a trick i thought up for voltage with feigao XL motors.
 

ok.
what you do to find the optimal voltage for a decent speed setup is.... you look at the rating...? of the motor like the rating of a 7xl would be 7. for a 14xl it would be 14. ok? now divide that voltage by 2 and you get the number of cells you should use. ill make a table for ya. for all the odd-rated feigao's, 7xl, 9xl, 11xl, and 13xl just divide by 2 and, since you cant have 1/2 of a lipo cell, round down. heres the table. all setups are based on en emaxx in 2nd gear with 20/72 gearing.

rating voltage rpm speed
6xl: 3s 30846.9 32mph
7xl: 3s 26440.2 28mph
8xl: 4s 30843.2 32mph
9xl: 4s 27424.4 29mph
10xl: 5s 30858.0 32mph
11xl: 5s 20846.0 30mph
12xl: 6s 30858.0 32mph

its a bit slow, but its a cool, good setup.

Most of your motor speeds are between 5k and 9k rpm, too slow. Optimum motor speed for a feigao xl is around 35k rpm.

Best thing to do is to take the kv of the motor and divide it into 35000, that will give you the total voltage. Take that number and divide it by 3.7 (1 cell lipo nominal voltage) and that will give you the number of cells.

So:

6xl :2779kv = 3s (4s is too high, over 40k rpm)

7xl :2382kv = 4s = 35253 rpm

8xl :2084kv = 4s = 30843 rpm or 5s = 38554 rpm(5s is over 35k rpm, but it works fine)

9xl :1853kv = 5s = 34280 rpm

10xl :1668kv = 5s = 31228 rpm or 6s = 37473 rpm (depends if you round up or down)

11xl :1516kv = 6s = 33655 rpm

12xl :1390kv = 7s = 36001 rpm

So we see that some motors are good for a certain cell count, while others (8xl, 10xl) are good for two different cell counts. Spinning the motors between 35k and 40k rpm is acceptable, as long as you are gearing for around 40mph, or have a very light vehicle. If you try to gear up you may have heat issue with these motors, as they are less effecient. My specs are for something like a revo or a truggy that is not super heavy.

Now, I will await the comments that a 7xl is too hot to run 4s lipo. I have a feeling this is due to the fact that it can pull some serious juice on 4s and really good batteries are required to keep up, or else everything heats up. Don't get me wrong, it will get warm, and be gearing sensitive, but what feigao is not like that? Maxamps are not going to keep up, trust me I know. Trakpower/flightpower, neuenergy or kokam are going to be required to run this.

You can spin the higher quality motors (neu, leher, hacker c50) faster and gear down lower, which produces more torque via gearing. This also amplifies the benifit of switching to a better built, more effecient motor. Lower gearing and higher motor speeds are much more responsive than the opposite.

I have recently tested a hyper buggy on 4s lipo with a hacker c50 8l motor (3300kv). Motor speed is 48840rpm :gasp:, but the motor stay cool, as gearing is 12/46. Top speed is near 45mph, I have an 11t pinion on order to drop the speed and pick up some runtime. That said I still got 25mins or runtime from a 4900 mah 25c battery. This buggy is not very light, but has tons of power!

Very good info guys! please keep it coming.

So linc would it be a bad idea to run the 7XL on 4s or do I just need the appropriate C rating (im guessing 25C)

I run a 7xl in a revo with a slipperential on 4s lipo. I use 2 trakpower 2s 4900 gold packs and they handle the load fine. I also have a tower terror conversion with a 7xl and it works fine too. They are both geared for low 40mph and I get at least 25mins runtime.

I have a 7xl hacker b50 motor for the revo to try. I have a feeling the better quality motor will work better than the 7xl.

I second that. I run a 3S A123 on my 5700kv T4. It's geared 13/87 and goes like heck, plenty of runtime even with 2300MAh cells. Everything runs nice and cool too. I think many folks overgear their systems.

I am right there with you on the overgearing. Use higher voltage, spin the motor faster, gear back down and it will run better. Plus the various pieces of equipment will run cooler. I see people trying to get a cool system by running a 10xl on 4s, gearing for 40mph and then wondering why it runs hot. Too much load!

good advice guys ill keep it in mind.