[WIP] 4x4 Rock Crawler -- BuWizz + Buggy motors

[Teo LEGO Technic]

Hello EB friends!

Today, I'm getting to work on a new project - a 4x4 rock crawler to compete in our upcoming Toronto Truck Trial in August. I've always wanted to try my hand at a rock crawler, but somehow never came around to it until now. It seems like the perfect opportunity to make the best offroad vehicle - faster than a large 6x6 or 8x8 trial truck, but still having excellent offroad capabilities with large ground clearance, huge suspension travel, oversized wheels, and the perfect excuse to omit bodywork entirely and make a light, nimble machine. 

To make the best rock crawler I can, I'm going to draw inspiration from a couple of my past creations (the parts of them that went well) as well as from some other builders' MOCs. First of all, I'm going to reuse the general axle design I used in my Praga 6x6 Trial Truck, which was inspired by @Attika's design in his offroader:

To power this crawler, I want to get the best power-to-weight ratio I can. Since I'm limited to 2 x buggy motors and a BuWizz 2.0 - that's all I've got and I don't plan to invest more money at the moment - the only solution is to keep the truck as lightweight as possible. In my latest RAM pickup truck I was very pleased with the performance it achieved in low gear using 2 buggy motors and planetary hubs. Because I will be using larger crawling tires, I will reduce the ratio further for this build compared to the pickup, using the same 12-tooth bevel gear, 28 tooth bevel gears pairing as in my Praga at the axle, for a ratio of 2.33:1, rather than the ratio with the 20-tooth bevel gear, 28-tooth differential, as on my pickup, which has a ratio of 1.4:1. Overall, because my 3rd party crawler tires have about double the radius of the 62.4 x 20 tires on the pickup, and so double the circumference and double the speed, the ratios will more or less cancel out and the crawler will have the same speed and torque as the pickup, which was plenty. Indeed, it may be a bit faster, but it will also be lighter to account for it. 

If I got any of that math wrong, please feel free to correct me, but it makes sense to me that the speed is proportional to the wheel circumference and therefore the wheel radius, by the formula 2 * pi * r for circumference.

Next, I'm going to just omit differentials entirely. I think the truck should be light enough to not need them, although the grippier tires may slow it down in the corners where diffs are necessary. For the suspension, I want to try my hand at building a four-link triangulated suspension, partly because it looks fun, and partly because it offers the advantage of being able to connect the axle to a small body, that doesnt need to extend all the way over the axle to accomodate a Panhard stabilizer rod. I think I will take inspiration for that from @PunkTacoNYC's awesome Chilli Crawler:

In summary, then, here are the current specifications, ideas and goals for this build:

1. 2 x buggy motors mounted in the body for drive, PF servo in the front axle for steering; BuWizz 2.0 for power
2. Lightweight build overall to keep weight minimal and improve power/weight ratio - little to no bodywork, and short wheelbase
3. Triangulated 4-link suspension to enable a smaller, lighter body, using the suspension links on the front axle to create a mild caster angle 
4. Planetary hub live axle suspension, as from my Praga truck, inspired by Attika - I just can't think of any way to improve on these, they're so simple, robust, and elegant, and have very precise steering and great ground clearance

The first step is to adapt the front axle for this build. I will make it two studs wider to account for the larger tires, and I have to figure out mounting points for the suspension links and shock absorbers. Other than that, I will leave it untouched, as it just has it all. The planetary hubs and bevel gears together create a mechanical reduction of 12.6:1, which is excellent for how compact it is. The hub + defender rim combination keeps the pivot very close to the center of the tire, making the steering more effective. Attika's clever design gives it excellent ground clearance. And, importantly, mounting the servo on the axle gives it excellent precision with return-to-center steering, something rare for a heavy-duty axle, and it is very robust with the double-racked steering mechanism - I can't remember who recommended that to me in the past, but it's brilliant. Here, then, is the beginning of the front axle, widened by 2 studs:

I'm excited for this build - I think it's going to have some nice performance! As always, I'm happy to take tips and suggestions from you guys - please pitch in! 

-Teo

 

Edited June 23, 2025 by Teo LEGO Technic
Add short wheelbase goal

[bruh]

Looks pretty dope! I hope it goes well for you. Definitely gonna be keeping an eye on this project!

[2GodBDGlory]

Looks like a good start! 

I expect this thing to be a fearsome competitor at our race!

[Teo LEGO Technic]

 

13 hours ago, 2GodBDGlory said:

Looks like a good start! 

I expect this thing to be a fearsome competitor at our race!

15 hours ago, bruh said:

Looks pretty dope! I hope it goes well for you. Definitely gonna be keeping an eye on this project!

Thanks guys! I hope it'll be worth your time 

Edited June 20, 2025 by Teo LEGO Technic

[lmdesigner42]

It's always fun to push the boundaries of Lego performance. Your math is correct, longitudinal speed is directly proportional to tire radius. However, you may want to consider some additional gearing down, since heavier or softer tires have additional power loss from higher inertia and rolling resistance.

[Teo LEGO Technic]

50 minutes ago, lmdesigner42 said:

It's always fun to push the boundaries of Lego performance. Your math is correct, longitudinal speed is directly proportional to tire radius. However, you may want to consider some additional gearing down, since heavier or softer tires have additional power loss from higher inertia and rolling resistance.

That's a very good point about the tires. I think I will try a quick prototype before I build the final model to see how much torque it has with the current gearing, and adjust from there. I can just hook up two axles with no suspension, add about the amount of weight the final model is expected to have, and see how it does. 

[Teo LEGO Technic]

Update: June 22 

I spent quite a while adding additional bracing to the front axle to get it ready for the heavier rock-crawling tires and to ensure the added two studs of width don't introduce any wobble. This included bracing the steering rack longitudinally to the 5x7 frame to reduce steering play and adding robust mounting points for the 4-link triangulated suspension. This added a little extra weight compared to my previous design, but I think it's worth the added toughness, given that the axle has to handle larger tires and drive at higher speeds than my Praga trial truck, and therefore will absorb greater impacts. 

I did a quick informal test, and I'm happy with the strength of the steering and its rigidity. I also tested the torque and speed if I mount this axle directly as-is to buggy motors. @lmdesigner42 I think it has enough torque, I don't need to reduce it further. I think there was still lots of torque to spare on the pick-up truck form before with a similar setup, and I don't want to compromise on the speed - this thing won't have massive torque to tow other trucks and such, but it will have more than enough to get up steep hills fairly quickly. 

I was also considering purchasing an additional BuWizz as I found the single BuWizz struggled to power both buggy motors under load, particularly in Ludicrous mode. However, I did some research, and it appears to me that the torque is still the same in Normal and Fast mode; it's just the speed that is reduced. Therefore, I think, for the sake of my bank account, I will stick to the BuWizz I have. In Normal or Fast mode, it can use its full torque to climb difficult terrain, and in Ludicrous, it can drive over flat surfaces, or where the ground is sandy and speed is needed to get up a hill, although it will not work well under load. Effectively, it will be like having a sort of "gearbox" - Normal/Fast mode is slower but uses full torque, and Ludicrous is faster but cuts out under heavy load and can't use the full torque. To all the electrical engineering buffs out there, if I am wrong in this, please correct me, and I will reconsider buying another BuWizz (though they sure are pricy!). 

The next step is to build the rear axle. Next, for the chassis, I think I will try to keep the wheelbase as short as possible to minimize weight. It also occurred to me that I can connect the suspension links on the front axle in such a way as to create a positive caster angle, so I will bear that in mind when I come to it.

As always, tips and suggestions are much appreciated

[gyenesvi]

Always good to see another rock crawler in the making :) I'm about to publish one soon as well!

About the gearing, you don't need more down-gearing. My rock bouncer had this amount of gearing from the slow output of two buggy motors and it had plenty of torque, but was a bit too slow for speed racing. Then my blue buggy had 1:1 gearing (12:12 diffs) to the planetary hubs from the fast outputs of two buggy motors, and it was still enough for not so extreme off-roading! However, another point you should keep in mind is controllability of speed. If you make it too fast, it will be hard to control on the rocks. It's good if it has some punch left for some climbs, but too much speed is not needed for climbing. I think the current gearing is a good start.

[Teo LEGO Technic]

2 hours ago, gyenesvi said:

Always good to see another rock crawler in the making :) I'm about to publish one soon as well!

About the gearing, you don't need more down-gearing. My rock bouncer had this amount of gearing from the slow output of two buggy motors and it had plenty of torque, but was a bit too slow for speed racing. Then my blue buggy had 1:1 gearing (12:12 diffs) to the planetary hubs from the fast outputs of two buggy motors, and it was still enough for not so extreme off-roading! However, another point you should keep in mind is controllability of speed. If you make it too fast, it will be hard to control on the rocks. It's good if it has some punch left for some climbs, but too much speed is not needed for climbing. I think the current gearing is a good start.

Thanks, I'm excited to see your new one as well! :) 

Your Rock Bouncer is awesome! Aptly named, that suspension is very bouncy indeed and great fun to watch. Correct me if I'm wrong, but because I'm using the 12/28 tooth bevel gear combination in place of the differential, whereas you used the 20/28 tooth combination with the differential, is mine geared down further? Controllability is another good point - I think because I am using BuWizz 2.0, I can add a dial to my controller to shift between Slow, Normal, Fast, and Ludicrous, and in Slow I can have lots of control and go slow.

If you don't mind, I think I will take some inspiration from your crawler too, it's a gem  I just have a few questions:

1. Did the open diffs cause you to lose traction over very uneven terrain, or did the suspension articulation make up for it?
2. I see that you are using longer, custom-built suspension links rather than the 9-long Technic links, which allows you to connect the shock absorbers directly to the suspension links rather than the axle. Do you recommend this? Did it give you greater articulation?

Overall, an awesome build   I particularly like how you captured all the angles of the exo-skeleton. 

 

Edited June 23, 2025 by Teo LEGO Technic

[gyenesvi]

4 hours ago, Teo LEGO Technic said:

Your Rock Bouncer is awesome! Aptly named, that suspension is very bouncy indeed and great fun to watch.

Thanks, glad you like it!

4 hours ago, Teo LEGO Technic said:

Correct me if I'm wrong, but because I'm using the 12/28 tooth bevel gear combination in place of the differential, whereas you used the 20/28 tooth combination with the differential, is mine geared down further?

Originally I built it with the 12/28 differential, because I was cautious with speed, that was my first build with Buwizz motors. But I realized it is too slow and it can take more and changed the diff to 20/28 in the final one.

4 hours ago, Teo LEGO Technic said:

Controllability is another good point - I think because I am using BuWizz 2.0, I can add a dial to my controller to shift between Slow, Normal, Fast, and Ludicrous, and in Slow I can have lots of control and go slow.

That's an option indeed, though I have doubts that a Buwizz 2 can power two buggy motors in a stable way, even a Buwizz 3 shuts down quite a easily in case of faster offroad models if the current limits are not set. Are you planning to use the Buwizz app to control it?

4 hours ago, Teo LEGO Technic said:

If you don't mind, I think I will take some inspiration from your crawler too, it's a gem  I just have a few questions:

Sure, go ahead and use it!

4 hours ago, Teo LEGO Technic said:

1. Did the open diffs cause you to lose traction over very uneven terrain, or did the suspension articulation make up for it?

The suspension articulation did make up for a lot of it, actually that is why I left it open to see how it works, but for a race I'd just go with locked diffs. Practically all (non-lego) offroad RC cars use locked diffs..

4 hours ago, Teo LEGO Technic said:

1. I see that you are using longer, custom-built suspension links rather than the 9-long Technic links, which allows you to connect the shock absorbers directly to the suspension links rather than the axle. Do you recommend this? Did it give you greater articulation?

Well it's a useful technique if you want a ton of articulation, especially if you have shorter hard springs only. Mounted around the middle of the link, it effectively doubles the movement of the end of the link. Whether you need that much also depends whether the axles will have space for all that articulation, because if something else will limit them sooner, then it's not required so much. I think I'll keep using this more for the rear axle, but I am also experimenting with more traditional setups for the front.

4 hours ago, Teo LEGO Technic said:

Overall, an awesome build   I particularly like how you captured all the angles of the exo-skeleton. 

Thanks, that was one of the main goals of the model!

[Teo LEGO Technic]

5 hours ago, gyenesvi said:

That's an option indeed, though I have doubts that a Buwizz 2 can power two buggy motors in a stable way, even a Buwizz 3 shuts down quite a easily in case of faster offroad models if the current limits are not set. Are you planning to use the Buwizz app to control it?

I also thought it wouldn't handle it, but it seems to work well enough in Normal and even Fast mode, just not Ludicrous. @Zerobricks told me it was doable and it worked well on my RAM pickup, although I never drove it in low gear under very heavy loads. Yes, I plan to use the BuWizz app as usual. 

5 hours ago, gyenesvi said:

The suspension articulation did make up for a lot of it, actually that is why I left it open to see how it works, but for a race I'd just go with locked diffs. Practically all (non-lego) offroad RC cars use locked diffs..

That's good to know, I'll stick to locked diffs then.

5 hours ago, gyenesvi said:

Well it's a useful technique if you want a ton of articulation, especially if you have shorter hard springs only. Mounted around the middle of the link, it effectively doubles the movement of the end of the link. Whether you need that much also depends whether the axles will have space for all that articulation, because if something else will limit them sooner, then it's not required so much. I think I'll keep using this more for the rear axle, but I am also experimenting with more traditional setups for the front.

That's a good point. I think I will be ok since I am using the long 9.5L soft shock absorbers, and if I place them at an angle, it increases the range slightly further still, which I will do. If I find it's still not enough, I can switch it up, but it seems to have worked fine for @PunkTacoNYC's crawler

Thanks for the thorough feedback!

[gyenesvi]

5 hours ago, Teo LEGO Technic said:

That's a good point. I think I will be ok since I am using the long 9.5L soft shock absorbers, and if I place them at an angle, it increases the range slightly further still, which I will do. If I find it's still not enough, I can switch it up, but it seems to have worked fine for @PunkTacoNYC's crawler

I agree, that's the setup I want to experiment more with for the front. I've used it for the rear on my Toyota Hilux truggy, and it worked well.

[Teo LEGO Technic]

Update: June 25

I have been hard at work to get this done on time. The rear axle is complete; it is robust, and the mounting points are well-positioned, so I'm satisfied with it. I then did some research to determine what the ideal geometry is for a 4-link suspension, particularly on a rock crawler. These are the guidelines I came up with to summarize my findings, mainly taken from this great video I found https://www.youtube.com/watch?v=MTozXC4hq04. The guidelines are as follows:

1. The top 2 beams form a triangle ideally centred on the axle to centre it. The bottom two should be fairly far outward on the body to reduce twisting movement
2. Upper links should be ~85% the length of the bottom links (this, unfortunately, I can't do, I'm stuck with the 9L links)
3. Vertical link separation at axle ~25% of tire size
4. A diagram of the ideal rear link geometry is shown below, which optimizes the force exerted by the wheels

Using this information, I built a prototype of the chassis, which was only to figure out how to satisfy the geometry. I will rebuild it afterwards to be more rigid and efficient.

On the front axle, I managed a 4-degree caster angle, which is decent - I would have wanted a bit more, perhaps, but I couldn't figure out how to get it just right without using some non-perpendicular link connections, which I didn't really want to venture into. I also had to attach the upper links quite high so that they wouldn't touch the steering components during suspension travel, but this should not be an issue. In addition, the driveshaft is already angled, and adding more caster would further angle it, which I don't want.

On the rear axle, I was able to get the geometry almost exactly right, matching the reference above. Here is my current setup:

There are a couple of open questions I currently have:

1. Should I increase the ride height, and therefore, the ground clearance? 
   1. For this, I opted not to for now, because any higher and the geometry is impossible to get right on the rear axle. This is about a realistic ground clearance for a rock crawler, and I think it should be plenty with the massive crawler tires I'm using. Further, keeping the ride lower keeps the COG (Center Of Gravity) lower and enables better climbing.
   2. Conclusion: Keep ride height as-is
2. Should I mount the buggy motors on their side to lower COG?
   1. As I finished the prototype, it occurred to me that with the vertical buggy motor setup, the heavy part of the motor is sticking out at the top, which is terrible for the COG. A better solution would be to turn the engines on their side. In a quick test I found the crawler currently goes fairly slow, as you anticipated @gyenesvi, which isn't really a problem as I want lots of torque, but it means I think I can turn it on its side and use the faster buggy motor output (~35% faster) and it should still have ample torque and better high speed. This will, however, mean that the driveshaft will be mounted 1 stud higher into the body, which means the geometry for upward suspension travel will involve more lengthening and shortening of the driveshaft, but I think this could be manageable given the long range of motion of the 4L CV joint.
   2. Conclusion: Turn the buggy motors on their side for lower COG
3. Is caster angle relevant on the rear axle?
   1. This is an open question to the engineers out there. I couldn't find a definitive answer online. My intuition is that it isn't, because there isn't steering, is that correct? I'm asking because my current setup has a very tiny bit of a negative caster on the rear, and I don't want this affecting performance.

So the next steps are to (1) turn the buggy motors sideways, (2) shorten the wheelbase slightly, and (3) purchase some original LEGO planetary hubs and CV joints; these AliExpress copies are just not good enough.

Questions, suggestions, comments, and funny jokes are welcome as always!  

Edited June 25, 2025 by Teo LEGO Technic

[lmdesigner42]

It's great to see some more progress on your rock crawler. Based on the amount of effort you're putting into the chassis design, it should perform very well!

With regards to the negative caster angle on the rear axle, I don't believe the caster angle really exists unless the axle is steered. That being said, possible effects are:

1. Increasing the angle on the axle-side CV joint (usually not good)
2. Adding a toe angle and tire drag, IF there is any camber on the rear axles. I think your design is close enough to vertical though that there shouldn't be an issue.

[2GodBDGlory]

Looks really good! I never really thought about looking up ideal crawler geometry, so that's really cool that you're working with that! I wonder if that same geometry is also considered ideal for scaled-down models, but I don't have any particular reason to think it wouldn't be.

Yeah, I think turning the motors sideways could be a good call, and I agree that you probably have enough ground clearance.

I'd also agree with @lmdesigner42 that castor angle doesn't really exist unless the axle is steered, so I wouldn't worry about that angle on the rear axle.

[gyenesvi]

Nice that you look up real world suspension geometry, I did also dig a lot in this topic!

21 hours ago, Teo LEGO Technic said:

There are a couple of open questions I currently have:

I agree that GC is enough and turning the motors side-ways is a good idea for extra speed and also to lower COG, that is more important.

As for rear caster, I think it's only relevant for the incoming CV joint's angle, as @lmdesigner42 says, but I think this should actually be taken seriously. The problem that can easily happen if your caster is negative (or moves to negative during articulation) at the rear is that the CV joint is pulled apart and its operating angle increases, and the sliding CV joint can actually easily slide apart completely. I think there's an unavoidable design flaw at that part, that if the gap between the two sliding parts increases big enough (one stud), then the axle piece from the non-sliding end can slide out, into the non-sliding end, and then fall out. For me this typically happens when the angle and the distance between the two pieces increases during articulation (allowing which is ironically its main purpose), so I find that the only way to minimize the chances of happening is to decrease the gap/angle between the two, which can be achieved by slight positive caster.

The front is more complicated, because the same trick at the front would lead to negative caster, which then is not so good for the steering geometry. So I find that the best we can do is to keep the caster around neutral at the front. Also, for crawlers, positive caster is not needed to improve handling; that's only useful for faster cars I think.

As I hinted already, another thing to consider about link geometry is how the caster changes during articulation, not only how it is in resting state. At least, equal length links help achieve keeping the caster the same when they are parallel.

[Teo LEGO Technic]

18 hours ago, lmdesigner42 said:

1. Increasing the angle on the axle-side CV joint (usually not good)
2. Adding a toe angle and tire drag, IF there is any camber on the rear axles. I think your design is close enough to vertical though that there shouldn't be an issue.

Thanks for the tips! Glad that my intuition was correct, and good point about toe angle. The setup does have a very slight negative camber, so it's worth making an effort to not have the caster too far off neutral.

14 hours ago, 2GodBDGlory said:

Looks really good! I never really thought about looking up ideal crawler geometry, so that's really cool that you're working with that! I wonder if that same geometry is also considered ideal for scaled-down models, but I don't have any particular reason to think it wouldn't be.

Thanks! Yeah, I figure I would go all out while I'm at it   I believe all the points should apply at 1:10 scale, looking at scale RC crawlers, they apply all these points as well

14 hours ago, 2GodBDGlory said:

Yeah, I think turning the motors sideways could be a good call, and I agree that you probably have enough ground clearance.

Sounds good!

2 hours ago, gyenesvi said:

The problem that can easily happen if your caster is negative (or moves to negative during articulation) at the rear is that the CV joint is pulled apart and its operating angle increases, and the sliding CV joint can actually easily slide apart completely. I think there's an unavoidable design flaw at that part, that if the gap between the two sliding parts increases big enough (one stud), then the axle piece from the non-sliding end can slide out, into the non-sliding end, and then fall out. For me this typically happens when the angle and the distance between the two pieces increases during articulation (allowing which is ironically its main purpose), so I find that the only way to minimize the chances of happening is to decrease the gap/angle between the two, which can be achieved by slight positive caster.

That's a great point I hadn't considered. What you're saying is that, under torque, if there is 1 stud or more space between the two CV joints - the 3L and 4L - the 4L axle will slide 3 studs into the frictionless 4L CV joint and so come out of the 3L CV joint, am I understanding that right? That's definitely something to consider. For my particular setup, the bottom links are almost in the exact same plane as the driveshaft, so the mounting of the bottom links is the main factor in the distance between the CV joints. I will rework it a bit so that it is closer to ~0.5 stud under normal suspension positioning, so that under flex, there is still enough space for the driveshaft to shorten, but not so much space that the shaft is able to come out of the 3L CV joint.

2 hours ago, gyenesvi said:

Also, for crawlers, positive caster is not needed to improve handling; that's only useful for faster cars I think.

This is a good point, a true crawler has neutral caster, as it is meant to drive slow and the steering mechanism is often something very powerful, like a hydraulic actuator. Rock bouncers, on the other hand, are slightly faster than rock crawlers and have a tiny bit of caster to enable improved steering at higher speed. For my setup I opted to make it a bit more similar to a rock bouncer, adding a very slight caster angle on the front for two reasons. (1) I built this thing to be able to achieve decent speed, at which caster does help keep the wheels centred, and (2) because the servo motor is not as powerful as a linear actuator or a geared down L-motor, I want to give it that tiny extra bit of mechanical assistance. 

2 hours ago, gyenesvi said:

As I hinted already, another thing to consider about link geometry is how the caster changes during articulation, not only how it is in resting state. At least, equal length links help achieve keeping the caster the same when they are parallel.

Definitely agree   For that reason, the front control arms are nearly parallel. The rear ones are angled a bit more so that the instant center points below the anti-squat line, as per the illustration from before. 

Thanks for all the feedback guys, much appreciated! Time to implement your advice now @gyenesvi and see what I can do about that driveshaft gap issue.

[Teo LEGO Technic]

Update: June 26

Moving the buggy motors sideways was fairly straightforward; it required adding a pair of 16-tooth gears, which only very slightly increases friction, so I'm fine with the compromise. Either way, I plan to lubricate the drivetrain with silicone lubricant when I run this thing.

This, however, shifted both front and rear driveshaft outputs by a 1/2 stud, so I had to redo the 4-link suspension attachment, again trying to ensure the correct geometry. After a lot of tinkering, I have set it up again as follows: 

On the rear, I took your advice @gyenesvi and found a solution to ensure the distance between the CV joints never reaches 1 stud, to prevent the axle coming out under torque - this is especially important on the rear axle, where most of the torque goes when climbing. To this end, on the rear axle, I moved the bottom links half a stud inwards and upwards, approximately, using the angled liftarm. Because this made the connection point too close to the top links, I lifted those a bit higher. I also moved it a stud towards the front on the axle itself, so that I can achieve near-0 caster using the connection points on the same liftarm, so I don't have to go crazy creating a new connection point for the top links. Overall, this had the unfortunate effect of changing the geometry such that the instant center is no longer below the 100% anti-squat line, as it was before. While this is unfortunate, I don't see another solution that prevents the driveshaft from slipping out, and I think this is more important. I'm not sure how much of an effect this new geometry will have, but if the drive axle slips out, that's definitely a problem 

I also tried to find some blueprints to use as reference for the dimensions of the chassis, but couldn't find much. In the end, I created my own using some photos I found online of the "1/10 RBX10 Ryft 4X4 RTR Brushless Rock Bouncer":

Overlaying it on some existing blueprints I came up with the following. Since I am not making a proper body for this, to save weight, I will try to follow the general dimensions of the exoskeleton and give it a good look that way, so the chassis also functions as bodywork:

The moral of the story is this - setting up 4 link suspension in Technic is a real pain. If you want to have (1) correct caster angle, (2) good travel, (3) correct anti-squat geometry, and (4) make sure the CV joints stay a similar distance apart at all times to prevent axle slippage - basically, you will lose your marbles  

I will let the options marinate in my head for a bit longer, I think - if anyone has any ideas for improvements, please let me know - and then move on to attaching the shock absorbers and test it all out. 

[gyenesvi]

7 minutes ago, Teo LEGO Technic said:

The moral of the story is this - setting up 4 link suspension in Technic is a real pain. If you want to have (1) correct caster angle, (2) good travel, (3) correct anti-squat geometry, and (4) make sure the CV joints stay a similar distance apart at all times to prevent axle slippage - basically, you will lose your marbles  

I can agree 100% ! I think link geometry is difficult on its own, and then the stud system of lego imposes constraints on that because you can't have any link length, just full stud lengths, and on top of that we are stuck with only a couple of actual lengths that exist (though the max length of 9 does not help too much at this large scale).

By the way, I found that there are 3 link setups that can work well in lego:

• parallel links: as they can work with the same link lengths, and don't introduce caster problems. However, in this case you need panhard rods for sideways stabilization, so this is only applicable for trucks and cars with a front/end, so not good for buggies, for them you need triangulation.
• semi-triangulated links: in which case the lower links are parallel and the upper ones are triangulated. In this case, the lower links are best to be constrained at the chassis end to move only a bit sideways (no ball joints, only pins/axles). This works well with long custom links built from liftarms, as any length can be made, and also this one allows springs to be attached to the links. This setup is actually used in real buggies, including the constrained sideways rotation of the lower links.
• double-triangulated links: the lower links are triangulated outwards from the chassis, while the upper ones are triangulated inwards. In this case it works with links of equal length, because the outward and inward triangulation can be made equal, and that makes the link shortening the same on the upper and lower, so the geometry can play out nicely without creating caster problems. Also, if you set this up well (links not too steep), then you can get away without making complex mounting points to the springs that allow them to rotate around two axles, which in itself can be a complicated thing to do solidly if there's not much space / point to mount to at the axle end for example.

Lego could improve sooo much in this area with more parts, like longer links and more ball pins/ball socket parts..

3 hours ago, Teo LEGO Technic said:

That's a great point I hadn't considered. What you're saying is that, under torque, if there is 1 stud or more space between the two CV joints - the 3L and 4L - the 4L axle will slide 3 studs into the frictionless 4L CV joint and so come out of the 3L CV joint, am I understanding that right?

Yes, you got that right. It only has to slide 1 stud out of the 3L CV joint to let this happen, which is not that much.

[Teo LEGO Technic]

Update June 27

I took your advice @gyenesvi , and made one more attempt at correcting the geometry, this time using a longer custom steering link. My solution is a semi-triangulated setup, where the bottom links are still slightly angled, but not forming a triangle; it is very similar to the Ryft RC crawler I mentioned earlier. I also added a few more geometric best practices based on some videos I watched about anti-squat requirements for crawlers:

1. The distance on the axle between the top and bottom links should be ~25% of the tire diameter. For my tires, this is 14 studs, so 1/4 * 14 = 3.5. On my setup, the distance is 4 studs, very close.
2. The top link should be ~85% the length of the bottom links. Bc I used the 9L top link, this meant the bottom link should be 9 / 0.85 = 10.6. On my setup, their length is 10, also very close.
3. I also moved the bottom links further outward on the axle for better twisting stability.

The final setup looks like pictured below. I added a basic chassis with shock absorbers at the right height, just to test it out; it's only a temporary mounting. As you can see, the instant center is closer to being below the anti-squat line, an improvement from before. Indeed, this is quite realistic geometry for a crawler; the reason I can't get it below the anti-squat line without lowering the ride is that my CG is so low, much lower than a real crawler. I'm satisfied with it at last; the caster is also very nearly 0. There is a tiny bit less than 1 stud of space between the two CV joints - if this causes the axle to fall out, I can shift the FM CV joint going into the axle out by 1/2 stud and add a 1/2 stud spacer, and that will fix it. 

Now for the bad news  ...

I tested this mock-up on some inclines to see if the torque holds up, and I wasn't satisfied. It's able to make it up a roughly 45º slope, but it starts to struggle. I think you were right @gyenesvi - the BuWizz 2.0 can drive the 2 buggy motors, but it can't power them quite enough under load. As I see it, I have a couple of options:

1. Reduce the gearing further coming out of the fast buggy output. Currently, it is 1:1. I can replace this with a 12:20 gearing, increasing torque by 67%, but also slowing the model down by the same ratio 
2. Purchase another BuWizz 2.0, or a 3.0 - too expensive 
3. Power it with some cheaper 3rd party components

Of these options, I think I favour option 3. It seems like a shame not to use the full potential of the buggy motors - after all, the whole point of adding servo steering and keeping the build lightweight was to make this crawler fast in addition to powerful. If it's not fast, I might as well use PF XL motors, in which case I've not really improved much in terms of performance or design compared with my Praga truck. 

The issue is I'm not very knowledgeable about electronics, I just never really took the time to learn. I will start looking into it on my own, but advice is much appreciated!

 

[lmdesigner42]

That's a shame to have your performance let down by the power supply, after all the effort you put into the chassis geometry. Would it be of any benefit to put a lower current limit on just one of the buggy motors? That way, both could still run fully for high speed/low torque driving, while for high torque crawling the current output would favor one motor and allow it to produce more torque before the BuWizz cuts out.

[Teo LEGO Technic]

37 minutes ago, lmdesigner42 said:

That's a shame to have your performance let down by the power supply, after all the effort you put into the chassis geometry. Would it be of any benefit to put a lower current limit on just one of the buggy motors? That way, both could still run fully for high speed/low torque driving, while for high torque crawling the current output would favor one motor and allow it to produce more torque before the BuWizz cuts out.

That sounds promising, how exactly do you mean? On the BuWizz app I can map the same vertical slider for throttle onto multiple BuWizz outputs, so let's say I connect the BuWizz motors on outputs 3,4 and map both of these to the throttle slider, as below (servo is on output 2):

 

I tried for fun to see what would happen if I increase the current limit now on both outputs 3,4 - I tried values slightly larger than the max defaults:

It seems to have much better climbing ability now! Is this fair play? To be honest, I'm out of my depth a bit here . Could this cause damage to the BuWizz? I would hope they have a system to shut it down if it overheats before damaging anything?

The other good news is the geometry seems to be working well! All the power in climbing appears to be getting transferred properly 

Edited June 27, 2025 by Teo LEGO Technic

[gyenesvi]

Well, we mostly lower the current limits, not increase it, to prevent shutdown of the buwizz unit. But it seems your problem is not that it would shut down, so until that happens, it's okay to increase it I guess.. Just test it properly (like try to sprint uphill suddenly) because if it starts shutting down during race, that's not going to be a pleasant surprise :)

Nice progress otherwise with link geometry, I always wondered whether going so far with those details like squat line actually matters in case of lego models. My gut feeling is that it only matters with more power/speed than typical lego motors provide, but at two Buwizz motors' power and smooth suspension flex, it can start to matter.

[Teo LEGO Technic]

4 hours ago, gyenesvi said:

Well, we mostly lower the current limits, not increase it, to prevent shutdown of the buwizz unit. But it seems your problem is not that it would shut down, so until that happens, it's okay to increase it I guess.. Just test it properly (like try to sprint uphill suddenly) because if it starts shutting down during race, that's not going to be a pleasant surprise :)

Nice progress otherwise with link geometry, I always wondered whether going so far with those details like squat line actually matters in case of lego models. My gut feeling is that it only matters with more power/speed than typical lego motors provide, but at two Buwizz motors' power and smooth suspension flex, it can start to matter.

It does seem to shut off if I run it in Ludicrous, so I will stick to Normal and Fast. I also added a tiny bit of ramp-up and ramp-down programatically through the BuWizz app for the throttle control, 0.13 seconds, in the hopes that that will reduce bursts of current that could cause it to shut down. 

Glad you like the geometry! Half lost my mind getting it right haha. I think indeed in general it doesn't matter much for LEGO, but I did notice on my RAM pickup that, with the rear axle angled quite a lot, and therefore the instant center high in the air, on steep inclines the rear axle would keep pushing itself off the ground and lose traction, making it harder to climb. This was bc the buggy motors had enough power to make this matter, check it out here (I linked it with a timestamp for the hill climb): 

That's what made me put the effort in for the crawler, and in testing, it seems to have paid off - it stays completely planted

 

Edited June 28, 2025 by Teo LEGO Technic

[gyenesvi]

On 6/28/2025 at 5:54 PM, Teo LEGO Technic said:

but I did notice on my RAM pickup that, with the rear axle angled quite a lot, and therefore the instant center high in the air, on steep inclines the rear axle would keep pushing itself off the ground and lose traction, making it harder to climb

I also noticed such behavior in some models, but my assumption was that it has to do with a combination of low weight on the back / too stiff springs / too much power / lack of low speed control. Curious how your final model will behave in this respect!