TECH: Car setup basics

 
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Elson23
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PostPosted: Wed Apr 13, 2005 10:59 am    Post subject: TECH: Car setup basics Reply with quote
One of the most common questions from beginning, and even seasoned, autocrossers [and track racers] deals with how tire selections, tire pressures, spring rates, camber angles, sway bars, etc... affect handling. This thread is here to hopefully give the basics, and then answer questions if any remain.

First, a bit of terminology:
Camber is the measure of how much "lean" a tire has compared with the ground, when viewed from the front or the rear. It is typically measured in degrees, through the center line of the tire tread, or by the angle that the carrier hub is held.

Spring rates are how "stiff" a spring is. For you physics type people it's really just the spring constant, and refers to how much weight [force] is needed to compress or stretch the spring a given unit from it's equilibrium position. For example, a 600lb spring needs 600lbs of weight to compress the spring one inch.

Sway bars are put on a car to reduce body roll. The way this is accomplished is by joining both sides of the suspension for the front or rear of the car together by the sway bar so that as one side is compressed, the opposite side is [theorhetically] compressed the same amount, which means that the front [or rear] suspension corners stay at the same level. Of course no material can be completely resistant to twist, so swaybars are made of steel, and their stiffness is guaged by it's main section diameter.

Tire pressures are measured in pounds of pressure per square inch [psi], and can directly affect the shape and stiffness of a tire, which will be discussed later.

Tire compounds are measured on an arbitrary scale of hardness, normally with a durometer. A super simple way to describe how a given value can be represented in a tire, a value of 40 would be a very soft tire that you could leave an impression in with your fingernail with very little effort, while a value of 100 would be the hardness of your kitchen counter. For the vast majority of tire consumers, these compounds are simplified into categories speed ratings, tread wear ratings, and temperature resistance.

Caster is similar to camber, but refers to the axis of rotation [and it's variation in degrees fore to aft] that the hub carrier is held when the car's wheels are turned. Caster can have a dramatic effect on the weight transfer of a car's chassis when entering a turn, but since this is largely un-adjustible even with camber plates on most FWD cars, it will not be discussed.

Toe-in/out refers to the amount that the tires in the front or rear set with their leading edges pointing towards, or away from each other, respectively.

Damping rates refer to the shock or strut's ability to control the oscillations of the spring. There is no real defined scale other than a very high rate prevents the spring from moving much, and the highest rate would prevent any movement, whereas a very low rate would allow the spring almost free movement without resistance.

Ok, now that everyone should know what I'm talking about, lets get down to business.

For starters, we will be dealing mainly with FWD cars, this being a Honda enthusiast site, and the vast majority of Hondas being FWD. The inherent handling of a stock Honda is that they understeer, badly. This is due to a high percentage of the weight being over the nose, which is to say that the rear tires don't normally have enough grip to force the front tires in the directions they are pointed. The other inherent handling trait is that Hondas exhibit lift-throttle oversteer, which again deals with the heavy amount of weight being over the nose, and the ability of the engine to slow the front wheels without affecting rear wheel momentum; result, the rear tries to pass the front. So what do we end up with? A car that doesn't want to turn in, but as soon as you lift off the throttle, tries to swap ends on you.

While this sort of behavior is quite driveable, it's usually not the comfortable way to drive at the limit, which is to say that it's also no the fastest way. In my opinion, if a driver isn't comfortable with thier car, they won't be driving it to it's fullest potential. The general preference for drivers is to have a car that has good on-throttle turn in, with a slight bit of oversteer when off-throttle, but tucks the rear in nicely when back on the throttle again. At no point during a turn should the car be twitchy, slow in response to inputs, or unpredictable. This is commonly referred to as neutral handling, although different drivers have different opinions as to what is neutral.

Great, we know what we're after, how do we get there?

For starters, most people try to make sure the suspension has as little slop or other variables that can play havok with handling as possible. This includes installing strut tower braces, lower tie bars, polyurethane bushings, spherical bearings, heim joints, and of course, making sure the unibody is straight and true before doing any modifications.

Ok, your suspension is only going to move in the directions you want, now what?

Most people like to get springs and shocks and worry about sway bars later, however, these items should ideally be paired and installed at the same time to get the greatest benefit [**disclaimer: any time you make handling changes to your car, you should practice EXTREME caution when learning the new limits to your car**]. Typically the goal is to reduce body roll and force the suspension to make greater use of the tires. This means higher spring rates and higher damping rates on all four corners of the car typically, and also means that you would need a heavier sway bar in order to force the car into cornering flat with the stronger springs. Spring rates are going to vary from application to application, however most find that heavier spring rates in cause the car to rotate easier in corners, so that is something to keep in mind. Additionally, sway bar thicknesses and availabilities are going to vary from car to car, but it should be noted that if the sway bar is thicker than necessary, the car will exhibit more "jittery" behavior over uneven bumps [track curbing, potholes, etc.].

Ok, your suspension now moves only how you want it to, corners flat, and forces the tires to do more work since the car is no longer unloading as much weight off of the inside of the car, now it's time to look at your link to the road: your tires.

Your handling is only going to be as good as your tires, so it's time to evaluate what you can live with. The ideal tire would be one that is as soft as a gumball, that never overheats, and wears like iron. Like I said, that would be the ideal tire because it doesn't exsist. Instead, it's time to make some compromise choices. For those on a budget, a tire that offers excellent grip and doesn't wear out very fast means that they are normally prone to overheating, which eliminates them as the best choice for high speed, tight corner courses, but could make them an excellent 35 second per run autox tire. Still on a budget, if you're looking for a tire that has excellent grip and doesn't overheat, you can kiss durability goodbye, this is because these types of tires normally cool theirselves off by shedding rubber from the tread area, sticky race slicks come to mind in this area. And finally, if you want a tire that doesn't overheat and will run forever, well then... don't expect the greatest grip, it just isn't doable with tires nearly as hard as stones.

Alright, suspension is done, and you have your tires picked out, time to go kick tail right? Not so fast. All of your new parts and handling just means you spent a bunch of money, which doesn't earn you competition wins unless you know how to tune them for greatest effect.

Certain tracks and courses like different settings. If you were on a rough track with poor grip, for example, you might want a softer spring rate to cope with the bumps, and you may want a slightly softer damping rate so that the tire can spend as much possible time on the ground. On the other hand, a fast, smooth track would allow you to run a higher spring rate with a higher damping rate, which means your tires will likely never get a rest from being buried in the pavement. Obviously most people can't change spring rates at the course to cope with the conditions, especially those on a budget. What you can do, however, is to start with a middle-range spring rate, which can be quite versatile, and adjust the handling characteristics with some adjustible damping rate shocks or struts.

So you're all tuned in the suspension, what can you do to make sure the tires will do their part?

First things first, you need to know how the tires are wearing. Ideally, you want the outside front tire to use it's entire treadwidth evenly in the hardest high speed corner on the course. This will insure that you never roll the tire over on the sidewall which can unload all the grip you'd built up to that point, which also means that the car will only use as much tread as needed in all the lesser speed corners, which helps reduce your rolling resistance in a corner. To accomplish this will take some experimentation. The best tools to use for this are chalk [or white shoe polish] and a tire pyrometer [if you can afford it, otherwise trust the calibrated touch/sight method]. If your camber is set correctly, you'll use up to the edge of the tread, but not over, and if your tire pressures are correct, you'll have an even temperature across the tire. Adjust both camber and pressures until all is well.

Ok, pressures are set, suspension is set, and the car still doesn't feel right.

It happens. What you have to decide is what specifically don't you like about what the car is doing, and what is going to be your plan to go about changing it for the better.

These kinds of complaints and solutions are going to vary widely from person to person, but the one thing that should absolutely be kept in mind is to only change one item at a time between tests if you change more than one thing at a time, how will you know what change had what effect? This leads into another point I'd like to stress on everyone: keep a detailed notebook of changes and effects. This notebook should include important information such as tire pressures, which tires you're using [if you change], what the track conditions are, what the weather is like, and biggie: what the car was doing. This is going to help you find a setup that works for any situation before you ever put the car out on course, which will save you time when you need to start tweaking for your particular event.
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Last edited by Elson23 on Fri Mar 31, 2006 12:08 am; edited 1 time in total
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kotomile
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PostPosted: Sat Apr 16, 2005 4:57 pm    Post subject: Reply with quote
Great post Elson, just wanted to point out that treadwear ratings are made by the manufacturer of the tire, treadwear is not measured by the DOT. So, for example, your 30CA v700s might not be any softer than my 40CA A3S04s.

Also, the most important nut to tighten is often the loose nut behind the wheel Smile. An investment in seat time is crucial if the winner's circle is your goal.
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STSHatch
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PostPosted: Sat Apr 16, 2005 6:11 pm    Post subject: Reply with quote
words of wisdom....and VERY true.
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