one for the gurus, i'm a little confused, as usual...

[Kozy]

Right, I AM looking into this, I'm just not well versed in live or swing axles (I only know double wishbones), and I am actually only just getting around to properly working with FBDs in my OU course. Hopefully I can make sense of it all.

[KevinK2]

TheLastPost said:

... As you suggest, the jacking moments are in opposite directions on either side of the car but, importantly, the tyre thrusts are different on each side (the more heavily loaded outside wheel generating a lot more thrust than the lightly loaded inside wheel), so the difference between them will result in a net vertical force, jacking the car up or down. This is why determination of the Force Based Roll Centre requires a knowledge of the distribution of forces between the car's tyres (which can be very difficult to determine accurately) and why people argue that the Geometric Roll Centre as a concept has limited value.....

I agree completely. I had a 68 GT-6 with swing axles and a transverse leaf spring, which was rigidly attached to the frame, by way of the differential case. I recall my 1st autocross, and that wierd feeling in a corner of the jacking effect. Looking in the rear view mirror, you see the background disappear, and next you just see the sky! On the road at 40mph as you ran an autocross slalom, it felt like you were "cocking" a lateral spring that wanted to shoot you back to the road center. I had heard about the "swing spring" oem fix, which placed the rear roll center at the center top of the leaf spring (at the pivot bolt). The net effect was to allow the chassis to roll at the pivot, and kept the vertical load on the tires about equal. So the lifting force at the inside tire was equal to the compressive force at the out side tire. This should yield a zero "jacking effect" as shown on FIG D. I think there is a typo error on that equation, and the mu inside the brackets should not be there.

I converted to the "swing-spring" from a Spitfire, and used the later Spitfire swing axles that increased rear track about 2". Another benefit of this method was the rear end and CG was lowered, and the wheels had abot 3 deg of neg camber. I foolishly went to my local HPDE track with no other changes, and at the sharp 50mph turn at the end of the long straight, the car would not turn at all. Went from some what balanced to mega understeer. I had to replace the aluminum pivots for the 1" front bar, with oem rubber bushings, installed with some free play. The fix was to fabticate a 14mm adjustable rear bar ... worked great and saved the track day. I had a very diverse track-box.

Edited by KevinK2 on Sunday 14th September 15:49

[KevinK2]

TheLastPost said:

It's complicated, but yes, it can. ......


... A further problem with lowered cars is that if the car has different suspension geometries front and rear, you can end up dramatically changing the roll axis inclination, which has a major effect on understeer/oversteer balance (and the steeper the roll axis slopes down toward the front of the car, the more diagonal weight transfer you get onto the front corner, promoting understeer)....

I learned suspension set-up from "how to make your car handle" by Fred Puhn, a fellow Mech'l Engineer and real racer. He discusses roll centers in detail, but does not get into "geometric weight transfer" per se. In "autocross to win", this is described as the shear moment for the sprung mass, ie for a single CG and RC at that location, it's:

total sprung lateral G force x height of CG / track width

The problem I have is making conclusions out of the slope of the roll axis. If you have the FRONT end AT the mass center, and the rear above the mass center there, you still have spring based weight transfer at the front end, using Puhn. He uses the front and rear roll centers, and front and rear masses acting there, for "geometric weight transfer". But for the "elastic" weight transfer, the chassis is assumed 10X stiffer in torsion, vs any spring or bar component stiffness. He then finds the single CG, and uses a virtual plum bob to find the singe roll center for the CG, to come up with the "roll couple" for the sprung mass. THEN, the front and rear weight transfers are proportional to the F and R roll stiffnesses. The body will roll.

Only if the roll axis is the same as the mass axis, will there be no body roll. Other than F1 type race cars, that would be a contorted suspension mod for an oem car.

Now, how does the slope of the roll axis figure in?

Thanx, KevinK2
( also single owner of a 1976 T160 Triumph Trident MC. )

[itdontgo]

TheLastPost said:

There is only one CoG for the sprung mass. There isn't one at either end. banghead

You can have as many CoGs as you like. They're a concept. You can invent one at either end and this helps you understand the effect of roll axis inclination. When considering handling it's best to think of the car as a dumbbell weight so you can see how much roll each end of the car is trying to produce given that end's roll centre

[MintSprint]

itdontgo said:

You can have as many CoGs as you like.

No, you can have as many two-dimensional mass centroids as you like (and they are all equally meaningless, if you've got a reasonably stiff chassis connecting them), but so far as centre of gravity is concerned [Sean Connery:Highlander]...there can be only one...[/Sean Connery:Highlander].

itdontgo said:

They're a concept.

They're not, you know: they're a measurable physical characteristic (Refer to Staniforth for the practical measurement of CoG position on a car).

They're a three-dimensional point and any single, rigid body has only one.

itdontgo said:

When considering handling it's best to think of the car as a dumbbell weight so you can see how much roll each end of the car is trying to produce given that end's roll centre

The dumbbell analogy is useful for understanding polar moments of inertia, but it's downright confusing and misleading when applied to roll moments (as amply demonstrated by this thread). It doesn't matter what the separate forces acting on two-dimensional centroids at either end of the car are trying to do, because they're acting through an effectively rigid structure to resolve themselves into a single force vector acting on the single centre of gravity.

To get a proper understanding of roll (and pitch) forces, you have to learn to consider both ends of the car simultaneously: you can't think about each end in isolation, and that's exactly what this nonsense with 'mass centroid axis' and separate front and rear CoG's encourages people to do.

[itdontgo]

What possessed me to come on PistonHeads?