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Old 08-14-2009, 07:57 AM   #1
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Default Gear Ratio Explanation???

Okay, i see all of this talk about ratio's and FDR's (wtf is FDR???)

i peeked around a bit and couldnt find much on it. could someone give me a little insight on gearing? i mean...if what you use gets you up to speed, whats the difference? thats how i see it right now.
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Old 08-14-2009, 08:14 AM   #2
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The fdr is the final drive ratio.

Rather than say, i'm using X tooth spur gear, X tooth pinion gear and X amount of internal gear ratio for a given situation, it is easier to say im using an FDR of 5.7 for example.

To get the FDR of any car, you need to do the following.

Spur gear teeth divided by pinion gear teeth, multiplied by the internal drive ratio of the car you're using. The TC3 for example is 2.5.

so using a 72t spur gear, and 30t spur gear would give you a final drive ratio of 6.0

Hope that helps
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Old 08-14-2009, 12:48 PM   #3
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It matters because the smaller spur bigger pinion the hotter your motor will get and it is also true if you are undergeared so keep an eye on that
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Old 08-14-2009, 01:11 PM   #4
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Quote:
Originally Posted by Double D Donuts View Post
Okay, i see all of this talk about ratio's and FDR's (wtf is FDR???)

i peeked around a bit and couldnt find much on it. could someone give me a little insight on gearing? i mean...if what you use gets you up to speed, whats the difference? thats how i see it right now.
It's what both of the above post said. It is important to know how to properly gear your car.

The simple formula is (spur gear size/pinion size)*internal drive ratio = FDR

Depending on the type of motor you use, size of track and type of car, your FDR will be different. If you are just driving your car on the street to have fun knowing your FDR isn't as important, although you want to make sure you don't burn up your electronics. But for racers knowing your FDR and how to gear your is a must.
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Old 08-14-2009, 02:27 PM   #5
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im just starting to get into racing, and i heard this talk of all of this and i was like wow, this stuff is serious, i thought you just change the pinion/spur and thats it. apparently theres alot more to it!

so how do i know my "IDR" then? i have a Corally RDX PHI. also, the higher the FDR im guessing it is the more revolutions of the tire per pinion revolution? or something along those lines.
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Old 08-14-2009, 02:38 PM   #6
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Corally RDX Phi - 1.71 (FDR)
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Old 08-14-2009, 02:39 PM   #7
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It should say in the car's manual. Most (all?) of the Phi series has an internal ratio of 2.0.

You are close with your definition of FDR. It is actually the number of revolutions of the motor per revolution of the tires.

Ex. A FDR of 5.2 means the motor will turn 5.2 times by the time the tires turn once.
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Old 08-14-2009, 03:16 PM   #8
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so normally you want to stay within 3-6 FDR? lower the FDR the higher the speed, yes?
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Old 08-14-2009, 03:18 PM   #9
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For the Phi/'09, this also depends on the size of the pulley. The larger Corally Pulleys, 42 teeth, would have an internal gear ratio of 2.0, while the smaller pulleys, 36, are 1.71. In addition, the center pulleys (stock 21 tooth on a Corally) would affect the FDR is changed to say a 17 or 18. Keep that in mind if your going to make a change so you gone burn up your motor.
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Old 08-14-2009, 03:35 PM   #10
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While I`m not an expert on this I have been lucky enough not to smoke a motor yet..........but there is always tomorrow..
The higher the fdr the more speed/less touque(punch)
the lower the fdr the more torque(punch)/ less speed
When I go to a new track I just ask around and find out what other drivers are running and start with that and just adjust from that to suit my driving style, checking the motor temp every 3 or 4 minutes.

Steve P.
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Old 08-14-2009, 03:37 PM   #11
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One other thing go to www.gearchart.com and print out ratio sheets.


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Old 08-14-2009, 06:56 PM   #12
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Quote:
Originally Posted by samstheman2 View Post
For the Phi/'09, this also depends on the size of the pulley. The larger Corally Pulleys, 42 teeth, would have an internal gear ratio of 2.0, while the smaller pulleys, 36, are 1.71. In addition, the center pulleys (stock 21 tooth on a Corally) would affect the FDR is changed to say a 17 or 18. Keep that in mind if your going to make a change so you gone burn up your motor.
Right, to figure out your car's outdrive ratio(that number you multiply with the spur/pinion ratio to get the FDR) on a belt-driven car, just take the number of teeth in the diff pulleys & divide it by the number of teeth in the center pulley. For example, on my Tamiya 416WE, I have 37 tooth diff pulleys & an 18 tooth center pulley, so my outdrive ratio is 2.055(37 divided by 18 is 2.055), & if I have a 96 tooth spur & want an FDR like say, 5.058, then I'd use a 39 tooth pinion(96 divided by 39 times 2.055 is about 5.058).
Hope that helps you get a better handle on it.....
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Old 08-14-2009, 09:51 PM   #13
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Originally Posted by srp67 View Post
While I`m not an expert on this I have been lucky enough not to smoke a motor yet..........but there is always tomorrow..
The higher the fdr the more speed/less touque(punch)
the lower the fdr the more torque(punch)/ less speed
When I go to a new track I just ask around and find out what other drivers are running and start with that and just adjust from that to suit my driving style, checking the motor temp every 3 or 4 minutes.

Steve P.
isit not other way around ? Higher FDR lower speed / more torque ??
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Old 08-15-2009, 03:42 AM   #14
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isit not other way around ? Higher FDR lower speed / more torque ??
Yes it is - you're right.

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Old 08-15-2009, 10:46 PM   #15
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isit not other way around ? Higher FDR lower speed / more torque ??
Basically yes, numerically higher(also referred to as a shorter gearing) FDR, better acceleration but less top speed, while a numerically lower FDR(referred to sometimes as being geared taller) means higher top speed, but slower acceleration, & it also makes for higher amp draw on the battery & can generate more heat from the electronics(if you go too far, the motor, ESC, etc. could overheat & if that gets bad enough they could become damaged)....
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