Influences that can change the way a dirt race track handles can be caused from.. Humidity, the type of soil, Out side temperature. All of these causes have a large influence on how the track will react to your race car driving on it during the day or night, also how the track is groomed. Preparation, in grooming and watering is a large factor in how the manager can help prepare the track for a race.
Anticipating the ever changing track in how it will hold up for the night is the key to keeping your chassis dialed in...and keeping your tires hooked up to go fast.
As the race night progresses...Walk the track..Observe how the soil behaves....watch other cars on the track and see what they are doing. Are they hooked up or having problems. as you walk the track pick up a little bit in your hands and squeeze it.. Does it clump together? or fall apart? if it falls apart and is not sticky plan on the surface being very slick during your race. The chances of the dirt getting sticky or clumpy during your race are slim so plan on how the surface is holding up to how you are going to set up your car. If the soil seems to hold up fairly well for the night you can more than likely plan on getting good bite. If you are seeing dust clouds on the track before your race you can plan on the top of the surface loosing its moisture and you know what that means..
If the air is hot and humid you can easily plan on the surface drying out. If you can get on the track, take a screw driver with you and push it down in the ground. Give it a turn so you can get a look at whats down below the inch or two mark. How moist is it?
A cooler humid temperature will help keep moisture in the surface.
Remember that the chassis has to be setup for the end of the race...Not the beginning..Remember that most all tracks are going to get slicker as the night progresses so plan accordingly. Plan on setting up your chassis to the way the surface condition changes. If you anticipate the track getting harder and slicker, you will have to start the race with your chassis set tighter than you would like it to normally be. You will go faster toward the end of the race when the track comes to your setup. When you watch what the track surface does during the night you can get an insight on how much stagger to use, how to set your wheel tracking, and how to set your tire pressure.
Dirt Track Adjustment Procedures
Depending on track conditions - wet to tacky to dry slick - how the car creates side bite for the right side tires and creates forward traction are the major tuning elements.
On a wet dry track, the car is naturally tight and pushy. One a dry slick track the car is always wanting to be loose and wants to skate outward toward the wall. How the weight is transferred to the right side tires is going to dictate how well the car corners. In the following sections we are going to examine the different conditions of the track and how to adjust your chassis for each condition.
The Wet Track
The track starts out wet, heavy and not packed. Remember that the track most likely will not remain this way, if you want to set your car up for this type of condition, you will want to loosen up your chassis at corner entry, and lower your right front brake bias as not to lock up the right front tire when you go into corner entry. Maximize your stagger to 3 1/2" or the largest stagger your tires will allow. With the top of the surface very slippery you need to lower the rear roll center of the chassis to create side and forward bite. The chassis has to force your tires to dig in and get the bite needed to turn in the corner. Also move your weights in the car to the right side and as high as you can get them. This will cause the rear to turn out on you and prevent push.
The Tacky Track
As a track gets more use, it will pack down reducing the moisture in the top soil. This will make for much better side and forward bite on the rear tires. Tacky tracks create a push in your chassis, the rear tires get so much bite that the car doesn't want to turn. The chassis has to be adjusted to free it up, it has to be adjusted to decrease the right rear side bite On this type of track side bite and body roll is not as critical as the dry track.
At this point, you want to move your panhard bar higher and have it placed more level, this will decrease side bite and raise your roll center. Changing the right rear tracking will also help reduce in side bite. Use a wheel spacer or change the wheels offset. This will help the car turn. Using a stiffer right rear spring and or moving your ballast to the left and as low as you can go will also help free up your car. A stiffer right front and right rear shock will help slow down the weight transfer and stabilize your car. Use a slightly stiffer shock than you normally would at those corners You may need to change your brake bias as the track dries out. A tacky track requires more rear bias than a wet track, you will want to start at 60 - 65%, brake pressure to the front should still be minimal.
The dry track
Its your turn to race and the track has dried out and lost its moisture, experience has shown that the rear of the car will become loose and difficult to get any forward bite.
When you run into this condition, you will want to drive more like its an asphalt track, keep your rear end of the car tucked in. Decrease your stagger to about an inch. Be aware that stagger changes a the corner changes the weights of the car. When a larger right rear tire is used, it adds weight to that corner. When a larger left rear tire (or smaller right rear tire) is used, more weight is added to the left side increasing its weight. This takes weight off the right rear and places on the left rear making the car tighter. So when you change the stagger for this type of track condition, you are tightening up the car. When the stagger is reduced your cross weight is increased.
Using cross weight as the track becomes drier and slicker is a compromise between tightening up the chassis sufficiently and preventing a push. You have to have the right balance between cross weight and getting the car to turn at the middle of the corner and in corner exit. If the car has too much cross weight it will push in the middle of the turn. The answer to this problem is to have the chassis properly balanced with springs and shocks so that cross weight becomes only a fine tuning element. With a dry and slick track make sure you use more front brake bias. You will need about 60/40 combination front to rear brake bias. To much rear brake bias will make the car loose during braking entry. The right front brake should be fully operational. Adjust the right rear tire in toward the car and the left rear out away from the car. This will increase side bite. Move the panhard bar lower and add angle to it, this will also help create side bite for corner entry and load the rear tires for traction at the corner mid and exits. Decrease your tire pressures by at least 2 lbs all the way around. This will give the tire more contact and help with grip. Shocks will also help the handling on a dry slick track. Use a softer shock to help the influence of weight transition quicker. Splitting valves in the shock will help fine tune your weight transfer but you must use them as a last resort. Usually a split shock will help if you have other problem areas that you cannot correct.
Using Ballast On A Dirt Track
The most critical element to good handling on a dirt track is getting the car to turn in the middle of the corner. The factors that affect this are spring rates, front to rear brake bias, and weight height placement. A minimal amount of ballast added to the chassis is used as a fine tuning device to dial in the chassis as the track changes. The drier the slicker track gets, the higher the ballast has to be placed to get downward weight transfer onto the right side tires to get them to stick.
On a wet tacky track where traction is obtainable, the weight doesn't have to be placed high in the chassis, if there is to much weight transfer to the right rear tire the car will push. Don't play with spring weights and shock dampening. Move the weight low and to the left side of the car.
On a dry slick track where traction is not obtainable, the weight needs to be placed as high in the chassis as you can get it and to the right side of the chassis, this creates right side bite and will help stop the car from skating through the turns. This tightens the chassis up on a dry slick track. A lower rear roll center also helps to increase the overturning moment to gain side bite.
Chassis tuning with air pressure
Tire pressures are varied depending on how much adhesion there is in the track and how abrasive the track surface is. If the track has a lot of adhesion, such as a tacky wet track, the tire pressures will need to be significantly higher, this makes the contact patch smaller and lessens the tire grip on the track surface. A higher tire pressure will loosen up the chassis, especially when used at the right rear. On a tacky track the starting right rear pressure may be as high as 14 lbs. As the track dries out the right rear pressure can be lowered to around 12 lbs, On a dry slick track the pressure for the right rear can be as low as 10 lbs. This increases side and forward bite by providing more tire patch contact. This is where records of previous races are needed. At the start of the race set the tire pressure to what the soil is dictating. Remember that an abrasive track will raise the tire pressure significantly.
Shock Travel Indicators
Shock travel indicators are a good rule of thumb guide for a chassis to see if it is handling as it should and if not where to look for the problem. When the travel of one corner is out of alignment with the other 3 corners it most always can point out a problem. The shock travel is the same for a wet, muddy track, a moist tacky track and a dry slick track. The difference is in the springs and shocks used in the car for these different track conditions to maintain the proper range of shock travel.
A good Ballpark shock travel number to look for:
Right front: 2 - 2 1/2"
Right Rear: 2 1/2 - 3"
These numbers are for a mildly smooth track, a track with heavy ruts and bumps will not give you the same numbers..and will not be a good indicator of chassis roll to determine understeer or oversteer. When checking suspension travel do not go over any large potholes and bumps in the pits to throw off your measurements.
Remember that these are guides for helping you adjust your chassis at the track and that racing is a dangerous sport so exercise extreme caution when using this guide.
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