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. :)
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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
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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