Saturday, April 18, 2009

Parts Prep - day 4

Today was a productive day. Having decided on the drivetrain made the next step pretty simply; overhaul the gearbox and differentials. I began with the front. Much to my surprise, I fully neglected putting any type of grease in the front diff. This is not like me so there must have been a reason I did this. This front diff had very smooth action and was still perfectly good for continued service. I decided that cleaning the bearings and changing out the casing for new was a worthwhile effort. I do remember all the deflashing that the bevel gears required in the past. The effort of filing the gear teeth clean makes the diff performance 10-fold improved. Notice that I use shield washers on the outside for the bearing to help keep the bearings clean. The bearings barely had a film on them.



During re-assembly, I used my favorite hobby grease: Parma Silicone Lube #7900. This lube applies thick but tends to ooze into all the crevices over time. This lube is safe for plastics and is tenacious about minimizing friction. In the long run, it will ooze out of wherever you put it so you want to use it sparingly. One bottle will likely last a pervasive hobbyist a lifetime.



Here is a shot of the assembled front differential. I reversed the drivers so the wear surface is virtually new. I try to take out all the sideplay when tightening the drivers on. I also use a hard hex wrench and make sure these are tightened very well. If you've been collecting Optimas for a while, you will notice that the setscrews used here are quite rare in your parts boxes.



Next, apart comes the custom dual chaindrive ball-diff. The unit was a bit raspy and definitely a bit dusty. If you've never worked on one of these, this section will be one for you to pay much attention to if you ever plan to install one.

I could easily have put a normal bevel gear style rear diff in this Mid. The shear beauty and performance of these units makes this little adventure well worthwhile. These can be set up as slippers [not recommended on 4WD] or simply dial in the drive you want to push onto the outside wheel. I set mine up to where the plates would slip only in case of a hard shock. I replaced the plates as the ones I removed were pretty rough. The bevel washers were still in good shape, and the small thrust bearing was in pretty good condition. The screw still had some thread lock on it, and the foam washer also was in good shape. Overall, this was simpler than I had anticipated. Please note the error in the photo... you cannot use a shim washer on the left side bearing! Doing so would keep the driver from engaging properly with the disk.



This is where the two shim washers should go in order to keep the bearing clean. Also notice the orientation of the bevel washers: <><><> ...followed by 2 special washers for the thrust bearing. The foam plug has a hole in it to make adjustments without removing it.



Here again, the Parma grease did its usual perfect works by holding the 8 balls in the thrust bearing. This was just what Doc ordered to slide this sub-assembly into the driver.



Always check the function of your ball diff after your first run when you've rebuilt the unit. These tend to require a little break-in period. One other word of warning about this ball diff... the part were the balls are installed must be on the drive side! Reversing this will diminish performance.



So here you have them, a virtually new set of differential. Sweet!

Next comes a perplexity that comes to light only after you try something new and have never really had to, or had the chance to correct the little mechanical nuances. In this case, the slack side of the drive chain is wearing the boss for the case screw. If I recall correctly, I had one track-side repair to replace the power side drive chain. I can imagine that slapping the chain against the case had something to do with that. The wear area is the "I I" on the right gear case.



This was my determined correction to the shortcoming. I ground away the plastic to make room for a hard rod end ball. It is not meant to spin, but it is there to make sure that the chain can glide past it and maintain its dimensional shape. A screw is used to keep the ball in place through assembly when you push the right screw through exchanging the ball's holding shaft.



When all is said and done, nothing has been changed except the expected lifetime of both chain and case. This setup has a near perfect fit for this chain-pitch. Backlash is quite minimal. Do notice that one of the case mods is removing one of the back screw bosses.



With a 7-18, or 2.57:1 ratio, this is pretty much the same as stock 14-37, or 2.64:1 ratio.

Here is a quick video of how efficient this drivetrain really is.



Now for a second shortcoming of my original build. Obviously, I never checked the spacing of the two pinions as compared to the differential itself. Turned out I needed to remove almost .050" from the idler pinion to have the two match up properly. I seem to have had the chain biased quite heavily to the right in operation. This might have been the second reason for the broken power drive chain. Today I put the idler on the lathe and matched it all up.



I have no excuse for this oversight other than sloppy work. I am glad I had the opportunity to fix this as this car will see a whole lot more power run through it than it was originally designed for. Remember, this was a car dedicated to ROAR 4WD Stock Class racing. I fully anticipate putting my Rev Tech Red on this car. This car has never flown as fast as this motor can take it.

Here is another piece of the how-to puzzle. This mystery car that sacrificed the needed technology to make this modification possible also had a simple and clean solution for the spur gear... a flange that the gear simply screwed onto. The flange has 2 setscrews the same size as your everyday pinions. The reason this flange is key is that the drive shaft is 4mm vs the 5mm of the Mid. Even the bearings have a spacer in it to make them 4x10's. I'm also happy with the fact that this is a 48-pitch gear... since I have a nice collections of 48-pitch pinions.



The next image is to preserve the knowledge of the sequence of all the spacers and shims to allow for minimal side play. I left out one of the shims, the second one to the left from the idler. Even though the idler is free-floating, it will rotate at the same speed as the drive pinion. If you look carefully at the 7T idler, you will see I had to cut into the setscrew holes.



Again, be sure the diff is installed correctly!

The next step is pretty straight forward... add bolts, motor plate and battery holder [H-bracket]. One bolt through the wing mount serves to replace the hacked case screw; and the new ball in the case is in place. Again, push the screw out holding the ball with the screw that goes through the case. That way you know it hasn't fallen free.



I played with the chain a bit to make sure it rolled freely over the 7T cog. The chain did have a rough spot where it was joined. Working with it a bit made it behave as I would expect it to.

Here is a shot of the diff cases on the chassis. Notice I also marked the connector signals for the ESC on the receiver.



I think I am pretty close to buttoning up the chassis and drivetrain. I have a little epoxy work to do on the chassis first. I also need to make new chainguards from Mylar sheets. Do you think I can put all the pieces together without breaking the chain and threading it through the chassis? We'll see!

1 comment:

  1. Howdy, very interesting read of this build and your performance mods! One of the weak links in these drivetrains (Optima/Turbo Optima/Javelin/Salute/Ultima) is the final pinion gear in the rear gear-box, the one on top attached with a set screw. On all models except the Ultima Pro, the stock part # OT-41 gets chewed up quite easily. An optional hardened final pinion gear is hard to find, part # OT-76. Do you happen to know any third-party manufacturers of a gear that can replace the stock OT-41 that has the same number of teeth & pitch & other specs?

    I can be reached at coelacanth97 AT yahoo DOT com

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