# Question on rollout and motors relationship...



## UntuchablSS (Aug 10, 2004)

Hello all, thanks in advance for your time...

I finally understand rollout and how important it is and what to do when changing tire size. But how about when changing the motor?

I'd like to know how to calculate the appropriate rollout when using any given motor at any given track. Primarily with 19 turn and stock motors. If I used a gear ratio to get a 1.60 rollout using a Trinity CO27 motor, would I use the same rollout if I were to use say a Green Machine, Binary or Revenge of the Monster, etc?

Motors are made differently and perform differently at different gear ratios and rpms, so what do I use from any given motor to help me determine the best gearing (or tire size) for any given track? Am I approaching this the right way? Can anyone out there give me a shove in the right direction?

Thanks for your help, I greatly appreciate it.
Val
UntuchablSS


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## tamiya_kid_ta04 (Dec 3, 2005)

ok say you run a 27 t motor with 23t pinion and 88t spur, so if your going to change to another 27t buy a pinion above + below, so test it on your track to see which has best performance, or if your going to lower turn motors say a 19t then id go 6t lower short and about 3t higher or same on a big track, not saying this is good but it works for me,p.s. havent burnt out a motor yet :thumbsup:


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## bsracing8 (Dec 14, 2004)

ok the co27 you are going to gear atleast 3teeth higher then any monsters that is a given!!! Are you racing track or are you just bashing around if you give me the run line of the track i can give you kind of and idea of what gears to run!!!'


Brandon


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## graeme_rsa (Jul 16, 2006)

UntuchablSS said:


> I finally understand rollout and how important it is and what to do when changing tire size. But how about when changing the motor?
> 
> If I used a gear ratio to get a 1.60 rollout using a Trinity CO27 motor, would I use the same rollout if I were to use say a Green Machine, Binary or Revenge of the Monster, etc?


I just bought a CO27 motor last week and unlike other motors I have bought, Trinity supplied some rollout specs with it!

Their rollout examples for a CO27 were 1.2 / 1.209 / 1.271 so your 1.60 would be a bit off! They also suggested that if you were running a 'Monster' stock motor than your gear ratios would be fine!

Cheers

Graeme


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## UntuchablSS (Aug 10, 2004)

Thanks for the pointer !
I've run against some of the Trinity guys locally (a friend of mine is on the team) and it seems they have the inside track when it comes to rollout. As fast a I run, they are MUCH faster. It's not batteries, or motor, it's gearing and rollout. I try to get my set up close to theirs and I'm missing something small but crucial. Thanks for the pointer with the CO27, I'm anxious to try it out.

My question about it is, whether to keep that rollout no matter the length of the tracK...what do you think?


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## zaner612 (Mar 23, 2005)

UntuchablSS said:


> My question about it is, whether to keep that rollout no matter the length of the tracK...what do you think?


Negative. As tracks are different, you'll have to change gearing. You might be able to get away with it if the tracks are relatively the same size and comprable layouts, but on different tracks you'll have to alter gearing to get out of the corner better or gain straightaway speed. Best bet is to ask the fast guys what a good target rollout is for your motor. The stuff included with the motor is recommended gearing for Trinity's test track, Jackson R/C which is a large, flowing track with a BIG straightaway. So if you're running on a little carpet track somewhere this winter, it'll be over geared. 

For on-road here's what we've found between the motors, all based on the Monster stock:
Cobalt- 3 to 4 teeth higher than Monster
Epic (Old style)- 1 tooth higher, or a spur gear change. Should just be a tick higher.


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## IndyRC_Racer (Oct 11, 2004)

I'll give an exaggerated example of how different motors require different gearing. At the Indy Velo (http://www.pdxracing.net) the run line is 1123 ft'. I have been trying to find the best gear ratio in 6-cell stock for CO27, P2K2, Epic, Monster, and Green Machine 3. This past weekend in practice I ran an Epic motor for a run of 18/4:10. With a C027 I went up 10 teeth for a similar run of 18/4:06. Both motors came off at a reasonable temp, even considering the air temp was 90+ and the track temp was 115+. 

Obviously most tracks aren't this long, but each type of stock motor does prefer different gearing to maximize power output. The gearing can vary from motor to motor. One CO27 might need more gear than another CO27. Another factor that affects gearing is how well a car gets through the turns. An effecient car in the corners can run more gear without bogging down the motor. Better batteries also have an effect on gearing. If the "team" drivers have better batteries than you, they might have more volts to push through their motors resulting in quicker lap times. Lastly, driving style/ability also affects gearing. A good driver can get through the corners better which might allow them to run different gearing. 

My best suggestion to you is to ask your friend that is on the team if he will swap you cars in practice. Try to set the cars up equally. Compare lap times of each car. Is he faster with yours? Are you faster with his? See if he will let you run his motor or batteries in your car. See if that makes a difference. Try to eliminate as many variables as possible, but be prepared to accept that the difference might be behind the wheel and not in the car.


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## graeme_rsa (Jul 16, 2006)

*Rollout Theory*

Technically if you use rollout to determine the gear ratio of your car, all other factors can be ignored (on the same track). 

I recently set up an HPI Mini to race in our Mini class using a Co27 motor and an IB3800 pack using their suggested rollout of 1.2 - 1.271 (30.16mm). The performance of the car was to say the least - pathetic! Even on the straights there was no speed. I then transferred the Co27 to my RC Lab mid motor tourer and set it up with the same rollout. The difference was incredible! The Lab went off like a rocket compared to the Mini. I'm talking about pure speed - not handling in corners etc. 

Rollout is supposed to compensate for gear ratios, tyre diameter etc. but perhaps does not take into consideration efficiency of the drive train. However comparing 2 belt drive cars - there should not be much difference in the efficiency of the drive train!

Why should the Lab be so much faster in a straight line than the Mini when everything else is similar?

Cheers

Graeme


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## CDW35 (Jan 17, 2006)

like bsracing8 said it is a given that you will gear it at least 3 teeth higher, to find the right gearing its really just test and tune, and ask what some of your fellow racers are running and what kind of gearing/rollout they are using then start out about what they are using and then go from there,
good luck and have fun


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## swtour (Oct 8, 2001)

This is a question I've asked way too many people...funny thing is..NOBODY seems to have a true answer, they all just throw out things like "With motor "A" you'd gear up 3 teeth." or something similar.

So ask yourself...

What IS rollout? If I'm not mistaken, it's the number of revolutions your motor spins for every INCH of travel. 2.45 rollout = 2.45 motor revolutions for every INCH of vehicle travel. (But here's the REAL question...  At WHAT Speed?)

To me there are a couple of information tid bits you need to know before creating a NEW rollout.

What is the motor RPM on a NEW vs. an OLD motor?
if the monster has 18,000 rpm @ 5.0 volts @ 30 amps and the
C027 has 14,500 rpm @ 5.0 volts @ 30 amps and the correct ROLLOUT for the monster was a 2.45 and that gear ratio was a 3.17 (108 x 34) the NEW gear ratio would be about a 2.55, and if the numbers are close, the new rollout should be about a 3.04 (Bigger rollout shows that the motor RPM is lower, so the rollout has to be higher)

However, without knowing the RPM change IMHO there is NO way to KNOW what the new ROLLOUT should be.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

As far as trying to create a NEW rollout, you need motor RPM (At the voltage and amperage you'd be running on the highest speed part of your track) (Stock Oval I use 25 amps, 19t I use 30-35 amps and Mod I use 35 amps, but since we only run 4 cell, I only use 5.0 volts)

You also need to know the SPEED you are trying to achieve.

35 MPH is usually good for a short track

35 mph = 51.33 Feet per second. and/or 3080 Feet per minute.

35 mph = 615.96 Inch per second

motor "A" turning 14,500 RPM (At race Amp and Voltage) would turn 241.66 Revolutions Per Second.

motor "B" turning 18000 RPM (At race Amp and Voltage) would turn 300.00 Revolutions Per Second.

At this speed and rpi, the rollout would be (615.96 / 241.66 ) = 2.548 Rollout for motor "A"

At this speed and rpi, the rollout would be (615.96 / 300.00 ) = 2.053 Rollout for motor "B"

(Now I know why people just try to SHARE rollouts and run similar motors...LOL )


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## erock1331 (Apr 1, 2002)

I have found its really tough to compare different types of motors and corresponding rollouts.
Its alot easier to find a rollout if you focus on one motor then the control number theory seems to work.

Here is a post by Big Jim Greenmeyer that discusses rollout and gearing in depth. The contol number makes sense mathmatically but in my opinion doesnt work when you switch to different motors because of how each motor handles heat, torque etc
For example I used this method once after finding a benchmark with a Monster stock then tried to relate the formula to gearing a P2K in a pan car. The RPM called for about 60 points higher or 10 teeth than higher than the Monster stock. The car was a rocket ship for 2 minutes but felt like it was going to dump the last minute. Motor was really hot after the 4 minute run.

*Big Jim's post*
*Gearing Formula*

*As for gearing you need to have a fast stock motor first (if you are doing stocks at the time) on a certain track to calculate gearing "for sure". *

*For an example. Say, you have a fast Monster at your local track that is really fast or is the fastest stock motor you have. You have experimented with your gearing so you are reasonably sure you have it geared properly when you run it. *

*Now, dyno that motor remembering what your overall gear ratio was when you ran it (figure in the tranny ratio if so equipped but leave out rollout). It's really easier to leave out the tranny ratio if you are going to use the same car all the time. Just use your pinion and spur ratio.*

*Next, pick a load figure in amps that you believe is around the average load on the motor during a 5 minute (or however many) race. Usually, 20A to 28A. I use 20A. Most racers pick a number too high but as long as you always use the same voltage and amp reading, it doesn't matter much.*

*Say at 20A that Monster is turning 26105 RPM (using your numbers). Now divide this RPM by your over-all gear ratio, say 7.8 to 1. Rounded off, you get a number of 3347. This is your axle speed at that load level and will be your "control number" for any stock motor at that track.*

*Now you have a P2K that you dyno and you want to know how to gear it to get the same speed. If the motor is about the same watts at the same load, this formula is almost right on. If the P2K is down on power a bit, you will have to adjust your gear but it will still put you in the ballpark.*

*The P2K's RPM at a 20A load and the same voltage is 22,125. Take your "control number" and divide it into the RPM of the new motor at that same 20A load. 22,125/3347 = 6.61(rounded off). This is the gear ratio you should use on your P2K to equal the speed of your Monster in that car on that track.*

*If your pinion and spur were 29/88 (64 pitch) in a TC3 you would be geared 7.8 to 1. So to gear your P2K using the same spur in the same car, you'd need a 33 tooth pinion to have the P2K geared properly based on the dyno input from your Monster that you know was fast.*

*That's a 4 tooth difference in pinion size. How many racers would think to start gearing the P2K by going up 4 teeth on the pinion?*

*This part is important. The gearing formula is almost dead-on "IF" your input figures from the Monster was accurate, you are racing on the same track and you are using a stock motor as your "new" motor. You can't use a stock motor formula for mods or 19t's. Each class must have it's own "fast motor" to use to get your "control number".*

*And if you race on a different track, you have to start all over again. The #1 thing you have to have is a good, fast motor FIRST because every motor from then on in that class at that track will be based on that fast motor's dyno readings.*

*Also, you don't have to worry much about tire size effecting your gear ratio. Well, not a smuch as most racers think. It takes a .070" difference in rear tire diameter to equal one tooth on a 64 pitch pinion. It takes a whopping .100" to equal one 48 pitch pinion tooth. If you are running hard rubber tires (we used to call them "caps" in oval) you can forget about figuring rollout into your ratio because your tire diameter won't change near that much from race to race.*

*Thge laid out formula is: *
*RPM (1)/ gear ratio = control number.*
*RPM (2)/ control number = new gear ratio*

*...or if you want to figure in tranny ratio and tire size:*
*RPM (1)/ gear ratio (pinion & spur) X TR (tranny ratio)/ tire diameter = control number.*

*RPM (2)/ control number = gear ratio (P & S) X TR/ tire diameter.*


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## UntuchablSS (Aug 10, 2004)

I am REALLY Happy to have started this thread !! 

See? the "asking" what everyone else is using approach is fine, but I don't necessarily agree to that. I do in that you should always try helping one another but you got to admit, will a team driver necessarily give you the answer you really need? Or one that you'd expect to hear? As a racer I'd like to figure it out myself and with your help I think I'm on the right path. I'm of the mentality that I'll try your way and someone else's way and I'll draw my own conclusions. 
Roll out is a very important part of racing, no matter if it's on or off road. Keep the theory coming, I'm absorbing it all !!!!!

Thanks to all that have "chimed in" !!

Val


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## Guest (Aug 6, 2006)

On my local track (oval stock) I found that by concentrating on using one style of motor (Epic Roar Stock) it made it somewhat easy to nail a good rollout. Then when using one's that had different power or amp draw or even a different style motor the gearing was changed based on either amp draw or dyno results and how it ran on the track.

Keeping notes also help in keeping track of changes that might be occurring too.


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## UntuchablSS (Aug 10, 2004)

Our track consistantly changes the layout, however the length of the track itself remains the same. Sometimes the track is tighter (more turns) than the previous layout, sometimes not. If I manage to nail the set up in the original configuration, and the following week it is changed to another layout, does rollout remain the same?


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## erock1331 (Apr 1, 2002)

UntuchablSS said:


> Our track consistantly changes the layout, however the length of the track itself remains the same. Sometimes the track is tighter (more turns) than the previous layout, sometimes not. If I manage to nail the set up in the original configuration, and the following week it is changed to another layout, does rollout remain the same?


you are talking on-road I take it?

I am an oval guy but I would assume the tighter the turns and the more you are letting off you would need a lower gear to get you up off the turns quicker.


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## CDW35 (Jan 17, 2006)

erock1331 said:


> you are talking on-road I take it?
> 
> I am an oval guy but I would assume the tighter the turns and the more you are letting off you would need a lower gear to get you up off the turns quicker.


that is right if the track is long and doesnt have that many turns, you will want a bigger gear then you would for a track with more turns. and yes if you change the gear and yes it will make the rollout different,


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## graeme_rsa (Jul 16, 2006)

swtour said:


> What IS rollout? If I'm not mistaken, it's the number of revolutions your motor spins for every INCH of travel. 2.45 rollout = 2.45 motor revolutions for every INCH of vehicle travel.


Other way round - rollout is the distance your wheels travel for every revolution of your motor!

Speed doesn't come into the equation! It's all down to gearing and tyre diameter! If the rollout was set to 30mm and if you were able to get the motor to turn 100 revolutions/per minute it would travel 100 X 30mm = 300mm=3m per minute. If you got the motor to turn 30 000 revolutions the distance travelled would be 30 000 X 30mm = 900m/minute

Each motor has a particular rollout. This is the starting point! From there you go up or down a pinion to determine the best for a particular track (or ask the guys at the track running the same motor!). 

The question is, how do I find out the rollout for a particular motor?

Ask! Read the specs that come with the motor. Trial and error?

Technically the rollout figure is supposed to be the gear ratio/tyre size that the motor will operate at most efficiently. From articles that I've read, the Stock 27T motor is the most difficult to work out as different motors have different properties despite being the same wind. Rollout for 27T Stock motors are anywhere from 30 to 34mm depending on whether they are wound for torque or revs.

Where rollout is particulary important is when you run foams and don't have a bank account that allows you to run a new set every heat! Get the calipers out and factor in the wear on the tyre - change the pinion/spur!

An example

Assume I have a car that performs best at a rollout of 29.78mm and I start off with tyres 63mm diameter and a gear ratio of 40:122. OK I win my heat  I then check my tyre diameter and find that has worn down to 61mm. I enter the figures into the laptop and find that the curent rollout is 28.83mm. For the next race I should change to a 41T pinion which will give me a rollout of 29.55mm. After doing this for all the heats I find that my tyre diameter has worn down to 58mm. For the final I must go to a 44/124 which will give me a rollout of 29.67mm which is as close as I can get to my optimal rollout of 29.78mm. 

OK I win the final :tongue: 

Cheers

Graeme


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## swtour (Oct 8, 2001)

sorry, my BRAIN and the METRIC system aren't on the same planet. Although I'm a mechanic and have to use tools that are in mm's that is where my brain STOPS...everything else is in Feet, MILES, or INCHES.

I'm sure I had the number backwards...but I know I heard someone once say it was known as rpi, which has never made sense to me.

Now, w/o using rollout if my gear ratio is 4:1 the axle would spin 1/4th turn per motor revolution.

my tire is 2.5" diameter (2.5 x 3.1416 = 7.854 inches) so then would the rollout be a 1.9635? (7.854 x .25)

Let's use the rollout calcs.

29th pinion/ 116 spur x 2.5 x 3.1416 = 1.9635

Cool. That makes a little more sense.

But, I still would have to disagree about speed.

There has to be a way to use motor RPM and SPEED as the desired factors to determine your initial rollout. (I've never used ROLLOUT to determine gear ratios, I've always found a gear I liked and refined from there...)

I still think if you know you want to pull 30 mph on a mid length straight away, you should be able to look at 

30 mph as 2640 ft. per minute
2.5 inch tire = .6545 ft.
2640 ft. / .6545 ft. per tire rev. = 4033.6 tire revs per minute.

motor revs per minute (at race amp load) 16,100

16,100 / 4,033.6 = 3.991 ratio

But you would need to be able to know what your RPM is at the needed amp load.


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## graeme_rsa (Jul 16, 2006)

swtour said:


> sorry, my BRAIN and the METRIC system aren't on the same planet. Although I'm a mechanic and have to use tools that are in mm's that is where my brain STOPS...everything else is in Feet, MILES, or INCHES.
> 
> I still think if you know you want to pull 30 mph on a mid length straight away, you should be able to look at
> 
> ...


THINK METRIC - it's easier! I remember my days at school when we had to work out problems in inches / feet / yards / miles (12 / 3 / 1760 / 1)! Metric - divide or multiply by 10!

I don't know how one calculates the recomended rollout for a particular motor. Is it math or is it trial and error? 

Speed doesn't play any part as rollout is simply a calculation based on 1 motor rev X pinion/spur ratio X internal gear ratio X tyre diameter with Pi chucked in somewhere to give the circumference of the tyre. 

In your example you have a final drive ratio of 3.9:1 - this would be impossible to achieve with a motor only capable to turning 16100 revs under track conditions as you would burn out the motor! Been there - done that! Perhaps on a LONG drag strip that 16 thou motor might reach 30 mph!

As mentioned, rollout is supposed to be the most efficient performance of a particular motor and is a general guideline as where to start your gear ratios - modifying if required for your track. Once you have determined the optimal rollout for your track it then allows you to compensate for tyre wear with foam tyres (which can go from 65mm[2.55"] to 62mm[2.44"] in one heat which might require me to go up a pinion size. With rubber tyres rollout is not really that important as tyre wear from new to insert showing is about 2mm [0.08"] total!

Cheers

Graeme


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## Big Papa (Aug 14, 2006)

*gear finder*

Hey everyone I am looking for a download that was called Gear Finder, I had it in my Computer but since it crashed I can not seem to find nor remeber where I had downloaded it before. Anyone know??
Big Papa


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## Jim Smith (Oct 1, 2001)

I have a copy, email me and I will send it your way. [email protected]

Or download it yourself at http://jimsmith.iwarp.com/RCDownLoads.html (Bottom of page).

Anyone know anything about this program. I'd like to give credit on the download page.


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