
12242008, 01:37 PM

#1

Tech Master
Join Date: Nov 2008
Location: Latrobe, PA
Posts: 1,086

Can someone explain
What rollout is? Im confused.
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l MOMO USA l OZ Racing l Willwood Brakes  Dunlop Tires 
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12242008, 02:06 PM

#2

Tech Master
Join Date: Feb 2008
Location: Alberta,Canada
Posts: 1,293

From the xray site...
Quote:
Gearing  All Models
Author: djnutz
Explanation
Gearing is one of toughest things to nail down. There are a lot of different types of RC cars, but they all have one thing in common and that is gearing. Your first time racing usually goes like this: You ask the person next to you what spur and pinion gears they are using; you try the same setup and then find you are more than a lap slower. Then you go back to your pit and try going up a tooth on your pinion. Now your car is faster, but you can’t finish the run because your pack went dead, or even worse, you cooked the motor. Finally, you get discouraged because you can’t seem to find the perfect balance between speed and runtime.
The problem is that you are asking for the wrong information. When you go to any track for the first time, you should ask around about each driver's rollout and not their gearing. If they can't or won't tell you, move on to the next person and keep asking. Once you find a driver in the same class you are running that tells you his rollout, you will then be able to match your cars rollout to theirs as closely as possible for a good base gearing setup to start with. Once you have done this, the only factors that will affect your lap times, outside of chassis tuning, are driver skill, power output of the motor and obviously changing the spur or pinion gears to a different size.
What is rollout? Rollout is the distance the car will travel in one revolution of the motor shaft. Pinion and spur gears are used to generate the rollout desired. If you take a shaft car and a belt car with the same exact motor and rollout, they will have the same theoretical top speed. The higher the rollout number is, the slower you will be off the line (less acceleration) and the faster the top speed will be. The lower the rollout value, the quicker you will be off the line, but with a lower top speed. Now, let’s get down to the business of how to calculate rollout and setup your gearing accordingly. I am using numbers from my car, an original T1 with modifications, for all of the following examples. What can I say, I love it and it’s still in one piece!
NOTE: Keep in mind that all of the numbered values in this example will not necessarily yield the same results for your model of car!
Calculating Rollout
1. Determine the Drive Train Ratio (DTR)  The ratio of all the internal gears from the transmission, including differentials and pulleys is known as the drive train ratio. The drive train ratio is usually different for each model of car. Your owner’s manual should provide this information, but keep in mind that some manufacturers use the words "transmission reduction" or "internal reduction" to indicate the DTR. The DTR in most cases cannot be changed, unless you are able to change the number of teeth on the pulleys and/or gears. My original T1 has the Xray low ratio pulley set which provides a DTR of 1.77.
2. Determine the Primary Drive Ratio (PDR)  The ratio between the pinion and spur gear is known as the primary drive ratio. This number is commonly rounded up to the nearest thousandth after the decimal point.
Spur / Pinion = PDR
My cars PDR: 93 / 24 = 3.875
3. Determine the Final Drive Ratio (FDR)  The ratio between the DTR and the PDR is known as the final drive ratio. This number is commonly rounded up to the nearest thousandth after the decimal point.
PDR x DTR = FDR
My cars FDR: 3.875 x 1.77 = 6.859
4. Determine the Rollout  Rollout is not affected by the motor, batteries, or electric components you are using. Rollout simply defines how all of the gears, belts/shafts and tires work together to make the car accelerate and reach top speed. Rollout is affected by your tires diameter, but don’t ever let anyone tell you that rollout only matters if you run foam tires. Rollout is calculated using tire diameter and therefore the diameter of both foam and rubber tires make a difference since both wear down. However, the diameter is more important if you run foam tires since they can be used anywhere from 64mm to 55mm. And as they wear and get smaller, your rollout value will change a lot more. Rubber tires do not wear down near as fast or as much so the change will be much less. The circumference is commonly rounded up to the nearest hundredth after the decimal point. The rollout is commonly rounded up to the nearest thousandth after the decimal point for American standard and to the nearest tenth after the decimal point for metric. My car has Take Off CS32 rubber tires with a diameter of 2.4375 inches (62 millimeters, although it’s common to find that other drivers use 63mm for a rubber tire diameter).
Tire Diameter X 3.14 (value of PI) = Tire Circumference
Tire Circumference / FDR = Rollout
My cars tire circumference: 2.4375 x 3.14 = 7.65 inches (or 194.68 millimeters)
My cars standard rollout: 7.65 / 6.859 = 1.115
* The car travels 1.115 inches for each revolution of the motor shaft
Or metric rollout: 194.68 / 6.859 = 28.4
* The car travels 28.4 millimeters for each revolution of the motor shaft
5. Determine the Top Speed  Theoretical top speed can be obtained by combining the maximum dyno’d RPM with the rollout number. However, this does not take into account the forces that will slow your car down, including aerodynamics, drive train efficiency and motor power. Most brand new motors have their maximum RPM on a label attached to the motor can. You can also have your motor dyno’d to find out what the maximum RPM is.
( ( max RPM / FDR ) * Tire Diameter ) / 336.135 = Estimated Top Speed in Miles Per Hour (MPH)
NOTE: The number 336.135 is used to convert the results to MPH.
Completed Testing
Using the same battery pack for all testing, I ran my car with three different gear combinations, while maintaining as close a ratio as possible. For each combination I made three runs to get an average runtime and top speed. The only discernable difference was that the change in spur changed the motor height in the chassis. The taller spur placed the motor lower in the chassis and provided faster cornering speeds.
Conclusion
Once you get the hang of the equations you will notice that some of the gear combinations will yield similar numbers. For example, some of my current gearing combinations below have really close rollout figures.
90 spur and 23 pinion results in a rollout of 1.106
93 spur and 24 pinion results in a rollout of 1.116
99 spur and 26 pinion results in a rollout of 1.136
Changing either the pinion or the spur to maintain a consistent ratio will not affect your overall speed or runtime. Increasing the number of teeth on the pinion by one is pretty close to increasing the number of teeth on the spur by 3 or 4. This knowledge is very useful with all of the Xray chassis previous to the FK05. This is due to the fact that the size of the spur and pinion dictate the height of the motor in the chassis. The lower the motor, the lower the center of gravity in the rear of the car.
I have a theory that applying the same amount of torque over a larger area should make for a more efficient use of motor torque. This theory would state that using a 93t spur and 26t pinion (1.208 rollout) would be better than using a 90t spur and 25t pinion (1.201 rollout). This is due to the fact that there is more surface contact between the gears where they are meshed, which will spread out your motor's applied torque across more of the teeth surface area.
There really isn’t anyone who can tell you the best gearing for any specific track since each driver and car setup differs. The best advice I can give is to ask the drivers that are turning the quickest laps what their rollout is. Then match your rollout to theirs and go for it.
If you find that your motor is too hot after the first ten or so laps first check that there are no parts binding in the drive train. You can do this by taking the pinion off the motor shaft and giving the spur gear a spin. If they don't free roll for a few seconds or it seems a little gritty then it is time for a good bearing, axle coupling and/or diff cleaning. All of these can trap dust and dirt or become damaged from a crash. I would also give the motor a good once over cleaning. If neither of these helps, drop one or two teeth on your pinion gear, and try to find the best driving line around the track. Here is an awesome site that deals with motor maintenance. The rule of thumb is CLEAN IS FAST.
http://www.motortuningsecrets.com
If your motor is less than 140 degrees Fahrenheit after at least a four minute run, you could try to go up one tooth on the pinion.
Here are two ways to get quick FDR and rollout numbers.
1. Online Gearing Website
http://www.gearchart.com
2. Gearing Spread Sheet
To make things easier, and since I have a full pinion set from 1835t and three different spur sizes, I created a Microsoft Excel spreadsheet to perform the calculations. Once I enter the pinion and spur gear values, Excel does the rest. Row one is my header row with the column labels in this order:
Spur(column A), Pinion(column B), Tire Diameter(column C), Final Drive Ratio (column D), Rollout (column E), Estimated Top Speed (column F)
Here are the equations that you can cut and paste directly to your own Excel spreadsheet if you like. I have listed the equation and then what each value stands for.
* Final drive ratio equation for all rows in column D and starting on row 2:
=A2/B2*1.77
Note: 1.77 is the internal reduction (DTR) of my cars transmission. Replace this number with your own models DTR.
* Rollout equation for all rows in column E and starting on row 2:
=(C2*3.14)/D2
Note: The first part of the equation determines the tire circumference
* Estimated Top Speed equation for all rows in column F and starting on row 2:
=((21529/D2)*C2)/336.135
Note: 21529 is an example of the maximum RPM for a good stock motor. Replace this number with your own motors maximum RPM.
Good luck at your next race!




12242008, 02:08 PM

#3

AEReedy
Join Date: May 2008
Posts: 780

Rollout is the distance the drive tire will move for 1 rotation of the motor.
This is a more accurate way of discussing motor gearing as it includes the tire diameter in the equation. It can be stated in inches or millimeters depending on which is used to measure the tire diameter. The equation goes like this.
tire dia. x pi (3.1415) / spur x pinion / transmission ratio(if needed)



12242008, 02:09 PM

#4

Tech Regular
Join Date: Apr 2008
Location: Jonesboro, Arkansas
Posts: 482

Rollout is how far the car will move with 1 rotation of the motor shaft. Here is the formula: pi * tire diameter/ gear ratio=rollout. So here is an example. your PDR(primary driver ratio) is 1.2. your FDR (final drive ratio) is 2.12. Then multiply those and you get 2.55. Then say your tire diameter is 60mm. Multiply that by pi and you get 188.5. So divide 188.5 by 2.55. That gives you 73.9. This is your rollout so the car will move 73.9 mm with one rotation of the motor.
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12242008, 02:35 PM

#5

Tech Master
Join Date: Nov 2008
Location: Latrobe, PA
Posts: 1,086

Thank guys, that helps alot
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Mark Ament
l MOMO USA l OZ Racing l Willwood Brakes  Dunlop Tires 
The Raceway at Beaver: Forever In Blue Jeans



12242008, 06:02 PM

#6

Tech Adept
Join Date: Dec 2008
Posts: 182

so does that mean higher the rollout = higher speeds, lower the roll out faster accelleration but slower high end speed?
or higher the rollout the harder the motor needs to work so motor has a chance of getting hotter compared to a low rollout??
hope i made sense
thanks in advance.



12242008, 08:34 PM

#7

Regional Moderator
Join Date: Jul 2006
Posts: 2,661

Quote:
Originally Posted by Jin
so does that mean higher the rollout = higher speeds, lower the roll out faster accelleration but slower high end speed?
or higher the rollout the harder the motor needs to work so motor has a chance of getting hotter compared to a low rollout??
hope i made sense
thanks in advance.

Generally, yes to all the above, but it really is motor dependent. Some brushless motors seem to defy the odds and heat up with lower rollout or run cooler with a higher rollout. Also, adding timing will generate higher RPM, less torque, but more heat as well. Lower motor timing will allow the motor to run cooler, run less RPM, and produce more torque.
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Last edited by CarbonJoe; 12242008 at 09:57 PM.



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