Why the RA has so big influence on steering precision/feel

[Bourgest]

Hi

Had to drive another cost reduction measure the other day. In a vehicle with a torsion beam RA with additional ARB it was suggested to remove the ARB with expected results.

So how could I theoretically explain why the steering feel is more compliant and the vehicle has poor precision. The same feeling as when you e.g. reduce low speed damping. I've been trying with weight transfer and tyre loads but didn't get anywhere. In "English" I could explain but what is the theory behind it?!?

//René

Best regards
René le Grand

http://www.bourgest.com

 

[BrianPetersen]

So the antiroll bar was removed from the torsion-beam rear axle, while front roll stiffness was maintained?

Reducing rear roll stiffness relative to the front will:
- reduce total roll stiffness, thus increasing body roll at a given cornering load, results in a loss of precision;
- increase the proportion of cornering loads due to weight transfer that the (probably already overloaded due to front wheel drive??) front wheels have to absorb, this leads to an even greater bias towards understeer.

 

[BrianPetersen]

Forgot one thing. Trailing-arm twist-axle rear suspensions generally have the rear roll center at or very close to ground level, and the twist-axle itself generally has low torsional stiffness (by design). This means that if you remove the rear antiroll bar, there will be almost no roll stiffness at the rear beyond that caused by the spring rates themselves.

You could stiffen the rear spring rates to bring back some roll stiffness, but stiffening them enough to completely compensate for the loss of the rear antiroll bar will probably make the ride quality poorer.

 

[NormPeterson]

I've been trying with weight transfer and tyre loads but didn't get anywhere. In "English" I could explain but what is the theory behind it?!?

Maybe you need to look at how these things vary with time. I suspect that the new roll frequency is somewhat lower than the original one. What happens if you add some low speed damping?


Norm

 

[GregLocock]

How about:

More load transfer at the front, so the pneumatic trail changes across the car, so the steering generates a different torque for a given Fy.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cibachrome]

Contrary to urban legend, twist beam trailing axles do NOT have roll centers at or near the ground. Only a very few do. The roll axis is defined by the location of the beam's mid-vehicle shear centerrelative to the LCA bushings. Do the figuring. That puts it very well above the ground in most cases. However, if your's is in fact one of the very few designs with a low roll center, you also probably have quite a big of roll oversteer in the rear suspension by the very nature of the mechanics of these axles. Allowing the vehicle to roll more per g revealed the ugly truth about the effects of roll oversteer in a rear suspension design (and its effect on steering precision). Its also quite funny to listen to the grand wizards at a famous Euro auto company (Still Ain't A BMW) claiming to have roll oversteer designed in for improved handling, only to be shown over and over and over again by KFC tests that it just ain't so...

 

[GregLocock]

What's a KFC test? Does it involve getting greasy fingers?


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

It sounds like finger licking good test to me.

To pass the car has to have a really spicy handling trait.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[cibachrome]

A KFC test refers to a K&C test for which our rear roll oversteering brethren are still is total denial about the often repeated results. Maybe just one more test will reveal the truth of their religious zeal to hype roll oversteer. In fact, we sometimes have found that one wheel has 0.0+ roll understeer and the other has 0.0- roll understeer. The benefits of a non-rigid axle and a Panhard bar at just the right trim. With some clever semantic manipulation, these results become 0.0- roll oversteer and 0.0+ roll oversteer, leaving them with happy faces....

 

[BrianPetersen]

I'm still at a loss as to what the "K" and "C" stand for ... and now I'm at a loss as to what's the significance of having a "+" or "-" following a "0.0". I read that as "zero". Curiosity has been aroused. Explain?

 

[Bourgest]

KFC= Kentucky Fried Chicken
K&C= Kinematics and Compliance

The difference between -0.0 and +0.0 I'm not so sure of either. + is toe-in and - is toe-out.

Thanks for your answers though. I think also it has a lot to do with the time delay in lateral acceleration build up between FA and RA.

Best regards
René le Grand

http://www.bourgest.com

 

[GregLocock]

SAAB have always had weird ideas about rollcentres, and some other GM departments have got infected with the same disease. I listen politely. That's not to say we don't have our own shibboleths, but they are at least in line with mainstream experience.







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kenweise]

You may want a new twistbeam that doesn't use a torsion bar to gain roll stiffness. Give Benteler Automotive a call. Their beams are typically lighter as well. They use a crushed center profile.

WRT roll center, the Benteler beam may have a problem with it being too HIGH. You will need to investigate it.

Ken

 

[GregLocock]

The only Bentler twistbeam I am familiar with has a torsion bar running though the cross beam. I can't see why this is a bad idea as it offers a simple way to tune the same twistbeam for different ride themes.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.