Speedrun high RPMs

[Ericehess]

So I have a brushless upgraded infraction mega with a 3600kv motor I run on 3s, runs great hits a little over 50mph. I want to try it on 4s for speed runs to get it into the 60s or faster. ESC is 4s capable. The motor is only rated to 50k RPMs. On 4s the rpms could get higher than that by a few thousand. If I'm only getting it to WOT for a few seconds do you think this could harm the motor?

 

[bill_delong]

If I'm only getting it to WOT for a few seconds do you think this could harm the motor?

Eff around and find out how many seconds it takes to delaminate the magnets, looking forward to the follow-up post on where to buy a new motor which will likely be the end result regardless since you have the need for speed...

 

[Ericehess]

Wow, a simple yes would have been sufficient.

 

[bill_delong]

Wow, a simple yes would have been sufficient.

You answered your own question in the OP, you know what the motor is rated for yet asked how long before it will fail so I helped put things into perspective

FWIW, I once ran a similar motor that was rated for 3S on 4S and I had some amazing speed runs, I think I got maybe 4 speed runs until the motor failed, I was going to replace the motor anyway ... I just effed around and found out for myself

 

[tudordewolf]

So I have a brushless upgraded infraction mega with a 3600kv motor I run on 3s, runs great hits a little over 50mph. I want to try it on 4s for speed runs to get it into the 60s or faster. ESC is 4s capable. The motor is only rated to 50k RPMs. On 4s the rpms could get higher than that by a few thousand. If I'm only getting it to WOT for a few seconds do you think this could harm the motor?

It's unlikely (impossible, in fact) that you're hitting the theoretical RPM in your speed runs (torque output at that speed would be zero), so you probably still have some margin of overhead. Using the RC speed calculator, figure out what RPM's your motor is turning at to hit 50mph. I was hitting 53mph with a motor going 39k RPM on 18/39 gearing, 13/38 diffs, and 65mm wheels, for example. Your 3600kv motor might not even be getting past 40k RPM under load.

However, just pushing a higher voltage through the motor is the brute-force way to try and get more power to the ground. You'll accomplish a lot more if you optimize your build, which can involve different KV's, gear ratios, and voltages. Generally, you'll want a somewhat lower-kv motor spinning the largest pinion it can handle on the highest voltage your ESC can take. By lower KV, I mean a 3300kv motor on 4S may very well deliver more power than your 3600kv would on 4S by making better use of its amps, which often is the real limiting factor in speed-run cars.

 

[tntpoof]

leave it on 3 cells try going higher in teeth on pinion.The real factor in speed runs IS not amps its aerodynamics!
No to 4 cells

 

[Ericehess]

I've never tried pushing a motor close to the theoretical rpm max so just wanted to find out what the probability of damage is. I figured the rpm calculation is biased on the high side due to load effects. I think I have a 22tooth pinion in as it is, not sure how much bigger I can fit in there. Anyone know if there is a smaller spur for the infraction than the 60t I have in there?
Sorry it's a 50t spur

 

[Grizz03]

Something else many overlook is the voltage sag. The harder a motor is pushed and/or lower quality batteries the farther from the max 4.2volts per cell is ever seen.

 

[Ericehess]

That might work in my favor

 

[Ericehess]

Did speedruns for 15 mins or so, only getting to WOT for a couple seconds at a time max, my 9y/o son hit 72 in the road in front of our house, better spot could easily hit 80.

 

[Mrgoodwrench35]

Nice! That's an impressive speed for that platform.

Just a little friendly advice. Get rid of all those battery adapters. Solder is your friend. You'll likely see gains from that alone.

 

[Grizz03]

Just a little friendly advice. Get rid of all those battery adapters. Solder is your friend. You'll likely see gains from that alone.

Wow. I didn't even notice how much spaghetti there was going on there.

 

[Ericehess]

Motor was warm not hot afterwards. 4s seems ok so far.

Haha, yeah I didn't have time yet and didn't wanna void ESC warranty until I was sure it was working. The only series connector I had was a deans connector, the esc is xt60 and the batteries are ec5. Had to solder a couple quick adaptors today. Gonna get it all sorted soon.

 

[J’sCrawler]

Go straight to ec5 on the esc then.

 

[Ericehess]

Yeah I plan on it, will either need an ec5 series connector or to buy a 4s battery.

 

[Mrgoodwrench35]

The single battery is a 'better' option. I know, better is subjective but I'm fond of the KISS principle.

 

[Ericehess]

Me too, spent too much money on RC lately though, may wait til spring, won't be running it much.

 

[Grizz03]

Nothing wrong with running with what batteries you have. I run series also in some of my vehicles because that's what I have now and when they get weak they'll be repurposed to be used in my crawlers.

But going to single adapter is going to be so much better than using multiple ones for speed runs. I'd be curious to see how much improvement that alone makes like @Mrgoodwrench35 mentioned.

 

[Ericehess]

Yeah I'm sure it'll be an improvement. Thanks for feedback.