OBX Helical LSD

[DJB]

All you need to measure pinion drag is a fine 0-20in/lbs torque wrench.

How do you do that? The spec is the drag while turning at a specific speed. The pictures show a special hand tool, so perhaps the speed isn't critical. Do you use a "clicker" torque wrench so that you don't need to read a needle or dial while turning?

(I still think my idea of putting torque wrench on the end of a power screwdriver could be an inexpensive yet accurate way of doing this.)

We always measure rear ends and order the proper shims here at work. I don't think a whole set is needed in stock to setup a diff.

Which part do you measure? You have to do the same torque-drag setup on the output bearings, changing the shims slightly each time to get the right preload. Then once you have the total shim thickness, you have to install the ring gear (hot from the oven) and balance it from side to side to get the right contact pattern on the ring gear. That seems to require at least few shims to select from.

Or do you measure the old center section of the diff, compare it to the size of the replacement, and just use the measurement?

I'm not trying to be argumentative here. In the past "never touch the insides of a diff, just spend $100 to buy a good used one" was reasonable advice. I expect that a bunch of e30 people will be looking at OBX torsens and there will be lots of questions about how to correctly set up the diff so that they don't burn up or howl.

[matt]

I don't think it's something the average joe can or should try to do in his garage... I am not a tech, so I have no idea how they do it, but we get told what shims to order for each particular project.

I would guess there are feeler gauges involved...

[grebnut]

The speed is 50rpm, and no it's not incredibly easy to do on your own unless you are very very handy and very patient.

I have been through Auto Tech at college, and we learned how to set them up. Basically, you have to get the overall bearing thickness right (using your torque gauge), then measure the backlash and change shims from one side to the other, without changing the overall thickness of the "shim pack".

That's assuming you haven't touched the pinion. If you have replaced pinion bearings or a pinion (i.e. wanted to change gearing in the same case), you have to get that shim thickness right to set the contact patch, which then messes with your backlash setting.

Not the easiest thing to do, but I know several people who do it all the time and they can do it in their sleep... Specialization of labor, ya know? ;)

Ben

[moatilliatta]

Has anybody tried this?

[mops]

Any update on this ?
I bought OBX LSD, but havent put it in yet. I want to get another open diff unit with 3.46 ratio and put it in there (so I keep my stock difff 'just in case).

I find it hard to get any specific information on how to setup those diffs.

Searching the internet for info about this, this is how i understand the procedure.
That torque gauge thing is only used when removing/reinstalling input shaft. AFAIK there's no torque measurement for output shafts. I understand that between input 'gear' (i think that's called a pinion?) and the other large gear (called rack maybe ?), there's is a certain amount of freeplay. that is called backlash and is adjustable with those spacers/shims. On some diffs it's like a screw adjustments but i think bmw diffs it's shim adjustment. that backlash is measured
with dial indicator, measuring big gear radial play. I was recomended to measure that before taking it aprt and then on reassembly make sure it's the same ot slightly tighter (10-20% max), but best it it was very similar. Do not touch input shaft.....

can anybody confirm that is the right way to do it ?

[Jean]

mops - update? I am going to pick up an extra diff for this task soon :)

[b*saint]

well its not hard to figure new shims. You measure the old bearing and the old shim and add them. You measure the new bearing, subtract the old shim dimension and PRESTO! you know what shims you need.

[mops]

well its not hard to figure new shims. You measure the old bearing and the old shim and add them. You measure the new bearing, subtract the old shim dimension and PRESTO! you know what shims you need.

can you please be more specific ? i dont think i follow.

exact thickness of shims is required for:
1. set backlash (this is basically amount of slack between rignand pinion gears), which basically how much offset there is between ring and pinion gear
2. set bearing preload - the diff head is suspended (sandwiched? ) between two conical bearings where shims regulate 'how hard they are sandwiched together'

both of these conditions have to be satisfied to have long lasting diff.
now, the tricky part is that once you have correct bearing preload you might find out that now you need to 'move' the whole diff head eg. 0.1mm to the left (or right), so you can grind 0.1mm off one shim, but the other you have to make a new one that's 0.1mm thicker...
so once you do it you make sure bearing preload is still ok and then you measure the backlash again and work out whether you need to make further adjustments, and then process repeats until you get both bearing preload and backlash in spec. Pita is that whatever you do you need different thickness shims so you either get a shim pack (nobody sells these, not even $tealer), or you have couple of spare shims, maybe from wrecker yard and surfact grinder handy.

there is a video explaining bearing preload (not mine, not even bmw diff, but all principles hold i guess). best to know what the 'drag' (measured in torque) it needs to be and then set it as required, but that requires specialized torque measuring tool.

http://www.youtube.com/results?search_type=search_playlists&search_query=too+much+bearing+preload&uni=1

I suspect for backlash there are is some way to calculate that eg, if you have 0.1mm backlash then you need to move ring gear closer my 0.xx mm, but i wouldn't know you would need to talk to diff specialists.

I tried to do that but i found that nobody was wanting to 'share their trade secrets', they just wanted to do it for me and chage me $$$, so if anybody can provide more detail then plz share.



Yes I put it in, and it works... I'm very happy with the way it works
it seems to have a bit of slack - in the diff head itself. I'm unsure whether that is how it's supposed to be or is it because of OBX's poor quality control. check thread in my sig for details.

[edit]
i'll fix my sig, but the link to my build thread is here: http://www.e30tech.com/forum/showthread.php?t=55733&
it's there on one of the pages somewhere :)

[b*saint]

To center the differential, I just used the factory specs that I measure when I removed the diff.

[Jean]

So here is some more info / pics on the box lsd unit http://rbryant.freeshell.org/obx_washers.htm

Also, why would you need to do anything with pre-load etc if you are replacing the center / lsd section and not doing anything with the gears or bearings??

[Wanganstyle]

Pita is that whatever you do you need different thickness shims so you either get a shim pack (nobody sells these, not even $tealer)

- really? I have actually ordered a giant pack of side shims from BMW before, it took like 6 weeks from germany and was expensive, but does exist yes! comes with tons of different thicknesses

you can also re-use shims if they are the needed thickness, they dont really wear out.....!