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Old 05-23-2019, 12:18 PM   #1
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Default TRX4 Overdrive vs Underdrive Gearing Options

After having installed both the overdrive (12/33) and underdrive (10/35) gearing in the axles of my TRX4 Sport, I decided that both provided too much difference in the front to rear rotations of the wheels...especially when sidehilling.

So...which end should be replaced with the stock gearing (11/34)?

Initially thought that it shouldn't make any difference...But, I was wrong as follows:

- Underdrive with a 10T pinion and 35T spur has a ratio of 3.50.
- Standard with an 11T pinion and 34T spur has a ratio of 3.09.
- Overdrive with a 12T pinion and 33T spur has a ratio of 2.75.

So, the difference between Underdrive and Standard is 0.41 compared to Overdrive and Standard being 0.34. So, some minor tuning is possible when picking between these two options because Underdrive gearing is a 20% greater gearing increase than Overdrive.

Thinking I will be keeping the Underdrive at this point.

Comments?

Last edited by Still Bill; 05-23-2019 at 12:21 PM.
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Old 05-23-2019, 12:25 PM   #2
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

You’ll want the front spinning faster than the rear, so put the stock up front and under in the rear.
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Old 05-23-2019, 12:30 PM   #3
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

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Originally Posted by Pant Danza View Post
Youíll want the front spinning faster than the rear, so put the stock up front and under in the rear.
This is what I did and it works great!
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Old 05-23-2019, 05:06 PM   #4
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

I went from overdriving the front to underdriving the rear. I like it much better.
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Old 05-23-2019, 07:01 PM   #5
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

The other advantage to UD's is lower overall gear ratio. I ran with UD and stock for a while and liked it. I just recently put in the OD's too and I like it even better, but i guess it all depends where and how you drive.


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Old 05-24-2019, 11:32 AM   #6
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

I like it with both over and under for the tighter steering radius. I've seen no adverse effects on performance.
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Old 05-24-2019, 03:28 PM   #7
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

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Originally Posted by Stormin2u View Post
I like it with both over and under for the tighter steering radius. I've seen no adverse effects on performance.
Same, I have both in one trx4 and just the OD in another (UD was sold out) and they drive similar enough. Iím never really going fast enough for the gearing distance to matter.
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Old 08-23-2019, 02:10 PM   #8
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

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Originally Posted by Still Bill View Post
After having installed both the overdrive (12/33) and underdrive (10/35) gearing in the axles of my TRX4 Sport, I decided that both provided too much difference in the front to rear rotations of the wheels...especially when sidehilling.

So...which end should be replaced with the stock gearing (11/34)?

Initially thought that it shouldn't make any difference...But, I was wrong as follows:

- Underdrive with a 10T pinion and 35T spur has a ratio of 3.50.
- Standard with an 11T pinion and 34T spur has a ratio of 3.09.
- Overdrive with a 12T pinion and 33T spur has a ratio of 2.75.

So, the difference between Underdrive and Standard is 0.41 compared to Overdrive and Standard being 0.34. So, some minor tuning is possible when picking between these two options because Underdrive gearing is a 20% greater gearing increase than Overdrive.

Thinking I will be keeping the Underdrive at this point.

Comments?
I'm new to this so forgive me, so what you're saying is you're leaving the standard gearing up front but putting the underdrive gears in the rear. This will give you 20% gear reduction, where if you had done overdrive in the front and standard in the rear the gear reduction percentage would have been less?

So it's better to go with the former set up with the 20%?

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Old 08-23-2019, 02:35 PM   #9
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

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Originally Posted by MaX-D View Post
I'm new to this so forgive me, so what you're saying is you're leaving the standard gearing up front but putting the underdrive gears in the rear. This will give you 20% gear reduction, where if you had done overdrive in the front and standard in the rear the gear reduction percentage would have been less?

So it's better to go with the former set up with the 20%?

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I think what he meant is that the change is 20% greater by underdriving the rear which I do no think it right. I think what he did was .41/.34 =1.2

But really you should be trying to understand the change vs the slower axle. So you underdrive the rear and leave the front stock and you end up with (3.50/3.09)-1 = 13.2% wheel speed difference.

If you overdrive the front and leave the rear stock you end up with (3.09/2.75)-1 = 12.4% wheel speed difference, so basically the same. Its not 20% different.

Over/under driving results in (3.50/2.75)-1= 27.2% difference in wheel speed.

I would imagine the advantage to underdriving the rear only would be that your overall gear ratios are lower than overdriving the front.

Last edited by mass-hole; 08-23-2019 at 02:45 PM.
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Old 08-23-2019, 02:54 PM   #10
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

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Originally Posted by mass-hole View Post
I think what he meant is that the change is 20% greater by underdriving the rear which I do no think it right. I think what he did was .41/.34 =1.2

But really you should be trying to understand the change vs the slower axle. So you underdrive the rear and leave the front stock and you end up with (3.50/3.09)-1 = 13.2% wheel speed difference.

If you overdrive the front and leave the rear stock you end up with (3.09/2.75)-1 = 12.4% wheel speed difference, so basically the same. Its not 20% different.

Over/under driving results in (3.50/2.75)-1= 27.2% difference in wheel speed.

I would imagine the advantage to underdriving the rear only would be that your overall gear ratios are lower than overdriving the front.
Ok, now I get it. Thank you.

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Old 09-15-2019, 12:47 PM   #11
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

If you’re only going to do one, under-drive the rear. I typically don’t do both OD and UD on any of my rigs. I’ve tried it on my Wraith, and if I was just straight rock crawling, I could see sticking with it. But, most of my rigs get trailed and crawled, so UD rear seems to be the sweet spot for me. A bigger % difference in ratio from front to rear can make the rig fickle when side hilling.
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Old 09-15-2019, 10:05 PM   #12
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Default Re: TRX4 Overdrive vs Underdrive Gearing Options

I use both. Love it.
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