I asked earlier,but a mechanic is doing some work to rear on my 1981, my question is do the half shafts hold the body up,the mechanic took only my half shafts off for me to paint,he wasn’t there when I picked them up,but noticed the car had dropped down in the rear like 3-4 inches,so either something was wrong by previous owner,or is that normal to pull the half shafts off and the body drops,meaning the half shafts have weight on them. Just curious.

 

Yes, indeed! VERY MUCH the half shafts are a stressed member of your rear suspension.
Yes they hold up the whole car. Frame, body, seats and all!
And why is someone else taking your car apart?

 

When I have my suspension disassembled for any reason. I wouldn't dream of taking it off the stands and setting it down.
Not until the suspension was reassembled would I set it down. The fact that you seen your car off the stands with I don't know, the tires on the inside of the fenders holding it up? The trailing arm on the bump stop perhaps? Lets hope so.
Unbelievable. This is why you work on your own car. No one loves your car as much as you do!

 

The halfshafts do not bear any of the vehicle weight, just like the upper control arms in front don’t. The springs hold up the car.

That said, the halfshafts, like the upper control arms in front, are an integral part of the suspension geometry so when removed, will allow the rear wheel camber to change, putting undue stress on the trailing arm bushings but not to the degree you mention.

Bottom line, the car should be supported on the frame while the halfshafts are removed.

 

No, that's just not true. The half shafts do indeed hold up the car. It's only a 3 point IRS. The half shaft is one of the 3 points. The upper one! It's like removing one leg from a 3 legged stool. Spring or not. That shaft bears weight.

 

Yea,I took it to a shop that has a lift for replacing the differential.

 

Follow the path of the force the weight is exerting on the rear wheels. You understand these cars so please don’t be offended if I am using simple terms.

As you know, the differential is hard mounted to the frame via a crossmember and the transverse spring is hard mounted to the differential. That force travels to the ends of the spring, then up the bolts attached to the trailing arm. From there, it goes to the bearing carrier, axle, wheel and, finally, the tire.

The halfshaft is not in that pathway. Yes, it is a 3-point IRS but the halfshaft only serves to locate the upper point of the trailing arm in the vertical plane. Removing a halfshaft has no effect on that force path (because it is in the unsprung weight category).

 

I understand what you’re saying,but,I drove my car into the mechanics shop,The rear fender is every bit of 6 inch gap between fender and tire,he puts it on his lift and removed only the half shafts,and then sits it’s weight back on tires,and the rear dropped at least 3-4 inches,now someone has replaced the spring at some point,it has one thick spring now,but that’s it,so,no other modifications have been done back there,so,should be what came from the factory,so I’m going to have to agree with 4-vette.but curious what kind of angle is supposed to be on the shafts,like 3% angle or more,I mean they aren’t suppose to be straight correct.

 

The shafts should be essentially parallel to the ground with full weight on wheels with a normal setup. Some folks twiddle with the bolts (installing longer to lower the body and installing shorter to raise the body) so in extreme conditions, the shafts will angle down toward the differential with a lowered body and angle upward with a raised body.

 

If the diff has been removed, then the spring has also been removed. No other way.
So he must have some blocks of wood in there holding it up. He probably needs his lift for other projects and blocked it so he could roll it. It's not being driven, so it should be fine. When he puts it all back together it'll be fine. Relax. I didn't know your diff. was out .

 

The half shafts, at the correct ride height angle down to the wheel about 2*. Or 1/2" lower at the outer H/S end.
The Chevy way of measuring the rear ride height is the rear strut rods. The difference between the inner joint and the outer joint is called the "D" height.
Spec there is 2.1" with zero passengers.

 

So I undertook a little experiment. I measured the distance for the floor of my garage to the top of the frame rail just in front of the halfshafts. Both left and right measured 29 7/8”. (Full disclosure - my chassis is sitting on wheeled dollys so don’t try to compare to).

I next removed the left halfshaft.

Remeasured the distance between the same points - exactly as before - 29 7/8”.


Left side with halfshaft installed

Right side with halfshaft installed

Left side with halfshaft removed

Scene of the experiment

 

Nothing is holding that wheel from swinging inwards or outwards. Put a car on it and get back to us.

 

Well, there are still 2 of the 3 attach points in place. The lower strut is holding it in the lower lateral plane and the bushing at the front of the trailing arm is opposing the twisting motion that occurs in the absence of the driveshaft so the “swinging” motion is restrained (subject to the flexibility of the bushing).

What you didn’t address is the lack of any change in the vertical plane which is precisely my point - the halfshafts do not bear any weight. While it is true that removing them on a fully assembled car, particularly with old trailing arm bushings, will cause the wheels to “tip” inward at the top (and thus a perceived loss of ride height), there is no actual change to the ride height.

 

It’s not out yet,and the only thing I see that has been done in the past,was replacing the spring with the one thick type spring,and no he lifted it with his lift,and like I said previously when he set the body full weight on tires the body dropped at least 3 inches,so maybe like 69L88 said,must of changed the camber or something else with the trailing arms,but I think he removing the differential tomorrow.but a new differential,new trailing arms,new bearings,new adjustable strut,new shocks,and just ordered for a 1984 corvette spring bolt,because it’s 10 inches long,and I will just cut off the end,I already had a 8 inch bolt in it,and it didn’t lower it any.

 

Bullshit the halfshaft is a stressed member. Yes, the spring holds it up. But against what?
like I said. Put a car on that. Put it down on the ground and get back to us.
Or, just look at the damage caused when a half shaft is no longer connected to a moving car.
Or explain to us why our diff yokes wear when there is no weight or pressure on them.

 

I could be wrong here but I didnt see him stay the halfshaft doesnt see stress, just that it doesnt directly support the weight of the car. I'm thinking of a radio antenna with support wires and the halfshafts acting much like a support wire to hold it in position from the side. it sees stress in the form of stabilizing sideload

 

69L88,I think you’re only missing the body(weight). Put the body on it,and pretty sure it would go down,you don’t have any weight on it now,it shouldn’t move.

 

I'm sorry I missed this yesterday. I found checking out the forces involved the rear IRS fascinating.
Here is my poor drawing:

Here is the best I can simplify the physics:

720 lb force car weight downward on 1 spring bolt (red)
equal resisting force upward in center of tire (red)
tire, axle & trailing arm resist 720# vertical force

but ~7 inches between points (blue)
the trailing arm bushing is the pivot point
720# force trying to rotate the trailing arm with a 7” lever arm
This is a 420ft-lbs rotational torque on the trailing arm from tire axle
top of tire tips in with 420# force with 12” lever arm (yellow)
the halfshaft is attached to the trailing arm pivot point
no sideways force at the halfshaft (small green) it is just a locator
rotational torque has to be resisted by something, half-shaft is at the pivot point, so it can’t, so it is up to the lower strut rod
lower strut rod (long small yellow) 840# force, only 6” lever, resisting tire rotation, due to car weight

Take the half shaft out as 69L88 did, and not much would happen.
Take the strut rod out, and the tire crashes into the upper frame with 420# force

Add a G cornering force of 500# at bottom of tire tread (green)
500# of horizontal force is resisted by both half shaft and lower strut rod, say 250# each
But also a 500 ft-lbs of rotational force on tire at the trailing arm, due to 12” lever
This is resisted almost entirely by the lower strut rod, 1000# of force, 6” lever
If cornering uses strut rod as a pivot point, half shaft tries to pull out of diff with 500# of force on one side of car, and is crushed into diff with 500# force on the other side of car. That causes diff pin wear, when the posi rotates under all that pressure.
(Wish I could find that video where the axle stub pops out of the guys diff in a corner)

Is it any wonder why the lower strut rod bushings and diff cross pins take such a beating?
They can get hit with up to 1000# of force, cycling on & off very quickly.
And the axles get rotated on the cross pins while under all that pressure.

Ok As I finished that I thought of an even easier way.
Think of the tire trying to rotate the trailing arm as a torque wrench.
It is 7" long. And set at 420 ft-lbs

As you push up at blue arrow, the car weight, spring, thru the spring bolt, push down at the red arrow.
I know all of you know how a torque wrench works.
If the spring is strong enough at red, the TQ wrench does not move vertically, but the handle rotates at blue.
There is virtually zero pressure right to left.
Or in this case, along the half-shaft.

Thanks for the mental exercise.
I needed it, and that one made me work!
Vette IRS is unique!

 

You guys are arguing the same point... the half shafts do not hold the weight of the car...while the suspensionis flat. They do locate the tire so they are seeing lateral forces, that's why the diff side yokes wear out. If you lift the car, remove the yokes and sit it down. Then measure where the tires end up they will camber out and will shift outward but the height will not be dramatically decreased due to the spring. If you remove the shocks and jack up the rear you will see the arc of travel. This is also why you have to roll your car forward and backward after a suspension change. The halfshafts are not going to get shorter so they need to settle in their arc but due to the tire friction cannot so the tires prevent the car from settling which is pushing in on the halfshafts and are supporting the weight of the car.

You are both correct and wrong it's just the point you are stuck on and at what point in a dynamic suspension travel you are at

Now quit arguing so I can go enjoy some sunshine at a Cops and cars event to support Veterans and first responders