SAI and wheelbase questions | FerrariChat
Buy

SAI and wheelbase questions

Discussion in 'Technical Q&A' started by fastradio, Sep 23, 2008.

This site may earn a commission from merchant affiliate links, including eBay, Amazon, Skimlinks, and others.

  1. fastradio

    fastradio F1 Rookie
    BANNED Professional Ferrari Technician

    Apr 26, 2006
    3,664
    New England
    Full Name:
    David Feinberg
    #1 fastradio, Sep 23, 2008
    I'm working with a Ferrari that appear to have a handling problem, although I'm not 100% sure if "it's real" or imagined. The car does appear to turn differently, right to left...versus left to right.

    All of the suspension wear components are new...and the alignment specs. are spot on. Tire wear is perfect.

    However, two potential red flags:

    The is an 8 degree difference in SAI, side to side. One side is at the factory specs, the other 8 degree greater.
    The wheelbase on the car is different by .236"

    The SAI difference points potentially to a problem in the front suspension, yet all components appear to be in excellent condition, with no evidence of any collision or frame damage.

    Q:
    Could the wheelbase difference account for the SAI difference?
    Is a .236" difference "significant", if the factory specs are 98"?


    Thanks,
    David
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,931
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #2 Steve Magnusson, Sep 23, 2008
    Are you sure about the numbers, or did you mis-type? An 8 deg difference in SAI is unbelievably large, and if the spindles are quasi-equal mirror images, you couldn't get the camber to be OK at the same time. Did you mean 0.8 deg or 8 minutes?
     
  3. fastradio

    fastradio F1 Rookie
    BANNED Professional Ferrari Technician

    Apr 26, 2006
    3,664
    New England
    Full Name:
    David Feinberg
    #3 fastradio, Sep 23, 2008
    Steve,

    Sorry to say, no typo.

    The exact difference is 7 degrees, 34 minutes....and the camber side to side is identical! The shim pack is slightly different on the two sides, but not more or less than I've seen on a few hundred other Ferrari's.

    I had replaced, awhile back, both the spindle and the front lower control arms on the side in question, as diagonal measurements indicated a slight difference which I believe to be corrected. If it weren't for the alignment machine giving me the SAI numbers, driving the car wouldn't reveal the potential magnitude of the problem.

    David
     
  4. jm3

    jm3 F1 Rookie

    Oct 3, 2002
    4,364
    United States
    Full Name:
    JM3
    #4 jm3, Sep 23, 2008
    The machine sounds like it is wrong.

    Post all of the specs and several us can probably help.


    Depending on what Ferrari you are talking about, the wheelbase IS adjustable independantly of camber and caster, so we need to know what car too.


    PS the alignment shop should know this, they should not be apprenticing on your car.


    Jay Morris
     
  5. jm3

    jm3 F1 Rookie

    Oct 3, 2002
    4,364
    United States
    Full Name:
    JM3
    #5 jm3, Sep 23, 2008
    PS the rear is adjustable too, so when you are measuring wheelbase are you measuring from front wheel to rear wheel or to a hard point?


    Jay
     
  6. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,931
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #6 Steve Magnusson, Sep 23, 2008
    David -- Amazing, but I can't believe that it's possible unless you have different componets on each side (i.e., different spindles and at least 1 A-arm of very different length). My argument would be that, assuming the upper and lower A-arm are about 10" apart (and all the "error" is in 1 A-arm) this would require the length of 1 A-arm to be ~1.25" different to get an 8 deg difference in the steering angle inclination. Have you tried just making rough measurements of the distance from the upper A-arm pivot axis to the center of the upper ball joint and the distance from the lower A-arm pivot axis to the center of the lower ball joint on each side. It's not like you are looking for a 1/16" difference -- ~1.25" should even be obvious just using a tape measure or ruler and eyeballing it.

    What model?
     
  7. fastradio

    fastradio F1 Rookie
    BANNED Professional Ferrari Technician

    Apr 26, 2006
    3,664
    New England
    Full Name:
    David Feinberg
    #7 fastradio, Sep 23, 2008
    Steve,

    This one has me confused, as well. The car has been on multiple alignment machines, of different types....
    Visually, there is no obvious bent components. If I understand SAI correctly, it is a calculated angle based on KPI and Caster, with a 20 degree turn-in. Or, in using the factory numbers on a BB of 9 for the KPI and 4 for the caster, should yeild a number of about 13 degrees. I read 13 on one side...and 21 on the other. What's very odd is that the side with an SAI of 21 has the new spindle...If my thinking and your input are correct, this is pointing to the upper control arms?

    I did take measurments last year, comparing the relative position of the lower ball joints, side to side, which resulting in the conclusion that the lower control arms were slightly tweaked. This repair was verified by diagonal measurements to the rear suspension. I did not take any measurements on the upper arms, as they are much shorted and showed no evidence of "bends" at the chassis mounting points.

    The car in question is a Boxer, which has "non-stock", but identical side to side alignment settings...hence I must use comparitive numbers to determine if a problem exists, or not. I have verified that the front and rear suspensions are in perfect alignment with one another, as the thrust line offset is less than 0.1mm.

    David
     
  8. jm3

    jm3 F1 Rookie

    Oct 3, 2002
    4,364
    United States
    Full Name:
    JM3
    #8 jm3, Sep 23, 2008
    with the wheel on the car, measure from the rotor to the lower ball joint. compare. (this is easy)

    do the same on the upper ball joint,you can probably do this through the wheel.

    If the cambers are the same LR, and the ball joints are the same distance from the rotor, then the KPI has to be the same.

    Have any of the print outs had "toe out in turns" printed on the sheet anywhere? thats what it usually says for the ackerman

    Jay Morris
     
  9. Newman

    Newman F1 World Champ
    Consultant Owner Professional Ferrari Technician

    Dec 26, 2001
    14,510
    Canada
    Full Name:
    Newman
    #9 Newman, Sep 23, 2008
    You have to measure the upper control arm pick-up points, something is bent, take out the tub in the front if you have to and measure everything. This setback and SAI issue is the cause of the instability for sure. The frames are surprisingly accurate considering the garage they were made in so something has changed since.
     
  10. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,931
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #10 Steve Magnusson, Sep 23, 2008
    Last edited: Sep 23, 2008
    You are calling the SAI something different than Jay or I. We are both calling SAI = King Pin Inclination with the steering pointed straight ahead (so the caster angle doesn't get into it); whereas; you are doing a more complete addition of the KPI and the caster angle at some amount of turning -- do you have a reference link explaining what physical angle you are measuring or showing a diagram?

    I don't think the KPI and caster angle would just add straight together (because they lie in perpendicular planes), and it might be asymmetric depending on the direction of turning -- i.e., turn 20 deg right and the LH side reads "A" deg while the RH side reads "B" deg; turn 20 deg left and the LH side reads "B" deg while the RH side reads "A" deg. Do you have measurements for both turning directions?

    I do like Jay's suggestion to just try to confirm that the KPI is roughly the same on each side by measuring the ball joint locations relative to the brake rotors (which is the same thing I was trying to suggest by measuring the effective lengths of the A-arms) -- if it truly was different by 8 deg, you would detect it easy.
     
  11. fastradio

    fastradio F1 Rookie
    BANNED Professional Ferrari Technician

    Apr 26, 2006
    3,664
    New England
    Full Name:
    David Feinberg
    #11 fastradio, Sep 23, 2008
    Last edited: Sep 23, 2008
    Yes...It appears that I am calling SAI something that is not the industry standard terminology. I cannot recall where I learned, or thought I learned that SAI was different than KPI....and that there was some math involved which revolved around a caster. angle calculation. Perhaps my memory is going away...as my ASE studies were about 20 years ago!

    Regardless, the alignment printout, from one of those Hunter Laser Machines specifically states SAI. So...you guys are on the same page, that I'm not. But I understand what both you and Jay are saying.

    If the ball joints to rotor distances are the same, left to right, then the spindles are ruled out of the equation. Then the next step would be to pull the front inner trim pieces and measure/imspect the welded upper control arm mounting points.

    As to more meaningful information on the alignment printout, such as toe-out on turns...There is none.

    Looks like I've got a bit of work ahead of me...
     
  12. Newman

    Newman F1 World Champ
    Consultant Owner Professional Ferrari Technician

    Dec 26, 2001
    14,510
    Canada
    Full Name:
    Newman
    #12 Newman, Sep 23, 2008
    The alignment machine will show toe out on turns, I guess they are just showing you the static straight ahead measurements. Someone that knows what they are doing would see that especially when the screen goes into the red when they swing the wheels left to right. Its easier to put it on an alignment rack again to see the actual readings full sweep then yank the tub to measure the frame.
     
  13. fastradio

    fastradio F1 Rookie
    BANNED Professional Ferrari Technician

    Apr 26, 2006
    3,664
    New England
    Full Name:
    David Feinberg
    #13 fastradio, Oct 4, 2008
    After studying the chassis dimensional drawings for the Boxer, it would appear that the upper control arm pick-up points are not centered on the chassis. One side is 10mm closer to the vehicle centerline than the other. Presuming the control arms are the same length, left to right, maybe the SAI difference the aligment machines are measuring is correct. Although geometry was never my best subject, as the upper control arm moves inward, the SAI would increase.

    On the other hand, maybe the control arms, right to left, are different lengths which would negate my previous theory.

    I'm having a friend with another Boxer get the SAI numbers off of his car to compare vehicles...

    Stay tuned,
    David
     
  14. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,931
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #14 Steve Magnusson, Oct 4, 2008
    Wow, that seems strange -- can you post or give a link?
     
  15. fastradio

    fastradio F1 Rookie
    BANNED Professional Ferrari Technician

    Apr 26, 2006
    3,664
    New England
    Full Name:
    David Feinberg
    #15 fastradio, Oct 4, 2008
    Last edited by a moderator: Sep 7, 2017
    Steve,

    I agree, it's a bit odd. The first picture is the front suspension, the second is the rear. Also, notice that the hight of the upper control arm pick-up points is different. I would suspect that compensating for "road crown" may come into play here?

    David
    Image Unavailable, Please Login
    Image Unavailable, Please Login
     
  16. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,931
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #16 Steve Magnusson, Oct 4, 2008
    Last edited: Oct 4, 2008
    David -- Can you post the other view showing where the Section A-A is taken? Sometimes they "jog" these section lines so the two sides (in the same view) aren't really showing the same thing -- i.e., looks to me like the thing labeled "2" is the upper A-arm mounting point, but the thing labeled "1" is the upper shock mounting point -- or vice-a-versa. And the things labeled "3" are both lower A-arm mounting points, but one is the forwardmost one and the other is the rearward one of the set.
     
  17. fastradio

    fastradio F1 Rookie
    BANNED Professional Ferrari Technician

    Apr 26, 2006
    3,664
    New England
    Full Name:
    David Feinberg
    #17 fastradio, Oct 4, 2008
    Last edited by a moderator: Sep 7, 2017
    The front end pick-up points are symetrical. One dimension is for the shock upper eyes, the other for the upper control arm pick-ups. The are offset in the chassis, like the drawing shows. Like I said, geometry and I never got along too well. Thanks for pointing this out.

    David
    Image Unavailable, Please Login
    Image Unavailable, Please Login
     
  18. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,931
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #18 Steve Magnusson, Oct 4, 2008
    Phew! You had me worried there for a second ;) -- it would be very, very unusual for a road car to be asymmetric L-R. Keep digging, I'm still very interested in your "alignment" problem, and would like to know more about what you find -- TIA.
     

Share This Page