Tuesday, February 21, 2012

(165) Crossing over

You may remember when I wrote about getting the new family car, a 2012 Subaru WRX, I think I mentioned that my dad was interested in taking it autocrossing.

Well, last weekend, we did.

Since I have no financial ability to race road courses on my own at the moment, autocross is a good way to stay in driving shape without spending very much money.

Basically, for 35 bucks you get a few runs at a course laid out in a parking lot with cones and chalk. Complete the course in the fastest time and don't hit cones because you'll get a time penalty for each one. Really simple, and a great way to have a lot of fun in everyday cars in competition. SCCA puts on the events, as do other clubs.

For the first run I coaxed mom to go for a ride. She was stony silent through the whole run, but she started shouting how awesome it was once we crossed the finish line.

I didn't find it quite as exciting as club racing, but you can't have as much for such a small cost.

We entered the car in the D Stock category. But since we are rookies, we elected to compete in the Novice class. The Novice class is one of the "indexed" classes. Basically, each class has a handicap that adjusts the final time based on a multiplier. Once the multiplier is computed, every class has equal footing to compete against each other.

Now you may be wondering why an experienced road racer is going into the novice class. The reason is because I have been briefly exposed to cone-dodging before. I know how different of a discipline it is.

In road racing, you have the opportunity to get practically unlimited attempts at perfecting the course prior to actually racing on it. Only money and scheduling is stopping me from going out to Infineon for a day of lapping in a race car in order to gain a better understanding of the track. Unless major construction occurs, the track will not change very much by the time I next race there.

Autocross travels to a variety of different sites, and those sites don't change very often, this is true. However, the course is always different for every event. A huge portion of the skill of a competent autocross driver revolves around extremely quick learning. I'm not familiar with learning a track in such a way. You literally cannot be methodical. A run may be around a minute long, and you may only get 3 or 4 runs. 4 minutes of track time and 4 attempts to set the fastest time possible. At a road race, I will have usually between 40 minutes to an hour of track time with around 20 laps of practice and familiarization prior to the first race. And that doesn't count previous visits to the track.

My dad, Scott, taking a run. The Subaru designers really outdid themselves. From this angle, with this lighting especially, the WRX sedan is gorgeous.

And the spontaneous nature of the sport is only half of it. Throw in another twist in the form of a very tight track at speeds less than 50 or 60 mph (when, in road racing in a Spec Miata, per-lap average mile per hour is in excess of 75 mph with top speeds approaching twice that of autocross), and you've got a very different discipline indeed.

I fully expected to do okay, but not to break any records.

My dad took the car for the first run to get acquainted with the course. I did the same soon after, taking mom for a joyride which she enjoyed immensely.

At first I turned the traction control system off. I wanted to feel how the car was set up mechanically first. It felt pretty good. Being an all wheel drive car, the engine sends half of it's power to the front tires and half to the rear. This is a recipe for some understeer and the car does push a bit. But it is controllable and it doesn't completely wash out the front grip. It is suitably agile.

The car isn't a tank but it's not a flyweight either. It weighs 3,300 pounds. Naturally that tends to pull the body around a bit as you brake, accelerate and turn,. The car has a tendency to really dig into one corner of the car at a time an that gives it a hoppy feeling as it bounds from corner to corner.

On the second run I turned the system on to see how that would fare. It didn't go well. The computer seems to have an aversion to accelerating while turning. It's so conservative, in fact, that it simply will not allow you to reach the limits of adhesion. If you enter a corner faster than it wants you to, it slows you down with additional braking. If you try to accelerate back up to the speed you intended to corner at, it won't let you, because it cuts the throttle according to how much steering you're using. And with the system on, any press of the brake will cut the throttle as well, so you cannot use the gas and the brake at the same time.

Thankfully this is all disabled by a button on the dash and the car becomes cooperative again. I will actually start disabling the system when I drive it on the street, because I do not have faith that the car will respond in a way that I am trained to expect if I have to make an emergency maneuver. This is not a fault of traction control as an idea, mind, it is just a fault of this particular system. A good traction control system will work with a driver, and I have experienced systems that do just that. This one tries to control the whole car by itself. Slightly over-reaching, in my opinion.

I don't think this level of cornering ability is possible with the computer activated. By the way, all of these lovely images are courtesy of Doug at hkophoto.smugmug.com

In any case, with the computer sleeping serenely via that special dashboard button, the car will corner like a champ. It has wonderful traction coming out of the corners and the engine is superb. I would not believe that the car was turbocharged if I didn't see the turbo under the bonnet.

Anyways, you probably want to hear about the rest of the day, so I will direct you to the video I got from my helmet camera.

SFR SCCA Solo 2012 Round 1 helmet cam footage.

There are still things to work on. My starts need to be a bit faster (I was trying to be gentle because I hear the 5-speed is kind of delicate), and there was probably at least a second on the table in my 3rd run.

So how did we do? Well, my dad, Scott, got 5th out of 16 in the Novice class with an indexed time of 37.852, and I got first place with an indexed time of 34.727.

I'm pleased with our results to say the least! I'm proud of my dad for doing so well at his first event and I can't wait to do it again.

Tuesday, February 14, 2012

(164) Driving According to the Chump #7: Stop Braking

A long time ago in this series, I said I would not focus on something so elementary as braking a race car. Braking is really simple - you press the pedal hard, and if you push too much, you lock up. So only push as much as you can without locking up a tire.

The complex part happens when you decide you want to stop braking. Since this is complex, let's talk about it.

Basic human instinct is to do things safely. Most racing drivers will probably forget (or refuse to admit) that the first time they drove on a track they naturally over-slowed the car for corners. With time and practice, and possibly a gene deficiency or two thrown in for good measure, they will reprogram themselves to carry more speed into the corners.

Once that hurdle is overcome (and it may take a while), you will begin to find that the brake pedal does some interesting things to the car as you begin to turn. You've discovered trail braking.

Trail braking can be a little hard to follow so I'll illustrate textually.

When the car brakes, the tires generate a force that turns the energy contained in the vehicle - the kinetic energy we gained from accelerating - into heat energy via friction between the road and tires, and friction between the brake pad and the brake rotor. If your brakes are suitably durable and your tires suitably sticky, your brakes may even glow red due to the relatively high amounts of energy being dissipated. The red glow is literally the energy being radiated outward.

During this whole process, the chassis and the body of the car is still trying to move forward. It is by the mechanical linkage to the tires that the chassis decelerates. As a result, the chassis tries to push forward in an attempt to separate from the tires and wheels. The linkage should be strong enough to arrest the chassis, which causes the nose of the car to dip, and the tail to lift.

The lift/dip is called weight transfer. In braking specifically, since the chassis is dipping to the front, the front tires will now carry most of the weight of the car, assuming a reasonably even weight distribution between the front and back wheels while at rest. This transfer of weight has a fun effect on the tires in the front - it flattens them out. It also brings weight off the rear tires, and those tires will become elongated vertically.

This directly affects the area of the tire that is in contact with the road. Since the fronts are now flatter, and the rears more stretched vertically, the fronts will have more material on the road, and the rears will have less material on the road. This enables the front tires to generate more deceleration, and the rear tires will generate less deceleration. For the purposes of a single-car example, this doesn't make a difference to the maximum achievable deceleration of the car. It would be desirable if we could get less nose dive by changing settings or parts on the car, but if we cannot, then our best choice is still to make the body roll as much as possible because by doing so we are generating the maximum amount of acceleration or deceleration possible.

This transfer of load from body roll also works from side to side and to the rear, when turning and accelerating in addition to braking.

So what we've done is we've taken grip from the rear and put it on the front. In a straight line, this makes little difference. We simply adjust the brake system so that more braking is done on the front tires so that we can stop most effectively - the front brakes clamp down a bit more and the rears clamp down a bit less, and we stop very well. But it gets complicated when we try to turn.

On the limit of grip, the car uses all four tires to turn. That is to say, all four tires are generating side-force. The amount of side force each tire (and for our purposes, each end of the car) produces depends on it's contact area. It also depends on it's heat, it's compound, and other factors, but let's keep it simple.

To illustrate this, let's imagine a car sitting on a flat surface. It is still - it is not accelerating, turning, or braking. It's just sitting there being fabulous.

The car weighs 2,000 pounds and it has absolutely perfect weight distribution when sitting still - 500 pounds rests on each tire. That's 1,000 pounds in the back, and 1,000 pounds in the front. Nice. The engineers outdid themselves this time.

The car also has symmetrical track widths, and tire widths. The suspension settings (spring rates, damper rates, camber and toe settings) are the same on all four corners.

The car is set up to have perfectly even grip generation capabilities between each end of the car. That is to say, when subjected to a turn with no acceleration or deceleration forces, the front and back tires generate (combined) the same forces in order to turn the car, assuming the car is being turned on the limit of grip.

If we begin to introduce acceleration (pressing the throttle), the car will raise it's nose and squat the rear. More grip potential will be on the rear tires, and less will be on the front. The total grip remains the same, but now the front tires will have less grip and the rears will have more grip. The nose will not be pushed "into" the corner as strongly, and the car will not respond as sharply. It's resisting yaw. We're understeering.

If we decelerate the car (brake), the opposite occurs. Just like braking in a straight line, the fronts will gain more grip because the nose is pressing down on them, while the rears will lose grip because the tail is lifting up. The nose will be pushed "into" the corner more strongly when we try to turn because they have more grip, and the rear tires will resist less strongly, causing the rear of the car to swing outward since the tires cannot counteract the total forces. Basically, the car is yawing, and the rear tires don't have enough grip to stop the yaw. It's trying to spin - oversteer.

So, we now know that the brake can be used to alter the yaw rate of the car as it turns. The amount of braking to use in order to encourage the car will therefore change depending on the static weight distribution of the car and the basic tendency for the car to try and spin out, or resist turning - whichever it ends up being. Depending on the exact weight distribution, this can become a quite complex dance. The yaw rate might have plus or minus peaks and valleys depending on how the brake is manipulated and how the weight is distributed in the car.

So when do we try to do this? That's the art, really.

Trail braking is not a technique to use all the time. Not every corner requires it. But it would be a very unusual circumstance indeed if the track you're on didn't have at least one corner where trail braking was needed.

I think the best way to figure out if you need it or not is to change how you view the purpose of trail braking. Most drivers probably view it as a rotation tool, meaning that it's design is to get the car to the angle where the tires are happy. But I don't think this is a wise way to look at it. It's too easy to trick oneself into over-rotating the car and simply scorching the tires.

My trail braking performance improved drastically when I started viewing trail braking in a slightly different way. I view it as an understeer control device. As you enter the corner, ask yourself if the level of understeer being exhibited by the car is acceptable to you. If it is acceptable, then you do not need to trail brake. If it is unacceptable, then trail brake a little bit, just enough to make it acceptable. I find that by sticking to this line of thinking, my corner entrances are more consistent and much better controlled. You may take to this as well, or you may not. Everyone's different.

I want to quickly touch on one more aspect of braking. The above is mostly about how to release the brake, but when should we start releasing it?

Conventional wisdom says that the brake release should happen as the wheel begins to turn for the corner, appeasing the law of the traction circle that says the car can only do 100% braking or 100% turning, not both, and as such, inputs should be blended together in order to smoothly transition between full braking and full cornering.

This is true. But we're forgetting something. There is a human element at work.

Humans, sadly, are not very good computers. Our calculation speed is quite low. Even the fastest thinker is still very far behind a simple calculator in terms of adding up numbers in a conscious sense.

So imagine coming down from 110 MPH to 45 MPH for a hairpin, in about 350 feet, during about 2 or 3 seconds. You are not fast enough to watch the speedometer tick down to the precise mile per hour you need to enter the corner on the limit and if you don't pay attention to it, as most people don't, you're very unlikely to hit that precise speed when decelerating that quickly on the limit of traction, using only your subjective sense of speed.

My racing coach suggests a different approach. Literally. Ric calls it thirds braking.

His theory goes that for any given large braking zone, it is advisable for the driver to divide it up into three sections, evenly spaced, from the point he begins braking to the point where he begins to turn. The first and second sections are full deceleration zones - braking with 100% capacity. The third and final zone, the last third of the braking area, is the fine-tuning zone. During this zone, the driver releases the brake pedal enough so that he slows his rate of deceleration so that he may more accurately choose the speed at which he enters the corner. How much to reduce the rate of deceleration depends on the driver's ability to choose an accurate speed.

I do not use thirds braking literally. In reality I use maybe the last 10 to 15 feet of the braking zone to come off the pedal a tiny bit and tune my speed. But the principle is the same - give the driver just enough time to choose his speed, instead of doing the whole thing off of timing which requires precision a human is not capable of.

The better the driver is at choosing his speed, the less he will have to release the brake and the closer he can get to appeasing the ultimate action of the traction circle theory, which is maximum traction use at all phases of the corner. By trading a tiny bit of braking efficiency for that extra rolling speed, you'll end up going faster. Braking at the limit is absolutely useless if you just waste the effort by scrubbing too much speed in the process. Entry, apex, and exit speed have absolute priority.

I believe the common phrase "let it roll into the corner" applies here. When drivers say this, they mean to come off the brake just a tiny bit sooner and carry that tiny extra amount of speed into the corner that results. I don't think too many drivers really understand what that phrase means, though I could be wrong, since I am not telepathic. Yet.

Ten tips for braking:

1. Don't just threshold brake all the way to the turn in point. Chances are you're over-slowing the entry. Back off the brake a bit before entering the corner and watch those entry speeds rise.
2. If you're finding your trail braking is resulting in a lot of oversteer or high tire temperatures, try changing the way you think about trail braking in order to get you to stop rotating so much.
3. Learning to come off the brake can take a lot of time. Patience is key.
4. When unsure whether to trail brake or not at a particular corner, prefer not trail braking, and then ask yourself if the level of understeer is acceptable to you. If it is not, trail brake a little bit until it is.
5. Try to envision (or better yet, work it out mathematically) how the size of the tire contact patch relates to the yaw rate of the car on the limit of grip.
6. A properly bled braking system can do wonders for your braking performance. The firmer the pedal, the easier it will be to fine tune your brake pressure - a light pedal requires very delicate feet.
7. In some cars, trail braking can actually cause understeer instead of oversteer. This is part of the peaks and valleys in the yaw rate I talked about. A rear-weighted car will be more likely to understeer at firm trail brake pressures, and snap oversteer when the brake is released.
8. Do try to train your left foot to brake. If you don't need to use your right foot to brake, don't.
9. When you initially apply the brakes, be direct. Don't gradually build up to threshold pressure. Hit the pedal hard and get to the desired pressure quickly.
10. Not every corner requires trail braking, but by the same token, almost every race track is going to have at least one major trail braking corner.

Saturday, February 4, 2012

(163) A little change of scenery

I've updated the blog layout and design today. That old format was getting, well, old. For now it's simple, but I may make tweaks and changes to it as time goes on.

Considering the rest of the blog, I have been thinking about putting ads on it for the past month or so. I have decided against it. While the traffic here could generate some income, I do not believe it is worth it to you the readers to have big advertisements plastered around. This blog is free for me to run, besides my simple time investment, and I see no reason to try to make money off of what is ultimately a hobby. I enjoy writing here since racing is my passion, even if my postings have gotten a little sparser lately.

Anyway, keep calm and carry on. I hope you like the new design.

Wednesday, February 1, 2012

(162) There is no such thing as a black art

I've heard a number of things been called a black art in racing. The most common example is the tire engineer being compared to a voodoo shaman. The argument goes that the racing tire contains so many unknowables that it is impossible to objectively develop it with any kind of expected precision.

Now, you're probably expecting me to write another sentence in which I "prove" (through my own force of utterance) that racing tires can be simply developed by taking into account some single variable that somehow applies to everything the tire does on the track. Unfortunately I half-expected myself to do this when I started thinking about this issue a week ago. That's how pervasive (and sometimes attractive) such gross simplification is these days.

No, I will say it right now - racing tires are complex. They may be the most complex single part on the car. As much as I understand them as a driver, much of their function yet remains a mystery to me.

But that does not mean the tire engineer has the same mysterious perception of the racing tire.

Racing tires are incredibly important to the car. This much is obvious. If you don't have tires you don't have grip. You can't accelerate, stop, or corner very well at all without them. Their wonderful characteristics are a perfect storm for shoes on a car - they make the ride more comfortable, they make the car more capable, they reduce noise, they make it easier to drive, and they do a bunch of other more minor things that are boring and that I can't really think of at the moment anyway. All I'm concerned with are how fast they are. Simple me.

We know why they're important. How do we make a good tire?

The first way is to have someone drive a car outfitted with our tires. As a driver, I will tell you right now, this will probably backfire. Drivers, even the good ones, are highly subjective analysis tools. The best test driver is usually totally different in personality to a championship winning driver. If you give a car to two different drivers you will usually get two different view points as feedback. This is not so much a problem when you have one car for one driver - just make it how he likes it and he'll go as fast as he can because he's comfortable. But this is a problem when you are developing a set of tires for sale to a bunch of different drivers and cars. We're not making a tire that one car and one driver has to like - we need lots of people and machines to like it. So we need to remove those faulty variables.

Meet the tire test rig.

Photo courtesy Engineering Dynamics Corporation.

This guy will take your tire through a number of normal operating ranges (called slip angles and slip ratios) at a number of speeds for any duration you want with almost any practical weight pressing down on the tire.

What it does is record the forces exerted on the arm of the machine when the tire deforms. Basically, it just measures the stickyness of the tire. By extension it can also test basic tread wear rates and simple durability in a controlled environment.

Using this rather intelligent-looking thing, tire engineers can experiment with the construction of any tire and observe the results. This invention is probably the single largest reason why tires these days are so good. There are large repositories of data from machines similar to this scattered all around the internet. One of the reasons for it is to use in simulations. In fact, you can glean a lot about how a tire works by trying to build a simulator (not that I have, but talking to other people who have is almost as good in some cases).

If you want a really good resource I stumbled upon recently, there is a document by Michelin that details exactly what a tire does. I think you will be surprised how much tire engineers understand about tires. They get paid to study them, after all. Here is a link to the document on a free download service called Mediafire.


(It was hard to find this, and I'm unsure if it was ever publicly released by Michelin, so if it was not intended to be shared please let me know and I'll take it down)

If you read that and still think developing a good tire is a black art, then I don't know what else to tell you. This is science. Not magic. We humans can smash protons together 250 million times each second at nearly the speed of light, and we know the exact probabilities of what will come out of the explosion and where it will go. You really think modern science wouldn't know how a tire works after doing quantum mechanics?