Rango - Build diary

**Max** · 5th April 2014, 22:26

I make that 26 miles per hour assuming 15krpm, 5:1 reduction and 75mm wheels.
Sounds too fast to me especially for a machine with so much grip.
But my calculations could be wrong!

**overkill** · 5th April 2014, 22:31

OK, so the KV is about 883 giving about 18,500 rpm on a 6S pack. divide that by 5.75 and the axle speed is a whopping 3,217 rpm - there's the problem!

**Ellis** · 5th April 2014, 23:08

Hi guys. The motors aren't that fast. They are no load 10,700rpm @ 18v, commercially rated as 600w, idle draw under 3a and stall amp draw of 65a, max torque of 541 mNm - according to the site from which I bought them. Through the 5.75:1, at 5s, with 75mm wheels, that's give or take a maximum theoretical of 16.7mph. The rev per volt is only a little higher than the graupner BB speed 900s.

I based my reduction on robots such as Satanix, which, being geared for a theoretical over 25mph, and others being geared for even more, seem fine. I'm not directly comparing to those setups - different motors etc, realise that. There is no longer any resistance to speak of in the reduction, it's smooth and rolls to a stop. It might well be a combination of things, the grippy tyres and the low reduction, plus the motors having to run at near-stall much of the time. But the reduction isn't that nuts. If the motors were straining so hard I'd expect them to get hot and they never even got warm, whilst the speedos did.

The problem has changed. Amp draw before was high because of the belts being too tight for the unsuitable wheel bushes, but now that is out of the equation. I do expect amp draws to be high considering it's a hungry drive setup, and it might well simply be too steep, but not in the amount of time I was poking it to like 1/4 throttle for about 1 minute before stopping, to feel the speedos. I didn't even turn much lol. Unless the amp draws are truly insane then surely the 85s would take it in their stride whilst the motors ate themselves?

^Which is why I suspected the braking function might possibly be related. It definitely rolls when it should brake, and it brakes when off the floor, so related to this or not the speedos are probably not very happy. I can see if this is relevant when I get the V2 tz85's and test them come Wednesday/Thursday.

**Revsmin** · 6th April 2014, 01:41

Speed 900s have a no load speed of 6500rpm, so the difference is a bit more than you might think!
Edit: derp, that's at 12v :P

Anyway, at 10,700rpm through a 5.75:1 gearbox and 75mm (2.95") wheels, I make the max speed 32.7mph... there's your problem.

Edit 2: if you double the rpm of a S900 you get 13000rpm; as I was running them at 6S on 7:1 through 120mm wheels, shouldn't that give a top speed of about 50mph!? It can't be right, the setup goes about 25mph as the rpm quoted on the S900 data shows... I'm confuse.

**Ellis** · 6th April 2014, 09:30

I think your maths must be a bit weird... you're getting roughly double the theoretical max speed for both setups. :P

**shakesc** · 6th April 2014, 09:46

Ellis, according to my calcs your 10700rpm with 75mm wheels with 5.75 reduction does give 16 mph
If you put some loses into that then the top speed should come down a little

Do you have a torque graph for the motors to see where they are operating in their performance curve?

**Revsmin** · 6th April 2014, 10:09

http://wahiduddin.net/calc/calc_speed_rpm.htm
:P

**Max** · 6th April 2014, 10:36

I think you were putting in 2.95" in the tyre radius box- 2.95" is the diameter so the radius is 1.475". This then gives an answer of 16.3mph.
It sounds like it must be the motors struggling if it works fine on the bench then gets hot on the ground. Maybe the specs you were given were wrong, like some of the fake s900 had unrealistic stats?

**Eventorizon** · 6th April 2014, 10:38

A Speed 900 can not do 10'700 RPM. When I ran them in Conker 1 and 2 as weapon motors the disc basically did the same speed at 6S ans 10S within I think it was 300rpm of eachother at around the 8000rpm mark. But when on 10S they had far more torque than 6S.

Going back, I agree with what Alan suggested abut removing the rear tyres and trying skid steering. Its far from perfect but may solve your drive issues. And it won't leave so many tyre marks in your swimming pool.