Weight transfer due to drive torque!

  • Thread starter R Power
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  • #26
14
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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).
 
  • #28
271
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if differential is exactly at the center of axle why the axle would turn?
 
  • #29
2,017
85
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 dont 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.
 
  • #30
271
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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????
 
  • #31
Ranger Mike
Science Advisor
Gold Member
2,153
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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 lenght 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.
 
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  • #32
271
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so i guess it is this difference of axle lengths with which axle rotates
 
  • #34
271
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I didn't understand the 2nd one
 
  • #35
271
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still i don't get a solid reason why axle twists
 
  • #36
2,017
85
Because there is a twisting motion input to it along the propshaft.

The diff doesnt 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.
 
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  • #37
14
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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.
 
  • #38
803
9
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.
 

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