How to Build a Diff, the right way?
 

I have a JRX-S and the diffs need to be rebuilt. I have felt some cars and the diffs seem to have some minor gritt to them, and others seem to feel like silk. I was wondering what techniques everyone uses to build diffs. I have a Sonic cleaner that I use for the bearings. Any tips or advice you guys might have to offer would be great. Thanks a lot

Chris

Nobody wants to help out? Have any special techniques?

Chris

Some Diff informations. Hope to help you

http://www.rctek.com/general/differentials.html

http://www.rc-car.com/detail.asp?id=99

http://www.rc-car.com/detail.asp?id=179

well i use 800+ grid sand paper to sand the diff rings.. its works okay....

Make sure all of your stuff are blasted clean.

Since its a rebuild.You should change the balls and the trust bearings and use the reveerse side of the plates to ensure its a new surface.

Reminber to run in your diff before running.

What do you mean by 'run in'?

Skiddins

To "run it in", I just take an old dog bone, chuck it up in a drill or Dremel, insert it into the diff (while it is out of the tranny), hold the other side of the diff with a screwdriver, and spin it up. After you do this, you will want to check the adjustment again.

Well this is kinda easy but one that very few actually do right.

If you diffs feel gritty, I would start over 100% fresh. New diff rings, diff balls and thrust bearings just incase any damage was done.

Buy good diff balls and thrust bearings. Do not go cheap here. Good diff balls and thrusts bearings will almost never need replacing if the diff is built correctly. I use Acer Ceramic in my stuff and it lasts forever!! I have a set of balls that are over 3.5 years olds and are perfect.

Some sand their diff rings, some do not. I have never noticed a difference in doing it so I stopped. It's a personal choice. If you do sand the diff rings, use a fine grit sand paper. The theory here is that the scratch marks the sand paper makes in the diff rings creates channels for the lube to stick too. Same principle with honing the cylinder of a motor on a real car.

First, only use enough diff lube to lubricate the diff balls. Use a good lube here. Associated Stealth Diff Lube or Losi (the Losi is much thicker) are my favorites and they are among the best. Most people use way too much diff lube and this attracts dirt. Not a lot is needed.

Second and most importantly, use Associated Black Grease on the thrust braring. Use a ton of it. Just when you think you've used too much, put a bunch more on. Seriously, pack it in. You want this area to be well lubed 100% of the time and you do not want any dirt or water hitting the thrust bearings. The thrust is responsible for the majority of the diff action. If this area is not well lubed, your diff willbe gritty and not work properly.

Other problems that people with diffs is that they tighten them down way to much and start streching out the diff bolt or compressing the spings or cone washers too much. When adjusting the diff, tighten it down a little at a time...work the diff a little back and forth, and then tight some more...
Also many think that a diff should not have to be rebuilt but once a year. But diffs require more maintenance than most think for proper, flawless operation.
A diff should be looked at and adjusted if necessary after every run. Open diffs should be rebuilt after each race or practice day IMO. Letting it go without maintenance is asking for trouble. This is when you start going through rings, balls and thrusts like they are nothing.
A rebuild does not mean new rings, balls and thrusts either. It means taking stuff arpart, cleaning it and reassembling it.

Also keep in mind that diff rings are supposed to have the little wear groves in them. You just do not want them to get too deep. When they do get too deep, just flip them over and use the other side!!

Another personal preference is breaking in the diffs. I used to break them in on the bench but stoped. My first pack on a new diff I run slow and bring the car in every 4-5 laps or so to check the tightness adn readjust if necessary. I personally think breaking it in on a bench is useless as the bench does not place anywhere near the same load on the car ad the track does. I would rather breaking in the car using the same load as it will see in race conditions...parts will seat better IMO.

Hope this helps

Thourgh guide to building a ball diff
 

Sorry for the long post but a little more detail on rebuilding an ball diff

http://www.teamirsrc.com/techtips.html

The link above is the best way to build silky smooth diffs I use ceramics balls instead of steel as the are less likely to flat spot and bed in quicker due to being harder than steel or carbide.

Note: This process take a bit of time but once done your diff can last a long time before needing a rebuild! And you just need to sand the groove out of the ring rather than buy a new set.

I would sand both sides of the diff rings with 400 gritwet and dry so that there are no low spots on the rings by sanding them in a circular motion on a flat surface due to the pressing process warps them slightly.This gives an even thrust spread across the balls. When tightening you will find that, you don't need to tighten the diff as would previouly to compensate for the uneven thrust load. Then i would finish with 1000 1200 grit wet and dry this give the diff rings a keyed surface so that the ball will roll rather than side over the diff rings. Unkeyed ring require more force to stop them slipping and cause diff to over heat and flat spot the balls if not bedded in.

Note: Clean the rings and hands after sanding!!!

To grease the balls; Note the grease is to act as an coolant rather than a lubricant. It also acts as friction inducer the stickyness of the grease allow the ball to roll against the ring rather than slip. You should lubricate the ball by putting a little grease on to finger and thumb(clean finger and thumb) and roll each ball betteen your finger and thumb to get the right amount of grease on before insersting in to the diff gear no more is need after that.
Thrust bearing the smaller one is under a high load of the diff screw it job is to stop the screw tightening or backing out of the diff it need a as much black grease or what ever the manufactures recomend to keep it cool and lubricate it the sheer force on the thrust bearing will cause it to slip ratehr than roll that why you pack it with grease.

I would bed it in on the bench with the diff set to slip slightly this is done to being to key a groove in the ring. Do this for 20sec on full throttle left wheel and right. To set the slip; tighten the diff down so it won't slip then back it off 1/4 turn.
Doing it with the diff fully tightend causes the balls to flat spot and slip.

Then i would tighten the diff (s) to so they won't slip at the with your thumb pushing the diff while hold the tyres down do the same for the front diff run it and tighten the front down to reduce oversteer.

Most people run the front diff tighter than the rear to reduce oversteer and stop it slipping to much on exit of the turn.

The end result is you should be able to spin a wheel and it should rotate the both in opposite directions for a revoultion or two.

WOw thats what I am talking about, awesome information

THanks a lot

I am off to rebuild my diffs

Chris R

Ok thats all good info, but how tight should a diff be.

I can test the pulley to make sure it doesn't slip at all, but how much tighter should it be?

Thanks
Skiddins

How tight is a personal preference. You don't want it so loose that it is slipping all the time. I think it was described above but for instance, hold the right rear wheel so it cannot rotate and also hold down the spur gear so it cannot turn. Now try and rotate the left rear wheel, if you can spin the left wheel with the right wheel and the spur locked, the diff is too loose. Tighten the diff just until you cannot spinn either wheel at that end of the car with the spur locked.

Also the tighter the diff, the more traction you will get at that end of the car.
For example, if you need more steering, tighten the front diff. This will help the front of the car pull more. If the car is pushing, loosen the rear diff, etc. Please keep in mind that this is no way is a substiture for a car that is not properly set-up.

Note that the torque produced from your arm/wrist will alway be greater than the torque of the motor due to the leverage your wrist and arm will provide so it will slip a little by turning the wheel and you will end up over tightening the diff!! so don't do that. lock the wheels and push the pulley with your thumb when you can't push or it stop slipping it it is tight enough.
The same way i do a 1/12th car.
Alway check it on it first run though, as the diff screw can back off (keeping it simple) when it beds in. Actuatully when it beds in the plates have to move in to compensate for the groove that why they feel loose after the first run but it depends on which model of car your using an wether they are known for the diff nut to come loose.

This is great information. Taking notes

Chris