What could be wrong with my setup?
 

Trying to figure out why my Jammin CRT .5 is not hitting the top speed it should be according to the online calculator.

Stock 46t spur and stock diffs. 25T pinion. Castle 2650kv motor on 2s and 3s. 4.5" buggy tires.

Calculator says it should go ~44mph on 2s and ~66mph on 3s.

My actual speeds are 28mph and 43mph.

The motors is less than 120F on both packs.

What am I doing wrong?

Tire diam?

Yeah, I got that correct too. Whats really confusing is the speed are off at the same degree.

The calculator says I should be getting 22mph per 3.7 volts of lipo power.

GPS says I'm getting ~14mph per 3.7 volts.

Also, on 3s it can pop wheelies from a stand still easily. I don't think its over geared given this and the low temps. Lastly, the batteries should be good for 125amps.

the motor isn't spinning 2650rpm/v then... do a kv check

How do I do that?

Would this work?

Method 3 (Colin Dedman's formula)

1. Drive the motor with a drill press.

2. Measure RPM and voltage generated (by one phase).

3. Kv = RPM / (volts x 1.41 x 0.95)

your speeds match up to 1650kv. weird.

So how do I get the kv of a brushless motor?


Update.

Just ordered a RPM-KV meter. Looks like a cool little device.

If you have a castle esc and castle link for pc you can do the kv check, it's part of the procedure for setting up Torque Control.

Really? Cool. I'll look into that tomorrow. Thanks:yipi:

What diff ratio does it have?

3.25 according to the calculator and google.

What are you using for battery voltage on the calc? 3.7v per cell should be the one to use.

3.7v

Also, its not my first time using the calculator. I've been able to match up my Savage Flux pretty good, its within 3mph.

Try exporting everything and pasting it here. Maybe there's just something being overlooked...

Differential Ratio: 3.25
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 25
Total Voltage: 11.10
Motor KV: 2650
Tire Diameter (inches): 4.5
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor Coil Resistance: 0.0035
Spur/Pinion Ratio: 1.84 : 1
Total Ratio: 5.98 : 1
Tire Circumference (inches): 14.14 inches (359.08 mm)
Rollout: 2.36:1
Total Motor Speed: 29415 RPM
Vehicle Speed: 65.85 mph (105.78 km/h)
Effective KV Value: 2650
KT constant: 0.51 oz-in/A
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...
Final Power: Amperage not specified...

The only thing that looks a little odd is the tire size. Looks like buggy tires size-wise. And the pinion tooth count is rather high. Other than that, everything looks ok. Since you other vehicle is pretty close speed-wise, it has to be something to do with this vehicle somewhere...

Yeah, its running buggy wheels and the pinion is high in hopes of 40mph on 2s.

Here's a pic with the 2s battery mounted. Its an older max amps 4200mah pack good for 84 amps. The 3s pack I used was a lot larger, 5000mah 25c turnigy pack.

http://www.rctech.net/forum/attachme...s-img_7436.jpg

Have you tried another 2s battery? What are the speeds on 3s?

The performance of the batteries are consistent
.

are you unloading on any of your diffs? I'm thinking maybe the center diff is a little loose.

I had the buggy apart last night and the diff feels tight. I don't know what weight fluid is in it but its definitely not thin. Also, it pops strong wing slapping wheelies.

I'm an idiot. I didn't have the esc retrained to my dx3. I used the kvm meter and it showed two motors not producing their defined kv. I then tried another radio and rx (a traxxas at that) and they both were just over their stated kv. Retrained the esc to the radio and with the current setup the calculator says I should be doing 56mph and I'm getting 52mph. I'll take that.


But yeah, I'm a bonehead for nothing thinking of this in the first place. I do like the kvm meter though.

simple...you have to deduct 10% loss in the efficency...
it says you will be doing 56...but its actually 52.
10% of 50mph = 5mph, there is your missing number...:yes:
well your loss is only 4 mph...
chris

That wasn't the problem chris. :smile: