Bw: Tmd.

[Mega Masher]

Hello there,
This is a the build diary of my new bw melty bot, TMD.
So, it's going to have a welded 2mm titanium chassis, 3mm polycarb lid, 2 250w brushless motors, 4s 1300mAh LiPo, and a huge weapon bar:

Motors:
http://www.hobbyking.com/hobbyking/s..._version_.html

Lipo:
http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html

These wheels (in 2", shore A45 and black):
http://www.fingertechrobotics.com/pr...o-wheel&cat=11

And this weapon:
http://i1280.photobucket.com/albums/...ps2892f122.jpg
http://i1280.photobucket.com/albums/...ps79bc9424.jpg
Yep, its a tree splitter thing
Please do let me know if I've made a huge mistake in the parrs I've chosen (not bought any yet)
John

[mrsam]

How heavy is that bar? It looks hefty

[Gaseraki]

1.2 kg according to the picture.

[Andy]

Looks good, and that bar should be deadly, but how are you going to get it in the weight limit?

[Flag Captured]

If that bar really is 1.2kg, I'm sorry to say that you'll have no chance getting that in the weight limit. If you're modifying the bar, however, that might prove viable...

And the two motors you've chosen have an amp draw of 20A-ish each, which means 40A. Your battery will only supply 32.5 if my calculations are correct, so you might want to get a higher discharge rate battery if you're going this route. Probably a higher capacity too, I'd say - maybe 1500-2000mAh would be better.

[Mega Masher]

I'm only going to use the front third of the bar

[harry hills]

Got a CAD?

[mrsam]

Motors seem usable although dual brushless setups can be a pain, might be worth just going for one more powerful motor and to echo what Matt said get a higher capacity battery and one that can supply more current than you need.

[Flag Captured]

If you only use the front 1/3 of the bar, that's a lot more reasonable, just make sure you mount it strong because that's still a good what, 400g? Sure you already know that though.

Definitely use a bigger battery though, I feel that the one you've suggested won't cut it, both capacity and current-wise. You'll want it to run for a good 4 minutes at the most, I'd say - just to leave yourself some lenience so you'll never run out of battery in a fight. I'd assume you'd be running those motors at full pelt for the full 3 minutes of the fight so that'll require quite a beefy battery I'd guesstimate.

[daveimi]

You're battery will be fine I ran a feather on a 1600mah pack n used to get 2 or 3 fights out of that

[mrsam]

I would suggest a bigger battery capacity definitely, i imagine David that you ran a drill powered feather off your battery? very little current draw. If John is using a dual brushless set up then he will be drawing a fair bit, especially at start up to get that blade up to speed, not to mention that with lipo's it's better to be safe as you don't want to over discharge them.

[daveimi]

Yeah, to be honest I know zilch about lipo's, it's why I stay away from them. Bit new fangled for me lol.

[Flag Captured]

There's a lot of numbers to them, but once you learn what they all mean, it becomes relatively second nature - you just need to do the calculations. Or use a 5000mAh battery for two drive motors which is a lot more than you need haha

the C rating is your discharge, times that by your capacity in amps to figure out total discharge available, 1.3A x 25 works out at about 32.5A discharge - since each motor's draw is about 20A on 4s, you'll just overdischarge and blow your mandatory fuse. Really, you'd be looking at maybe a 2.6mAh battery which would give you a 65A discharge there abouts and means you can run both comfortably without really taxing the battery. To be honest, I'd ignore the burst ratings and plan for the standard discharge figure since you don't want to be constantly running it in burst, you'll wreck it easily.

Though people with more experience can probably fill you in more, I'm a newbie to it all still too!

[mrsam]

What Matt said is basically all you need to know to get started though i'm by no means an expert i've just done my research. Lipos are fine and safe to use as long as you're smart with them, over discharging damages them and will most likely end in a fire which is why it's better to go for a bigger pack than you need just to be on the safe side. I shouldn't think you need a capacity more than 2000mah, for a single fight it should be plenty but try and find something with say a 35 or higher c rating, that means you'll be able to pull 70amps continuously which should be fine.

[daveimi]

I still race my Tamiya Hornet on nicads haha! but yeah see what you mean about them needing to be right for the application etc.