Negative camber on rear wheels of FWD - why?
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  1. #1
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    Default Negative camber on rear wheels of FWD - why?

    Hi All

    I see so many modern FWD cars, my own included, running what seems to be 1-2deg neg camber on the rear wheels. That would mean they likely dial in some toe out as well.

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    What are the advantages other than higher tyre wear?


    P

  2. #2
    1000+ Posts cam85's Avatar
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    Better cornering grip. Safer for the average driver. High speed stability.
    Sounds ok?
    94 205 Gti Classic #9
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    http://www.aussiefrogs.com/forum/res...-race-car.html

  3. #3
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    Yeah I guess I assumed that. But how does having neg camber on the rear achieve those things?

    In many cases there's more neg on the rear than on the front. I've thought that would simply induce understeer rather than improving cornering grip.

    The only thing I can think of is that with 1.5 deg neg it allows a rear toe out of, say, 1.5mm and this allows for better turn-in. If that's the case its seems like a pretty ham-fisted way of doing it.

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    Default Small cars

    The original Mk 1 Renault 5 ran a small amount of positive camber at the rear. It would swap ends very quickly being such a small wheel base.
    The second series from about 1980 had different rear trailing arms with a couple/three degrees of negative camber which was to prevent it spinning on a six-pence.
    I can guarantee the positive camber cars had this trait. I've not yet driven the later model variant to compare.

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  5. #5
    I might be slow... DRTDVL's Avatar
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    Have a look at Sandy's post here: http://forum.205gtidrivers.com/index...howtopic=56644 <--- camber and toe settings for various PSA cars.

    The below is most likely completely wrong.... Warning...
    I'd assume toe in on the rear of a front wheel drive to make the rear drive to the middle of the front wheels increasing it's stability in a straight line, it's camber will provide some improvements in corner grip

  6. #6
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    Quote Originally Posted by Exfrogger View Post
    Hi All

    I see so many modern FWD cars, my own included, running what seems to be 1-2deg neg camber on the rear wheels. That would mean they likely dial in some toe out as well.

    What are the advantages other than higher tyre wear?


    P
    Here are the answers to your questions:

    Most FWD cars, for reasons of cost, do not run unequal length wishbones on the rear. The obvious exception is Honda. The others run either McPherson struts, or trailing arms, which both exhibit 1:1 camber gain in roll - i.e. for every degree of roll, the wheel gains one degree of (positive) camber. Some static negative camber delays the transfer into positive camber untill the roll angle reaches the same angle as the amount of static negative camber. Positive camber in roll is bad, because the positive camber, and the forces acting on the tyre, act together to distort and minimise the contact patch, usually resulting in violent understeer, when on the front wheels (MGTC), or oversteer, when on the rear wheels (VW, Triumph Herald, both of which ran swing axles, which exhibit even worse camber gain than struts or trailing arms, but ARE the classic examples of rear wheel positive camber oversteer.)

    Your assumption about the toe out is incorrect. Toe out on the rear of any car, FWD or RWD, is unstable and dangerous. More or less, the ONLY cars to run toe out on the rear are Autocrossers (American for Motorkhana), where the drivers are looking for a very "loose"" car.

    Negative camber with toe out will result in heavy wear to the inside of the tyre. Toe in offsets the negative camber, resulting in negligible wear, and improved stability.

    The only FWD car I can think of which shows noticeable negative camber, out of the showroom, is the Mercedes "A" class, as a result of the infamouse "moose" test. There are, no doubt, others, but I don't notice them. A 205 or 306, and probably the later replacements, showing noticeable negative camber on the rear, is probably worn out.

    Tim

  7. #7
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    Thanks all

    DRT that was a handy link.

    Tim, thanks for the detailed story. I'm a little the wiser for the input. Do check out some of the fresh-off-the-showroom-floor FWD cars. The Golf in particular is quite pronounced.

    And yes, I did have the toe around the wrong way...

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    Very interesting stuff and great explanation Tim.

    Nice to see all the numbers there, would love to see the 405's toe as well.

    Not just wheelbase that makes my 306 feel safer than the old 205!

  9. #9
    Fellow Frogger! Karoshi's Avatar
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    Default Homework

    Year--Model--Rear toe in (mm)--Rear negative camber(degrees/minutes)

    1985-92--205 GTi 1.6+1.9/Rallye--3.60--0/50
    Wheelbase 2420 mm 95.3 in
    1985-92--309 base--3.10--0/50
    Wheelbase 2470 mm 97.2 in

    1986-90--309GTi--3.80--0/50
    Wheelbase 2470 mm 97.2 in

    1990-93--309GTi+GTi16S--5.00--1/15
    Wheelbase 2470 mm 97.2 in

    1994on--306 S16+GTi6--4.20--1/20
    Wheelbase 2580 mm 101.6 in

    1997on--306 1.8 16v--3.40--1/20
    Wheelbase 2580 mm 101.6 in

    1998on Xsara VTS--5.00--1/20
    Wheelbase 2540 mm 100 in

    1993 405mi16--3.00--1/20
    Wheelbase 2669 mm 105.1 in
    Last edited by Karoshi; 28th April 2011 at 11:51 PM.

  10. #10
    1000+ Posts schlitzaugen's Avatar
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    Just to add one more, the Opel Calibra has a very obvious negative rear camber/toe-in on the rear and yes, for the reasons explained above.

    In actual fact, rear toe in diminishes understeer on FWD cars as well.

    More cars have it today probably because it has become cheaper to build especially as most cars have done away with the solid rear beam suspension.
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  11. #11
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    I noticed that the updated Cruze runs the same transverse watts link suspension as the newest Astra.

    I assume the idea is to control transverse movements so you can run softer bushes in the torsion beam mount but still control the movements that affect toe change.

    It could be adapted to pretty much any torsion beam if the gains were worth it. Anybody seen anything similar?


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