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Quick Change Gears: What are they? How do they affect setup? What do the numbers mean?

In this blog post, we explain what quick change gears are, how changing the gears can affect the handling of a sprintcar, what the gear numbers mean and a few formulas.

What are quick change gears?

In sprintcar racing, quick-change gears refer to a specific type of rear-end differential gear setup that allows for rapid adjustments to the gear ratio. The quick change gear setup is highly prevalent in sprintcars due to its adaptability and ease of gear ratio changes, which can significantly affect the car's performance on different tracks. The rear-end differential housing contains a set of gears that transfer power from the engine to the rear wheels.

Tip when changing Gears

Ensure the bigger gear is always at the top as sprintcars run on the high side of the chart, or remember an "upside down snowman"!

How can gearing be used to adjust the car?

Changing to a higher gear ratio will tighten up the car for two reasons:

1) There is less wheel spin because the motor is bogging down, and

2) When the engine bogs down, it is working off torque, and the torque is trying to lay the chassis over onto the right rear tyre, adding traction to that corner.

The term "higher" gear ratio, as used here, means the ratio is lower numerically (such as going from 6:86 to 5:70), but the car will move a further distance for each engine revolution. A "low" gear ratio is numerically high (such as 6.86) but produces a slower speed at the same engine RPMs.

When the track gets slicker, many racers will go to a higher gear ratio. But be aware - the corner speed always slows down when the track gets slick. So, the car is going from a slower corner speed onto the straightaways. The driver doesn't necessarily have to go to a higher gear because he isn't carrying the speed through the corners that he was when there was some moisture in the track. The higher gear is only sometimes necessary when the track gets slick.

If the corner speed slows down and everybody is down on the bottom groove, stay with the original gear. If the groove moves up and there is still a bit of a cushion, then go a little higher on the gear ratio, so the tyres don't spin.

A lower gear ratio can be used to loosen a car up. When the driver gets back on the throttle, he can loosen it up by spinning the tyres. Plus, the motor won't be torquing the car over and creating more right rear traction.

So, for example, if you are racing on a tacky track and the chassis setup is really tight, and the driver can't get the car loosened up, use a little lower gear. This might sacrifice some top-end engine performance and lose some speed at the straight's end, but the speed gained by loosening it up through the corners will more than compensate for that. The momentum gained from increased mid-corner and corner exit speed will produce more straightaway speed for passing.

The final gear ratio is affected by the tyre size. If a racer uses a 102-inch tyre and is geared perfectly, then changes to a 106-inch tyre to correct the stagger, the gear ratio must also be changed. The larger tyre raises the overall ratio, so the gear ratio has to be lowered. The difference in gear reduction needed is the percentage of tyre circumference increase. In other words, going from a 102-inch tyre to a 106-inch tyre is a 4-inch difference. 4 divided by 102 equals 4%. This means the gear ratio has to be 4% lower. Say the driver was using a 6.24 gear with the 102-inch tyre. To find the new gear required with the 106-inch tyre: 6.24 x .04 (4%) = .25. 6.24 + .25 equals 6.49, which is the new gear ratio required.

What do the numbers on the gear sets mean?

You don't need to know this, but this is how it is worked out. The gear numbers are used to determine the final gear ratio in combination with the ring & pinion inside the diff. Sprintcar rear ends are commonly 4.11 or 4.86.

Once you know what rear end you have, you'll need to take a known gearset and divide the number of teeth on the large gear by the number of teeth on the small gear to give you the "Spur Ratio". Then you take the "Ring and Pinion" ratio and Multiply it by the "Spur Ratio", and it'll give you your final gear ratio.

i.e. Gearset #10 has 17 teeth and 23 teeth on the "Winters Gear Chart", so you'll divide 23/17 = 1.3529, then multiply this by the Ring and Pinion ratio (I'll use 4.86), which is actually a 34/7 Ring and Pinion (4.857).

So the final drive is 1.3529 x 4.857 = 6.57 final drive. Luckily, you don't have to work this out; we have a chart that does this for us. The winters gear chart is in the front of our setup record book that you can buy by clicking here.

The RPM Gearing Formula

The only formula I have used in our wingless division is the RPM gearing formula, as we have a 6000 limiter. This is your current RPM divided by the gear ratio times by the new ratio, which gives you your new RPM.

Say I have a max RPM of 5600 with a 6.57 gear ratio. To get this closer to 6000 RPM, I must increase the ratio from 6.57 to something higher, like 6.97.

Using the formula 5600/6.57 = 852

Times by my new gear ratio of 6.9 (852 x 6.97) is 5938 RPM.

Gearing is only one part of the overall setup of the car. It is important to keep a note of what gearing you have previously used at each track you have been to and under what conditions.

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