DMSBD Tech Tips

Race Preparation & Driving Strategy - by Ian Carsten

Even if you have made all the legal mechanical refinements to your car, you still havenít maximized the performance of its most important componentóthe driver. Many new race teams fail to recognize the critical role the driver and crew play in determining success in derby racing. For example, if your driverís posture is incorrect, then your car may not have as low a center of mass (or cm) as it could, and that may prevent it from attaining top speed. To illustrate, if your driver is sitting too upright, then his/her cm, and therefore, the cm of the car/driver system will be higher than it could be. To correct this, the driver can tuck the upper body down as low as possible and still be able to just barely see above the front edge of the cockpit and safely control the car. This will lower the driverís cm and that of the car/driver system as much as possible insofar as driving posture is concerned. Also, consider the seating position. Is the driver sitting on an unusually high seat pad or on a tall ballast weight? If so, then you should modify your setup so he/she is sitting as low to the floorboard as possible to lower cm. In any type of gravity-powered racing, such as soapbox derby, pinewood derby, or sledding, lowering the cm is an essential ingredient to attaining top speed.

The Resisting Forces
Now consider the resisting forces that act upon your racer. There are two resisting forces that you have little control over. Both the wheel bearings and the rubber tires generate resistance as they roll under a load. For a given load, wheel and bearing resistance is fixed and does not vary as speed increases. However, the resistance does increase as the weight load is increased. For this reason, it may be a bad idea to greatly load the rear axle more than the front. In fact, the AASBD has in previous years restricted cars to no more than 15 pounds greater load on the rear axle than the front. The other resisting force is air resistance. This increases as your carís speed increases. And air resistance is what engineers call a nonlinear force. For example, as your car increases speed from 10 mph to 20 mph, a doubling of speed, air resistance significantly more than doubles. It is air resistance, more than anything else that limits the speed of a derby car.

Your track practices the ďwheel swap/lane swapĒ to eliminate one particular carís having a superior wheel set as a reason for dominating other competitors. And the lane swap is intended to prevent one car from winning simply because it always gets to run in the fastest of lanes. Since all cars within a division have the same shape, their air resistance should be the same. But once again, the driverís posture plays an important role in the performance of the car, this time, in determining how aerodynamically efficient the car is. For example, a first time driver may sit too upright. If so, the upper body sticking out of the cockpit acts like a sail on a sailboat and will slow the car considerably. Of course this upright position raises the cm, which also contributes to slow performance. Next, lets consider how the driverís position in the cockpit can affect air drag. When the car starts moving forward, the air flowing past the car will also flow around the part of the driverís body exposed to the air stream. Since the driver sitting in position does not completely fill the cockpit opening in sit up and lean forward cars, there must usually be a gap between the driver and cockpit, either at the front, rear, or both.

The least desirable case is a gap at the rear. The air flows along the driverís head, and back and is directed down into the gap where it must abruptly loop back upward. This gap at the rear of the cockpit produces a significant drag force at speed and is believed to produce an effect similar to a parachute. To reduce this drag, the driver can scoot back until the back is in contact with the rear of the cockpit opening thus closing off the gap. Now the airflow can make a smooth transition from the driverís back to the top of the body shell just behind the driver. Of course, now there is a gap at the front of the cockpit, but an examination of the shapes involved suggests that the air flowing into the driverís head and shoulders, especially when the driver is tucked down as low as practical, should be much less deflected than in the previous case. Therefore, we can expect less drag with a gap at the front of the cockpit than one at the rear.

Also, you should use the rear cockpit foam to help close off as much gap as possible. And for a superstock car, if your driver is small enough, then you should also try to use the side foam as well to help close off the cockpit. These ideas can help to minimize air drag due to the fit of the driver in the cockpit opening. In order for your car to cut through the air with the least resistance, it is necessary for the driver to tuck down as low as possible. It would be very useful to carefully watch the fastest drivers and pay attention to how they position themselves into their cars. You should compare the posture of your driver and his/her faster opponents, as they are poised on the ramp. These fast, experienced drivers are not getting down low to look cool; they are doing so to generate the least possible air drag and have lowest cm and your driver should too. 

Driving and Car Handling
Now consider how the driver and crew can make a difference in how the car is handled. When the front wheels are turned to make a steering correction, a sideways thrust must be generated against the pavement to change the course of the car. This necessarily creates a resistance force that slows the car. The more suddenly this is done, the greater the angular change, or the sooner this is done after launching the car (the closer to the starting ramp) then the greater the resistance. You can reduce this by being very careful when you set up the car so that it is aligned in precisely the correct direction in the starting ramp, (which isnít always straight down the middle of the lane). Then little or no correction will be necessary until the car has traveled a considerable distance and built up as much speed as possible. Further, donít forget to align the front wheels on the ramp so the car will start in a straight line when the gate drops. Some crews place a small reference mark on one, or both, branches of the steering cable, such as a small spot of nail polish that is even with the clearance cutout when the front wheels are straight ahead. If the front wheels are not in the straight-ahead position, the car will roll harder and the front wheels will have to be straightened rather close to the start. That will cause you to unnecessarily lose time. Also, if any steering input is needed, then the gentlest possible correction, carried out over a relatively long amount of car travel, will generate the least possible resistance. Additionally, if any steering correction is needed, then allowing the car to travel in a straight line (even though it may not be the required course) for as long as possible, and allowing the car to build as much speed as it can before beginning any correction, will result in less time lost. For example, if your driver sees that the straight line the car was launched on will cause it to kiss the curb at 350 feet out from the start, then no steering correction should be made at all for at least 200 ~ 250 feet, allowing the speed to build as quickly as possible. Only then should the driver just barely begin to make a very slight correction designed to alter course to miss the curb by no more than a foot or so. In making this very gentle, slow correction, the curb will have been avoided and the least time will have been lost. This idea applies to all steering corrections the driver feels necessary. To do this smoothly, consistently, and safely requires practiceóthe more, the better. 

 

Drop Fast and Avoid the Rough Stuff
Obviously, when a gravity car goes downhill it builds speed. And the faster it falls vertically, the quicker the speed builds. The fastest racers know that if there is a sudden dip or one side of the lane falls more rapidly than the other, by steering into the dip or steeper decline, they will gain a corresponding increase in speed. So, you and your driver should arrive early at the track and walk slowly down each lane looking for any such dips or steeper side areas. When you know where they are, you can plan a line of travel that will take the car down into those dips or steeper declines, and thereby, gain a bit of speed. If you canít get there early, then watch the line the fastest racers take down each lane. By using the same line as they do, your car will likely be close to an optimum path. Since these fast drivers are experienced with this particular course, they have already determined the quickest pathóyou might as well use it too. Likewise, you should be aware that if there are some unusually rough spots, cracks, or gravel on one side of a lane, they will slow the car down and are to be avoided, if possible. For example, if your driver allows the tire to ride on a painted lane marker, that is usually rougher than the surrounding pavement, and will slow down the car somewhat. Try to avoid this to minimize speed/time loss. Further, the hotter the track surface, the faster your car will be. You may be able to take advantage of this at some tracks where one side of the lane is in shade and the other side is baking in bright sunlight. If that area is otherwise level from side to side, then steering onto the hottest side will allow your car to be slightly faster. Also, some new drivers unconsciously press down a bit on their brake pedal as they drive, lowering the brake pad into the air stream below the floorboard. Discuss this with your driver to be certain he/she does not do this, as it will slow the car somewhat.

 

Race Preparation
Preparing for a race entails many things, involving the driver, crew, car, tools and any accoutrements you bring with you. Both the driver and crew should arrive at the track well rested. Racing should be an interesting, challenging and fun experience for everyone on your team. Itís too easy to forget something or make a race-loosing mistake if any of your team is overly tired from lack of sleep the night before a race. Each member of a competitive race team must stay focused to ensure a coordinated effort and that requires everyone to be well rested at the start of the race day. Several days prior to a race, the car should be thoroughly checked to verify everything is adjusted and all screws, nuts, and control cables are tightened, as you want them. If you previously raced the car, the spindles may have been roughed up a bit by many wheel swaps. If so, you may wish to touch them up with aluminum polish. Buffing them to a high luster will make them as slippery as possible prior to the race. Likewise, you should bring whatever lubricants you choose to place on the spindles and wheel bearings. Any such lubrication should be applied just before you submit your car for inspection. Also, it is a good idea to check the spindles for proper alignment both horizontally and vertically. Further, you should carefully make a checklist of all the things you can possibly think of that might be necessary to have with you at the track. Then, when you pack your stuff into your vehicle, you can check it off from the list to verify it has been loaded. For example, if you think you might need to check the torque settings of your kingpins, be sure to include your inch-pound torque wrench and all necessary sockets in your race kit. Likewise, it is important to bring any tools or spare parts, especially easily lost items such as spare wheel clips, and wheel washers with you on race day. If you think you may need to remove the body from the board, it is a big time saver to bring a battery-powered drill with a # 2 phillips bit. A drill with a variable torque clutch adjusted to a low setting is the best choice. Final tightening should be done with a #2 phillips screwdriver by hand to avoid stripping the thread in the wood. Further, if you need to remove the body from the floorboard, or an airfoil from an axle, itís easy to loose a screw and/or washer, so bring spares. Donít forget to have some spare brake pads and mounting hardware handy. An even better idea is to have a spare brake plunger with a new pad already mounted and the screws, trimmed flush to the top of nuts. In this way, by simply swapping plungers, a new brake pad can be installed in about one minute. Also, since the 5/64-inch allen wrenches used to secure the setscrews on the cable clamps are so small and easily lost, you should take several to the race. Hereís a partial list of the things derby teams typically bring with them to a race. Lightweight folding chairs, portable awning and frame or beach umbrella (many derby tracks have no shade) insulated cooler for drinks, sandwiches, ice, or any food that has to be kept cool, goggles for the drivers and sunglasses for everyone. The suggestion that the driver wear goggles is especially important because occasionally a driver has to retire from a race due to getting some junk in an eye. Thatís very easy to do at the 30 mile per hour speeds typical in a derby race. Also, youíll need sunscreen lotion, simple first aid kit, aspirins, antacid tablets or any medicines needed, paper towels, napkins, pre-moistened hand wipes, blankets, pillows, toys, games books etc. for young children, notebooks, pencils, erasers, insect repellant, cameras and plenty of film or video camera with spare batteries and tapes, cell phone, waterproof tarp to cover your car and other stuff and weights to hold it down in the event of a sudden thunderstorm which may be accompanied by strong wind. This list may be too long for you or you may want to bring even more, depending on your particular needs. One very important thing youíll need is a pre-planned response in the event of a rapidly forming storm. For example, you should have each team member responsible to quickly cover and secure everything and then get into your vehicle for safety until the storm subsides.

Also, stock and superstock drivers need maximum flexibility and range of motion to get into an efficient racing position. That requires practicing stretching on a regular basis before and during the racing season. Then the driver should actually get into his/her car with an assistant to visually check that the position is really low enough and that the back closes off the gap at the rear of the cockpit. If you do this regularly, then you should easily get into a nice low racing position on race day. When you transport your car, be sure the wheels are not exposed to the air on an open trailer or in the back of pickup truck. Otherwise, the high-speed air stream at highway speed can force dirt or sand into your wheel bearings and spoil them. Likewise, appliquť exposed to such air speed can start to peel away from the carís body. If your vehicle is open to the air, it is a good idea to tarp it and wrap the tarp with elastic bands to prevent the tarp from flapping. Additionally, when you arrive at the track, it is an important safety practice to re-check the tension of the setscrews on the brake and steering cable clamps.

This Concludes Article 7.

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