please help...bad cogging

[ncnguyen123]

i just got my gmaxx up and running on brushless and notice bad cogging mainly when i hit full throttle from stand still or go slowly. for some reason, it doesn't cog when i run on stand without wheels nor cog as bad when i hit full throttle in reverse. also, it doesn't cog as bad when i loosen the strobe but of course this nearly melts the pads. i even tried adjusting EPA but no luck.

here's my set-up:

BK 9920
Feigao 8xl
UBEC
14 cells, GP3300s
Strobe, 51t and 16t pinion

please advice...

[sleebus.jones]

Have you checked the driveline for drag in the forward direction? That would be my first thing to take a look at.

Your motor choice, cell count, gearing and ESC all seem reasonable. Are you running it in 2nd gear only? Have you tried 1st to see how that works?

[ncnguyen123]

yes, rolls very smoothly in forward and reverse. and yes, i'm also using the g1 single speed conversion.

[glassdoctor]

I don't know much about the 9920, but I would not be surprised if you are pulling 100+ amps in that situation, so maybe it just can't handle the load?? Or... the GP3300s voltage may be dropping so bad it's causing a problem with the BEC or ESC.

It may be worth adding a capacitor on the rx (you could wire it directly to the UBEC output leads)... and/or add caps to the 9920.

[sleebus.jones]

I agree, the amp draw is likely to be high. I've used GP3300's in a similar setup (12 cells with a 10L, and 18 cells with a 10XL) and found that they drop a LOT of voltage under load...as much (or more than) 7 volts. Yours is geared to about 40 mph, so maybe try dropping a few teeth to see what happens?

I'd also check all your connections to make sure they are clean and tight. I'm assuming you're using deans plugs or similar. Also, how old are the packs? They might not be up to snuff.

[Finnster]

The other thing I may check is to see if there are any broken connections to the capacitors, as they seem prone to breakage on the 9920. Obviously your problem is tied to heavy loads on the controller, and there only so many things that would cause that.

Edit: Had another thought... sometimes I got cogging when running a ubec even w/ an FM radio. The DSM radios are the only ones that ensure smooth operation. Have you tried using a RX pack in place just to check this out? What radio

[ncnguyen123]

thanks all! i think you are right about the batteries. I recharged the batts and ran the truck with batts hot off the press. cogging almost went away. however, things got progressively worse as batts drained. looks like i may need lipo, but a little too much $$ for me to spend now. i'll try and gear down to see if it helps.

this forum is awesome!

i'm using a futaba FM radio.

[lincpimp]

I had a very similar setup and I think that gearing down will help alot, maybe a 13 or 14t pinion would be better. Those batteries may have some issues moving the truck around properly with taller gearing.