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F355 suspension analysis

Discussion in '348/355' started by Mitch Alsup, Apr 2, 2004.

  1. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    This is a 16 part diatribe concerning the suspension of the F355, and what it means to align it, and what it might means to lower it. Please wait until you see the entry pertaining th the Vette C5 suspension before joining the thread; Allowing me to complete the throught train before jumping in.

    In the first part, I present the front suspension of the F355. This was constructed from a scan of the Users Manual. On the picture, I have measured all of the suspension pickup points and made various notes concerning the suspension. The data is read off and inserted into a 3D suspension analysis program in order to compute the caster, camber, and toe jyration as the suspension moves in dive and roll.

    In particular notice the instant centers for the front suspension are very long (about 300 inches), and the roll center is just below the ground surface.
     
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  3. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    In this installment we take a look at the rear suspension of the F355. All of the same measurement are taken and fed into the 3d suspension analysis program to extract camber and toe as a function of dive and roll.
     
  4. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Here we see the output of camber from the suspension alanysis program, graphed in 3d form showing the relationship between dive, roll and camber.

    All of the later computations are based on this shape with various gyrations of dive, roll and other suspension alignments.
     
  5. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Here we see the output of camber from the suspension alanysis program, graphed in 3d form showing the relationship between dive, roll and camber.
     
  6. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    At this point everybody is pretty much lost, but bear with me for a few more entries and I will get to the point.

    The camber graphs are what suspension engineers have been working with for years. However, what a driver feels is traction at the 4 contact patches. So, how can we get from a 3D camber graph into traction: Simple: use each point on the camber graph and interpolate the following traction function found in may books on suspension design and analysis:
     
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  8. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Now, the front traction situation 'looks' like:

    Notice the shape is like a doughnut, and there is a particular surface where traction is best. Each collor represents 1% of traction, so the magenta-brown is 99%-100%, then brown is 89%-99, and so on.

    We want the suspension to not move off the peak traction surface (very far).
     
  9. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Now, the rear traction situation 'looks' like:

    Notice the shape is similar to the front, but different some how.
     
  10. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Ok, now what do we do to use this in any sensible way?

    What we need is a way to look at what all 4 tires are doing at the same time. Looking at 4 different 3D graphs and making sense out of them simultaneously is --lets just say-- not easy. So I flatten the 3D into 2D as in: and add in directional indication in textural form on top of the data:: front traction:
     
  11. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    And similarly for the rear
     
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  13. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Ok, still doesn't help, but bear with me:

    Now lets manipulate the graphs by seeting them on the same scale and orrenting them so that the direction vector for deceleration is the same front and rear, and similarly for left/right.

    Here I add in a F355 drawing to remove confusion as to the traction profiles

    In all cases braking (deceleration) has a vector pointing towards 9:00 o'clock, the outside tire in a turn has a vector pointing towards 12:00 o'clock, acceleration creates a vector pointing towards 3:00 o'clock.
     
  14. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    You might have noticed, I embossed a traction circle at the operating point and within the traction circle, a 90 degree turn path in blue and a lefr hander followed by a right hander in red. I set the amount of movement equal to 2" of suspension travel equals 1 Gs of force--a reasonable street value -- but uncalibrated nonetheless.

    Now we are at the point where we can talk about what a f355 suspension 'feels' like.

    The picture shows the traction profile of an f355 at stock suspension settings for front and rear simultaneously.

    At the front, notice that the operating point was chosen such that one does not run out of traction under 1 Gs of braking. And that as the car moves from pure braking into cornering with trail braking that it remains at 100% traction. The corresponding section on the rear tires looses 3% traction, and this gives teh trailing throttle twitch teh f348 and f355 are known for. Under pure cornering, the front is at a 4% loss to the rear (safe predictable understeer), and remain so accelerating out of a turn.

    At the rear, notice the operating point is a careful balance between traction during steering input (as above) and traction while accelerating out of corners. This delicate balance is the achilles heel of lowering this suspension geometry with stock springs.
     
  15. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    THis next picture shows what happens when the suspension has camber added front and rear

    The handling imporves as the steering with brakes twitch is greatly reduced with a small issue with respect to putting down power as the turn becomes straight (power on oversteer fishtailing).
     
  16. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Now lets see what happens with lowereing of the car.

    This first picture shows pure lowering without realigning the tires to the new ride height. In some respects, it looks better, and if a casual driver drove such a car they would think if 'feels' fine. However, when pushed, it has a bad traction issue when power is applied from overcambering of the rear tires.
     
  17. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Now lets realign the suspension back to stock specs at the new ride height:

    While this does dial in a balance between loosing traction off the corner entry phase with the corner exit phase. Notice the SIZE of the traction peak is smaller than with the car raised.
     
  18. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Ok, you say this screws up the suspension--so how come challenge cars are both lowered and <blah blah blah>

    Ok, lets look at this: the challenge car has stiffer springs and antiroll bars. stiffer suspension moves less when encountering the same amount of force, so the whole traction circle is compressed. I graph a suspension setup twixce as stiff as the road car.

    Notice at this stiffness, the circles fit necely in the traction peak and thereby the suspension is delivering optimal traction at any point in time.

    Notice that it can use a bit of camber without loosing any traction and perhaps helping the overall balance of the car.
     
  19. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    So, then, why can a car like the Corvette C5 be lowered as much as 2" an camber added <and so on>

    Well, the Vette hss different geometry, and the doughnuts are shapped differently. The following picture shows the C5 vette traction profiles. Notice how long and wide they are. One can add up to 2 degrees of camber ate either end with only gains in performance. In addition one can raise or lower this suspension with little effect to the geometry.
     
  20. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,679
    Ok, so why does Ferrari use such geometry?

    At the front, the twitch I mentioned earlier can be controlled with ride height and camber to dial in as much or as little as the driver likes to control how quickly the car reacts to steering input. This makes the car lively under brakes and at steering initiation.

    Notice that the Ferrari rear tractions curves are shaoped such that traction is optimal when applying power and turning. In NASCAR parlance, this is 'forward bite' and what makes driving these things out of 100 MPH tunrs with you foot flat on the floor so much fun. They bite and carve big lurid arcs.

    Taken together, the F355 suspension can be adjusted to whatever the driver likes with very tiny changes to its ride height and camber settings.

    Notice the vette does not have this, and must resort to bigger tires (or stiffer springs and camber) to get reasoanble amounts of forward bite. The biggness of the tires impacts the ability to ';dance' with the car.

    ----I'm done--have at it.
     
  21. Hubert

    Hubert F1 Rookie

    Jan 3, 2002
    2,642
    The Left Coast
    What about motion ratios?
     
  22. Tipo815

    Tipo815 F1 Rookie

    Nov 1, 2003
    3,493
    Newport Beach, CA
    Full Name:
    Jeffrey
    I'm sure glad I don't work for Ferrari's F-1 Team. Now I know what their engineers must have to analyze to set up Schumy's car. Enough to make a sane and normal person go nutty!! :) Nice analysis (I think). ;)
     
  23. Hubert

    Hubert F1 Rookie

    Jan 3, 2002
    2,642
    The Left Coast
    F1 suspensions setup comparions w/ street cars after you say "... it's got 4 wheels.." After that, there's no comparison.
     
  24. Tipo815

    Tipo815 F1 Rookie

    Nov 1, 2003
    3,493
    Newport Beach, CA
    Full Name:
    Jeffrey
    I agree. Just meant the extensive statistical analysis of graphs and geometry, etc. No doubt that your analogy is 100% correct!
     
  25. fatbillybob

    fatbillybob Two Time F1 World Champ
    Consultant Owner

    Aug 10, 2002
    20,575
    socal
    Hubert,

    The motion ratios just tell us the relative chassis height change at any corner per turn on the spring collar. Mitch's comment on ride height is not assuming keeping factory ride heights or lowering with factory chassis rake. The huge tuneability he talks about is due to the ability to set each corner ride height per driver specs. Those things are partly taken care of and what Darrin does when he corner weights our cars with us sitting in them, I.e. our weight changes ride height and thus corner weight, weight transfer.... Still there are infinate other tuning issues with alignement specs, tires, shocks, springs.....
     
  26. tonyc

    tonyc Formula 3

    Oct 19, 2003
    1,483
    Monterey, CA
    Full Name:
    Tony C
    Hi Mitch and thanks for the graphs. Although I have not spent enough time with them to say I understand them or to be able to answer your question,
    but I have one of my own :)

    I recently spent about 3 hours having my 355 aligned with me in the drivers seat and a half a tank of gas. We had to make a lot of adjustments and remove some shims from the front right side. I asked the shop to make my setup for street use and not the track. The car tracks very straight on roads even at high speeds with hands off the wheel.

    Here is the my current geometry:

    Left Front Right Front
    Camber: -.8 -.9
    Caster: 7.1 7.6 (out of recommended spec)
    Toe: .16 .14

    Cross Camber: .1
    Cross Caster: -.4
    Total Toe: .3

    Left Rear Right Rear
    Camber: -2.3 -2.4 (out of recommened spec)
    Toe: .11 .13

    Cross Camber: .1
    Cross Caster: .25
    Thrust Angle: -.01

    Do the above numbers fall into the circle you plotted ?

    I do not have the old numbers handy right now, but I always felt like I was fighting the car on the street, even though on the track at Laguna Seca it felt fine.

    thanks again !
    --tony
     
  27. Schatten

    Schatten F1 World Champ
    Rossa Subscribed Owner

    Apr 3, 2001
    11,237
    Austin, TX
    Full Name:
    Randy
    Mitch, good post - just as reflected in the tech session you led at Moorspeed last year. I was wondering if you wouldn't mind posting the xls sheets that these are derived from. If they are over the limit, drop me a note and I'll host them. Thanks!
     
  28. rexrcr

    rexrcr Formula 3

    Nov 27, 2002
    1,572
    Kalamazoo, MI
    Full Name:
    Rob Schermerhorn
    Fan-f*ing-tastic Mitch... well done analysis, great presentation of an extremely complex system.

    Rob
     

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