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Help tire pressures on 355

Discussion in '348/355' started by alanhenson, Mar 21, 2004.

  1. alanhenson

    alanhenson Formula 3

    Dec 2, 2003
    1,335
    I need some help guys. I have a 1999 355F1 Spider. It is running 18 inch HREs with Michelin Pilot 225 45 18s on the front and 265 35 18s on the rear. I took it out of the garage for a drive and the tire pressures were around 35psi. The tires have a max pressure rating of 51psi. I inflated them to 48 psi and the car feels a little out of control, like it slides a little. Doesn't feel confident. My question is, What is optimal? Any experience? Thanks fellas.
     
  2. GTO84

    GTO84 Formula Junior

    Dec 13, 2003
    562
    Go with 40 psi front and 38 to 40 rear depending on traction. Over 40 will wear the middle of the tire down.
     
  3. alanhenson

    alanhenson Formula 3

    Dec 2, 2003
    1,335
    What about handeling though? That's what I am really concerned abut.
     
  4. nzo4re

    nzo4re Karting

    Mar 13, 2003
    197
    San Jose, CA
    Full Name:
    Tom Lassen
    The reason your car slipping around is because you have the tires way too hard. Go for around high 30s.
     
  5. sherpa23

    sherpa23 F1 Veteran
    Silver Subscribed Owner

    May 28, 2003
    9,500
    Colorado
    Full Name:
    Bastuna
    You do know that the stock size is 225/40 and 265/40, right? It's not a big deal but you've gone higher profile in the front and smaller in the rear, effectively raising the front slightly.
     
  6. alanhenson

    alanhenson Formula 3

    Dec 2, 2003
    1,335
    they were on the car when I got it. When I get new ones I am going to swap them. Thanks guys. The lower tire pressure was right.
     
  7. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    6,982
    Your rear tires are too small (265/35ZR18s) This causes the rear end geometry to be all screwed up (less rake than factory, and lower rear ride height). {I had this problem too when I first got my F355.} This causes -- oversteer and unstable behavior. You have 2 choices: raise the rear ride height with the spring perches, or change the rear tires to a more appropriate rolling radius (265/40ZR18 or 275/40ZR18).

    The chassis/suspension on the F355 is adjustible to the point where any set of tires that fit on the wheels can be made to work well with equal tire pressures front and rear.

    When I run on the track, I like to come off the track with the tires stinking hot and have 41 PSI in them. This relates back to the cold tire pressures of 33.5 PSI to 34 PSI sitting in the garage overnight. Any cold tire pressure above 35 is indicative that the suspension is not setup correctly.
     
  8. alanhenson

    alanhenson Formula 3

    Dec 2, 2003
    1,335
    Thanks mitch. Very informative. Any idea on my other problem. Rythmic Thumping coming from the passenger footwell.
     
  9. Corsa

    Corsa Karting

    Nov 1, 2003
    109
    Stockholm
    Full Name:
    Peter
    Interesting, Mitch.
    Do you have any idea of what in the rear suspension that makes it unstable/lose grip when it’s lowered? Is it the aerodynamics due to rake change, track changes? And is it applicable on 328?
     
  10. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    6,982
    As you move the rear end up or down, you end up changing the roll center--a virtual point about which the rear of the car rolls in turns. It ends up that the distance from the roll center to the center of gravity acts like a lever arm in converting latteral thrust at the tires into roll on the chassis. Since you are moving only the rear end, the lever arm of the front end remains constant. Therefore, as the rear end ride height changes, you are adjusting how much of the roll is carried by the front suspension and how much is carried by the rear suspension.

    At/close-to the factory ride height is a balance point with factory sized tires and factory sized anti-roll bar. With this setup the car is decidely neutral under power in turns. As the rear end is raised, the roll center is raised faster than the ride height is raised, so the lever arm is reduced in length, and therefore the rear end becomes reletively stiffer in roll and the outside rear tire becomes overloaded==oversteer.

    The general philosophy is applicable to the 328 and any short/long a-arm suspension, but all sorts of geometry is different with the 328. However, raising the rear should reduce the lever arm and induce oversteer (and vice versa) however the amount to raise or lower will change due to the geometry differences.

    Aerodynamically, if the nose gets too low on the F355, you can feel the center of aerodynamic pressure move forward even as you back off the gas over 160 MPH, and it allow the car to become unstable under brakes in the highly arestable range of speeds these cars are capable.

    However, a more insideous geometry issue is present in the F348/F355 front suspension with stock springs. Under heavy braking, the front tires camber in to a near perfect aspect profile for braking, and the rear tires attain a nice flat profile at the rear as the suspension extends. This is just perfect for braking; however if you add steering input while under heavy braking, the front end retains its high traction on the road while the outside rear tire looses traction as it transitions from extension into compression with roll. This causes the car to oversteer and confuses the novice driver while the expert uses this effect to speed turn entry. Adding as little as 3mm to the front ride height eliminates 90% of this effect, calming the car without getting rid of the quickness in the steering.

    Since we are talking about ride height adjustments in the 2mm-3mm range, one should be cautious about lowering these cars with stock springs or even changing the rake without a plan. Indeed, when I got my F355, it had all of these effects, and took me a year to figure out that the ride height was the culpret--and very tiny amounts of ride height to boot. A car like the Vette has different suspension geometry and is practically imune to these effects under as much as 2" of lowering.
     
  11. murph7355

    murph7355 Formula 3

    Nov 30, 2002
    1,691
    SE England Yorkie
    Full Name:
    Andy
    Just checked mine today - the sticker on the car states 2.6bar front and 2.3bar rear. Which I'm pretty sure equates to 37.5psi/33psi respectively.

    I can attest to 355s being very sensitive to pressures. You only have to get the balance out by a pound or two across tyres and the handling goes to pot (more unforgiving in this respect than other cars I've driven).
     
  12. jakermc

    jakermc Formula 3
    Owner

    Jan 17, 2004
    1,778
    Palm Beach, FL
    Full Name:
    Rob
    Its worth noting that the owner's manual states different tire pressures depending on the tire manufacturer. For the Pirelli P-Zeros I have it states 32 Front and 29 Rear. Pirelli's own literature states the same thing. The specs for Michelin and Dunlop were a lot higher, similar to those indicated in the previous post.
     
  13. Corsa

    Corsa Karting

    Nov 1, 2003
    109
    Stockholm
    Full Name:
    Peter
    Thanks a lot, Mitch. Very informative, as always.

    I didn't realise that roll center changes could have an impact on the handling of that amount. This means that I have to reconsider the 4WA I will do on the 328.

    I have Ohlins shocks and springs on the 328 now. I'll change springs and do a 4WA. I have a "funny" behaviour at the rear end that I want to get rid of. Maybe roll center changes could be part of it. The car is lowered about 20 mm.

    Ciao
    Peter
     
  14. Corsa

    Corsa Karting

    Nov 1, 2003
    109
    Stockholm
    Full Name:
    Peter
    Lowered 20 mm front / 15mm rear if my references are correct.

    Ciao
    Peter
     
  15. 355f

    355f Formula Junior

    Nov 1, 2003
    305
    If you running Pirelli it shold be 2.2 bar front 2.0 bar on rear
     
  16. murph7355

    murph7355 Formula 3

    Nov 30, 2002
    1,691
    SE England Yorkie
    Full Name:
    Andy
    Cheers 355f.

    I have Bridgestones at the moment. Will be playing with the pressures by a pound or three over the next week or so to see what suits it the best.
     
  17. paulie_b

    paulie_b F1 Veteran
    Consultant Owner

    Jan 13, 2003
    6,792
    Jupiter, FL
    Full Name:
    Paul Bianco
    I am not a mechanic but are they the original size for car? if so, check what Ferrari recommends. plus I usually add 2-3 more pounds than they suggest for normal driving. on the track you will want to go softer.
    **A tire professional once told me that the car manufacture will suggest one tire pressure and the tire manufacturer will suggest a DIFFERENT tire pressure.
    Why....the car company wants you to have a smooth and soft ride while the tire company wants you to have longer lasting tires.
    Go figure!
     
  18. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    6,982
    I run different pressures depending upon application. For the street, I run 35/35, at TWS I run 33.5/34 and at MSR I run 32/33. This is with B S03s after I setup the rear ride height to allow even pressures f/r on the street. A different brand of tires will LIKELY require slightly different tire pressures to feel just right.
     
  19. jdb

    jdb Formula Junior

    Nov 16, 2003
    273
    Northern California
    Full Name:
    Jeremy
    I know there have been a lot of threads on this topic, but I have a few questions.

    1. Ideal tire pressure for '97 355 Spider with new Bridgestone S-03's. I'm using the OEM size in the front (225/40/18), but I had to use a slightly larger size in the rear (275/40/18 vs. 265/40/18 OEM). From what I've read, if I have the OEM sizes, I want to run equal pressure front/rear. However, since I have larger rears, I want to run those at 1-2 psi less than the front. So am I "right" in running, say, 34 psi front, 33 psi rear? Any suggestions would be appreciated.

    2. Why do tire pressure recommendations differ so much for similar tires on different cars? Example: Michelin Pilot Sport PS2's on an M3, I've been told to run at 35/37 psi F/R; Michelin Pilot Sports on a Viper, I've been told to run at 29/29 psi F/R.

    3. Finally, are the tire manufacturer's maximum tire pressure ratings for maximum tire pressure while cold?
     
  20. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    6,982
    It actually depends upon the ride height at each end. Tire pressure is then used to trim the handling to balance the car. If you are at factory ride heights (after putting these tires on the car) then you can use my above recommendations {assuming you are on stock springs, shocks, and antiroll bar}

    Because different cars have different amounts of weight on each corner of the car, and different camber, toe, and finally different suspensions change camber and toe differently. With this much difference its basically impossible to get a single number for a tire. You are lookking for balance under various conditions.

    Max tire pressure is (legally) max, however every manufacture builds in margin.
     

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