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Choosing the right brushless motor

Old 06-09-2016, 02:57 PM
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Default Choosing the right brushless motor

My first post, and somewhat longer than intended.

I'm looking for some advice on what recommendations / expectations I should have for a brushless system in a Kyosho Mini Inferno buggy. I'd like to increase real use top speed, but not make it insane or risk damaging the stock transmission or spend more than its worth (it's for my kids in the back garden - and me!) . I'd prefer to avoid Lipo for now.

Current setup is a hobbywing ESC, stock motor, and a modified battery layout to take 8x full size AA batteries with the space gained from smaller electronics☺

I measured the current car /motor speeds at
No Load (suspended) = 47km/h (29mph) = 41,000rpm
Carpet = 21km/h (13mph) = 18,000rpm
Paving = 20km/h (12mph) = 17,000rpm
Cut Grass = 13km/h (8mph) = 12,000rpm
(gearing is 10.8:1, speed measured using a 50fps camera at the end of an 8 metre run-up).

My question is - what spec of brushless motor would give a reasonable speed increase without needing other upgrades beyond an ESC? Is the limiting factor actually acceleration / torque in this case rather than raw rpm? Would something fairly low like a 3000kv motor be better than a higher kv rating? (reducing theoretical top speed, but increasing actual speed due to the torque). Would I get longer run times? Would the NiMH batteries quickly limit any performance gain?

Or am I not going to see a worthwhile improvement and so shouldn't bother? Should I just get a better bruhed motor instead?

Many thanks
Old 06-10-2016, 03:27 PM
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In my opinion the AA batteries are a limiting factor even with the stock motor. AA's are usually better in lower amp draw setups and a rc vehicle is a comparatively high amp draw setup. I'd try making the switch to something like the link below first. I believe they're called sub A cells, either way they should be able to take higher amp draws while maintaining a higher voltage under load. This means more torque and rpm potential.

Choosing a brushless motor isn't always easy but you can usually get pretty close to ideal. Then you fine tune the gearing to make everything perfectly happy. If you pick a 5000kv motor you'll have to run a smaller pinion than say a 4000kv motor. Obviously the 5000kv is going to be spinning faster but the smaller pinion will keep top speed the same as a 4000kv with a larger pinion. If you're going to be running 9.6v you'll want to stay on the lower side with kv. Not really that familiar with the mini inferno so can't really make any kv recommendations. I wouldn't run brushless on AA's though as amp draw is higher from a stop than it is with brushed.
Old 06-11-2016, 06:15 AM
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Thanks Maj_Overdrive. The post you made doesn't have a link, could you send it?

Did you mean Sub-C cells rather than Sub-A? (apparently there's also 4/5 Sub-C which is presumably 20% lighter). I think you are probably right that the batteries are limiting current, I've not measured it but it was in the back of my mind - I'll have a look with a DVM.

I checked battery weights, I think AA are about 25g ea, so 200g total for the car whereas Sub-C are about 55-60g ea, which at 350g for 6x may negate the gains (it's only a 1/16) - I'd already converted it to a mini Tamiya plug so I'll try and check that option with a 1/10 pack I have.

I guess my main concern was I was surprised that the top speed was so far below the Kyosho stated speed (21 vs 28km/h). To be honest it it could hit 30-40km/h I think the kids would be delighted with it, especially if it got to 20ish in the lawn. I wondered if that Kyosho speed claim was a bit like VW's emissions stats and TV companies energy efficiency claims (i.e. either plain rigged, or set using a set of conditions the user never obtains). I'd expect to be a bit slower as my 8x full size AAs add 50g on top of the rather pricey Kyosho 8x 2/3 AA battery packs, but it's not enough to slow it down by 25% much.

Would there be a high power brushed alternative? It'd save a new ESC, but there seems to be little info around on these options, most inferno info seems centred on turning the car into a mini-rocket with brushless + lipo + repeated rebuilds for shredded transmission - which seems to cost a fortune!

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