Rollout, Final Drive Ratio (FDR), and Transmission Ratio

In the world of RC car racing, you'll often hear the terms "rollout" and "FDR," but what exactly are they?  And how do they help?

Usually when you hear people talking about "gear ratio" it refers to the ratio between the spur and the pinion gear (the "secondary ratio".) But in many classes of racing, particularly on-road, people talk more about the final drive ratio or the rollout because it lets them compare gearing across cars of different makes. This is because cars of different makes have different internal ratios (aka transmission ratio or primary ratio).

For instance, if you're driving a TLR 22 5.0 and you know your spur/pinion ratio, you can't directly compare that to an Associated RC10 B6.4 because their internal ratios are different. However, if you include the internal (transmission) ratio in your calculation, you can compare ratios across cars. This is where the Final Drive Ratio comes in.

FDR

The Final Drive Ratio is the ratio of how many times the pinion rotates for every rotation of the car's wheels. The higher this number, the more times the pinion rotates per turn of the wheels, producing more torque and acceleration but less top speed. If this number is lower, the wheels turn more closely to the rotation rate of the pinion, producing more top speed, but less torque and acceleration. Looking at it the other way, larger pinions produce a lower FDR, and vice versa.

The Final Drive Ratio is calculated by multiplying the internal/transmission ratio by the secondary ratio, the ratio of the spur's teeth to the pinion:

final drive ratio = internal ratio * ( spur / pinion )

So, let's say you know one of the fast girls at the track has a certain FDR. You can figure out which pinion to use to get that same FDR on your car even if you have a different car by reversing the equation:

pinion = ( internal ratio * spur ) / final drive ratio

For recommendations on a pinion to use for your car, check out our free RC Gear Recommender App.

For help calculating FDR or rollout, try our free RC Gear Ratio Calculator App.‍

Internal/Transmission Ratio

For many cars, you can find the internal ratio in the manual, but not always. If it isn't there, you can look for the size of the gear on the differential (or axle) and divide it by the size of the gear that drives it.

The differential/axle gear may be driven by a belt or by a direct gear-to-gear contact - either way, you can divide the gear on the differential/axle by the number of teeth on the gear that drives it via belt/gear. This gear is usually called a pinion, and we'll cal it an internal pinion for clarity: 

internal ratio = differential gear teeth / internal pinion teeth

If you're using our RC Setup App, the internal ratio is probably already provided in the Drivetrain section of your setups. Or, you can check our RC Gear Ratio Calculator which has many internal ratios listed for popular cars.

Important: cars with just a pinion and spur and no transmission have no internal ratio! If you are doing calculations, you can treat their internal ratio as "1." These are typically on-road "pan" cars such as 1/12th scale pan, 1/10 scale pan, and F1.

Rollout

Now, let's say you're driving a 1/12th scale pan car with brand new foam tires you just mounted, and you look over, and the racer next to you has the same pinion and spur, but they just finished sanding down their tires, so they're 5mm narrower. How do you think this affects the car?

Larger tires have the same impact on the drive ratio as using a larger pinion - larger tires produce higher top speed and less acceleration (and they also change your ride height!), so even though you may have the same gear ratio is that other driver, you have a lower effective gear ratio.

To account for this we use a number called rollout which is a true measurement of the effective gear ratio.

If your tires are always the same size (for rubber tires), FDR is all you need, but if your tires can change sizes (usually only significant for foam tires) you need to use rollout to understand your effective gear ratio.

Rollout definition: how far the car moves for a single rotation of the motor (or pinion.)

rollout = tire circumference / FDR or, for pan cars with no transmission:

rollout = tire diameter * pi / (spur / pinion) or

rollout = tire diameter * 3.1415 / (spur / pinion)

A higher rollout number means the car moves farther every time the motor turns which would have the same effect as a lower FDR or larger pinion.

Again, let's say the racer next to you is going crazy fast, so you ask them what their rollout is. When they tell you, then what? What pinion should you use? Backing the formula around, you get:

pinion = (spur * rollout) / (tire diameter * 3.1415)

For recommendations on a pinion to use for your car, check out our free RC Gear Recommender App.

For help calculating FDR or rollout, try our free RC Gear Ratio Calculator App.