Why at least 4S lipos in 1/8th

[charles08]

Hey,
Just got Ft rc8 roller and which to convert it to electric, i was wondering why can't we use 2S lipo and a 1/10th esc with a high Kv 1/8 motor ?
Btw what are the size of them, 1/10 are 540 what are 1/8th .

Thanks .

[Crazy L]

Originally Posted by charles08

what are the size of 1/8th .

Thanks .

Frackin' big son.

and because 1/8th vehicles are bigger, they need a bigger motor to haul the thing, thus more oomph to juice the motor, otherwise it's like popping a 125 engine in a Goldwing.

[Marvin]

The reason why most people use the higher voltage batteries is twofold:

1.To get a decent run time you need a whopping capacity battery on 2S, which aren't that easy to find. This is due to the equation Power=Current * Voltage.

• E.G: 7.4V 5000mAh battery. Using the above formula, 5Ah * 7.4V = 37Wh.
• E.G 2: 18.5V 5000mAh battery. 18.5V * 5Ah = 92.5Wh

There is a lot more potential energy stored in the higher voltage pack of the same capacity. I use a 18.5V 6600mAh battery to get just over 20 minutes race time. This pack has 122.1Wh. So, as you can see, even the largest 1/10 battery has nowhere near enough power for long run times.

2. High current causes high resistance. Using the power equation again, say your set up has a maximum power output of 2000W (it is possible in 1/8!). At 7.4V that's 270A drawn, whilst at 14.8V that's 135A drawn. The higher current causes heat build up and higher resistance. Therefore more energy is wasted.

Your best bet would be to run two 2S packs in series for 14.8V. This keeps temperatures lower, extending run time, and ensures you don't need a high kV motor, where your choice is limited in larger sizes.

The reason why people wouldn't use 1/10 ESCs stems from above. If you are drawing 270A, no 1/10 ESC is capable of providing that. Some ESC manufacturers give ratings of like 1080A or something but that is absolute rubbish. The largest plane/heli ESCs peak at about 150A - and they've been doing brushless far longer than the car manufacturers.

Most 1/10 ESCs do, I would guess 60A continuous, and 100A burst, though this is quite generous. Try running that in 1/8 and you have a melted ESC.

I know at the Neo Adam Skelding was running an 1/10 LRP ESC, but this had many extra capacitors on it to take 14.8V, and therefore the current would be less.


As for motors, 1/10 motors are, as you say, 540 size. 1/8 motors aren't so clean cut. It all depends on the application, they are normally picked by the length, and the kV. Also, they can vary in diameter too. For a 36mm can, most people run 60/70mm in a Buggy, 70 or 80mm in a Truggy, and 80mm+ in a Monster Truck. Greater diameter means you don't need so much length.

A useful website for figuring stuff out is http://www.scriptasylum.com/rc_speed/_top_speed.html

[JJ The Boat Wizzard]

Originally Posted by charles08

Hey,
Just got Ft rc8 roller and which to convert it to electric, i was wondering why can't we use 2S lipo and a 1/10th esc with a high Kv 1/8 motor ?
Btw what are the size of them, 1/10 are 540 what are 1/8th .

Thanks .

Hi m8

We use atleast 4cell as a 2cell 5000 would last aslong as a 4cell 2900 witch isnt very long , the more Volts the Less Amp's are needed i think thats what i have been told some 1/10th spedos can run 1/8th motors just make sure there spec'd up to 20V at atleast 150-200Amps

I also suggest you run 4.00mm corallys for the motor and battery conections as you will cook anything else and that can be expensive
if i were to buy some new batts i would go for a 5or 6 cell 4000 and they will last absolutely adges mabie even enough for a 20min final

Flightpower are good cells
Hyperon are good as you can charge them at 5c

(You wouldnt be able to tell that ive only been doing 8th electric for just over a week would u )

John

[JJ The Boat Wizzard]

OH NO I TOOK 2 LONG TYPING MARVIN BEAT ME TO IT

[Marvin]

And a much longer, more technical post too

[charles08]

Well guys thanks a lot for all the infos, it all makes sense .
I'll the go for 2s packs in series since i'm already in 1/10 .

Good to know that most of the esc manufacturers lie about their specs.
I was going to cook mine if i tried ...
Well as long as it works for 1/10th that's fine, i'll have to get a new one for 1/8th .

Thanks again .

[JJ The Boat Wizzard]

Originally Posted by Marvin

And a much longer, more technical post too

But we said the same thing and
ive been doing this a week

[niggs98]

[QUOTE=charles08;254951I'll the go for 2s packs in series since i'm already in 1/10 .

.[/QUOTE]

thata what i did for neo, ran 2 track power 4900 25c to get 14.8v. still lasted the 15 min final with batery to spare. do advise you get a 1/8th speedo tho as they are just soooo much more beefed up to cope with the abuse they get

[charles08]

yep, will certainly do that.
15 minutes, wow that's a lot i guess the fight is against heat !
Any idea if 2 saddle packs would fit in the tray of a conversion kit ?
Thanks .

[Marvin]

They probably won't be a straight fit, but you can get custom made battery trays from RC-Monster.

[greggy]

1 quick question, im converting my 8th

can i just wire a "Y" lead (so to speak) and plug 2 5000maH trakpowers into it , or would that blow speedo's up ?

[Marvin]

It depends what sort of ESC you have, and what kind of y-lead you're going for. See my D8 buid - I'm using a parallel lead there, which gives twice the capacity at the same V, or a series lead, which gives twice the V for the same capacity.

[HandyRacing]

Just and FYI - The Castle Mamba Max (1/10 speedo) holds up quite nice under 4-cell powered balls to the wall racing in 1/8 buggies.