For stock class racing, making...
For stock class racing, making gear changes at the track may not be feasible. But many teams keep several assembled rearends handy with different ratios for running different racetracks.
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 are using producing the fastest lap? And, what if the track requires a different tire than last year? Here's 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 rethink 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.
There are two basic rearends that are used in circle track racing. There is the OEM-type ring and pinion in a pumpkin case, 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, which 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.
Quick change and not-so-quick...
Quick change and not-so-quick change rearends both need to have the correct gear ratio in order to have maximum performance.
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'll use the quick change rearend as an example. 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's simply a matter of switching third members and a team must have several of these units assembled and ready to install.
This is a rearend gearset...
This is a rearend gearset that is based on the Ford 9-inch rearend. It's durable enough for high horsepower applications. It's not easy to change at the track. Professional teams keep many sets of these units on the trailer in case they need to change their gear ratios.
We should always know and consider the highest and lowest rpm in our power band when choosing the gear for our cars. You need to know where the useful engine power band starts and ends for your motor. Then look at what the engine rpms are at each point around the track for the gear you are currently running. You don't want to begin to accelerate off 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 power band somewhere down the straightaway. A bigger mistake might be to begin to hit peak horsepower just as you are 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.