Axle Gearing Question - Ratios

[bilinvic]

I'll be the first to admit, gearing ratios have me pulling my hair out when I try to figure it out!

A stock AR44 axle ring & pinion is 8T / 30T
A stock BOM axle ring & pinion is 11T / 33T

Can I assume the ratio between these two are the same?

A Hot Racing or Boom Racing AR44 overdrive axle ring & pinion is 8T / 27T

Here is how my brain works...if I add three teeth to the stock AR44 r&p, that equals the BOM ratio....If I add three teeth to the HR/BR OD r&p, I get 11T / 30T, which to me will be overdrive compared to the stock BOM 11T / 33T ratio....am I correct or way out to lunch???

I'm thinking of replacing the BOM stock front axle (the whole assembly) with an Axial front axle with an overdrive gear set.

Some please steer me in the right direction!!!!

 

[JSterrett]

30/8=3.75
27/8=3.375
33/11=3.0

So 30/8 would overdriven by 25%
27/8 12.5%

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[bilinvic]

Thank you JSterrett. I guess if I swapped out the front axle, I don't even need an AR44 overdrive gear set, the stock AR44 gears are overdrive compared to the stock Gmade GOM gear set.

 

[89redranger]

All of the above is correct except your OD percentage would actually be under driven. The numbers are right, you're just thinking about it backward. At 3.75:1, you would want the 10.2 axle in the rear for 25% underdrive.

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[JSterrett]

All of the above is correct except your OD percentage would actually be under driven. The numbers are right, you're just thinking about it backward. At 3.75:1, you would want the 10.2 axle in the rear for 25% underdrive.

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You're right, my bad. Got confused for a moment...

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[JSterrett]

Thank you JSterrett. I guess if I swapped out the front axle, I don't even need an AR44 overdrive gear set, the stock AR44 gears are overdrive compared to the stock Gmade GOM gear set.

89redranger is correct, I mixed up OD/UD. The AR44's are the lowest of the 3 ratios...

If the stocks are 33/11, you would need the 27/8 OD gears in the front AR44 axle to get a 12.5% OD front.

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[bilinvic]

Thanxs for clarifying 89redranger...this is good news because I would rather swap the rear axle out and keep the offset front BOM axle. I wonder is 25% rear underdrive would be too much...opinions?

 

[89redranger]

I'll get back to you with some numbers in a bit. Tapacrap is throwing fits and deletes my post every time I try to close the app to copy and paste numbers for ya.

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[89redranger]

Alright, so this may help you form your own opinion...just comparing OG AX10/SCX10 gears because most know what these combos feel like by now.

OD- 36/14, 2.57:1

HD- 38/13, 2.92:1

UD- 43/13, 3.31:1

So, the most popular combo is standard ratio HD up front, 2.92:1, and UD gears in the rear. This combo yields 13.35% UD, or 1.1335:1 front/rear. The inverse of this would be running OD gears up front and standard ratio out back for 13.6% UD. If you were to shoot for the moon and go all-in you'd be looking at 28.7% UD with OD gears up front and UD gears out back.

Personally, I think 25% is a bit much for a trail rig. But I'll leave that up to you.



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[JSterrett]

Thanxs for clarifying 89redranger...this is good news because I would rather swap the rear axle out and keep the offset front BOM axle. I wonder is 25% rear underdrive would be too much...opinions?

In my TRX4 I'm running standard 11/34 (3.09:1) gears in the front and underdrive 10/35 (3.50:1) in the rear, so the front is about 13% overdriven, and I plan on adding the 12/33 (2.75:1) overdrive gears to the front soon because I feel like it could use more of a split.

My Wraith is running OD 36/14 (2.57:1) in the front and UD 43/13 (3.31:1) in the rear, for a 28% overdrive in the front, and I like that split a lot better than the 13% on the TRX4.

Just my $.02

PS - I'm also shooting for less trail and more crawl/climb on these trucks, so that plays a factor. For most stuff the 13% in TRX4 has worked fine...

 

[89redranger]

I guess my numbers should also be figured as a percentage of OD, not UD. Or even better, we all just need to figure that the number cranked out from your calculations should be expressed as a ratio to begin with, and not really a percentage. Like I showed above, the difference between HD gears and UD would be 1.1335:1. That would best describe that fact that for every turn of the rear axles the fronts would turn 1.1335 times.

Edit: between helping you guys and doing my own stuff here I may have worked myself into a circle. But you see where I'm going with this. And I think we're all on a similar page.

In my TRX4 I'm running standard 11/34 (3.09:1) gears in the front and underdrive 10/35 (3.50:1) in the rear, so the front is about 13% overdriven, and I plan on adding the 12/33 (2.75:1) overdrive gears to the front soon because I feel like it could use more of a split.

My Wraith is running OD 36/14 (2.57:1) in the front and UD 43/13 (3.31:1) in the rear, for a 28% overdrive in the front, and I like that split a lot better than the 13% on the TRX4.

Just my $.02

PS - I'm also shooting for less trail and more crawl/climb on these trucks, so that plays a factor. For most stuff the 13% in TRX4 has worked fine...

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[bilinvic]

Thank you again for this very helpful info. I also run UD / OD in my TRX4 rigs, but only UD gears in the rear axle on my 2.2 rigs with the Axial gears. I may just try using a 10.2 rear axle with the 30T/8T gear set because my focus is rock crawling.

 

[bilinvic]

If the stocks are 33/11, you would need the 27/8 OD gears in the front AR44 axle to get a 12.5% OD front.

Is this statement correct? Should it read in the rear axle to get UD rear?

Sorry guys, this ratio stuff isn't my thing!!!!

 

[89redranger]

Kinda...The 10.2 axle would need to go in the rear. Everything else is correct.

Is this statement correct? Should it read in the rear axle to get UD rear?

Sorry guys, this ratio stuff isn't my thing!!!!

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[89redranger]

But, that gives you two OD/UD options with one rear axle. 12.5% and 25%. What're ya gonna do with choices..? [emoji23][emoji23][emoji23]

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[bilinvic]

That's what I'm trying to figure out right now! I think I will try the 30T/8T gears for more underdrive.

 

[OSRC]

I think 25% is about max for a trail rig. That's a pretty high overdrive percentage. IMHO of course. About 15% is what I shoot for.

I tried running one of my trucks with RC4WD's t-case set for overdrive which was 47% - it was like driving a front wheel drive truck. Basically dragging the rear end.

 

[ferp420]

Im trying to mellow out a 1.8:1 split in the trans the front output is overdriven 1.8:1 and the rear outout is 1:1 im tyring to get it driveable by playing with the axle ratios i know i will have to run underdrives up front and overdrives in the rear but will that be enuff to be drivable im looking for mostly rock crawling performance on a very heavy rig 15-20lbs with a 17"-20" wheel base i want to run this set up because the front output is almost a inch lower than the rear So i can get more traval in the front end with out the drive shaft binding at full droop

 

[89redranger]

You're gonna have a hard time making up for 80% OD/UD. Even using currently available gear options to "adjust" that bias, you're gonna be somewhere around 50-60% over driven still. I hate to say it man, but I think you need to look at other t-case options.

Or even download the Fusion 360 trial and design/3D print what you need. You'd be amazed what can be accomplished with a CAD/3D modeling program, some pinions gears, bearings, 5mm shafts, and an STL file sent to Shapeways for printing...

Im trying to mellow out a 1.8:1 split in the trans the front output is overdriven 1.8:1 and the rear outout is 1:1 im tyring to get it driveable by playing with the axle ratios i know i will have to run underdrives up front and overdrives in the rear but will that be enuff to be drivable im looking for mostly rock crawling performance on a very heavy rig 15-20lbs with a 17"-20" wheel base i want to run this set up because the front output is almost a inch lower than the rear So i can get more traval in the front end with out the drive shaft binding at full droop

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[ferp420]

You're gonna have a hard time making up for 80% OD/UD. Even using currently available gear options to "adjust" that bias, you're gonna be somewhere around 50-60% over driven still. I hate to say it man, but I think you need to look at other t-case options.

Or even download the Fusion 360 trial and design/3D print what you need. You'd be amazed what can be accomplished with a CAD/3D modeling program, some pinions gears, bearings, 5mm shafts, and an STL file sent to Shapeways for printing...

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I might need to make some kind of underdrive for the rear output on the upside i will be able to move the rear output alittle for better shaft angles