[WIP] 42099 B model 'Rocky II' - 4WD RC Race buggy

[Didumos69]

For my 10th anniversary at work I got a second 42099 set. So I figured I might as well experiment with another B model I have in mind. This is still work in progress and there is no guarantee whatsoever this is going to be a success. This time I did want to do a WIP-topic, simply because that fits me better and I can use the feedback.

The idea: Gear up as much as possible. My believe is that when you want to use XL motors with differentials in a speedy model and you don't want the differentials to slip internally, you need to reduce torque by gearing up the differentials and then gear down towards the wheels. With the new portal hubs that come with 42099 the gearing down part has already been taken care of, so what rests is the gearing up part.

To gear up, I will be using the turntables as gear. I could fixate the grey side of the turntable directly to the motor house and connect the black side to the motor output, but then I would get quite some friction from the two turntable halfs. So I ended up using the turntable as one big gear. Next I tried to mash a 12 tooth gear with the turntable. I used a 2x4 (center to center) distance (similar to a 2x1 distance which can be used to mash a 16t gear with a 20t gear), but the distance is slightly too small (4.47L instead of the ideal 4.5L for a 60t-12t gear mash) and made the mash suffer from too much friction.

So eventually I opted for a 60t - 20t gear mash with a 3x4 distance (which matches the ideal 5L distance). Overall the gearing up ratio is 5.95:1 and with the portal hubs gearing down by 1:5.4, the effective ratio will be 1.1:1.

The whole model at this stage. Now the challenging part will be to brace the motors and gears well in a rigid chassis. I plan to put the battery / control unit in the back, behind the motors.

Edited November 7, 2019 by Didumos69

[nerdsforprez]

Wow.  I'll be paying close attention to this.  This looks like some massive up-gearing. Cant imagine it will work.  But your skills are amazing. I'll be eager to see if you can make it work. 

[gregorski904]

is the plan to turn turntable as a whole or just black part of it?

[Didumos69]

1 hour ago, gregorski904 said:

is the plan to turn turntable as a whole or just black part of it?

As a whole, just the black part would give unnecessary friction.

[Attika]

That is an ambitious upgearing there, very peculiar concept. Looking forward to see it on your rug. 

[schraubedrin]

Seems like a great project, i'm curious what speeds you will achive.

Although I understand the space constrains, i suggest to support all gears on both sides and to turn the motors 90° so you only have one bevel gear connection.
Supporting the gears on both sides greatly reduces the bending forces in the axle (and possible gear misalignment) and removing a bevel gear reduces friction as they introduce axial bearing forces.

[Didumos69]

11 minutes ago, schraubedrin said:

Although I understand the space constrains, i suggest to support all gears on both sides

I certainly will.

11 minutes ago, schraubedrin said:

and to turn the motors 90° so you only have one bevel gear connection.

Gearing would be better yes, but I don't see a way to work my way around the big turntables with the few liftarms I have left. They are practically 9 studs wide.

[Attika]

I keep thinking about it, and this doesn't let me be still:

In the original model the same electrics were geared up 2.77 before the diffs. (20/12x20/12). In your setup this number is 8.333 (before the diffs). That is -by the nature of the available ratios- exactly 3 times faster. So we expect to have third of the torque compare to the original. I did not buy the it, but I've had an occasion to play it aroud back in the late summer and it was rather modest on the torque department to begin with. Hence the gearswaps came to be popular. 

What do you think?

[Permo]

I wonder if this setup will trip the current limiters in the new motors / controller ?

 

[Didumos69]

8 minutes ago, Attika said:

What do you think?

That there is a big chance this will fail.

[Attika]

Just now, Didumos69 said:

That there is a big chance this will fail.

Oh, come on! That would put me in a very grinchy position. 

[Didumos69]

42 minutes ago, Attika said:

Oh, come on! That would put me in a very grinchy position. 

Well, to be a bit more nuanced, this started with a wish / dream I had about a fairly simple model that would have a spark of this:

Soft suspension and speed. So that's what I aim for. The front suspension has 8l long suspension arms, which gives a very nice soft feel. I still need to redesign the rear axles to get the same feel there. And then much will depend on the speed it can develop. It doesn't really need to be off-road capable, just this tilting in turns, is what would make me smile. But sofar it's just a 3D idea. I have not seen anyone build anything speedy with the new hubs, so maybe it's just wishful thinking. But somehow I have the feeling that 2 of those new XL motors should be more capable than what we've seen sofar.

Edited November 11, 2019 by Didumos69

[Zerobricks]

1 hour ago, Didumos69 said:

 have not seen anyone build anything speedy with the new hubs, so maybe it's just wishful thinking. But somehow I have the feeling that 2 of those new XL motors should be more capable than what we've seen sofar.

Ehm:

 

[Didumos69]

10 hours ago, Zerobricks said:

Ehm:

I meant with XL motors and Control+.

[Zerobricks]

3 hours ago, Didumos69 said:

I meant with XL motors and Control+.

Problem with the new hubs is they have quite a bit of friction, and the faster you drive them, the more friction you have to overcome... Also since new PU XL and L motors have nearly identical characteristics I suggest you use 4 or more L motors for drive because they will take up less room.

[Philo]

2 hours ago, Zerobricks said:

Also since new PU XL and L motors have nearly identical characteristics I suggest you use 4 or more L motors for drive because they will take up less room.

Yes, but that's not an option for a 42099 B-model

Edited November 12, 2019 by Philo

[Attika]

Still in my head  In case if it "will fail", there's room to play with gear ratios. Had an idea, I'm sure you've been thinking of it too. Reducing the number of gears and keeping the 60/20, what goes 20/12 into the differential. That gives 5:1 before the diff. That's a reasonable step back from 8.33 and still above the  2.77 featured in the original A model. Sort of a"Plan B", I guess. 

 

 

[Didumos69]

34 minutes ago, Attika said:

Still in my head  In case if it "will fail", there's room to play with gear ratios. Had an idea, I'm sure you've been thinking of it too. Reducing the number of gears and keeping the 60/20, what goes 20/12 into the differential. That gives 5:1 before the diff. That's a reasonable step back from 8.33 and still above the  2.77 featured in the original A model. Sort of a"Plan B", I guess. 

I did think of that yes. I could just skip the 12t on the side and have the 20t go in the center frame directly. I have also thought about skipping the diffs and using the 12t from the side for a 12:20 reduction instead of the 20:28 the diffs give. That would drop your 5:1 ratio further down to 4.2:1.

Problem is that I have little time to build right now. Will take some time before I can try some things in real life.

4 hours ago, Philo said:

Yes, but that's not an option for a 42099 B-model

What he said.

Edited November 12, 2019 by Didumos69

[Attika]

Just now, Didumos69 said:

I did think of that yes. I could just skip the 12t on the side and have the 20t go in the center frame directly. I have also thought about skipping the diffs and using the 12t from the side for a 12:20 reduction instead of the 20:28 the diffs give. That would drop your 5:1 ratio further down to 4:2.

The 5:1 comes as 60/20 x 20/12.  With differential (20/28) it goes down to 5 x 0.71 = 3.55:1. If 12/20 replaces the diff, then it gives 5 x 0.6 = 3:1 in my books. 

I guess the differential is handy if you want to keep the speed while cornering. I'd keep it. 

 

26 minutes ago, Didumos69 said:

Problem is that I have little time to build right now. Will take some time before I can try some things in real life.

What he said.

I can wait.... 

It won't fail, just comes to balance with the new XL motors and electronics.   Talking of the wish/dream: The main barrier is the overall power difference between 4 PF-L on buwizz vs. 2 "over protected" C+XL's. Just getting my first steps with a borrowed buwizz and it makes a difference, I have to admit. So does the absence of it in this case. If the dream can be rearranged along with the gears, then it'll be ok. 

[Didumos69]

11 hours ago, Attika said:

I guess the differential is handy if you want to keep the speed while cornering. I'd keep it. 

Good point, I will keep it.

11 hours ago, Attika said:

If 12/20 replaces the diff, then it gives 5 x 0.6 = 3:1 in my books. 

Replacing the diff gives (28/20) x (12/20) = 0.84. 5 x 0.84 = 4.2. I mistakenly wrote 4:2, where I meant 4.2:1

Edited November 13, 2019 by Didumos69

[Attika]

11 hours ago, Didumos69 said:

Replacing the diff gives (28/20) x (12/20) = 0.84. 5 x 0.84 = 4.2.

The 5:1 ratio comes before the differential. Replacing a diff can't effect this ratio, as it appears on the input axle of the differential . When we add the differential and bring it into the math, then the ratio on the output axles of the diff is (5x.071)  3.55:1. 

In the other case, if we replace the diff with the 12/20, and bring that in the math the ratio is (5x0.6) 3:1 on the output axle again. The ratio for the replacement is still valid: 3 / 3.55 =0.84, but logically, the differential replacement quotient (0.84) shouldn't be  applied on a ratio (5:1) what doesn't include the differential. In other words the 4.2 is a theoretical number here. If you compare the motor rpm to any axles on the way down the drivetrain, non will give you this ratio. 

Sorry if I'm too fixated on this question, my ocd kicks in when being around numbers.  

Also just realised:  "Good point, I will keep it."    That makes the math around the replacement rather pointless.

 

 

 

Edited November 13, 2019 by Attika
short attention span

[agrof]

Interesting concept, but somehow I can not make myself familiar with the idea to gear up the drivetrain for sake of compensating the portal hubs.

Technically it is a nice challenge, and also like the concept so far, but I am afraid there will be enormous amount of friction, which with the high rpm together will eat up the parts... silicon spray could be handy here.

p.s.: just curiosity... could be your crawler B model modded for 4 wheel steering? That would be the ultimate flick to TLG. 

[gyenesvi]

Hi @Didumos69, did this project lead anywhere? I have just finished working on a similar idea and came across this thread (interesting one with those turntables as gears). However, similarly to what @Attika suggested, I worked with a less ambitious gearing, speeding up 3x with a ratio of 20:12. I put the drive to the hubs without a differential, only rear wheel drive, one XL motor for both rear wheels (so final ratio is 0.85:1). Speed is better than the original model, but as expected, the torque is pretty much lost, though it works on a flat surface. However, the hub does start to blink orange when quickly changing directions (forward-backward), so it is probably limiting the motors. Also, as I wanted to build sort of a dune buggy out of it, I created softer suspension with longer swing-arms. Here's an image of the drivetrain and the full model. You can find more info about it on Rebrickable. Let me know what you guys think!

 

[Didumos69]

On 1/26/2021 at 9:14 PM, gyenesvi said:

Hi @Didumos69, did this project lead anywhere? I have just finished working on a similar idea and came across this thread (interesting one with those turntables as gears). However, similarly to what @Attika suggested, I worked with a less ambitious gearing, speeding up 3x with a ratio of 20:12. I put the drive to the hubs without a differential, only rear wheel drive, one XL motor for both rear wheels (so final ratio is 0.85:1). Speed is better than the original model, but as expected, the torque is pretty much lost, though it works on a flat surface. However, the hub does start to blink orange when quickly changing directions (forward-backward), so it is probably limiting the motors. Also, as I wanted to build sort of a dune buggy out of it, I created softer suspension with longer swing-arms. Here's an image of the drivetrain and the full model. You can find more info about it on Rebrickable. Let me know what you guys think!

No this did not lead to anything yet. I haven't had time for LEGOs the past half year.

But what you made is really cool and is about as good as it can get I think, performance-wise.

[gyenesvi]

Thanks, hope you can back to Lego shortly.