F355 suspension Analysis

This thread is dedicated to an analysis of the F355 suspension. There is a similarly titled thread in the General discussions section where comments, questions, disagreements and the like can be discussed. This will leave this thread for the presentation of the F355 suspension so that it can be extended over time and remain readable.

See: http://www.ferrari-talk.com/discus/m...tml?1158280360

My story:

When I first got my 1995 F355B just a little more than 5 years ago, the suspension was not well sorted. Being an engineer by trade, and well versed in physics, and proficient in eXcel; I decided to find out what was wrong and how to fix it. The knowledge gained in this journey is condensed into this thread for anyone what want to understand the F355 suspension and more importantly, why ti feels the way it feels while driving. Later in this thread, I will show how the original F348 suspension felt different and why, and I will wrap up with a few statements on why Ferrari suspensions are the way they are.

Being a semi-proficient mechanic, I decided to investigate why my car felt so weird. It had symptoms of being overly nervous while driving on straight flat uncambered road surfaces, was actually dangerous when braking hard from from super-super legal speeds.

The first course of events is your typical suspension alignment. I found that the rear cambers were imbalanced and agressive, and that the front suspension was pretty close to factory specs in the camber department. I also found out how easy it is to change the camber and toe on the F355 suspension, so I proceeded to 'straighten' out the rear end back to factory specs. This procedure had worked for me on numerous sports cars I had owned in the past.

Unfortunately, just getting the camber set correct did absolutely NOTHING with respect to the feel of the car, and indeed had made the car even more nervous in a straight line. Something fishy was going on in Denmark (with no disrespect implied to those fine people living in Denmark).

After a cuple of more tries in the camber and toe adjustments to the rear of the car resulted in no forward progress, I conned a technician with a Hunter alignment rack and access to some corner weighting gear to attempt to get the suspension sorted. We spent an hour taking measurements of where the suspension currently was setting (which disagreed a little bit with the measurements I had performed at home), and we decided upon some resonable numbers for each end of the car. Finally, we adjusted each corner of the car to some reasonable agreed upon specs with the basic properties of::

Front Caster:: equal side to side
Front Camber:: -0.8d and equal side to side
Front Toe:: 0.12 toe-in and equal side to side
Rear Camber:: -2.0 and equal side to side
Read Toe:: 0.12 toe-in and equal side to side.

At this point the average car would be driving straight down the road and be quite happy doing it. No such luck here. Mine was still nervous, and worst of all still had the high speed braking instability. Drat.

By this point in time I was taking the car to road race tracks and starting to learn about vehicular dynamics. I had noticed that the rear end felt soft with respect to other cars I had driven in anger, but that the shocks were absolutely perfect for the amount of spring in both the front and rear of the car.

So, I got out the old reference manuals on high performance driving, and suspension setups that I rad read (oh so) many years ago. I have all of the Carrol Smith books on race car preperation, setup, tuning,..., Race Car engineering and Mechanics by vanValkenburg,and just to be sure I picked up a copy of Race Car Vehicular Dynamics by Milliken and Milliken, and a copy of Race Car Aerodymanics, Designing for speed by Katz. I recommend that each person who really wants to understand suspension stuff, get these (kind of) books, read them and attempt to understand them.

Due to the use at the race tracks, my rear tires were showing cord, so I odered up a set of the same tires in the same size as was on the car, had them mounted and balanced hopint an invisible tire defect was interacting with the suspension alignments in some unknown way.

Result: My car was even worse than before! I first noticed how much worse at MSR in Cresson while driving through Big Bend with the steering wheel more than 15 degrees in the opposite direction as the arc the car was driving. No wonder the car felt no nervous when I shifted up to the next higher gear, the rear end had to step back in a whole foot when the power came off!

Something had to be done! I went to an old mechanic and expained the problem to him, one of this shop mechanics had worked on the Jaguar race team while Jaguar was running the LeMans Spice car (that lovely V12 in the GTP white body circa 1990). He indicated that they had all sorts of high speed instability issues with that car until they got the front end at the correct ride height.

This piqued my curiosity, so I went back to my books and looked up suspension setup. In each and every book there was a similar sentance:: The first step in setting up a suspension was to set the ride heights and then work from there.

OK, but what ride heights would be appropriate? I had no idea.

I talked with several other people who use/operate high performance cars, and ask them for guidance. One of them pointed me to David Moore at MooreSpeed a race car preperation facillity and Porsche mechanic. I took my car to David, explained the situation; and we agreed to do a complete grounds up suspension setup to factory specs. This included setting the ride heights, adjusting all the normal stuff, and then finishing by getting the corner weights set correctly with my weight in the drivers seat.

Within 10 feet of leaving his facillity, I knew the problems were vastly better, and by the time I got home, I was considering this the best money I had ever spent on mechanics work! The car was completely transformed. It sould drive straight down the hiway, it displaied no hints of instability, or nervousness, and one could hit the brakes at unreasonable speeds with ones eyes closed (if the car was pointed straight before braking).

OK, now that the car was sorted, what did we learn, and how does this apply to other cars with double a-arm suspensions? Stay tuned!

Since I happen to have an F355 and the owners manual, I tok a scan of the front suspension, and loaded it up into a program where I could take some measurements, apply scaling, and otherwise embelish points of intrest.

The following figure shows a scan of the F355 front drivers side suspension, with some important features annotated:



The first thing to notice is that The longitudinal steering axis inclination (shown in red) intersects at almost at the exact center of the loaded tire. This means that all forces from the chassis to the tire are centered in the contact patch. So, when the outside (or inside) edge absorbs a force for the road, the driver can feel it even when massive forces are in play (lake during maximum cornering). Summary: driver can feel contact patch, but driver does not (so much) feel the chassis loads. We call this feel.

The next thing to notice is that the shock is pointing almost directly at the center of the contact patch. Thus the shock damps the movement of the contact patch, not the movement of the suspension arms! Since the shock is only controlling the contact patch, there are minimal added side loading in the A-arm pivot points due to the springs, and thus low friction.

We know that the A-arms are made of two stamped steel parts welded together. This makes for a lightweight but strong and stiff control arm. Add to this the Magnesium wheels and low profile tires (by 1995 standards) and we have a very precisely controlled, stiff, light way of holding the contact patch to the roads surface. This enables the suspension to move faster in response to road surface irregularities, thus higher road speeds are encountered before the suspension looses grip on the road. Some other measured (from the scan) data are included.

Since we have a scale model of one side the the F355 suspension, and we know the track of the car; it is a straightforward excersize to flip the scan over and position it at the track specified in the owners manual. In order to know the scale of the drawing we keed to find something on the scan we can measure on the real car. I show the tire diameter specification and scalld the track to the equivalent size. Presto, and accurate representatin of the F355 Front end:



A double a-arm suspension allows the center hub to move as if that hub were on a fixed length radius rod pivoted on the point where the upper and lower controls arms intersect. To determin this point plot a line that passes through both upper a-arm pivot points and extend many car widths to the other side of the car (the red lines). Do the same for the lower a-arms (the blue lines). The intersection point is not showd due to its large dimension. The intercection point is some 300inches to the inside of that side of the car. The radius arm is long indeed.

This point (and how it moves as the suspension moves) determins many properties of the front end.

Another interesting point is found by drawing a line from those intersection points to the contact patch from each side. Where these line cross is known as the roll center. The roll center tells us a lot about the suspension of the F355. It just so happens that the roll center of the F355 is slightly below the road surface and that the a-arm geometry keeps the roll center below the road surface under reasonable amounts of pitch, dive and roll.

Having the roll center on the road surface (F355 is a good approximation) means that center of the front end does not rise or fall as cornering forces rise and fall. And has the consequenct that the a-arms do not carry the vertical loads from the contact patch, the springs do! Another friction minimization. I will return to this later when we talk about the rear end.

Finally note that we can extend the virtual steering arm and note where these croos n the center of the chassis. This important thing to note here is that the steering arms cross exactly in the middle of where the upper arms cross and where the lower arms cross! This minimizes bump induced steering. So while the driver can feel the edge of the contact patch as the front hits a pot hole, the steering kickback is imune from the vertical wheel movments involved in this feedback mechanism!

The numeric pairs on the right hand side of the figure are scan-measured coordinates from surface of the road in the center of the car.

By looking at the hub carefully, we see that the wheel bearings are almost at the center of the wheel-tire combination, and connected by a short-fat axle to the rotor and wheel. This centralizes all the rotating inertia.

The rear end can be analyzed in the same manner. The follwoing figure is the F355 rear suspension as found in the owners manual:



Notice, here, that the suspension to hub pivot points intersect towards the outside of the rear tire contact patch. When large braking forces are applied to the contact patch the area inside the point of intersection is larger than the area outside, so the tire toes-in under brakes. The toe-in under brakes compensates for the reward pull on the suspension under brakes that naturally toes the rear end out. Conversely, as power is applied the rear tire toes-out.

Notice that once again the shock is pointing directly the center of the contact patch, minimizing side loading on the suspension pivot points.

Once again the wheel bearings are just outside of the center of the wheel, the brake rotor is centered, and the whole axle assembly is short and fat (that is stiff!).

Next we notice that the rear suspension a-arms are more steeply convergent than the front a-arms.

Finally there are some ride height numbers measured from the scan after scaling.

Once again we flip the scan over, scale it toa measured size adn then position one scan to the other so that the track is correct. The following figure is a reasonable representation of the F355 rear suspension:



By tracing the upper control arms (red) to the inside of the car, and by tracing the lower control a-arms (blue) to the inside of the car, we find the point of intersection called the instant center.* Notice that the instant center is close to the car and that the radius arm is much shorter that at the front. It is this short radius arm and the relatively shoft springs that give the rear end of the F355 that soft feel.

Fron the instant center we draw a line (green) to the contact patch and where these two line intersect is the roll center. The roll center of the F355 is about 4.75 inches above the road surface, while the roll center at the front is some 0.66 inches below the road surface for a total rake of 5.41" a rather steep rake.

With a cente of gravity somehwere in the 18" above the road surface, and a rear roll center of about 5 inches, about 5/18 of the vertical forces from the contact patch are found in side loadings of the a-arm pivot points, and about 13/18 of the forces are taken directly by the springs.

These side loadings and the added load of the weight at the rear end (compared to the front) is why the reaar suspension is so much more beefy than the light low friction front suspension.

If you remember, the front roll center is just about at the surface of the road, so 0/18 of the vertical forces are found in side-forces in the a-arm pivot points, while 18/18ths of the forces are taken by the springs.

A consequence of the rear ride height is the jacking force. Consider a large side loading at the contact patch. This load is transfered to the roll center on the line connecting same. The veritcal component of this force is the force that jacks up the rear of the car while cornering. This ends up being one reason that the car is so much more stable cornering with power applied to counteract the jacking force. But I am getting ahead of myself.

A consequence of the steep roll center rake of the car is that cornering forces are shifted from the inside rear contact patch to the outside front tire. When teh tire is operating below its maximum grip level, this added load enables more steering (e.g. oversteer) while when operating at maximum load this added weight inducing a small measure of understeer while cornering. Thus during turn in, this weight transfer makes the front end respond quicker, while at mid-corner, this slow down the front end response.

OK, all of this makes sense; so why do these cars have a reputation for "getting away from the driver"? Stay tuned for tomorrow night's continuation; where we dive into camber and how it changes over suspension movement, what this does to traction, and utimately how the F355 suspensions feels in graphic detail.

[*] should have mentioned this in the front end description.

Camber is a funny thing, most "car guy" thinks they know something about camber, and 95% of them are wrong. So, let us start with the basics. Camber is defined as the outward leaning of a tire with respect to the road surface. This is why the camber angles we dial in durring a suspension alignment are negative values. This denoting that we want the top side of the tire to lean inwards (not outwards). Just to be on the safe side, the following figure shows the definition of camber found in Carrol Smiths' Tune to win:



Some things to notice. A) camber is defined with respect to the flat road surface. One simply cannot measure camber unless the car is absolutely (positively) level to small fractions of a inch (better than 1mm). It is a rare occurance that a garage surface is sufficiently flat to accurately measure camber. B) camber is measured AFTER the ride height is set.

Camber is an easy measurement to make/take; one can do a precise job with simple tools such as a pulmb bob and a mechanics ruler! Most such measurements fail due to the lack of a flat and level surface, not because the plumb bob and ruler are imprecise.

And jujst to be on the safe side, let us take a look at the another critical definitions with respect to suspension alignment:



Toe is measured by setting up a parallelagram around the car, with the property that distance from the center of the left front wheel is the same as the distance from the right front wheel, and that the distance from the left rear wheel is the same as the right rear wheel, and that the strings are held equi-distant at both ends of the car. Once again, simple equiptment can suffice. In this case tow bars with slots cut at the same time and suspended at the height of the centerline of the wheels. With this arrangement toe can be measured directly. Front toe is measured by comparing the distance from the front of the wheel to the string denoting the aforementioned parallelagram. The measurement can be expressed as a linear measurement directly, or when divided by the width of the wheel (where the measurement was taken) and then multiplied by ArcTangenet( 1 degree) = 57.3 it can be expressed as an angular measurement.

Toe-in means that the front of the wheel is closer to its opposite wheel than the rear of the same wheel is to that same opposite wheel. Toe-out means the opposite.

Both of these measurements are static measurements. As a car accelerates, decelerates, corners, hits bumps, pot holes, or even crests hills or crashes through depressions, the camber and toe will change based on the geometry of the suspension and stiffness of the suspension bushings and the control arms themselves.

Camber is an intrieguing issue, most people understand that a liffle camber help road holding and few understand why. The following figure was scanned from Carroll Smith's "Tune to win".



Although this figure shows that the absolute grip of a tire is maximized at some small negative camber, it is (strictly) only valid for that particular racing tire. I am going to use this camber grip in a numerical analysis a little later on.

Each tire/wheel combination will have its own grip with respect ot the cambers employed. Wider tires enjoy LESS camber (static and dynamic), and even the height of the tire sidewall strongly interacts with the traction envelope, shorter tire (lower numerical aspect ratios) like LESS camber (static and dynamic).

Another variable is the tire contact patch to the wheel rim measurements. Consider a very wide tire on a narrow rim. Here the sidewally are buldging outwards in a static situation. Now when the car enters a corner, latteral forces build, distorting the contact patch. The inside edge of the outside tire will lift the inside contact patch off the road surface as the car builds cornering forces. The converse happens with a narrow contact patch on a wide wheel.

Camber gain is the amount of negative camber a tire/wheel combination attains as a car more heavily loads said tire while cornering. Some suspension designes have a lot of camber gain (front end suspension of 1960 American V8 big luxury cars), and some suspension designs have little camber gain (strut suspensions with parallel lower control arms.) While the former is well suited with wide tires onnarrow rims, the latter is well wuited with wide tires on narrow rims.

Suspension design and optimization has to deal with all of these variables to achieve a well balanced suspension.

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