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Opinion: Bigger Spur or Bigger Pinion Gear

Old 08-01-2013, 11:49 AM
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The OP and wingracer can also seee that my saying ”amount” really meant ”ratio” and not amount of teeth... If you got to keep the same fdr, you will have to go up or down on both sides , pinion and spur, period.... Don't twist the words here by claiming I meant ”amount of teeth” while I only wrote ”amount”.... You guys would make bad lawyers !!!!
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Old 08-01-2013, 03:51 PM
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As they say on Mythbusters...

WARNING - SCIENCE CONTENT

I did a little bit of calculating. Using gear combinations that will actually fit (on a TC6) that both are 4.0 FDR, the inertia values for small vs. large gear combos are as follows (assuming 48P aluminum pinions):

33P/66S = 0.0000002908 kg-m2
40P/80S = 0.0000005054 kg-m2

So it sounds like smaller is waaaay better, right? Only one problem. The reflected inertia of the entire car (1400g) is approx. 0.0007879 kg-m2. Which is 1559 times the inertia of the gears. So if you think a 0.06% increase in acceleration will help...... go for the small gears. Otherwise, I'd suggest going with a spur size that gives you some adjustment range to either side of where you start at.
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Old 08-01-2013, 04:13 PM
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Originally Posted by Kevin Marcy View Post
As they say on Mythbusters...

WARNING - SCIENCE CONTENT

I did a little bit of calculating. Using gear combinations that will actually fit (on a TC6) that both are 4.0 FDR, the inertia values for small vs. large gear combos are as follows (assuming 48P aluminum pinions):

33P/66S = 0.0000002908 kg-m2
40P/80S = 0.0000005054 kg-m2

So it sounds like smaller is waaaay better, right? Only one problem. The reflected inertia of the entire car (1400g) is approx. 0.0007879 kg-m2. Which is 1559 times the inertia of the gears. So if you think a 0.06% increase in acceleration will help...... go for the small gears. Otherwise, I'd suggest going with a spur size that gives you some adjustment range to either side of where you start at.
I think only Ronald Volker and Marc Rheinard can even FEEL that 0.06% in acceleration.
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Old 08-01-2013, 08:21 PM
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Its simple logic. Ratio is ratio and that's it. You can achieve it any way you want and its not gonna change the output as long as the FDR is the same.

If you go thru pro drivers setup sheets, some of them does mention that they used bigger spur for more forward motor location or smaller for the opposite. That is what its all about. Nothing said about acceleration/speed/temp.
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Old 08-01-2013, 08:31 PM
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Remember e=mc2.... If you take into consideration the dynamic acceleration and deceleration difference of the lighter drivetrain versus the heavier drivetrain, you can see how the 0.06% can become 0.6sec less every lap.... It's the compounding of the small weight savings that you get faster laptimes, and the gears are the first things to make lighter/smaller...
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Old 08-01-2013, 08:45 PM
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0.6 seconds a lap from a gear size change? Are you running 30 minute laps?

You would change the gear weight before you looked at your diff weight? Saving 0.5 gram on a small rotating part vs saving. 10+ grams on a much larger part?
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Old 08-01-2013, 08:59 PM
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Yes truly amazing how many replies aren't getting to the point of the OP.
SAME FDR, bigger gears vs smaller gears.
Two things involved in this discussion are seperate.
1. The effect of the gears themselves as far as rotational mass (of the gear NOT the car), teeth friction, so on and so on.
2. Where that places the motor in relation to the car (rotational mass of the car).
I am frankly not convinced that it makes that much difference in either case. At least for the average racer.
Me... eh, doesn't matter one squat if I make an error in my driving.
So has anyone bothered to put in a BIG combo and measure the position of the motor or the weight distribution ? Same for SMALL. Then compared that ? I 'd like to see actual data on that. Back up the mental myths with real facts and not math theory.
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Old 08-01-2013, 09:06 PM
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In this hobby nothing is negligeable... Every milligram counts. Lighter gears make your Car accelerate and decelerate faster while conserving energy and decrease battery voltage loss throughout the run... It's a chain reaction that can make a big difference...
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Old 08-01-2013, 09:13 PM
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Originally Posted by bertrandsv87 View Post
In this hobby nothing is negligeable... Every milligram counts. Lighter gears make your Car accelerate and decelerate faster while conserving energy and decrease battery voltage loss throughout the run... It's a chain reaction that can make a big difference...
Totally agree. Which is why I would generally use smaller gears and change the cars rotation by other means. Let's not forget to confuse the matter even more, not all gears of the same "size" are the same weight, thickness, material and so on so friction and mass of the gears can be DRASTICALLY different.
I'm still gonna concern myself with being a better driver, thats the final truth in winning.
edit: if I wasn't so tired I would dig out some gears and do some weights. Compare a hogged out chamfered Alu pinion vs a fat steel one, as well as some thinned out spurs vs the standard fatties. Yes, big difference in weight, huge if you ran 12th stock with a 4 cell and a dustbuster motor. The difference was visible.
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Old 08-02-2013, 05:34 AM
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Originally Posted by asc6000 View Post
Yes truly amazing how many replies aren't getting to the point of the OP.
It isn't helped by someone constantly posting plenty of confusing, irrelevant and contradicting information.

Last edited by terry.sc; 08-02-2013 at 01:19 PM.
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Old 08-02-2013, 07:08 AM
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Originally Posted by oldrcr View Post
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.
Decreasing/increasing the spur and pinion by the SAME NUMBER OF TEETH will give you a different FDR, as this changes the ratio of the teeth on one gear to the teeth on the other.....

Multiplying the number of teeth on each gear by the SAME CONSTANT will give you the same FDR.
Using an internal gear ratio of 1.85
80 spur, 30 pinion gives you a ratio of 4.93
Using 0.9 as the constant in this example,
80*0.9 = 72
30*0.9 =27
using an internal gear ratio of 1.85(same as before)
72 spur and 27 pinion gives you a ratio of......... 4.93

So now, just increasing the the number of teeth on both by 10...
90 spur and 40 pinion with an internal ratio of 1.85 gives you an FDR of 4.16 (significantly lower than 4.93)
Theres the math .

Regarding the mass distribution stuff... i don't know about you guys, but when my car isn't working, the first thing i generally think of changing on my setup isn't to change the spur and pinion to achieve a very slight difference in front/rear weight distribution... but hey thats just me.
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Old 08-02-2013, 10:23 AM
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Originally Posted by terry.sc View Post
It isn't helped by someone constantly posting plenty of confusing, irrelevant and contradiction information.
So you're implying we should 86 the comments from Mr.87?
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Old 08-02-2013, 12:38 PM
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Originally Posted by CarbonJoe View Post
So you're implying we should 86 the comments from Mr.87?
touche'
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Old 08-02-2013, 05:31 PM
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Originally Posted by bertrandsv87 View Post
Remember e=mc2.... If you take into consideration the dynamic acceleration and deceleration difference of the lighter drivetrain versus the heavier drivetrain, you can see how the 0.06% can become 0.6sec less every lap....
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Old 08-02-2013, 05:45 PM
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Originally Posted by JamesL_71 View Post
I had a good laugh from his post as well.
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