For a CO27 I usually run 48-52mm rollout on a 100x48 track or as little as 38-40mm on a 58x36 track. I don't run brushless but most people usually gear a 13.5 into the mid 60's which you'll never reach with a 100 tooth spur.

Rollout = (tire diameter*3.14)/(spur/pinion)

You should get an 88 tooth spur, you'll strugle to get to the proper rollouts with that tall a spur.

1.750-1.800 for rollout on a stocker. Toss the 100 if your going 13.5. Its just not gonna happen unless you go to a much smaller spur. 78 tooth spur from RW is nice to use if your going with a 13.5 and roll it out in the 60-65mm range like what was posted earlier.

Here's a roll-out tutorial I just sent to the new 1/12 racers in our club. I really do believe that doing the math a couple times helps it all make more sense...plus you get a better idea how changes in this or that affect your roll-out. A tooth on the pinion or a mm of tire diameter make a couple percent difference, a couple teeth on the spur don't make much difference, etc.

This comes under the heading of "give a man a gear combo he'll know roll-out for the day...teach him to calculate roll-out (and what it means) and he'll know roll-out forever. AND be able to teach someone else!

Please feel free to correct if you find any errors. My typing ain't all it could be.

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I sent some information to Neal without really thinking that he (and maybe you guys) probably have never been exposed to roll-out. This is the kind of stuff that will drive you bat-shit because nobody ever really just spells this stuff out. OUR big goal, though, is to keep you guys from getting frustrated (like we invariably did).

Here's a bit of a primer on roll-out.

Gearing a foam-tire on-road car is ENTIRELY dependent on the current diameter of your tires. What you're trying to find is how far the car moves forward for every revolution of the motor. To express it any other way is hopeless because of the fact that foam tires change diameter where rubber tires, effectively, do not. Also we'll see why the question "what gear to run" is impossible to answer without some of the other variables answered.

Just as a reference point I've found for brushed motors in 1/12 that a 96T spur gets pretty much all the combos you need for our tracks. Then it's a matter of measuring your rear tire dia. and consulting the roll-out chart to find desired roll-out and what size pinion gets that for you. I will show you these charts Sunday next. BUT...like a lot of things in life you will gain a FAR better appreciation and understanding for this phenomenon if you do it the hard way a few times. Remember that prior to PC's and easy to download Excell share-sheets we had to manually calculate roll-out and this is a good skill to have because even now I'll end up with an oddball spur gear or a value that doesn't show up on the chart. Or I'll leave my charts someplace. Or... Plus the mathematician/math teacher in me LOVES this stuff!

This is where your high school geometry comes in handy (you remember...the stuff you and every other kid on earth said "but I'll never use it"). Roll out is calculated using good ol' pi (3.14159...). You multiply the tire diameter by pi and that gives you the tire circumference (distance around the tire). Then divide your spur gear by the pinion gear to get your final drive ratio. Finally, divide the tire circumference by the fdr and you have roll-out. The roll-out charts all do this but it is performed on an Excell spread sheet.

What you're usually trying to do is find what pinion to use. If you know your tire dia/circumference, spur gear teeth and desired roll-out value you can identify the pinion size that yields your desired roll-out with a little algebraic manipulation.

The base formula is (remembering that / means "divided by"):

(pi X tire diameter) / (spur teeth / pinion teeth) = roll-out

Let's call (pi X tire diameter) "tire circumference" and (spur teeth / pinion teeth) "final drive ratio" (though we probably won't use this for now).

As I said, the mystery number is usually the pinion teeth so we want to segregate that variable.

Our base formula is now:

(tire circumference) / (spur teeth / pinion teeth) = roll-out

Noting that we have a "double division" in this base formula (basically a fraction in the bottom of a fraction) our first step is to take the bottom fraction, (spur teeth / pinion teeth), invert it and multiply by the tire circumference, giving:

(tire circumference) X (pinion teeth / spur teeth) = roll-out

Now we multiply both sides by spur teeth giving us:

(tire circumference) X (pinion teeth) = (roll-out) X (spur teeth)

Finally, divide both sides by (tire circumference) and we get our desired result:

pinion teeth = (roll-out X spur teeth) / tire circumference

Seems like the long way to get there but THIS is the point where you'll start. This last is the only formula you'll need if you know everything BUT the pinion gear size. I just showed the math to get there as proof. Please note that there are a couple ways to get from there to here but the result will be the same.

So, say you're like me and have a 96T spur gear and new 48mm dia rear tires. You want to be a little conservative so you're going to start with a 40mm roll-out (I think I'm running about 42-43mm in Stock). The pinion you need is found by:

pinion teeth = (40mm X 96 teeth) / (48mm X pi)

Remember that pi is approximately equal to 3.14159 (you can really stop at 3.14, but what's the fun in that?)

pinion teeth = (40mm X 96 teeth) / (150.8mm)

pinion teeth = (3840 mm teeth) / 150.8mm)

When we divide out the right side the mm cancel leaving only teeth AND the desired result.

pinion teeth = 25.46 teeth

NOW...a 25.46 tooth pinion is a bear to find, and it usually doesn't mesh worth CRAP even if you do locate one. You'll need to round up or down, especially since you're nearly exactly in the middle. This little difference probably won't be catastrophic (provided your initial assumption was "conservative" as I did) either way, so I'd probably do a 26T pinion and settle for the 40.84mm roll-out.

NOTICE...the roll-out we "accepted" in this example is almost a full mm more than what our target was. THAT'S how quickly things that seem really small can change things once multiplied back out. This difference is about a 2% variance but such is what we're left with since we can only go to full tooth intervals on pinions. The only alternative would be to true the tires down a nick. I can find that amount (if I was really concerned...in this case I really wouldn't be but will make the entire case for our discussion) by going back a step in our earlier calculation to where we had:

(tire circumference) X (pinion teeth) = (roll-out) X (spur teeth)

And divide both sides by pinion teeth to yield:

tire circumference = (roll-out X spur teeth) / pinion teeth

tire circumference = (40mm X 96 teeth) / 26 teeth

Multiplying and dividing (this time "teeth" cancel) we find that the tire circumference to hit EXACTLY 40mm roll-out would be 147.7mm. Divide this by pi and we see we need to true the rear tires to 47mm...taking a full mm off of them. Again, a bigger result than anticipated, but that result is multiplied by over 3 X to enter the equations.

Once you're comfortable with roll-out we'll tackle ride height adjustment.

hth guys,

Scottrik

Scott-
All I can say is...Whew!

That's whatcha get when you tell D.Powell I'm a "math geek"

Scotrick you got a pm from fl.

ROTFLMAO

Then again, my bud Bambi (yes, her real name...and she's not a stripper) just says I'm pedantic. She's probably correct.

I for one can hardly wait for Scott's tutorial on body clips.

You know I've already seen this, I quit reading when he said take out a Trinity Spring Thingy and check the spring tension!

"NOW...a 25.46 tooth pinion is a bear to find, and it usually doesn't mesh worth CRAP even if you do locate one"

that has to be the line of the day

thanks for posting that though becuase i cant tell you how many times i ask someone how to gear my car at a race.

i acutally copy and pasted that into my race notes on my computer for future ref.

thanks bud

That wasn't a Trinity thingy, baby!!

And you were PLENTY interested when I was bending the mil-spec ti wire with it...

Alright, back in my cave.

What about rollout for 19T...? C2 or Money from Checkpoint..?

Tire question....I heard, the new shores for the 12th are F/R Pinks...is that true? I am still stuck with Purple Front and Pink Rear, and with the new bigger rims from Jaco, CRC, Parma....I want to try some new setting...thanks.

Pink front and double pink rear I think.