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

[Rusteh101]

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.

[TexasSP]

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.........

[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.

[RC-Monster Mike]

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. :)

[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.

[snellemin]

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

[BrianG]

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.

[Thomas]

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?

[BrianG]

Originally Posted by Thomas

...Anybody care to smash my assumptions?

Of course! 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.

[suicideneil]

Originally Posted by BrianG

Of course! 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 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)...

[RC-Monster Mike]

A123 packs are recommended because of their low resistance (not so much their ability to put out power, but rather their ability to absorb it). They allow power to be "put back" into them very quickly(ie:they can be charged in about 10 minutes or so with the A123 charger). This ability to absorb the energy back is what makes them "Castle Approved". The energy doesn't "get caught" in the ESC because the path to the batteries offers low resistance.

[lydiasdad]

Hmmmm...... I know I saw on another brushless esc manufactures website that a123 cells have a comparatively high internal resistance. They did not want a123 cells being used with their esc's. I'll try and find a link.

[snellemin]

Originally Posted by lydiasdad

Hmmmm...... I know I saw on another brushless esc manufactures website that a123 cells have a comparatively high internal resistance. They did not want a123 cells being used with their esc's. I'll try and find a link.

Hmmmm, I'm interested in who this manufacturer is.

[lydiasdad]

Originally Posted by snellemin

Hmmmm, I'm interested in who this manufacturer is.

Found it. This pdf applies to this thread. Page three is where they talk about the a123 cells. http://mgm-compro.com/pdf/en-motor-braking-050409-g.pdf