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Old 07-20-2006, 06:20 PM   #1
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Default How to figure rollout for a speciffic track?

Ok I have the equation for rollout and you can ask around and get 100 different opinions about how to gear a particular motor for a particular track.

I also understand about gearing for the more important part of the track. For example if you are at a track with a super long straight, you need to gear for it, or if it is mostly short straights, then you can adjust.

My question is that how does that final number you calculate relate to the amount of straightaway you have? Or does it?

I know what it is you calculate, you solve for how far the car will roll with one revolution of the motor, great. What does that tell me, and how to I effectively use this information?

I know ask someone who runs the track, but I would like to know how to figure it out myself.

This does not have to apply to foams either. Same question with rubber tires.

I am not asking about a particular track or motor or car, none of that should matter at this time. Just in general, how do you measure what gear to throw on a car when you show up at a new track?

Last edited by ahsikes; 07-20-2006 at 07:55 PM.
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Old 07-20-2006, 07:10 PM   #2
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First off, you should have a general idea about where the motor likes to be geared, ie its powerband. From there you have to decide whether to gear more for the straightaway or infield, ie where are you going to cut the most time off your lap? A good starting point should be to gear so that your motor accelerates and tops out near the end of the straightaway. Check your laptimes to see how it changes if you gear up or down.
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Old 07-20-2006, 07:37 PM   #3
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Hey I appreciate that.
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Old 07-20-2006, 07:40 PM   #4
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it really depends on the motor.i always start with a somewhat standard gearing for say 19 turn,mod or stock that i would normally run.say for mod on asphalt i would start at say 9.0 FDR with a 7x1.from there i know that the worst thing for a motor like that is to over rev it so i listen to it.if it sounds like its spooling up too quick i will go up a tooth or keep changing until it seems safe to not throw a wind.now in 19 turn,i start at about a 6.0 FDR.19's can be tricky.some like more gear,some dont.it usually takes me about 2-3 packs to figure them out.this is the same with stock.if they spool up too quick(under geared)they will run hot.this can lead you to believe the motor is overgeared when gearing by temp.this is why i go by performance.the motor needs to have enough load to keep it from getting hot.when you find this point the motor will be at its ultimate performance for the full race.so as you can see,start with a somewhat standard gearing you use and adjust from there.its all trial and error.especially with how inconsistant alot of these motors are.
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Old 07-20-2006, 07:44 PM   #5
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Thanks to both of you. I do those very things and it does work, I was just hoping that there was that calculation that would come into play. Guess if it were easy, everyone would do it.

Guess just keep on Guessing.
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Old 07-20-2006, 07:44 PM   #6
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Try going to This site it will calculate it for you, just type in the type of car and the rest of the info needed.
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Old 07-20-2006, 07:47 PM   #7
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yeah,there really is no calculation.you can calculate your roll out or FDR but that doesnt mean you are running the right one trial and error buddy.the standards get you in the ballpark though.
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Old 07-20-2006, 07:53 PM   #8
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Everyone seems so quick to give charts. nothing directed at you slot. If the answers were in a chart, I would already have it.

The question was about gearing upon going to a new track. What reference do you use to judge what gear to thow on?

I think Jason was the closest to what I was looking for, with the exception of not having an equation to determine a desired ratio based on the straightaway size.

I realize all motors are different, even the same brands, but I was just hoping something like this existed.

I do exactly what jason said and listen to the motor as it goes down the straight, and I can always judge once on the track with other drivers, I just wanted a way to determine this quicker. and like Jason said, maybe that is why this question has not been answered, cause there is not a way to do it exact. Too many variables.
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Old 07-20-2006, 09:28 PM   #9
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Rollout calculations are used as a reference. Once you determine where your motor is happy, you will be able to calculate that particular rollout and adjust it to different tracks. Rollout is mostly used for foam tires where the diameters are constantly changing. If you had a 1.80 rear tire diameter and you found a gear ratio that you felt good with, you could then calculate the rollout and adjust your gearing to maintain that rollout as the tire diameters change. If you just run rubber tires the diameters will be constant, so Final Drive Ratio is used. Its kind of the same equation without figuring in the circumference of the tire. Rollout is the distance the car will travel per motor revolution, and FDR is the number of times the motor will turn to make one revolution of the tire. The formula for rollout is (tire diameter x 3.14 x pinion gear)/spur. FDR is (spur/pinion) x gearbox ratio or pulley ratio.
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Old 07-20-2006, 09:35 PM   #10
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By the way, I realize this is not a direct answer to your question. If you keep a logbook of your rollouts or FDR's in relation to a given track, you will eventually develop a feel for what will work at other tracks. You are correct in assuming that there is no formula to determine the optimum gearing for a given motor on every track. Ask the fast guys where they are gearing, start there, then make your own adjustments to fine tune. When you figure it out, write it down. You'll soon be able to estimate a starting point pretty well.
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Old 07-21-2006, 01:19 AM   #11
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I think the problem is that he is trying to ask a question that doesn't have a formulaic answer. It's like asking at what point should i turn the wheel with a one-way opposed to a spool, etc. It's just something you have to try. It's not like you can show up to a track, measure the length of the straightaway, how many turns there are in the infield, measure the total radial distance of the turns, curvature of each section, etc etc and then plug it all into some formula and voila, get some magic number that should be your optimal gear ratio.
The most you can calculate is rollout so that you can figure out where your motor performs best and then be able to keep that same setting regardless of tire size or track size.
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Old 07-21-2006, 05:06 AM   #12
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I just take a good look at the track and other cars in practice on a track,then visualise if they are gaining more time on the straights or on the corners,then take things from there. Its pretty crude I know,but it works ok for me.
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Old 07-21-2006, 05:06 AM   #13
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try this....

http://home.comcast.net/~k.bojarski/Roll_out.xls

its a spreadsheet I made a couple of years ago...it shows you the calculation you need to find your roll-out as well as what pinion is needed (based on spur, IR, etc) to make a certain roll-out...

I also put a section in there that's a "handy-dandy pitbox reference sheet"

Hope it helps...any questions about the spreadsheet, shoot me an email...

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Old 07-21-2006, 06:27 AM   #14
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thanks for the input.

Guess no substitue for practice sessions.
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Old 07-21-2006, 05:21 PM   #15
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Please stay in school, I have guys at my track deep into their thirtys that dont use roll out because the math is too hard
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