Passive rear steering

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batwad
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Joined: 26 Aug 2003, 16:37

Passive rear steering

Post by batwad » 25 Nov 2004, 22:32

So I got to wondering: how does that work? Which part "passively steers"? Is it the whole sub-frame, just the stub axles or what?
What are the disadvantages of this setup?
Educate me please [:D]

tomsheppard
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Joined: 19 Dec 2002, 15:46

Post by tomsheppard » 25 Nov 2004, 22:51

Easiest way of explaining it is the Cordiano nee Chapman strut.
Assume that the rear wheels toe in and that they have negative camber.
As the body rolls, the toe in becomes greater on the outer wheel, increasing the turn into the bend.
The disadvantages of the setup are a slight increase in rolling resistance and tyre wear.

Dave Burns
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Joined: 14 May 2001, 05:30

Post by Dave Burns » 26 Nov 2004, 01:55

The popular fallacy is that the subframe some how waggles about on big fat rubber mountings, wonder what norp of a motoring journalist started that one off.
Dave

VisaGTi16v
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Post by VisaGTi16v » 26 Nov 2004, 02:32

heh thats what I always thought after someone telling me. So how exactly does the rear tow in increase? there must be something flexible on the subframe that moves to allow this or is it just with the natural movement of the arm upwards and so the toe in will be greater at a higher speed on the same corner/turn angle?

tomsheppard
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Post by tomsheppard » 26 Nov 2004, 04:31

Yes, the mounting angle of the rear suspension allows the toe to change with body roll.

alan s
RIP 2010
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x 5

Post by alan s » 26 Nov 2004, 06:38

I had passive rear steering on my BX, but I fitted a new set of swing arm bearings and got rid of it!![:D][:D][:o)][:o)]
Alan S [;)][:p]

DoubleChevron
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Joined: 22 Sep 2003, 18:06

Post by DoubleChevron » 26 Nov 2004, 09:07

Yep,
only Alans passive rear steering also worked when driving in a straight line Image Image Image Image Image
seeya,
Shane L.

tomsheppard
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Post by tomsheppard » 26 Nov 2004, 14:01

Alan,I was trying to avoid that one! I always found it to be rather active, actually.

David W
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Joined: 30 Apr 2001, 17:49

Post by David W » 29 Nov 2004, 14:21

Guys,
You are joking????
<<The popular fallacy is that the subframe some how waggles about on big fat rubber mountings, wonder what norp of a motoring journalist started that one off.>>
Could it be the designers at Citroen who told the world that is exactly how it works...actually I think you know it anyway Dave?
If anyone has ever changed rear subframe mounts you'll see the odd construction that allows this flex to move the whole axle assy. I think it is under 5deg steer to get the effect we experience.
I can't point you to a link on the Internet for a description/diagram but there is one for certain in the earlier Xantia brochures.
David

Dave Burns
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Joined: 14 May 2001, 05:30

Post by Dave Burns » 30 Nov 2004, 04:11

Hows your trig. DJW, ok so ther's going to be an imaginary pivotal point somewhere between the rear wheels, don't know how wide the axle is but at a guess 1500mm from outer edge of each wheel.
You rekon rear steer is under 5 degrees, it sure as hell wants to be because 5 degrees would give some 65mm of fore and aft movement of the wheels as the whole subframe waggles from side to side as the car corneres, each wheel would need scope to move 130mm in total, so you can't get anything approaching 5 degrees by letting the subframe waggle about on lumps of rubber.
A mere 1 degree of steer would produce about 13mm of wheel movement in each direction, 26mm in total, still far too much, a more realistic figure would be about 8mm total movement, that would come from only 0.3 of a degree, whats 0.3 of a degree of rear steer going to do for you.
Dave

David W
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Joined: 30 Apr 2001, 17:49

Post by David W » 30 Nov 2004, 05:13

Errr...I guessed or sort of remembered *under* 5deg from something I'd read somewhere. The only real point I was making is that it is the bushes that give this tiny movement in degrees that has such a good effect...more so on the Xantia I reckon.
I have a picture in a 1994 Xantia brochure in front of me where they represent one rear wheel moving forward of its static posn and the other side rearwards....it is shown to produce a twist to the whole axle giving the rear steer effect.
I'd always believed Citroen in this matter...supported by the construction of the bushes when you look at them... and the fact that shot bushes will allow a serious wander at speed due to the uncontrolled extra movement.
David

tomsheppard
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Joined: 19 Dec 2002, 15:46

Post by tomsheppard » 30 Nov 2004, 07:26

Yes, the degree of movement on the axle is tiny as the majority of steering is done by the front wheels. The passive setup only improves stability. Chapman's strut, copied by the Fiat 131 Rally (Trawl for it because there is a good decription somewhere) is an extremely subtle piece of geometery relying to a great extent upon the exact angle of the top strut mount. Genuine Lotus Elevens (Of which over 300 of the original 164 survive)are verified by this amongst other subtleties.