FDR (Final Drive Ratio) is the ratio of the motor rotation relative to the wheel rotation. If the FDR is 10 or 10:1, for every 10-motor rotation, the wheel will rotate once. In RWD RC drifting, FDR will dictate the ‘feel’ of your throttle to the rear wheel, which will translate into controlling the traction of the rear wheel. FDR will have a great impact on how the chassis drives and feels.
FDR should always be related to the motor you are running. FDR 10 with 8.5T brushless motor and 17.5T brushless motor, the final RPM at the wheel is different.
Higher the FDR number, is considered ‘Low’ or ‘Short’ gearing, and lower the number it is considered ‘High’ or ‘Tall’ gearing.
WHAT FDR SHOULD I USE?
Let’s get right to the point. There are two great write ups in the Team Yokomo blog about FDR, or Gear Ratio, which I always recommend to people when they need a good ‘reference’.
Based on the most common brushless motor turns used in RWD RC drifting, these are some ‘references’ for FDR recommendation.
1) Plastic Tiles, Concrete
13.5T to 10.5T motor, FDR 10 to 13
2) Asphalt
13.5T to 10.5T motor, FDR 8 to 10
3) Carpet
10.5T to 8.5T motor, FDR 7 to 9
Again, these are just some ‘references’ and final setup is up to you to find the right FDR for the track and your driving style. If you are going to a new track, it would be easier to check with the local drivers what the spec tires are, and also check the FDR / motor turn to get the chassis dialed in quicker.
HOW DO I CALCULATE FDR?
This is one of the frequently asked questions in RWD RC drifting that many people struggle to remember. The math is very simple;
FDR = Spur Gear / Pinion Gear x Ring Gear / Drive(Bevel) Gear
On a Yokomo YD-2 or YD-2S, the Ring gear / Drive gear multiplier is 2.6. (Ring gear = 52, Drive gear = 20, the two idle gears are 1:1 so does not change the gear ratio)
With a stock 84 spur gear and 20 pinion gear, the FDR is 84/20 x 2.6, = 10.92. This fits right in the recommended FDR by Yokomo for plastic tiles and concrete tracks, which is more popular in Japan.
Most chassis will come with a FDR or gear ratio table showing the calculation, but if you know what the rear gearing multiplier is, the math is very simple.
HOW DO I ADJUST FDR?
On a Yokomo YD-2 or YD-2S chassis, the rear gearing is pretty much fixed at 2.6 regardless of if you have a spool, gear diff or ball diff. On other chassis platforms, you will need to count the number of teeth on the ring gear and divided it by the number of teeth of the drive gear or the bevel gear. On a stock Yokomo DP or DP conversions, the standard gearing is 40/17 = 2.35. There are optional reduction gear sets from Yokomo (originally designed for CS front gearing), D-LIKE, MST and other brands that offer 40/15=2.66, 40/12=3.33, 40/10=4.00, which enables you to get lower gearing easier by adjusting the rear gearing so you have more gearing options with spur and pinion. The rear gearing should be in the range of around mid-2 to mid-3 range to be practical to achieve FDR of around 8 to 10 to put the spur gear / pinion gear ratio of around mid-2 to mid-3 range as well. Some brands offer under 2, which will make it difficult have flexibility in adjusting. These were originally designed to increase the rear wheel RPM on CS platforms and not well suited for RWD application.
Once the rear gearing is set, you can adjust the FDR with the combination of spur gear and pinion gear. Changing the pinion gear is usually the easiest and most effective. Changing spur gear can be time consuming on some chassis.
Changing 2 teeth with the pinion gear will have more impact to FDR compared to changing 2 teeth in spur gear, for example;
Stock YD-2
Change pinion from 20 to 22 Change spur from 84 to 82
Spur gear 84 / Pinion gear 20 x 2.6 = FDR 10.92 Spur gear 84 / Pinion gear 22 x 2.6 = FDR 9.92 Spur gear 82 / Pinion gear 20 x 2.6 = FDR 10.66
Keep in mind that some chassis will have interference by using larger spur, or the motor mount adjustable range may restrict certain combinations of spur gear and pinion gear. Therefore, the manufacture FDR table has a recommended range.
I highly recommend having a good range of pinion and spur gears in your tool box. 1-2 teeth increment in pinion gear and maybe 4 teeth increment in spur gears, ideally from the same brand.
TUNING TIPS?
In RWD RC drift cars, there is no 1st or 2nd speed gears, and only one gearing to run the full range of the motor. Therefore, is it up to you to find the right ‘feel’ by adjusting the FDR and to find the ‘sweet spot’ for traction, speed, and control. There are many combinations of gearing, even trying to achieve the same FDR, and this is where you will need to trust your ‘feel’ to find the setting that fits the track and your driving style.
To achieve FDR 8 on a YD-2, you have various options;
Spur gear 90 / Pinion gear 29 x 2.6 = 8.06
Spur gear 84 / Pinion gear 27 x 2.6 = 8.08
Spur gear 78 / Pinion gear 25 x 2.6 = 8.11
Even with the similar FDR, the chassis should ‘feel’ different between these combinations. In ‘general’, smaller pinion will have quicker response from your throttle control, larger pinion will be slower to respond. If you are struggling with the tight turns on the track or too fast on the track, try going smaller in pinion. If you are not able to make the big sweeper on the track, or too slow on the track, try going to a larger pinion.
The load on the motor will be different with FDR. Lower the FDR, less load on the motor, and you will get longer run time on the same battery. Higher the FDR, more load on the motor and will generate more heat on the motor and ESC.
Each motor also has their characteristic in power and torque. Higher motor turns like 13.5T and even 17.5T, will have more torque but less RPM. 8.5T and 9.5T motor may not have as much torque, but more power and higher RPM range. Some people may think why torque is needed in drifting, but higher turn motors will allow high FDR in the 5-6 range maximizing the torque in the motor in lower RPM ranges. The same RPM at the wheel can be achieved with low FDR on a lower turn motor, but the feel will be different. Each motor will have its ‘sweet spot’, and adjusting the FDR to utilize this ‘sweet spot’ is also a good tuning technique.
Another element is the material of the gears. Spur gears are usually plastic. Axon/Yokomo makes spur gears with open center swirled rib which is designed to transfer power more effectively. Using lighter aluminum pinion gears versus heavier steel pinion gears will also change the ‘feel’. The rear gearing may also have options for different material. Plastic ring gear versus aluminum or heavy steel gears will have different feel and also different rotational mass.
Happy drifting!
January 20, 2018 / Charlotte NC