Higher kv vs. taller gearing

I killed the 7xl in my yusa buggy when I geared it up to do some speed runs, the rotor separated from the shaft. I had been wanting a lower kv motor for it anyway. I plan to buy another feigao for it (because I cant afford a neu right now), and Im wondering how much lower on the kv I should go. Im thinking I should do a 9xl geared 18/52, but I could also do a 10xl geared 20/52 and get almost the same speed. Im wondering which setup would run cooler/ be more efficient, or if there would be a difference. Any opinions would be helpful.

[lincpimp]

I would use the 9xl and your gearing, rather than the 10xl and higher gearing. Are you planning on staying with 4s? Maybe an 8xl would be better? I have 7 - 8 - 9xl motors and have not had any issues with them yet. I plan on running my 8xl on 5s lipo in a lsp truggy.

[aqwut]

I'm using 20/46 gearing with 6-8S and a 12XL, I really like this setup for my Mayhem....

[sleebus.jones]

I aim for a motor RPM of 35,000. (I think this was BrianG's suggestion) First, I decide how many cells I want to run. Divide 35,000 by volts of your pack and *pow* there's your KV value. Find an XL motor that matches and you're good to go.

I've found that the motor you get when you follow the above formula is very tolerant of gearing. I'm running 5S on my hyper7 with a 10XL, it's geared 13/46 has tons of acceleration and refuses to get more than barely warm. I think I'd have to get to 15/46 to get any real heating to start, but that'd be more speed than is usable on the track.

Your choice of the 7XL fits right in with this.

As for which motor to pick from the choices you listed, well...they're going to be pretty much the same. You see, you're dealing with a set amount of power...your battery. Without changing that, going outside of the "normal setup" really isn't going to bring you much benefit. To really make a change, you have to change the power avalible to the system, that is add a cell. Then dropping the KV will make some sense.

The bottom line is that if you drop the KV and change the gearing to bring the speed back up, the same amount of work is being done, so I wouldn't expect much change.

Either that or I'm all wet. :)

Id like to stay at 4s because im using an mm. This should probably be another thread, but whats everyones opinion on running the mm on 5s? I do have 3s and 2s 4000s that I could wire up. That would only be capable of the same watts my 4s 5000 is, so Ive stayed under what is known to be safe for the mm. Ive heard people say the older ones are good for 5s, and this one is pretty old.
Based on the opinons here, I think Ill go with an 8xl if I stay on 4s. I understand the 7xl would fit the 35k rule better, but Id like to keep the motor temps down. Geared for about 40, my 7xl was running close to 150 most of the time. Ambient was about 30, and I have 3 fans on it, but the MM never got over 80ish, so I dont think it was over geared. Im hoping dropping the motor rpm while keeping the vehicle speed the same will bring the motor temp down closer to the esc.

[sleebus.jones]

Ah ok, I see now. Well, the 8XL will help, but only if you don't try to gear it up to the same speed. I'd try gearing for 30 or 35 and see how that works. Either way you go, you have to lower the work the motor has to do, so that means gearing lower and reducing your top speed.

[DrKnow65]

Ok, can I restate the question?

If I have a 6 pound buggy, want it to go 40mph, use 4S lipo and not get hot....
Would a higher kv motor geared 16t to net 40mph on 4S be better than a lower KV motor geared 20t to still net 40mph as far as heat goes?

In pondering the question, I suppose all things constant, the motor that is closer to its most efficient RPM would heat up less rght?

^Yes, thats exactly what Im asking. So I guess my next question is, how do we determine what rpm range a motor is most efficient in?

[What's_nitro?]

I think you could go with the 9XL and keep the 4s batt. for now. I've noticed it's a fairly common setup in this forum. I'm not familiar with the Yusa buggies. Are they as heavy/heavier that a Revo? That's what I see the 9XL/4s combo in most often. You should be able to gear it for the same speed as with the 7XL and notice a large drop in motor temps. I'd advise against any lower kV to avoid having heat issues with the MM and being back to square one, unless you want to up the voltage a little...

[DrKnow65]

I think at least for feigao's, from what I can gather, 35-40K is where they perform best. So for 4S (14.8v) a 6XL @ 2501KVx18.8V=37,014RPM or a 7XL @ 2145KVx14.8=31,746RPM.

If it's right that 35-40K is optimum for a feigao than the 6XL would be the best bet (for 4S), which means a smaller pinion for the same speed than any of the lower KV feigao's. I think this would load up less. Now the question turns to how much pinion does a 6XL need to get 40mph in your RC? (if if if if I am correct in in my understanding that feigao's are most efficient between 35-40K)

I have to imagine that the amps this setup is pulling could be the root of the problem... mabey 4S just isn't enough voltage for pushing the rig to 40mph without huge amps? Have you looked into modding the MM to run on 6S? I've got mine modded heavily (heat sink on both sides of the power(fet) board, pusher/puller 25mm fans in a "wind tunnel" setup, better capacitors on the power board, a zenier diode and 4 large capacitors on the power supply to the esc) I only run 3S but according to BrianG and GriffenRU these modds will make it run 6S without issue. Or just hold out another few years (sarcastic) and wait for the Monster Mamba Max to come out (early 08') it's made to run 6S...

[sleebus.jones]

Originally Posted by DrKnow65

mabey 4S just isn't enough voltage for pushing the rig to 40mph without huge amps?

IMHO, you are correct.

[BrianG]

Personally, 35-40k rpm is on the high side for feigaos. Of course, that depends on how long you stay at top speed. I usually shoot for 35k as the top end of the rpm range, but tend to like the 30k-32k range. Any loss in speed can be regained with gearing within reason.

4s is good for 30-35mph without large currents, but that depends on vehicle size/weight, drivetrain drag, etc. Gotta remember it takes power to move your truck, and power is Volts x Amps. If it takes, say, 1500w to achieve the top speed you want with the acceleration you want, it will take less current to get that power at higher voltage.

Originally Posted by B15

^Yes, thats exactly what Im asking. So I guess my next question is, how do we determine what rpm range a motor is most efficient in?

It depends on the quality of the motor (materials used, construction technique) and how many poles it has. The 35k rule that people refer to should be the 230/poles rule IMHO. If I were to (attempt to) run my 8 pole motor over 30k, it'd overheat very quickly (if my ESC can even drive it beyond 30k...)

I don't know how many poles the Feigao motors have. Neu 1500 has 4 poles, Neu 1900 has 8. Ideal motor for our cars would have 6 or 8 poles due to gearing limitations. (difficult to gear for motor RPMs over 40k)

Don't forget ESCs have RPM limits too. Most will handle 250-300k/pole, but some much less. (BK controllers?)

[BrianG]

Feigaos are 2 pole, as are MM motors and LMT motors.

The "35k rule" is basically for the Feigaos as they tend to lose efficiency rapidly over that and simply heat up. Motors like the Neu and LMT are higher quality and seem to run just fine at higher rpms. Usually, I like to stick to max rpms that are around 75% or less than the max rpm rating.

My mamba has holes through the case with 25mm pusher/ puller fans and a 30mm fan on the heatsink. When I killed my 7xl, it was geared for about 48mph, and the mamba still didnt get over 90ish after 7-8 minutes of running. Thats what made me think this car has more speed in it. But maybe it doesnt without a higher efficiency motor or higher voltage.