Spinning Brace Torque bug

#1
TL;DR: Attaching a brace to anything that spins causes extra unwanted torque. This torque is NOT symmetrical across counter-rotating coaxial systems (making it impossible to create a coaxial helicopter that doesn't yaw unnecessarily [without active compensation, anyway].

Skip to post #8 for a better demonstration of the problem than the posts below.

Full version:

Make two machines like the pictures below. The only difference is that one of them has two additional unpowered wheels that are braced to the powered wheels (ignore the bit of brace on the side, that's just where the mouse cursor was).

Try both in simulation and see what happens.

The one with the unpowered wheels wheelies (despite having more weight on the front end).

This is very counter-intuitive. It feels like the addition of the extra unpowered wheels results in masses of torque being transmitted to the machine. The same thing happens with big, heavy machines; it almost seems independent of machine weight. It also happens with the large unpowered wheels.

It's not due to the extra weight of the unpowered wheels - if you replace them with ballasts, it doesn't wheelie.

It's not due to extra grip either, because the unpowered wheels lift off the floor during the wheelie. Plus, if you scale the unpowered wheels down with EasyScale so that they're not touching the floor, the problem persists.

Also, with the large wheels, the number of braces you use changes the severity of it - for example a single brace through the centre causes some effect, but 4 braces from the outer points of the powered wheel to the outer points of the unpowered wheel causes a huge effect.

If you play with this bug, you can actually make a machine that hovers one end purely based on torque...it's insane.

Can someone else confirm this bug? Or maybe explain what's going on?

Besiege_08_31_2016_08_51_08.png
Besiege_08_31_2016_08_51_21.png
 
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ITR

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Staff member
#2
Have you tried without the unpowered-wheels, but with the braces?
EDIT: wait, nvm, I think I misunderstood how you set it up, can you share a .bsg?
 
#3
You're right. If I delete the unpowered wheels, the problem persists - it's the braces that are the issue. Make this and see what happens:

Besiege_08_31_2016_09_29_21.png


It also happens with cogs, so it's definitely the braces that are at fault.

This makes me wonder if some new engine could be created by exploiting the torque caused by spinning braces...
 
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ITR

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Staff member
#4
You're right. If I delete the unpowered wheels, the problem persists - it's the braces that are the issue. Make this and see what happens:

View attachment 418

It also happens with cogs, so it's definitely the braces that are at fault.

This makes me wonder if some new engine could be created by exploiting the torque caused by spinning braces...
I could be wrong, but I believe they use this for what they call a reaction wheel :p
 
#5
Further research shows that adding braces actually increases the power of the wheel.

I made a vehicle that holds boulders - without any braces on the wheels, it can carry 25 boulders up the ramp in the sandbox

WITH braces on the wheels, it will carry 54 boulders up the ramp! So that's over twice the power (note: twice the POWER, not twice the torque - the wheel speed was the same)

Also the braces do not have to go outwards like the last screenshot I posted. Just bracing from one point of the wheel to another is sufficient.
 
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Shade

Active Member
#6
I made a vehicle that holds boulders - without any braces on the wheels, it can carry 25 boulders up the ramp in the sandbox
WITH braces on the wheels, it will carry 54 boulders up the ramp! So that's over twice the power (note: twice the POWER, not twice the torque - the wheel speed was the same)
Also the braces do not have to go outwards like the last screenshot I posted. Just bracing from one point of the wheel to another is sufficient.
This is interesting, gonna check how to utilize this
 
#7
I am also wondering if a combination of this bug + the symmetry brace bug (sometimes the 4th brace doesn't get placed) is the source of unwanted yaw in coax rotor systems...maybe it affects the spinning block as well. Maybe brace length affects the power - most of my coax designs are based on gears and thus one 'axle' brace is longer than the other, which could potentially mean that one rotor is exerting more 'back-torque' than the other...

I will investigate tonight.

As for how to utilise it...I think that the only time where it becomes useful (instead of being a pain in the ass) is when you have a very heavy vehicle or lots of friction, and thus need that extra power. Or for torque/reaction wheel attitude control systems (which I am trying to get away from as I feel they are 'cheating' somewhat :p ).
 
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#8
Update:

  • Spinning blocks, steering blocks, and saws all also have this bug, so it looks like the problem is definitely with the concept of spinning a brace, not with the wheels or blocks
  • This bug is ASYMMETRICAL IN CONTRA-ROTATING COAXIAL SYSTEMS! I.e. if you have two wheels spinning in opposite directions, BOTH with identical braces attached, the net torque is NOT zero! This is responsible for the unwanted, illogical yaw in coaxial helicopters that we're all suffering from.
  • Indirect attachment of braces also causes this (e.g. build a block on a wheel, then attach a brace to that block - you still get the increased power)
  • Adding more braces increases the asymmetry

If you create a machine like the pic below, this demonstrates the last two points - you will see that the starting block rotates when you power the wheels; if you remove the braces, the starting block remains still when the wheels are running. Adding more braces to the each ballast equally results in the starting block spinning faster. Reduce the number of braces and it spins slower.
Besiege_09_08_2016_01_28_09.png
 
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#10
This sounds a lot like a reaction wheel. These aren't exactly new, because there is a reason behind the rotations, and I've been using them for s very long time. In fact, there are several in my profile pic. Every action has an equal and opposite reaction, so if an object spins one way, whatever it is attached to will want to spin the other way. Let's say that A spins on B. When A spins, B will automatically begin to spin the opposite direction. If weight (in your case, braces) is added to A, it will be harder to spin, and B will begin to spin faster as compensation. I've used these in steering, and it can also be a problem when making planes, because it's rotors can act as a reaction wheel, and causes the whole plane to roll.
 
#11
I think you need to re-read the thread a little.

Try this: make a machine that has a wheel with a ballast attached to the wheel. Set the weight of the ballast to be the weight of 1 brace (0.5). Now make a 2nd identical machine, remove the ballast, and put a brace in it's place instead.

When you spin both wheels, you will see a HUGE difference in the amount of torque induced on the supporting frame.

Also refer to the experiment I conducted earlier in the thread - adding braces to wheels actually increases power - a vehicle with braces attached to the wheels can carry TWICE the number of boulders up a slope compared to an identical machine with no braces on the wheels. Try it if you don't believe me. That's absolutely nothing to do with reaction wheel theory.
 
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