Quick-change and "not-so-quick-change" rearends both need the correct gear ratio in order
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?
Differences in the track, tire sizes, class rules, and other factors can cause you to rethink your selection of gears, possibly from week to week and even from the beginning to the 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 Primer
There are two basic rearends that are used in circle track racing. There is the OE 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 rearend that is designed for much easier access to the drive gears and therefore facilitates quick changes to your gear ratio. The reasons for making these changes are varied, but can be necessary in order to maintain performance levels.
To say the quick-change is superior would not be entirely accurate. It can be less durable than the OE rearend, but holds up fine for most short-track racing with regular maintenance. For larger cars that run long and fast racetracks and have high-horsepower engines that run at high rpm, the Ford 9-inch rearend is almost mandatory.
For our discussion, we will use the quick-change rearend. Some of the discussion will also relate to the OE rearends as we talk about reasons for gear change, not necessarily the process of making those changes as involved with each type.
In this exaggerated example, we can see where the use of a lower gear (5.61) might improve
Choosing the Correct Gear Ratio
We should always consider the highest and lowest rpm in our powerband when choosing the gear for our cars. You need to know where the engine powerband starts and ends for your motor. Then, look at the engine rpm for each point around the track for the gear you are currently running. You don't want to begin accelerating off a turn below the rpm where the power starts to build.
Likewise, it may not be the best thing to run out of the powerband 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 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 choose a gear that will pull from 6,000 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 6,000 rpm and 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 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, we will be better off (at least to the flag stand).
For the last half of the straightaway, if we have gained half a car length, 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 slowed 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.
It may be better 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 stopwatch, 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 improve your performance.
Gear changes in the OE rearend are harder than the quick-change rearend, but doing a littl
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 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 spinning, so make sure you can tighten the car off the corners sufficiently to take advantage of the lower gear.
This is especially true for running on dry-slick tracks. 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 in 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 have their higher ratio gear in the car that will 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 go 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 touring Late Model drivers have said they often used no more than half throttle for the whole race for many of their wins on dry-slick tracks.
To calculate the drive ratio for a quick-change, divide the number of teeth in the top gea
Running Past the PowerBand
If you go to a lower gear and put yourself way beyond the point where the motor produces good horsepower halfway down the straightaway, you've probably gone backward. Restricted motors are especially known for a dramatic drop in horsepower at the end of the powerband. Not only is the powerband running out, but also the restricted intake won't suck any more air at the higher rpm.
For tracks 31/44 mile or longer, using a higher gear might produce more top speed for a larger gain since we stay at the elevated speed longer on those tracks and acceleration is not a significant factor.
This is where thinking that higher rpm produces higher horsepower may be wrong. Torque is a factor in producing a higher top speed, and the torque curve may fall off before the horsepower curve does. Gearing for maximum torque may produce a higher top speed on longer racetracks.
Let the Stopwatch Decide
The best rule is to let the stopwatch determine the best gear. The fastest gear on the watch may not look or feel fast, but the lap times will tell the true story.
Divide the track into segments and time the car from the points on the track where the driver starts to accelerate to where the driver lifts going into the next turn. Use the same point on the racetrack for every measurement of elapsed time. Compare your times to your competitor's times to judge how each change stands up.
To calculate the final drive ratio (that includes the tire size) we divide the tire size b
Final Drive Ratio
The common interpretation of final drive ratio for a quick-change rearend is the combined ratio of the axle ratio and the QC gear ratio. We prefer to call this the drive ratio. We will use the word "final" later to include the tire size used in combination with the gear ratios. To find the drive ratio, we multiply the QC (quick-change) gear ratio by the axle ratio.
Tire size affects our true drive ratio. If we include the tire size in our final drive ratio, we will have the whole enchilada. To find this number, we divide the tire circumference by the drive ratio. If we have a 4.11 axle ratio, a 30-tooth over a 22-tooth QC ratio being 1.3636, then our drive ratio (DR) is 5.6045. If we then divide the tire circumference of 85 inches by the DR, our final drive ratio (FDR) is 15.1663.
The reason this number is significant is we might find ourselves having to use a larger tire some weekend. Suppose we arrive at the track late and all that is left in the tire truck are 87-inch or larger tires. Do we need to make a gear change? You bet. Here is why.
If we do the math in reverse and divide 87 (the new tire size) by our FDR number, we get a new gear ratio of 5.74. That is what our rear gear should be in order to keep our FDR at 15.1663. That means we need to install the closest gear to that-a 28-tooth over 20-tooth, 5.76 QC gear-to maintain our previous performance. If we don't, the car may be flat off the turns and our lap times will suffer.
Always keep a record of your FDR. This is the ratio that includes tire circumference information. You may find the right gear ratio for your track and then lose the advantage when you are forced to run tires of a different size. This is especially true when a track changes tire brands. We have seen the sizes of each brand differ by two inches or more.
In this example, we have a new tire size of 87 inches in circumference, up from the 85 inc
When to Make Gear Changes
The following are indicators that it may be time to make a gear change:
1) When the track changes, which may be true for dirt or asphalt tracks. On dirt, it is obvious when the track changes. For asphalt, temperature changes affect bite and gear choices.
2) When the rear tire sizes change.
3) When we go to a different racetrack.
For dirt tracks, you might need to make two or three changes to your gear ratio, depending on how much the track changes during the event. When the lap times change by 3-4 seconds, the lowest and highest speeds the car will attain will necessarily change. To keep the engine in the proper powerband, gear changes must be made.
Hot summer temperatures make some asphalt tracks slick. A lower gear might produce more wheelspin and slow down lap times. A higher gear would allow more throttle application without tire spin. For the cooler months, a lower gear combined with the added grip the track provides could produce better gains off the corner.
Always calculate your gear needs when changing rear tire sizes. A change in gear ratio equal to our example represents a 150 rpm difference at 6,000 rpm. If you have dialed in just what you need for optimum lap times, maintain the FDR by making the necessary QC gear change.
The dirt Late Model cars are more open in the area around the rearend, making gear changes
When we travel to different racetracks, we need to consider several factors when selecting a gear. Gear needs are influenced by the following: 1) track length, 2) track banking angle (speed through the turns), 3) turn radius and length of straightaways (tighter turns equal less speed and rpm, long straights equal high-end speed and rpm), and 4) surface bite availability, especially for dirt tracks.
For longer tracks, we are less concerned about acceleration off the corners because the exit speeds are much higher than on shorter tracks. We can tune our gear selection to the rpm desired at the end of the straights.
Tracks with high banking angles will allow more turn speeds, just as the longer tracks do. Therefore, our exit rpm will be higher and the added speed will be carried over the entire lap. The method of gearing slightly lower can be applied to short tracks with higher banking as well as low-banked tracks. It will probably be more successful in this situation because traction off the corners will be less of a concern with the higher banking and downforce created.
Gear selection and final drive ratio maintenance is very important and can improve your performance if done correctly. The math is simple, and the process of finding the correct gear that will produce the quickest acceleration is a process that is best done during testing.
We have shown where you can run different gear ratios and still end up with nearly the same beginning and ending rpm. Shooting for just an rpm number may not lead us to the fastest gear. Don't leave out this tuning process that is a critical part of the overall performance package.