Driver Log

First, let me correct a mistake in the previously published DE schedule.  We will NOT be at Watkins Glen from September 18 through 29. Twelve days in a row at the Glen has a certain appeal, but human, mechanical, and financial fatigue do take a toll. We’ll have to settle for the 18th and 19th.
UPCOMING EVENTS
On February 26 we will hold our annual instructor and driver seminar. This year we are very happy to have as speaker Cass Whitehead, Head Instructor for the Porsche Sport Driving School.  Please see the ad on page 5 for further details.
Registration opens on February 8  for our first DE of the 2017 season, which is, of course, at Lime Rock. We’ve been able to move the event one week later in April. That is no guarantee of tropical weather, but it is, as a former boss of mine used to say, directionally correct.
Registration for our advanced DE at NJMP Lightning opens on February 25. That event used to take place on a Monday and Tuesday. Last year we got it moved to Sunday and Monday. This year I’m pleased to report that we have now secured Saturday and Sunday. So, for those of you who do not require an instructor, why not enjoy a spring weekend on the track with NNJR?
THE TAO OF TRACK TIRES
Tires are the single most important factor in vehicle handling. Don’t believe me? Equip your 991 GT3RS with narrow, long life run flats and let’s see how well you do at Mid Ohio. I often get asked about which track tires are best. My answer – it depends – is not always well received. Let’s take a look at what a tire is and how it works. Let’s also take a look at tire options for the track.
The function of a tire is multifold. It supports the mass of the car. It allows the vehicle to travel over a surface with a low level of energy lost to friction (rolling resistance). It is able to transfer longitudinal forces (acceleration, braking) and lateral forces (steering) to the pavement, thereby altering the velocity (speed and/or direction) of the vehicle. It augments the spring and shock absorber as the sidewalls have characteristic spring and damping rates.  The tread acts as a pump to remove water from the contact patch, allowing much greater traction when the pavement is wet.  It must function under a variety of loads and environmental conditions and be resistant to puncture and impact. This is, from an engineering standpoint, a pretty tall order.
How does the tire support the car?  An uninflated tire has sidewalls that will buckle under moderate load.  Once inflated, however, the
sidewalls are put into tension and are able to support much higher loads. Too little pressure, and the entire tread, specifically the center portion, will not maintain contact with the road, reducing traction and generating unwanted and potentially dangerous heat buildup. Too much pressure, and again the tread will deform, this time bulging in the center and lifting the edges and, again, decreasing adhesion.
How does a tire transfer forces to the pavement? There are two primary factors at work.  There are atomic level forces that cause adhesion between the tire and the pavement. These are lost if the pavement is wet. There are much larger scale forces caused when bits of the tire surface mechanically interlock with road surface irregularities. These forces continue to work when the pavement is wet, up to a point. If the amount of water exceeds the ability of the tread as a pump, the tires ends up separated from the pavement by a layer of water. This is call hydroplaning, and when this happens you may as well be on ice.
Let’s consider tire options for use in DE as a function of driver experience.
Novices typically start with street tires. Street tires are designed to provide reasonable levels of grip through a wide range of temperatures (with the range being narrower for ultrahigh performance). Consequently, they have grip as soon as you begin your session, regardless of ambient temperature or if the track is damp. Once their maximum grip is exceeded, it falls off gradually, not precipitously. They have treads that are able to pump out a lot of standing water, making it safer to run in the rain. They also have the wonderful characteristic of making noise as they approach the limits of traction, so you don’t need to be highly skilled to know when your tires are close to giving up. Because they have lower levels of grip, you push the car into understeer (front tires on holiday) or oversteer (rear tires on holiday) or a four-wheel drift (all four on holiday simultaneously) at lower speeds. This enables novice drivers to gain critical car control skills at non-scary speeds.  They are in fact the best choice by far for learning track driving.
A lot of drivers want to go as fast as possible as soon as possible, and hence wish to use track tires immediately. In fact, some new Porsches come with so-called R compound tires, which are street legal but optimized for track use. Tread is mostly eliminated and sidewalls are stiffer, which improves response time to steering inputs. The compound is much stickier, enhancing grip. If you know what you are doing, you can corner much faster. If you don’t, you can still corner faster, but watch out, as there is no free lunch. Going to these tires too soon is a mistake, because it causes the driver to miss steps in the learning process. It is easier and safer to learn car control skills – like skid recovery – at lower speeds or in the rain. Very sticky tires, coupled with electronic stability systems, will allow a novice driver to drive through turns with very poor trajectories, up to
pretty high speeds. Unfortunately, beyond that, the tires and the car will give up very quickly, and the novice driver likely will not have acquired the car control skills needed to prevent an off-track excursion. These tires will typically require changes to the suspension setup that may make the car feel nervous on high crowned, uneven New Jersey back roads. They are also expensive and don’t last very many miles, and they are nearly useless in standing water. I know.  I once drove on Pilot Sport Cups through 300 miles of Pennsylvania, on the way to Mid Ohio, in the pouring rain. It was probably the world’s longest drift, and not fun. My advice is not to be in a hurry to move to R compound tires.
Slicks are the ultimate track tires. They have no tread whatsoever and are hence useless when there is standing water on the track. They use a compound that is very, very sticky when hot. When cold, they have less grip than a street tire. For maximum grip, slicks must operate over a rather narrow temperature range – and thus over a very narrow range of inflation pressures when hot. Outside that range, grip falls off precipitously. There is a reason why you see guys in street tires or even R compound tires blow by cars with slicks on warm up laps. If the ambient temperature is 45 degrees or less, and especially if it is cloudy and hence the track stays at ambient temperature, it becomes impossible during a DE to get enough heat into the slicks to make them stick.  Getting slicks to work properly requires a suspension system designed for them – typically with parameters that are impossible to obtain with reasonable modifications to a street car (I run over four degrees negative camber on the front of the cup car). Settings must be precise and are checked by measuring core temperature across the tire when they are at full operating temperature.  This requires a tire pyrometer and a helper to take the measurements immediately after pulling into the pit lane. Slicks will typically lose a lot of their grip well before being worn out because multiple heat cycles (heating up during a run and then cooling down is one heat cycle) change the rubber and makes it less sticky. Slicks give you the impression of limitless grip. Until they let go, that is, and when they do, it happens quickly with no audible warning and recovering requires lightning fast and precise corrective steering input. Astonishing grip, expensive, difficult to master at the limit, and short-lived.  What more could you ask for?One more practical consideration: will I get more grip if I put wider tires on the car? The answer is: yes, but probably not for the reason you think. The area of the tire that is in contact with the pavement is called the contact patch, and it is, to a first order of approximation, determined only by the weight of the car and the tire inflation pressure (Force = Pressure*Area, so A =F/P). If you put on wider tires, you will not change the contact patch area.  You will, however, change the shape of the contact patch – wider tires will have a wider but shorter patch. According to tire engineers, in hard cornering, a wide but short contact patch will have a lower percentage of the contact patch slipping than a long and narrow one. But wide tires are not a panacea. Beyond minor increases in width, suspension and other vehicle changes become necessary.
As you may have guessed by now, track junkies never tire of talking tires. You knew that was coming