Opinion: Bigger Spur or Bigger Pinion Gear
 

I'm sure everybody has thought about this before, but now I'm really wondering which is better. Certain cars have gear covers and will only allow certain size gears, my question is does it matter what gears you run to get final ratio? If you get to your ratio with a bigger/smaller pinion does it put more strain on the motor or do you go by temp of the motor? I'm looking more at electric cars, but I guess this could go for nitro cars as well :nod:

Tried this several years ago in Offroad, as the thought generated couriosity...the smaller spur/larger pinion seemed to put more of a strain on the motor and resulted in worse overall performance. Have not since tried it in Onraod....

I am not an engineer. My logic tells me that Big Pinion & Small Spur combo should be OK for both On-Road and Off-Road as long as the motor can handle the workload. I would go further out and imagine that in competitive racing Sensored Brushless Motors are better poised to take advantage of larger pinion gears over Non-Sensored Brushless Motors.

As long as you have the FDR you want, with a smaller pinion and smaller spur you have less rotational mass. It might just be a few tenths of a gram lighter but the pinion is spinning at 30k rpm, it might a give you a slight performance advantage.

The only real noticeable difference, will be where the weight of the motor sits in the car.

Its my understanding and experience that you never want a pinion that bigger then the spur. If the pinion is getting bigger and the spur is staying the same or getting smaller, your engine is working harder and harder. Once you get to a 1:1 ratio, then you defeat the purpose of gearing and you also loose more and more torque. A smaller pinion gives you more torque and less speed and a bigger pinion will give you more speed and less torque. On a super light car like the 12 stock or 12 mod, you can afford to run a larger pinion to the point where you have to shave the pillow ball mount to be able to move the motor further up front so as to fit a larger pinion. In this 12th class, the car is so light and the torque is so high that we were running something like a 1.5:1 or 2:1 ratio. I don't believe that this is possible to do off road or on larger scale cars like 10th or larger. The key to gearing is to find that middle ground where you're getting the sweet spot in toque to RPM range. This will both keep your motor from being overworked which in turn will keep the heat down and keep it from frying. Also, you wont eat up gears on crashes or hard landings. But that's just my opinion. In the time ive spent on the track, ive managed to fry zero motors, stripped one spur and I place well in the pole. But that's just my opinion..

I don't think you understand what the OP is asking. The OP is asking if it matters if you go with a smaller pinion / smaller spur, or a larger pinion and larger spur... final drive ratio being fixed/unchanged/right where you need it.

He isn't asking about gearing taller, or gearing shorter.

Thanks... I for one, misunderstood the OP's question.

But thank you to Cpt.America, M3Roc and the others who added to this thread. I've learned some new stuff and can't wait to try out some new pinon/spur combos and see how it affects the motor and balance of my car. I especially like the concept of rotational mass and weight distribution. I've been trying to get my center of gravity lower by lowering the ride height, now I can try gearing as well.

The difference in rotational mass is still very very very tiny. I have run larger gear combos, and shorter gear combos in efforts to shift weight forward or rearward in a number of cars, both onroad and offroad, and I can't feel any noticable difference is preceived lag or change in acceleration due to the difference in rotational mass.

Great question
 

Ok, issue at hand is to determine if there's a difference in performance if the FDR stays the same and the gear size relationship changes. To me the answer is YES. With a any device it boils down to efficiency. This is simply calculated as output/input. I think of it like a pulley all a out leverage. In conclusion it is my belief that a smaller pinion to spur ratio is more efficient.

I use this as a tuning aid quite often. Lets say the sweet spot on the motor is 4.0fdr. Now the traction is lower . A smaller pinion to spur ratio would be better as it provides less "rip". It for me helps to control the undesired wheel spin.

Do you get higher temps? Yes but not that I worry about. Heat is usually the end result of energy being converted. More heat less efficiency

All I can say is that you need to keep the distance between the middle of the motor shaft and the point of contact between pinion and spur as short as possible to make it easy on the motor: so smaller pinion/smaller spur is the without a doubt... It's always best not to kill the torque from the source by reducing the initial moment arm. It will allow the motor to pull more load with less current draw while running the same gearing...
Also, you must consider the total area of contact between spur and pinion. The more teeth make contact, the more power lost. The wider the pinion teeth, the more power lost. The more the pinion and spur material stick to One another, the more power lost... The more the pinion and spur vibrate or move back and forth, the more power lost. The bigger pinions and spurs displace more air, and therefore cause more power loss. ..

hmmm.
 

Without getting into the technical differences with friction because of teeth size and moment arms...

S(spur)/P(pinion) = FDR (Final Drive Ratio)


80S/10P = 8.0FDR > high rip, lower top speed, low temp - more rip

80S/20P = 4.0FDR > med. rip, med top speed, med temp
{These two will perform basically identical since FDR is the same}
40S/10P = 4.0FDR > med. rip, med top speed, med temp

40S/20p = 2.0FDR > low rip, higher top speed, high temp - less rip


So,

If FDR is higher, rip is higher, top speed is lower, temps will be lower.

If FDR is the same, performance, rip, temps. are the same.

If FDR is lower, rip is lower, top speed is higher, temps will be higher.


Simple.:)

The thing we are talking about here will not change your laptime by a thousand of a second if you use the normal competition stuff and have the right FDR. The most important aspect mentioned is the rotational mass as it affects the handling of your car. So this is one point for the small pinion/spur combination. As the higher friction/drag theory is not wrong, the "loss" of power is so small (if even existent in comparison) that it will not affect laptime. That effect may even hit back if pinon/spur are very small and only two teeth are touching for a longer time! But what counts here is durability; more theeth touching, the force shared with more teeth when using a strong motor; and weight balance; the position of your motor in your car, as it is the second heaviest component.

Wow, alot of good information I am reading about this. Now I'm glad I asked because I can look at it from more points.

Using bigger or smaller gears might have made a difference in efficiency 20-30 years ago, when we were using 1200mah nicads and we had to squeeze every last ounce of efficiency out of the car. Anyone remember having to take your car over and put it down on the start line so you didn't waste that half a lap of power getting there? Today that's not really a problem.

What changing gears does is move the motor on the chassis. The weight of a buggy or touring car means means the small amount of movement isn't going to make much difference relative to the rest of the car, but in pan cars where the motor is the biggest part of the unsprung weight, moving the motor forwards makes the car smoother and easier to drive, smaller gears so the motor is nearer the rear axle makes it more aggressive and harder to drive.