What makes the rear wheels sit forward in their wells?

[jmortensen]

Thrust angle is a function of the front toe vs the rear toe. Sounds like your rear toe was screwed up and you fixed it and that fixed your thrust alignment. No mystery there.

The front of the control arm is still in the same location regardless of where the rear is. There will be a slight movement in the hub when the toe angle changes, but you're not going to get a lot of movement there. This may be easier to visualize on cars that have monoball pivots instead of bushings. With the monoball, there is no slop in there and when you adjust you can really see that the control arm just has no freedom to move fore and aft.

Several of us made what amounts to adjustable uprights for our cars, you can see them here: http://forums.hybridz.org/index.php?showtopic=24100 We took the idea from Ron Johnson who fixed a toe issue in his car by slotting the upper holes in the upright and moving it to set the toe.

[madkaw]

When the previous owner installed the r200, how did they connect the rear of the rear control arms?

The reason I ask is that the r200 won't fit with certain brackets that connect the rear control arms. The PO might have made his own bracket and made it too wide causing toe in.

Edited January 23, 2012 by madkaw
Yep

[Mr Camouflage]

Looks like the previous owner may have used the 260/280 struts, which would explain why the rear end is so high.

[doradox]

Thrust angle is a function of the front toe vs the rear toe. Sounds like your rear toe was screwed up and you fixed it and that fixed your thrust alignment. No mystery there.

The front of the control arm is still in the same location regardless of where the rear is. There will be a slight movement in the hub when the toe angle changes, but you're not going to get a lot of movement there. This may be easier to visualize on cars that have monoball pivots instead of bushings. With the monoball, there is no slop in there and when you adjust you can really see that the control arm just has no freedom to move fore and aft.

Several of us made what amounts to adjustable uprights for our cars, you can see them here: http://forums.hybridz.org/index.php?showtopic=24100 We took the idea from Ron Johnson who fixed a toe issue in his car by slotting the upper holes in the upright and moving it to set the toe.

Isn't thrust angle a function rear toe alone?

The hub will move for and aft if you only move one LCA bushing location. The whole LCA pivots about the fixed bushing swinging the wheel side of the LCA. A small amount but it will move approximately by..

(the amount the rear bushing was moved ) * ((Distance from LCA shaft to hub)/(distance front to rear of the LCA bushings))

Steve

[jmortensen]

Isn't thrust angle a function rear toe alone?

Yes, essentially, since the front toe isn't going to have a thrust component to it. The fronts are going to go straight and the steering wheel will turn if the "front thrust" is off. The rears can't compensate since there is no steering, so they continue to point in one direction or the other.

The hub will move for and aft if you only move one LCA bushing location. The whole LCA pivots about the fixed bushing swinging the wheel side of the LCA. A small amount but it will move approximately by..

(the amount the rear bushing was moved ) * ((Distance from LCA shaft to hub)/(distance front to rear of the LCA bushings))

Exactly the point I was trying to make. Yours is a much more technical and accurate way of saying: "There will be a slight movement in the hub when the toe angle changes, but you're not going to get a lot of movement there."

[Jetaway]

Doradox wrote: The hub will move for and aft if you only move one LCA bushing location. The whole LCA pivots about the fixed bushing swinging the wheel side of the LCA. A small amount but it will move approximately by..

(the amount the rear bushing was moved ) * ((Distance from LCA shaft to hub)/(distance front to rear of the LCA bushings))

Jmortensen replied:

Exactly the point I was trying to make. Yours is a much more technical and accurate way of saying: "There will be a slight movement in the hub when the toe angle changes, but you're not going to get a lot of movement there.

Sorry about the unusual quoting style, but I couldn't figure out how to quote a quote and this was the relevant passage.

Jon,

I think you overlooked that I didn't move just one bushing location. I had loosened every bolt attaching the transverse link / control arm to the frame and those attaching the differential carrier to the frame. I only whacked that one point with the little sledge, but I never intended it to be, nor was it, the only point that moved. I don't have before and after pictures, but I assure you that the support plate (one of the two) that holds the rear of the differential carrier went from a definite angle, with the bottom of it closer to the front of the car than the top went to perpendicular, or at least damn close to, after my ministrations.

Toe? Whose toe? More accurately, which toe?

When we talk about toe-in or toe-out we are assuming that, at zero toe, each wheel's rotation is parallel to the frame of the car and parallel to it's partner on the other side of the car. So, when the rear's toes are identical, even if non-zero, the resulting vector, or thrust remains parallel to the frame of the car. But in my case, the first assumption was wrong. While each rear wheel was parallel to each other, they were not parallel to the frame of the car. Even if both rear wheels had exactly zero toe, the thrust vector was out of parallel with the frame of the car and also with the front wheels. After moving back one side, the rear wheels became parallel with the frame of the car and remained parallel to one another. The thrust vector then became parallel with those of the front wheel, even though the rear wheel's toe remained unchanged.

Chris

[jmortensen]

I think you overlooked that I didn't move just one bushing location.

You cannot move the front pivot of the control arm, as its location is a welded part of the frame. If you move the rear pivots, you can change the toe. You can't significantly change the wheelbase. Using doradox's formula and estimating the control arm length fore/aft at 12 inches and the width at 14.5" (I know the width is correct), a 1/4" toe change (which is a big change) gets .3" of movement at the hub. You said that the upright moved .1". That would translate to a hub change of .120".

I had loosened every bolt attaching the transverse link / control arm to the frame and those attaching the differential carrier to the frame. I only whacked that one point with the little sledge, but I never intended it to be, nor was it, the only point that moved. I don't have before and after pictures, but I assure you that the support plate (one of the two) that holds the rear of the differential carrier went from a definite angle, with the bottom of it closer to the front of the car than the top went to perpendicular, or at least damn close to, after my ministrations.

The differential is hung on the mustache bar in the rear, and it doesn't actually connect to the uprights at all. So again, loosen up all the bolts that hold the diff mounts (although I think you said you didn't mess with them), tighten them all up, and you might get a slight change in orientation of the bushings, but this adjustment is WHOLLY unconnected to the location of the wheels in the wells. In the front, it's attached to the crossmember. The front crossmember actually holds the front control arm bushings to the corresponding cups which are welded into the frame, so again I don't think that loosening these bolts and hitting stuff with a hammer and tightening them down wouldn't make any difference.

If you're again talking about the rear suspension uprights moving, as I said before, you're not going to get a really major movement of the wheel forward or aft, because the front bushing cannot move unless the frame gets bent. Squaring up those uprights as you did is great and it can fix a toe issue/thrust angle issue. I don't see how it could significantly change the wheelbase though. Again, that was what Ron Johnson did and it led to that toe adjuster that a bunch of us made, but he slotted the holes in the upright in order to get more movement out of it, since the holes that are there are sized to the bolts, and don't allow for much movement.

Not sure what your point is with the toe clarification. Toe is the relation of the angle of the two tires. Thrust angle is their relation to the centerline of the car. In a Z, the rears have a thrust element and the front do not.

[Jetaway]

You cannot move the front pivot of the control arm, as its location is a welded part of the frame. If you move the rear pivots, you can change the toe. You can't significantly change the wheelbase. Using doradox's formula and estimating the control arm length fore/aft at 12 inches and the width at 14.5" (I know the width is correct), a 1/4" toe change (which is a big change) gets .3" of movement at the hub. You said that the upright moved .1". That would translate to a hub change of .120".

The differential is hung on the mustache bar in the rear, and it doesn't actually connect to the uprights at all. So again, loosen up all the bolts that hold the diff mounts (although I think you said you didn't mess with them), tighten them all up, and you might get a slight change in orientation of the bushings, but this adjustment is WHOLLY unconnected to the location of the wheels in the wells. In the front, it's attached to the crossmember. The front crossmember actually holds the front control arm bushings to the corresponding cups which are welded into the frame, so again I don't think that loosening these bolts and hitting stuff with a hammer and tightening them down wouldn't make any difference.

If you're again talking about the rear suspension uprights moving, as I said before, you're not going to get a really major movement of the wheel forward or aft, because the front bushing cannot move unless the frame gets bent. Squaring up those uprights as you did is great and it can fix a toe issue/thrust angle issue. I don't see how it could significantly change the wheelbase though. Again, that was what Ron Johnson did and it led to that toe adjuster that a bunch of us made, but he slotted the holes in the upright in order to get more movement out of it, since the holes that are there are sized to the bolts, and don't allow for much movement.

Not sure what your point is with the toe clarification. Toe is the relation of the angle of the two tires. Thrust angle is their relation to the centerline of the car. In a Z, the rears have a thrust element and the front do not.

I understand and accept all you have written.

Eppur si muove

Chris

[BTF/PTM]

I love when my threads turn into technical discussions I always end up learning much more than I thought I would.

The comment about the PO having used 280z struts makes me wonder. It's been a long time since I've crawled under the car. Time to get the car out of storage and tinker a bit, methinks. I'll need an enclosed trailer, though. Damn you, road salt!