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      07-24-2009, 08:29 AM   #27
lucid
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Drives: E30 M3; Expedition
Join Date: Apr 2007
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Quote:
Originally Posted by r53s65e90 View Post
First of all many thanks for a great thread!

I was reading this thread over at e90post where Mr.5 demonstrates pad change by removing the pad carrier along with the caliper instead of just the caliper as shown in this thread. Whenever I changed my pads on my Mini I use lucid's method of unscrewing the caliper pins and not the pad carrier bolts. I haven't changed pads on the bmw yet. Mr.5's thread got me thinking. I was wondering what is the right/easier procedure to follow. Here are my questions and I would appreciate any opinions/insights.

1) From the pics and the schematics it seems to me that the stopping force is transferred to the pad carrier and not to the caliper pins. Do you think this is correct? The caliper pins look too flimsy and their torque specs are low (22lbft).

2) If 1 is correct it follows that the caliper is not taxed with stopping forces but just with compressing the pads onto the rotor, which would explain the size and torque specs of the caliper pins. Does this make sense?

3) Somebody on the e90post thread mentioned that the caliper pins are only guiding the caliper so it would be better to remove the caliper instead of removing the pad carrier with the caliper. In the latter case one would need to unscrew the pad carrier bolts that are experiencing all the stopping force if 1 is true and are torqued much tighter than the caliper pins. So, remove the caliper or the pad carrier along with the caliper? (I vote for the caliper only as I have done many times. Although it seems easier to handle the pads when the caliper is still attached to the pad carrier).

Thanks in advance.
Here is my understanding (without having the caliper system in front of me):

The pins should not be carrying any significant braking load. If you look at the picture in Step 10 of this DIY, you can see that the caliper body positions the pads so that the top and the bottom ends of the pad back plates are either making or about to make contact with the carrier. When you brake, the pads generate the braking force by resisting the movement of the rotor. The pads "want" to rotate counter-clockwise together with the rotor when the piston applies pressure on the inside pad, which is also experienced by the outside pad since the caliper body is rigid and free to move along the axis of the two pins, but since the pads make contact with the carrier, which is fixed (non-rotating), they can't. So, the resulting friction force is transmitted to the carrier through the large contact area between the top of the pad back plates and the carrier as can be seen in the picture in Step 10. There might be some resultant forces that are not orthogonal to that contact surface, which might be transmitted to the carrier body itself, but they must be small.

I cannot see why it would be necessary to remove the carrier. The carrier bolts are torqued high and it is not easy to get to them with a breaker bar without really jacking up the car. The first time I replaced pads, I must not have pushed the piston back far enough and could not remove the caliper after removing the pins, and unbolted the carrier. It was much harder to do. Then JAJ pointed out that I didn't have to do that, so I learned the hard way.
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