Torsion beam roll centre 2

[BrianGar]

Or sometimes called twist beam.

How is the roll centre calculated?

Google is failing me, as is Fred Puhn, Ive been looking now for days.

I hope this is not a stupid question, may be the reason Im not finding an answer!

Is it at wheel centre height?

Is it at pivot height?

Is it somewhere else?

Id also like to know the calculation if the 'trailing' arm back to stub axle is not directly inline with the front pivot bushing, as in, trailing up/down as it extends to the rear when viewed from the side.

Thanks in advance.

BG.

Beam in question below,

 

[autogyro46]

Here are two references I came across. The first one is based on an ADAMS simulation and is pasted below:

http://www.mscsoftware.com/support/library/conf/adams/euro/1996/EURO9604.PDF

the second one is more "hands on" and provides some simple calculations which can also be applied to de Dions, like the old pre-war Mercedes Grand Prix cars, and the infamous Rover 2000 to name two. It's in the attachemnt
((eng-tips seem to let attach only one link at a time)

Hope this helps.

 

[BrianGar]

It helped loads. The second link is what Im going to do. I have the beam dimensions/locations here beside me and a pen full of Indian Ink ready to go

Thanks a million, I dont know why I could not find the second link, I did find the first one.

To the drawing board...

BG

 

[autogyro46]

Aahh.. Old School!
I'm at mine this weekend too. (I use pencils, however..well.. actually, more like erasers.)

 

[BrianGar]

Indeed very retro!!

I was drawing a suspension the last day on my room wall, and the intersection points of two lines ended up following on into the other room which involved removing a door and some paintings to pull a string line. LOL.

The joys of the pen! Still...never crashes, breaks down, or gets a virus! Except of course, when I get all those symptoms!!

Brian!

 

[NormPeterson]

I'm pretty sure that the Millikens' RCVD book has two or three slightly different versions of twist beam illustrated.


Norm

 

[cibachrome]

SAE Paper: "The Design of Trailing Twist Axles" by Terry Satchell. It's a very simple analytically defined location as is the roll by steer coefficent. No need to do any handwaving or flashy references. Just simple math...

 

[cibachrome]

More detail..

The Design of Trailing Twist Axles

Date Published: 1981-02-01Paper Number: 810420



Author(s):

Terry L. Satchell - Pontiac Motor Div. General Motors Corp

Trailing twist axles represent a relatively new type of rear suspension system that is becoming more common with the increasing number of front wheel drive vehicles.

The design of the axle structure must account for the complex interactions of the placement and type of sub-components used to meet the load carrying, geometric, and roll control requirements of the suspension. The crossbeam placement is the primary variable in controlling suspension parameters.

 

[GregLocock]

Hah, good catch, SAE 810420

Well i'll wait for the photocopy before deciding how much handwaving is involved.

Three questions

1) will the properties of the bushes affect the RCH?
2) Will the torsional stiffness of the beam affect the RCH?
3) will the tire vertical stiffness affect the RCH?



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[cibachrome]

I do not believe the bushing, beam or tire properties could be changed enough to measureably impact the vehicle roll gain unless there are special cases: The VW "thinking Bushing" (remember that ploy?) Wedges in the bushings that were supposedly lateral load dependent. Trouble is, no real effect ever showed up in K&C tests of properties vs lateral load cases. Oops. Since the bushing stiffnesses control the amount of lateral force oversteer, a very soft bushing would have horrific cornering results. Yes their are voided bushings stuffed in in some cars because the ride is so bad with a solid bushing. Also, since these are generally FWD suspensions, the majority of roll gain (influenced by RCH obviously) is controlled by the front. Roll stiffness is another effect to consider, though. The beam properties have a very great influence on the steer by roll (roll steer) coefficient since the warping properties of the cross beam are the mechanics of the controll arm deflection. This situaton greatly depends on whether the beam is open or closed section, though. The Opel/ VW "V" beams with the nose down had roll oversteer, requiring special, high cornering stiffness tires to give the car reasonable handling. The US versions with the nose up, had a reasonable amount of roll steer understeer which allowed lower stiffness (and lower rolling resistance) tires to be utilized. The rotational orientation of the V-beam had some major structural and durability influences, which Terry and a few others were able to solve. Obviously the weight of the vehicle is a big factor in the RCH and roll steer and roll stiffness coefficients, because the need for a Panhard bar becomes evident when lateral deflections/loads become higher. The track bar, crossbeam, and control arm mechanism is overconstrained. so the arm flexibiulity now is a factor as well.

But, the ability to have a decent, load dependent roll understeer coefficient (as the payload increases, the total vehicle understeer can essentially be maintained constant) makes this suspension design quite attractive.

Still, the ability to sense and measure the effects on the amount of roll is so dwarfed by the influence the same parameters have on lateral force steer and camber and roll steer, make the study of such things a good academic exercise only, IMHO.

The coolest thing you can do to teach new handling evaluators is to connect the cross beam of a twist axle to the control arms using something similar to a pipe flange using, let's say 6 variable bolt positions. By rotating the V beam around these 6 positions and driving the car with the same tires, your Grandmother could tell the difference bewteen the best and the worst case. Anybody who claims roll oversteer is good for a car's handling rating will have to admit the error in their thinking.

I'm a firm believer that a twist axle would make a very nice package for a small pickup truck because of the load dependent geometric properties. Having a constant steering gain truck, no matter what the load, is a nice, safe package. Just don't put in Level Control to screw up the advantages...

 

[BrianGar]

Greg,

''Three questions

1) will the properties of the bushes affect the RCH?
2) Will the torsional stiffness of the beam affect the RCH?
3) will the tire vertical stiffness affect the RCH?''

You really did have to ruin it and throw all that into the mix didnt you...

cibachrome, Ill have to digest all that over coffee later, thank you.

Brian,