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ABS and negative scrub radius

Discussion in 'Technical Q&A' started by snj5, May 17, 2004.

  1. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
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    Russ Turner
    Well, if you clicked on this thread with that thrilling title, you are a tech guy...

    Anyway, was reading about ABS and the author made the statement that ABS required a negative scrub radius to work. I know this is borne out in Ferrari Practice when they went to the large offset wheels on the ABS 328s to enable a negative/zero scrub radius.

    While he did not go into why ABS needs a negative scrub, I'm guessing that with much of a positive scrub a differential application of the ABS left to right would tend to yank the steering wheel away and increase instability, while a zero or negative scrub would be more tolerant of L/R differential braking caused by ABS.

    Does this sound right?

    In addition, I would also guess that the Mondial came with almost zero scrub to help with the steering effort, while the 308 series carried ( by my rough calculation - please correct me if I missed) about 7/8 - 1" of positive scrub radius for road feel and bite. Would make sense as I have often heard that 308s have some noticable bump steer.

    Any clarification to help understanding ABS geometry better would be great
    Many thanks
    Russ
     
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  3. don_xvi

    don_xvi F1 Rookie

    Nov 1, 2003
    2,920
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    Don the 16th
    Your reasoning is correct, but ABS doesn't dictate scrub radius; you don't have to have 0 or negative scrub, but the more scrub, the more it will yank the wheel around on split mu surfaces. So then you have to reduce the ABS efficiency on a split surface like that, increasing stopping distance. It's all tradeoffs......
     
  4. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    Russ Turner
    Many thanks - While I have read that increased scrub radius (e.g. using front wheel spacers) will increase bump steer, I've also read that turn in sharpens a little due to weight transfer characteristics. The author said that a little scrub radius - 1 inch or so - is good for driver feedback and control, especially at the limit. Have also seen some printed Mustang specs with scrub radii of 4 inches. Anyone with practical experience with putting front spacers or decreased offset wheels and the qualitative, if any, effect on handling?
    best
    rt
     
  5. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,005
    With a scrub radius (either positive or negative) when the ABS kicks in to modulate braking AND the steering wheel is turned (from straight), the ABSing will induce kickbacks into the steering wheel and degrade the steering precision at exactly the time you need precision.

    In the F348/F355 both front and rear contact patch loads are centered in the kingpin inclination of the suspension. This means that the tire does not toe out or in durring application of power or brakes. With offset in the rear, power would toe the contact patch in and brakes would toe the contact patch out.

    Very low scrub is an outgrowth of the larger wheels (17"+) allowing bigger hubs inside the wheel which in turn allow longer suspension arms, and bigger brakes, and lighter unsprun weight. This cleaned up the airflow around the inside of the rear wheels in race cars.

    At the front you want a little negative scrup or a little positive scrub but not zero. This scrub. Large scrub will make it very hard to manipulate the steering when applying large (braking) loads at the front. Negative scrub induces understeer and calmness, while positive scrub induces oversteer and liveliness. When the ABS is doing its thing you most definately want calmness as the situation is already "lively".

    Note: on suspensions with monoball pivot points (not bushings) these rules do not apply unless the suspension arms are weak (not a Ferrari issue) since deflection of the bushings has been removed and no toe effects are incuded by high load effects at the contact patch.

    Finally, the ABS in F355 and newer is not a full on/full off system like found in many lesser cars. The Ferrari system bleads down until it senses wheel acceleration, then bleads back up until it senses wheel deceleration back and forth. This is a "gentle" manipulation rather than the harsh application found elsewhere. The gentleness of this system preserves the steering feel and supension dynamics in difficult (race) situations where ABS is required all the time for maximum performance.
     
  6. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    Russ Turner

    Thanks! - great informative instruction.
    best
    rt
     
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  8. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
    2,149
    way north california
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    chris morse
    I cannot stand the fascination any longer, I am going to take the a arms down to the shop and continue with the bushing replacement tonight.
    BTW, has anyone charted the bump steer curves of the 308???
    Are there any other Carroll Smith fans out there???

    with high regard,
    chris
     
  9. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,005
    I am a Carroll Smith fan (may he rest in peace).
     
  10. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
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    Philip
    Corky Bell (does he need an intro?) put me on to Carroll's books. I just bought the set. What great (essential) reading.
    Philip
     
  11. Prova7

    Prova7 Formula Junior

    Nov 17, 2003
    254
    Dallas, TX
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    DamonB
    I'm a big Carroll Smith fan and racecar suspension fan in general.

    Scrub radius also plays a big part in steering feel along with other factors such as steering axis inclination, caster and trail. If you had zero scrub, no trail and no caster there is no resultant force about the kingpin to present feedback to the steering wheel; in this case the driver would have no feel through the steering wheel for the front tires.

    It is all too easy to completely screw up the scrub radius on a car by fitting wheels with an improper offset. Too many people feel that if a wheel tire combo will bolt on and not rub anything then it must be ok and they look no further.

    Racecar Engineering has had some great articles about steering geometry in the past year or so. Mark Ortiz is a contributing author and most of his articles for the magazine are all available for reading here:

    http://www.auto-ware.com/ubbthreads/postlist.php?Cat=&Board=knowledgebase
     
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  13. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
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    Russ Turner
    Interesting reading; will roust up some some Carroll Smith books.
    Anyone know some examples of scrub radii values?

    Many thanks to everyone.
     
  14. ferrarifixer

    ferrarifixer F1 Veteran
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    Jul 22, 2003
    8,520
    Melbourne
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    Phil Hughes
    The 360 N-GT has 75mm greater front track than a 360C. This is achieved only from greater wheel offset, with no other changes. Of course, this makes the scrub radius 37.5mm larger.

    You may think that it would feel significantly different, but it doesn't. Our Aussie cars have run with or without the ABS used and the feedback remains the same.
     

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