Braking vs Camber

[gnosez]

Am I right to assume that the more negative camber you have the less the tire patch is and therefor the harder it is to stop a vehicle using the brakes?

 

If so, has anyone found a compromise camber angle that gives good turn-in and doesn't effect braking as much?

[blueovalz]

My understanding of contact patches is that the patch is directly related to weight on the tire, and inversely related to air pressure in the tire. This means that the contact patch area should remain the same, but that the configuration, or shape, of the patch will change. With the rigid sidewalls of todays tires, I would find it hard to believe that the patch does not reduce in size once the tire is tilted over a significant amount. I believe there is a correlation between shape and force, but I am lacking in absolute knowledge on it. There is a means of addressing some of this problem though, and that would be through the caster. With the Z suspension, one can increase the camber curve (increase in camber as the wheel is turned) by increasing the caster. Depending on the circuit, and your suspension, increasing the caster and decreasing your static camber could help you maintain a more optimal contact patch.

[johnc]

OK, this is going to be a long reply with a big quote included from Mark Ortiz (a chassis/suspension consultant that is a regualr contributor to Racecar Engineering magazine).

 

Terry is right about contact patch area remaining relatively constant and he's also right about tire construction itself affecting contact patch area and shape.

 

There are a couple things that can contribute to reduced braking distances when tire camber is increased (remember though, we are talking about fairly extreme camber angles (+3 degrees negative).

 

1. Directional instability. Increasing a tires camber changes the contact patch shape and changes the pressure distribution across the tire carcass when the tire is not in a corner. This pressure distribution change causes the tire to be extremely sensitive to road imperfections. The tire will tend to defelct or follow any imperfection in the road and variations in load transfer also induce changes in the tire's slip angle - thus affection directional stability.

 

2. Contact patch shape. By running the tire on the inside of the tread we are changing the contact patch shape from wide and narrow to thinner and longer. This affects tire grip. Read the following:

 

WHY ARE WIDE TIRES BETTER?

 

It has been recognized for about 40 years now that wide tires provide more grip, at least when we are not limited by aquaplaning. One might suppose that this effect would be be well understood by now, on a theoretical level as well as a practical one. Yet the matter seems to be receiving a lot of attention from various authors lately. This seems to be due in part to the need for mathematical tire models to be used in computer simulation. I have encountered the question at least twice in the past month, once in a seminar presented by Paul Haney, based on his recent book about tires, and once in Paul Van Valkenburgh’s November Racecar Engineering column. The issue has also come up in my work as an advisor to the UNC Charlotte Formula SAE team.

 

On the face of it, one might wonder why there is any controversy about this, and also why it took people until the 1960’s to try wide tires. More tire, more rubber on the road. More rubber on the road, more traction – right? Why wouldn’t this be obvious?

 

Essentially, there are two reasons it wasn’t obvious. First, according to Coulomb’s law for dry sliding friction, friction is independent of apparent contact area. It depends instead on the nature of the substances in contact, the normal (perpendicular) force, and nothing else. Second, a tire’s contact patch area theoretically doesn’t vary with its width anyway. If we widen the tread, the contact patch just gets shorter, and the area theoretically stays the same.

 

Let’s consider each of these notions. Coulomb’s law applies quite accurately to hard, dry, clean, smooth surfaces. However, a tire tread is a soft, tough, sometimes tacky substance in contact with a hard, rough surface. When two hard, smooth surfaces are in contact, they actually touch only at a small percentage of their apparent or macroscopic contact area. Friction depends on molecular bonding in the small microscopic contact zones. As normal force increases, the microscopic contact area increases approximately proportionally, and consequently friction is directly proportional to normal force.

 

With rubber on pavement, however, there is not only the usual molecular bonding but also mechanical interlock between the asperities (high points) of the pavement and the compliant rubber. Sliding then involves a combination of shearing the rubber apart and dragging the asperities through it as the rubber reluctantly oozes around the asperities. The interface somewhat resembles a pair of meshing gears. With gears, when we increase the size and number of teeth in mesh, we increase the force required to shear off the teeth. It would be reasonable to expect a similar effect with the interlock between the tread and the pavement.

 

With increasing normal force, this interlock gets deeper, as the asperities are pushed further into the rubber. However, we might reasonably expect that at least beyond a certain point, the asperities are pushed into the rubber to pretty nearly their full depth, and further increase in normal force does not proportionately increase the mechanical interlock. With greater macroscopic contact area, it should take a greater normal force to reach this region of diminishing return.

 

A tire typically does show characteristics that would match this hypothesis. It will often have a range of loadings where its coefficient of friction is almost constant; where friction force is almost directly proportional to normal force. Above this range, the tire exhibits much greater load sensitivity of the coefficient of friction. The curve of friction force as a function of normal force goes up almost as a straight line for a ways, then begins to droop at an increasing rate.

 

Of course, the contact patch does not remain the same macroscopic size as load increases. It grows as we add load. Nevertheless, this contact patch growth is evidently not enough to keep the coefficient of friction constant.

 

The contact patch growth is interesting in itself, and a bit counter-intuitive. A tire can be considered a flexible bladder, inflated to some known pressure, and supporting a load. If such a bladder is extremely limp when uninflated, like a toy balloon, and we inflate it, place it on a smooth, flat surface, and press down on it with a known force, the area of contact with the surface is equal to the normal force divided by the pressure: A = Fn/P.

 

If a tire approximates this behavior, then it follows that the contact patch area depends only on the

load or normal force and the inflation pressure. If we make the tire wider, then at any given load and pressure the contact patch doesn’t get bigger, it just gets wider and shorter.

 

Accordingly, much discussion of the reasons a wide tire gives an advantage focuses on reasons we might expect a wider tire to yield greater lateral force than a narrower one, assuming similar construction and identical pressure, tread compound, and load.

 

One theory, advanced by the late Chuck Hallum and evidently picked up by Paul Van Valkenburgh in his recent column, is that a tire is primarily limited by thermodynamics. It generates drag when running at a slip angle. The drag times the speed equals a power consumption, or rate of energy flow. This energy is converted into heat. For the system to be in equilibrium, the heat must be dissipated as fast as it is generated. Even short of the point of true equilibrium, the tread compound needs to be kept below a temperature where it softens to the point of being greasy rather than tacky. If the contact patch is shorter, that means that each square inch of tread surface spends less time getting heated and more time getting cooled.

 

Also, when a tire is operating near its lateral force limit, the front portion of the contact patch is “stuck” to the road and the rear portion is a “slip zone” in which the tread moves across the pavement in a series of slip-and-grip cycles. The slip zone grows as we approach the point of breakaway. Beyond the point of breakaway, the entire contact patch is slip zone. The slip zone generates less force and more heat than the adhering zone. A shorter, wider contact patch is thought to have a larger adhering zone and a smaller slip zone at a given slip angle, and wider tires are also known to reach peak force at smaller slip angles. Therefore, a wider tire is not only better able to manage heat, but also generates less heat at a given lateral force.

 

This all makes sense, but it fails to explain why wide tires give more grip even when stone cold.

 

There is little doubt that they do. If you have a street car with four identical tires, and you replace the rear tires and wheels with ones an inch wider, using the same make and model of tire, with no other changes, the handling balance will shift markedly toward understeer. You will see this effect at all times, from the first turn in a journey to the last. Surely this effect is not coming from heat management.

 

Paul Haney explains this by the larger-adhering-zone theory described above. The tire makes more efficient use of its contact patch, even if the contact patch isn’t larger.

 

As much sense as the above theories make, they ignore some real-world effects that have a bearing on the situation.

 

First of all, the degree to which tires follow the A = Fn/P rule varies considerably. A very flexible tire, at moderate load, may have a contact patch as large as 97% of theoretical. A fairly stiff tire may

 

 

be well below 80%. We are all aware of run-flat tires currently being sold, which will hold up a Corvette with no inflation pressure at all. As P approaches zero, Fn/P approaches infinity. If A does

not approach infinity, and the tire does not go flat, the contact patch area as a percentage of theoretically predicted area approaches zero.

 

One might suppose that the effect of carcass stiffness would be significant mainly in street tires, with run-flats being an unrepresentative extreme. Yet I have seen dramatic differences in carcass rigidity in different makes of racing tires intended for the same application. The Formula SAE car run by the University of North Carolina Charlotte uses 10” wheels. Hoosier and Goodyear both make 6” nominal-width tires for the application. The stiffnesses of these tires differ dramatically. The Hoosiers are much more flexible than the Goodyears. The Goodyears are so stiff that they will support the front of the car (without driver), with little visible deflection, when completely deflated – run-flat racing tires! How closely do these tires approximate A = Fn/P in this load range? Not very closely at all.

 

My point here is that tire stiffness, vertically, laterally, and otherwise, is not purely a function of inflation pressure, so it is a bit risky to try to infer contact patch size from pressure and load. Therefore, we don’t necessarily know that two tires differing only in width do have the same contact patch area at the same inflation pressure and load, or even that tires of the same size do.

 

Anyway, if it is approximately true that A = Fn/P, it follows that a wide tire will have greater vertical stiffness, or tire spring rate, than a narrow one, at any given inflation pressure. It will also have a smaller static deflection at a given load, which is why the contact patch is shorter. The flip side of this is that for a given static deflection or tire spring rate, a wide tire needs a lower inflation pressure. Consequently, if we compare wide and narrow tires at similar static deflection or tire spring rate, rather than similar pressure, they will have similar-length contact patches and the wider one really will have more rubber on the road, just as we would intuitively suppose from looking at them.

 

As we make a tire wider, not only does vertical stiffness increase for a given inflation pressure, so does the tension in the carcass due to inflation pressure. A tire is a form of pressure vessel. We may think of it as a roughly cylindrical tank, bent into a circle to form a donut or torus. Borrowing from the terminology of pressure vessel design, we may speak of the “hoop stress” in the walls: the tensile stress analogous to the load on a barrel hoop. For a given inflation pressure, the hoop stress is directly proportional to the cross-sectional circumference, or mean cross-sectional diameter. When the carcass is under a higher preload, the tire acts stiffer laterally. This effect can easily be seen in bicycle tires. A fat bicycle tire will feel harder to the thumb than a skinny one, at any given pressure. If we try to inflate a mountain bike tire to the pressure we’d use in a narrow road racing tire, the tire will expand its bead off the rim and blow out. So when we compare narrow and wide tires at equal inflation pressures, the wider one will be stiffer laterally as well as vertically, and it will achieve this at no penalty in contact patch size.

 

Finally, there is the question of tread wear. As we have noted, if the contact patch is longer, it has a larger slipping zone near the limit of adhesion, and it also spends a greater portion of each revolution

 

in contact with the road. Not only do these factors influence how hot the tire runs, they also influence how fast it wears. Therefore, assuming good camber control, a wide tire should last longer than a

narrow one, with similar tread compound. The astute reader will see where I’m headed with this. If we need to run a given number of laps or miles on a set of tires, then with wider tires we can trade away some of the inherent longevity advantage, and run a softer compound.

 

Okay, summing up, what does a wider tire get us?

1. It runs cooler, and/or

2. it makes more efficient use of its contact patch by having a greater percentage adhering, and/or

3. it can run at lower inflation pressure and therefore actually have a larger contact patch, and/or

4. it can have greater lateral stiffness at a given pressure and therefore keep its tread planted better, and/or

5. it can use a softer, stickier, faster-wearing compound without penalty in longevity.

 

Note that most of these effects in turn play off against each other. We can blend and balance them, and get a tire that is somewhat cooler-running, has a somewhat lower operating pressure and somewhat larger contact patch, has somewhat greater lateral stiffness, and survives long enough with a somewhat stickier compound, all at the same time. That would explain an improvement in grip, wouldn’t it?

[gnosez]

Thanks to you both. I'm sure a few others will enjoy the info as much as I did.

 

I am at 3 degrees neg and have tried braking at 0 degrees and 3 neg and have found that it takes more time (read distance) to stop. The car is also less stable while under threshold braking at speeds over 100 mph with camber set at more than 2 degrees. Not to the point where I feel the car isn't under my control but where I sense it and therefor it demands a part of my attention when I should be thinking other thoughts (like did I remember to buy my wife's birthday present, etc.) as I make my turn in.

[blueovalz]

Excellent JohnC!! All the while I was reading this I am trying to compare an optimal patch for drag racing verses road racing. Initially, my assumption was that if a wide, short pattern is good for lateral loads, then a narrow, long pattern would be good for longitudinal loads (drag racing patterns and road racing patterns having patterns perpendicular to each other), but obviously you don't see drag racers with narrow tires. So somewhere in this information a hint must indicate why the two types of racing work well on essentially the same pattern. My guess is that the slip angles (zero for the drag tire and minimal for the road tire) must be so low that they in essence can be thought of as the same or similar, thus the fat tire works with both. The heating/cooling efficiency caught my eye on this as well, but it is hard to accept when half a tire goes up in smoke on a burnout prior to a drag race. Balled up rubber (which is not mentioned here) must be considered as well. A wide, short pattern will not "roll" over sheared rubber (marbles) as much as a narrow, long pattern would. Thus the contact patch following the area that is initially subjected to shearing is shorter, and hence, the marbles are dispersed more quickly. A long, narrow patch would seem to keep the contact patch over the sheared rubber longer.

 

This article created as many questions as it answered, but thanks for the post!

 

GnoseZ:

 

This is why I went to approximately 7º of castor on my Z. The camber increase at this setting gave me a better camber change (overall) than the 3º OEM setting did. For the most part I was able to reduce the camber settings to less than half of what I needed prior to the change. The type of circuit, and suspension (springs and bars) has a lot to do with how well this works, but on my car it was a good compromise between the tighter courses and the high speed sweepers.

[Drax240z]

For those interested, Mark Ortiz puts out a free internet newsletter (the name is escaping me at the moment) and it is excellent reading on all things suspension. I'm sure a quick websearch will find it for you.

[JMortensen]

Try a little toe in out back. I'm running 3/16" total. It should help control the sketchy feeling out back and may make the difference for you. At some point there is a balance between more negative camber makes you turn faster and less enables you to brake harder, but I'm with John on this one and I don't think 3º of camber should be affecting the car terribly.

 

Caster makes a HUGE difference as Terry said. I don't run as much as he is, I've been running about 5º with -3º camber in front.

[Mikelly]

Yup, Excellent read and I enjoy all his!

 

We found with Steve and Ian's 2000# 240Z that less negative camber and more positive caster worked wonders for handling. Steve and ian eventually ended up with 6.75 + Caster up front with .75 -Camber and zero to 1/8th toe. On the back, they were running around -2 degrees camber and 1/8th toe. Those boys are SMOKING Quick!

 

Mike

[johnc]

But, since Steve and Ian were (are?) running bias ply racing slicks, their alignment settings (in particular, camber) are very different then what folks running DOT-R tires need. When Bryan Lampe and I ran bias ply Hoosier racing slicks on his ITS 240Z for OTC 2002 when went from about 3 degrees negative in front to, at most, 1 degree negative.

 

Ultimately you need a tire pyrometer to determine what camber settings you need. When I went from 245s to 275s on my car I actually reduced camber from negative 3.2 in front to negative 2.75 based on tire temps.

[Drax240z]

Are any of you achieving significant caster gains with using urethane bushings in the front control arms? I'm curious to know how much binding is going on there. I am at 2.9* caster and not sure I can get much more out of the front without modifying a-arm to accept a spherical bearing of some sort.

 

I really should get some more caster though, I think that would solve a lot of my low speed cornering push.

[260DET]

Stability under braking may be influenced by many things, just one of which is camber.

 

Using R tyres with up to 3.5 degrees negative camber my car is completely stable under high speed braking up until lockup. While the suspension settings have been changed over time, braking stability has remained consistently good.

 

I've never heard of any Z braking instability problems other than those due to incorrect proportioning. Seems that the Zed is inherantly stable under braking and will remain so if kept in sound condition.