Xjeepguy
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So I have some custom upgrades in mind, so disassembled my Grom and broke out the calipers to get some measurements.
Here’s the skinny on the Grom’s design.
Steel driveshaft, metal diff rings and acorn gears, full bearings, and the chassis is super solid and well built.
The steering servo mounts directly to the ESC housing. I’m going to have to machine some mounts, not only because of this, but also because the Spektrum servo has an oddball measurement between the base of the spline and the mounting tabs. So to keep proper steering geometry, an upgraded servo has to be spaced differently.
The way the chassis is designed, access to the diffs are simple. I’m very impressed with the overall design. It’s extremely easy to work on, but is very durable.
Now the bad….
The motor mount does not allow for much choice in gearing. It’s solid, but I’d rather be able to run whatever pinion I want.
And the really bad.
Arrma designed a truck with a bombproof drivetrain, for the trucks size and weight…. Until they got to the diffs. An all metal drivetrain with plastic diff gears. Seems a bit strange to have everything strong except the hardest part to get to.
Could they be strong enough for big brushless power? Maybe. Who knows? But I’d have rather seen metal gears in this thing. Maybe somebody in the aftermarket will make stronger options.
The diffs are not sealed, which is fine for a vehicle this size, but I did add a thicker grease to add a bit more tension.
Instead of cross pins, the diff uses a molded plastic center ring that the acorn gears fit into.
So metal dogbones with plastic cups. And I am not particularly happy with how the drive cups attach to the sun gear. It relies on two tiny plastic tabs to maintain connection. If one of these outdrives has these tabs shear off, there will be no meaningful connection between the diff and the dogbone. I guess I’ll continue with my plans and see how it goes, but I’m pretty sure I’m going to junk some rear diffs.
Here’s the skinny on the Grom’s design.
Steel driveshaft, metal diff rings and acorn gears, full bearings, and the chassis is super solid and well built.
The steering servo mounts directly to the ESC housing. I’m going to have to machine some mounts, not only because of this, but also because the Spektrum servo has an oddball measurement between the base of the spline and the mounting tabs. So to keep proper steering geometry, an upgraded servo has to be spaced differently.
The way the chassis is designed, access to the diffs are simple. I’m very impressed with the overall design. It’s extremely easy to work on, but is very durable.
Now the bad….
The motor mount does not allow for much choice in gearing. It’s solid, but I’d rather be able to run whatever pinion I want.
And the really bad.
Arrma designed a truck with a bombproof drivetrain, for the trucks size and weight…. Until they got to the diffs. An all metal drivetrain with plastic diff gears. Seems a bit strange to have everything strong except the hardest part to get to.
Could they be strong enough for big brushless power? Maybe. Who knows? But I’d have rather seen metal gears in this thing. Maybe somebody in the aftermarket will make stronger options.
The diffs are not sealed, which is fine for a vehicle this size, but I did add a thicker grease to add a bit more tension.
Instead of cross pins, the diff uses a molded plastic center ring that the acorn gears fit into.
So metal dogbones with plastic cups. And I am not particularly happy with how the drive cups attach to the sun gear. It relies on two tiny plastic tabs to maintain connection. If one of these outdrives has these tabs shear off, there will be no meaningful connection between the diff and the dogbone. I guess I’ll continue with my plans and see how it goes, but I’m pretty sure I’m going to junk some rear diffs.
. 
