Spring fatigue? | FerrariChat

Spring fatigue?

Discussion in 'Technical Q&A' started by JohnMH, Jan 3, 2022.

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  1. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,632
    Dubai / Bologna
    #1 JohnMH, Jan 3, 2022
    A question aimed at those who restore older cars, is there an age or mileage at which the original shock springs fatigue to the point of needing replacement?

    My youngest car (TR) is 1991, my oldest (BB) is 1978. None have high mileage, but they all seem a little soft. Dampers have been rebuilt or replaced and the rubber bushings are pretty new.
     
  2. afterburner

    afterburner F1 Rookie
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    Jun 20, 2008
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    #2 afterburner, Jan 3, 2022
    Are the ride heights / ground clearances correct?
     
  3. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,632
    Dubai / Bologna
    #3 JohnMH, Jan 3, 2022
    Good question. Visually, the stance is correct, but I am not sure what the clearances between the tops of the tires and the fenders should be.
     
  4. afterburner

    afterburner F1 Rookie
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    #4 afterburner, Jan 3, 2022
    Actually, the distance between the ground and the chassis as defined in the workshop manuals is the relevant dimension.
    The body work can be off by a lot...
     
  5. mahlerfree

    mahlerfree Karting

    Dec 19, 2019
    224
    #5 mahlerfree, Jan 3, 2022
    springs can have fatigue, and can be measured to verify by adding load on a bench and measure its hegiht change
     
  6. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
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    #6 Steve Magnusson, Jan 3, 2022
    The specs for what the spring load should be versus the spring length (for three measuring points of both the front and rear springs) are given on page F25 of the TR WSM along with the nominal spring rates (i.e., the "flexibility") if you are concerned enough to get them measured. Usually "sagging" would go along with "softness".
     
    afterburner likes this.
  7. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,632
    Dubai / Bologna
    #7 JohnMH, Jan 3, 2022
    Thanks, I had noticed that in the manual, but the information on rates is confusing. I can measure the free length easily enough, but have never encountered the units of measurement for spring rate quoted in the manual. I can find a place where I can measure the stiffness of the springs, but the answer will be in Kg/mm.

    From your experience, how often have you needed to replace springs in an old Ferrari? Who makes their springs? Are the OEM units generally pretty good quality? I doubt you could order new springs from Ferrari after 30 years.

    It is odd, the older (but lower mileage) BB springs seem 'normal' (hard to discern spring rate with those tall sidewalls), while the TR (which has a very comfortable ride) seems to want to use up so much more of its rear suspension travel when it encounters a bump. It seems the softer of the two (maybe intentionally?) In comparison, both my 42 and 40 year old Countaches (not sure what the plural of Countach is) are very firm and rigid, which is consistent with the design of the car.
     
  8. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,149
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #8 Steve Magnusson, Jan 3, 2022
    Never? ;) Has just never gotten to the top of my "this needs to be fixed or investigated" list.

    The shocks are probably frozen ;). Seriously, I don't have any information about that comparison. Free length is the least important spec. Much more important to measure the Cs load when at the Ls length and the Cd load when at the Ld length. It does seem that Ferrari managed to mangle their calculation of the spring flexibility spec in that table on page F25. Using their values for Cs, Ls, Cd, and Ld gives:

    TR front spring stiffness = 27.81 N/mm = 2.836 Kg/mm = 158.8 lb/in (this would correspond to a spring flexibility spec of 0.360 mm/daN)

    TR rear spring stiffness = 30.68 N/mm = 3.128 Kg/mm = 175.2 lb/in (this would correspond to a spring flexibility spec of 0.326 mm/daN)

    Those don't seem too crazy to me for a street car with relatively light front end and a very heavy back end (as, with two springs at the rear on each side, it's effectively 350 lb/in at the rear). Would always be interested in seeing whatever data you might get as they might have mangled the Cs, Ls, Cd, and/or Ld values on F25, too.
     
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  9. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,632
    Dubai / Bologna
    #9 JohnMH, Jan 4, 2022
    Thanks for the conversions, I can check those. Fascinating.

    Given where I live, I choose a project for each summer while the car is off the road. 2022 will be the summer of Testarossa suspension. I have access to spring testing equipment and will report back on what I learn about my springs.
     
    moysiuan likes this.
  10. Ferrarium

    Ferrarium F1 Veteran
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    #10 Ferrarium, Jan 4, 2022
    If shocks have adjustable colors to set right height I'd be careful about using bumper or fender height to determine spring sag. People can and do adjust those things all the time over the ownership of a car. It can have a spring sag then tighten it up and push the car back up, likely could happen when they're doing an alignment as well.

    I found brand new OEM springs for my 348 4006 a piece. I'm just going to replace them they're probably old and probably have some sag but they're a little bit scraped up and I don't want to powder coat them for risk of annealing them.

    When in doubt just replace them it'll look spiffy.

    Sent using FerrariChat.com mobile app
     
  11. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,632
    Dubai / Bologna
    #11 JohnMH, Jan 4, 2022
    I will remove the springs from the car and test them on the bench for spring rate. I agree, checking them on the car is not going to provide any meaningful data.
     

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