Autocross Set-up Tips
We often dog ear or cut out magazine articles about suspensions, tires, sway bars, etc., but when it comes time to find them... This page is place to keep track of such items by topic with the hope to saving some time and anguish when starting the next project.
The April 2008 edition of SportsCar has an excellent article that put tire "truths" to the test. The four areas are wheel size, slicks v. treaded tires, full tread v. shaved/worn-out tread, and lightweight wheels.
Wheel Size. Bigger rim diameters and low-profile tires look cool, but what differences can you expect on the course after making this modification?
Stock Wheels and Tires: Much easier to drive because the larger sidewalls provide more cushion for overly-aggressive inputs (starting, braking, and turning.) Stock rims are "friendlier" and make it easier to lay down consistent runs. After extensive testing between 15-inch and 16-inch set-ups, the 15-inch setup was approximately 0.4 seconds faster.
Bigger Rims and Low-Profile Tires: Short stiff sidewalls make it very easy to exceed the threshold of grip. You will notice more wheel spin and lock-up of the brakes. However, short sidewalls make turn-ins and transitions easier and crisper. The low-pros have a sharper "edge" and give little cushion for overly-aggressive inputs. Overall, the stiff sidewalls make the tires harder to drive to the limit and often leave quick runs "dirty."
Both tire set-ups came up even in the lateral g-force department.
Slicks v. Street Tires. Most autocrossers know that DOT-rated slicks are legal in stock autocross classes. How much time can you expect to gain by using tires that you won't want to use driving to and from an event?
After extensive testing that also included messing around with sway bar sizes, R-compound slicks beat treaded tires by a 1.7 to 2.0 second gap.
Full Tread v. Shaved/Worn Out Tread. Shaved/worn out tires average 3 pounds lighter than full tread tires and clearly place more rubber on the road. Full tread tires also squeal like a pig. However, in the end, the full tread tires were quicker by .2 seconds.
Lightweight or Light Price Wheels.
Testing between a 21-pound wheel and a 16-pound lightweight wheel failed to show
any measurable time gained or lost due to the use of the lightweight wheel,
however, using racing tires, gains in the area of 0.1 to 0.2 were noted with the
lightweight wheels. The testers concluded that the light wheels could
help, but probably wouldn't hurt. (8/08)
UTQG (Uniform Tire Quality Grading) Ratings.
These ratings are made up of three components. The ratings on the sidewall of the tire will appear at follows: Treadwear Grade/Traction Grade/Temperature Grade (i.e., 220/AA/A)
Treadware Grades. The treadwear grade is a comparative rating based in the wear rate of the tire. A tire graded 200 would wear twice as long as one graded 100. The stickiest autocross tire have a treadwear grade of 40. Racing tires (R-compound) are usually around 100. Some race governing bodies consider tires with wear ratings less than 140 not to be street tires. Normal tires have treadwear grades in the 300s. Performance street tires are around 200-220. R-compound tires will pick up a significant amount of sand and rocks when they are hot. Autocross tires with treadwear grades of 40 will pick up everything whether they are hot or cold, i.e., install mud flaps if you want the save the paint on the body panels behind your tires.
Traction Grades. Traction grades, from highest to lowest, are AA, A, B, and C. They represent the tire's ability to stop on wet pavement. Full-treaded street tires are typically rated AA, while racing slicks are rated C. The traction grade is based on braking traction tests and does not include cornering (turning) traction.
Temperature Grades. Temperature grades, from highest to lowest, are A, B, and C. They represent the tire's resistance to the generation of heat and its ability to dissipate heat. Sustained high temperatures can cause the material of a time to degenerate and can lead to sudden failure. Accordingly, the higher the grade the better. (8/08)
TIRE WORKING TEMPERATURES AND LATERAL GRIP
Autocross tires are designed to reach optimum traction at 120
to 140 degrees F.
Racing tires are designed to reach optimum traction at 180 to 220 decrees F.
The October 2008 issue of Grassroots Motorsports contains an excellent article testing the relationship of tire temperatures vs. grip. The subject tires were 205/45R16 Hoosier A6 tires mounted on a Mini Cooper S. To keep things consistent, Tire Rack's 200-foot skid pad circle was used to provide steady-state cornering, perfect for testing lateral grip. Conventional wisdom states that a race car's tires will gain grip with increased tread temperature. Likewise, many of us have experienced how cold tires can lead to decreased traction in those first few turns of an autocross. The result of the test was that although the A6 tires are optimized for lower temperatures (120 to 140), they remained fast even during high temperatures and extended lapping sessions. The tires were abused to temps of 200 degrees, but the lap times never changed significantly. Although the grip did not change significantly, as the temperatures built, the slip angle required to generate maximum grip increased along with a general loss of precision in steering feel. While ultimate grip is important, precision and confidence are mandatory for fast autocross laps, which might explain why autocrossers like cooler tires. (9/08)
TIRE PRESSURES V. LATERAL GRIP
The November 2008 issue of Grassroots Motorsports included an excellent article exploring how tire pressures impact grip. Similar to the previous test that measured grip vs. temperature, an autocross-ready Mini Cooper S running Hoosier A6 tires was used together with a 200-foot skid pad. Rear tires were held constant at 55 pounds and the fronts were raised and lowered in 5-pound increments from 50 to 20 and back up again. Despite the pressure spread of 25 pounds, the lateral grip only ranged between 1.012 and 1.057. However, from behind the steering wheel, the testers noted quite a bit of difference. At the highest lateral grip (1.057 at 30 pounds pressure), Steering precision and feedback was significantly reduced, the outside front tire emitted a noticeable low groan or growl, and the tire was really rolling over onto the shoulder. In conclusion, the article indicated that a setup that feels good might not be the fastest, and the fastest setup doesn’t always feel good. Back to the white shoe polish, I guess.
WATER V. TRACTION
The December 2008 issue of Grassroots Motorsports probed the issue of choosing the right tire for a wet autocross. Making your wet tire choice should start with an analysis of the track conditions:
If the course is saturated with water and no visible track texture can be seen:
Use a full wet tire or deeply-treaded street tire.
If the course is fully wet, but you can begin to see pavement's texture: Continue to use a full wet tire or deeply-treaded street tire.
If the rain has stopped, but there is no sun to dry the course: Use intermediate tire such as a radial wet tire shaved to partial depth.
After the rain stops and the track has a matte finish with no shine and a few small puddles: Use dry tires, but at 50-60% of dry performance.
If the track is a bit more wet with unavoidable puddles or water streaming across the track: Use dry tires and take the slip with the grip.
When Mother Nature obliges with some rain during your next practice event, don't just hide in the trailer to stay dry. Use the time to test which tires work best for you.
Tips for Setting up a Stock Class Solo Car
The May 2007 issue of SportsCar has an excellent article about setting up a stock class solo car. In the article, a panel of autocross experts were asked a series of questions and told to rate various stock class "allowances"--wear items that may be replaced--in order of importance. You will want to read the article, but here is a recap of the recommendations:
1. Tires and
an alignment. Get the best, stickiest R-compound tires you can
afford. An alignment is one of the least expensive modifications
you can do. Usually, you want to max out the front camber, add a
little toe out up front and zero in the rear.
2. Dampers. Performance shocks are generally better than OEM, but it is crucial that you properly test with them to make sure.
3. Swaybars. A larger than OEM swaybar is always better overall.
4. Wheels. Go for the lightest wheel you can afford with a +1/4" offset all around.
5. Brakes. For autocross, current OEM pads are probably better than racing compound pads.
6. Power Adders. Using a performance cat-back system will usually save weight and use any kind of air filter--as long as it's clean.
7. Maintenance. Don't mess with the ignition systems or thermostats. Use fresh spark plugs and the best fluids and lubricants you can afford.
8. The Driver. Strongly consider a full race harness, a video system and a data logger. For the beginner, there's nothing like seat time. Any driver will benefit from driving instruction. (6/07)
Removing Pylon Scuff Marks. The
best way to avoid pylon scuffs is to avoid the cones. If you are unable or
unwilling to do that, make sure that your car has a good coat of wax (or two)
before hitting the course. Once a scuff is on there, a quick spray with a
penetrating liquid oil like WD-40 or a cleaning solvent like "Goof Off" followed
by a swipe of a rag or paper towel should do the trick. Some extreme
scuffs may require some buffing with a paint-friendly polish. (9/08)
Helmets. Do you dread that part of tech inspection when the safety steward looks inside your helmet for the Snell sticker? You may also want to check out page 32 of the 2008 SCCA National Solo Rules (Rule 4.3.1) which mandates that helmets must meet the current or two immediately preceding Snell Foundation standards. Snell updates their standard every 5 years, so the current standard is 2005. It would appear that your helmet is legal if you have a 2005 sticker (current), 2000 or 1995. In 2010, the 1995 stickered helmet will no longer be legal.
PAX/RTP Scoring. PAX is a method of handicapping cars. It's also known as RTP (Racers Theoretical Performance). Cars are assigned handicaps based on their make/model/year and level of modification. These handicaps are based on actual performance of cars that have run in SCCA National events, and are determined by the national PAX administrator. They are used to judge performances at the national level, and are accepted by all competition entrants. The most modified car is "A Modified", which gets a score of 1.00, so it's time would not be adjusted at all. Everything else gets multiplied by a figure that you could call a PAX handicap. PAX values are adjusted yearly. Click here for the most current PAX factors.
DATA ACQUISITION FOR AUTOCROSSERS. A spring 2009 issue of Grassroots Motorsports contains an excellent article on GPS-based data acquisitions systems for autocrossers. The article notes that data acquisition was formerly only for teams with infinite budgets. However, technological advances have made the systems smaller and their prices have dropped significantly. While you will want to read the entire article, the upshot is that the resolution of most GPS-based systems is often not precise enough for autocross. Most systems will place the car within three meters, but for autocross, that still leaves a fair bit of uncertainty. Make sure you buy a system that can superimpose multiple plots for comparisons so subtle differences between driver (or runs) can be shown. One of the most popular units for autocross is the MaxQData. Software in the system also displays acceleration and braking along the mapped route. The small GPS receiver sends its information to a PDA via Bluetooth. The data can be displayed on the PDA or downloaded to a computer. Grassroot’s advice: Also run one of those new portable cameras so you can see exactly where you are placing the car.
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