IRS alignment setting for wide rear tires

[Michael Ghia]

Bruce,
Point taken... for some reason I was thinking that it was the inner leg which the bolts were on
Going from the outer TA being longer to being shorter ... as in pivot point... yes it reverses the action.



Chers

Mike

Originally Posted by Bruce2

Positive Mike (pun intended)

As the stock suspension articulates, the pivot point is not the TA's inner pivot. It is also not the center of the torsion bar at the spring plate. It is along a line drawn between these two points. Because this line is not parallel to the torsion bar but tilted back as it extends from the spring plate's axis to the center of the car, the TA goes negative camber when the TA goes up.

Now loosen the 3 bolts between the TA and SP. The new axis is along a line from where the 3 bolts are to the TA's inner pivot. This line is tilted opposite of the one above, so therefore camber changes are opposite.

[Sandeep]

AWESOME information in this thread. I'm foregoing the camber boxes for now ... no time to complete it but will perform the TA/SP alignment mentioned.

I will reort back with my findings in about 2 weeks

Sandeep

[RonRyon]

Bruce, reading your posts I realize that you’re a lot smarter than I. So, I’d like to bounce a couple of things off of you to see if they make sense. First of all, the way I see it, the reason that camber changes as the TA moves up and down is the angle of the pivot axis relative to the center line of the car. That axis runs thru the center of the inner pivot bolt to the intersection of the spring plate with the center line of the torsion bar. If the pivot axis was perpendicular with the center line on the car, there would be no change in camber. If it were parallel, there would be extreme changes in camber. I think this agrees with what you have said.

As far as moving the inner pivot up, I like this idea because it would provide anti-squat for lowered cars. But it seems to me that in doing so you would be increasing positive camber unless you also moved up the outer pivot. Since my car is lowered I’m thinking of raising the pivot points (inner and outer). I’m using narrowed TA’s and spring-over-coils so this shouldn’t be too hard to do. What do you think?

Also, looking at the picture above of the “BugPerformance red tube chassis beetle” where the outer pivot is not on the same axis as the inner pivot, doesn’t this setup cause unnecessary stress in the suspension components?

[Bruce2]

Ron, I appeared to have you fooled. It took me a long time to figure that out. I first saw how the relationship works about 5 years ago. A guy I know didn't install the 3rd bolt attaching the TA to the SP, and the rear of his TAs tilted up causing lots of +ve camber. I tilted it down and got -ve camber. At that time I didn't understand why this happened, I just knew the relationship.

Then when Mike posted the opposite a couple of nights ago, I again thought of why and it finally came to me.

In your first paragraph, you have it exactly right. When the pivot axis is parallel to the torsion bar there is no camber change. A perfect example is the front suspension on a torsion bar Bug. This design exhibits absolutely no camber changes of the susp relative to the car's chassis. Unfortunately body roll causes the tire to be positive camber relative to the ground.

If you move the inner pivot up, how does this give you "anti-squat"? Squat as I know it is caused when there is weight transfer, like when you dump the clutch. How can moving the pivot up prevent weight transfer?
If you do move it up you get radical positive camber. I have a friend who converted his 54 swing axle pan to IRS by welding in the pivot points. By mistake they got the pivots in too low. Its not much, but it caused horrible negative camber. Even after flipping the TAs left to right, he still had negative camber. If your chassis is an original IRS one, raising the inner pivot would be difficult. Raising both inner and outer would be even more difficult. I don't see the point. The geometry change wouldn't be any different from stock.

In looking at the red tube chassis suspension, I think the long link (that replaces the spring plates) is that long for convenience. It had to be that long to reach the chassis. Just a guess. Being so long is bad for camber change. Small changes in ride height will cause massive camber changes. It doesn't look like the car is finished, so he probably doesn't know what's going to happen. I think you are right. Its going to be hell on the inner rubber pivots. Worse if he's got urethane. If you look at the stock pivot bolt, its axis, if extended, would probably intersect with the center of the torsion bar at the spring plate. Thus there is no binding. But move that outer pivot that far forward, you're in trouble.

[RonRyon]

Bruce, if the TA pivot axis is higher than the center of the rear wheels, the force transferred from the wheels to the TA tends to cause the TA to rotate about the pivot axis in a clockwise direction on the driver’s side and counter-clockwise on the passenger side. The result is a lifting force at the pivot points, which works to reduce squat. If the pivot axis is lower than the center of the wheel, the opposite is true and the car will have more of a tendency to squat excessively under acceleration. Have you noticed at the drag races how the entire car is “lifted” under acceleration? This is caused by the lift bars. If you crawled under one of those cars you would see that the rear of the lift bars are lower that the front. Therefore, under acceleration, the wheels are causing the lift bars to rotate such that the entire chassis is lifted upward. The Bug’s TA can act like a lift bar.

With the stock Bug chassis, the rear of the TA is lower than the front as it should be -- no problem. However, if you lower the rear of the car much, the rear of the TA will be higher than the front, and handling will suffer. I have seen some people argue that squat isn’t all bad since it helps with weight transfer. I think they are getting confused, thinking that squat some how aids weight transfer. The amount of weight transfer is dependent on the height of the cars center of gravity, the higher the better. When a chassis squats, the center of gravity is lowered. Anyway I hope this all makes sense to you, if not please let me know.

[Bruce2]

I see what you are saying Ron. But you are making a fundamental error. The reactive torque is not absorbed by the TA, but by the engine/transmission unit. This is why the nose of the trans wants to rise on acceleration. There is no rotational torque applied to the TAs. They just support the bearings.

The phenomenon you describe occurs in solid axle rear ends like 60s and 70s American cars have. When they install ladder bars (which look like the spring plate substitute on that red chassis car 2 pages back), the ladder bars are fixed to the axle housing, which is fixed to the diff housing. When they launch, the front of the rear end wants to rotate up. By installing ladder bars, the counter rotational force causes the whole rear end to be forced down, creating more traction. In a VW swing axle, some guys do install links that look exactly like the ladder bars on an American car's rear end. But they don't have any function except as a trailing arm. This is because the VW's axle tube is not rigidly connected to the rear end housing (trans case).

[RonRyon]

I’m not talking about torque causing the lifting action but rather the force transmitted to the TA from the wheels which push the car forward. Physics would say that the force exerted by the wheels to the chassis through the TA has two components, a horizontal force and a vertical force. The horizontal force pushes the car forward and the vertical force pushes the car either up or down depending on the angle of the TA. The amount of the vertical force is dependent on the angle of the TA relative to the horizontal.

Or, a another way of explaining it --- looking at the passenger side for example, if the TA angles down to the rear, the force exerted by the wheel will try to cause the TA to move in a counter clockwise rotation. In order to do so it must lift the car.

It would be easier to explain this with a drawing but I don’t know how to do it on my PC.

Not that it matters, but don’t ladder bars lift the car? I don’t believe I have ever seen a ladder bar design that could push the rear of the car down. Maybe if excessively long, push the front end up and thereby helping weight transfer to cause the rear of the car to squat from the additional load. Also, it seems to me that the counter rotational torque is absorbed by the ring gear in either case (fixed axle or IRS). But then I have been wrong before.

The bottom line is, I think that among other handling problems created when lowering a Bug, anti-squat should also be given serious consideration.

[AIRSICK]

Originally Posted by Sandeep

AWESOME information in this thread. I'm foregoing the camber boxes for now ... no time to complete it but will perform the TA/SP alignment mentioned.

I will reort back with my findings in about 2 weeks

Sandeep

Hello SAndeep, I'm new here. Did you ever get this adjustment to work for you? I am going to try it on my 69 bug. Can you tell me for sure if the TA goes up or down for more + Camber?

I have / \ and want | |

Thanks

[Sandeep]

Yes the mod did work. You have to rotate the rear of the trailing arm up in relation to the springplate.

Sandeep

[AIRSICK]

Originally Posted by Sandeep

Yes the mod did work. You have to rotate the rear of the trailing arm up in relation to the springplate.

Sandeep

Thanks for the reply Sandeep. I took my rear suspension apart last night and fond that I have 4 bolt on the spring plate. The previous posts only mention 3.
The bolt closest to the torsion housing is threaded into the trialing arm and the bolt does not stick through the inboard spring plate. The other three bolts go in trough the inboard spring plate, through the trailing arm and out past the outboard spring plate where there is a spring washer and a nut.

This brings up two questions:

Which bolt do you want to use as the pivot point for the TA to rotate about?
Why can't you slot the TA holes vertically instead of over sizing the holes in the spring plates so you have movement in two axis? (There's not much meat around the rear spring plate slots.)

[boygenius]

I think that only the 1969 beetle had double sided spring plates while every year after that had an easier to work with single spring plate. The single spring plates only use three while the double spring plates use 4 bolts.

When you enlarge the the spring plate hole you allow the TA to rotate either counter-clockwise or clockwise which changes the camber of the rear suspension.

Am I correct???

[AIRSICK]

Thanks for the info on the spring plates. I thought that because the slots in the spring plates are elongated horizontally already for toe adjustment that I could elongate the holes in the TA vertically and that way I would have adjustment both up and down and front to rear without over sizing the spring plate holes.

But I still need to know if I use the middle bolt for the rotation center or the forward one closest to the torsion housing.

[juse]

Originally Posted by Sandeep

Yes the mod did work. You have to rotate the rear of the trailing arm up in relation to the springplate.

Sandeep

I´ll better go trough what I`ve learned from the earlier posts:
-When you lower rear IRS suspension, tires get negative camber.
-For even tire wear and good handling performance, you want to get rear tires near zero camber.
-When you put those two statements together you get:

After lowering your IRS rear suspension you need de-cambering, right?

-One way of de-cambering is to move the inner pivots of trailing arms upwards.
-This can be done with a camber-box familiar from Porsches, or maybe by fabricating a DIY mounting, higher than the original one.
-Again we couple the two earlier statements and get a question that I`m interested of:

If you move the inner IRS mount upwards the same amount that you have lowered your rear suspension, do you get stock camber???

Justin

[Panelfantastic]

My bus has bolt on IRS pivots, so I was able to loosen and turn them to get the negative camber out... but, the amount I rotated them did not seem consisitent with the amount the bus was lowered. Several inches on lowering but it took a lot less than that to correct the camber. Sorry I didn't take exact measurments at the time but I would say it was less than half the amount. No way to gaurantee that this would translate the same to your project but maybe it helps?

Jeff.

[juse]

My project is a `63 pan and a ´61 body. A mixture by coincidence, nothing special.
But I would like to get The IRS conversion welded into it before getting it sandblasted and painted.
Has anybody else any thoughts or experiences on this, "rear-lowering to pivot-lifting ratio"?
Justin