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Old 07-19-2010, 04:56 PM   #1
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Default Car Gearing vs Motor Turns vs track size

hey guys,

i have a question and i think many do have the same question.

let me start first by saying that i DO understand the whole formula to calculate the final drive ratio. and i do know that each motor has its own FDR range.

But my question is, how to choose the right FDR from motor turn and most important the track size???

for example, i run a 4.5T and the recommended FDR is a range of 8 to 10. thats alot of gearing in between, if our track is carpet, both technical and fast and the straight is around 27meters = 89 feet.


i know my FDR should be between 8 and 10 but what do i really need for such track??? well, someone recommended me a FDR of 9.5 but why?? and how did calculate or know that for a 90' track, you need a 9.5FDR running a 4.5T??

is there a way to know aproximatly the FDR needed for each track depending on the size??

sorry for the long question but its been hunting me for while

cheers
Ali
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Old 07-19-2010, 07:00 PM   #2
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1) Experience... lots of experienced racers can just look at the track and be immediately in the ballpark as far as gearing. Obviously, this doesn't work for those new to electric.

2) Ask others running the same motor what gearing to start with. Err on the side of being under geared, and start adding teeth until one of the following happens: a) lap times don't get any faster, or b) motor temps start to get out of hand. Then back off one tooth for safety.

3) Play with gearing until the car stops accelerating about 2/3 - 3/4 of the way down the straight. You might be able to calculate approximate gearing if you know the speed of the other cars given the same motor. Something like the Novak Sentry data logger can help here as well.

4) If the track has a lot of slow turns and short chutes, err towards the smaller pinion size to get more rip in the infield, since that is where most of the time in the lap is spent. Lots of long straights and fast sweepers, go the opposite way.

5) It's even more complicated for the spec motor classes (17.5/13.5/10.5), since they generally play with motor timing/boost/turbo as well. For modified, not so much, since you generally have way more power than you need.

Hope this helps...
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Old 07-20-2010, 12:07 AM   #3
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thanks for the great explaination,
yes it helps alot, now i understand more what i should do to gear up my car, so its always better to start with a lower FDR value not to burn the motor. watching the temp and car's acceleration, i should gear up 1 tooth at a time (pinion) or 2-4 teeth (spur).

sounds great, thanks again

Ali
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Old 07-20-2010, 01:25 AM   #4
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Originally Posted by KWT-NITRO View Post
thanks for the great explaination,
yes it helps alot, now i understand more what i should do to gear up my car, so its always better to start with a lower FDR value not to burn the motor. watching the temp and car's acceleration, i should gear up 1 tooth at a time (pinion) or 2-4 teeth (spur).

sounds great, thanks again

Ali
On the contrary. You need a high FDR (low pinion ) to start.
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Old 07-20-2010, 01:36 AM   #5
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On the contrary. You need a high FDR (low pinion ) to start.
why will you start with a high FDR??? the safe side is to start with a lower FDR value and start increasing it untill you get to the desired value.

High FDR = shorter ratio = faster acceleration = less top end = higher temp

cheers

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Old 07-20-2010, 02:14 AM   #6
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why will you start with a high FDR??? the safe side is to start with a lower FDR value and start increasing it untill you get to the desired value.

High FDR = shorter ratio = faster acceleration = less top end = higher temp

cheers

Ali
FDR = Spur / Pinion x Internal Gear. The higher your pinion, the higher the temp. So the lower your FDR the higher your temp.

You want to find the ideal FDR without blowing your engine. So start with a small pinion and then increase it. No point blowing your engine with a big pinion and then deciding you needed to go down one tooth..
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Old 07-20-2010, 02:22 AM   #7
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FDR = Spur / Pinion x Internal Gear. The higher your pinion, the higher the temp. So the lower your FDR the higher your temp.

You want to find the ideal FDR without blowing your engine. So start with a small pinion and then increase it. No point blowing your engine with a big pinion and then deciding you needed to go down one tooth..
you got it all mixed up buddy.

FDR = spur/pinion * DTR (Drive train Ration) given by manufacturer

for example (XRAY T3): 116/25 * 1.9 = 8.816 FDR
with a smaller pinion 116/22* 1.9 = 10.018 FDR

10.018 is shorter ratio than 8.816 so you will creat more heat with the 22

you start with 25 and gradulay decrease the size of pinion.
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Old 07-20-2010, 02:42 AM   #8
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Originally Posted by KWT-NITRO View Post
you got it all mixed up buddy.

FDR = spur/pinion * DTR (Drive train Ration) given by manufacturer

for example (XRAY T3): 116/25 * 1.9 = 8.816 FDR
with a smaller pinion 116/22* 1.9 = 10.018 FDR

10.018 is shorter ratio than 8.816 so you will creat more heat with the 22

you start with 25 and gradulay decrease the size of pinion.
Mate. in that case start with a 45 pinion and when you blow up your motor don't come asking what happened.
Heat is due to overgearing. Do reduce the gearing you reduce the pinion or increase the spur, which increases the FDR.

From Novak website: "Over gearing the motor will result in excessive operating temperatures, which may result in internal damage. This type of damage is not covered under the productís warranty."

Heat is not due to the motor spinning faster. It is due to needing too much power to spin a bigger pinion.
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Old 07-20-2010, 02:44 AM   #9
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Quote:
Originally Posted by KWT-NITRO View Post
you got it all mixed up buddy.

FDR = spur/pinion * DTR (Drive train Ration) given by manufacturer

for example (XRAY T3): 116/25 * 1.9 = 8.816 FDR
with a smaller pinion 116/22* 1.9 = 10.018 FDR

10.018 is shorter ratio than 8.816 so you will creat more heat with the 22

you start with 25 and gradulay decrease the size of pinion.
Sorry bro, I would hate to see you start nuke'n motors, but lower FDR numbers are 'taller' gears and put more load on motors than 'shorter' or higher FDR numbers.

I am reaching here, but trying to make a point: Gears multiply leverage or torque, it is the same principle how pullys work with rope. The higher the FDR number the more leverage the gear exert, the less work the motor has to do to move the car.

Temp shows itself in 'U' shaped curve, too low and FDR or too high an FDR can raise temps. You would notice quite too much a drop off in top speed before you got into temp trouble with too high and FDR. You could go too low, love the speed, but puff the motor from over temp fairly easily.

Just my feeble attempt at helping explain the process. The best way to understand is to experiment yourself. It will become obvious quickly.
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Old 07-20-2010, 02:52 AM   #10
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thanks for the answer,

i understand what you are saying, but what would be safer to start with a very high FDR or low FDR?? you dont want the motor revving up alot with a high FDR.
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Old 07-20-2010, 02:57 AM   #11
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Quote:
Originally Posted by tigerdim View Post
Mate. in that case start with a 45 pinion and when you blow up your motor don't come asking what happened.
Heat is due to overgearing. Do reduce the gearing you reduce the pinion or increase the spur, which increases the FDR.

From Novak website: "Over gearing the motor will result in excessive operating temperatures, which may result in internal damage. This type of damage is not covered under the productís warranty."

.

thanks for the explaination. i aint talking 45 pinion and 20 pinion. not that much of a difference. i'm between 23 and 25.

and thanks for the info, i didnt know the last part you said ''Heat is not due to the motor spinning faster. It is due to needing too much power to spin a bigger pinion''

i thought when the motor revs high, the temp goes up, didnt know a bigger pinion will load the motor.
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Old 07-20-2010, 02:59 AM   #12
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thanks for the answer,

i understand what you are saying, but what would be safer to start with a very high FDR or low FDR?? you dont want the motor revving up alot with a high FDR.
"but lower FDR numbers are 'taller' gears and put more load on motors than 'shorter' or higher FDR numbers. "

You want to start high FDR. Smaller pinion means less load on the motor as there's less resistance.
Imagine taking off on a push pike with a very high gear..you ll hurt your muscles pretty quickly.
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Old 07-20-2010, 03:14 AM   #13
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Here it is in a nutshell.

the actual values will depend on the size of your track, setup (especially ESC setup) and everything else as per usual.
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Old 07-20-2010, 03:15 AM   #14
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Quote:
Originally Posted by tigerdim View Post
"but lower FDR numbers are 'taller' gears and put more load on motors than 'shorter' or higher FDR numbers. "

You want to start high FDR. Smaller pinion means less load on the motor as there's less resistance.
Imagine taking off on a push pike with a very high gear..you ll hurt your muscles pretty quickly.
sounds good.

okay, then what happens if my FDR is too high?? higher then needed?? i mean what should i know or notice to stop going smaller in pinion size??

cheers
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Old 07-20-2010, 03:23 AM   #15
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sounds good.

okay, then what happens if my FDR is too high?? higher then needed?? i mean what should i know or notice to stop going smaller in pinion size??

cheers
If your pinion is too small, you ll get to your max speed almost right at the beginning of the straight.
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