[MOC] Mini 8109 - Flatbed Truck

[Ngoc Nguyen]

 

2011 was a wild year for Technic. It is best remembered for the first licensed flagship, namely the 8110 Unimog, and all the novelties that it introduced. But that's not the only noteworthy set about 2011. 8109 is another set from that year that to me is no less, if not more, ingenious and even magical than 8110.

 

THE ORIGINAL 8109

The function I like the most about 8109 is the tow bar. Thanks to an ingenious combination of gears, liftarms, and a rubber band, the motor drives two functions at the same time in an exact order.

 

THE MINI 8109

The original model is on the scale of 15 studs long. My mini replica is on the scale of 9 studs long. At this scale, there's no space for motorization, linkage, or rubber band.

And now I will attempt to reproduce the same function within those constraints.

 

THE MAGIC

At this scale, to control two functions from one input, there's only one way to do it.

 

Here's the transmission.

 

The idea is to transfer motion into one end of the differential. While fully retracted, the tow bar is restricted on the top size by the bed, so it can't open. Motion therefore can only go to the unobstructed path, which is the differential case. The red 28z gear will drive the linear actuator to push the tow bar out. After the tow bar is pushed out, it can open, so motion will go to that path instead, which is the other end of the differential. Thanks to the frictionless red 8z gear, motion can still be transferred after the tow bar is pushed out.

THE BED

The bed in the original 8109 comes with a nice side effect: the rear wheels lower as the bed is pushed back. In the mini replica, there's no space to do that, but I put it a different effect.

In the original 8109, the moving bed is controlled through motorization and worm gears, so it can stay in its position. In the mini replica, there are no such things, so the bed tends to fold up or even collapse back into the body by itself. Therefore I made it lockable when it reaches the furthest position through the past-the-midline lock technique.

 

 

Also, since the diameter of the differential is larger than 3 studs, I remove the floor section under the winch to accomodate the differential.

 

INSTRUCTION

https://reb.li/m/261523

 

 

 

 

 

[jorgeopesi]

Epic miniature, the one I have liked the most so far.

[Timewhatistime]

You picked one of the best official sets for one of your best miniatures.

The use of a differential is very clever. Is there a reason you didn't choose one of the other differentials (3L or 4 L) which is a bit smaller in diameter?

[Timewhatistime]

And yet another question - concerning the backwards motion of the "magic": How do you guarantee that the tow bar folds in before the mini-LA pulls it back beneath the bed? I don't see any friction pins or similar contraptions to control the order of the two motions.

[Ngoc Nguyen]

3 hours ago, jorgeopesi said:

Epic miniature, the one I have liked the most so far.

Thanks a lot!

 

2 hours ago, Timewhatistime said:

Is there a reason you didn't choose one of the other differentials (3L or 4 L) which is a bit smaller in diameter?

I'm not sure if there's any 3L-long differential that has a diameter of 3L. The 4L-long differential is not chosen for a different reason.

Usually for models at this scale, I try to keep a ground clearance of 1 stud. In other words, there should not be anything that goes below the base line. You can see that approach in my mini 42128.

 

For the mini 8109 I have no choice but to take up the 1 stud space below the base line because of the complexity of the mechanisms involved.

 

If I use the 4 stud differential, which has a 24-tooth gear, the diagonal gear that can mesh with it is the 12-tooth gear, which has a larger diameter than the 8-tooth gear, and thus will scratch the ground.

 

46 minutes ago, Timewhatistime said:

How do you guarantee that the tow bar folds in before the mini-LA pulls it back beneath the bed? I don't see any friction pins or similar contraptions to control the order of the two motions.

For the extension, it doesn't really matter if the tow bar folds in the middle of the extension or at the end of the extension. It's only during retraction that the tow bar needs to fold before it retracts.

To achieve that, there is actually extra friction for the transmission to the mini LA to guarantee that the tow bar folds before the mini LA pulls it back. I just didn't include it in the transmission diagram. You can see the highlighted parts that create the extra friction here.

Edited May 14 by Ngoc Nguyen

[Timewhatistime]

18 minutes ago, Ngoc Nguyen said:

For the mini 8109 I have no choice but to take up the 1 stud space below the base line because of the complexity of the mechanisms involved.

 

If I use the 4 stud differential, which has a 24-tooth gear, the diagonal gear that can mesh with it is the 12-tooth gear, which has a larger diameter than the 8-tooth gear, and thus will scratch the ground.

I see... however, couldn't the 24t gear be meshed directly with a 8t gear in a straight line beneath it (so no diagonal meshing)?

This solution inverts the sense of rotation, but this can be compensated by swapping the tan 12t bevel gear at the end of the tow bar.

So the ground clearance would stay the same, whereas a little bit of space above the differential could be saved...

Edited May 14 by Timewhatistime

[vascolp]

This is very cool!

[Ngoc Nguyen]

1 hour ago, Timewhatistime said:

1 hour ago, Ngoc Nguyen said:

For the mini 8109 I have no choice but to take up the 1 stud space below the base line because of the complexity of the mechanisms involved.

 

If I use the 4 stud differential, which has a 24-tooth gear, the diagonal gear that can mesh with it is the 12-tooth gear, which has a larger diameter than the 8-tooth gear, and thus will scratch the ground.

Expand  

I see... however, couldn't the 24t gear be meshed directly with a 8t gear in a straight line beneath it (so no diagonal meshing)?

This solution inverts the sense of rotation, but this can be compensated by swapping the tan 12t bevel gear at the end of the tow bar.

So the ground clearance would stay the same, whereas a little bit of space above the differential could be saved...

The space wouldn't be saved anyway since the big gear has 24 tooth, so its diameter is also larger than 3 studs.

Regardless, I guess the 4 stud differential can perform similarly here. But what you see here is just the final version after a lot of trials and errors as well as fine tuning. And in the beginning I went for the 28 tooth differential right away, simply because of the intuition that space is premium, especially in 9 stud trucks.

So in the end the short answer is legacy.

 

1 hour ago, vascolp said:

This is very cool!

Thank you!

[Anio]

Clever design.

Thank you for sharing. :)

[Ngoc Nguyen]

5 hours ago, Anio said:

Clever design.

Thank you for sharing. :)

Thank you for your support!

[Ngoc Nguyen]

I got a share from Master Grohl!!

[Timorzelorzworz]

Superb parts selection! The yellow L-beam with perpendicular holes in the cab feels as if it was eveloped solely for this purpose.

[Jurss]

This is really good downsizing.

[Ngoc Nguyen]

1 hour ago, Timorzelorzworz said:

Superb parts selection! The yellow L-beam with perpendicular holes in the cab feels as if it was eveloped solely for this purpose.

Thank you! The newly available yellow frictionless pins help too.

 

1 hour ago, Jurss said:

This is really good downsizing.

Thanks a lot!

[Saruzeufel]

I love the solution with the diff, such a clever and creative mechanism. And the model looks great, too!

[Ngoc Nguyen]

44 minutes ago, Saruzeufel said:

I love the solution with the diff, such a clever and creative mechanism. And the model looks great, too!

Thank you for your support!

[Paul B Technic]

Great work, your best one yet. I was a massive fan of 8109 when it first came out.

[Ngoc Nguyen]

1 hour ago, Paul B Technic said:

Great work, your best one yet. I was a massive fan of 8109 when it first came out.

Thank you Paul!

[gyenesvi]

Pretty cool mechanics, and it looks great too! Great addition to the series!

[Ngoc Nguyen]

15 hours ago, gyenesvi said:

Pretty cool mechanics, and it looks great too! Great addition to the series!

Thank you!

 

@Timewhatistime Your suggestion got me curious, so I replaced the 28Z differential with the 24Z differential to see how it works. The result is that the version with 24Z differntial works almost no differently from the version with the 28Z differential. The only difference in performance is that, for some reasons I'm too lazy to investigate, when I retract the tow bar, the differential skips gears occasionally. That didn't happen with the 28Z differential. Therefore I'll keep the 24Z in my personal build. The official model on Rebrickable will still use the 28Z one.

The only good thing from this version is that I can finally put the 24Z differential to use. I have 3 of those 24Z differentials that I've been trying to get rid of through incorporating them into my MOCs. I was hoping that I could put it into my mini Zetros or G Wagen or Unimog, but all of them need the 28Z gear.

Edited Monday at 06:42 AM by Ngoc Nguyen

[Timewhatistime]

10 hours ago, Ngoc Nguyen said:

Thank you!

 

@Timewhatistime Your suggestion got me curious, so I replaced the 28Z differential with the 24Z differential to see how it works. The result is that the version with 24Z differntial works almost no differently from the version with the 28Z differential. The only difference in performance is that, for some reasons I'm too lazy to investigate, when I retract the tow bar, the differential skips gears occasionally. That didn't happen with the 28Z differential. Therefore I'll keep the 24Z in my personal build. The official model on Rebrickable will still use the 28Z one.

The only good thing from this version is that I can finally put the 24Z differential to use. I have 3 of those 24Z differentials that I've been trying to get rid of through incorporating them into my MOCs. I was hoping that I could put it into my mini Zetros or G Wagen or Unimog, but all of them need the 28Z gear.

Glad to hear that my proposal brought some results and insights.