What is Torque Twist (TT) and how to fix it.

[opek]

What is Torque Twist

The differential:
So there are two gears inside the differential (AKA pumpkin). They look like this, the ring gear is the big one that looks like a ring :ror: and the pinion is the little one. The shaft that sticks out of your differential is the shaft of the pinion gear. The driveshaft bolts to the pinion gear and spins it, forcing the ring gear to turn. The ring gear is attached to your locker/spool which drives the axles.

The history:
Remember how things in motion remain in motion unless acted on by another force? (in space shit keeps going forever, on Earth we have to battle air resistance and gravity). Well the opposite is true to, if things are not moving they don't want to start moving. This resistance to changes in motion is called inertia. The fridge doesn't want to start moving! :lmao:

More history:
Imagine you are the fridge, and this dude is laying on top of you. Not only do you have to overcome his inertia to get his drunk ass off of you, but gravity is pushing him down. You have to work extra hard to get him off. Just in the same way the weight of the whole rig has resistance to move. This resistance is increased when climbing up steep inclines or when the rear end is bound up. The more resistance to movement is present the more torque twist will rear it's head.

What happens:
When the motor starts to spin and applies a force to the driveshaft/pinion, the inertia in the wheels/tires makes the ring gear not want to move. Just like pushing into a fridge, the transmission is like a little kid trying to spin a giant nut driver attached to the top of a fridge. Only your chassis isn't standing on the ground so we have to change the analogy a little bit: the kid is suspended in the air by a bunch of spings and dampners. :lmao::lmao: When he tries to spin the driver, he himself will just rotate.

Another factor:
The motor itself plays a small roll. The part that spins inside the motor has mass and accelerates very quickly. When it suddenly speeds up or slows down it wants to rotate the case. The case is mounted to the chassis. When the motor changes speed quickly it will try and rotate the chassis.





How to stop torque twist?

Weight distribution
EeePee had a great idea to mirror the transmission, so the weight of the motor hangs off of the right side of the chassis. This makes it harder for torque twist to lift the right side of the chassis.

Dealing with the resistance to movement

Springs and Shocks
Picture the kid suspended in the air again, if we used stronger bungees and beefier dampeners he would rotate less as he tried to spin that giant driver. This would be akin to beefier springs, thicker oil, and a shock angle more inline with the path of the axle. You can also use a smaller hole or fewer holes in the shock piston. Softer springs up front will let the chassis flex more during TT and will help keep your right front tire down.

Antisquat
You can set your links up for more antisquat to keep the rear from compressing on acceleration. If you need help with antisquat, see this thread: http://www.rccrawler.com/forum/general-crawlers/362032-diagram-anti-squat.html Some people like to run different antisquat values on either side of the truck (left to right) to help combat the twisting of the chassis.


Weight
You can weight down the front axle, or the front right wheel. Knuckle weights or weights far out, away from the diff, will make it harder for the chassis to pick up a tire.


Diff Gearing
A higher ratio ring/pinion combo will make more torque and the driveshaft will use less force to make the wheels spin/rig move. Underdrive gears all around will make the biggest reduction. You can also combine underdrive rear and overdrive front which will help reduce TT when going forward by stretching the rig out. The overdrive in the front will make the front tires spin faster than the rear (I have read it's 9 front tire revolutions to every 8 rear). This stretching will compress the suspension a little bit.





If I need to add anything else, or if my explanations are not up to snuff, let me know and I'll make things more clear and complete.

 

[opek]

Re: What is Torque Twist - check. How to fix TT - need help

Reserved in case final explanation extends beyond post 1.

 

[TSK]

Re: What is Torque Twist - check. How to fix TT - need help

Run a Felsenfest counter-rotation unit on the back axle"thumbsup"

 

[ax10tn]

Re: What is Torque Twist - check. How to fix TT - need help

if your running a 4 link the put 1 link on top of your 4 link plate and 1 underneath the plate that will help also.

 

[opek]

Re: What is Torque Twist - check. How to fix TT - need help

if your running a 4 link the put 1 link on top of your 4 link plate and 1 underneath the plate that will help also.

Antisquat seems to play a big role in torque twist also. I wonder if it's it just a band aid.

 

[TSK]

Re: What is Torque Twist - check. How to fix TT - need help

Everything is a band aid. There is no real cure for TT, only adjustments that reduce it's tendencies.

 

[bamr33]

Re: What is Torque Twist - check. How to fix TT - need help

I too can relate to the guy lying on the refrigerator.

Mounting the motor to the axle will help with reducing torque twist .

 

[Hardline]

Okay, I'll play. I don't think a lot of that is right. But I'm probably wrong.

I'd claim torque twist isn't related to inertia. Inertia has to do with moving bodies, and torque twist can be observed in a totally stalled shafty on an angle board. The rig just sits there with it's right front hovering off the ground like a backwards puppy. Note that no part of the rig is actually moving, but the stalled motor is still generating an amount of torque.

Every element in the drivetrain has an in and out. For example the pinion shaft receives a twisting force from the driveshaft, but delivers it to the ring gear. So they cancel each other. No net effect on the chassis.

I'd suggest that the source of torque twist energy is the motor and mounting. If it were unrestrained, the motor body would spin instead of the output shaft. It is however restrained by connection to the chassis, and the chassis restraint of the motor is what results in torque twist.

 

[Duuuuuuuude]

Here is the easiest way I've found to explain torque twist: chuck up a 1/2" drill bit into an electric drill. Clamp the bit into a vice. Hang on to the drill and pull the trigger.

What happened?

Either:

A. The drill twisted out of your hands.
B. The vice twisted off of the table because you didn't bolt it down.


Why did it happen?

Because one object is exerting a twisting force onto another object, and, since the two are not rigidly connected, one (or both) of them will react.


How do you make it go away?

You don't. That force will always be present as long as the motor is spinning and there is resistance to it. The best that you can do is counteract it.

 

[TSK]

Okay, I'll play. I don't think a lot of that is right. But I'm probably wrong.



I'd claim torque twist isn't related to inertia. Inertia has to do with moving bodies, and torque twist can be observed in a totally stalled shafty on an angle board. The rig just sits there with it's right front hovering off the ground like a backwards puppy. Note that no part of the rig is actually moving, but the stalled motor is still generating an amount of torque.



Every element in the drivetrain has an in and out. For example the pinion shaft receives a twisting force from the driveshaft, but delivers it to the ring gear. So they cancel each other. No net effect on the chassis.



I'd suggest that the source of torque twist energy is the motor and mounting. If it were unrestrained, the motor body would spin instead of the output shaft. It is however restrained by connection to the chassis, and the chassis restraint of the motor is what results in torque twist.

I'm confused. are things moving or not moving?

Here is the easiest way I've found to explain torque twist: chuck up a 1/2" drill bit into an electric drill. Clamp the bit into a vice. Hang on to the drill and pull the trigger.

What happened?

Either:

A. The drill twisted out of your hands.
B. The vice twisted off of the table because you didn't bolt it down.


Why did it happen?

Because one object is exerting a twisting force onto another object, and, since the two are not rigidly connected, one (or both) of them will react.


How do you make it go away?

You don't. That force will always be present as long as the motor is spinning and there is resistance to it. The best that you can do is counteract it.

You forgot...

C. The vise and table are bolted together and that ish flipped over and a table legged poked out your eye because you have gorilla grip.




And again,

Devise a counter rotation unit like the one EGRESSor had on his AX and you should have a cancelling effect band aid applied to the gaping TT wound.

Counter Rotation unit (noun) unit that counter roates ish)

If you look at Christian's old AX tuber, he had a thing bolted to the front of his trans that sorta looked like a dig without a servo. It basically had a gear on the original output shaft, which drove another gear that had an output shaft which spun in the opposite direction. He had to flip his rear axle upside to maintain proper drive direction of both axles, but it sounds like it will work in theory.


Christian 's counter rotation unit:

 

[Hardline]

I'm confused. are things moving or not moving

Stalled means nothing's moving. Even though the armature is making torque, no part of the rig is moving. And the tire is still in the air due to on-going torque twist. Release the trigger and then the wheel drops.

I agree with Duuuuuuude...TT is unavoidable. On the other hand, I have to wonder what would happen if the motor were mounted crosswise. Motor mount reaction would now lift the front or rear instead of the right side. Power delivery would probably get ugly, but I'm thinking TT would be gone. Or maybe just different.

 

[EeePee]

EeePee had a great idea to mirror the transmission, so the sudden acceleration of the motor will spin the chassis the opposite direction, for a moment counteracting the torque twist caused by the inertia of the wheel/tire/rig combo.

I did feel smart. :mrgreen:

Actually, the idea is that it's putting the weight of the motor on the side of the chassis that wants to lift up, making it harder to lift up. There's nothing opposite about it really (other than it's on the opposite side of the chassis I suppose), as the motor spins the same direction as before the mod. It's a minor tweak, and simple as you've either got weight over here, or you've got weight over there. Might as well use it to your advantage.

 

[Duuuuuuuude]

On the other hand, I have to wonder what would happen if the motor were mounted crosswise. Motor mount reaction would now lift the front or rear instead of the right side. Power delivery would probably get ugly, but I'm thinking TT would be gone. Or maybe just different.

It wouldn't change anything really, the twisting force between the transmission and the axle is still the same.

There is some snap from the motor if you just hammer the throttle, but we don't drive like that all the time anyway, so its not really a relevant thing.

It's a minor tweak, and simple as you've either got weight over here, or you've got weight over there. Might as well use it to your advantage.

Kinda like when everyone runs to one side of the sailboat so it doesn't tip over. :mrgreen:

 

[EeePee]

Kind of like placing a fat ass NASCAR driver on the left side of the car and only making left turns.

Edit: Is that why they run counterclockwise tracks?

 

[Duuuuuuuude]

Kind of like placing a fat ass NASCAR driver on the left side of the car and only making left turns.

Edit: Is that why they run counterclockwise tracks? :ror:

Hillbillies naturally veer left when drunk.

 

[EeePee]

This is a tech forum, you know, like science and stuff. Do you have anything to back that up? Or at least a wikipedia link?

 

[Duuuuuuuude]

This is a tech forum, you know, like science and stuff. Do you have anything to back that up? Or at least a wikipedia link?

Dude, I fawkin' live in Arkansas. I should know what a damn drunken hillbilly looks like.

 

[opek]

I did feel smart. :mrgreen:

Actually, the idea is that it's putting the weight of the motor on the side of the chassis that wants to lift up, making it harder to lift up. There's nothing opposite about it really (other than it's on the opposite side of the chassis I suppose), as the motor spins the same direction as before the mod. It's a minor tweak, and simple as you've either got weight over here, or you've got weight over there. Might as well use it to your advantage.

Thanks, the first post has been updated.

If we put an infinitely heavy block spaced infinitely far away from the chassis (pushed off to the passenger side) would TT disappear, or would the torque pass through the chassis and to the front axle?

 

[Hardline]

It wouldn't change anything really, the twisting force between the transmission and the axle is still the same.

I'm not so sure. Imagine power delivery like a motorcycle with sprockets and chains. I see nothing there that would induce axial chassis twist. If you choose to deliver power with bevel gears and such, I think again all input torques would balance output torques with no net effect on the chassis -- just the initial motor mount reaction.

Proof is left as an exercise for the student.

 

[opek]

I'm not so sure. Imagine power delivery like a motorcycle with sprockets and chains. I see nothing there that would induce axial chassis twist. If you choose to deliver power with bevel gears and such, I think again all input torques would balance output torques with no net effect on the chassis -- just the initial motor mount reaction.

Proof is left as an exercise for the student.

Imagine you are the kid suspended by the ropes. You grab onto a street sign and try to twist it like a screwdriver. What will happen? You will spin. That is what torque twist is, it has nothing to do with gears. That pic is there so people who aren't sure how things work can feel a bit more comfortable. "thumbsup"

A sprocket setup would twist the chassis in a different way, pushing/lifting the front end up. Proof is left as an exercise for... naw! Imagine the chain, front and rear sprockets on a dirt bike.

To go forward the front sprocket will turn clockwise, which will pull the chain and make the rear sprocket also turn clockwise. If the rear sprocket cannot move, the front sprocket will turn clockwise and walk itself along the top side of the chain. Since the distance between the sprockets cannot change, either the chain snaps or the front sprocket raises up and the whole bike loops over. Suspension compression typically slackens the chain, this would happen until the suspension can no longer operate at which point the bike would loop over.