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Thread: Underdrive for the R1... Finally

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Old 05-11-2014, 07:33 PM   #1
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Default Underdrive for the R1... Finally

Don't know why I even post on this forum anymore. Aside from Dravpnir's Wrapter thread (which is awesome), this forum is as quiet as the grave. Kind of makes you wonder if anyone is buying/building/driving Gmades anymore.

Even after everyone pooped on the idea of using the ring and pinion (differential bevel gears) from the Sawback to affect an underdrive in the rear of the R1, I just could not give it up. The Sawback bevel gears (GM52106) are only about $8 a set so I took a chance and ordered them.

Installing the Sawback ring and pinion in the rear differential of the R1 turned out to be a lot more work than I expected. However, it was not an unreasonable amount of work, and the amazing results made it worth while.

As you can see, the Sawback pinion is much thicker than the R1 pinion and the ring hub on the Sawback ring gear is shorter than that of the R1 ring gear.




I had to file the Sawback pinion down to the same thickness as the R1 pinion to get it to fit and mesh properly with the ring.




As for the rings, both the Sawback and the R1 rings are the same diameter and have the same number of teeth (32). However, the hub on the Sawback ring is shorter than that of the R1 ring. This turned out to be advantageous since it gives you some play with which to adjust the position of the ring and ensure proper mesh with the pinion. I used washers at each end of the ring hub to shim and position the ring.




The shorter hub on the Sawback ring allows the axle shafts to be inserted further with the undesirable result that the outer ends of the shafts (especially the left shaft) don't quite reach the outer support bearings in the portal housing. To fix this, I had to drill and tap (M3) a hole (for an M3x0.5 the recommendation is a 2.5mm drill bit, but I have used a 7/64 inch bit with good results) through the center of the ring hub and install a set screw that could be adjusted to prevent the shaft from being inserted all the way. You can see the set screw in this picture. (Edit: A better solution would be to use cytogen's HD locker solution Poor Man's HD locker. This will remove slop from the drivetrain and will provide you with the ability to precisely adjust the locations the diff output shafts relative to the ring carrier.)



And here is an image and explanation of the adjustment.



With the pinion filed and the ring adjusted, the mesh is perfect.



Test drive revealed a marked improvement in turning radius and lowered and stretched suspension as expected. I can't wait to test on the rocks. My calculations indicate that for each turn of the transmission output the front wheels travel 0.5 inches (~12mm) further than the rear wheels. There is no discernible spinning of the front wheels or dragging of the rear wheels except in sharp turns. The drive ratio front-to-rear is a little greater than that for an Axial Wraith with underdrive. However, it is less than that for a Wraith with both overdrive and underdrive installed.

Last edited by supermoturd; 06-27-2017 at 06:28 PM.
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Old 05-11-2014, 10:38 PM   #2
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Default Re: Underdrive for the R1... Finally

Great stuff, thanks for posting this!
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Old 05-12-2014, 01:00 AM   #3
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Yea very nice man


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Old 05-12-2014, 03:07 AM   #4
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Default Re: Underdrive for the R1... Finally

Another reason I should get an R1
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Old 05-12-2014, 09:56 AM   #5
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Default Re: Underdrive for the R1... Finally

Thanks, guys! I tested it out at my favorite spot this morning and made it up some lines that I have never been able to make it up before. The only time that it hurts you is when you are trying to creep up steep, flat faces. The wheel spin in the front robs you of a little bit of traction. So far, though, I can still climb everything that I could before. I'm experiencing fewer back flips and my turning radius is pretty insane.
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Old 05-12-2014, 11:41 AM   #6
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Default Re: Underdrive for the R1... Finally

I've never owned or worked on any G-made vehicles, so I'm not sure if what I'm looking at is correct,

But, does the D-shaped hole in the differential gear/carrier locate off of the axleshafts that are bearing supported.

Meaning, is the diff gear/carrier directly supported by bearings, OR, is the diff only located by the axleshafts themselves which are then supported by, I assume, an inner and outer (portal) bearing?

Regardless, that is some good old fashioned grit and determination to get-it-done. Nice write up as well!

I don't know what a G1's portal ratio is but what is the combined final drive numerically of the front and rear axles?
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Old 05-12-2014, 12:12 PM   #7
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Default Re: Underdrive for the R1... Finally

Quote:
Originally Posted by meatmonkey View Post
I've never owned or worked on any G-made vehicles, so I'm not sure if what I'm looking at is correct,

But, does the D-shaped hole in the differential gear/carrier locate off of the axleshafts that are bearing supported.

Meaning, is the diff gear/carrier directly supported by bearings, OR, is the diff only located by the axleshafts themselves which are then supported by, I assume, an inner and outer (portal) bearing?

Regardless, that is some good old fashioned grit and determination to get-it-done. Nice write up as well!

I don't know what a G1's portal ratio is but what is the combined final drive numerically of the front and rear axles?
Thanks for the complements! Only the shafts ride on bearings. I would say that the shafts locate on the gear carrier since the gear carrier hub (at least the one on the stock ring) fits perfectly (but not tightly) between the bearing carriers in the walls of the diff.

The portal gears are 14T/16T. The stock reduction from diff input to axles is 2.15:1 (17T/32T, 14T,16T). For the underdrive, the reduction is 2.61:1 (14T/32T, 14T/16T).
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Old 05-12-2014, 10:22 PM   #8
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Default Re: Underdrive for the R1... Finally

Awesome work.

Between this and your knuckle weights, you're becoming the R1 innovator on this forum.

The R1 isn't dead, and thanks to you it will live a lot longer.
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Old 05-13-2014, 10:07 AM   #9
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Default Re: Underdrive for the R1... Finally

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Originally Posted by Jesse XR10 View Post
Awesome work.

Between this and your knuckle weights, you're becoming the R1 innovator on this forum.

The R1 isn't dead, and thanks to you it will live a lot longer.
x2!

good job man!
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Old 05-13-2014, 09:07 PM   #10
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Default Re: Underdrive for the R1... Finally

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Originally Posted by Jesse XR10 View Post
Awesome work.

Between this and your knuckle weights, you're becoming the R1 innovator on this forum.

The R1 isn't dead, and thanks to you it will live a lot longer.
Thanks, Jesse!
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Old 05-26-2014, 08:27 AM   #11
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Default Re: Underdrive for the R1... Finally

got my parts ordered to do underdrive.what size drill bit do you use for 3mm set screw.I'm thinking I will do front and rear axles to get better crawling ratios.
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Old 05-27-2014, 08:22 AM   #12
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Default Re: Underdrive for the R1... Finally

I don't understand the benefit of having different gearing front to rear for crawling. I know the mud guys will do that so the front pulls the rear. But why for crawling?
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Old 05-27-2014, 02:45 PM   #13
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Default Re: Underdrive for the R1... Finally

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I don't understand the benefit of having different gearing front to rear for crawling. I know the mud guys will do that so the front pulls the rear. But why for crawling?
For an M3x0.5 the recommendation is a 2.5mm drill bit. However, I have used a 7/64 inch bit with good results.

There are a couple of advantages. Though much less pronounced than true front dig, the slower rotation of the rear wheels facilitates more responsive steering. You essentially get some of the advantages of front dig without actually locking the rear axle. Our rigs have locked differentials, which makes turning somewhat problematic. When you turn the front wheels, the rear wheels continue to shove them in the forward direction, causing them to scrub and side-slip, creating what is essentially understeer. By slowing the rear wheels down, they don't shove the front wheels along as much during a turning maneuver. This leaves the front wheels with more traction to pull through the turn.

The other advantage of underdrive is that it stretches your rig; causing the suspension to settle. This draws your center of gravity downward and makes your rig more stable. With the settled suspension, the rig is much less likely to flip over when climbing and side hilling.

One of the disadvantages of underdrive comes in side hilling. I have not verified this, but it seems plausible. Namely, that the inherent slip at the front wheels could cause the rig to side-slip down the grade.

All that said, I have seen a marked improvement in my rig's performance after implementing underdrive.
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Old 05-27-2014, 03:50 PM   #14
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Default Re: Underdrive for the R1... Finally

Added this edit: "A better solution would be to use cytogen's HD locker solution Poor Man's HD locker. This will remove slop from the drivetrain and will provide you with the ability to precisely adjust the locations the diff output shafts relative to the ring carrier."
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Old 05-28-2014, 06:57 AM   #15
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Default Re: Underdrive for the R1... Finally

okay thanks for the info super . looks like I will do just the rear.waiting on my parts don,t know how long it will take had to order from jun fac also waiting on new servo . when all that comes in there will new pics
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Old 06-11-2014, 06:11 PM   #16
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Default Re: Underdrive for the R1... Finally

would the r1 ring gear work with the 14t pinion (after thinning) or does the adjustibility make the sawback ring the better option?

Great tutorial, thanks for putting it and the steering ones up.
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Old 06-18-2014, 09:13 PM   #17
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Is the underdrive really needed


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Old 06-19-2014, 08:50 AM   #18
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Default Re: Underdrive for the R1... Finally

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Originally Posted by joneail View Post
Is the underdrive really needed


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If you've ever driven a crawler with under drive in the rear it's night and day. The steering, control, suspension predictability, etc is amazing. It's worth it.

Supermoturd, awesome work man! This has been a complaint of mine on the R1 axles since they came out!

Now, if someone could figure out a way to tighten up the slop in the portal boxes, I'd be one happy camper! I often find my rig getting pulled into unintended lines waiting for the rear tire i need to have traction, catch up with the gear slop. It's like I have 3 wheel drive for a quarter turn of the tire.
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Old 06-19-2014, 09:14 AM   #19
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Default Re: Underdrive for the R1... Finally

I don't own an R1 anymore but if I did, I would be doing this mod.

Great work!
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Old 06-20-2014, 01:23 PM   #20
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Default Re: Underdrive for the R1... Finally

Thanks, guys. It was well worth the effort.
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