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Differential preload adjustment

Discussion in 'Technical Q&A' started by Difaz, Mar 30, 2005.

  1. Difaz

    Difaz Karting

    Mar 30, 2005
    114
    Sydney Australia
    Full Name:
    Phill
    I was wondering if anyone has any information regarding the differantial and in particular the method used to obtain the required preload.
    Does it utilize a bevel washer to hold tension on the plates and has anyone ever experimented with reducing the preload?

    The preload setting seems to be quite high (about 40ftlbs) and I thought 20 to 25ftlbs would reduce some of the under steer felt in tight corners.

    If anyone has any experience etc I would be very gratefull for your input.

    Thanks for your time
     
  2. Difaz

    Difaz Karting

    Mar 30, 2005
    114
    Sydney Australia
    Full Name:
    Phill
    I suppose I should mention that the Differential in question belongs to a 1979
    308 GTB
     
  3. Gary48

    Gary48 Guest

    Dec 30, 2003
    940
    brian, are you refering to the pre-load on the tapered roller bearings that hold the carrier assembly?
     
  4. Difaz

    Difaz Karting

    Mar 30, 2005
    114
    Sydney Australia
    Full Name:
    Phill
    Hi Gary

    No I am refering to the locking preload between the 2 drving wheels.

    Phill
     
  5. Gary48

    Gary48 Guest

    Dec 30, 2003
    940
    Difaz, if its the limited slip differencial that you are refering to, then I,m afraid I can't help you there

    Gary
     
  6. Nuvolari

    Nuvolari F1 Veteran
    Sponsor Owner

    Sep 3, 2002
    5,082
    Toronto / SoCal
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    Rob C.
    The best guy to probably answer your question is Phil Hughes (aka. ferrarifixer). He has a lot of experience and ran 'Fluffer' in a 360 Ferrari in Australia. Good luck with it.

    Rob Calisi
     
  7. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,558
    Chicago
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    Philip
    Pre-load or limited slip percentage?
     
  8. ferrarifixer

    ferrarifixer F1 Veteran
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    Jul 22, 2003
    8,520
    Melbourne
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    Phil Hughes
    Robert Nuvolari !!

    How do you know Allan then. The fluffer nickname is from the Palmer school isn't it...??

    Anyway, the diff.

    The static preload is actually quite low I think at around 40lbs slip, and tightening it up further by re-arranging the clutch plates order, or fitting thicker ones can help traction a fair bit. It is too low to cause understeer, and I'd say that's more a wheel alignment / tyre thing in most cases. Total lock will cause low speed understeer, but that's a long way from where the std Ferrari units runs.

    The clutch plates / static lock can be adjusted by changing the number of friction surfaces that work against each other. The more faces the tighter it gets. But it's not an exact science, and much time can be absorbed for very little gain. New plates always settle a LOT, and it's better to re-use old ones for consistency, so long as they're healthy of course.

    The dynamic lock (on/off power) is controlled by the ramp angles on the cross spider shaft, where it locates to the housing. Typically, most Ferrari diffs are quite good, in that they have a fairly loose overun slip, and tighten up nicely under power.

    Frank Capo of Modena Engineering (here in Melbourne) makes really nice diffs, of adjustable (by dismantling) locking, and can be configured just about any way you like, from open to fully locked, and just flip the cups/housing over to reverse the torque loading direction.

    In short, unless you're racing VERY seriously on slicks, a std Ferrari diff in good condition works very well. You may gain some traction by upping static slip to around 65-75 lbs, but you'll probably need thicker clutch discs or shims to do this.

    On 246 and early GT4, there is no static slip preload, and locking is achieved solely by more aggressive ramp angles. I think static preload became the norm around late '75, but as always with Ferrari, there are no doubt exceptions out there.
     
  9. Difaz

    Difaz Karting

    Mar 30, 2005
    114
    Sydney Australia
    Full Name:
    Phill
    PMA1010 / Philip

    Differential preload is what I am questioning as I can't seem to find any specifiacations on this. The percentage lock on power I believe is 40% from what I can see in my workshop manual.


    Thanks Ferrarifixer Phil for the info, I didn't know what the std preload setting was supposed to be.

    Difaz
     
  10. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
    5,379
    NWA
    Full Name:
    Paul
     
  11. gdbsti

    gdbsti Formula Junior

    Oct 10, 2004
    283
    Nor Cal
    Full Name:
    Bruce
    Is the push during overun entry/mid corner on power apex out?

    Changing the overun ramp angles will help entry push but as the guys mentioned above, it sounds like the diffs in these cars are well balanced to start with.

    Reducing the power on ramp angle should help exit push but may make it a bit loose.

    If the problem is more like a lack of entry rotation, I'd start by taking some rebound out of the rear dampers. Might unsettle your braking a little, but like any chassis tuning, it's a matter of balance and driver style...
     
  12. ferrarifixer

    ferrarifixer F1 Veteran
    BANNED

    Jul 22, 2003
    8,520
    Melbourne
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    Phil Hughes
    I build them with new discs on the bench with oil, then spin them until they seem to settle, (or as long as I can be bothered to stand there and do it), then strip, clean and rebuild again. It usually settles about 10lbs, but it can vary wildly depending on many things.
     

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