Clutch power is the magic that makes LEGO work. It is “the ability of its bricks to snap together tightly while also being easy to separate, thereby readily allowing for de- and reconstruction”. In engineering terms, it is an interference fit. The distance between the outside of the studs is slightly larger than the distance between the inner walls of the brick. When you push a stud into the gap between inner walls and tubes inside, then these elements have to deform slightly to make it fit. A combination of friction and the pressure caused by the elastic deformation hold the studs in place. (The deformations we are talking about here are very small, they do not interfere with the surrounding bricks when building so they are within the 0,1 mm tolerance.)
When we look at the shape of a 2×4 brick, and assume a situation where all 8 stud positions are used, then we can imagine the outer studs pushing the wall outwards and causing a deformation. This deformation moves the wall outwards and away from the center studs. That means that the fitting is less tight at that location, and the studs exert less force against the wall. The center studs contribute a bit less to the clutch power of the entire brick in this situation.
This is visualised in the image above. The red arrows represent the force exerted by the stud on the walls and tubes. Those elements exert an equal force in the opposite direction on the stud, but I have omitted those from the image for clarity.
If we assume a situation where only the center studs are used, then we can imagine that the outer wall deflects outwards easier then when the corner studs are used. The wall corner acts as a stiffener preventing deformation. A corner stud might grip slightly better than a center stud.
If you would want to reduce the deformation, then there are basically two options: change the material, or change the shape.
- There are several possibilities to change the shape, but not all are suitable to use here. Changing the stud or wall size is not an option because all parts still have to fit the other parts in the LEGO system, so only very minor changes are possible. Adding a cross-support limits the outward deflection of the wall and helps to preserve the fit of the stud connection. Because the situation is not entirely symmetrical (there is a difference between the thickness of the wall and cross-support), this solution is not perfect. Using two or three cross-supports could improve the situation a bit more, but that comes at a cost because more material is needed. LEGO did some experiments in this direction, and I believe this is why. The cross-supports were already used on the long, thin beam bricks and using those on more bricks might be a logical direction to explore if they were trying to optimise the clutch power of the new bricks with tubes. Another option that was tried is to make the center tube thicker than the outer tubes. The additional deformation of the wall might be compensated by a reduced deformation of the center tube.
- Soon after the experiments with the CA bricks with cross-support the CA material was replaced by ABS. The new material deformed less and probably made the cross-supports obsolete on small parts. Soon after, the ABS stud fitting was optimised using the Bayer 1xABCD bricks.
In 1958 LEGO had already considered a design with thin walls and vertical ridges. They never used it until 1970 when LEGO introduced Minitalia as a cheap alternative for LEGO System in Italy. These were the first production bricks using thin walls and split tubes. This design change was probably made to reduce the material cost. Also, a cheaper material was used for these bricks.
A few years later, material cost went up during the oil crisis. I think it is no coincidence that the brick design with thin walls and split tubes was introduced to the regular LEGO System at this time. LEGO was not completely satisfied by the performance of this design and reverted to the previous thick-walled design.
When the thin walls returned in a later design, the most obvious design change was the return of the cross-support. This is not surprising, because the thinner wall offers less resistance to deformation.
[Original post and comments on Flickr.]