Suspension - Part 2 - Ride and Handling

Measuring Handling

There are several ways to test handling, some more subjective than others. One of these tests, the skidpad, is a great place to measure ultimate “steady state” cornering ability. It’s a simple test, where you drive around a pre-measured circle at the very limits of traction. By steady state, we mean that we try to keep the conditions of the test constant, by keeping the throttle and steering wheel as steady as possible while trying to stay centered on the prescribed circle. Based on your elapsed time for one lap around the circle (with a running start), the lateral acceleration of the car can be determined. This quantity is expressed in terms of acceleration due to gravity, or “g”. One “g”, (1.0g) which is a benchmark of sorts, means that the car generates sideways grip that is equivalent to the weight of the car. I like to describe 1.0g by noting that if you were turning right at 1.0g with no seatbelt, the force pushing you against the driver side door would be enough to hold you up if the seat were removed from underneath you. Kind of like the amusement park ride that spins you around in a big drum and then drops the floor out from under you, leaving you pinned to the wall with no visible means of support.

Another test is the slalom, where cones are spaced at even intervals in a straight line. This is a great test of a car’s “transient” handling ability. By transient handling, we mean the ability of the car to transition, or change direction, in a controlled manner. The driver enters the slalom (again with a running start) and weaves back and forth through the cones as fast as possible without hitting cones or losing control, the time from the beginning to the end of the cones is measured, and an average mile per hour speed is determined.

A third test would be comparative lap times around a racetrack or other controlled course, which would create a combination of both steady state and transient handling conditions. The trick to getting good results in any of these tests, of course, is to have an experienced, consistent driver, otherwise, the results could be less than optimal.

Measuring Ride

Ride, being a more “subjective” subject, is a little more difficult to define, and each individual has his own perceptions. There are actually calculations that can be made, based on the weight of the car, the spring rates, and the shock absorber characteristics. The results of these calculations are compared to many years of data gathered by car manufacturers and testing facilities about what most people find to be comfortable or harsh. The most common calculation used to analyze ride is what’s referred to as ride frequency, which is actually a simple calculation that looks at the relationship between the spring rate and the vehicle weight. Ride frequency is usually expressed in terms of cycles per second (commonly known as Hertz, abbreviated Hz). If you have ever checked your shock absorbers using the “push down on the bumper and see if the car goes down and up more than once” test, you've taken a rudimentary step toward measuring your ride frequency. If the shocks are completely worn out, and the bumper goes up and down several times, it will do so at the ride frequency. This number is usually calculated mathematically, though, to eliminate other influencing factors.

While ride may be a personal preference issue, much has gone into the study of how the human body reacts to various inputs – certain parts of your body actually vibrate in harmony with certain frequencies that a car chassis can produce, sometimes with uncomfortable results. If you’ve ever been to a live concert and been close enough to the sound system that you really feel the music as much as you hear it, especially the heavy low notes that sometimes make your chest feel like it’s vibrating, you are experiencing the same kind of thing I’m talking about.

Camshaft Function and the Four Cycle Engine - How Does a Camshaft Work?

Camshaft Operation

There have been thousands of pages written about camshafts, but I’m going to talk about them here, because it relates to my cylinder head articles. This is going to be basic stuff, so feel free to bypass these posts if you already have a thorough understanding of the underlying theory. I promise not to be offended.

The camshaft is probably the single most important component when it comes to determining the power characteristics of your engine. No other single component has as much influence on how your engine performs. The camshaft controls the opening and closing of the intake and exhaust valves in the cylinder head, either indirectly through pushrods, rocker arms, or followers, or sometimes directly.

Why, you might ask, is the camshaft so important? (Please ask).

As I stated in my first cylinder head modification article (http://hiperfautotech.blogspot.com/2009/05/cylinder-head-modification-part-1.html) , your engine is just a big air pump. The more air you can pump into and out of your engine, the more potential you have to make power. The camshaft not only opens and closes your valves to let air in and out, but determines when and for how long the valves remain open. With this in mind, let’s talk about what happens as the engine spins. What follows next is a basic explanation of four-cycle engine operation, described in relation to the four valve events. For each rotation of the cam, we have four valve events. The crankshaft rotates twice for each revolution of the camshaft, so four valve events happen for every two revolutions of the engine.

Event 1 - Intake valves opening

The camshaft opens the intake valve, and the piston moves down the cylinder. As the pressure drops in the cylinder, the air starts moving past the intake valve to fill the cylinder. This period of the engine cycle is known as the intake stroke.

Event 2 - Intake valves closing

At some point, usually after the piston reaches the bottom of the intake stroke, the intake valve closes. The piston moves up the cylinder, beginning the compression stroke and compressing the fuel/air mixture within. At some point, usually before the piston reaches the top of the compression stroke, the spark plug ignites the mixture, causing it to burn and expand rapidly. The crankshaft has rotated once at this point.

Event 3 - Exhaust valves opening

After the piston reaches the top of the compression stroke, pressure from the burning, expanding mixture pushes the piston back down the cylinder. The exhaust valve starts to open, usually before the piston is all the way down, allowing some of the burnt gasses to exit the cylinder. This is commonly referred to as the blowdown phase. The piston begins to move back up, forcing the rest of the hot gas out of the cylinder.

Event 4 - Exhaust valves closing

As the piston moves back up the cylinder, the exhaust valve remains open, usually until slightly after the piston reaches the top of the cylinder. We refer to this as the exhaust stroke. As the piston reaches the top again, the intake valve begins to open again, often before the exhaust valve is fully closed, and the whole cycle begins anew. The period when both valves are open simultaneously is referred to as “overlap.” The crankshaft has now gone around twice.

In a nutshell, here’s how it all happens.

Overlap
Intake stroke
Compression stroke
Blowdown
Exhaust stroke
Repeat

Next post we can tie this together with the cylinder head article. Thanks for reading.

Cylinder Head Modification Part 1

To many, cylinder head modification is a black art. Indeed, it is a complicated undertaking, especially if you are attempting it yourself, but do you really know why you want to take a grinder to such an expensive piece of hardware? The obvious answer is “I want more power,” but that answer is far too general in terms of determining what to grind, and where. Much has been learned, through trial and error, as well as through the application of sound engineering principles. In the days when the V-8 was king, and Chevrolets, Fords, and Mopars ruled the streets, a new breed of men was born – those with grinders in their hands and a hunger for power in their hearts. Much of the technology they developed on these two-valve pushrod motors has been refined and re-applied to the modern four-valve overhead camshaft engines of today, and with success. Many of those involved with 60’s and 70’s muscle cars are today applying their prowess to the sport-compact market – but is there more to be gained?

When talking about cylinder head porting, everybody talks about flow and velocity. Flow is, basically, the amount of air that passes through some portion of the cylinder head, usually through an intake port or an exhaust port, and measured at differing amounts of valve lift. Flow is often measured in cfm, or cubic feet per minute. Velocity is simply the speed of the airflow. Both are important. Both are related. Knowing how they relate to one another and how they affect performance is the key to unlocking the cylinder head’s potential. Flow numbers can be deceiving, however, and don’t tell the whole story. You’ll see many different tuners talking about flow numbers at different levels of vacuum, at different valve lifts, so it’s hard to equate them. We’re not going to talk about hard numbers here – we’re going to talk about what really happens with airflow inside your engine.

The key to making power is to create energy in the combustion chamber. This energy works to drive the piston down in the cylinder and cause the crankshaft to rotate. The more air/fuel mixture you can get into the cylinder, the greater your potential for creating more energy, and more power. That’s why turbochargers and superchargers work so well – they cram more fuel laden air into the cylinder while the intake valve is open. In a naturally aspirated vehicle, the intake mixture must be drawn into the cylinder by the pressure drop (suction) caused by the piston moving down in the cylinder. The mixture is then compressed by the piston, and ignited by the spark plug. The burning mixture expands, pushing the piston down again. The next time the piston comes up, the exhaust valve is open, and the spent gases are forced out through the exhaust system. Then it all begins again, and I’m sure you seen the diagrams and heard the explanations a dozen times or more.

Your engine is actually an air pump, and it is very easy to determine what the maximum airflow requirement of your engine is. Let’s use a hypothetical engine to illustrate what we mean. I like using round numbers, so we’ll use a displacement of 100 cubic inches, which is about 1640 cubic centimeters – a little over 1.6 liters. This is a size that most sport compact enthusiasts can relate to. We’ll assume that it’s a 4 cylinder engine, so each cylinder displaces 25 cubic inches. (Six and eight cylinder guys can multiply our answers by whatever factor they need to get to their displacement). Since we’re going to assume that most engines have one intake and exhaust port per cylinder, we only analyze one cylinder at a time to get a feel for what’s going on.

Let’s set the rev limiter at 8000 rpm, and figure out how much air can possibly flow into one cylinder at that engine speed. We’ll round things off to whole numbers as we go, to keep it simple.

25 cubic inches per cylinder x 8000 rpm = 200,000 cubic inches per minute.

We’re talking about a four cycle engine, so the intake valves are only open every other crankshaft revolution, so we divide by two.

200,000 ÷ 2 = 100,000 cubic inches per minute.

Seems like a lot, but we have a little more math to do. We’re going to convert from cubic inches per minute to cubic feet per minute (cfm), since that is a very commonly used unit. There are 1,728 cubic inches in a cubic foot.

12 inches x 12 inches x 12 inches = 1728 cubic inches

So -

100,000 cubic inches per minute ÷ 1,728 cubic inches per foot = 58 cubic feet per minute (cfm)

That’s how much air the we need, every minute, for a 25 cubic inch cylinder to be completely filled on every intake stroke at 8000 rpm. Multiply by 4 and you need about 232 cubic feet of air for all 4 cylinders. That is roughly equivalent to a box that measures just over 6 feet on every side. For those of us who grew up racing V-8 musclecars, a little easy math tells us that a 400 cubic inch motor would need over 900 cfm of air at 8000 rpm (if you could rev it that high without floating the valves or ventilating the engine block with a connecting rod).

That intuitively feels right to those of us who used to race small-block V-8’s. To us, a 650 cfm carburetor on a 302 cubic inch motor that never saw more than 6000 rpm was enough for good performance. These days, we think of throttle bodies and fuel injection, but the same theories apply.

As usual, more later. I'll put up a hubpage again and suggest some selected reading when I get a chance.

Suspension - Part 1 - A Definition

Since I spent so many years as an engineer in the suspension industry - from off-road trucks to formula cars, and everything in between, I'm going to start by discussing suspension. No surprise, right?

Suspension – What it is and What it Does

Webster’s Dictionary defines suspension as: "n. Act of suspending; intermission; abeyance; deprivation of office or privileges for a time. " In regards to your car, about the only thing in that definition you can apply is the “act of suspending”. The definition of suspend is: "vt. To hang; to cause to cease for a time; to debar temporarily; to stay." Again, applied to your car, only the “to hang” section seems to fit. It seems, then, that the definition of suspension in automotive terms is “the act of hanging.” This slightly twisted definition is actually reasonably accurate, if you ask me, because, in my mind, suspension is what you use to hang the wheels on your car. It would also be accurate to state that suspension is used to hang the car on your wheels. While these last two statements seem to say the same thing a little differently, they actually have individual significance when looked at by themselves, and at some point in the future, I’ll explain why, but for now go ahead and file that bit of information in your gray matter somewhere.

So how do I define suspension? My definition is actually quite simple. Suspension is all the things that connect your car’s chassis to the wheels. Shock absorbers (more correctly called dampers), McPherson struts, coil springs, leaf springs, torsion bars, control arms, radius rods, spindles, knuckles, bushings, ball joints – the list is almost endless. I tend to lump everything into my definition, and then weed out what I don’t need at the moment. Even things like brakes and steering components might fit, depending on what I’m looking at. In some cases, I might even consider the tires and wheels part of the suspension, so my definition might actually be “all the things that ‘connect’ your car’s chassis to the ground.” (Hey, it’s my definition, and it may be a little vague, but I’m stickin’ with it!)

So now that we know what it is, or at least how I define it, we can talk about what it does. I use two favorite words to describe what suspension does – handling and ride. In my book, handling is the most important function that the suspension has. Simply defined, handling is keeping the tires in contact with the ground. If the tires don’t have good, consistent contact, your car won’t accelerate, turn, or stop very well. On a smooth, straight road, the job is pretty simple. On a bumpy, twisting road, it can be a difficult task. The other job that the suspension has, ride, is the isolation of the chassis (including the driver, passengers, and cargo) from road inputs – by road inputs I mean anything that the tires may encounter - like bumps, dips, rocks, roadkill, etc… Both ride and handling have characteristics that can be measured, but ride is a more subjective thing. Some people may think your car rides too firmly to be comfortable, while some may think it’s too soft. Ride and handling are also usually related. Although not always the case, a car that rides more firmly usually handles better than the same car that has a softer suspension. The exception to this rule is usually at the extremes – a car that is too stiff will handle poorly, and a car that is too soft will ride poorly, in the sense that it makes you uncomfortable.

For selected reading on this subject, see my recommended titles at Amazon.com by clicking HERE. I especially recommend the books by my friend Don Alexander.

For those who can't wait, here's a little more: http://hubpages.com/_suspension/hub/HiPerfAutoTech

Stay tuned for more...

Welcome !

To everyone who has found their way here, thanks for stopping by. Hopefully, you'll find some of the things I talk about fresh and informative. As you may have guessed from the name, this blog is all about high performance, and I look forward to sharing some of the things I've learned over my years in the industry. Feel free to comment and criticize, but please be polite and constructive if you can!

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