# Opinion: Bigger Spur or Bigger Pinion Gear

#

**31**
Again another person that does not understand the original question... Wow... You can't decrease the size of One gear and increase the size of the other without changing the FDR !!! To keep the same fdr, you need to decrease or increase both gears by the same amount !!! Too many folks here who don't understand ENGLISH !!!

#

**32**Tech Elite

iTrader: (115)

To answer your question. No. The ratio is the ratio.

There will be no effective difference in rotatianal mass, as if you decrease the size of one gear and increase the other, the difference cancels out. Assuming the gear pitch was kept the same, and the mesh was optimal, the only effect would be WHERE the motor is in the car (further forward or backward in comparison to the original position). If you multipy the number of teeth on each gear by a constant, the gear ratio will remain the same.

Not sure why people seem to think that changing sizes will give you some other advantage than mass distribution.

Bizkit, you run whatever gears will fit in your car and give you your desired FDR.

There will be no effective difference in rotatianal mass, as if you decrease the size of one gear and increase the other, the difference cancels out. Assuming the gear pitch was kept the same, and the mesh was optimal, the only effect would be WHERE the motor is in the car (further forward or backward in comparison to the original position). If you multipy the number of teeth on each gear by a constant, the gear ratio will remain the same.

Not sure why people seem to think that changing sizes will give you some other advantage than mass distribution.

Bizkit, you run whatever gears will fit in your car and give you your desired FDR.

gearchart.com still works last time I checked, anyone can go there and play with the numbers to see what combinations of spurs / pinions will give the DESIRED FDR. Pick an number to aim for and see what options you have to reach that particular number. If you are able to reach an FDR a few different ways, and want to try those different combinations to move weight forward or backwards, then great. Fine tuning. As stated before "the ratio is the ratio".

#

**33**Tech Addict

iTrader: (5)

There will be no effective difference in rotatianal mass, as if you decrease the size of one gear and increase the other, the difference cancels out. Assuming the gear pitch was kept the same, and the mesh was optimal, the only effect would be WHERE the motor is in the car (further forward or backward in comparison to the original position). If you multipy the number of teeth on each gear by a constant, the gear ratio will remain the same.

Not sure why people seem to think that changing sizes will give you some other advantage than mass distribution.

Bizkit, you run whatever gears will fit in your car and give you your desired FDR.

#

**35**
Still the question is misunderstood, and will stay that way... I guess ignorance is bliss for some.....lol... I'll pass them all forever anyway. Let them stay ignorant....

#

**36**Look at this example where the number of teeth has been decreased on both gears by the same amount;

90s/30p = 3.0 FDR

80s/20p = 4.0 FDR

the FDR changed. Keep in mind we are working with a ratio in most cars that is not 1/1.

There is only one case where increasing or decreasing both the spur and the pinion the same amount maintains a constant FDR, 1.0 or 1/1, both gears would need to start out with the same number of teeth;

90s/90p = 1.0 FDR

80s/80p = 1.0 FDR

70s/70p = 1.0 FDR

and so on...

The math is simple.

I won't get into the understanding of english or not.

*Last edited by oldrcr; 08-02-2013 at 06:56 PM.*

#

**37**Tech Champion

iTrader: (68)

Lets try not to confuse people with bad information.

Look at this example where the number of teeth has been decreased on both gears by the same amount;

90s/30p = 3.0 FDR

80s/20p = 4.0 FDR

the FDR changed. Keep in mind we a working with a ratio in most cars that is not 1/1.

There is only one case where increasing or decreasing both the spur and the pinion the same amount maintains a constant FDR, 1.0 or 1/1, both gears would need to start out with the same number of teeth;

90s/90p = 1.0 FDR

80s/80p = 1.0 FDR

70s/70p = 1.0 FDR

and so on...

The math is simple.

I won't get into the understanding of english or not.

Look at this example where the number of teeth has been decreased on both gears by the same amount;

90s/30p = 3.0 FDR

80s/20p = 4.0 FDR

the FDR changed. Keep in mind we a working with a ratio in most cars that is not 1/1.

There is only one case where increasing or decreasing both the spur and the pinion the same amount maintains a constant FDR, 1.0 or 1/1, both gears would need to start out with the same number of teeth;

90s/90p = 1.0 FDR

80s/80p = 1.0 FDR

70s/70p = 1.0 FDR

and so on...

The math is simple.

I won't get into the understanding of english or not.

#

**38**Probably for touring car, there will not be any diff. In the 1/12 threads, there is a theory around using the largest spur to aid on car balance. Maybe for 1/12, there is some diff. I personally switched from 88t to 96t spur, maintaining the same ratio and didn't notice any diff.

For anything else, it really doesn't matter one bit. The gears are light enough that the mass and moment of inertia differences are pretty negligible. The only real exception being that you don't want to go below a certain pinion size or the mesh gets really inefficient. That's not usually a problem since for 48p (not sure what the point is in other pitches) that point is like 14 teeth or so.

#

**39**
Well I believe I got my answer, as far as fdr goes. This is going in a FF03 and the front to back weight transfer doesn't matter, unlike a touring car or buggy/truck. Thanks for all of the insight and for giving me more to think about in the future.

George

George

#

**40**Look at this example where the number of teeth has been decreased on both gears by the same amount;

90s/30p = 3.0 FDR

80s/20p = 4.0 FDR

the FDR changed. Keep in mind we a working with a ratio in most cars that is not 1/1.

There is only one case where increasing or decreasing both the spur and the pinion the same amount maintains a constant FDR, 1.0 or 1/1, both gears would need to start out with the same number of teeth;

90s/90p = 1.0 FDR

80s/80p = 1.0 FDR

70s/70p = 1.0 FDR

and so on...

The math is simple.

I won't get into the understanding of english or not.

In other words, take a 20/60. That's a 3 to 1 ratio. If your smallest pinion is 30, what spur do you need? Well this case is so obvious that anyone can tell you a 90. 30/90 is also a 3 to 1. Notice something about the change? Pinion went up by 10. Spur went up by 30. 10/30 is also a 3 to 1 ratio.

#

**41**Tech Champion

iTrader: (68)

Yep, but notice that to maintain the same ratio, you have to change both gears in the same direction by the same RATIO.

In other words, take a 20/60. That's a 3 to 1 ratio. If your smallest pinion is 30, what spur do you need? Well this case is so obvious that anyone can tell you a 90. 30/90 is also a 3 to 1. Notice something about the change? Pinion went up by 10. Spur went up by 30. 10/30 is also a 3 to 1 ratio.

In other words, take a 20/60. That's a 3 to 1 ratio. If your smallest pinion is 30, what spur do you need? Well this case is so obvious that anyone can tell you a 90. 30/90 is also a 3 to 1. Notice something about the change? Pinion went up by 10. Spur went up by 30. 10/30 is also a 3 to 1 ratio.

#

**42**Tech Master

iTrader: (34)

What was being talked about though was going with a larger pinion and spur or smaller by increasing or decreasing the same # of teeth. Like going from a 35p 85s then going 40p 90s by adding 5 to each. This is just an example though. They both give different ratios. The only way to do this is by having the pinion and spur the same number of teeth.

#

**43**Tech Elite

iTrader: (115)

That was not what the original post was asking. He wanted to know if there was an advantage in using different size gear combinations (larger pinion & smaller spur or smaller pinion & larger spur) and maintaining the same FDR. He was trying to determine if the rotaional weight of a pinion gear, or gear mesh, has any effect when choosing what he uses.

#

**44**
#

**45**