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-   -   Brushless kv choice for Bomber (http://www.rccrawler.com/forum/axial-rr10-bomber/592580-brushless-kv-choice-bomber.html)

jdcooper 12-06-2017 10:30 AM

Brushless kv choice for Bomber
 
As a theoretical matter how does one select the appropriate kv brushless motor?

I have all the parts for a Bomber on the way and picked the Castle 1406 2850kv combo after reading some build threads. However after reading the "What's Powering Your Bomber" thread, and others, it seems like many people are running 3300+kv motors. Anecdotally seems like folks are running low 2000s kv up through 4000+kv.

Is there a theoretical approach to choosing a motor kv? To put it another way, can anyone explain clearly the tradeoffs associated with varying the kv, assuming they're willing to change gearing or battery voltage?

Thanks!

Alexander_0_1 12-07-2017 08:03 AM

Re: Brushless kv choice for Bomber
 
The use the lower kv are the crawler guys, often enough I’ve read that they use 4s, they tend to gear lower, on the count of the extra volts. I think I’ve read that using a 4s lipo is more power efficient than 3s...

I myself am not one of the crawler guys, I use 3s lipos (that all I have to work with), so I decided on using a higher kv motor, but I couldn’t decide when I was assembling my bomber so I order the 3500 standard, the 3500 xl, and the 4100kv R from HolmesHobbies (No, I don’t have a whole lot of money, I just sold off some 1/8, and 1/10 brushless tekin motors so I could try HolmesHobbies).

svt923 12-07-2017 08:25 AM

Re: Brushless kv choice for Bomber
 
Assuming you are selecting from various KVs of the same motor, you choose the one that gives you the desired wheelspeed using the desired voltage at the lowest practical gearing.

Decide what voltage you are going to run and how fast you want your truck to be then plug the numbers into a gearing calculator and see what KV gets you to where you want to be.

R/C Calculations

JatoTheRipper 12-07-2017 08:52 AM

Re: Brushless kv choice for Bomber
 
I prefer my Tekin ROC412 3100kv for trailing. The Holmes Puller Pro 3500kv is faster and a little too fast for my liking on the trails with the same gearing.

jdcooper 12-07-2017 08:52 AM

Re: Brushless kv choice for Bomber
 
Quote:

Originally Posted by Alexander_0_1 (Post 5773642)
I couldn’t decide when I was assembling my bomber so I order the 3500 standard, the 3500 xl, and the 4100kv R from HolmesHobbies (No, I don’t have a whole lot of money, I just sold off some 1/8, and 1/10 brushless tekin motors so I could try HolmesHobbies).

Which one did you end up liking best in the truck?

jdcooper 12-07-2017 08:56 AM

Re: Brushless kv choice for Bomber
 
Quote:

Originally Posted by svt923 (Post 5773643)
Assuming you are selecting from various KVs of the same motor, you choose the one that gives you the desired wheelspeed using the desired voltage at the lowest practical gearing.

Decide what voltage you are going to run and how fast you want your truck to be then plug the numbers into a gearing calculator and see what KV gets you to where you want to be.

R/C Calculations

Hm, so this opens up 2 questions -

1. Why would I not run 4s instead of 3s? I already have a pile of 4s batteries from quadcopters so it would be convenient.
2. Why is the lowest practical gearing for a given target wheelspeed the thing you want to optimize for? (To put it another way, what are the tradeoffs between lower/higher gearing assuming each is set up for the same target wheelspeed?)

Cheers!

jdcooper 12-07-2017 08:58 AM

Re: Brushless kv choice for Bomber
 
Quote:

Originally Posted by JatoTheRipper (Post 5773653)
I prefer my Tekin ROC412 3100kv for trailing. The Holmes Puller Pro 3500kv is faster and a little too fast for my liking on the trails with the same gearing.

Thanks. I will mostly be using this as a trail truck / crawler to hike along with my kid and missus who has expressed interest in driving the SCX10.2, so slower is probably good. But will also appreciate the ability to bash a bit and jump, etc. The SCX10.2 is ... not good at jumping :)

Jim85IROC 12-07-2017 09:13 AM

Re: Brushless kv choice for Bomber
 
Quote:

Originally Posted by jdcooper (Post 5773355)
As a theoretical matter how does one select the appropriate kv brushless motor?

I have all the parts for a Bomber on the way and picked the Castle 1406 2850kv combo after reading some build threads. However after reading the "What's Powering Your Bomber" thread, and others, it seems like many people are running 3300+kv motors. Anecdotally seems like folks are running low 2000s kv up through 4000+kv.

Is there a theoretical approach to choosing a motor kv? To put it another way, can anyone explain clearly the tradeoffs associated with varying the kv, assuming they're willing to change gearing or battery voltage?

Thanks!

It really comes down to personal preference, intended usage, and also, what supporting hardware you're using. I've got the 2850kv motor in my Bomber and I find it to be just where I want it... any faster and it's not going to be controllable with locked axles, and too much slower might make those long straight sections of trail a little boring. Other guys that run in bigger, more open areas may want more top speed. Guys who spend their time in tight trails may not need as much top speed.

But... that's all assuming that we all use the same gearing. One benefit to a higher kv motor is that if you gear down at the axles and/or gearbox and/or pinion & spur, is that you can get more low speed resolution by gearing down a faster motor.

To try and be more clear, based on what the Castle guys have said, their whole line of sensored motors, regardless of kv rating, all have the same startup speed. There's a minimum speed that those motors can start up at. Based on that minimum motor speed, that's going to translate into a minimum wheel speed based on your vehicle's overall gear ratio. If you can raise that numeric ratio, that will lower the minimum wheel speed of the vehicle.

So, for example, let's say my Bomber has a total gear reduction of 8:1 (I'm not sure what it really is, I'm just throwing this number out there as an example) and uses a 2850kv motor. If I used a 3800kv motor, I could get the same top speed with a 10.8:1 total gear reduction. That's a 35% gear reduction, which means that my minimum startup speed would also drop by 35%.

Of course the drawback to all of this is that you have to sink a ton of money into new gears, which is why I just bought the 2850.;-)

svt923 12-07-2017 09:14 AM

Re: Brushless kv choice for Bomber
 
Answers below

Quote:

Originally Posted by jdcooper (Post 5773655)
Hm, so this opens up 2 questions -

1. Why would I not run 4s instead of 3s? I already have a pile of 4s batteries from quadcopters so it would be convenient. Nothing is stopping you from running 4S, if you have them and they meet your needs then run them.
2. Why is the lowest practical gearing for a given target wheelspeed the thing you want to optimize for? (To put it another way, what are the tradeoffs between lower/higher gearing assuming each is set up for the same target wheelspeed?) More gear reduction provides better low speed control, improves brake hold, increases torque, and decreases the load on the motor which allows it to run closer to maximum efficiency. Less gear reduction will make the truck faster at the expense of everything else. That is why it is better to use the lowest practical gearing then either use a faster motor or more voltage to get the top speed in the range you want. This is the basis for the classic Holmes mantra "Volt up, gear down".

Cheers!


Alexander_0_1 12-07-2017 10:55 PM

Re: Brushless kv choice for Bomber
 
Quote:

Originally Posted by jdcooper (Post 5773654)
Which one did you end up liking best in the truck?

I put the 3500 xl in my bomber, it’s got a bit of a hard start, but it fits what I use my bomber for. The 3500 standard found a home in my BMRC fully independent suspension wraith, and for no reason I put that 4100 in a scx10 w/ RCBros burly mud truck plates...

Teng 12-07-2017 11:58 PM

Re: Brushless kv choice for Bomber
 
Quote:

Originally Posted by jdcooper (Post 5773355)
As a theoretical matter how does one select the appropriate kv brushless motor?

I have all the parts for a Bomber on the way and picked the Castle 1406 2850kv combo after reading some build threads. However after reading the "What's Powering Your Bomber" thread, and others, it seems like many people are running 3300+kv motors. Anecdotally seems like folks are running low 2000s kv up through 4000+kv.

Is there a theoretical approach to choosing a motor kv? To put it another way, can anyone explain clearly the tradeoffs associated with varying the kv, assuming they're willing to change gearing or battery voltage?

Thanks!

If you are in high gear, In my humble opinion 2850kv is still very powerful with 3s. kv thing only personal preference. On my hearvy Wraith I ran 4s with 2200kv Puller Pro and I am happy with that :)

*Old* 12-08-2017 11:35 AM

Re: Brushless kv choice for Bomber
 
I'm very pleased with the Holmes 540 Brushless 3500kv on 3S. I play on many types of terrain, not just rocks. So I like the 3500 as an all-around trail performer.

Maybe I'm too dumb to know better, but the brushless allows this to go as slow as I need it and pretty fast when I need it to.

Jboucher 12-08-2017 03:23 PM

Re: Brushless kv choice for Bomber
 
Quote:

Originally Posted by *Old* (Post 5773998)
I'm very pleased with the Holmes 540 Brushless 3500kv on 3S. I play on many types of terrain, not just rocks. So I like the 3500 as an all-around trail performer.

Maybe I'm too dumb to know better, but the brushless allows this to go as slow as I need it and pretty fast when I need it to.

Mamba X esc?

Sent from my Nexus 6 using Tapatalk

sportbikeco 12-10-2017 01:57 AM

Re: Brushless kv choice for Bomber
 
id want to be around 2500kv on 3s

durok 12-10-2017 02:41 AM

Re: Brushless kv choice for Bomber
 
Holmes makes a 2700v Puller Pro just for you, then!

Always good to have a bit extra...

I have a 2200kv and 3500kv of the same, haven't yet hooked mine up - 2200kv would be a very slow top end crawler-oriented motor even on 3S, could go up on cell count to increase speed.

The 3500kv is what's going in my Bomber, however - with 3S - I have under-driven diffs front and rear and will start out at 12t pinion / 64t spur - but have 11t & 10t pinions if needed.

I'll likely be starting out with a very tall tire - 5.75" - which also affects overall gear ratio - but I also have a 2nd set off wheels with the stock height kit tires to switch out - they are 5.43" tall.

Jboucher 12-10-2017 06:52 AM

Re: Brushless kv choice for Bomber
 
Keep us updated durok.
I will be using 11/64, stock tires and normal diffs.
If 3500kv is too much on your rig it would be way too much on mine.

Sent from my Nexus 6 using Tapatalk

durok 12-10-2017 07:28 AM

Re: Brushless kv choice for Bomber
 
It's not too much - John Holmes himself has stated that stock Bomber gearing can be used with 3S and a 3500kv Puller Pro Standard 540 without issue.

Lots of people here run that setup...

BTW - you don't want to wait for my input - have you noticed how long ago I bought my RR10 kit?

(26-Jul-2016 per my TowerHobbies account history)

It's been sitting in pieces on my coffee table since then as I every so often buy yet another upgraded part.

So far all I really haven't replaced are the cage, the skid, and the CVDs up front.

I will likely get another of each of those items later so I can build a 2nd Bomber that will be nearly stock...

I already have a few upgrades that I bought 2 versions of - so I just may be the worst procrastinator on this forum.

Don't worry, at least I have my K10 Ascender to drive for now!

(Which reminds me, I need to install the handful of upgrades I recently bought for it...) :);-):ror:8):oops:

mikemcE 12-10-2017 07:41 AM

Re: Brushless kv choice for Bomber
 
For 3s the 3500 is sweet, if you go 4s the 2700 is perfect. I like the 3500’s wheelspeed


Hang up and Drive

Lightning Bomber 12-15-2017 07:42 PM

Re: Brushless kv choice for Bomber
 
i used a 2200kv puller pro with 2s and it was slow,so i went to a 3300kv trailmaster and it is great on 2s, i my go to 3s soon:D

Yonzariq 12-18-2017 02:04 AM

Re: Brushless kv choice for Bomber
 
I run a 3400KV only on 2s and think that already is more speed than I ever need...
Had to lower the punch from 7 to 5 (and can go up to 10) as the front wheels would not stay on the ground when accelerating, now only when from stand still to full throttle it will still wheelie :lmao:

But I built mine for more of a trail runner, no slow rock crawling or high speeds.

Might try 3s some time soon 8)


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