[WIP] Building a krAZ 255 6x6 Truck

[AutoBacon]

Hi there. So I was inspired by @keymaker and their awesome krAZ 255b build to create my own krAZ 255 truck at a similar scale (56x26 Wrangler tires), first a bit of back story. I've been tinkering with compact driven, steered, and suspended axles for quite a while, with the initial intention of making an RC chassis for the 42122 Wrangler or 10290 Creator 50s Pickup Truck. Anyway, the result was a pair of 11 Stud wide axles, but it took me so long that by the time I created a chassis, I had moved countries and hardly have any Lego with me. Therefore this new project will be for the most part CAD only (apart from some component testing) until I can get some more parts. 

Goals:

• I wanted the scale to be as accurate as possible using the 56x26 tires, which means an axle narrower than my current design is required, preferably 9 studs.
• All 3 axles should be fully driven and suspended.
• The body and cargo bed should be brick built if it's not too heavy, and the body/bed should sit as low as possible on the frame rails so as to not look like it has a suspension lift kit.
• The axles and chassis should be constructed favoring simplicity and low part count.
• The drivetrain should somewhat resemble the real truck, i.e. offset drive, dual rear drive lines, where the 3rd axle's drive line passes over the 2nd axle.

 

I'll also point out that I don't mind modifying parts slightly if I think it is worth it, especially at such a small scale. Anyway, as keymaker did, I'd like to start with the front axle because it has the most constraints.

After some experimenting, it became apparent of a few restrictions due to the scale.

• Springs are too big, rubber connectors should be used as springs and can give a maximum of one stud travel, however, these are very soft.
• The steering angle will be limited due to the steering rack contacting other parts. If the steering rack was filed down a little it could increase the angle but may end up causing the wheel to contact the frame anyway...
• A 20-tooth bevel gear is too big and too much effort to integrate, so no gearing down in the axle. Hopefully, with the light rubber springs, this doesn't lead to too much axle torque roll under load or CV joints breaking.
• Wheel flex could be an issue but again hopefully the overall weight is not too high and the amount of flex won't be too tragic.

I'm not too worried about these, however, as I think it's just what happens when you shrink things, compromises have to be made. I also tried shortening the axle of the female CV end for the drive shaft to 1 stud, just because it looked better, might be a bit wobbly, who knows.

Positives:

• Chassis rails are only one stud above the top of the tire, which I think is close enough to the blueprint
• There is nothing protruding above the rails that could interfere with the cab or motor mounts
• The axle is firmly held in place and lateral movement should be minimal.

That is all for today. I have already almost finalized a rear pair of axles, however, I didn't want to make this post too long. Thanks for reading :D Thanks again to keymaker for the inspiration.

 

[gyenesvi]

This is a nice tight start, I like the general idea and how the driveshaft comes in, but wonder how the axle front/rear/top/bottom sections are connected, because I don't see much possibilities for that in a stable way. For example what is the main load bearing 7L cross beam connected to? How is the rubber piece connected to it in a way that it will transfer the force from the wheels? If you push the wheels up, and the rubber is pushed down by the body, how will the two not separate?

Also one note, I believe your axle in the other thread is considered 9 stud wide (not 11), because it's the steering pivot that matters, not where the wheels are mounted, that can go inside the rim. So I'd consider this one a 7 wide, which is pretty rare and tight I guess.

Oh, and one more thing, I'd check that the wheels don't hit the lower links when steered before moving on! It's quite close. Though as you say, the steering may be limited, so you might be able to get away with it, but still, a cross check would be a good idea!

Edited November 23, 2022 by gyenesvi

[keymaker]

I'm glad my MOC can be inspiration for other builders I'm curious what you will create with even smaller scale, but you didn't mention what this scale would be. My KrAZ 255 is 1:23 which leads to 15 studs wide body. This gives me 11 studs of place with standard rims for axles (or as in my KrAZ 2.0 13 stud axles with special rims).

From what you write I see you will face or already faced problems I had. I really want to see how you are going to solve them, especially something like this in such small scale :D

6 hours ago, AutoBacon said:

The drivetrain should somewhat resemble the real truck, i.e. offset drive, dual rear drive lines, where the 3rd axle's drive line passes over the 2nd axle.

 

[AutoBacon]

12 hours ago, gyenesvi said:

This is a nice tight start, I like the general idea and how the driveshaft comes in, but wonder how the axle front/rear/top/bottom sections are connected, because I don't see much possibilities for that in a stable way. For example what is the main load bearing 7L cross beam connected to? How is the rubber piece connected to it in a way that it will transfer the force from the wheels? If you push the wheels up, and the rubber is pushed down by the body, how will the two not separate?

Yes, you are right about this. Two friction pins are what keep the two parts from separating. Once I have some physical parts and the chassis is rolling I will have to test its rigidity, I'm really not sure how to get around this. I tried some stronger designs but they all result in a 1 stud higher profile. We will see how the rest of the build goes and see if its a major problem.

7 hours ago, keymaker said:

I'm glad my MOC can be inspiration for other builders I'm curious what you will create with even smaller scale, but you didn't mention what this scale would be. My KrAZ 255 is 1:23 which leads to 15 studs wide body. This gives me 11 studs of place with standard rims for axles (or as in my KrAZ 2.0 13 stud axles with special rims).

 

I don't have the dimensions of the real wheel so I'm not sure about the scale, however, given the size of the tires used in the blueprint I found, it would seem a total width of 14 studs for the axle and 15 for the widest part of the body (the cargo bed). Upon closer inspection, it looks like we are using different blueprints :D

[AutoBacon]

For the rear axles, I really liked the idea of replicating the dual drive lines which I think would be really cool, and if anything, would distribute the motor load across more cv joints for better reliability.

So I started with a mock-up, to see if there would actually be room. Straight away I decided that I would need to use some shortened suspension links (4 studs) to achieve the desired spacing. I also liked the idea of using the soft axles as leaf springs but they seemed too complicated to mount and would require an additional torque arm to stabilize the axles on both axis.

I decided the drive system was viable. The hard part would be making the axles stiff enough. After a few iterations, I decided that the topmost links that would form the 'leaf' would also need to be shortened by a stud, which shouldn't be too laborious with a blade and file.

Again, the axle is subject to a real-world build test, however, I am quite satisfied with the general design. The first axle is slightly less stable than the rear due to only being connected to the top beam by 3 studs rather than 4, which is to allow room for the second drive line to pass through. I did some basic tests and I think the front and rear bracing between each 'hub' using the 7L thin liftarms should be sufficient. The mounting of the central pillar to the frame rails will probably have to be improved a little to increase rigidity. The only concern, apart from the usual overly soft suspension, is the tires rubbing on the frame during full travel. I'm hoping only one stud of travel should mean the axle doesn't pivot too much.

I quickly mocked a simple geartrain to get the drive shafts rotating in the correct direction, and here is a rough assembly:

Next will probably be figuring out how to mount the motors in a reasonable way. I'd like them to be central in the chassis, perhaps the steering motor under the hood and the drive motor behind it, all concealed by the cab, and the battery box positioned behind the cab next to the spare wheel.

Thanks again for reading and any thoughts are appreciated. Also, if anyone is interested in throwing together one of the axles in real life that would be amazing :D

Edited November 24, 2022 by AutoBacon

[keymaker]

8 hours ago, AutoBacon said:

Upon closer inspection, it looks like we are using different blueprints :D

Below you can check blueprints I used. Nevertheless there are very interesting solutions and I'm looking forward to see more. And you should not hesitate to build it in real life, because thanks to my experience with such solutions based on renders I already see what can be a problem when you will put it together :D

Edited November 24, 2022 by keymaker

[gyenesvi]

14 hours ago, AutoBacon said:

I'm really not sure how to get around this.

I can see that you are not really afraid of 'manufacturing' all those parts for yourself that Lego forgot to give us :) (that 1stud shorter (or longer) control arm would be good to have, glad someone else also realizes that). Anyways, for many structural problems, flip-flop beams in all sizes can provide answers, as they allow connections in perpendicular directions, it is often possible to form-lock stuff in a smaller space so that it cannot come apart in a certain direction. Since this won't be a pure lego solution anyway, I suggest you buy a bunch of CADA flip-flop beams on Ali Express. They are available in all sizes as a pack for about $15 in black, something like 10 pieces each size, I have bought one pack for such experimental builds.

About the suspension travel, I don't understand why you expect 1 stud, the rubber pieces are 2 studs long, one half is fixed, the other half can compress 1/2 studs max.

Anyways, I like your direction of thinking with quite clean structure, it would be awesome if it was possible to pull off in a stable enough way for the scale. BTW I'd definitely try to put two motors to where those tanks are on the sides and keep the cabin space for seats. Also the steering will need a way to route to the front axle in a way that does not collide with the drivetrain.

[AutoBacon]

Since I can't build and test the chassis at the moment, I decided to spend some time mocking up a cab and bed. Bodywork has never been my forte, and I've never tried using bricks, so this is a first for me. This is just a mock-up to see how the scale would work, so the cab and bed are not fixed to the chassis yet, as are the fuel tanks and front fenders/bumper. I'm not quite happy with the fenders yet due to the high placement of the front lights, and I reckon there will be some tire rub at full lock. So next I will be experimenting with fender designs to rectify these issues unless I have to compromise. Another compromise was the middle windscreen pillar, which I couldn't get to look right on this model, so I went without it.

The single large motor for drive has been placed under the hood, which is long enough for it to be completely contained. This leaves some room for a bench seat that sits directly above the drive shaft out of the motor. Three 16t gears move the output down 6 studs, the middle gear of which is a red clutch gear to allow the steering axle to pass through. I chose to use a cada micro motor for the steering, which based on the rpm specs, I should be able to use un-geared which means it takes up hardly any room and can be mounted inline, in between the frame rails.

On 11/24/2022 at 3:46 PM, gyenesvi said:

I'd definitely try to put two motors to where those tanks are on the sides and keep the cabin space for seats. Also the steering will need a way to route to the front axle in a way that does not collide with the drivetrain.

I did consider L motors mounted in place of fuel tanks but it doesn't really keep with the style, plus it might be overkill in terms of torque, and with the un-geared drivetrain, I'm quite nervous about ripping cv and u-joints.

Lastly, I couldn't find a suitable place to mount the battery. I will be using a MouldKing Batterybox because the physical controller it comes with has proportional control which I really like, but it is a little larger than a buwizz. I may have to put it into the bed, either right at the front or dress it up as cargo.

Having never designed bodywork before, I'm quite happy with how it's shaping up, but I am aware that there will be a point where I need to build it outside of the computer so I can figure out how to build it in a strong way, and perhaps fix some parts that don't work in real life. I think I am getting close to this point but I still need to buy some parts or a set to salvage from!

Also, I appreciate the feedback and tips so far, and any ideas for the cab will be well received :D

 

Edited December 7, 2022 by AutoBacon

[gyenesvi]

 

It is looking quite good, I like it!

4 hours ago, AutoBacon said:

I chose to use a cada micro motor for the steering, which based on the rpm specs, I should be able to use un-geared which means it takes up hardly any room and can be mounted inline, in between the frame rails.

I'd test this out IRL as soon as possible because I'd be surprised if that motor just worked well ungeared. May happen, but it just sounds too good to be true :) It could be too weak, it could be too fast, etc (or both). I just designed on paper a steering system based on an M motor and a 24T clutch gear, which I have seen in other builds a few times, and it just did not work for my purposes IRL (too fast, too much slack).

4 hours ago, AutoBacon said:

I did consider L motors mounted in place of fuel tanks but it doesn't really keep with the style, plus it might be overkill in terms of torque, and with the un-geared drivetrain, I'm quite nervous about ripping cv and u-joints.

Yeah, you are right about the style, and it may be an overkill indeed. An XL should be enough for this.

[keymaker]

13 hours ago, AutoBacon said:

I did consider L motors mounted in place of fuel tanks but it doesn't really keep with the style, plus it might be overkill in terms of torque, and with the un-geared drivetrain, I'm quite nervous about ripping cv and u-joints.

I like the body, but I wish to know what is the weight of whole truck (even based on program calcualtions) :D Because if you are only planning to driving only on flat, hard surfaces than one L motor will be ok, but If not, then... well :) From my experience, if you pass over 1kg, then one L motor won't be enough for anything different than flat hard floor. And as you probably know, KrAZ is not the typical truck for highway.

On 11/24/2022 at 4:33 AM, AutoBacon said:

I'm not sure If you saw this, but such using of connector (6536) will block your axle, one or another.

 

I strongly advise you to check your solutions in real life, it helps a lot and verifying many ideas, good luck! :)

Edited December 7, 2022 by keymaker

[gyenesvi]

18 minutes ago, keymaker said:

I'm not sure If you saw this, but such using of connector (6536) will block your axle, one or another.

I also saw that, but I thought it's so obvious that he probably meant it as a mock for positioning things, and the driveshaft will be suspended from the top for example.. Curious if that was the real plan or not :) But actually this part may help in such cases. But even with that part, in that setting, it will be hard to properly fix that driveshaft going to the second axle; it will put friction on the cross axle of the first axle. So suspending the driveshaft from the top seems like a better idea for me.

Also, the 2L axles going into the 'differential', only supported by half a stud would worry me. Especially if the model is getting heavier..

[keymaker]

59 minutes ago, gyenesvi said:

Also, the 2L axles going into the 'differential', only supported by half a stud would worry me. Especially if the model is getting heavier..

That is why I would paraphrase sentence of popular "orange" president:
"Looks good, doesn't work" :D

[AutoBacon]

1 hour ago, keymaker said:

I like the body, but I wish to know what is the weight of whole truck (even based on program calcualtions) :D Because if you are only planning to driving only on flat, hard surfaces than one L motor will be ok, but If not, then... well :) From my experience, if you pass over 1kg, then one L motor won't be enough for anything different than flat hard floor. And as you probably know, KrAZ is not the typical truck for highway.

You have me worried now! hehe. The calculations say 840g at the moment, and that is without the battery. It is quite a light battery, I don't have it to hand to measure, so let's say the final weight will be 1000g including any part strengthening. Can 1 ungeared motor power that mass offroad? You have given me doubts :D I am using a MouldKing L motor, which according to youtube testers has about 10% more torque than the Lego version. Also, Ingmar Spijkhoven's 1:17.5 scale KrAZ 255b was powered by 2 L motors, weighed very nearly 2kg, and could tow a 500g trailer. Obviously this was with some down gearing, perhaps with my model I could squeeze in a 20t/12t combination into the differentials to help somewhat. When I return home later this month I will mock up a test bed to see how much weight the single motor can shift.

34 minutes ago, gyenesvi said:

I also saw that, but I thought it's so obvious that he probably meant it as a mock for positioning things, and the driveshaft will be suspended from the top for example.. Curious if that was the real plan or not :) But actually this part may help in such cases. But even with that part, in that setting, it will be hard to properly fix that driveshaft going to the second axle; it will put friction on the cross axle of the first axle. So suspending the driveshaft from the top seems like a better idea for me.

Also, the 2L axles going into the 'differential', only supported by half a stud would worry me. Especially if the model is getting heavier..

Yes, you are correct about the blocked axle, it was only a mock-up to see if there was room :D Space is at a premium here so I don't think I have room for your above recommendation, although I could be wrong. That would be even more difficult if I need to use a 20t gear on the axle though. I also agree the 2L axle into the diffs is worrying. To be honest this whole 'build only in CAD' has included so many assumptions and speculation that I'm nervous the whole drivetrain will crumble in real life!

[Zerobricks]

2 hours ago, AutoBacon said:

You have me worried now! hehe. The calculations say 840g at the moment, and that is without the battery. It is quite a light battery, I don't have it to hand to measure, so let's say the final weight will be 1000g including any part strengthening. Can 1 ungeared motor power that mass offroad? You have given me doubts :D I am using a MouldKing L motor, which according to youtube testers has about 10% more torque than the Lego version. Also, Ingmar Spijkhoven's 1:17.5 scale KrAZ 255b was powered by 2 L motors, weighed very nearly 2kg, and could tow a 500g trailer. Obviously this was with some down gearing, perhaps with my model I could squeeze in a 20t/12t combination into the differentials to help somewhat. When I return home later this month I will mock up a test bed to see how much weight the single motor can shift.

Yes, you are correct about the blocked axle, it was only a mock-up to see if there was room :D Space is at a premium here so I don't think I have room for your above recommendation, although I could be wrong. That would be even more difficult if I need to use a 20t gear on the axle though. I also agree the 2L axle into the diffs is worrying. To be honest this whole 'build only in CAD' has included so many assumptions and speculation that I'm nervous the whole drivetrain will crumble in real life!

 

From my experience both the red and blue U joints will simply slip out and your model will drop the 12 tooth bevel gears. I have a lot of real-life experience professionally designing models and even then I still test the individual modules for real when designing a model in LDD.

Edited December 7, 2022 by Zerobricks

[pow]

@AutoBacon Ever since I saw the build, I've been haunted by that one part you use on the rear axle as the swing arm. I mean the part that connects the red ball pin (6628a) to the 2L axle connector (6538c). I mean this magnifying glass with the cross axle(?). I can't find that anywhere. Does this have a Lego part number?
Greetings

[AutoBacon]

15 minutes ago, pow said:

@AutoBacon Ever since I saw the build, I've been haunted by that one part you use on the rear axle as the swing arm. I mean the part that connects the red ball pin (6628a) to the 2L axle connector (6538c). I mean this magnifying glass with the cross axle(?). I can't find that anywhere. Does this have a Lego part number?
Greetings

It's actually just a 2739a suspension link, which has been cut in half, shortened, and then glued into the 2L axle connector. The link itself is basically just a cross axle so it fits well.

[pow]

Damn, that could have been a very useful part. The closest could be a combination of an axle pin without friction (3749) + pin with pin hole (15100) ...instead of ball pin + magnifying glass.

Edited December 12, 2022 by pow

[gyenesvi]

5 hours ago, AutoBacon said:

It's actually just a 2739a suspension link, which has been cut in half, shortened, and then glued into the 2L axle connector. The link itself is basically just a cross axle so it fits well.

Hmm, that's a neat way of manufacturing such a 4L link :) and a 5L can also be made with a 3L connector. And it can even be glued together in a perpendicular way! Lots of combination possibilities, even longer ones can be created by glueing more connectors together with 2L axles... I always wanted 11-13L versions for larger scale suspension links.

[Fluwoeb]

@AutoBacon, I have an idea that I have tried. It is simple but could possibly break under the combined stress of torque and weight. What do you think?

 

 

[AutoBacon]

On 12/22/2022 at 1:14 AM, Fluwoeb said:

@AutoBacon, I have an idea that I have tried. It is simple but could possibly break under the combined stress of torque and weight. What do you think?

@Fluwoeb Hi! Is this regarding the 4L suspension link? To be honest, I'm already using some non-Lego parts so I'm quite happy fabricating some 4L links out of regular 6L links :D

Small update: I have just received a delivery of mixed-length flip-flop beams from Ali-express as suggested by @gyenesvi, as well as a Cada micromotor. I can confirm that such a motor can drive a steering mechanism ungeared, as long as it is paired with an analog controller such as the mouldking 6.0 battery system. With a regular IR remote for example, it would be far too fast. It does seem though it would be beneficial to add an inline friction clutch.

I have also managed to redesign the axles a little bit to integrate 12t/20t gear down in the 'diffs' using Zerobricks' advice. It looks like I should be able to do some real-life mockups of the drivetrain in January.

[keymaker]

Looking forward to check your photos of real-life mockups. Even if you are using some parts unavailable in lego directory, the ideas are universal and can be handy :)

[AutoBacon]

Hi! Whilst back at home for the holidays, I have finally been able to build the chassis for real! I'm happy to say that with initial testing, it works really well. I can't wait for some proper off-road tests! Below are some testing notes.

Good:

• One Mouldking L motor can easily power the chassis, and the very long wheelbase allows for some very steep hill climbs. The gearing allows for a good top speed, and analog control also allows for precise slow-speed control.
• Cada Micromotor is strong enough for steering without any gearing and using the analog controller, the steering is adequately precise.
• The suspension performs very well, with very good articulation on the front axle, no binding of tires at full lock and full compression, neither against the chassis rails nor 9L links.
• The front suspension can be fully compressed without breaking. I used two of these to attach the upper suspension mounts to the 7L beam - they seem to have a much stronger connection than normal friction pins. I also changed the design ever so slightly featuring a 3L flipflop beam for extra rigidity.
• 4 rubber pieces for the rear suspension actually prove to be very sturdy, and the truck should be able to support a decent weight over the rear axles.
• The steering angle is much better than expected.
• I thought not having suspension extension limiters would cause issues but actually, it just improves articulation in a pendular sort of way, and the drive shafts into each axle act as a sort of bump stop, so they don't droop very far when lifting the model from the ground.

Less good:

• I forgot how weak CV joints are, I think I will try small zip-ties wrapped around the female ends to stop them from flexing apart under load. It's not a big issue but with the extra weight of a cabin, it may become one.
• The cabin may be too heavy and cause the front wheels to flex apart too much. Further testing is required.
• Rear tires occasionally bind on the lower 4L suspension links, but it doesn't seem to happen when driving, only when manually manipulating the axles.
• The front diff is not the strongest, It holds, but it will slip under some conditions. It's not ideal but I'll have to see how it fares with the additional weight of the cabin.

Apologies for the questionable quality of the photos and video!

Not sure when I will be able to start on the cabin. I think first I will try to sort the front fenders.

 

[keymaker]

It is great to see photos, can you tell why you decided to add 20T gear to the front and resigned from this part? After all, there is no more solid bracing like using one part to do that.

[Aleh]

Double crankshaft is very cool and realistic!
i wonder what are the short suspension arms? Do they genuine lego? Sorry, if already asked.
 

[AutoBacon]

1 hour ago, keymaker said:

It is great to see photos, can you tell why you decided to add 20T gear to the front and resigned from this part? After all, there is no more solid bracing like using one part to do that.

I decided to trade some solidity for the improved reliability of CV joints. Since the beginning, I was not happy with putting full torque from the motor on CV, so any gear reduction after CV is beneficial, especially as more weight is being added with the cabin. Using 20T also means the female CV end can remain 2 studs long which should address the concerns of ZeroBrick's comment above.

38 minutes ago, Aleh said:

Double crankshaft is very cool and realistic!
i wonder what are the short suspension arms? Do they genuine lego? Sorry, if already asked.
 

Thanks, yes, as a small model with few functions, replicating the original driveshaft layout was an important goal :D The upper arms are 5L arms that have been shortened by one stud with a Dremel. The lowers are made using 6L arms shortened and joined using a 2L axle connector :D

Edited January 9, 2023 by AutoBacon