A Question of Physics ?
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Melting Slicks
Joined: Jun 2009
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From: Anderson SC
A Question of Physics ?
When you dump the clutch and the car begins it's "hole shot" - the driveshaft is turning "clockwise" as the side yokes rotate in the "forward" direction.
Now, at the moment of "go" when the inertia of the car is a dead weight - there is the twisting force of the differential carrier casting itself. This causes a stress on the carrier cross member.
My question is - During the moments of "go" does the carrier casting twist in the same direction as the driveshaft rotation - or opposite driveshaft rotation?
(My instinct is that it would be in the same direction as the drive shaft rotation - because if you welded the side yokes to the housing - the whole "brick" would turn with the drive shaft rotation.)
-W
Now, at the moment of "go" when the inertia of the car is a dead weight - there is the twisting force of the differential carrier casting itself. This causes a stress on the carrier cross member.
My question is - During the moments of "go" does the carrier casting twist in the same direction as the driveshaft rotation - or opposite driveshaft rotation?
(My instinct is that it would be in the same direction as the drive shaft rotation - because if you welded the side yokes to the housing - the whole "brick" would turn with the drive shaft rotation.)
-W
Thread Starter
Melting Slicks
Joined: Jun 2009
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From: Anderson SC
I know, that's what lead me to ask the question at all. 
But I keep coming back to: What direction it would want to spin, if I welded the rear wheels still. And in that case, the housing would follow the driveshaft rotation - right?
-W
But I keep coming back to: What direction it would want to spin, if I welded the rear wheels still. And in that case, the housing would follow the driveshaft rotation - right?
-W
Melting Slicks
Joined: Jul 2006
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From: Salmon Arm, BC
If you had an open rearend the right rear tire will spin at the point of dumping the clutch. This is because the rearend housing is being torqued or twisted counterclockwise and increasing weight or traction on the left rear tire, and decreasing weight or traction on the right rear.
As SanDiegoPaul said the rearend housing is reacting opposite to the direction the driveshaft.
As SanDiegoPaul said the rearend housing is reacting opposite to the direction the driveshaft.
Melting Slicks
Joined: Sep 2005
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From: Warminster PA
When we used to race, I could recite pages from a book by Dave Morgan called "Door Slammers: The Chassis Book", Great stuff which I have forgotten most that I learned (funny how that happens as we get older......)
Anyway, when the car launches, the engine twists at the engine mounts causing the chassis to twist and lift the left front tire. This twisting action is clockwise viewed from the rear. The rear axle housing reacts in an "equal and opposite" direction rotating counter clockwise unloading the right rear tire.
Tom
Thread Starter
Melting Slicks
Joined: Jun 2009
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From: Anderson SC
My differential cross-member has a little bit of torque tweak to it. I'm not sure how it was oriented - but I'm sure I want to put it back so that it gets the opposite stress this time around.
-W
Thread Starter
Melting Slicks
Joined: Jun 2009
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From: Anderson SC
Anyway, when the car launches, the engine twists at the engine mounts causing the chassis to twist and lift the left front tire. This twisting action is clockwise viewed from the rear. The rear axle housing reacts in an "equal and opposite" direction rotating counter clockwise unloading the right rear tire.
As viewed from the rear of the car.. the driveshaft is turning counter-clockwise with the car in forward gears. I just checked that now.
So if the differential housing is having an opposite reaction.. then it's trying to turn clockwise - as viewed from the rear of the car.
Are we on the same page now?
-W
Last edited by Clams Canino; Apr 4, 2010 at 09:56 PM.
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Le Mans Master
Joined: May 2002
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From: Lebanon Township New Jersey
That's right. That is why, on a solid rear axle car with slapper type traction bars, the front of the bars move up toward the leaf springs as the axle housing rotates - clockwise looking at the drivers side of the car.
Melting Slicks
Joined: Sep 2005
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From: Warminster PA
Melting Slicks
Joined: Sep 2005
Posts: 2,713
Likes: 902
From: Warminster PA
I think I'd put a straight crossmember back in. Especially if you plan on racing it. If not, you'll be chasing gremlins trying to get the car to hook.
Tom
Thread Starter
Melting Slicks
Joined: Jun 2009
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From: Anderson SC
I'd be a little apprehensive about using a bent crossmember (torque tweak, bent, same thing). Since the differential housing bolts directly to the cross member, if there is a bend in the crossmember, the diff housing will be at an angle also. This will affect the rear spring causing more load on one side.
I think I'd put a straight crossmember back in. Especially if you plan on racing it. If not, you'll be chasing gremlins trying to get the car to hook.
Tom
I think I'd put a straight crossmember back in. Especially if you plan on racing it. If not, you'll be chasing gremlins trying to get the car to hook.
Tom
Placing the differential on a flat surface and and setiing the crossmember on top of it there is only 1/4 inch difference in height out at the ends of the crossmember. This means in the real world that the differential housing might be tilted a whopping 3 degrees off level as mounted. I never would have seen the "tweak" if I wasn't actively "looking for trouble". But now that I've seen it... I want to reverse the force.
-W
Last edited by Clams Canino; Apr 5, 2010 at 02:33 PM.
Thread Starter
Melting Slicks
Joined: Jun 2009
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From: Anderson SC
So we are all in agreement now that:
If when I stand behind the car and I observe that the driveshaft would be rotaing counter-clockwise in forward gear. That I can conclude that the differential carrier would be "trying" to turn clockwise on a launch?
We all agree on that??
-W (wants to bolt it up and move on - today)
If when I stand behind the car and I observe that the driveshaft would be rotaing counter-clockwise in forward gear. That I can conclude that the differential carrier would be "trying" to turn clockwise on a launch?
We all agree on that??
-W (wants to bolt it up and move on - today)
Le Mans Master
Joined: May 2002
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From: Lebanon Township New Jersey
I don't agree.
The engine block will try to turn clockwise - opposite crankshaft spinning direction - looking from behind the car. The front of the differential is trying to go up (pinion climbing the ring gear)
The engine block will try to turn clockwise - opposite crankshaft spinning direction - looking from behind the car. The front of the differential is trying to go up (pinion climbing the ring gear)
Thread Starter
Melting Slicks
Joined: Jun 2009
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From: Anderson SC
Lets keep the engine twist and torque out of this entirely and limit the discussion to the car from the driveshaft back. Just to keep this simple enough for me to grasp with both hands. 
So... The driveshaft is spinning counter-clockwise (now by magic) as viewed from the rear of the car.
Yes.. the front of the diff is trying to go up... I can see that easily myself. (which means I have hope)
But assuming it's also trying to rotate on the other axis... which way and why? So far the overwhelming feeling is that it would tend to try to twist opposite the driveshaft rotation.
My gut (and BK) seem to tell me that if it can't flip up in the front (and it really can't) - then the next force it sees (presuming the wheels are good and stuck) would be a desire to turn in the same direction as the driveshaft.
But it seems I'm almost alone in seeing it that way...
-W
So... The driveshaft is spinning counter-clockwise (now by magic) as viewed from the rear of the car.
Yes.. the front of the diff is trying to go up... I can see that easily myself. (which means I have hope)
But assuming it's also trying to rotate on the other axis... which way and why? So far the overwhelming feeling is that it would tend to try to twist opposite the driveshaft rotation.
My gut (and BK) seem to tell me that if it can't flip up in the front (and it really can't) - then the next force it sees (presuming the wheels are good and stuck) would be a desire to turn in the same direction as the driveshaft.
But it seems I'm almost alone in seeing it that way...
-W
Last edited by Clams Canino; Apr 5, 2010 at 04:00 PM.
Melting Slicks
Joined: Jun 2009
Posts: 2,709
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From: Savannah GA
I don't think this exercise will net any gain since my 1977 crossmember is drilled in such a way that the diff will only mount one way...... eliminating all hope of installing it backwards. 1968 might be different, check it out.
Last edited by markids77; Apr 5, 2010 at 04:44 PM. Reason: spelling