Chain links 3711 / treads 3873

[Sylvain Sauvage]

Hi all,

3711/3873 both have a span (distance between connectors) of 16LDU.

IRL, the distance is slightly bigger.  It’s about 2.7% more.  (Measured while digital-building 10303: it needs 262 links in digital, while 255 (222+33) are in the set / instructions.)
That would make a span of about 16.4LDU.

So, two questions:

1. Am I the only one bothered with that?  I.e. not having the correct number of parts in digital builds.

2. Could it be changed?  Or is there a rule (that I didn’t find) somewhere saying that it can’t?
I mean, excluding backward compatibility rules.  So to rephrase the question: Could a similar part be built & officialized with a span of 16.4LDU?

[N. W. Perry]

1. Am I the only one bothered with that?  I.e. not having the correct number of parts in digital builds.

Nope. If that happened in my model, I'd be forced to scale the links up so there's the correct number of them.

[Rene Rechthaler]

I think its maybe that there are many links and every of these many connections has a bit of tolerance and play...
That and geometric inaccuracies in all chain drives (except for the basic chain drives of two identical gears and the right distance) its impossible to get a perfect fit (same in reality, its either too loose or too tight...).

[Sylvain Sauvage]

2. Official LEGO sets are designed based on specific guidelines and requirements. These guidelines include considerations for stability, compatibility with existing parts, manufacturing constraints, and other factors. The current standard for connectors with a span of 16LDU is 16LDU, and it is unlikely to be changed to 16.4LDU in an official capacity.

If the set were designed on a 16LDU span, it would come with 262 links, not 255, because 262 is the mathematically exact number of links the build needs in all mathematically exact LDraw tools when you use a mathematically exact 16LDU span¹.  So, as the real LEGO set comes with 255 real links (and the instructions say to use 255 links), it’s proof the official LEGO set was designed to use links that really span 16.4LDU   

So, LEGO knows² the real span and uses it to design sets.  It’s weird that we don’t.  It’s weird to stick with exact in-System lengths when it’s proven they don’t work and nothing would be lost in the addition (I didn’t even ask about a change).

———
¹ More precisely: The chain turns around 2x 3648 gears whose centers are exactly 2000LDU apart.  2000/16 = 125 links. Multiplied by 2 to get both sides of the chain, you get 250 links.  That leaves you 255 − 250 = 5 links to make the two half-turns around the gears.  Two half turns around the gears is the same as 1 turn around one gear.  Not possible with 5 links.  You need 12 links for that.  And 12 + 250 = 262.
The other way around: You need 12 links to make the half-turns at the ends of the chain.  That leaves you 255 − 12 = 243 links to cover the 4000LDU on the sides of the chain.  4000/243 = 16.46LDU.  (The difference with 16.4LDU is rounding and the fact that it actually takes a bit less than 12 links to make the turn.)

² True, it has happened that LEGO changed designs between production runs to add or remove a link here and there.  So their knowledge is rather fuzzy, but it’s still not 16LDU 

[Rene Rechthaler]

I just measured mine with calipers: the pin has 1,1 mm and the clip has 1,3mm...
These 0,2mm are the tolerance for that (0,5LDU).
Thats the same why bricks are in real 7,9mm wide and in LDraw 8mm (20LDU).

Try laying a chain flat on a table and bend it sideways, then you can see how much play these links have.

[Gerald Lasser]

As we will get a new chain element (3249 with the bar) I got myself all the chain and tread elements avaialble on the instructions app.

Interestingly, the share the exact same geometry as our part, but do have a 0,186 LDU gap between pin and clip, which is just under 0,1 mm

What about adding a gap to the parts and a !HELP line to highlight the slag.

I am currently subfiling the existing parts to use as much as possible what is common in 3711/14969, 3873/15379 and 3249