How to calculate final drive ratio

[Panther6834]

I've been searching for this formula, but have been coming up empty. Honestly, I think there's are a LOT of others wanting/needing this info. Without this very important piece of information, it can be near-impossible to determine what tooth count one needs for the motor's pinion gear. So...does anyone know the formula for calculating the final drive ratio...with, and without portal axles?

I'll give an example. Starting with an AX10-style 3-gear transmission (say, 56T spur), using the standard 30T/8T ring & pinion gears in the axles, with portals having 0.875:1 in the front, and 1.14:1 in the rear. Another example would be the same transmission, and standard axles, but using the 27T/8T OD gears in the axles. In both examples, even if I had the formula, I wouldn't be able to calculate the FDR, as I have no clue what the standard AX10 3-gear transmission output ratio is.

This is a formula that a LOT of people could use...but, at the same time, a formula that few people (relatively speaking) seem to know.

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[durok]

These may not have everything that you need but certainly can help ya get there:

from Voodoobrew ~

https://docs.google.com/spreadsheets/d/1-XXoHF5DLj9Q2tsf8AOyyCYe8rx3_iXO_rnkfjELZhA/edit#gid=453959559

And here's something simpler to play with (the dropdowns have common vehicle diffs/trannies/spurs/etc listed) ~

https://scriptasylum.com/rc_speed/top_speed.html

Axial transmission guide (slightly outdated) ~

Axial Racing - Axial Transmission Guide

I think this Axial speed calculator may also be from Voodoobrew ~

https://docs.google.com/spreadsheets/d/1urRjDrMoRWxICO7Q1opixZHsroD5TICVaz99ZbXstI4/template/preview?usp=drive_web

 

[Panther6834]

Thank you, although this really didn't help me. All of the data presented in these is based on standard axles, as opposed to portals. Then, there's no motor kV mentioned. As a "base", I'd guess the 13T...but, again, that's not taking the portals into consideration, not does it take any particular motor kV into consideration.

~ More peace, love, laughter & kindness would make the world a MUCH better place [emoji1690]

 

[durok]

Sheesh. I'm not handing you the exact info on a shiny silver platter, no... ;-)

It's effin' math, and portals are relatively new to R/C. It just requires a couple more variables to be added to the equations.

It certainly gives you SOME of the numbers that you mention needing, and there are drop downs for the standard AX10 3-gear transmission output ratio in this one:

https://scriptasylum.com/rc_speed/top_speed.html

These forums typically are a collaborative effort - hopefully someone else will chime in and provide some additional info. This isn't the first time I've provided the "building blocks" of information to try to help answer one of your questions only to receive a "Thanks, but no thanks" answer. I will try to remember that rather than dig up all the gearing charts that I have saved over the last few years. Your answer was so quickly posted that it's obvious to me that you didn't actually check out all the features included in each link I provided.

Info on the AX10 I snipped and saved and attached is below:

 

[Panther6834]

Sheesh. I'm not handing you the exact info on a shiny silver platter, no... ;-)

It's effin' math, and portals are relatively new to R/C. It just requires a couple more variables to be added to the equations.

It certainly gives you SOME of the numbers that you mention needing, and there are drop downs for the standard AX10 3-gear transmission output ratio in this one:

https://scriptasylum.com/rc_speed/top_speed.html

These forums typically are a collaborative effort - hopefully someone else will chime in and provide some additional info. This isn't the first time I've provided the "building blocks" of information to try to help answer one of your questions only to receive a "Thanks, but no thanks" answer. I will try to remember that rather than dig up all the gearing charts that I have saved over the last few years. Your answer was so quickly posted that it's obvious to me that you didn't actually check out all the features included in each link I provided.

Info on the AX10 I snipped and saved and attached is below:

First, thank you for what you did present. Thanks to you, I now know the AX10's output ratio (2.6:1), as well as what I believe to be the axle ratio (3.75:1). Unfortunately, this, alone, isn't enough to determine possible pinion gears, as I still don't have a formula. Even if I did have the formula, I also wouldn't know how to incorporate the portal ratios into the formula, nor do I know what an "optimal" FDR range should be. If I had the formula, as well as an "optimal" FDR, or FDR range, then I'd be able to determine the pinion gear tooth counts I should consider. I'm sure I could calculate potential FDRs, as well as determine possible pinion gears...but, in order to do so, I'd need the formula (which I still don't have).

Second, there was no need to be rude...I wasn't rude in my reply. I simply stated that the info didn't answer my question, and I explained why. Where you come up with thinking that's a "Thanks, but no thanks", I haven't a clue. I did check all the links you provided, including the 3rd & 4th links you added duringng edits (I was checking the 2nd when you added the 3rd, and checking the 3rd while you were adding the 4th). Only the 2nd link was a calculator, but it's too determine potential to speed.


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[Joshs4x4toyz]

(Pinion and spur ratio*Trans ratio*Tcase Ratio*Axle Ratio*Portal Ratio)=FDR

Here is the formula for FDR, just multiply each ratio together. The portals change nothing as they are just another component with a ratio. If you were picking a pinion based on the formula go with your OD ratio as that is the one that will cause the most drag on the motor.

The tough part (kind of as were trying to get a little over technical on finding the right one)to your question, is finding the right pinion. For pinion, no matter what, I always run the smallest with an overpowered motor on too many volts, dial it back if need be.. problem solved:mrgreen:. The motors can be installed in anything so its difficult to say whats best based on a formula. There is also batteries, weight, tire size, driving style (basher or trail which is relative to what? But very important on the demands of your electronics). Could someone make a formula, sure, will it be anymore accurate for your specific rig than whats available already? Not likely

As Durok said the ratios are out there. it just depends on what youre looking for. The cars manuals have the gears and tooth count in them too, so you can figure out the ratios that way as well if you needed to.

Hopefully more guys chime in though. If never dug into things much further than described above, so I am curious myself.

 

[Panther6834]

(Pinion and spur ratio*Trans ratio*Tcase Ratio*Axle Ratio*Portal Ratio)=FDR

Hopefully more guys chime in though. I've never dug into things much further than described above, so I am curious myself.

Thank you. This is the biggest part of what I was looking for (tho, I could still use knowing what an "optimal" FDR range would be).

Please, correct me if my calculation is wing, but, based on the formula you presented, and using a 13T pinion for this example, this is what I calculated:

13T pinion & 56T spur ratio = 4.308:1
AX10 trans ratio = 2.6:1
30T/8T axle ratio = 3.75:1
Front portal ratio = 0.875:1
(just for reference, rear ratio is 1.143:1)

Based on this, the front FDR would be 36.75:1, and the rear ratio would be 48:1. If I went with a 14T pinion the ratios would be 34.125:1 & 44.577:1. With a 12T, we're talking 39.813:1 & 52:1.

It certainly gives you SOME of the numbers that you mention needing, and there are drop downs for the standard AX10 3-gear transmission output ratio in this one.

Nothing against you, Durok...and, honestly, I do mean that. I realize you were trying to help. Unfortunately, the info presented didn't really help. They did give me two very important "starting blocks" (to use your words) - the transmission & axle gear ratio - and, for those, I DO thank you. Unfortunately, they didn't give the formula needed to calculate possible FDRs. Now that I have more info, I'm going to head back over to that calculator you provided the link for, and do some "test calculations"

Normally, I'm great at researching info like this. Heck...to friends, family, and others in the various forums I'm a part of, I'm often one researching & providing info to others. However, due to too many days worked, overly long days, as well as 'personal matters', these past couple of weeks, my brain is over-taxed, I'm a bit stressed, and concentrating had been a bit on the difficult side.

To both of you...and anyone else reading this...as I'm still somewhat new to crawlers, allow me to reword my second question this way: What would be considered an "optimal" FDR range? Hopefully, my question makes more sense worded this way. Again, I'll be running a 2300kV HH PP BL 540 on 3S. I'm (obviously) not looking to race this thing on a track, but I also don't want any snails passing my vehicle.

EDIT: Using info presented by Durok & Josh, I went back to the "max speed calculator x, plugged in the numbers, and, based on 5.75" tires (Hyrax 2.2), I can't up with the following theoretical top speeds:
10T - 8.98mph, 47.775:1
11T - 9.88mph, 43.43:1
12T - 10.78mph, 39.813:1
13T - 11.67mph, 36.75:1
14T - 12.57mph, 34.125:1
15T - 13.47mph, 31.85:1

Now, I can't know if these top-speed calculations are correct, but I'd assume the FDRs are correct. Realistically, based on these speeds & ratios, what would you be targeting? Btw, I dbl-checked my 1st Capra, and, running stock, except for the TGH OD portal gears, I'm using a 9T pinion, and I think the speed is quite perfect...not too fast, and not too slow.

~ More peace, love, laughter & kindness would make the world a MUCH better place [emoji1690]

 

[svt923]

What would be considered an "optimal" FDR range?

So here is the thing, unlike most racing classes that have a narrow FDR range that everyone uses, there really isn't an "optimal" ratio. The best you can do is run the lowest gear ratio that is practically possible for a given chassis and choose the motor KV or battery voltage based on desired wheel speed (the classic Holmes "gear down, volt up" mantra). This will produce the most torque at the wheels, best low speed control, best brake hold, and most efficient condition for the motor's operation.

As to what the desired wheel speed should be, that boils down to personal preference. Some people feel any faster than a walking pace is a waste and some want enough speed to jump stuff. Personally, I find a gearing with a top end in the 6-8 mph range for a given motor is what I prefer in pretty much all my trucks. This works out to about a 45:1 ratio for a 2200kv motor on 3s up to about 60:1 for a 3500kv motor on 3s. If you have a truck that is a known good setup for you, calculate the ratio on that and use it as a baseline for another truck.

 

[Panther6834]

So here is the thing, unlike most racing classes that have a narrow FDR range that everyone uses, there really isn't an "optimal" ratio ... choose the motor kV/battery voltage based on desired speed (Holmes "gear down, volt up" mantra). This will produce the most torque at the wheels, best low speed control, best brake hold, and most efficient condition for the motor's operation.



Personally, I find a gearing with a top end in the 6-8 mph range...is what I prefer. This works out to about a 45:1 ratio for a 2200kv motor on 3s up to about 60:1 for a 3500kv motor on 3s. If you have a truck that is a known good setup for you, calculate the ratio on that and use it as a baseline for another truck.

Thanks for the info. While some of it is helpful to now, I know all of it will be very helpful with future builds. Coming from off-road & on-road race vehicles, I know exactly what you mean about the extremely narrow FDR range.

As for the "go slow" side, my only fully-operational crawler (for now) is my first Capra, as the Trail King is still missing the ESC, wheels, tires, and a body. As such, the "use it as a baseline for another truck" part doesn't yet apply...but, it will, especially since this Capra will be more SCX10.2 than Capra.

While I was originally thinking a 13T pinion, based off info some of the previous links provided, I'm now thinking (because of the 45:1 FDR you mentioned) that, maybe, an 11T...and, maybe a 10T...might be more appropriate. At least it gives me a great starting point...plus, since I happen to already have an 11T laying around (having replaced the 1st Capra's pontoon with a 9T), it won't cost me anything extra.

~ More peace, love, laughter & kindness would make the world a MUCH better place [emoji1690]