The following is compliments of Devon Brown of BOAS
Brown Oval Auto Sports
Brown Oval Auto Sports
Things that make you go hmmm.
January 2, 2012 · 1 Comment
The following is compliments of Devon Brown of BOAS
Brown Oval Auto Sports
The Five Most Important Elements to the Performance of a Circle Track Race Car. In order of importance!
I once read that if you can run at the front in your current division, the only thing you need to compete at a higher level is money. I disagree. I think knowledge of any race car, at any level is more important than money. If the driver does not know how to feel a car, and be able to relay that information into positive adjustments, he will never be competitive. That being said, to compete at any level takes a certain amount of knowledge, so maybe it will just take some new equipment to move up. But I’m willing to bet most teams will be required to gain new knowledge of a different style of car, with different adjustments available, and possibly a different tire. The step up usually means a race chassis with more adjustability, and you need to know those adjustments, and what they will do. Fortunately, being able to feel a race car is a talent easily acquired with thousands of laps of seat time. This is commonly referred to by the technical term, “sperience”. And there is nothing of more value, except, knowledge, which goes hand in hand with experience.
Rule #1 - Do not adjust a race car chassis, just because you aren’t winning! First, you must know where you are getting beat. You must know if the car is loose, or the car is tight, and you must know exactly where the car is not handling as well as your competitors. You must start with handling on corner entrance, from the time you lift off the accelerator, until corner exit, when you are full throttle. As a driver you must be able to divide each corner into three distinct area’s, entry, middle, exit, and be able to “feel” what the car is doing at each of these segments. A push in any one area may be a completely different adjustment than another. Never adjust for a loose or a tight condition, without knowing where the problem is occurring, and always adjust in order, entry, middle, exit. A loose entry can cause a tight middle because of steering input to the right to correct the loose entry. And a tight middle can lead to a loose exit because of excessive steering input to the left, the “tight – loose” condition commonly referred to.
Rule #2 – Be Smooth! Every car handles flawlessly at a certain speed, exceeding that speed, will leave you in an out of control condition. This is detrimental to yourself, as well as your fellow competitors. Making 25 laps one second slower than the fastest car on the track, worse case scenario, will put you 25 seconds behind the leader. Best case scenario, if you start inverted, racing for the win! Wrecking your equipment, and possibly somebody else’s, trying to run a car half a second faster than its, and faster than your capabilities, will score you a DNF. It’s a pretty easy choice for me to make.
Check for binds! Nothing ruins a setup as fast as something not moving the way it is supposed to. Check suspension travel, shock travel, swaybar travel etc. Make sure everything moves freely through the full range of suspension travel, plus a little. Make sure the shock doesn’t rub on the spring or spring spacer.
Do not believe you do not have to pay attention to geometry, because you are limited to a stock frame with stock parts! Geometry is the largest contributing factor to the handling of any race car, even if it was designed in Detroit. Knowing that Geometry, is knowledge that will make you and your team better. I am not going to tell you what parts you need, and then offer to sell them to you; I am going to try to give you the knowledge you need to make those descisions yourself.
First and foremost, in the front suspension, camber, and camber gain are the most important. Static camber can be checked with a straight edge on the wheel (make sure the wheel isn’t bent), and an inexpensive angle finder. Dynamic camber can be checked with a pyrometer, gauging treadwear, or even with a line drawn across the tread of the tire for short tests. Every dual A arm car has some amount of camber gain through bump travel built in. This is a good thing for circle track racing. For example, if you set the right front tire at negative seven degrees static camber, it will increase to ten degrees through two inches of bump travel. On the left front, if you start with positive three degrees of camber, it will increase to five degrees through two inches of rebound travel. This only holds true if the upper control arms are shorter than the lower control arms, which they always are, and the lower control arms are near level from the inner frame mount pivot, to the ball joint pivot. The upper control arm inner pivot must be lower than the ball joint pivot. This angle is the major contributing factor to camber gain. In most cases, once initial camber is set, the left upper control arm will be on a steeper angle from level, because the spindle is more vertical. The lower control arm ball joint pivot can be up to one inch below and up to three inches higher than the inner pivot without having any large negative effects. It is recommended the ball joint pivot be about one inch higher than the inner pivot. Most OEM style suspensions react better with a stiffer spring rate in the front, that reduces body roll, because they do not have enough camber gain built into the design to compensate for much body roll.
Moment centre! Roll centre! Same thing, different times. It refers to a point in the front suspension, where lines drawn through your control arm pivots and tire centrelines intersect. Do not take the time to draw it out. If you really want to know your moment centre, purchase moment centre software, or find someone that has it. I personally feel that the most important part of the moment centre, is that it dictates your camber curves. The moment centre should be located about two to five inches above ground, and about six to twelve inches left of centre. On a lot of stock chassis cars, that require OEM control arms and mounts, with minimal modifications, two to three inches in height is a good as you are going to get. With a conventional setup, where the car has roll, the moment center usually moves to the right, with a setup that has more dive than roll, such as BBSS or on a highly banked track, the moment center will migrate left.
The rear of the chassis also has a moment centre. Its location is dictated by the method it is mounted. As with the front, I recommend purchasing software, taking measurements, and plugging in numbers.
Caster is the amount the upper ball joint, is leaned back, as compared to the lower ball joint. You can set your caster with a tape measure! Just make a mark on the firewall, directly back from the upper ball joint, and measure from that mark to the grease fitting on the balljoint. This will not give you degrees, but will give you a baseline measurement, to adjust from or return to, after you have had your caster set. If you have power steering, you can run up to 10 degrees of caster. The only detriment to added caster, is more steering effort. A little caster split, with more in the right side, will help the car turn to the left, but will make it harder to counter steer. Caster recommendations, for power steering, usually run in the range of four degrees left, and six degrees right. Caster will also make a slight variance in wheel weights, effecting crossweight slightly when turning the wheels. Positive caster will reduce crossweight when steering to the left, add crossweight when steering to the right. An added bonus of caster is added camber when turning the wheels.
Ackerman? Is the difference between the left tire turn radius, and the right tire turn radius. Most steering systems have some Ackerman built into them. An example would be if the left wheel were turned ten degrees to the left, the right wheel would be turned nine degrees to the left. You can use turnplates, toeplates, or string the front wheels to measure the amount of Ackerman in your steering system. The amount of Ackerman in your steering system will be influential to where you set your initial toe. I personally prefer to run minimal toe out, with a degree or two of Ackerman. There are very few ways to legally change the Ackerman in a street stock type class. Running a shorter steering arm on the left will increase Ackerman when turning left, but this method is not recommended for dirt, where you counter steer a lot. Moving the centerlink towards the crossmember will also increase Ackerman, but involves moving the steering box and idler arm, or using longer pitman and idler arms. Fortunately, most OEM suspensions have a significant amount of Ackerman built in, and may even benefit from reducing it. The amount of Ackerman you run, as with toe out, will be based on the track, and tire you are running.
Bump Steer is caused by the tie rod travelling in a different arc than the spindle steering arm through bump and rebound travel. Every pivot point in the front suspension contributes to this. It can be corrected with different centerlinks, tie rods, tie rod ends, and steering arm alterations, but is very hard to do within OEM rules. I recommend measuring it, and using it to help with your static toe settings. Bump steer can be measured with a jack, 20” piece of angle iron, plumb bob, a piece of cardboard, and a laser level! Which one of those don’t you have? That you can purchase for less than $50? Or borrow from a friend, or work? With the car at ride height, measure the lower control arm, near the ball joint, to the ground, record this measurement. Remove the shock and spring and reconnect the spindle to the lower control arm. Block the frame at ride height, and jack the lower control arm back to the previously recorded measurement. Centre the steering, and lock the steering column with vice grips. Bolt the angle iron to the hub, level it and clamp the caliper with a c clamp to keep the rotor from turning. Clamp the laser level to the angle iron facing forward. Drop the plumb bob to the ground, at the center of the spindle, and make a mark on the floor, this mark is your baseline for the side scrub. Mark a vertical line on the cardboard, and stand it with the vertical line directly on the laser dot,ten feet from the spindle centerline. Move the spindle through bump (up) three inches, and measure the distance the plumb bob moved in from the scrub baseline. Measure the distance the laser moved from the vertical baseline on the cardboard. The difference, divided by five (vertical baseline is ten feet away, bump is measured in two feet) is the amount of bump steer in three inches. If the plumb bob moves in one inch, and the laser moves in one inch you have 0 bump. If the scrub moves in one inch and the laser stays on the mark you have one fifth of an inch bump out. If the scrub moves in one inch and the laser moves in 2 inches you have one fifth of an inch bump in. You can measure at each inch. An even easier method, is to use a carpenters square from the floor to the angle iron on the spindle, or 2 plumb bobs, drawing marks on the floor as you progress through suspension travel. You can also use a bump steer gauge, but personally I like the laser level, it’s cool watching the laser move. Check the right side through three inches of bump, and the left side through two inches of bump and two inches of rebound. This will help you see what your front wheels are doing during dive, entering the corner, and roll, during cornering. Bump out on the right front is acceptable, up to one quarter inch maximum. As well a little bump out, in rebound on the left front is acceptable, up to one eighth of an inch. Just keep track of how much and at what point, and you may discover between Ackerman and bump, you should be setting your toe near zero.
A great starting point for your rear end is square and centered to the chassis centreline. The easiest way I have found to square the rear contact patches to the front, is to use laser levels, placed against the rear tires (possibly with a spacer to get the laser out past the body, and measured out from each front tire sidewall to the laser (you can even check toe out to some degree using this method). But don’t think the rearend remains square and centered through chassis movement. It is a good idea to map your rear suspension travel as you did with the front, it may tell you why your current setup may not be performing as intended. All links travel through arcs, and these arcs dictate the direction, and amount, of rear roll steer you are getting.
Moving the differential to the left of center of the front tire contact patches will tighten the chassis, moving it to the right will loosen the chassis, especially through the center and exit of the corner. It’s easy to visualise the effect of this by pushing something across a desk, push in the center, it moves straight. Push on the left side, and the object will turn to the right. Push on the right side, and the object will turn to the left.
This adjustment can easily be made with wheel spacers, offset wheels, or a track bar length adjustment. Skewing the differential left or right with different length trailing arms can also have a similar effect. In this case the chassis will run in yaw, but the effect of moving the rear tire contact patches in comparison to the front is similar.
Thrust Angle! Thrust angle is a force acting on one end of a lever, in a direction not in a direct line with the lever. These levers include both upper and lower front control arms, and the rear axle trailing arms and track bar. Next to camber curves, and rear steer characteristics thrust angle is the only reason front and rear moment centers are important. Forget the fancy “voodoo” the books are telling you, there isn’t a specific location the moment center needs to be, rather, there is a general location it is usually near when the camber curves and thrust angles are optimum.
Chassis setup can be summed up easily in one sentence, the tire with the most available traction (not necessarily weight), controls the direction the car will want to travel.
Weight is one of the most important elements of chassis setup. Total weight, weight placement, and weight distribution. Weight placement is the only way to control the amount of weight transfer, not where the weight transfers, but how much transfers. The least amount of weight transfer possible is usually the best scenario.
It is very important to be at or near the minimum weight allowable by your rules. I feel that being close to minimum weight is more important than being near suggested weight distribution. Many people feel that rear weight in a circle track car is important to tighten corner exit. While rear weight may help slightly with forward bite, it will also loosen entry, and mid corner. Weight wants to keep going straight when you turn the car (g forces). Ballast placed near the rear of the chassis will want to continue in a straight line, thus making the chassis feel loose. A front heavy car will have the opposite effect. Place any required ballast weight within the wheelbase of the car. On a Street Stock type car having forty five to fifty percent rear weight is acceptable. Around forty eight percent is optimum. Try to get as much left side weight as is allowable or possible, and remain near the minimum required weight. You should easily be able to attain fifty three to fifty five percent left side weight.
In general terms, Weight placed on the left side will loosen entry and tighten exit. Weight placed low will loosen the chassis, while higher placed weight will transfer more to the right side, tightening the car. Weight placement for a wet tacky dirt track should be similar to an asphalt track, low and left. A dry slick track will have higher weight placement, and/ or further to the right.
Cross weight is the amount of weight on the right front and left rear tires, as a percentage of total weight.Static cross weight distribution will depend on the chassis design, front and left weight percentages, and the springs and sway bar used. For dirt track racing crossweights from forty six percent to fifty six percent is normal. For asphalt fifty to sixty percent. More crossweight will tighten a car, less crossweight will loosen a car.
Spring rates will dictate what crossweight will work, with a given chassis and geometry.More right front and/or left rear spring rates will require less crossweight and Less RF and LR spring rates will require more crossweight. Crossweight also goes hand in hand with stagger. More stagger may be balanced with more crossweight, and less stagger with less crossweight.
Keep in mind you cannot control how much weight transfers with springs, you can only control where it transfers too. Corner entry relies mostly on the front spring rates. With the RF spring being a higher rate than the LF, the RF will be loaded more as weight transfers forward during deceleration. In this case the RF will gain more traction than the LF, and will tighten entry. Swap the spring rates, with the LF spring being the higher rate, and you will loosen entry.
Mid corner leans on the right side springs. This is what I call the coast section, where you are done braking, and just starting to accelerate. A stiffer RF spring rate, or lighter RR, will tighten the car in the centre. A lighter RF, or stiffer RR will loosen the chassis.
Corner exit is when the car starts to roll back onto the rear springs. As with the front springs, on corner entry, whichever rear tire is loaded more will push the car. If the RR is loaded more it will loosen the car, if the LR is loaded more it will tighten the car. A stiffer RR spring will loosen the car, a stiffer LR spring will tighten the car.
A spring that requires a specific weight to compress it one inch also requires the same amount of weight to be removed to extend it one inch. With this knowledge, a spring can also be used to “tie down” a corner of the car. If you car transfers four hundred pounds from the left rear to the right rear mid corner, a two hundred pound spring will extend two inches, a four hundred pound spring will extend 1 inch, generally speaking.
A sway bar, in simplest terms, adds wheel rate to the right front of the car, by taking wheel rate from the left front. Adding a sway bar will tighten a car, preload to a sway bar will add crossweight, increasing the size of the sway bar will add more RF wheel rate. In some cases, with larger sway bars, increasing the left front spring rate will also increase the right front wheel rate through the sway bar. I recommend using a sway bar, with adjustable links, with the option to disconnect one link.
As well as controlling the oscillation of the spring, shocks can have the same effect as spring rates, but momentarily. Stiffer shocks will momentarily load a corner during a weight transfer period. A lighter shock will also allow a corner to transfer weight to another quicker. It is recommended to let springs control bump travel, and shocks control rebound travel.
Buy the best tires your rules allow, and your budget can afford. Tires work considerably better if you use the tread, rather than the sidewall, on the track surface. Use the proper air pressures for the tire type, and weight of car you are running. Tire stagger is the difference of the circumference of the tires from the left side to the right side of an axle.
Front stagger does nothing more than help adjust crossweight in a car with limited adjustments. The only time front stagger affects the chassis otherwise, is when the brakes are applied. Having front stagger, with the left smaller, will help the car turn into the corner when braking. This is because the smaller tire has less leverage on the brake caliper, and the brake slows that tire faster. I recommend running the minimum amount of stagger possible on the front.
With a locked rear axle, rear tire stagger is critical, and every track has a specific shape that requires a specific amount of stagger for a given car. More stagger, with the left smaller will loosen the car, less stagger will tighten the car. You can compare rear stagger to rolling a Styrofoam cup, it will turn towards the smaller circumference. Never run reverse stagger, with the RR smaller, to try to tighten a car, this is a disaster waiting to happen. If you get loose, and have to lift on corner exit, the car will snap back, directly into the wall. Between one and three inches is in the normal rear stagger range. Two inches can be made to work successfully on all tracks in Saskatchewan.
First and foremost it has to be reliable and easily maintained within your time frame and knowledge. Spend your money on knowledge, setup, parts, tools, and equipment, and tires, not HP. Not until you have everything else.
Setup techniques – Track tuning
Two phrases often used in circle track chassis tuning are “neutral” and “balanced”. They are not the same thing.
Neutral refers to a chassis with a near equal amount of grip available front and rear, through the entire corner. Neutral does not always mean fast, it just means you’ve acquired a near equal amount of traction, not necessarily all of the traction available.
Balanced means a setup that uses all 4 tires as equally as possible. A balanced setup may not always be neutral, and may not produce the fastest short term speed, but almost always produces the fastest long term average speed. A balanced setup is usually also easier to find a neutral range, has a larger range of adjustability neutral handling is available, and usually stays closer to neutral over a longer period.
A balanced setup may not be possible within the rulebook you run. A balanced setup also requires a geometric balance that may not be possible for the style of suspension you are required to run. This means you will be attempting to compensate for geometric differences with a combination of setup items that may be out of the normal range. In this case consistency is still the key. A setup that may not be balanced, but is fast and will remain neutral for the shorter races you run may be the best compromise for certain types of racing.
One thing I try to do with every new setup, is make laps at mid corner speed, usually between 3500-4500 rpm, without braking or accelerating to test and adjust the mid corner balance of the car, as well as check mid corner peak speed. This speed (mid corner rpm reading) can later be used to test if you are over braking, and costing yourself mid corner speed, when making laps at full speed. Having tape wrapped at the top center of the steering wheel will also help you judge steering input from slow laps to laps at speed. If the tape is further left of center at speed, than it was during slow laps, you are having to apply more steering input, and the setup may be too tight. If it’s further right, your setup is possibly loose.
If you find that your chassis routinely runs fastest with a slight push, it is most likely a front geometry issue.
Tags: Trevor's Rant