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.........

[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.

[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.

[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.

[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]

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