[sebasadam]

Hi everybody, I would like to get some information on those following things :
What does changing the pinion tooth's change exactly and what are the effects on temperature ? For example if I go up 17-18 instead of 16 what will happen to my torque, speed and temperature ? what if I go down 14 or 15 ?? Same goes for the spur gear... I am just starting into RC world and I am quite noob I would really appreciate if you guys could help me out a bit on those terms, also how about differential fluid ? how does it affects the vehicle exactly ? For example if I add thicker oil in the back diff, will I have more torque but less speed ? Or in the front will it handle better ? Anyways hopefully you guys understand my concerns and can understand me.

Thanks a lot !!

[waylander]

What pinion and spur combination you use will depend on the vehicle, motor choice, where you're running it and what exactly you're looking for.

Increase in pinion gear will give higher end speed but lower torque, a decrease in pinion gear will give the opposite.

The spur is opposite of the pinion.

To understand this imagine the motor is putting out a constant RPM, an increase in pinion gear teeth means it will turn the spur gear faster as the teeth in the pinion will come around faster. With 15 teeth the spur gear will turn that number of teeth in one revolution of the motor, with 17 teeth the spur will turn that much with one revolution.

Now let's assume the pinion remains the same and you change the spur gear, an increase in teeth in the spur gear means that the output shaft on the spur will rotate less for the same number of teeth engaged. Less teeth on the spur gear will make it rotate more. So if you have a 54 tooth pinion you need to engage 54 teeth to make it rotate once, with a 60 tooth you need to engage 60 to make it rotate once, that means the motor has to rotate the pinion gear more times.

Simple math. A 15tooth pinion on a 54 tooth spur will require 3.6 revolutions of the pinion (and therefore the motor output shaft) to turn the spur gear once. A 17 tooth pinion only needs 3.18 revolutions to turn the spur once. So assuming a constant rpm from the motor the 17 tooth will turn the spur more. Now for a constant pinion gear of say 15 tooth, remember it needs 3.6 revolutions to turn the spur once. With a 60 tooth spur it needs 4 revolutions to turn the spur once.

Trying to increase speed with the same motor, regardless of whether you use the spur or the pinion to do it, will generally increase the temp of the motor.

For differentials, let's look at both extremes. With no differential fluid the power will all go to the wheel with less traction as it's the easier path, so that wheel will spin while the other wheel won't turn at all. With a spool (locked differential) the power is evenly split as both wheels will turn at the same rate regardless of traction. So what you want is a differential fluid that is somewhere in between, it will provide more rotation to the wheel with less traction while providing some power to the wheel with good traction. Which "density" you use is a personal choice. Differential fluid is cheap so you can try a few different ones to test. It has less to do with handling and more to do with traction.

Handling will be affected by toe in / toe out, type of shocks and the oil in the shocks (keep wheels on the ground) and camber. Those are a completely different discussion and a lot of information can be found on google.

[sebasadam]

Hi Chang,

Thank you VERY much for all your extremely useful information. I will try different settings to see what would be the best but is there any kind of chart telling me what gear would be good with what motor ? For example I run a SC10 4x4 FT 6.5t Novak ballistic racing 550 (see link) 3400RPM with a 16T-0.8mm pinion and a 32P-62T Spur gear. First, what does the P (pitch i assume) change as well as the mm in the pinion ? and second, do you think this is a good setup or should i try lowering my pinion and or my spur ? I'm currently bashing mostly with the vehicle I really like to do jumps but I also like speed therefore i need a good middle with top speed and torque. Anyways I'll definitely try some settings since I now understand better how it works but if you or anyone know a chart I would really appreciate seeing it.

Once again thank you

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