Overdrive discussions
Last Updated on Monday, 12 October 2009 07:13 Written by SlowSteady Wednesday, 21 January 2009 17:02
Question by Jas1 
Right just a quick question, I'm a rubber driver, and had little experience with setting a foam car up, in the past I've just been told what size to true my tyres down to and just run the car, and never got involved in over drive ratios. So ideally what sort of over drive do I want to be running?
Also when running solid or diff in front do you still run over drive or not?
Answer by InitialD
Well actually the more technical amongst us is Master Julius. *Twisted Evil*
Usually, the the front overdrive or underdrive thingy is use when you're on oneway front. That's because when you're off throttle, the front tires which would turn at a different rate either because of the different front pulley ratio or different tire diameter would "disengage" from the rest of the drivetrain.
Julius did mention that if you use overdrive with a front solid or a front diff, it will create some kind of drag brakes when off power which could prove very useful on certain tracks with certain driving style.
You can achieve front overdrive by two ways...
1. Using bigger front tires 2. Playing with front / rear crown gears (for shaft driven cars) or pulleys for belt driven cars
The 1st method is pretty easy and you sometimes get this accidently when you run in a long main when the rear tires get smaller than the front tires. The handling of hte car changes of course.
The 2nd method is rather tricky. You really need to know how to calculate the front / rear internal drive ratio of a particular car. I give some examples below;
Serpent 710
35T
22T 19T
22T 24T
45T
Pardon the pulley arrangement but you will see that the front internal drive ratio is as follows;
Front internal DR = (35/19) x (22/22) = 1.842 Rear internal DR = (45/24) = 1.875
In this case, you will notice that the Front internal DR is less than the rear which means if I use equal size tires, I get more drive in the front wheel i.e. front wheels turn more times than rear which simply means front overdrive.
When the front wheels turn at a faster rate than the rear wheels, then the tire rollout that results from a single engine revolution (at a particular tire diameter) in the front will exceed the tire rollout from the rear wheel.
Therefore in order to get the same front and rear tire rollout with different front and rear internal drive ratios, you need to use different tire diameters to balance this out. A higher front tire rollout as a result of lower internal drive ratio would need a relatively smaller tire diameter to match the rear tire rollout with a relatively larger tire diameter.
Front Tire Rollout = PI x Front Tire Diameter / (Gear Ratio) / (Front Internal Drive Ratio)
Rear Tire Rollout = PI x Rear Tire Diameter / (Gear Ratio) / (Rear Internal Drive Ratio)
In this case, we can calculate and derive that the 710 requires about 1 mm split (i.e. front tires 1 mm smaller than rear tires) to get 1:1 overdrive.
Other examples;
Kyosho V1RRR
27T
24T 18T
18T 21T
42T
Front internal DR = (27/18 ) x (24/18 ) = 2 Rear internal DR = (42/21) = 2
Same front and rear internal drive ratio with stock pulleys. With optional 23T side pulley instead of 24T, you need about 2.5 mm smaller front tires to get 1:1 overdrive.
Mugen MTX-3
32T
24T 16T
19T 19T
48T
Front internal DR = (32/16) x (24/19) = 2.526 Rear internal DR = (48/19) = 2.526
Same front and rear internal drive ratio with stock pulleys. With optional 16T and 19T used as side pulley instead of 19T and 24T, you need about 3.5 mm smaller front tires to get 1:1 overdrive.
32T
19T 16T
16T 19T
48T
Front internal DR = (32/16) x (19/16) = 2.375 Rear internal DR = (48/19) = 2.526
Team Magic G4 / Kyosho V1RR
32T
27T 18T
19T 19T
48T
Front internal DR = (32/18 ) x (27/19) = 2.526 Rear internal DR = (48/19) = 2.526
Same front and rear internal drive ratio with stock pulleys. With optional 26T side pulley instead of 27T, you need about 2 mm smaller front tires to get 1:1 overdrive.
HPI R40
36T
23T 18T
19T 19T
46T
Front internal DR = (36/18 ) x (23/19) = 2.421 Rear internal DR = (46/19) = 2.421
Same front and rear internal drive ratio with stock pulleys. With optional 22T side front pulley instead of 23T, you need about 2.5 mm smaller front tires to get 1:1 overdrive.
Kyosho FW05R
40T
13T
13T
40T
Front internal DR = (40/13) = 3.08 Rear internal DR = (40/13) = 3.08
Same front and rear internal drive ratio with stock ring gears. With optional front and rear ring gears like 39T, 38T and 37T, the internal drive ratios are 3, 2.923 and 2.846 respectively.
You can experiement and play with the using different front and rear ring gears to give you different tire split;
39T ring gear front / 40T ring gear rear = 1.5 mm tire split
38T ring gear front / 40T ring gear rear = 3 mm tire split
37T ring gear front / 40T ring gear rear = 4.5 mm tire split
38T ring gear front / 39T ring gear rear = 1.5 mm tire split
37T ring gear front / 39T ring gear rear = 3 mm tire split
37T ring gear front / 38T ring gear rear = 1.5 mm tire split
With the Kawahara conversion, the 0.8 ring gears given will be a new set as follows;
45T
17T
17T
46T
Front internal DR = (45/17) = 2.647 Rear internal DR = (40/13) = 2.706
With the above config, you need about 1.5 mm smaller front tires to get 1:1 overdrive.
NTC3
40T
16T
16T
40T
Front internal DR = (40/16) = 2.5 Rear internal DR = (40/16) = 2.5
Same front and rear internal drive ratio with stock ring gears. No known optional front and rear ring gears to change internal drive ratio.
NOTE:- Readers may be interested to know that in the Downloads section, under RC Cars, there are numerous programs and spreadsheets that will help you understand as well as calculate overdrive ratios and roll-out numbers. The original thread link is here.
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