Higher pinions = safer MMM's? Or no? Someone's lying.... - RC-Monster Forums

RC-Monster Mike · 07:48 PM

"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. :)

J57ltr · 10.06.2009, 01:03 AM

Quote:

Originally Posted by RC-Monster Mike

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. :)

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:

Quote:

Originally Posted by J57ltr

It will go faster if you gear it down (numerically higher). I have a friend that has the same combo I have and he will eat my lunch if I am geared too high. You are geared for about 75mph, it'll never get there. For sure this is the reason you are have weak brakes. If my friend is geared 87/15 and I am 87/20 I can't even come close to catching him. We both run A123's and the 7700 in a rustler. It won't even pull a wheelie with that gearing in my truck
(87/20) Even his top speed is faster than mine if I am geared too tall. And I usually have to have 100% for the braking force and it will slow the truck from top speed but as the truck slows the braking force drops.

Jeff

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

RC-Monster Mike · 11:04 AM

Quote:

Originally Posted by J57ltr

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.