brake function and the importance of freeplay

[skids]

https://youtu.be/SY3-_2AloBU

Keep watching this video through the technical explanation of the brake function. It gives another possibility for brake lock-up besides "spooge" in the vent hole.

[dbeardslee]

I know how brakes work, but for some reason that video just mesmerized me. Good one, Skids!

I put a couple standards that have been sitting for several years back on the road this past summer, and that entailed rebuilding the brake systems on both. During that process I discovered a couple other things that can make 'em stick.

The first was relatively straight forward, but something that often gets ignored, and that's the caliper boot - the little rubber boot that allows the caliper to slide back and forth. Those things get hard and sticky over time, and the hole they live in gets crudded up too. And when that happens the caliper can't slide like it's supposed to. I found that removing the boot and "massaging" it with silicon lube returns it to a nearly new condition. I clean out the holes with a battery terminal brush, and when reassembled you can really tell the difference in how the caliper moves.

The other problem was more unexpected. I found that the brake pads for the rear (or for the front on 78/79's) had a retaining hole that was too doggone small for the retaining bolt that goes through it. The result was that the bolt applied pressure to the top of the pad and wouldn't allow it to release. A quick trip to the drill press to enlarge the retaining hole a bit solved that problem. That's the first time I've run into that problem, but it's now on my list of things to check when renewing brakes.

[SFerinTEXAS]

retaining bolt that goes through it

I puzzled the purpose of that screw ( at least on the rear brake ) and could not find any good reason it's there so now I don't even put it in .

[dbeardslee]

I puzzled the purpose of that screw ( at least on the rear brake ) and could not find any good reason it's there so now I don't even put it in .

The only thing that holds that type of caliper on is one bolt, and that bolt is riding on a rubber boot. If the boot is working properly it allows a little "wiggle room" for the caliper. The brake pads themselves are held down by the caliper mounting bracket, and that little screw is there to keep the caliper down on the inside and to help insure a "perpendicular push" from the piston. Since it's the only thing securing the calipers on the inside I personally wouldn't run without 'em. You might be able to get away with it, and then again, you never know when that darn Murphy is going to show up.

[MaximPhil]

I have always just re-used the screw that came with mine. It has a reduced end which I now assume is factory original or some PO did a really good job of modifying. As I have only purchased aftermarket pads that have all come with a regular screw. I now wonder if OEM have the custom screw in the package?
Phil

[dbeardslee]

I have always just re-used the screw that came with mine. It has a reduced end which I now assume is factory original or some PO did a really good job of modifying. As I have only purchased aftermarket pads that have all come with a regular screw. I now wonder if OEM have the custom screw in the package?
Phil

The parts fiche shows the pads, anti rattle clips, and screws as all being included under part #4K0-W0045-01-00, brake pad kit.

It was the screws with the reduced end that wouldn't fit through the hole on the pads, and I attribute that to the holes just being too narrow. After drilling out the hole I replaced the reduced screws with some that came with an aftermarket set of pads, and those don't have a reduced end.

If you ran into this problem you could fix it in one of two ways - either reduce the end of the screw farther, or open up the hole. To me it was just easier to drill a quick hole than to grind the end of the screw.

[MaximPhil]

The parts fiche shows the pads, anti rattle clips, and screws as all being included under part #4K0-W0045-01-00, brake pad kit.

It was the screws with the reduced end that wouldn't fit through the hole on the pads, and I attribute that to the holes just being too narrow. After drilling out the hole I replaced the reduced screws with some that came with an aftermarket set of pads, and those don't have a reduced end.

If you ran into this problem you could fix it in one of two ways - either reduce the end of the screw farther, or open up the hole. To me it was just easier to drill a quick hole than to grind the end of the screw.

If there is enough metal in the pad for a bigger hole it makes me wonder why the factory did not just use a normal screw in the first place ?
But as you say drilling the hole is certainly the most convenient solution for most of us.
Phil

[dbeardslee]

If there is enough metal in the pad for a bigger hole it makes me wonder why the factory did not just use a normal screw in the first place ?
But as you say drilling the hole is certainly the most convenient solution for most of us.
Phil

Personally I think the screws with the reduced end are a better choice. The pad is going to wiggle around on the screw a bit, and if it's wiggling on threads they can act like a saw blade, which is my guess as to why the factory screws are reduced. But I think the pads would wear out long before the non-reduced type became a problem. I've used both kinds, and either type of screw works as long as it's got enough clearance in the hole.

[MaximPhil]

Personally I think the screws with the reduced end are a better choice. The pad is going to wiggle around on the screw a bit, and if it's wiggling on threads they can act like a saw blade, which is my guess as to why the factory screws are reduced. But I think the pads would wear out long before the non-reduced type became a problem. I've used both kinds, and either type of screw works as long as it's got enough clearance in the hole.

I like you reasoning Doug : smaller as a result of removing the thread. then the pad hole made to fit securely to the pin.
Phil

[dbeardslee]

I like you reasoning Doug : smaller as a result of removing the thread. then the pad hole made to fit securely to the pin.
Phil

Something you have to watch for even with the reduced screws - if you screw one all the way in a caliper and look at the business end you'll see that the screw may extend out far enough for some of the threads to be exposed. And it's back on that end of the screw where the brake pad sits. Probably best to make sure you have a thick enough lock washer so that only the reduced part of the bolt extends out of the caliper, but not so thick that you reduce the amount of engagement in the hole.

I mention that because on the bike I encountered the problem the bolt was reduced on the end, but the threads extended beyond the caliper. The threads made contact with the hole, which wasn't big enough for it, and it applied pressure to the pad. I would have liked to reuse the reduced bolt, but it was kind of chewed up and I opted for a new one. Hence the enlarged hole in the pad.

[ViperRon]

Screw threads could wear into the side of the pad causing the pad to grab when pressure was applied. That is why you need a smooth surface. Most would put a soft screw in but if you put a hardened one in this could be a result.

[dbeardslee]

Screw threads could wear into the side of the pad causing the pad to grab when pressure was applied. That is why you need a smooth surface. Most would put a soft screw in but if you put a hardened one in this could be a result.

The problem I had was that the threaded portion of the screw was bigger than the hole in the pad. Once the pad is in it's proper place the pad isn't going to move "in and out" in relation to the caliper. The back of the inboard pad lays flat against the "hooked" part of the caliper and it pretty much stays there. The screw just keeps the caliper from lifting up on the inside. As such I don't think it makes much difference whether the end is threaded or smooth as long as there's enough clearance in brake pad hole. I've used both types of screws and never had any problems with the threaded type until I encountered pads with a hole too small for the threads.

As far as being hardened, I've only ever used screws that came with a set of pads. Just looking at them they've got the gold coloring of a grade 8 bolt, but I don't know for certain if they're grade 8 or not.

[motoman]

The problem I had was that the threaded portion of the screw was bigger than the hole in the pad. Once the pad is in it's proper place the pad isn't going to move "in and out" in relation to the caliper. The back of the inboard pad lays flat against the "hooked" part of the caliper and it pretty much stays there. The screw just keeps the caliper from lifting up on the inside. As such I don't think it makes much difference whether the end is threaded or smooth as long as there's enough clearance in brake pad hole. I've used both types of screws and never had any problems with the threaded type until I encountered pads with a hole too small for the threads.

As far as being hardened, I've only ever used screws that came with a set of pads. Just looking at them they've got the gold coloring of a grade 8 bolt, but I don't know for certain if they're grade 8 or not.

Original screws ARE grade8. DOT regulations specify and brake retainer, frame bracket mount bolts, bumper bolts, and suspension related retainers ARE grade #8.