Higher pinions = safer MMM's? Or no? Someone's lying....

[shaunjohnson]

eagle tree can be set to below 1 second if needed i think...deffo not 10secs though!!
there is a TVS in the MMM to protect the FETS from the voltage spikes i think.
large voltage spikes are caused by the battery not being able to absorb the charge and hence the ESC has no where to dump the power, then the voltage rises and blows the TVS off sometimes.
then the next time the TVS is needed, it inst there, so the FETS suffer.
i'm not a castle teach but this is what i believe to be happening.

getting an MMM is a good move but you need to run good batteries period!!

[RC-Monster Mike]

Originally Posted by J57ltr

No it doesn't but it does change the way braking works The amount of power delievered back into the system is the same, but with the smaller pinion it will be delievered faster (Force over time) with the samller pinion. braking will be quicker than it would be with a larger pinion. After all we are only talking about an 8.62 mph difference.

This question I have posed was answered by Patrick is:

Now on the large pinion smaller pinion issue is it safe to say that having a numerically higher gear ratio will cause more back emf which could exacerbate the problem if the batteries are sub par?:'

and his answer was:

In 99% of the cases, that is exactly true.


Yes the motors are spinning at the same speed given everything but the pinion, but the amount of energy dissipated (mechanical to electrical) is higher during braking with the one with smaller pinion will get rid of it’s energy more quickly than the one with the larger pinion.

And with the low KV motors you guys use (under 3K) the generated voltage is higher. I switch between motors all the time and depending on what I am running I may have to take the braking force to 30% but on a high KV motor I might have to have it at 100%, even though the gear ratio changed to correspond to the motor and batteries being used.

So unless the batteries have a low resistance then the voltage will start to rise to a dangerous level. If Regen is occurring, then it must be a higher voltage than the power source. And since 6 S is just about at the limit anyway so the back EMF is going to be higher (but for a shorter period of time) than it would be with the larger pinion.

So unless someone has some voltage numbers on the decel rates at these differing gear ratios we aren’t getting anywhere.

Jeff

Patrick's statement was further clarified by saying that a smaller pinion causes less ESC Stress in 99% of all cases - indicating the opposite of what you said(and your "numerically higher" gear ratio comment was likely misconstrued - which was also discussed).

Why(how) does the vehicle with the smaller pinion get rid of more energy more quickly? My math says the opposite. In this example, force is actually NOT the same - the vehicle with the larger pinion would be moving faster and would therefore require more force to stop itself(same mass at higher velocity has more inertia and requires more force to stop). In the vehicle with the smaller pinion, the vehicle speed is lower and the motor has more leverage at a given rpm. If anything, the faster vehicle with the larger pinion will have to generate more force stopping itself. I don't see how it could be any other way.

[BrianG]

I think what people are saying is this: Yes, a higher geared (larger pinion) does require more work for the motor to propel the vehicle (less rpms per mph), but when braking, the vehicle is now spinning the motor less rpms as well. Likewise, a smaller pinion makes the motor spin faster per mph, and when braking the motor spins faster creating more back-EMF. I can understand the thought process, but the few tests I've run show that low gearing creates the highest back-EMF voltage, but these bursts are short and current is lower; while high gearing creates lower back-EMF voltage, the highest back EMF current, and the pulses lasts longer.

There are several ESC-killing issues; all this talk of one thing is ignoring the rest. All are affected by battery and setup choices:

1) Overcurrent. This is just the result of a setup that is geared too high and/or too heavy using a motor with too high a kv on whatever voltage. Nothing to do with ripple, braking, etc. I think most decently-designed ESCs can handle these bursts currents, but the resulting excessive heat from losses at high currents does shorten component life. Besides, a setup like this is more "on the edge" and if anything goes wrong, too much current can easily be drawn.

2) Braking. This seems like the biggest controversy here. We all know that braking dumps energy back into the battery. Obviously, a better battery has lower resistance, which helps reduce the voltage spike, but brake current can still be very high. The MMM TVS is supposed to clamp the voltage to a safe level (~28v IIRC) so the battery/FETs are not seeing huge voltage surges. But, if that TVS device goes bad (and it's not physically noticeable to the user), those voltages are no longer clamped and excessive voltages can then be present. Personally, no matter how you are geared, braking while running 6s on a larger vehicle is gonna stress everything - that's a lot of energy to shed! Solutions: reduce braking force and/or increase the time span in which braking is applied, use lower voltage (but don't gear up to compensate for lost speed), use mechanical brakes, or use more TVS devices and caps to help deal with those transients.

3) Ripple current. Simply the "AC" current generated by the ESC's PWM pulses. The caps filter these out, but if battery resistance is too high, the increased amplitude of the ripple current overworks the caps, heats them up, and things blow. No matter what the throttle position is, the instantaneous current of the PWM pulses is going to be very high. Lower throttle just means there are fewer ripples, which means overall lower average ripple.

[RC-Monster Mike]

I agree with everything you say Brian - except the very 1st statement. You, like Jeff, are asuming the same speed with different gearing. This is not a proper assumption(if we change gearing, vehicle speed must also change with everything else the same). When braking a given vehicle that is being run on a given voltage - the motor rpm has not changed regardless of the gearing. Only the vehicle speed has changed(due to the gearing). A 2200kv motor on X voltage runs at x rpm regardless of the gearing. At a given vehicle speed this is different, but this would not be apples to apples, as the discussion started with a given voltage, vehicle and motor(all things the same except gearing, which results in a different vehicle speed).Your tests demonstrate exactly what I have said - higher gearing results in lower voltage at higher currents and vise versa at a given vehicle speed. At a given input voltage, however, I don't see how it could be. The speed changes with the gearing - as does the load(be it braking or acceleration).

[BrianG]

I agree Mike, and as I said, that is what I thought people were saying.

BTW: you have a few emails from me, but only read the last one...

[Pdelcast]

Originally Posted by J57ltr

I asked if having a smaller pinion could cause more back EMF that could cause a failure if the batteries are up to the task, to which the asnwer was



So basically to me he is saying that in 99% of cases having the smaller pinion can cause this problem with the regen braking. But he ends with the statement of smaller equals less stress.

Those two statments given that Patrick are opposed to one and other unless he meant 2 different things.

Small pinion+bad batteries= damaged ESC

And Small pinion= less stress on the ESC under load.

Or he missunderstood higher numerical ratio.


Conclusive answer?

sorry -- I guess I misread the original statement. Higher gear ratio (smaller pinion) = less stress (lower reverse currents to the battery, lower ripple voltage.)

Any time you let the motor rev more freely, it is easier on both acceleration currents and braking currents.

[J57ltr]

Originally Posted by shaunjohnson

eagle tree can be set to below 1 second if needed i think...deffo not 10secs though!!

That's 10 times a second or 10 Hertz or every .1 second or 100 milliseconds. It's still not fast enough.

Mike,

I guess what I am trying to say is that the people that are having these problems more than likely don't make any other changes than the pinion. They don't change braking force all they do is slap in the new pinion and go for it.

I see it as downshifting, your rolling down the street at 40mph and shift to 3rd then you slow to about 30 and drop it in second the engine reves much higher than it does when your at 30 then you were at 40, basically you are changing the gear ratio.

I never missed that they are traveling at different speeds but when I ran some numbers I didn't some up with enough of a difference.

Could you show your math? I would be interested to see what you came up with. I (according to the speed calculator Brian has made) I only say an 8.62mph difference between the 2 pinions. I would really like to see what you have for total energy that is being dissipated to come from top speed to a complete stop from the 2 different gear ratios with the weight involved.

I also see times where the RPM of the motor is not the same given only the gear ratio change I see a lot of times if the motor is slightly overgeared that it doesn't reach it's calculated rpm.

"Likewise, a smaller pinion makes the motor spin faster per mph, and when braking the motor spins faster creating more back-EMF. I can understand the thought process, but the few tests I've run show that low gearing creates the highest back-EMF voltage, but these bursts are short and current is lower; while high gearing creates lower back-EMF voltage, the highest back EMF current, and the pulses lasts longer."

Brian, is that low gearing (numerically high) and high gearing (numerically low)? Also what equipment were you using to make these observations?




Granted I don't know what the ratios are in the Flux and haven't really looked, I just used the Savage as the example with all things being equal except the pinion size.



As Brian pointed out that the MMM is to close to it's limits on 6S which it is rated for. Patrick even said that adding an external TVS and cap setup would be a great idea. This is something that tells me that the unit is not sturdy enough for the general population (I feel for Castle tech, I really do).

That's one of the sticking points I have with the MMM. Not enough real useful information on operation. At the onset before the MMM came out there was no mention that the 2650 should be used for 4S and the 2200 for 6S, there wasn't anything about the batteries other than A123. Granted we already went over this and I know that Castle can't test every battery but there isn't anything on their website as what are good readings and what are bad readings for batteries. All you see is what the MMM is used with in the RC rags. There simply is not enough technical information to be had. I know that Patrick has said that people glaze over and they might lose a sale to China. What? I can't even understand the manuals since they are in Engrish, if I have a warranty problem do I really want to wait for 3-4 months for a replacment shipping overseas. No I don't.

I think Castle really needs a better manual even if you alienate a few people.
I have said it before I have a ton of experience in many different fields, but if I can't get a handle on everything involved, what makes you think Joe 12 pack is?

Patrick,

Thanks for the clarification.


I have asked for clarification on many points, not because I doubt you guys, but the old "trust but verify" is something I live by.

Jeff

[RC-Monster Mike]

"Mike,

I guess what I am trying to say is that the people that are having these problems more than likely don't make any other changes than the pinion. They don't change braking force all they do is slap in the new pinion and go for it.

I see it as downshifting, your rolling down the street at 40mph and shift to 3rd then you slow to about 30 and drop it in second the engine reves much higher than it does when your at 30 then you were at 40, basically you are changing the gear ratio.

I never missed that they are traveling at different speeds but when I ran some numbers I didn't some up with enough of a difference.

Could you show your math? I would be interested to see what you came up with. I (according to the speed calculator Brian has made) I only say an 8.62mph difference between the 2 pinions. I would really like to see what you have for total energy that is being dissipated to come from top speed to a complete stop from the 2 different gear ratios with the weight involved.

I also see times where the RPM of the motor is not the same given only the gear ratio change I see a lot of times if the motor is slightly overgeared that it doesn't reach it's calculated rpm."

People are having problems - but i don't think it is fair to say or assume that gearing down(smaller pinion) is the cause of the problems. In fact, the opposite is far more likely to be true. The problem may have shown itself coincidentally when they geared down, or perhaps they were never geared up to begin with. I don't think there is any information or data out there that can make the case for higher failure rates with lower gearing(again, the opposite is likely to be true). Can you show me any significant data that suggests this to be true?

As far as "seeing it as downshifting" - there is NO GEAR CHANGE happening in use, so no downshifting. We don't have a 3rd and 2nd and 1st gear to "shift down to". These are single speed vehicles. Whatever ratio we start the run with, we finish the run with. If it happens to be a smaller pinion, then the resulting vehicle speed will be lower, as will the braking load required to slow the vehicle. I don't see how this can be disputed.

What math do you need to see? Do you really need specific numbers to understand that it takes more energy to slow the same mass from a higher velocity? really? And there is a 12.4 mph difference between a 20t and 25t pinion on the Savage Flux when switching from a 20t to a 25t pinion(with a 5.5 inch tire and 44t spur). With 11+ pounds of mass, this results in a significant amount of extra energy along with less mechanical advantage with the higher gearing. Even the 8.62mph difference at 11+ pounds is significant (run the truck into your shim at 8.62 mph to see the energy it carries).

Regarding the reduced rpm with a taller gearing - perhaps. Unless the gear ratio was extreme and/or the batteries were poor, this is not very significant in my experience(when trying to set a speed record, yes - in daily use - not so much). This speaks to less than ideal batteries, though - which is the primary cause of most problems related to this topic(has been discussed numerous times). Gearing too low is not likely to cause a blown ESC. :)

[TexasSP]

Just wondering but does the force to slow something down at higher MPH compound like the power it requires at higher MPH.

ie: I know that if say 2HP will get you 40 MPH you will need much more than double to get you 80 MPH. This is factoring in aerodynamics, wind resistance, etcetera. I do not remember the equation but remember reading about this extensively a couple of years ago. So the question is if it takes x amount of force to stop a vehicle at 30 mph does it take 2(x) to stop it at 60 mph or is it more? It seems to me like it would.

[lydiasdad]

Originally Posted by Pdelcast

No, I'd suggest running BIG, high quality batteries. The best way to prevent voltage rise on the bus is to have low impedance batteries.

Lowering the voltage is an option, but the batteries are more important -- insufficient batteries will cause high voltage rise on the line even with lower voltages.

Thanx!

Why do you suggest using a123 cells? They have high internal Resistance and drop heavily under load. Even in 2p configuration.

[J57ltr]

Originally Posted by RC-Monster Mike

People are having problems - but i don't think it is fair to say or assume that gearing down(smaller pinion) is the cause of the problems. In fact, the opposite is far more likely to be true. The problem may have shown itself coincidentally when they geared down, or perhaps they were never geared up to begin with. I don't think there is any information or data out there that can make the case for higher failure rates with lower gearing(again, the opposite is likely to be true). Can you show me any significant data that suggests this to be true?

As far as "seeing it as downshifting" - there is NO GEAR CHANGE happening in use, so no downshifting. We don't have a 3rd and 2nd and 1st gear to "shift down to". These are single speed vehicles. Whatever ratio we start the run with, we finish the run with. If it happens to be a smaller pinion, then the resulting vehicle speed will be lower, as will the braking load required to slow the vehicle. I don't see how this can be disputed.

What math do you need to see? Do you really need specific numbers to understand that it takes more energy to slow the same mass from a higher velocity? really? And there is a 12.4 mph difference between a 20t and 25t pinion on the Savage Flux when switching from a 20t to a 25t pinion(with a 5.5 inch tire and 44t spur). With 11+ pounds of mass, this results in a significant amount of extra energy along with less mechanical advantage with the higher gearing. Even the 8.62mph difference at 11+ pounds is significant (run the truck into your shim at 8.62 mph to see the energy it carries).

Regarding the reduced rpm with a taller gearing - perhaps. Unless the gear ratio was extreme and/or the batteries were poor, this is not very significant in my experience(when trying to set a speed record, yes - in daily use - not so much). This speaks to less than ideal batteries, though - which is the primary cause of most problems related to this topic(has been discussed numerous times). Gearing too low is not likely to cause a blown ESC. :)

1. I never said that it was a fact in fact I asked if it could make a problem worse.

2. Data? No I don't I was asking a question. And giving my thoughts on what could happen. I also didn't say it was the problem I only said that with subpar batteries that maybe this could possibly be a problem.

3. I never said there were any gear changes. It was an analogy to show that a difference in gear ratio from the same speed causes the motor spins faster at the same given speed. Which could cause more back EMF.

4. You said according to your "math says the opposite" leads me to believe that you calculated something. It will take me some time to get some accurate calculations but when I get back I'll bone up on my ME books and see what I come up with and post how much power it takes to stop from different speeds. I don't think you have calculated anything because you still don't see that going from accel to decel changes the mechanical advantage on the motor.

5. I am not sure what a shim is (other than a thin piece of material). Not to be smart, but I can't tell if it is a spelling error or what.

6. a 5 tooth difference gives me this:

Originally Posted by J57ltr

It will go faster if you gear it down (numerically higher). I have a friend that has the same combo I have and he will eat my lunch if I am geared too high. You are geared for about 75mph, it'll never get there. For sure this is the reason you are have weak brakes. If my friend is geared 87/15 and I am 87/20 I can't even come close to catching him. We both run A123's and the 7700 in a rustler. It won't even pull a wheelie with that gearing in my truck
(87/20) Even his top speed is faster than mine if I am geared too tall. And I usually have to have 100% for the braking force and it will slow the truck from top speed but as the truck slows the braking force drops.

Jeff

the only difference between these 2 setups is the body EVERYTHING else is the same same number of cells even charged on the same charger. We usually do top speed runs but my setup always changes from a 3.3Kv all the way to a 10.5K motor and I run everything from .8 to 1.0 to 48 pitch gears depending on what I am doing at the time. I run anywhere from 2S A123 to 4S A123.


To TexasSP: Yes to stop in the same distance it takes much more power, but I'll have to get back to you on that for an approximate value


To lydiasdad

What is the typical internal resistance of a lipo? I know my 4S1P A123's come in at .032 ohms. Or about .008 ohms each and that's with the connections. I don't know off the top of my head of the 2P packs.

Jeff

[RC-Monster Mike]

Originally Posted by J57ltr

1. I never said that it was a fact in fact I asked if it could make a problem worse.

2. Data? No I don't I was asking a question. And giving my thoughts on what could happen. I also didn't say it was the problem I only said that with subpar batteries that maybe this could possibly be a problem.

3. I never said there were any gear changes. It was an analogy to show that a difference in gear ratio from the same speed causes the motor spins faster at the same given speed. Which could cause more back EMF.

4. You said according to your "math says the opposite" leads me to believe that you calculated something. It will take me some time to get some accurate calculations but when I get back I'll bone up on my ME books and see what I come up with and post how much power it takes to stop from different speeds. I don't think you have calculated anything because you still don't see that going from accel to decel changes the mechanical advantage on the motor.

5. I am not sure what a shim is (other than a thin piece of material). Not to be smart, but I can't tell if it is a spelling error or what.

6. a 5 tooth difference gives me this:



the only difference between these 2 setups is the body EVERYTHING else is the same same number of cells even charged on the same charger. We usually do top speed runs but my setup always changes from a 3.3Kv all the way to a 10.5K motor and I run everything from .8 to 1.0 to 48 pitch gears depending on what I am doing at the time. I run anywhere from 2S A123 to 4S A123.


To TexasSP: Yes to stop in the same distance it takes much more power, but I'll have to get back to you on that for an approximate value


To lydiasdad

What is the typical internal resistance of a lipo? I know my 4S1P A123's come in at .032 ohms. Or about .008 ohms each and that's with the connections. I don't know off the top of my head of the 2P packs.

Jeff

1. - You did not say it was fact. But...you won't accept that it is not true, either. I have tried to explain through various analogies, which you are reluctant to accept. You revert back to a similar vehicle speed with a smaller pinion, which requires an input voltage change along with the gearing change - this is not the topic being discussed, or did I miss something? I thought we were discussing a simple gear change and its likelihood of being the cause of failure on an esc.

2. - subpar batteries IS the problem - not a smaller pinion

3. - you suggested gearing down as an example, which indicates a gear change. Without the gear change, if the vehicle is going the same speed with shorter gearing, the motor HAS TO BE spinning faster to begin with(higher input voltage), which is not apples to apples here. The discussion was regarding a simple gearing change. This gearing change will also change vehicle speed, and therefore required braking force. I don't need an example to show that a different gear ratio results in different motor speed at a given vehicle speed - this is not only obvious, but has been my point all along. At a given vehicle speed, a shorter gearing requires higher motor rpm. However, if driving a vehicle at a given voltage, a gearing change ALSO changes vehicle speed. I am not disputing the fact that higher motor rpms yields higher back voltage. I am simply disputing the higher rpms to begin with(unless you change input voltage, output rpms won't change with a given Kv motor).

4. - When you do your calculations, you will plainly see that the motor's mechanical advantage DOES NOT CHANGE from acceleration to deceleration (accept in your car scenario when you physically change the advantage through a gear shift). Without a mechanical change, the mechanical advantage does not change. I look forward to any data that refutes this.

5. Shim is a spelling error. Sorry for the confusion. I meant to say run the vehicle into your SHIN at 8.62 mph to see the energy that must be disippated. Then try it at 0 mph and let us know if there is a difference in force applied to your shin. :)

6. - your data verifies my statement - The A123 cells are subpar for the task at hand AND the ratio is extreme. And it is a speed run. That is exactly what I said(subpar batterries, extreme gearing etc.). In 1p configuration, the A123 cells drop voltage significantly, as has been well discussed. If you want to go 75mph in the Rustler, you need more than 2s1p A123 packs(unless the rustler is in the back seat of your real car). Still, lower gearing in all cases results in less load on the electronics.

[crazyjr]

I got to agree with mike here, just because you are running 30,000 rpm on acceleration, doesn't mean you are running 40,000 rpm on deceleration. The load ratio does flip, but doesn't effect RPM. If you are running 30,000 rpm on acceleration, the moment you hit the brakes you will still be running 30,000 rpm. I think cheaper motors have something to do with it as well, I used to have eagletree graphs with 7 and 9xl fegaio's that show 27-28 volts and 250 amp surges, this was done on polyquest twenty's and flightpower 20c 3700 packs. I thought at the time it was a glitch and deleted the graphs, wish i had kept them, because they show exactly what people describe. I also used quark 125's that have never been modded in any way and they are still in use to this day

[RC-Monster Mike]

Originally Posted by TexasSP

Just wondering but does the force to slow something down at higher MPH compound like the power it requires at higher MPH.

ie: I know that if say 2HP will get you 40 MPH you will need much more than double to get you 80 MPH. This is factoring in aerodynamics, wind resistance, etcetera. I do not remember the equation but remember reading about this extensively a couple of years ago. So the question is if it takes x amount of force to stop a vehicle at 30 mph does it take 2(x) to stop it at 60 mph or is it more? It seems to me like it would.

http://www.crashdamagerepair.com/calcs.htm

11 pound Savage stopped in a distance of 15 feet
at 50.89mph(20t pinion) = 63.53 pounds of force
at 63.61mph(25t pinion) = 99.26 pounds of force

Note that the 25% gearing change (5 tooth pinion increase) resulted in a 56% increase in force required to stop in the same 15ft distance.

[RC-Monster Mike]

Also note that the 11 pound Savage travelling at 8.62mph that is to hit Jeff's shin will require his shin to absorb 136.71 pounds of force in order to stop it in 0.2 feet(2.4 inches). This would likely hurt(please don't really try this, Jeff - 8 pounds of force can fracture the shin bone). :)