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#1 cck42

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Posted 04 December 2020 - 08:04 AM

Is there any performance difference in gearing when the gear ratio is the same ex
12:36 vs 13:39
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#2 Dominator

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Posted 04 December 2020 - 08:43 AM

13/39 will give you more top end but less acceleration.  Depending on the track and type of car one ratio may work better than the other.


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#3 Dave Crevie

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Posted 04 December 2020 - 10:01 AM

They are the same ratio. The only advantage is that you will have more tooth area in contact with the 13/39, theoretically spreading the load out over more surface area and reducing wear and breakage. 


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#4 MattD

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Posted 04 December 2020 - 10:39 AM

Dave is right, technically the performance will be the same.  In reality, the bigger crown and pinion you use, the smoother the mesh, resulting in less friction and quieter car.  That makes a small performance advantage.


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#5 Isaac S.

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Posted 04 December 2020 - 11:58 AM

You have to use bigger tires then. On vintage cars that isn't to big of a problem though, but I don't know what you're racing. 


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#6 MSwiss

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Posted 04 December 2020 - 12:42 PM

13-39, as mentioned, will most likely give you a slightly better gear mesh.

But ease of installation, and as also mentioned, ground clearance should be considered.

A 39T spur is approx. .630" in diameter, so if you were running with .050" minimum clearance, .730" diameter tires would be the smallest you could use.

Back to performance,12-36 will require a bigger motor angle, which will affect weight distribution.

Depending on the car, tires, track, that could be a positive or negative, but I'm not sure anyone but a real experienced racer could tell the difference.

12-36 would be a lighter gear train, so your rotating mass would be reduced, so in theory you should see increased speed.

Imperative words, "in theory".

It would have to be tested to see if it was significant.

That's quite a bit of the the fun for serious racers.

Testing, and finding improvements.


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#7 Dave Crevie

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Posted 04 December 2020 - 12:48 PM

We are, of course, talking 64 pitch, and a 39 tooth spur should be just within realm of a typical flexi-car. If you run smaller tires, like for Eurosport, you will want to run 72 pitch or even 80 pitch gears. In fact, to run the tiny pinions those guys like to use, you absolutely will have to go with 80 pitch.



#8 Fast Freddie

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Posted 04 December 2020 - 01:30 PM

The 13x39 combo will stress the motor less and reduce your ability to dial out brakes especially with motors like the PS4002FK and Phoenix.  Another very popular 3:1 gear set is the 11x33.  Mostly used by FNRS series racers due to the mandatory 11T pinion.  It's usually run using ..710-700 dia. tires.  The gear set reduces the brakes a lot but the small tire diameter allows  some brakes to remain and increase as tire diameter decreases due to changing FDR (final drive ratio). I have raced GTP cars using this combo and had no problems.  Qualifying on a Gerding King in the low 3.6 range while other racers using the same combo in the high 3,5 range.  But having a choice I would run the 12x36 with .700 tires.  I believe it would stress the motor less and the FDR is the same as the 11x33 gears. 


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#9 MSwiss

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Posted 04 December 2020 - 01:41 PM

If I don't ask it, someone else will. Lol

How does 13-39 "stress" the motor less?

Is that in reference to the mesh being minisculey superior?
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#10 MattD

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Posted 04 December 2020 - 04:43 PM

As Mike said the differences are so small, most of us  couldn't tell the difference.    Get some gears and go play and see  what you like.    So many variables, from gear mesh,  axle lubrication., you just have to go play and experiment.


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#11 slotcarone

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Posted 04 December 2020 - 11:32 PM

Same ratios. Only difference will come from smoothness of the mesh and the difference in the weight of the gears.
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#12 Fast Freddie

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Posted 05 December 2020 - 02:19 PM

It may sound silly to all you seasoned racers but I call it a stress number.  The higher the number the less stress on the motor.  It may be unsettled science but here's how I get the number. Keep in mind it's all based on running the same size tire size when comparing two different gear sets.  So if you compare 11x33 using .700 tires then 12x36 will have to have .700 tires also.  If comparing 13x39 needing .730 to tech then 12x36 would also need .730  I get the stress number by subtracting the pinion teeth from the spur teeth,  11-33=22, 12-36=24, and 13-39=26  With all else in order gear mesh, bushing alignment, and tire type.  Measure the heat of the motor after say a normal 2-3 minute heat.  Which gear set causes the least amount of heat in the motor.  It may be a small difference.  I look at it as a leverage thing.  When you use a lever to move something the pivot point (gear contact point) is placed as close to the object as possible, the short end of the lever (pinion).  The long end of the lever (spur) allows the least amount of power (stress) to move the object. If you increase the length (+ tooth count) of each side of the lever by the same ratio the amount of power it takes to move the object is reduced.  If you decrease the length (- tooth count) of the lever on each side by the same ratio you will increase the power (stress) needed to move the object. It's just my way of doing things and it has worked for me over the years.  So let it fly if you think I'm out to lunch or maybe agree.  I'm a big boy mentally and physically so I can handle it.


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