View Full Version : Diff tips thread

2016.12.08, 01:31 AM
A member had asked me to explain some of the tricks that I do for my f1 differentials. Fundamentally, all of the differentials on the 2wd cars are not balanced very well. If you look at the diff simply, there is the portion that connects directly to the right wheel, and the other connects to the left.

If you take the mass of each portion, the left wheel has more mass on it than the right. This means that there is more inertia at play on the left rear wheel than the right. While the shaft has less inertia due to the fact that it is on the center, it does still play a role. When you get on throttle, the right wheel will spin up faster....

There is also the fact that the axle bearing only give direct resistance to the left side. One of the primary reasons that most layouts are run clockwise, since right turns are typically smoother and left tend to hook due to the differential design of the 2wd chassis.

I noticed on my f1, and LM cars that the car would typically pull to the left on throttle, and right off throttle. The more motor you have and more drag brake you have, the more apparent it is. I theorized that the decreased mass of the right portion of the differential was allowing the wheel to spin up faster.

I have quite a few different differentials and parts at my disposal, and went about trying to balance the differential better.

My primary focus was to lighten the left side of the differential as much as I could. This meant using qteq differential parts, which at the time were the lightest differential parts available. I don't believe that they are available anymore, so substitutes may be needed now. Normal mr03 racer differential parts work on the left side of the diff, but not the right.

Titanium shaft is a must.

Small racer diff ring on left side

On the right side, Kyosho f1 diff half with larger f1 diff ring.

I used reflex 64p spurs because I never had to worry about them rubbing on the larger f1 diff rings like pn or atomic spurs do since the reflex spurs were completely flat. Again, it would be difficult to find them since reflex has moved up scale and is no longer producing mini-z parts. I would assume that pn spurs should work if you cut the little lip off and face that side (sand it flat with fine grit) and put that side towards the f1 right.

Qteq made a couple crucial parts that helped lighten the left side wheel and have good alignment without using any bushings or bearing in the hub. The left wheel hub had a built in spacer, and there was a conical wheel nut that held the wheel in place very well.

In my first attempt, I used a standard Kyosho differential, with a plastic left wheel, and alloy right wheel. The car tracked straight, which gave me proof of concept...

The problem is that the different materials have different handling characteristics, and the alloy wheel tends to cool the tire too quickly, especially because the carcass is so thin, this isn't very good. Also, it ties you to one wheel, which means that you cannot easily try different types of tires that you may have pre mounted on wheels.

The completed differential gave me a balance that I had never really felt before on the f1. I didn't have to shim the side plates the way they were before, and I was able to adjust my transmitter to dial more steering back into the left turns.

Is it perfect, no... But it is a noticeable improvement over a standard f1 differential.

What is important to note, is that I used Kyosho diff plates on both sides. They are a harder diff ring than atomic and pn. They also have a different slip rate than other manufacturer's, so you may not want to mix different manufactures diff rings in one diff.

Basically, if you already have a Kyosho f1 diff, use a titanium shaft with the lightest diff parts on the left side while keeping the right side stock.

The qteq diff parts are compatible with Kyosho diff rings... Not sure about the atomic and pn diff parts. its been a while since I have played with these things.

Hope people find this useful :)

I wrote this on my phone, so please don't mind the auto correct errors of there are any...

2016.12.08, 05:05 AM
Welcome back and nice write up. :)

2016.12.08, 03:03 PM
Always good to see your write-ups EMU. Welcome back buddy

2016.12.08, 04:47 PM
Thanks guys... Wish I was coming back to my regular racing regime, but I don't really have much time in the schedule... Just can't stay away from mini-z for that long.

2016.12.08, 06:46 PM
You are never farther away than an internet connection.

Somewhere I still have the newspaper headline that stated "Emu runs wild in South Carolina(with pictures). I need to find it and send it to you.

2016.12.09, 10:15 AM
Cool article... welcome back...

2016.12.09, 04:17 PM
Great info EMU - what about the gluing of pressure plates to hardware? I've read that somewhere.

2016.12.09, 06:29 PM
Great info EMU - what about the gluing of pressure plates to hardware? I've read that somewhere.
I didn't mention that here, yet... But will touch on it.

Some differentials have slotted diff rings, with a D shape stamped in the inner edge of the ring which mates with a similar notch on the side half and prevents the plate from spinning. These types of diffs should get just a little medium weight grease between the rings and diff halves to secure them.

When I build a diff, the first thing that I do is prepare the diff rings. Even new rings need to be prepped. Take a brand new piece of sandpaper, 600-800 grit should be good. Place it on a flat surface (glass tables are very flat), and place the diff ring on top. Make circular patterns with the ring. Stop every once in a while to examine the ring. You want to ensure that there are micro scratches throughout the entire surface, if there are still areas that have the factory glossy surface, then you must continue.

After completing one side, I flip it over and do the same on the reverse side. A little overkill, but it will ensure a flatter surface on both sides of the rings. After you finish, clean with a little motor spray (if you have a bearing cleaning container, a little dip with the mesh I the fluid, then dry with lint free cloth works perfect).

Clean the inner area of the diff halves I the same fashion. Allow to dry completely, which should take a minute or so once dried with cloth.

Using thin CA, make a small pool of glue in a little container... Plastic packaging for mini-z parts work well for this, so keep them in your tool box instead of tossing them. Using a very small pick tool, I use a cheap jewellers/eyeglass screw driver, dip into the glue, and place a few miniscule drops (3 or 4) evenly spaced on the inside of the diff half where it will mate with the diff ring.

Place the diff ring on the diff half, and as you seat it, twist the ring to spread the glue more thin. The twist is crucial as it helps keep the diff ring as flat and parallel as possible with the diff half. If one side had more glue than the other, the twist will help even it out.

Handling time after the application of the diff ring and diff half is a few seconds, you will feel that you can't spin the ring after maybe half a spin, if you don't have a good seat, remove the ring, clean both, and try again.

Now it is time to start to build the diff. If you have a small block of foam, or something to hold the diff shaft vertically, it makes it easier.

Place the left side pin in the shaft of its not there already. Put a drop of blue thread lock on the threads of the shaft where the adjustment nut will go, then pat it with a paper towel to remove some of the excess. I also apply a drop to both ends threaded areas to ensure a good lock on my wheel nuts. Wait for it to dry before installing the adjustment nut with o-ring. Screw on just a few turns. Place the left side diff half with attached ring, aligning groove for pin with pin.

If the spur requires a central bearing, now it's the time to clean it, give it a small drop of oil, and press it into the spur. Place the spur on the a different shaft, and give it a few spins to spread the oil. Then remove and pat with a paper towel to remove any excess.

I use associated black grease to build my diffs, but there are plenty of good greases out there. Apply a liberal amount of grease to the balls.

I typically use ceramic balls since they never deform, steel balls should be replaced when you rebuild the diffs, ceramics can be reused.

I use a hex driver to pick up the balls and place them into the spur gear. If I am using a PN spur, with 10 holes, I will use 5 balls for stock motor cars and all 10 for modified. The grease will keep the balls in the spur pretty well, just keep an eye and try not to move it too abrupt or they may fall out.

I use a new right side bearing whenever I rebuild my diffs to ensure smooth operation. So press bearing in right diff half, and then place on shaft on top of spur gear. There should be a small gap above the bearing before the groove for the circlip (e-clip). if the bearing is too close or covering the groove, back off the adjustment nut a bit until you have enough room to install the clip. I use needle nose pliers to install, applying pressure between the center of the clip and the shaft holding my pliers so that the nose faces down. After it seats, adjust the adjustment nut tighter until the bearing pushes the clip to the top of the groove.

Then remove the diff from the holder, and hold the spur with one hand while spinning with your fingers. You should have smooth operation. If the right half doesn't spin the opposite direction to the shaft, tighten the adjustment nut slightly until it does so.

If you notice any rough movement, it it's usually either the bearing or that the diff rings are not glued on straight. If when spinning, you notice a wobble in the rings, you will need to do it again.

This next part is very important.
What I like to do now is install the right wheel with bearing. Press the wheel down fully. Then look at the edge of the bearing around the shaft. Compare that to the lip on the shaft at the edge of the threads.

If the lip is considerably inside the bearing, when you tighten the wheel nut, you will essentially tighten the diff. Look for a slightly thinner bearing (awd hub bearing is .5mm thinner). If the lip sticks out further than the bearing, it means that as tight as you can tighten the wheel nut, the wheel will be loose. Here is where I would advise installing the wheel nut backwards. Where the nyloc insert is will fit over the lip slightly and allow a tighter fit reducing wheel wobble. There are also some wheel axle shims that could help, 3mm inside diameter, 5mm outside diameter. The goal is to get the lip and bearing to be as close to flat as possible.

Different wheel manufactures may have slightly different widths, so do take that into account. Some atomic 2wd wheels are a little wider than others. You may need to adjust the differential tension a little depending on the wheel installed.

After right wheel nut is installed... Hold the spur and right wheel in the same hand and lightly spin the shaft. Of it spins easily, increase tension on the adjustment nut. If it doesn't slip, loosen the adjustment nut a little. This will give you a good starting point. If you have a Dremel or drill, chuck the diff into it and put it on a low setting hold the right wheel from spinning for about 10s. This should be smooth, there shouldn't be any roughness in the spin. If that passes the final test, you can install the diff and its ready to drive.

Installing the diff, I like to allow a tiny space for the diff to move laterally. Maybe .1mm or so. This ensures that the differential isn't too tight on the axle bearings. The method that I use is to install the axle hub, tighten it about 1mm away from the bearing, install left wheel with wheel nut. Then loosen the hub screw just a little and gradually tighten the when nut until the hub is where I want it. Then tighten hub screw, and give the wheel nut a nudge more.

When I rebuld my diffs I like to clean or replace my axle bearings in the motor mount.

I tend to run my differential a little tight. I don't like to hear much slip at all. Some people use their diffs as slippers, and you can hear Them spool up when throttle is applied. That isn't good at all. It will wear out the rings, deform steel balls, and heat up causing damage to the spur over time.

You want to run the diff as loose as you can without slip. Place the car on the track, reverse slightly then tap throttle and release. If you hear the motor spin up and the car doesn't move much, then it's too loose. Its easier to start loose, gradually tightening until the point where you have minimum slip in this test. Then drive a few slower laps, and see how it feels in the corner.

In general, a looser diff will give more entrance steering and push out of the corner. A tighter diff will give less entrance steering with more exit steering on throttle. So there is a little adjustment to do for the driving style and setup. Although, it should not be used to compensate for other issues I the setup.

If you have any questions, shoot away.

Edit: some differentials need modification to the inside of one or both of the diff halves for compatibility with spurs that have an internal bearing. Since the bearing is thicker than say the stock Kyosho spur, the diff half needs to be sanded down much like how you would prepare the surface of the diff rings. Since it is aluminum, the material would be removed faster than on the diff rings, but a more coarse paper will speed up the process.