Originally Posted by snoopyrc
It was a 77spur and I cant remember what pinion but I was aiming for a 3.5 ratio. Thats FDR not the motor wind, for use with a hacker 17.5.
Can you ballpark the pinion and what pitch?
I got an equation directly from RRP that works for figuring out the diameter of any gear.
John, to find the dia.of 64p gear take the number of teeth on gear add two then divide
by 64. 78+2=80 divide 64=1.25 hope this helps
Without a distance from center to center you have no starting point, but that can be found. If you have a pinion of any pitch that's fits, It can be worked backwards.
I'm guessing it's a 48 pitch spur?
(77 + 2) / 48 = 1.65 inches diameter. The radius is 1/2 the diameter, .825 Inches.
Now, center the motor, slide any old pinion on that and makes contact with the spur and that you know the pitch of, metric will not work. The teeth will not mesh if they mismatch, so don't turn the gearing. All that's needed is the pinion tooth number.
For arguments sake, the pinion number we need to have will put us at 1 1/8 inch.
Again, (77 + 2) / 48 = 1.65 inches diameter. The radius is 1/2 the diameter, .825 Inches.
.825 (the spur radius) + Pinion Radius = 1 1/8 inches (1.125).
So the pinion radius is 1.125 inch - .825 = .3
we need to reverse the equation above. First thing is to double the radius for the diameter, .6 inches.
Now we have (P+2)/48 = .6, this becomes P = (48 x .6) -2. P = which points to roughly a 27 tooth pinion.
Run the equation again for the pinion and add the 2 final numbers together. You now have the distance from the center of the spur to center of the pinion. Now we need the internal gear ratio of the car. I know my TC3 is a 2.5, my e4js is a 2.0588.
As an example my e4js:
(Spur / Pinion) X <the internal ratio of your car> = FDR
(88 / 52) X 2.0588 = 3.484
Now to work it all backwards so the FDR meets the pinion to spur center to center distance. You'll have distance, the FDR and the internal ratio of your car. So lets change the example and keep calculating until we meet the magic distance number. This first one is for a Spur that is too large which is where most of us start from with these insane FDR numbers. This all trial and error on paper, but with this you buy a couple spurs and gears ONCE.
The distance number we need to achieve is still 1.125 inches.
(110 / P) x 2.0588 = between 3.4 and 3.5 P = 65 Sweet! (NOT)
Spur (110 + 2) / 64 = 1.75 inches diameter
1.75 / 2 = .875 inches
Pinion (65 + 2) / 64 = 1.05 inches diameter
1.05 / 2 = .525 inches
.857 + .525 = 1.4 inches or about 3/16 of an inch too big.
So decrease the spur size to calculate the new pinion.
lets try a 104 spur.
(110 / P) x 2.0588 = between 3.4 and 3.5 P = 61
Spur (104 + 2) / 64 = 1.66 inches diameter
1.66 / 2 = .83 inches radius
Pinion (61 + 2) / 64 = .984 inches diameter
.984 / 2 = .492 inches radius
.83 + .492 = 1.322 inches or about 1/18 of an inch too big. Closer though.
Keep going until you can be less then .075 of an inch up or down from your magic distance number. This will allow you to slide the motor around as needed to adjust for good gear mesh.
I found using this method that 94s are the largest spur to consider and 88s the smallest. My pinion collection goes from 40 to 52. I can hit anything from a 3.484 to a 4.838 as needed. I went overboard, I know.
Now to make anyone who has bothered to put up with me this long rest a little easier. I have a spread sheet that shows all of this. It can be adapted to any car, it includes rollout in millimeters and inches based on the FDR and Tire diameter. Not only that, the formulas are built into it. If you put in a spur, a pinion and your car's ratio, it spits everything else out for that row.
PM me your car's internal ratio, or the car make and model, with a valid email addy and I'll send it off.
Spur Pinion Center Ratio FDR
88 40 1.031 2.20 4.529
88 41 1.039 2.15 4.419
88 42 1.047 2.10 4.314
88 43 1.055 2.05 4.213
88 44 1.063 2.00 4.118
88 45 1.070 1.96 4.026
88 46 1.078 1.91 3.939
88 47 1.086 1.87 3.855
88 48 1.094 1.83 3.774
88 49 1.102 1.80 3.697
88 50 1.109 1.76 3.623
88 51 1.117 1.73 3.552
88 52 1.125 1.69 3.484