355 ssuspension rebuild - DIY attempt

Measured front spring constant is 180lbf/in; rear is 260lbf/in. This jives with the WSM published spring compliance data (stated as "flexibility" in mm/N).

Front motion ratio (also in WSM) is 0.70, rear is 0.85.

For reference, see my blog on "soft" suspensions http://www.separationofcontrols.com/posts/why-are-suspensions-so-soft-wheel-rate-motion-ratio-and-frequency/ "Most Ferrari’s are comfortable, even on long drives, and sporting enough to be better in many ways than the competition. Ferrari improves on the average sporting car with a bit more suspension damping."

Plug in the spring specifications into the old suspension spread sheet and you'll get a Wheel Rate of 90lbf/in front and 188lbf/in rear. Very comfortable yet sporting enough. The "sport" part comes in with the shock damping characteristics.

Best,
Rob

 

Rob, awesome! Thank you for that confirmation. Half the battle is figuring out what is the stock baseline and you've just brought clarity to that question. Wow! 180lb/in front spring rate, 90 lb/in front wheel rate, and motion ratio^2 = .49, so that establishes the relationship between the two. This car is a real sofa up front.

What is your opinion about ride height for the person who uses the car for both the street and track? One of the reasons why I was advocating a higher increase in front spring rate over the factory rate instead of an incremental increase is because of the basic math to limit weight transfer to the outside front wheel. The math says we can either lower the CG, lower the weight, or increase the track width to reduce weight transfer (and thus avoid overloading that corner.) But when the car is lowered, the available suspension travel to bump is reduced, so typically a more aggressive jump in spring rate is required or some careful selection of bump rubbers. I'm more curious about hearing your philosophy on the front setup. As I posted earlier, I have not invested in actually cranking out any of the math for myself. Thanks in advance!

-Greg

 

Hi Guys,

This is a great discussion started by J and has my wheels turning for more blog material.... I don't have much time for a long post, sorry.

I understand y'all wanting to see my spring selections and damper dyno plots for this project; I would ask that you please consider that this is my business, I make my living performing this service for my customers. Without customers I don't have a business. If I "lift the Kimono" for everyone to peek I've eliminated the sweat needed for someone else to recreate this suspension system. What I am happy to do is write articles on method and make suggestions for further reading.

Carl (FBB) hit it on the nose with his earlier post regarding suspension frequency and damping and that there's info on the 'net and calculators available; this is basically exactly what I do when designing plus add in my own hard earned knowledge via trial/ failure/ success.

Plug numbers into said spreadsheet and you'll find that Ferrari designed the F355 with a natural ride frequency of about 1.2Hz front and 1.5Hz rear. This suspension frequency in ride equates exactly to above stated wheel rates (90lbf/in and 188lbf/in). This follows best practices for a road car in that primarily you need a "flat ride" (hit a bump and the car rises in heave and settles in heave with little or no pitch movement at all). To achieve "flat ride" the car must have a higher rear ride frequency vs. front, otherwise you will create a pitch movement that the owner/ driver/ passenger will not tolerate in a road car.

Knowing how J will drive his car, his desires and experiences with other cars, track driving etc. I have chosen to increase the suspension frequencies closer to 2Hz, near the upper limit for a sporting road car without ground effects. Of course I am retaining a frequency difference front vs. rear as the car will be driven on public roads, not raced competitively.

I also perform anti-roll calculations as another parameter to ensure I'm "on track" for a well balanced chassis in cornering too. Some call this "roll couple distribution".

As for damping I'll offer up this: in my two decades experience as driver and engineer one of the most common mistakes is overdamping the suspension. Happens so often that one of the first pieces of advice I give someone after hearing their tale of handling problems is to go back on track with shocks set full soft and see if that makes an improvement.

Overdamped/ underdamped can be relative terms so I suggest Googling "critical damping", plus I have this blog post on damping here: http://www.separationofcontrols.com/posts/what-is-critical-damping-of-shock-absorbers/

With the F355 and it's Ferrari/ Bilstein computer controlled system valving is more challenging when tuning but suffice to say that when increasing spring rates an increase in rebound damping is appropriate.

Okay, out of time...

Best,
Rob

ps, I always keep in mind the fact that I don't know everything, there's always something else to learn (I just spent two days of continuing education two weeks ago with one the the top racing vehicle dynamics business consultants). Also please keep in mind that I have great respect for data; statements not backed up by data or examples of best practice are sometimes hard to chew on.

 

I hope my teasing about engineers didn't step on your toes Rob, that was not my intention, nor was it to peek under your Kimono

Like you, I also have great respect for data, in fact my guess is there is no other team in America that has compiled more miles of Ferrari Challenge testing or more pages of Ferrari Motec data than us (Level 5 Motorsports).
As a test driver I have been lucky to work with Jeff Braun (Ferrari 333 Championship engineer), and Rick Meyer, (Risi Ferrari 430 GT Championship engineer). Together we have set up 360's and 430's that have earned 4 SCCA National Championships, more wins in the 430 Challenge era than any other team and a host of other accolades. So when I tease you "engineering types" I do it with great respect and from the perspective of working with some of the best!

My only purpose was to help J see the set up process from the drivers perspective and not get overwhelmed by a bunch of what must sound like voodoo to the uninitiated. (OK I have been racing for 30 years and some of it sounds like Voodoo to me too!)

In any case, it sounds like J is in good hands with you and I am sure the car will work great! I have to say I am surprised just HOW soft the stock spring rates are based on your report...there is certainly lots of room to go up from there and still remain streetable for sure.

 

It's all good! And thanks for the kind words.

My data comments are more in regards to the speculation of what's good / bad/ too much/ not enough... I had a professor who said constantly "without the data it's just opinion".

Once upon a time I worked with your gearbox guy Chas and Jeff set up some GT1 shocks for me in the 90's...

Best,
Rob

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To all... I'm always happy to discuss suspension, shocks, Ferrari s, etc, call or email. Best time to get me is 9am - 4pm Eastern. 888.407.5122.

Also, I'll be in Indy this Thursday - Friday for the IMIS (imis-indy.com) show if anyone wants to chat in person.

Best,
Rob

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I'll have to make alignment, ride height and tuning methodology specific to F355 a blog post soon.



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I did it a few years ago so cant remember what I did step by step but they do press apart... and come completely apart... those are the rears.

Here is another pic... it may help.
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I think the Lathe was $600. It is good for small metal parts. The Delrin was about $300. It was a winter project. Very pleased with the way the car handled on track after that mod.

 

Very savvy! When determining/ designing damper specifications one must also take into account ARB rates, especially if rates are 'relatively' high vs. spring rates. Often it is enough to perform all damper specification calculations only in ride, but there are times when roll rates dictate an increase in damper forces (rebound). This is very true for a car with soft springs and much higher rate anti-roll bars.

 

I love these threads. It's one of the main reasons I joined here in the first place. Nothing more interesting than DIY projects by the members. Keep it comin'!

 

Oh the challenges of the engineer... I've worked as a contractor for many different manufacturers as a "pro-driver trainer"/ facilitator on interactive automotive events (aka ride and drives). At these events I'm always asked "why did <insert manufacture name> do this rather than that", like "why is that one supercharged and this one turbocharged?", or "why didn't they use aluminum here 'cause then it would be a faster car?".... Ultimately the answer to nearly every question is "cost". There's a budget and the design team must work within said budget (or get fired). This is true even for Ferrari.

In regards to F355 Challenge suspension specifications Ferrari learned a great deal with the 348 Challenge program. One thing learned is the chassis responds favorably to increased roll stiffness in front vs. rear. The first prep thing we did on 348 Challenge is swap springs front to rear to increase front anti-roll torque. Ferrari took a larger leap for F355C and dramatically increased spring/ wheel rates in front, a decidedly front biased anti-roll distribution. Why do it this way? Many reasons, here are two:
1. Known practice for race car designers to strongly favor roll rates and anti-roll forces at the front vs practical road car practice of biasing spring/ wheel rates/ frequency in ride toward the rear
2. Cost vs. performance (remember 'time' is also a cost factor)

An alternate (not saying better/ worse) approach would be to retain "softer" spring rates and increase anti-roll forces and move anti-roll bias toward the front via anti-roll bars. Packaging an ARB in the F355 of the size needed would pose an engineering challenge greater than packaging larger springs/ tender springs and a packer/ bump stop system.

Running much higher ride frequencies in the front vs. the rear, in violation of conventional road car practice, is okay/ acceptable trade off for a race car in many circumstances. The resultant pitch movements in ride is accepted because the lap times improve and to h*ll with driver comfort! It is also common to run much softer rear ride rates in attempt to increase tire grip (bite) in RWD chassis for acceleration at corner exit.

Good questions!

Best,
Rob

 

Agreed. It's all about balance/ compromise in design and build. Often times it's difficult to "freeze" the design and move forward 'cause there's always that one more thing to test, one more idea, one more data set.

That said, just look at the dramatic increase in quality of ride/ handling/ comfort/ civility of "today's" sporting automobiles vs. just 10 years ago, let alone 20 years ago, for every manufacturer; you CAN have it all in a car these days. Note how Ferrari is using magnetorheological (MR) shocks, they're 'magic' in how they allow the design team to balance comfort and responsive ride/ handling.

Link to NOVA on non-Newtonian fluid technology (starts at 25:30):
http://www.pbs.org/wgbh/nova/tech/making-stuff.html#making-stuff-smarter

 

Doesn't this stiff front/soft rear setup cause a lot of understeer? Or do you compensate for this by changing alignment?

 

No, when set up well this is neutral to overseer. "Compensate" no, optimize alignment - yes.

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Do not bang aluminum hat or you bend it = $$$
squirt penatraing oil or wd40 in the stud holes to soak in behind the hat. Heat the hat uniformly but remember that you decrease the strength 50% at 500*f. But the hat is probably made for that because the rotors transfer heat etc. Just don't crazy cook the thing. Heat will draw the wd40 between the aluminum and the hub. Rinse and repeat and lightly ding with a hammer and it should come free.

 

Yes! and that's why there is Rob to get you very close and new tires, stop watch, and open wallet to dial it in. How fast do you want to go? How much money do you have?

 

Bearings are a wear item. If. You don't do them now you will later.