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Old 04-06-2013, 09:45 AM   #1
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Default Does kv matter with a fixed fdr?

Trying to settle a debate. Lets assume you have the same exact car, with every single thing identical, including the FDR. The only thing different is the kv rating. Lets say car A has 1800kv and car B has 2000kv. Will A have more torque less speed and B have less torque and more speed. Or will the overall speed not matter, only the torque will be affected?
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Old 04-06-2013, 11:38 AM   #2
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Power is what matters. KV is not a measure of power.
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Old 04-07-2013, 03:34 AM   #3
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Please explain...
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Old 04-07-2013, 04:08 AM   #4
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I'm not sure, but for me, KV are the turns per volt per minute.
For example, if you run a 3200kV motor with 7.4V, it will turn 3200x7.4 times in a minute.
That isn't the power
Power is explained in Watts.
I think this is what sosidge means.
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Old 04-07-2013, 04:53 AM   #5
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Where the FDR puts the motors in regards to efficiency will be another factor too, as well as track size. An extra 5000rpm means diddly squat , if you take forever to get there , or never do......oh so many variables still
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Old 04-07-2013, 09:49 AM   #6
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Yes Kv matters. Here is a real world example.

At my local track I run VTA and stock sedan. Both Xray 3 (same internal ratio) and initially ran the same gearing on both (47/96). This equates to a 3.88 fdr.

With the sedan running a 17.5 (~2200kv) and the vta running a 25.5 (~1800kv) I can turn laps about 2 seconds quicker, the straightway speed is much greater (with a sedan body and tires you can pick up about a second)

Bottom line, more kv (withing reason) you will have more speed. Moving from a 25.5 to a 5.5 is a whole nother question and should have another set of rules.
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Old 04-07-2013, 04:26 PM   #7
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Quote:
Originally Posted by mkiiina View Post
Yes Kv matters. Here is a real world example.

At my local track I run VTA and stock sedan. Both Xray 3 (same internal ratio) and initially ran the same gearing on both (47/96). This equates to a 3.88 fdr.

With the sedan running a 17.5 (~2200kv) and the vta running a 25.5 (~1800kv) I can turn laps about 2 seconds quicker, the straightway speed is much greater (with a sedan body and tires you can pick up about a second)

Bottom line, more kv (withing reason) you will have more speed. Moving from a 25.5 to a 5.5 is a whole nother question and should have another set of rules.
I think you will find he is asking in regards to the Kv ratings within a single turn bracket. I run in a 10.5T blinky class. I have 3 10.5T motors, Kv ratings on them are 3250, 3400, 3600. Which is better and which is fastest really depends on what track i am running on and whether it is on road or off road. And it all comes down to do i need torque or rpm or a balance of both to make the fastest laps that i can from the track.

Does Kv matter? Not really, as pointed out above power, or the total amount of work that can be performed under a fix set of conditions is what really matters.

If you are quite new to this and racing, just buy any old motor and toss it in the car and go race and have fun and worry about these sorts of technical details once you get good at driving.
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Old 04-07-2013, 11:01 PM   #8
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Quote:
Originally Posted by RogerDaShrubber View Post
I think you will find he is asking in regards to the Kv ratings within a single turn bracket. I run in a 10.5T blinky class. I have 3 10.5T motors, Kv ratings on them are 3250, 3400, 3600. Which is better and which is fastest really depends on what track i am running on and whether it is on road or off road. And it all comes down to do i need torque or rpm or a balance of both to make the fastest laps that i can from the track.

Does Kv matter? Not really, as pointed out above power, or the total amount of work that can be performed under a fix set of conditions is what really matters.

If you are quite new to this and racing, just buy any old motor and toss it in the car and go race and have fun and worry about these sorts of technical details once you get good at driving.

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Old 04-08-2013, 01:22 AM   #9
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Your getting more rpm's out of a 2000 KV motor than a 1800 KV motor, therefore your going to be faster. That is, in an ideal rc world, there would be noticeable differences, but still very slightly. However, in the real rc world, there are so many factors will prevent noticeable differences. I run F1 with a fixed rollout and all the guys here in Japan are running Yokomo 21.5 series motors have 2200 KV. However, I'm running a speedpassion 21.5 V3 Ultra-Sportsman with only 1800KV. To be honest, I see no difference at all.

Knowing you, I would worry about your driving skills first.
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Old 04-08-2013, 06:17 AM   #10
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Hey Ed. When you coming back? Happy to see u found a place to race there. I should be back end of June. Just getting ready for gt2.

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Old 04-08-2013, 07:24 AM   #11
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I don't believe kv really has a major difference. In vta, I've run both a trinity d3 25.5 and a Novak ballistic 25.5. Besides the trinity running cooler bc of its design, there wasn't a noticeable difference at all with regard to power or speed.

This is with both cars geared at 3.8fdr.

Also I've been doing some testing with 17.5 motors as well. So far I've tested a revtech 17.5 with HT rotor (3300 kv according to trinity) against a Turnigy trackstar 17.5 (which has the same design as a reedy sonic, 2270 kv). In my opinion, this Turnigy motor blows the revtech away despite having a 1000 less kv rating. It has more torque and speed at the same fdr.

Thus, IMO, kv is BS.
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Old 04-08-2013, 07:48 AM   #12
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I would say, pick a motor and run it. I've had great success with the speed passion MMM series motors. Lots of rip and straight away speed. IMO, when it comes to any stock/fixed gear class, its about setup and being able to maintain the cars momentum around the track. Oh, and don't hit anything.

As for coming back to Canada, I'm not to sure when.. maybe next year.. 2014... The outdoor racing here in Japan is amazing. Also, the indoor facilities are top notch too! Currently, running a 1/12 Kyosho Plasma Ra with a 4.5 motor - need for speed! I also have a Tamiya F103, its much slower, but way, way competitive over here. People fight tooth and nail for F103's because they're so fast.

Hope all is well and good luck with your motor choice. If I was to choose a different motor I would go for Schuur Speed 21.5 or Viper 21.5
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Old 04-08-2013, 05:13 PM   #13
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Oh the other thing about Kv ratings is that you do not know in most instances whether the number being quoted is the load or no load Kv rating.

Quote:
revtech 17.5 with HT rotor (3300 kv according to trinity) against a Turnigy trackstar 17.5 (which has the same design as a reedy sonic, 2270 kv)
In this case the revtech has to be a no load rating as it has more kv than some of my 10.5T motors.
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Old 04-08-2013, 05:34 PM   #14
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kV is simply a unit to measure the final RPM's on a test stand if the motor is supplied 1 volt of potential difference.

It is not really fair to say kV has an effect on acceleration (remember- racing around a circuit is about maintaining the highest possible acceleration, not top speed.)

Acceleration can be seen in terms of power and torque, while top speed can be seen in terms of kV.

So in conclusion, kV doesn't matter so much if the difference in torque from two different motors is greater than the comparative difference in kV. Torque (highest possible magnetic flux on motor rotor @ a specific RPM) and power are important in determining the lowest lap time. Hence, you purchase the "high torque rotor" . Power is directly effected by torque (Power=Torque*Angular Velocity).

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Old 04-09-2013, 04:13 AM   #15
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KV numbers are supposed to be quoted at no load.

Given the number of variables (rotor, inbuilt timing, magnets, etc), I guess KV is just a sort of orientative number (read shorthand) that describes the motor power (not torque) by proxy (suggesting what the RPM-Voltage curve will do). In principle though, I think the higher the rev count per volt, the lower the torque (which is pretty much how all electric motors work). Practical experience supports the idea, of course.

Back to the question, I am not sure that on the same track, same car, same ESC you'd ever end up with the same FDR if using motors of different KV ratings. From my experience, I can tell you I have found wild torque differences between such motors which in turn required wildly different gearing. One example that comes to mind is a 17.5 Hacker motor (I think it was rated at 1700KV or thereabouts) that had such a huge torque I run out of room in the car before I found a pinion to make use of it. Compared to a Tekin or an SP this motor was a monster. The problem is the final speed achieved was inferior to the other two (with the largest pinion that fit in the car's gearbox). No doubt I could have geared it higher as far as the motor was concerned but am not sure how much higher I had to go to achieve comparable speeds to the other two. Of course, acceleration was mental.
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