How flexible are your cars?

[Lipko]

Hi all!

My current studless car WIP model twists quite a bit. About 10-15 degrees under its own weight (when resting on its two opposite wheels) measured between the two ends of the chassis (body parts not taken into account). The model doesn't have PF elements, but it's quite heavy anyway. The wheelbase is 34 studs long.

The twisting doesn't seem to affect functions (gears won't slip), the model isn't for outdoor use and isn't for offroad-ish use. The model seems to bear repeated twisting (doesn't disintegrate), hatches stay well aligned etc.

So the question is: should I be concerned about this, is it a noobish mistake, if it's a mistake at all? How about your studless cars or official studless car sets (for example 8070)? The Unimog seems to twist quite a bit too, but it's not fully assembled at this time.

Thanks for any answers in advance!

[allanp]

I found the unimog, once fully assembled, to be quite stiff for a studless model, but still not as stiff and a studded model. 8070 twists guite alot, 42000 even more so. I don't think it's so much of a noobish mistake, more of a charachteristic of studless constructions. I am now getting my MOCs fairly twist resestant if only to improve the way the suspention works and to keep drive trains running freely. Twists in the chassis can cause quite alot of friction. This used to be the case in the excavator i've started working on, took some tweeking to get it stiff. I've had gearboxes lock up completely when they were twised.

[andythenorth]

Flex is an integral part of many (not all) engineering structures.

[Ape Fight]

I over-engineer my MOCs big time (not a good thing), but this does mean they don't twist/flex, whether studded or studless. My earlier creations used to, but the more big creations I've built the more solid they've become; now they're basically one solid beam.

Looking at your chassis (which looks great by the way), there are a few places where there's only 2 beams running longitudinally. This isn't enough to stop twist. Find a way to add another two and the flex will all but disappear.

[Lipko]

Thanks for the replies (keep them coming)!

To my experience, adding more longitudinal beams is much less effectively stiffens the chassis than adding stiff corners (one of the edges of the corner should be longitudinal). My chassis has quite a lot of longitudinal beams, the images may be misleading. Even the "cover" of the gearbox adds beams, because the half-stud offset is achieved by the placement of the gears. There is always at least 6-8 beams running parallel. At the gearbox, there is a second layer of frame 2 studs above the bottom. This second layer can1t be seen on the images.

The problem with corners is that corner parts doesn't fit in the chassis, but maybe I'll do something about it.

The question is that if twisting is not a big deal, then I don't have to sweat too much on reinforcing it when (if) I redesign the chassis.

Edited January 3, 2013 by Lipko

[jorgeopesi]

I try on my MOCs to add strength through the roof and try to no integrate gearbox in the chassis to prevent deformation and malfunction on it, the latter was Sheepo advice I had no idea...

[shadowhearth]

Actually andythenorth is quite right.

It's hard to believe, but your every day car has quite a lot of flexibility in its body. That is done for a few reasons, but most important it is for ride quality.

A lot of rally or racing cars are getting a lot rewelding done to them to increase its sturdiness. It makes for a lot better driving and handling characteristics, but ride quality suffers a lot. Though we don't know that racing machines are not for comfort ;).

I would allow some flexibility in lego technic sets. Obviously I would try to make as solid as possible, but when you work with plastic like this its hard to do fully solid construction. It's not like Otis needed.

[Lipko]

I know flex is normal, I'm just wondering how flexible are other builders' cars. I know I can make the model as solid as possible, but if most of the Lego cars are this flexible, then I wouldn't worry about it too much. If I had the money I'd just build one of Paul's or Nathanael's cars to see those for example.

[Zerobricks]

My black cat supercar bends only some 5 degrees along the length when resting on diagonal wheels.

I think the low twist is due to the diagonal beams in the lower chassis:

[jorgeopesi]

I know flex is normal, I'm just wondering how flexible are other builders' cars. I know I can make the model as solid as possible, but if most of the Lego cars are this flexible, then I wouldn't worry about it too much. If I had the money I'd just build one of Paul's or Nathanael's cars to see those for example.

If you build a RC car with gearbox you will need to build as solid as possible but with no RC cars I don´t think more important the flexibility.

[Alasdair Ryan]

Doesn't this this question depend on how well designed your models are?

Studded models are in most cases stronger but it depends on how well there made.

[Blakbird]

The standard geometry of an automotive chassis (parallel longitudinal rails, 4-5 lateral cross members) has excellent rigidity in bending but very poor rigidity in torsion. This is much worse in studless models because the torsional shear all has to be transferred through pins which have some play in them. The best way to improve torsional rigidity in this type of structure is to add a shear web. With a studded model, this is as simple as adding a couple of plates between the rails. Note that these can't be 1x plates because they can rotate at the ends. The plates must be at least 2x. The wider the better. For a studless model, this type of reinforcement is impossible. Therefore, the next best thing is to add diagonal members. If you can create an "X" between the rails of your chassis and support the X in a way that the ends can't move, it will make a massive difference. Most official LEGO models do not do this and therefore the chassis can be very springy. The 8041 racing truck is an excellent example of a model that does NOT reinforce the chassis and stiffness suffers. 8258, 8285, and 8436 are other examples. Compare this to 8868 which is like a brick by comparison.

Most modern cars don't even have a chassis. The body forms part of the structure of the shell, including the roof. This is why convertibles tend to be heavier because using only the lower for frame for stiffness is much harder. Trucks still have an old style frame and you will almost always find diagonal bracing.

[Moz]

Blakbird, can't you use the 5x7 rectangle part to give at least some torsional rigidity? It's not diagonally braced, but it's (IME) more rigid than a set of liftarms in the same arrangement. It would be even better if we could get 3x16 or 5x16 plates with a stud missing in the middle of each end (so they'd plug straight into the 5x7 frames).

The extra weight of convertibles is somewhat notorious in the car world. It's why you get so many "oversize sunroof" semi-convertible cars, and high step-over into the doors (alternatively: doors that are very sticky to open, and cars that fall apart if you open the door while driving). Just leaving a couple of beams at the top of the car helps a lot. If you want to see lack of torsional rigidity, look for photos of trucks rolling over. Often you can see one end of a flatbed semitrailer rotated 30° or more from the other. You more rarely see that with box trailers, even though the superstructure is typically quite flimsy because it's quite high/wide compared to the chassis.

[Boxerlego]

I can show you some flex just let me take a picture real quick like.

[Blakbird]

Blakbird, can't you use the 5x7 rectangle part to give at least some torsional rigidity? It's not diagonally braced, but it's (IME) more rigid than a set of liftarms in the same arrangement. It would be even better if we could get 3x16 or 5x16 plates with a stud missing in the middle of each end (so they'd plug straight into the 5x7 frames).

Yes, the 5x7 frames are a reasonable substitute for diagonal members. The key is to have corners which are fixed at 90 degrees, which the frame accomplishes. Pinned corners allow flexibility. You could also use a series of 3x5 L-beams to attach your crossmembers which would help a lot.

[Boxerlego]

I start with the 9398 and how the flex is removed by having this horizontal connection.

Now on this truck there is no horizontal beam connection like the crawler has..

This is what things look like when you have all four tires on flat ground.

Now when you start to put the suspension to use. The vertical beam will begin to flex. You can use your mouse arrow to measure the distances on the two beams in the two pictures and see that the bottom one is definitely closer.

Edited April 12, 2013 by Boxerlego