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Spur to Pinion ratio for Reedy S-Plus Spec competition motor

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Spur to Pinion ratio for Reedy S-Plus Spec competition motor

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Old 07-02-2019, 08:48 PM
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Default Spur to Pinion ratio for Reedy S-Plus Spec competition motor

What is the best spur to pinion gear ratio to get the best top speed out of a Reedy 17.5 S-Plus Spec competition Motor? I race on road touring and I'm looking to get max speed out of that motor since I'm currently running a heavier chassis than the competition.
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Old 07-02-2019, 11:53 PM
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Originally Posted by JBMotorsports29 View Post
What is the best spur to pinion gear ratio to get the best top speed out of a Reedy 17.5 S-Plus Spec competition Motor? I race on road touring and I'm looking to get max speed out of that motor since I'm currently running a heavier chassis than the competition.
We'd need to know the internal ratio of your chassis to be able to answer that. Also, a heavier chassis will accelerate slower, so gearing for top speed will slow you down in the corners even further.
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Old 07-03-2019, 01:04 PM
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Originally Posted by gigaplex View Post
We'd need to know the internal ratio of your chassis to be able to answer that. Also, a heavier chassis will accelerate slower, so gearing for top speed will slow you down in the corners even further.
Correction : Heavier chassis and heavier drivetrain will accelerate slower.....
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Old 07-03-2019, 01:45 PM
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There’s no one simple answer. What works on pavement will be different from carpet. Track layout will also be a factor. Tight turns might require heavy braking and acceleration. Where a large open track you can stay on throttle. And only use limited brakes.

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Old 07-03-2019, 07:48 PM
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These are the dyno values for inertia flywheel only. Not Chassis dyno so friction efficiencies and inertia are not here to help you, if you set the timing to 40 degrees on the end bell and accelerate until the car no longer accelerates take that end rpm and replace the one in the curve with your number. Peak hp is about half of that rpm by looking at the curve. Set your pinion and spur to the ratio (FDR) and roll out to keep the motor at that peak. If your track is 1000 feet and it takes 10 seconds you want that peak to result in 100 feet per second. This is peak power not peak efficiency.

if it gets to hot switch to the second curve (efficiency). Take you max rpm and substitute it in the curve. Set the fdr to the 2/3 of this curve rpm. Again 1000 feet in 10 seconds is 100 feet per second. Somewhere in between the two FDRs (efficiency and power) will be peak power that doesnt overheat.
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Old 07-04-2019, 05:59 AM
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Originally Posted by JBMotorsports29 View Post
What is the best spur to pinion gear ratio to get the best top speed out of a Reedy 17.5 S-Plus Spec competition Motor? I race on road touring and I'm looking to get max speed out of that motor since I'm currently running a heavier chassis than the competition.
This question has been asked so many times before and there is no magic answer, period. If you are racing, you must use Final Drive Ratio (FDR) to obtain optimum settings and there are multiple other variables to consider. Are you running indoors or out, on carpet or asphalt, with rubber or foam tires, what chassis are you running, what motor timing are you using, what ESC boost (or perhaps blinky mode), etc....?

When I run a 4wd sedan indoors on a carpet track with Sorex 28R rubber tires I like to start with an FDR around 3.8 and after a few runs adjust up/down from there. The 17.5 class has used blinky ESC settings for some time now so there is no software boost/turbo on the speedo, but you can still play with motor timing. I am not familiar with that Reedy motor but it sounds like it might have fixed timing which means one less thing to fiddle with.

This should leave you with only spur and pinion to change to obtain a favourable FDR. I prefer 64 pitch gearing indoors as the adjustments are finer and I can dial in my car better. 48 pitch still works though so don't worry.

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Old 07-04-2019, 06:53 AM
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Originally Posted by JBMotorsports29 View Post
What is the best spur to pinion gear ratio to get the best top speed out of a Reedy 17.5 S-Plus Spec competition Motor? I race on road touring and I'm looking to get max speed out of that motor since I'm currently running a heavier chassis than the competition.
for me i use the largest spur that i can to achieve the FDR that i need for the particular motor
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Old 07-04-2019, 06:55 AM
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Originally Posted by Marcos.J View Post
for me i use the largest spur that i can to achieve the FDR that i need for the particular motor
Shift that motor weight up front eh pal The Axon spurs are the lightest incase you aren't already using them
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Old 07-07-2019, 09:59 AM
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Originally Posted by SteveM View Post
This question has been asked so many times before and there is no magic answer, period. If you are racing, you must use Final Drive Ratio (FDR) to obtain optimum settings and there are multiple other variables to consider. Are you running indoors or out, on carpet or asphalt, with rubber or foam tires, what chassis are you running, what motor timing are you using, what ESC boost (or perhaps blinky mode), etc....?

When I run a 4wd sedan indoors on a carpet track with Sorex 28R rubber tires I like to start with an FDR around 3.8 and after a few runs adjust up/down from there. The 17.5 class has used blinky ESC settings for some time now so there is no software boost/turbo on the speedo, but you can still play with motor timing. I am not familiar with that Reedy motor but it sounds like it might have fixed timing which means one less thing to fiddle with.

This should leave you with only spur and pinion to change to obtain a favourable FDR. I prefer 64 pitch gearing indoors as the adjustments are finer and I can dial in my car better. 48 pitch still works though so don't worry.

I refitted a redcat Drift chassis. Shaved out the inside of the motor mount to fit a brushless motor and shaved out excess parted of the chassis to make it lighter. It is still heavier because it is a plastic chassis as opposed to the graphite kits i'm competing against. I don't expect this car to match their speed, but I am looking to get the most out of it. I am building another car on an Xpress XQ1 chassis which is burning up the track. I would make this my backup car if I wreck the Xpress car and can't have it ready by the next race. I just do not want a mountainous falloff in speed between the two. I do not believe that I am getting the most out of my motor. I am currently running gears at 32:88.
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Old 07-07-2019, 10:06 AM
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Originally Posted by DaSilva3525 View Post
Shift that motor weight up front eh pal The Axon spurs are the lightest incase you aren't already using them
My kit already has the Motor in the front. The rear is lighter with the chassis adjustment I made. Just need to get the max out of my motor.
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Old 07-07-2019, 11:58 PM
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Originally Posted by JBMotorsports29 View Post
What is the best spur to pinion gear ratio to get the best top speed out of a Reedy 17.5 S-Plus Spec competition Motor? I race on road touring and I'm looking to get max speed out of that motor since I'm currently running a heavier chassis than the competition.
Yes, it feels good to get the most out of a motor! As has been mentioned, to get the best advice for gearing, you will want to let us know what the internal "gear" ratio of your car is and the size and type of track you are racing on.

To try and figure out your internal ratio, I looked up the manual for a Redcat drift car. The manual says that the center pulley has 17 teeth, but it doesn't say how many teeth the differential pulleys have. To find out your internal ratio, you will want to count the teeth on your differential pulleys (The teeth that connect to either your front or rear belt). Then you divide the number of teeth on you differential pulley by the number of teeth on your center pulley. For example, if you differential pulleys have 37 teeth, you would divide 37 by 17 for an internal ratio of 2.18. If your differential pulleys have 38 teeth, it would be 38/17 = 2.34 internal ratio.

After you know your internal ratio, you can ask people what final drive ratios (FDR) they run with an S-Plus 17.5. Then you can find spur and pinion combinations to achieve that FDR. For example a fast guy at my track who runs a Reedy 17.5 S-Plus uses a 4.5 FDR on our asphalt track with a 100ft straight. If your internal was 2.18, you could run a 48 pitch 72 tooth spur with a 35 tooth pinion to achieve an FDR of 4.48. The math would be 72/35*2.18 = 4.48

In addition to asking questions here on rctech, I'd encourage you to get to know some fellow racers at your track and find one who is helpful. They can be good resources to help you figure out your gearing.

If you have the budget, it would be very tempting to buy one of the sport versions of the Xpress. They are very reasonably priced, and you can have a whole car just for spares to keep your other Express going. May be less hassle than trying to keep two different brands going. Also, handling is more important than overall speed for getting fast laps, and a drift car is likely going to handle a lot different from a current race car like the Xpress.

Good luck to you.
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Last edited by bshookup; 07-08-2019 at 12:20 AM.
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Old 07-08-2019, 08:02 PM
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Originally Posted by bshookup View Post
Yes, it feels good to get the most out of a motor! As has been mentioned, to get the best advice for gearing, you will want to let us know what the internal "gear" ratio of your car is and the size and type of track you are racing on.

To try and figure out your internal ratio, I looked up the manual for a Redcat drift car. The manual says that the center pulley has 17 teeth, but it doesn't say how many teeth the differential pulleys have. To find out your internal ratio, you will want to count the teeth on your differential pulleys (The teeth that connect to either your front or rear belt). Then you divide the number of teeth on you differential pulley by the number of teeth on your center pulley. For example, if you differential pulleys have 37 teeth, you would divide 37 by 17 for an internal ratio of 2.18. If your differential pulleys have 38 teeth, it would be 38/17 = 2.34 internal ratio.

After you know your internal ratio, you can ask people what final drive ratios (FDR) they run with an S-Plus 17.5. Then you can find spur and pinion combinations to achieve that FDR. For example a fast guy at my track who runs a Reedy 17.5 S-Plus uses a 4.5 FDR on our asphalt track with a 100ft straight. If your internal was 2.18, you could run a 48 pitch 72 tooth spur with a 35 tooth pinion to achieve an FDR of 4.48. The math would be 72/35*2.18 = 4.48

In addition to asking questions here on rctech, I'd encourage you to get to know some fellow racers at your track and find one who is helpful. They can be good resources to help you figure out your gearing.

If you have the budget, it would be very tempting to buy one of the sport versions of the Xpress. They are very reasonably priced, and you can have a whole car just for spares to keep your other Express going. May be less hassle than trying to keep two different brands going. Also, handling is more important than overall speed for getting fast laps, and a drift car is likely going to handle a lot different from a current race car like the Xpress.

Good luck to you.

Thanks for the help. This has been a crash course for me in Touring. I'm traditionally a Pan car/oval racer so I've been learning on the go on the touring circuit. The guys at the track have been an immense help. They are the ones that sold me on the Xpress chassis. It is the most common chassis at the track that isn't Associated or XRay. I'm enjoying the experience learning not just from the guys at the track, but on this forum as well. Thanks everyone for your help.
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Last edited by JBMotorsports29; 07-09-2019 at 02:32 PM.
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