan66]

You have had tough problems like this before...only time and patience will do the job on this MOC!!

 

 

 

 

Edited November 28, 2018 by AFOLegofan66

[gate]

Sounds really interesting and legit, but if you start digging closer - many problems will arise. Such as the amount of gears, 2 u-joints for each wheel. The final result would have to be less than ~1.5 kilo I'd say to make it work? To me it already sounds unrealistic.

But that's just an imho after a hard day. If somebody can do that, that is you Didumos

[Attika]

It is an experiment worth to follow. Nicely presented and executed.   I do have my doubts here and there. Having spent a very lazy year on the motorized 4x4 subject, I'm a bit worried about those looong input axles and the number of gear meshes. Plenty of torque they  can eat up. If that will be the case a portal axle can still save the day. 

 Yet, you pulled off some non braeking 12t bevel in the 4WD buggy (remains to be an X-file to me), so-as I said- I'll follow this.

[zux]

6 hours ago, Didumos69 said:

I inserted a 1L Technic brick inside each differential - idea from @Madoca 1977's Toyota Land Cruiser 80 - to prevent the bevel gears from popping out.

Why not the 1L Technic "beam"-connector instead? This would have less contact surface with middle gear and still keep other two from going anywhere. 

[brunojj1]

I´m always very pleased to digest those sweet renders from you . There is much to learn from the accurately thought out concept, at least in theory. Having big doubts about the gearbox working well under stress & in rough terrain though. I guess the differentials will slip anyway, even more if not being secured from both sides. Good luck however with this journey .

[Aleh]

Wow! New level of lego technic complexity!!

[Didumos69]

6 hours ago, Rudivdk said:

This looks like a real challenge. Concept seems sound enough, but I wonder if you can get everything formlocked and without slack to prevent misallignment. This will keep you busy allright

Yes, bracing everything well will be the next hurdle. I left space for some transversal beams and all axles are 'in grid'. Hopefully that will help.

6 hours ago, pagicence said:

This is some mighty engineering. I was hoping that for your next project you will pick something that hasn't been done before, and you did. Kudos. As for the what kind of vehicle it would be may I suggest let your imagination run wild and see what happens, or just ask your kids.

That's a nice suggestion ?. I will ask my kids.

3 hours ago, AFOLegofan66 said:

You have had tough problems like this before...only time and patience will do the job on this MOC!!

I fear this is the most challenging build sofar. I really want a model that performs and can shift gears while driving.

1 hour ago, gate said:

Sounds really interesting and legit, but if you start digging closer - many problems will arise. Such as the amount of gears, 2 u-joints for each wheel. The final result would have to be less than ~1.5 kilo I'd say to make it work? To me it already sounds unrealistic.

But that's just an imho after a hard day. If somebody can do that, that is you Didumos

Yeh, the U-joints are probably the weakest link, but they only need to handle a quarter of all the power. Also in my Greyhound I used U-joints and bevel gears at the end of the drive train.

The number of gears may seem overwhelming, but the number of gear meshes involved in each gear is 4, which is really the minimum in a 4-speed AWD model. I managed to make sure each gear uses the same number of gear meshes.

1 hour ago, Attika said:

It is an experiment worth to follow. Nicely presented and executed.   I do have my doubts here and there. Having spent a very lazy year on the motorized 4x4 subject, I'm a bit worried about those looong input axles and the number of gear meshes. Plenty of torque they  can eat up. If that will be the case a portal axle can still save the day. 

 Yet, you pulled off some non braeking 12t bevel in the 4WD buggy (remains to be an X-file to me), so-as I said- I'll follow this.

I hope this model will be a good marriage between a crawler and a race buggy. Hence the gearbox.

We'll see about the long axles. They are geared up, I hope they can handle the reduced torque.

1 hour ago, zux said:

Why not the 1L Technic "beam"-connector instead? This would have less contact surface with middle gear and still keep other two from going anywhere. 

I will first try your suggestion, thanks!

3 minutes ago, brunojj1 said:

I´m always very pleased to digest those sweet renders from you . There is much to learn from the accurately thought out concept, at least in theory. Having big doubts about the gearbox working well under stress & in rough terrain though. I guess the differentials will slip anyway, even more if not being secured from both sides. Good luck however with this journey .

Thanks! I am probably going to learn a lot during this project ?.

[shadow_elenter]

That looks VERY interesting

I'm not a big fan of car MOC's in general because overal they tend innovate very little (partially inherent to the limitations of cars) and copy a lot but this I wil follow!
I do expect real-world issues, but Personally, even if it does not work I applaud your out of the box thinking.

I'd much rather watch an out of the box WIP ultimately fail than another re-combination of proven drive train/suspension elements succeed.

Keep it up!

[schraubedrin]

This is a great idea, i'm looking forward to your progress on this project.

8 hours ago, Didumos69 said:

 

 

Why do you use additional gears to achieve the locking differentials if you just could connect the driving rings to the differential housing?
This way you'd save space and gears.

Am i guessing that the fake engine is in the rear of the car? If so, you may reconsider the placing of the cross-gearing: the gears subtract torque from the axles and standard 4WD-vehicles have more torque in the rear (for better acceleration)

For the type of vehicle, this seems to be a perfect base for a SUV: not too much suspension travel but still sporty.

 

[Didumos69]

1 hour ago, shadow_elenter said:

That looks VERY interesting

I'm not a big fan of car MOC's in general because overal they tend innovate very little (partially inherent to the limitations of cars) and copy a lot but this I wil follow!
I do expect real-world issues, but Personally, even if it does not work I applaud your out of the box thinking.

I'd much rather watch an out of the box WIP ultimately fail than another re-combination of proven drive train/suspension elements succeed.

Keep it up!

Thanks! I appreciate it.

1 hour ago, schraubedrin said:

Why do you use additional gears to achieve the locking differentials if you just could connect the driving rings to the differential housing?
This way you'd save space and gears.

Because I want the axles coming out of the differentials to be supported by a liftarm directly next to the differentials. This is to prevent slipping gears.

1 hour ago, schraubedrin said:

Am i guessing that the fake engine is in the rear of the car? If so, you may reconsider the placing of the cross-gearing: the gears subtract torque from the axles and standard 4WD-vehicles have more torque in the rear (for better acceleration)

You are guessing right. I get your point. Will take an other look, but in the current setup moving the gears to the front would make them interfere with the stepper.

1 hour ago, schraubedrin said:

For the type of vehicle, this seems to be a perfect base for a SUV: not too much suspension travel but still sporty.

I think so too. It should be able to continue driving with one wheel off the ground, which won't even happen when it would have too much suspension travel.

[woodstock]

[removed]: I got my answer after looking at the picture more carefully :)

Edited November 29, 2018 by woodstock

[Jurss]

Interesting and something fresh.

[schraubedrin]

Thanks for answering my questions so thoroughly.

12 hours ago, Didumos69 said:

This is to prevent slipping gears.

That's a good reason, this drivetrain may be very torquey.
Would it be possible if you flip the differentials? This way you could still support the input side directly while using the 24-tooth side as locking-side.

12 hours ago, Didumos69 said:

in the current setup moving the gears to the front would make them interfere with the stepper

maybe if you move the gearbox nearer to the XL-motors? Using the old 2L driving ring might grant you more space. Although you might have to rebuilt the fake V8 input.

 

On a completely unrelated note: could you maybe make a screen-recoding of you working in LDD? (e.g. with OBS)
I think digital examples are one off the best methods to exchange ideas. It's just that i'm so slow using it, that i normally don't bother.
Therefore i'm interested how you're building such complex, free floating structures.

[Aleh]

Can't stand admiring this stunning and pleasant renders!

[pleegwat]

23 hours ago, gate said:

Sounds really interesting and legit, but if you start digging closer - many problems will arise. Such as the amount of gears, 2 u-joints for each wheel. The final result would have to be less than ~1.5 kilo I'd say to make it work? To me it already sounds unrealistic.

But that's just an imho after a hard day. If somebody can do that, that is you Didumos

You could replace the u-joint closest to the wheel by using the half-joint which snaps into the hub. That feels stronger, though I'm unsure whether it is.

On the inner side, you could make the bevel gear housing tilt with the suspension? Though that would probably be tricky in other ways.

[BusterHaus]

Nice start! You have some pretty ambitious goals, although I don't think they are out of reach.  When I saw the dual drive my mind immediately went to skid steering, but the wheel base would have to be shorter to make turn easier.  Are you planning on driving it in the snow?

[gate]

22 hours ago, Didumos69 said:

The number of gears may seem overwhelming, but the number of gear meshes involved in each gear is 4, which is really the minimum in a 4-speed AWD model. I managed to make sure each gear uses the same number of gear meshes

Are you not counting the idle gears here? Cause I can see at least 6 different connections per wheel. But then again if you are adding the 4 speeds, probably it can't be done with a less amount of gears.

A slight offtop question - have you considered maximizing efficiency by removing fake engine, may be dropping speeds to 1 or 2 max? Offroad vehicles do like and benefit from that. Judging by my experience - the best gear setup will be a super gear reduction from start or may be 1:1 composition (not counting further reductions here, just gearbox). One speed will always work better then others. May be not much, but still you gain better performance, less idly spinning gears, less weight etc. Really interesting what you think about it

[letsbuild]

I really like this concept, and from your past projects, I think this one will be great.

[SilenWin]

On 11/28/2018 at 9:47 PM, zux said:

Why not the 1L Technic "beam"-connector instead? This would have less contact surface with middle gear and still keep other two from going anywhere. 

Using a technic-brick instead of 1L connector can create an limited slim differential effect in some sense. Maybe there is no sufficient friction difference between this two parts, but when speaking about offroaders, I prefer to create a little bit more friction in a drivetrain (in differentials and after them, i.e. portal or wheel reductors), sometimes it helps a lot in crawling. 

[Didumos69]

On 11/29/2018 at 1:29 PM, schraubedrin said:

Would it be possible if you flip the differentials? This way you could still support the input side directly while using the 24-tooth side as locking-side.

I'm afraid not. The 8t and 16t gears along the sides of the differentials are on different axles. The differential locks need to make use of the axle with the 16t gear, otherwise the output of the differential would be forced in a speed different from the differential itself. So the gear locks need to sit at the 16t side of the differentials. I also tried with the locks in front of the diffs, but then the 90-degree stepper would have to move to the back side of the gearbox which causes all kind of problems. I think I will leave it as is.

On 11/29/2018 at 1:29 PM, schraubedrin said:

maybe if you move the gearbox nearer to the XL-motors? Using the old 2L driving ring might grant you more space. Although you might have to rebuilt the fake V8 input.

Yeh, it is the V8 input that makes the distance. Btw, I am not so much afraid of too much torque in the transmission. My approach is 1) to gear up the XL-motor output with 3:1 and 5:3 to get the inputs for the gearbox, and 2) to gear down the transmission output close to the wheels. This means the transmission (gearboxes) spins faster with less torque. The downside is that there will be more friction, but I'm putting in everything I know about getting gears and axles to run smooth. Really everything.

On 11/29/2018 at 1:29 PM, schraubedrin said:

On a completely unrelated note: could you maybe make a screen-recoding of you working in LDD? (e.g. with OBS)
I think digital examples are one off the best methods to exchange ideas. It's just that i'm so slow using it, that i normally don't bother.
Therefore i'm interested how you're building such complex, free floating structures

Nice idea, I'll do that during my next session.

On 11/29/2018 at 1:55 PM, Aleh said:

Can't stand admiring this stunning and pleasant renders!

Thanks!

On 11/29/2018 at 6:34 PM, pleegwat said:

You could replace the u-joint closest to the wheel by using the half-joint which snaps into the hub. That feels stronger, though I'm unsure whether it is.

On the inner side, you could make the bevel gear housing tilt with the suspension? Though that would probably be tricky in other ways.

Actually, I have no clue on what kind of suspension I will put in. The u-joints are just place-holders for the axles. I also don't know if I will be using the standard LEGO wheel hub. As for making the bevel gear housing tilt; I could do that, but I'm afraid that when the housing is only supported by the drive train itself, I will get more friction. I think I will first try without suspension at all, just to see how it perfroms on a flat surface.

On 11/29/2018 at 6:42 PM, BusterHaus said:

Nice start! You have some pretty ambitious goals, although I don't think they are out of reach.  When I saw the dual drive my mind immediately went to skid steering, but the wheel base would have to be shorter to make turn easier.  Are you planning on driving it in the snow?

I'm actually thinking of something like this (Zarooq SandRacer), but I really have to see how everything works out in real-life. I will certainly drive it on ice when I get the chance.

23 hours ago, gate said:

Are you not counting the idle gears here? Cause I can see at least 6 different connections per wheel. But then again if you are adding the 4 speeds, probably it can't be done with a less amount of gears.

A slight offtop question - have you considered maximizing efficiency by removing fake engine, may be dropping speeds to 1 or 2 max? Offroad vehicles do like and benefit from that. Judging by my experience - the best gear setup will be a super gear reduction from start or may be 1:1 composition (not counting further reductions here, just gearbox). One speed will always work better then others. May be not much, but still you gain better performance, less idly spinning gears, less weight etc. Really interesting what you think about it

No, I didn't count the idle gears ?. I'm putting in everything I know about getting gears and axles to run smooth, so I hope that will help. I also have no clutch gears transferring drive while not engaged with the axle they are attached to. Only where the drive trains cross, but there will minimal speed difference between the axle and the gear, especially when driving straight.

I did think of all these options and it may very well be that I have to change my plan, but for now I will aim for a 4-speed model with high spin / low torque in the transmission. If this won't work, I can quite easily change the gearbox input ratios, for instance to 1:1 and 5:3. I might also add an M-motor for the v8.

22 hours ago, letsbuild said:

I really like this concept, and from your past projects, I think this one will be great.

Thanks!

5 hours ago, SilenWin said:

Using a technic-brick instead of 1L connector can create an limited slim differential effect in some sense. Maybe there is no sufficient friction difference between this two parts, but when speaking about offroaders, I prefer to create a little bit more friction in a drivetrain (in differentials and after them, i.e. portal or wheel reductors), sometimes it helps a lot in crawling. 

I prefer to have the differentials open by default, so I will first try @zux's suggestion.

Update: Yesterday evening I spent some time on the main structure. I worked my way from the area around the differential locks to the back and to the front. The main structure is 7 studs high, which allows me to make it really rigid. I'm following my standard layered approach: I work lengthwise on odd layers (counting from bottom to top) and transversal on even layers. By sticking to that guideline consistently, you end up with many opportunities to put long liftarms through the structure.

I added attachment points for two BuWizzes to the sides (the studded plates and pins above).

Edited November 30, 2018 by Didumos69

[KevinMoo]

my major concern will be the central differentials.

I have built a third iteration of my dual-driveshaft car, which features dual central diffs and many other functions. All functions work fine except the central diffs. Here are some renders of the drivetrain (sorry I can’t show you real photos):

I was hoping a dual-central-diff-setup will reduce the possibility of gear slipping inside the diffs, but it still happens every time I put it on high gear (low torque). The complete chassis weighs only 1.08 KG.

I really can’t figure out how to improve it anymore, could be the result of too much frictions. I’ll probably give it up for future projects.

So my suggestion is to be very careful with central diff, it could be the weakest part of the whole chassis even with two of them.

[BusterHaus]

4 minutes ago, KevinMoo said:

So my suggestion is to be very careful with central diff, it could be the weakest part of the whole chassis even with two of them. 

A custom one would be stronger, although would take much more place.