IRS alignment setting for wide rear tires

[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.

[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.

[Bruce2]

Oh yes, of course! There will be a force pushing the TA down due to the forward force on the TA. And the higher the front pivot, the greater the force. When you break it down to the horiz and vertical components its clear.

Ladder bars on American cars do lift the rear of the car. They do it by planting the rear end down harder. Have you ever noticed how some of them don't squat at the line?

[RonRyon]

Bruce, that is what I was trying to say when I said the thing about drag cars being lifted as they come off the line. So, I guess you agree that the angle of the TA can have an anti-squat effect? I have personally observed this on the previous cars that I have built. The benefits were very noticeable and very positive not only for straight-line acceleration but also for overall handling.

[Bruce2]

The only problem I see is that when you raise the inner pivot, you will get positive camber. Raising both will require you to raise the whole torsion housing (a big job). Then it also raises the engine/transmission.

[RonRyon]

The plan would be to raise both the same amount. I'm not using the torsion bars so it will be less complicated. I'll let ytou know how it goes, but don't hold your breath, I move pretty slow these days.

[Sandeep]

Well, I've completed Bruce2's method of alignment to get +ve camber and I am happy to report that it works !

I've only completed the mod on the passengers side rear and went from -2.93 deg to -1.38 deg measured using a bubble level and some trig. I filed the two rear bolt holes from slots into what looks like a "D" rotated 90 deg CCW. I could get more +ve camber if I wanted but my arms got tired from all the filing work !

So thanks again Bruce2 for the suggestion and direction as you've probably saved me atleast $1000 in rear tires. :bowdown: :haveadrin

Sandeep

[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