Weight transfer due to drive torque

[R Power]

hi guys
I read something interesting yesterday(atleast to me) that weight transfer can occur in a car due to drive torque from driveshaft to axle which causes axle to rotate. I found it strange that why drive torque in differential will rotate the whole axle(a small angle however).Can u tell me how it occurs?

 

[Mech_Engineer]

Here's a good example of some significant weight transfer due to drive torque and acceleration:

http://img87.imageshack.us/img87/4149/mustangtorquedemotivaticg2.jpg

 

[xxChrisxx]

I love demotivational posters.

R Power, simple mechanics casues this. Draw a free body diagram of a car and put on the forces and moments.

All should become clear.

 

[R Power]

I am talking about lateral weight transfer due to drive torque on axle. The weight transfer in the given poster in longitudnal which i can easily understand. But i am talking about lateral weight transfer due to drive torque. Plz explain.

 

[Ranger Mike]

great poster..me being a dumb country boy..pls explain what you mean by lateral? becasue their is drive shaft weight transfer from left front to right front and left rear that takes place when you step on it ...

 

[R Power]

By lateral i meant from from one side of wheels to other. e.g left to right. I mean to say about lateral axis of car.

 

[Ranger Mike]

the engines firing order (american V8) rotates in a counter closewise direction. this means the left side of the engine will lift. ( i broke motor mounts and had to CHAIN the left side to prevent this). Efforts have been made in the past to make a reverse rotation engine but this requires a trick crankshaft, cam shaft and ignition.

what you have is a high torque engine, trying to rotate in a counter closewise direction thru the clutch , transmission. this counter closewise direction is converted to a 90 degree direction by the differential. This counter closewise direction torque continues to the wheels and tires. if the tires are really hooked up to the asphalt, little or no tire spin occurs and the twisting effect has to go some where...so the tires spin or the clutch slips or engine motor mounts break. some times drive shaft U joints shear, some times transmissions break.

 

[R Power]

Please tell clearly how the torque which rotates the axle and thus wheels through the differential, tends to transfer lateral loads.

 

[Danger]

I had this problem with the Roadrunner. In fact, it's why I had to park it.
The engine & tranny twisted over so far during acceleration that the torque tube for the clutch popped off of the pivots. I chained the intake manifold down to the shock tower to eliminate that, and the stupid thing started pulling the left front wheel off of the ground. After I switched to radial tires, it did both.
Some idiot teenaged girl in a turbo 'Vette started harassing me when I was peacefully heading home from lunch. I tried to ignore her all across the mall parking lot, but when we got to the lights that turned onto the main street, she got really aggressive with burping the throttle and glaring at me. I figured that it was time to tune her in. We were on a dual left-turn intersection, with her in the right lane.
When the light changed, she smoked around the corner. I eased out until I was straight, then romped on it. So far, so good. I went past her pretty quickly. A block later, just passing through the first set of lights, I power-shifted into 2nd at 65 mph. Much to my delight (not!) the clutch pedal stayed on the floor. The irritation of that was offset by seeing the 'Vette slink off onto the cross-street.
(For anyone who knows Calgary, this was coming out of Sunridge Mall onto northbound 36th.)
Anyhow, I couldn't afford to fix it. That was over 25 years ago, and it's been sitting in my yard ever since.

 

[Ranger Mike]

I know what you mean...i bought my Roadrunner because some zitty faced pencil neck geek in a 1992 Buick with NO FEAR decal in the rear window bugged me at a traffic light..
funny how once you race you always hanker to race!
bottom line is torque is real! chain that puppy down!

 

[Danger]

The weight transfer in the given poster in longitudnal which i can easily understand. But i am talking about lateral weight transfer due to drive torque. Plz explain.

Take a closer look at the picture. Notice that the left front wheel is about twice as high off of the ground as the right one. That is the result of driveline torque acting upon the chassis and body. Given how much that body is twisted, think about what the effect upon the rear wheels is.

Hey, Mike. Yeah, it do get into your blood. I can't stand the way kids do it nowadays, though. That's just stupid and deadly, to themselves and innocent bystanders. My friends and I were always very careful that we had unobstructed good pavement in a straight line, with lots of deceleration room. In the episode that I outlined, for instance, we were the only 2 cars within a couple of blocks. I knew the synchronization of the lights, so there was no worry of the first intersection going red on me. Just the same, if a car had been approaching on that side-street within half a block, I would have backed off in case it didn't stop. (That was a T-street, by the way, so I had only to watch one way on it.) These guys who chase each other through the streets like rats on bennies are a disgrace.

 

[Ranger Mike]

Yeah I know..earlier this year i was pretty bummed out about this younger generation..until i had my toys for tots volleyball tournament on labor day..it was raining very heavy but about 30 kids 20 to 30 years old showed up to play in the rain, on their day off work just to get a cheap plastic trophy ..after the final game and after the award cerimony ..they ALL stayed around to continue to play..in the rain and in the mud...so there is hope !

 

[Danger]

I can't speak toward their common-sense, as to not coming in out of the rain, but it certainly beats what they could have been doing instead.
By the bye, was that a typo? Did you mean toys for tots? I've heard of that, and think that it's a great cause. The way you typed it, it sounds like a kindergarten dating service.

 

[Ranger Mike]

good catch Danger..

 

[R Power]

OK I can see the left wheel is twice high. But why?
I mean can u explain me physics of this??

 

[Ranger Mike]

the engines firing order (american V8) rotates in a counter closewise direction.
this means the left side of the engine will lift.

what you have is a high torque engine, trying to rotate in a counter closewise direction.
this rotating force goes thru the clutch , transmission.
this counter closewise force changes direction by 90 degrees by the differential. This counter closewise torque continues to the wheels and tires. if the tires are really hooked up to the asphalt, little or no tire spin occurs and the twisting effect has to go some where

the result is the twist you see in the photo
either the tires spin or the clutch slips or engine motor mounts break. some times drive shaft U joints shear, some times transmissions break.

 

[Danger]

I'm not sure why, Mike, but after looking at that picture I have a sudden urge to go out and pick a couple of lemons.

R Power... I am going to assume that you know how a car engine works. (I don't mean that insultingly... some people don't know.) In North America, at least, the working bits rotate counter-clockwise as viewed from the driver's seat. Whatever amount of torque is applied to the crankshaft, flywheel, clutch, transmission etc. is countered by an opposite torque applied to the engine block, motor mounts, K-member, rear suspension... (action/reaction, in obedience to Newtonian law). The whole vehicle tries to twist in the opposite direction to that of the crankshaft, which sets a pre-load condition upon the round rubber bits that connect it to the road.

edit: I see that you beat me to it, Mike. Good post.

 

[xxChrisxx]

It's comedy how honkin great V8's can twist the chassis up like a corkscrew.

 

[Danger]

It's comedy how honkin great V8's can twist the chassis up like a corkscrew.

Only until you have to shell out a few thousand bucks to alleviate the problem.

 

[Ranger Mike]

teachable moment- this into compact unibody cars..e funny car scene hit in the early 1960s. was NHRA class A/FX which was Factory eXperimantal. this was Detroit stuffing huge 400 plus cid V8s into comapct unibody cars...these unibody desighns did not have the heavy full channle frames on which the body was bolted. unibody deign used a stamped sheetmetal floorpan and relied on the windshield and sheetmetal roof / pillars to tie the body together...anyway..when you nailed the throttle the twist would crack the windshield so we have to tie the front and rear sheet metal channels together to reinforce the body...NHRA mandated a full roll cage that took care of this problem, we welded in tubing to connect the rear spring mount points to the main roll cage hoop and ran tubing to the front end to tie in the whole pan. this reinforced the floor pan and assisted in weight retransfer to the rear. we still had twist but could control this with traction bar setting or pinion snubber setting , springs and shocks..

 

[Lsos]

http://img135.imageshack.us/img135/440/328lvl.jpg

Just thought I'd throw in another example of torque weight transfer...

 

[Ranger Mike]

case study in weight transfer

 

[R Power]

Danger can u explain a bit more...
I mean reaction to the action of crankshaft will be in the form of friction on clutch and other parts which engine has to overcome while rotation. So why whole body rolls!
It amazes me!

 

[Danger]

Maybe the easiest way to explain this is to tell you to get your mitts on an electric drill. Use a large-diameter bit and bore a hole into a log or thick plank. What you feel in your hand is the reaction torque to the motor, which is the same thing that the car feels in reaction to the rotating mass of the engine.

 

[Lsos]

Helicopters have the rear rotot in order to stop from spinning out of control due to the reaction torque from driving the main roror.

Propeller airplanes also have reaction torque, but they can counter this using their wings. I understand some of the more powerful airplanes with big, nose mounted engines (such as the WWII fighters), had so much torque that under full throttle, the pilot had to pretty much try to wrestle the plane into a full opposite turn, just to keep it level.

 

[CKwik240]

Danger can u explain a bit more...
I mean reaction to the action of crankshaft will be in the form of friction on clutch and other parts which engine has to overcome while rotation. So why whole body rolls!
It amazes me!

It was explained, but not clearly pointed out. The rotational motion of the motor and driveshaft is easy enough to understand. Why one side of the car jacks up is because as the tires grip the road under the load, the mass of the car resists the force provided by the motor. Since there is a "resistive force" transmitted back through the axle and differential, the rotational force at the driveshaft will no longer transmit the full force into accelerating the car, but rather starts to twist the entire rear axle around the axis of the driveshaft. To see it more clearly, try imagining such a car with the tires glued to the groud so that the tires cannot rotate. If the tires can't rotate, any torque provided by the motor through the driveshaft will cause the axle to rotate along the axis of the driveshaft in the opposite direction of the driveshaft. As a solid rear axle is suspended from the vehicle, it is allowed to move somewhat independently of the chassis and will jack up the side that is pushing down on the ground. Do keep in mind that the motor is attached to the chassis up at the front of the car, so that is the point where torque force acts opposite to the axle.

Lastly, you won't see this occur in a car with an independent rear suspension as the differential is essentially bolted to the chassis and the axles aren't able to transmit force in the direction of the ground (since they are jointed at each end).

 

[Ranger Mike]

yeah..it will never happen with them independent rear suspensions
note: these vehicles have independent rear suspensions

 

[R Power]

if differential is exactly at the center of axle why the axle would turn?

 

[xxChrisxx]

You are talking about lateral weight transfer arent you?

There is a twisting motion. The diff can only cope with so much so quickly, if you send a lot of torque throught it the entire axle pivots.

CKwick is right ish, if you don't have the solid coupling of a live axle you don't get the lifting motion of the body as the half shafts can pivot. Meaning you'll get much smaller effect, one wheel will rise but it's unlikely you'll get the entire body rolling. /the effect is not entirely eliminated though.

 

[R Power]

But why axle rotates ?
To rotate anything we need a torque. Now we have force from drive shaft but the prependicular distance from driveshaft or differential to center of rotation of shaft is 0 , so where this torque comes from?

 

[Ranger Mike]

as usual XXXchris XX you are spot on..it still twists but the independent rear suspension permits the torque to be BETTER transmitted to the pavement. there is still lateral and longintudenal twist going on though not so readily apparent. The independent swing arms " flex" to better accommodate the weight transfer where as the solid differential can not conform to varying weight transfer AS WELL and hence the wheelie and tork twist.

r power - most solid rear differentials have different length axles. the right side being shorter
then the left side. independent diffs have CV joints and same length axles and the diff is mounted at vehicle center line.

 

[R Power]

so i guess it is this difference of axle lengths with which axle rotates

 

[Ranger Mike]

maybe this will help

 

[R Power]

I didn't understand the 2nd one

 

[R Power]

still i don't get a solid reason why axle twists

 

[xxChrisxx]

Because there is a twisting motion input to it along the propshaft.

The diff doesn't transfer ALL the twisting motion to the wheels. The point where the propshaft connects to the diff acts as a pivot.

It's that simple.

 

[CKwik240]

still i don't get a solid reason why axle twists

Find a rigid object in your house that has a "T" shape to it. A T-Handle tool might be a quick find if you have one (If you don't, just try and follow this conceptually). Grab the object by the leg of the "T" and spin it along the axis of the leg. The top of the "T" will twist. This is pretty similar to typical live axle. The difference is that under no load (wheels off the ground), the torque gets transferred out through the drive wheels. But if you were to lock the drive wheels (perhaps using the parking brake; assume the parking brake can hold any torque the motor can provide), torque can no longer transfer out as it can no longer spin the wheels. At this point, the system will act just like that "T" shaped object and spin about the axis of the leg. These two scenarios describe what happens when the resistance to acceleration of the drivewheels are at 0 and infinity. What you see when a car twists like that is somewhere in between. Since the drivewheels aren't locked in place, torque from the driveshaft is transmitted to the wheels. But since there is resistance to forward motion of the vehicle, there will be resistance that tries to prevent the drivewheels from turning. When more torque is applied, there will be more resistance. So as more torque is applied, the results begin to appraoch that of that "T" handled tool and tries to spin the ends of the "T" around the axis of its leg.

 

[Lsos]

It's a simple matter of "for every action is an equal but opposite reaction". If the engine twists the drive shaft, the drive shaft will twist the engine just as strongly, but in the opposite direction. This in turn makes the whole car twist, which in turn presses one side harder into the ground than the other, which we see as weight transfer.

This effect can be seen in anything that spins.