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

[TexasSP]

Originally Posted by RC-Monster Mike

Lets all just walk away with the knowledge that better batteries and lower gearing most often leads to successful funtime with our brushless cars and trucks. :)

And if everyone followed this simple philosophy things would be much better in the BL world.

[J57ltr]

Originally Posted by RC-Monster Mike

At any given voltage, the motor rpm does not change with the pinion. For this reason, the very premise of your theory is an impossibility. With a given battery and a given motor, the gearing does not impact the motor speed(aside from loading that may lower input voltage and therefore motor rpms, but this is another discussion altogether). The 'force' required to stop the vehicle is different with different gearing(ratios, mass, etc. all come into play), but at a given voltage in practical use, the motor rpms do not change by simply installing a different pinion, which is the very premise of your thought.

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

[shaunjohnson]

jeff,
i tryed to look at these voltages yesterday but the eagle tree V3 wont log quick enough to really log the voltage spikes from breaking.

all of that aside, therefor the super low gearing, low KV and super high voltage guys, if they are running low quality packs then they should use mechanical breaks and disable the MMM's correct?
therefore there is no regenerative voltage.

i think this thread has taken it too far, too many seemingly big brains here and not enough balls to go test it ourselves!!

from what ive seen, my revo only demands 70A from my cheap turnigy when i give it the beans, but i rarely do that cause it's scary
it's all about the breaks isnt it, too many voltage spikes and pop goes the TVS.

[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...

[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.

[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.

[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 crazyjr

The load ratio does flip, but doesn't effect RPM.

No - the load ratio does not flip. It does not change at all. The direction of the force changes(or flips if you want to call it that), but the mechanical ratio between the motor and the "load" remains unchanged. The ONLY way to change this mechanical ratio is MECHANICALLY. The same mechanical ratio that exists on acceleration also exists on deceleration(opposite of acceleration - just like reverse is the opposite of forward). To state that the ratio somehow changes when brakes are applied is akin to saying that when you engage reverse, the gear ratio is different than when in forward. The motor must apply force in the opposite direction, but it is using the exact same mechanical connection and therefore the same ratio.

Freezebyte - all you need to know has been stated. YES, a smaller pinion is less stressful on the ESC.

[crazyjr]

Originally Posted by RC-Monster Mike

No - the load ratio does not flip. It does not change at all. The direction of the force changes(or flips if you want to call it that), but the mechanical ratio between the motor and the "load" remains unchanged. The ONLY way to change this mechanical ratio is MECHANICALLY. The same mechanical ratio that exists on acceleration also exists on deceleration(opposite of acceleration - just like reverse is the opposite of forward). To state that the ratio somehow changes when brakes are applied is akin to saying that when you engage reverse, the gear ratio is different than when in forward. The motor must apply force in the opposite direction, but it is using the exact same mechanical connection and therefore the same ratio.

Freezebyte - all you need to know has been stated. YES, a smaller pinion is less stressful on the ESC.

I am inclined to agree with you, i just figured if the load is off the wheels (wheels pushing the motor) the gearing might look different.

[RC-Monster Mike]

Originally Posted by crazyjr

I am inclined to agree with you, i just figured if the load is off the wheels (wheels pushing the motor) the gearing might look different.

The load is "off the wheels" no matter if you are accelerating or decelerating. the wheels are either driven forward for acceleration, or driven backwards for reverse or braking. The wheels are the link to the road when applying power in either direction. :)

[J57ltr]

Originally Posted by BrianG

I don't think he's thinking that the ratio changes from forwards to brake, but rather the braking energy changes with gearing. Like if there is X amount of EMF on a given setup, but then you gear down (or up) X changes accordingly. I can see a lower geared setup generating more EMF on braking if the initial speed was the same just before braking. That would mean if the lower geared setup was at WOT before braking, then the higher geared setup would have to be somewhat less than WOT to have the same initial speed.

Originally Posted by BrianG

Yeah, no matter what, a vehicle has X amount of kinetic energy "built-up" that is affected by vehicle weight and speed. If you stop on a dime, that energy will be "drained" quickly into the batteries, which results in a huge voltage and current spike, but the spike only lasts a short amount of time. If you stop gradually, that energy will be drained slowly into the batteries, which results in much lower voltage and current, but lasts a lot longer. Just like how batteries drain: pulling 500A for 10 seconds results in the same Ah draw as pulling 16.66A for 5 minutes. And just like batteries, it is pretty obvious which one is going to be harder on a setup.

Finally, this is what I was trying to say all along but apparantly I was not able to get this into the words that were coming out of my hands. Thanks Brian.

It was never about both cars going at different speeds.

Mike I don't use 2S1P a123 packs at all they are 2S2P, I do have 4S1P, and a couple of 3S1P packs and I change configurations all the time. I seldom run a 1P pack. And since Castle says they recommend A123 I guess they are good enough.

Sorry I was not more clear.

Jeff

[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). :)