Can someone explain preload & backlash? How is it measured?

I am hoping to install a TBD myself this summer. I see a lot of talk about preload and backlash; however, it is not covered hardly at all in the article section. Could we make this a thread about specifically what preload and backlash are along with how to measure for them and adjust to factory spec? I would really appreciate this information, as Im sure others would too. Thanks.

The Porsche Service Manual devotes 17 pages to this topic (Section 39 08 15 Adjusting the drive set). I'm not going to transcribe it here. I suggest you get a copy of the Service Manual and study it before you tackle this job. The procedure uses 12 special tools. Some of these are common, like a dial gauge holder, but others are special precision tools that may be hard to obtain.

Hmm, in other words this really is not a DIY application like the article section teases. We know our way around the garage...and my dad has been tearing apart and rebuilding Porsche 's since the 70's, but this sounds more difficult than the TBD article suggests.

The Porsche procedure may be overkill, someone who is a competant Porsche mechanic posted some pics of his method of doing the job in the topic thread about the TBD article several months back as I recall, you might check those out. I don't recall his procedures being that complicated that it required so many specialized tools.

Preload is the amount of force pressing the tapered roller bearings, mounted on the ends of the differential, together. Preload is used to pre-compensate for wear in the bearings (prevents the bearings from getting loose as they wear), and is defined as the amount of rotational drag induced by the load on the bearings. It is measured as the amount of torque required to turn the diff slowly. For best results, preload should be measured without the seals, and either before the pinion is installed or before the ring gear is installed on the differential. Backlash is the amount of space between the teeth of the pinion gear and the ring gear. Measured as the amount of rotation of the differential, forward and backward, with the pinion locked in place. In actuality, this is really an angular measurement, but everyone "converts" it (knowingly or not) into a linear measurement by measuring the amount of distance a specified-length pointer will move (angle x radius = circumferential distance, but since the distances are small relative the pointer length, it is simply treated as tangential distance). They can both be measured with the tranny in the car, but it would take a different setup and technique than specified in the workshop manual, which was written expecting the tranny to be on a bench. Also, preload and backlash are adjusted by changing the shims that space the outer roller bearing races that are fitted to the tranny housing and diff cover.

Bill - slight clarification. Preload is used to *prevent* wear, rather than just compensate for it - which it does as well. At the same time, it helps solidify the assembly of which the bearing(s) is part of. Too tight will wear the bearing too fast, as well as increase drag. Too loose will also wear the bearing too fast and may decrease drag at first until the bearing goes 'kaplooie'.:cheers:

I have looked thru the articles several times and could not find something directly addressing how to go about measuring preload/backlash with the tranny in the car. Obviously, I see that it can be done without removing the transmission, but I just want to see the steps and make sure I know what I'm even looking for w/ regards to preload/backlash. Do you have the link to the thread and pictures you are talking about?

Just a couple of details: 1. Porsche employees (or whomever puts the diff in) fine-tune the shim setup by ear- they set up the unit and listen for noise and adjust accordingly. They then make a notation by hand on the ring gear as to what lash was ideal for this unit. 2. I used an unusual but direct method to calculate the shims I would need: I took the old and new units to a vo-tech that had a CMM (Coordinate Measuring Machine, which are accurate to 1/10,000") and had them get the distance between the bearing landings and the ring gear landing for me. That told me how much to add or subtract from each side in terms of shims. I pulled out the old shims and measured them with a caliper, and ordered new ones to effect the change dictated by the measurements. This eliminates the errors inherent in indirect measurements such as those taken by the factory tools, as well as the error in the 'art' of using those tools either on the bench or in the cramped non-ideal space under the car. PS: my diff has been in about 7000 miles without problems. 3. A poor shim job often announces itself as a bad noise in the tranny long before you destroy anything.

HA! Bill C and I are on the same wavelength.

Exxxcellent! :banana:

Oh- and my way of explaining the two is this: preload=amount of interference between (diff+shims) and housing, or how much the diff is 'squeezed'; too little or too much results in accelerated wear and perhaps bearing destruction backlash=how close the diff is to the pinion; too far or too close and the ring gear doesn't mesh well with the pinion resulting in noise and wear

Thanks guys. This thread definitely has some great information. Im seeing that the closer the TBD is to the original differential (I will be using the same bearings), the easier this whole process is. I'm still hoping to install it myself, but might try to get someone who has done this before to aide in the process.

My experience with ring & pinion installs has taught me one thing. Let a professional do it according to the correct procedures. I personally have tried to do it myself on a couple of BMW 2002 diff's with lousy results. In the Phoenix area, I have only 1 individual who I would trust and he won't work on a R&P in the car. What I have seen on CC leads me to believe that the Cayman transaxle is far more involved. Really good R&P guy's make the process seem easy but they just have a feel for it which is usually not achieved by measurements but more by feel & sound. Of course when they are done the measurements are dead on. If I were considereing a LSD I don't know who I would get to do the work in Phoenix. I'd have to do a bunch of research before proceeding. The installer is probably more important that the parts as a seasoned installer will know if the parts are quality and whether the install will be reliable and long-lived.

WMF; Me too..! (the same experience as yours).. therefore, leave it to the experts! As a proud Alfa Milano owner with a factory ZF LSD (clutch type), I know that my CS needs a LSD, (when funds allow it) I will be installing a clutch type LSD. There are two or three Porsche repair shops that specialize in P transaxles and they both estimate $1,300-$1,500 (everything).. However being a certified gearhead (a cheap one) I am going to remove the gearbox myself and bring it to the repair shop on the back of my wife's Volvo V70 to let them install the LSD (the proper way), and "only" pay around $800-$900 for labor .

Let's get this thread back on topic... The original poster asked for explanations of "preload" and "backlash." Ahem... Preload , noun, The 2 liters of beer drunk by a Cayman enthusiast as he tries to screw up his courage enough to drop $1500 on a differential installation. Backlash , verb, The actions taken by the Cayman enthusiast's wife when she discovers that he spent $1500 to have a differential installed. :cheers: