It conclusively answers the question - read it again:
The question was weather higher gearing with larger pinions = death to ESC's
The conclusive answer:
In 99% of the cases, that is exactly true. Smaller pinion = less ESC stress.


While your back EMF question may be a point of confusion for you, Freezebyte's questions was conclusively answered. Smaller pinions = less ESC stress.

Regarding the higher rpms with a lower pinion yielding higher back EMF - not sure I buy it. On a given voltage, the rpms of a given kv motor will be the same, regardless of the pinion installed on the motor shaft. If braking from a given vehicle speed - yes, the smaller pinion will have had to be rotating faster(if everything else emains the same) to achieve the speed, but the mechanical advantage of the smaller pinion is still there on decel as it is on accel, so less torque on the motor shaft proportional to the increased speed(less motor current to achieve the higher rpms and vehicle speed and less "reverse current" to slow it). A 10 pound truck requires x amount of braking force to stop in x distance, regardless of how the force is applied(high rpms with high reduction or low rpms and low reduction). Force = force, doesn't it? The higher motor speed would result in a higher voltage spike at a lower current level - force would be the same. That is how i see it anyway. :)

"the mechanical advantage of the smaller pinion is still there on decel as it is on accel, so less "force" on the motor shaft proportional to the increased speed"

The mechanical advantage is higher on the motor during braking with a smaller pinion so there is more force (energy) being generated during a stop. Think about this, I don't know if you have ever push started a manual transmission but you don't do it in first gear you do in in second. the reason is that in first this has the numerically highest gear ratio and is very hard to get started. Why? Because the load is super high when you pop the clutch (slam on the brakes in RC). You are basically swapping the gear ratio when you are braking, this is what I am asking. Could this be a part of the problem (ya, ya the batteries).

We are not talking about controlled enviroments here these things are being handled (put under stress) by a lot of people that don't have a clue, they are in every hobby, it's just the way it is. No one is doing a test where they are topping the truck out and comming to a marker, applying maximum (impending lockup)breaking force recording the results and doing the same for the other pinion and calculating what they need to stop in the same difference. I haven't put the math to but I see your point. But I also see that there a difference, a force over time difference.

Besides as I see it since Patrick quoted me it was a response to me and not Freezebyte.

Jeff

I think your push start analogy is not proper(you are mixing up the motor and the load here). The gear ratio does not reverse when braking - only the direction of the force reverses.
A better analogy or perhaps more clear idea of how this is working is using a 10 speed bike. If you had superior balance, you could roll the bike backwards in 10th gear and try to stop it by pedaling forward(this would simulate a large pinion or low reduction). Then try again with the bike in 1st gear(to simulate a small pinion). At a given vehicle speed, the pedals will be moving faster in 1st gear(voltage), but will require less torque(current) to stop the bike and vise versa. The mechanical advantage or gearing does not change or reverse - only the direction of the force. The mechanical advantage that exists on acceleration also exists on deceleration.

And while Patrick was responding to you...he happened to answer Freezebyte's question - the question was still answered, though. :)

I don't know how my description is wrong other than not specifing any gear other than 1st.


Well going from say a 14:1 to a 1:14,is a big difference, because you are spinning the motor with the wheels instead of spinning the wheels with the motor, so the motor spins faster with a smaller pinion with all other things being equal. This produces higher voltages that could possibly damage the ESC. The motor is acting as a generator at this time, and it's voltage is proprotional to it's RPM.

The question was:Higher pinions = safer MMM's? Or no? Someone's lying.... :wink:

Jeff

I think I worded my title poorly, it probably should have been someones "misinformed" instead of lying

OK. Lets try this again. In your push start analogy, your scenario proves my point rather than yours. The gear ratio doesn't work in reverse as you are trying to illustrate. In your example, the engine compression represents a constant braking force if you will(force used to slow the vehicle). This braking force is STRONGER in 1st gear due to the mechanical advantage. At the SAME BRAKING FORCE represented by the engine compression, the lower gear(more reduction or smaller pinion if you will) more easily slows the vehicle - this is why you select a higher gear to start the car(higher pinion if you will). The car engine represents the motor in this case - not you trying to push the vehicle(this represents the inertia we are trying to slow). The engine (which is simulating the motor)has the mechanical advantage.

Now, I think I see what you are trying to say - higher motor rpms will deliver higher voltage . You could take the stance that a higher voltage can damage a component, which is later destroyed by current, which has been mentioned in this thread. While this statement is true, it simply is not applicable in this scenario. The motor rpm doesn't change simply by installing a different pinion.
Your very theory is an impossibility in actual practical use. If using a 2200Kv motor on 6s, the motor rpms will be the same when driven by the end user regardless of the pinion selected. Therefore, the motor speed when brakes are applied will also be the same. Yes, the truck speed will be different, but that isn't the point. The motor Kv is a constant. 2200Kv at 22 volts yields 48400 motor rpms with any pinion or no pinion at all. Unless you are suggesting that it is dangerous to quickly change to a smaller pinion immediately before you try to slow the vehicle? Nobody would argue the dangers of this actvity, but how do you do this?! :smile:
The motor does not know what gear is installed onto it. 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.
I think you may be too smart for your own good. :intello:

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. :)

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

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

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:lol:
it's all about the breaks isnt it, too many voltage spikes and pop goes the TVS.

Your right the Eagle Tree is slow 10 times a second is way too slow to pick this up inless you just happen to catch this. The only other data logger I have will only measure at 240 hz which is better (but isn't exactly small and needs to be connected to the computer at all times), but I think you need something in the 1K range to see this. I have several O-scopes, but they are all old and don't have a way to capture information (been thinking of a chassis dyno for some time now).

I also don't have a RC this large my stuff is 1/10th scale or I would find a way to test it. I don't even have a MMM becuse of all the problems I have seen. I will get one, but I think I am going to wait a little longer.

The way I see it ( I don't have schems in front of me to study the exact circuits so I have to guess based on what Patrick says and my knowelege of electronics). If the voltage rises enough to cause the damage to it then the power is only kept in check by the batteries. One question to Patrick I have is how much internal resistance is too much?


I don't agree that the thread has been taken too far, as it invites interaction between the actual manufacturer who gives good information and helps us as a group understand the working of the system on a better level. I also don't see this as arguing I see it as a discussion.

Jeff

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!!

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.

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.

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).

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

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

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.

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

"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. :)

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.

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

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.

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

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.

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). :)

Does that mean my girlfriends leg actually hurt as much as she said it did? E-Revo to the shin at about 15mph, on video too.

My head hurts trying to comprehend all this tech talk. All I wanted to know was wether smaller pinions were less stressful on the ESC. Everyone else turned it into an circuit board engineers meeting.

Thanks Mike! That is what I figured that physics would dictate. I was too tired last night to look it up and took the lazy route and asked here.

I do know that an rc truck at any speed to the shin hurts like hell no matter what force is applied.........

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. :yes:

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.

I guess when they keep saying that the mechanical ratio flips("changing from accel to decel changes the mechanical advantage on the motor"), it makes me wonder if they are crazy or trying to say something other than what they are writing!

Braking energy DOES change with gearing because speed changes with gearing. At a given speed, however, it takes a given amount of energy energy to stop the car in a given distance(see my examples). The gearing makes no difference at all - energy is energy, regardless of gearing. 10volts at 1 amp or 1 volt at 10 amps is the same amount of power. 100 pounds of required force to stop the mass is 100 pounds of force to stop the mass. A pound of feathers is the same weight as a pound of bricks. A single dollar bill has the same monetary value as 4 quarters. A gallon is the exact same volume as 4 quarts - all just different expressions of the same thing! How many more examples can we come up with to say the same thing!?

I agree that a lower geared setup traveling at the same speed as a higher geared setup would have to be making more motor rpms(given that other factors are the same) and would therefore create a higher back voltage, though at a lower current than the higher geared setup. The total force required to stop a given mass at a given velocity is the SAME, however. Pretty sure I have mentioned this a couple times here, though. :)

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.

Man, all this for a pinion question. Gonna hafta start a new thread for a spur question!

That's how it is around here. If people just want to hear "gearz up FTW" with no support data, they can go someplace else. :smile:

Reading all this makes me want to waste some time posting.

1. Do you really think these Savages reach 63 mph just bashing around? I can imagine that with the 20t pinion, you have a good chance of reaching the same or higher speeds in most gravel pits, compared to running overgeared with a 25t pinion. This because of
a) less acceleration with the taller gearing, but limited space -> less top speed.
b) more current draw -> more voltage drop (even more with poor batteries) -> lower motor RPM.

2. The stopping distance is not constant. You should be more concerned about power than energy. Power = Force * Velocity. With shorter gearing, the stopping distance is shorter (motor torque * gear ratio = torque at the wheels). I would say it's even disproportionately shorter.

3. Assuming both motors are the same, they can provide the same (braking) torque. With the higher motor RPM (because of 1.b) and assuming limited space, also 1.a)) in the 20t pinion setup, the recuperated power is larger because of Power = Torque * Speed.

Conclusion:
If applying full brakes at full speed is killing the ESC, then the 25t pinion is better because the motor RPM is lower.

Anybody care to smash my assumptions?

Of course! :smile: But only on point 1b, and therefore your conclusion. Yes, higher current draw = more voltage drop = less motor rpm, but at near top speeds, you won't have the huge currents you do when you start. So, I think motor RPM will be pretty darn close whether geared high or low, the taller gearing will just take longer to get there.

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.

I concur.

I also concur with posts #111 & #112- thats how it works out in my head.

When you gear up, there is less mechanical advantage from gearing, meaning when you slam on the brakes, the motor has to work that much harder to stop the truck on that blasted dime; if you gear down but travel at the same speed, the truck has the advantage of gearing to help it stop that muich quicker (kinda like a naturally induced drag brake). Thats what I observed many years ago in my old TXT-1 setup when farting around with the gear ratios- has interesting effects on the brakes gearing up way high with a weedy motor (hv4400)...

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. :)

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