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

[J57ltr]

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....

Jeff

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

[RC-Monster Mike]

Originally Posted by J57ltr

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....

Jeff

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?!
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.

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. :)

[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.

[J57ltr]

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

[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.