It's been a while since I touched this Bronco because of my SCX10 II build and the continuation of my TF2 build, but I had a chance to get it out this weekend...finally!
We have been absolutely bombarded with rain in the Northeast this spring and summer so I haven't been able to get out on the trails nearly as much as I've wanted.
After we made plans to go trailing I made a late night decision to install the rear Underdrive gears that I've had laying around for months now. It's so nice how Traxxas has the name "Underdrive" and gearing stamped right into the gears. No having to count teeth!
Because of the complexity of the TRX-4 axles with the portals I almost decided against doing this upgrade late at night. But I decided to forge ahead and I'm glad I did because it was extremely simple - much simpler than it first appeared.
For step 1 I removed the links and the driveshaft from the rear axle so that it was only attached by the remote locking diff cable. I didn't want to have to struggle to get to the diff cover screws which are screwed in from the back. Then you have to remove the portals and axles so the diff pulls out. Each portal is held on by 2 screws...2 screws! So simple to remove them. More great engineering by Traxxas. It literally takes more time to get into the portals than it does to remove them.
So after the axle was free I removed the diff cover. You can see the diff assembly as well as the plastic, remote diff locker. Would you look at that complicated mess?! Or is it...
Here is the diff removed from the axle housing. It isn't too bad when you look at it by itself and breaking it down by part. We just aren't used to seeing much more than a spool in scalers. From left to right you have the bearing, ring gear, diff cup that contains the spider gears and the engagement dogs for the locking diff, another bearing, the locking slider, and another bearing. Simple and efficient really.
I separated the diff from the assembly so I could replace the ring gear. Notice the 4 "tabs" on the stock ring gear. These go into the recesses on the plastic diff cup to hold the spider gear shafts into place. Keep that in mind.
Here is the underdrive ring gear next to the stock gear. It doesn't have the 4 tabs. I was surprised to see this. And there aren't any extra parts included to keep the spider gear shafts located in the diff cup. I was really surprised by this. Usually there is something on the diff gear or elsewhere that keeps the spider gear shafts in place.
Here is the underdrive pinion gear compared to the stock pinion gear. Notice it has the gearing and the word "Underdrive" stamped right on it as well. Awesome touch!
After this I buttoned it all back together and did a bench test. Everything worked fine. I should have taken more photos, but I didn't.
This whole process took only 20 minutes or so and some of that time was spend researching if there was anything supposed to be included with the Underdrive gear set to hold the spider gears in place. I didn't see anything and everything worked like it was supposed to during my bench test and the trail run the next day.
