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Speed/Motor/Esc Question.

Speed/Motor/Esc Question.

Old 09-08-2010, 10:23 PM
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Default Speed/Motor/Esc Question.

I feel like I already know the answer but I'm going to ask anyways...

I just bought a B4.1 RTR and my friend bought a Bandit VXL. We used the same battery (SMC 5200 50C+) but his FLEW while the B4 kinda went eh...

Now I know the VXL motor is 3500kV, and the Reedy is 3300kV. I have a feeling 200kV difference isn't really doing it... We both are on stock gearing as well, and I don't feel like that is doing it either... I guess my point is asking: am I right in thinking the ESC plays a big role?

The VXL system ESC is rated at 200A continuous and 320A Peak.
The Reedy system is rated at 45A max continuous...

So I'm guessing the VXL ESC is gaining more of an advantage from the 5200 50C battery than my XP is?

If I were to upgrade my motor to the 3900kV one and left the ESC the same, I'm guessing that wouldn't have a HUGE impact on my speed either? I'd need to upgrade my ESC then correct? Or am I waaaay off base.

Thanks!
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Old 09-08-2010, 10:55 PM
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Stock gearing difference between the Bandit and the B4 could be night and day. Look at Traxxas' site. All they promote is how fast each model will go. They probably have them all geared to the moon for top speed runs, while your B4 is setup more for racing which could be more conservative for reliability. I think my B4 originally came with an 82 tooth spur, the Bandit has a 76 tooth. I ended up gearing mine at 72/30 with a 13.5 brushless and LiPo and that thing would haul balls.

As far as current is concerned, amperage is pulled not pushed. If your motor is not demanding X number of amps, the battery or ESC is not going to provide it. Just because your ESC is rated 1/4 of what his is doesn't mean either of you is pulling that number. This all goes back to the first paragraph. If you are geared really high, you are stressing the system more and will need the current carrying capacity of the ESC and battery.
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Old 09-09-2010, 12:07 AM
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Well what I do know is that the B4 is 21/81, and the bandit is 26/76. Sooo the ratio for the B4 is .689 and the Bandit is .869. I went ahead and replaced the 21 with a 23 and made it .755. Will that make a noticeable difference or will it not? I've always heard you want it as close to 1 as possible. Is that true? I've never really messed with gearing before... (obviously)

Sorry I feel stupid now haha .
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Old 09-09-2010, 01:18 AM
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The Bandit has a transmission ratio of 2.72:1, along with 26/76 pinion/spur this gets you 7.9:1 overall. With 7.4V, 3.5" tires, and 3500Kv the buggy could run about 33-35 mph.

The B4.1 has a transmission ratio of 2.6:1, along with 21/81 pinion/spur this gets you 10:1 overall. With 7.4V, 3.2" tires, and 3300Kv this buggy could run about 23-25 mph.

The 1:1 setup you hear about is rollout not gear ratio. It is the amount of motor rotations needed to move the tires 1 inch. This is typically a good starting point for racers, but not someone wanting more speed.

Like Klaymon said, your buggy is set up for racing, while the Traxxas is set up for pure speed. The 23T pinion will improve your speed a bit, but if you want to run with a Bandit VXL you will need to make a major change like 30/72. This will put you on a much more level playing field.
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Old 09-09-2010, 08:19 AM
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Originally Posted by Mach Nine
Well what I do know is that the B4 is 21/81, and the bandit is 26/76. Sooo the ratio for the B4 is .689 and the Bandit is .869. I went ahead and replaced the 21 with a 23 and made it .755. Will that make a noticeable difference or will it not? I've always heard you want it as close to 1 as possible. Is that true? I've never really messed with gearing before... (obviously)

Sorry I feel stupid now haha .
To calculate your gear ratio, you divide spur by pinion, not the other way around, so 81/21 = 3.857
To calculate final drive ratio, you multiply gear ratio by drive train ratio( also called internal ratio) so 3.857 X 2.6 = 10.03 This means your motor is turning a little over 10 times for every 1 revolution of your tires.

Your buddy's motor is turning just over 7 times for every 1 revolution of the tires.

burnineyes is correct in the numbers, but didn't really explain the calculations, so I thought you might like to know for future reference.

oh and rollout is pi times tire diameter divided by FDR
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