Gear selection resources

[DMDUK]

I’m trying to understand gear choices.
What’s the best online resource to learn about how select the correct pinion/main gears for a particular setup?

Thanks.

David

 

[Diem Turner]

The best resource is going to be your speed calculator.

There are a couple of variables that will flow into your gearing calculations and, in the end, you'll just need to decide at what speed you want to begin the iterative process of getting faster and go from there.

The variables: diff ratio, wheel size (if you're running radials, having a speed calculator that allows values to account for tire ballooning is a nice feature), how many cells you plan to run on and having a handle on how far the voltage sags under heavy loads and your motor (i.e. KV rating).

The first thing you'll want to decide is how many cells you want to run on (I'd recommend starting on 3S, 4S at most because there's a somewhat steep learning curve in the beginning and there will, more than likely, be the one or other odd crash and the slower you're going when they happen, the happier you're going to be).

This is my speed calculator of choice.
https://scriptasylum.com/rc_speed/top_speed.html

Next enter the cell count and for the voltage per cell choose something around 3.6-3.7V/cell to account for voltage sag under load. For the motor's KV value, take the KV printed on the motor and multiply it by 0.8 to simulate 80% efficiency under heavy load (this has proven to be a good place to start from, a little tweaking is always necessary and data logs are where you get the information to make corrections).

Enter the diameter of your tires (GRPs are going to be around 3.85", Contact foams right around 4" and so forth. As I said earlier, with radial tires it you can account for ballooning. Depending on how fast you're going, 0.05-0.15" have done well at compensating some of the differences you see between the speed calculator and real life).

For the diff ratio enter 3.3 or 2.8 or whatever the ratio of your particular diff is. Transmission and Additional gear ratio, just enter 1. Now we're down to pinion and spur. Enter something in both fields and then hit calculate.

Now begins the iterative process to A) get faster B) understand where the car is having trouble and how to address it.

Play around with it for a bit and if you have any questions, fire away. I'll be happy to help anyway I can.

Here's an example of how I'd fill one out when using GRP tires so you can see what it should look like when you hit calculate.

Edit: It just dawned on me that there's a serious error in my suggested value for tire ballooning. I suggested values between 0.5-0.15...which doesn't even make sense. It should read 0.05-0.15. I normally default to 0.1 but, so far, I've only taken a car to around 120mph on GRPs. Depending on your speed this will, of course, vary. I've edited the post accordingly.

 

[DMDUK]

Fantastic!
Many thanks for taking the time to respond and provide such a detailed and useful post. Thank you, Diem, much appreicated.
Let the learning commence!!!

 

[Diem Turner]

My pleasure David, I bid thee much happy gearing and may your passes be forever toothful.

 

[Johnny b]

The best resource is going to be your speed calculator.

There are a couple of variables that will flow into your gearing calculations and, in the end, you'll just need to decide at what speed you want to begin the iterative process of getting faster and go from there.

The variables: diff ratio, wheel size (if you're running radials, having a speed calculator that allows values to account for tire ballooning is a nice feature), how many cells you plan to run on and having a handle on how far the voltage sags under heavy loads and your motor (i.e. KV rating).

The first thing you'll want to decide is how many cells you want to run on (I'd recommend starting on 3S, 4S at most because there's a somewhat steep learning curve in the beginning and there will, more than likely, be the one or other odd crash and the slower you're going when they happen, the happier you're going to be).

This is my speed calculator of choice.
https://scriptasylum.com/rc_speed/top_speed.html

Next enter the cell count and for the voltage per cell choose something around 3.6-3.7V/cell to account for voltage sag under load. For the motor's KV value, take the KV printed on the motor and multiply it by 0.8 to simulate 80% efficiency under heavy load (this has proven to be a good place to start from, a little tweaking is always necessary and data logs are where you get the information to make corrections).

Enter the diameter of your tires (GRPs are going to be around 3.85", Contact foams right around 4" and so forth. As I said earlier, with radial tires it you can account for ballooning. Depending on how fast you're going, 0.5-0.15" have done well at compensating some of the differences you see between the speed calculator and real life).

For the diff ratio enter 3.3 or 2.8 or whatever the ratio of your particular diff is. Transmission and Additional gear ratio, just enter 1. Now we're down to pinion and spur. Enter something in both fields and then hit calculate.

Now begins the iterative process to A) get faster B) understand where the car is having trouble and how to address it.

Play around with it for a bit and if you have any questions, fire away. I'll be happy to help anyway I can.

Here's an example of how I'd fill one out when using GRP tires so you can see what it should look like when you hit calculate.

View attachment 294880

I just woke up, got a cup of coffee, sat down, read this post and my mind already shut down for the day. Going back to bed.

 

[Diem Turner]

I just woke up, got a cup of coffee, sat down, read this post and mind already shut down for the day. Going back to bed.

Rest easy brother and have a beautiful Sunday

 

[Senton_Prime]

I just woke up, got a cup of coffee, sat down, read this post and my mind already shut down for the day. Going back to bed.

Get your @$$ up and re-read Mr. @Diem Turner post until it sinks in.. you know what, your gonna learn something today booooy!!

 

[Diem Turner]

@DMDUK there's one more thing regarding gearing that's worth an honorable mention. Building up a well sorted set of quality gears is probably one of the most neglected and underreported hidden costs of speed running. High quality, hardened steel gears will, on average, run you between $15-20 each so you can see how it's possible to spend $400-500 just on gears without even trying hard. This cost is multiplied if you're running a motor with a 5mm shaft because some gears you'll need to purchase twice since the spool needs 8mm gears. It's one of the reasons why I recommend motors with an 8mm shaft so you can just share gears between motor and spool.

Beyond that, 8mm rotor shafts make everything fit just a little stiffer and tighter with the added bonus that 8mm gears use M5 grub screws instead of the M4 you find in most 5mm pinions. You can tighten down M5 grubs sooo much harder than M4 with less risk of stripping out the hex. Come to think of it...I don't think I've stripped even one M5 grub. The M4 I lost count of.

Anyways, just wanted to lay that out. Don't skimp on good quality gears. They don't need to be SAGA gears, there are a number of other manufacturers that make hardened steel gears for between half and two thirds the price of SAGA gears that do the job well.

 

[DMDUK]

@DMDUK there's one more thing regarding gearing that's worth an honorable mention. Building up a well sorted set of quality gears is probably one of the most neglected and underreported hidden costs of speed running. High quality, hardened steel gears will, on average, run you between $15-20 each so you can see how it's possible to spend $400-500 just on gears without even trying hard. This cost is multiplied if you're running a motor with a 5mm shaft because some gears you'll need to purchase twice since the spool needs 8mm gears. It's one of the reasons why I recommend motors with an 8mm shaft so you can just share gears between motor and spool.

Beyond that, 8mm rotor shafts make everything fit just a little stiffer and tighter with the added bonus that 8mm gears use M5 grub screws instead of the M4 you find in most 5mm pinions. You can tighten down M5 grubs sooo much harder than M4 with less risk of stripping out the hex. Come to think of it...I don't think I've stripped even one M5 grub. The M4 I lost count of.

Anyways, just wanted to lay that out. Don't skimp on good quality gears. They don't need to be SAGA gears, there are a number of other manufacturers that make hardened steel gears for between half and two thirds the price of SAGA gears that do the job well.

Thanks again Diem.