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Hawk 7 oddity

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#51 dalek


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Posted 03 January 2018 - 10:56 AM

Didn't I say that in Post #39?  The Final Drive Ratio (FDR) is much more important then the gear ratio.  The FDR is readily changed without removing the motor or gears, just change the tire diameter.  The FDR also changes during the race due to tire wear and this must be taken into account before you decide on beginning tire size. 


Rule of thumb:  .020" = one tooth.


IOW, regarding FDR, a change in tire diameter of .020" is roughly the equivalent of a one tooth change of the spur gear.


To be exact, when using a 10t pinion, if you change to tires that are .02632" bigger in diameter, the FDR will be the same as staying with the original tires and using a one tooth smaller spur gear.


However, when the pinion is 14t, .01587" tire diam change = 1 tooth spur change.


The average is about .021" (approx. rule of thumb).


If you don't know, but are curious as to what Final Drive Ratio (aka rollout) is, it's the ratio of armature revolutions per 3.14" of car travel.


It's easy to find rollout charts but here's my version:  


Attached File  Gearing - Final Ratios ver.06.pdf   170.5KB   24 downloads

Dale King
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#52 bluecars


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Posted 03 January 2018 - 04:21 PM

If you don't have a chart the formula for FDR is crown-spur/pinion/tire size=FDR

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Robert "Red" Valantine :diablo: 

#53 Shiggy


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Posted 03 January 2018 - 10:35 PM

D. "shiggy" Person

#54 Fast Freddie

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Posted 04 January 2018 - 12:01 PM

Proslot has a very good FDR chart on their site.  It's the one I use all the time.  Check out the FDR that the retro guys use for the HR motors.  9/28 48P gears with .8125 tires.  You can find an FDR very close to this using 64P gears and a different size tire.  Try it and see what you think.

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Fred Younkin

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