We need to choose the correct gear for our particular racing and track configuration, and we need to maintain that overall ratio when we make changes. Finding the right gear ratio for a particular track in a certain class of circle track racing may be as easy as asking your competitors. Most racers who regularly run the same racetrack each week will settle on the same gear and run it the entire season. But is the gear you’re using producing the fastest lap? And, what if the track requires a different tire than last year? Here is how you can keep straight with your gear choices.
Differences in the track length and shape, tire sizes, class rules, and other factors can cause us to re-think our selection of gears, possibly from week to week and even beginning to end of an event in the case of dirt track racing. The reasons for this may become more apparent as we study the whole concept of gear selection.
Basic Gear Information
There are two basic rearends that are used in circle track racing. There is the OEM type ring-and-pinion in a pumpkin case type where gear changes are rather difficult and racers are less likely to make week to week changes. The other is the quick-change–type rearend, and is designed to be much easier to access the drive gears and therefore facilitate quick-changes to your gear ratio. The reasons why we would make these changes are varied, but in many cases necessary in order to maintain performance levels.
To say that the quick-change is superior would not be exactly accurate. It is, in fact, less durable than the OEM-type rearend for higher horsepower applications, but for most short track racing, holds up fine with regular maintenance. For larger cars that run long and fast racetracks and with high horsepower engines at high rpm, the Ford 9-inch based rearend is almost mandatory.
For our discussion, we will use as an example the quick-change rearend. Some of the discussion will also relate to the OEM rearends as we talk about reasons for gear change, not necessarily the process of making those changes as involved with each type. The OEM rearends are built to a specific gear ratio and are carefully matched so that the pinion and ring gears are meshed correctly. This unit is changed out to make gear changes to the car. It is simply a matter of switching third members and a team must have several of these units assembled and ready to install.
Choosing the Correct Gear Ratio
We should always know and consider the highest and lowest rpm in our powerband when choosing the gear for our cars. You need to know where the useful engine powerband starts and ends for your motor. Then look at what the engine rpm’s are at each point around the track for the gear you’re currently running. You don’t want to begin to accelerate off of a turn below the rpm where the power starts to build and/or the torque is near peak.
Likewise, it may not be the best idea to run out of the powerband somewhere down the straightaway. A bigger mistake might be to begin to hit peak horsepower just as you’re getting ready to brake into the corner. There is a compromise that may produce a faster lap.
There may be two or more gear ratios that will produce the same rpm at the lift point at the end of the straightaway. Each will begin at the same rpm off the turns and each will end at the same rpm at the lift point going into the next turn, but one will be faster. How so? Here’s how.
If we chose a gear that will pull from say 5,000 rpm just past mid-turn to 7,100 rpm at the end of the straightaway, we might feel we have the right gear if the horsepower curve goals are met. That means our engine horsepower curve comes on at 5,000 rpm and it peaks at 7,100 rpm. It sounds like we have the right gear, but maybe not.
On some tracks where acceleration off the corners is critical for passing, a lower gear (higher ratio) might help us get off the turns better while not necessarily hurting us at the other end. The lower gear will accelerate the car quicker and if we don’t lose traction with the switch, at least to the flag stand we will be better off.
For the last half of the straightaway, if we have gained a half a car length by now, our speed will be mostly peaked while our competitor will still be accelerating. We will still be pulling away from the other car because the other car has not yet reached top speed/rpm.
Our turn entry will be much smoother because we will have reduced our acceleration before we lift to brake while the other car is still accelerating when the transition to braking occurs. It is very disruptive to make the change from hard acceleration to hard braking. It is much smoother to transition from steady speed to hard braking. Just ask any high performance driving instructor and they will agree.
So, we might be better off to install a lower gear, beat our competition off the corner and have a better corner entry all by doing a little experimentation. All it takes is a little effort and testing with a stop watch and the results can be evaluated. Most teams don’t know what areas to test at a test session. This is one of those areas that may well improve your performance. Just work your stop watch from the mid-turn to the lift point at the end of the straight.
Power vs. Wheelspin
A lower gear will pull much better, but only if the engine is putting out sufficient horsepower at the low end, and if you can get the pulling power to the racetrack. Being able to pull great off the corner is all lost if the wheels end up slipping. So, make sure you can tighten the car off the corners sufficiently to take advantage of the lower gear.
This is true especially for dirt racers running on dry, slick tracks. But for most dirt tracks the condition of the track is constantly changing. If we know the track is going to have much more bite early in the event, during qualifying, and on into the early heat races, we might be able to use that to our advantage when selecting gears.
When the track is tight, we can utilize a lower gear since we will have more traction available. If the other cars still have their higher ratio gear in the car that work best under the dry slick conditions that will come later on in the evening, we might qualify better and run better in the heat race. That will get us a much better starting position for the main event.
If the track has gone dry and slick, we can then make a gear change to a higher gear that is less prone to spinning the tires to maintain grip off the corners. Power availability at the other end of the straightaway becomes less important on dry slick tracks. Many top late model touring drivers have been heard to say they often used no more than half throttle for the whole race for many of their wins on dry slick tracks.