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Old 05-05-2009, 02:01 AM   #1
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Default more speed?

hey all,

i have a yokomo bd5, onroad with hitec radio gear.
the car is very light due to the chassis and bracing being made of carbon fibre, and the rest of the cars parts are plastic and aluminium alloy.
the car had a 17t brushed motor, and novatech esc.

ive now ordered (and will soon fit) a castle creations sidewinder 5700kv esc motor combo.
ive heard this system getting trucks up to 40mph! they weight alot more than my little carbon car, so what is an expected top speed with stock gearing and 5000mah, 7.2v nimh battery.

also, im looking to get a 2s lipo to maximise its performance. what type of battery would be the best to buy for top end speed(and fairly good run time), with the sidewinder combo.
also, what is a decent gear ratio(can you post pinion and spur sizes), for top speed and moderate acceleration.
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Old 05-05-2009, 06:42 PM   #2
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Old 05-06-2009, 01:05 AM   #3
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Weight has practically no effect on top speed, the only difference is how long it will take to reach that speed.

AS far as gearing goes, the Castle website has gearing recommendations that you should follow.
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Old 05-06-2009, 06:03 AM   #4
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i checked the castle site about gear ratios, and all they mention is to use a 16-12T pinion, and a 'large' spur gear.

i was hoping some1 could tell me exact spur gear, and pinion gear teeth.
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Old 05-06-2009, 06:58 AM   #5
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You could work out exact speeds just by using a little maths, since the castle creation motors are rated in kv (Revs per volt) then you just plug some numbers in and you get the speed out.


5700kv * 7.4v (Lipo) = 42,180 rpm (Motor Shaft RPM)

Gearing is Spur / Pinion * Internal ratio (chassis dependent)
so if you take a 84 tooth sput and 28 tooth pinion and I guess the internal ratio of the BD5 is near 2 then:

84/28 * 2 = 6

This means that when the motor turns 6 times, the axle turns once. 1 Turn of the axle is 1 turn of the wheel, which makes you travel pi * d where d is the tyre diameter.

Not sure how big a tyre is exactly but the rulebook states they cant be larger than 67mm so I will go with 65mm diameter.

Then 1 turn of the wheel is 204mm.

Then 42,180 motor rpm converts to 1,434,120mm per minute.

This is 86 kph or roughly 54mph.

You can play with the spur/pinion to see the effect on top speed but remember that the higher the top speed the longer it takes to get there, and the hotter (in general) your motor will run, so gear appropriately.


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Old 05-07-2009, 05:58 AM   #6
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thanks neal,
best response ive had from multiple forums.

i guess i'll go out and buy some different spur and pinion gears, and see what i can do.
and you calc's are pretty close to the yokomo. internal ratio is around 2(very close), and tyre diameter is about right.
obviously that speed will be less, due to drag.

if any1 does have an exact gear ratio, that would be great.
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