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

[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.