Gearing, motor kv, and heat question....

[Joey]

I thought I understood how voltage, current, motor kv, gearing, and heat all related but apparently I missed something.

Lemme see if Ive got this right. If I have...

2200kv motor
4S lipo
~33000 RPM


...I will get a particular current draw and efficiency and heat. But say I now keep the same exact gearing and have...

1500kv
6S lipo
~33000 RPM

...the efficiency will increase, and current draw will decrease, right?



Ok, so Ive run my savx conversion stats through an rc speed calculator. Due to a slight design error I cant use a pinion smaller than 17-18T. So assuming I leave the spur at 48T and use and 18T pinion, with the above motor/batt combo I would be geared for ~63mph. Way too fast for me. SOOOO.... say I take combo #2 and change it to this....

1300kv
6S lipo
~28800 RPM

...this would get me to about 55mph. Still a little too fast I think but would keeping the batteries and gearing the same while dropping motor to 1300kv increase efficiency or decrease it due to running at lower RPM compared to the 1500kv? I know these motors make peak power at 35-40k RPM right?

[_paralyzed_]

what do you want to run? I'd go atleast 6s in a heavy savage. You can run a larger spur and smaller pinions. Lowering speed will lessen the load and increase efficiency (not really efficiency, but it'll run longer) In general, higher voltage means less amperage.

p.s. 35-40k rpm is good for neu's and medusa's
over 35k rpm with a feigao equals burnt
castle motors seem to spin forever

[Joey]

Originally Posted by _paralyzed_

what do you want to run? I'd go atleast 6s in a heavy savage. You can run a larger spur and smaller pinions. Lowering speed will lessen the load and increase efficiency (not really efficiency, but it'll run longer) In general, higher voltage means less amperage.

p.s. 35-40k rpm is good for neu's and medusa's
over 35k rpm with a feigao equals burnt
castle motors seem to spin forever

All I want is to keep the amps/heat down and get about 50mph max. 45 max would even be great. I guess I could get a bigger spur, that does seem like the smartest thing to do!

But would keeping the batteries and gearing the same while dropping motor to 1300kv increase efficiency (or reduce heat/amperage) or decrease it due to running at lower RPM compared to the 1500kv?

[_paralyzed_]

the above rpm numbers put the motors at their greatest efficiency. I can't exactly tell you specifically what you are asking, but a good basic rule is to turn your motor at 35k rpm with whatever voltage you have.

I would run atleast 6s in a savage, so get whatever motor that spins at 35k rpm on 22.2 volts

[Nek]

You will get better efficiency with 6S and can use a 1500-1550kv motor to spin at 35k rpm.

Better change your spur to get more gearing options. Limiting your motor rpm to lower your top speed isn't a proper fix :)

[Joey]

I can only find up to a 50T sput for a 8ight/8ight-t. Are there any other manufacturers spurs that will fit it that are 52-55T??

[_paralyzed_]

not knowing your specific vehicle or tire I can't use the speed calc for you, but going up 2 teeth on the spur should mean you can go down 2 teeth on the pinion. retry the speed calc with a 16/50 ratio and see what you get. I'm not sure why your gearing is limited, but you'll need that adjustibility to dial in your setup. If you're just a basher all of this isn't critical, you've got enough knowledge to make your vehicle work.

Why are you so wrapped up in efficiency? That might help us tell you what you want to know.

my first option would be the spur change(its the cheapest). Did you try the RCM speed calculator I put in your info and got about 43 mph

[Joey]

Originally Posted by azjc

my first option would be the spur change(its the cheapest). Did you try the RCM speed calculator I put in your info and got about 43 mph

I used this speed calculator:

http://www.scriptasylum.com/rc_speed/_top_speed.html

Not sure how you got 43 mph on a CD, 48 spur, and 18 pinion at 33000 rpm!

Originally Posted by Joey

I used this speed calculator:

http://www.scriptasylum.com/rc_speed/_top_speed.html

Not sure how you got 43 mph on a CD, 48 spur, and 18 pinion at 33000 rpm!

i thought you had a tranny...sorry

I did have your setup at

1300kv motor
5s lipo
52 tooth spur
18 tooth pinion
this got your speed down to 44 mph

[lincpimp]

You definitely need a larger spur... Or more reduction in the diffs. A motor that spins around 30k rpm is fine if you are geared for 40, but geared for 50 you will have a ton of startup load, and you may have issues...

[_paralyzed_]

Originally Posted by lincpimp

You definitely need a larger spur... Or more reduction in the diffs. A motor that spins around 30k rpm is fine if you are geared for 40, but geared for 50 you will have a ton of startup load, and you may have issues...

maybe 2.2 tires

[Joey]

Originally Posted by azjc

i thought you had a tranny...sorry

I did have your setup at

1300kv motor
5s lipo
52 tooth spur
18 tooth pinion
this got your speed down to 44 mph

No prob Azjc :) Didnt mean to come off sounding rude.

The larger spur is the way to go. But again... what spurs will fit the 8ight/8ight-t CD? The ones I find that are made for it only go up to 50T. Are most 8th scale spurs a "universal" fit with each other?

[lincpimp]

Originally Posted by Joey

No prob Azjc :) Didnt mean to come off sounding rude.

The larger spur is the way to go. But again... what spurs will fit the 8ight/8ight-t CD? The ones I find that are made for it only go up to 50T. Are most 8th scale spurs a "universal" fit with each other?

The 8ight uses 8x14 bearings, and the diff cup looks to be very similar to a ofna/hb cup... I am not sure if the mount holes will line up, but a crt spur, 62t, will accept a bearing with 8mm id and should not be too thick. Do you have any pics of you mounts?

[Joey]