Brakes Grabbing in Reverse

I started this new thread (as per Bigfitz' suggestion to not hijack the previous one) to try to resolve a problem with the front brake pads grabbing and locking the front wheel up while backing the bike up. It is worse when the wheel is turned to the left, not as bad when the wheel is turned to the right. The pads have only one season on them, and as shown in the pic below (pads in a vice, indicating position) there is a bit of a skew front to back. This is a 550 Seca, so the rotor is on right side of the wheel.

So, to start with I'll clean up the caliper slide, although it moves freely now, and I'll check again to make sure there is no rotational free play of the caliper housing, only sliding. I'll also clean up the piston chamber. Other suggestions of what to check are welcome.

 

I know the pads are suppose to have a taper to them, at least the ones I've replaced on a couple early '80s 650s did.

I'd be looking at the caliper, piston, mounting, bearings, stuff like that. The pads sound like they are wedging between the rotor and caliper. Look for a sloppy fit there.

 

check your axel sounds like something is maybe bent or out of alignment

 

That's why I asked him to start a new thread.

650 Maxim pads are supposed to be tapered, this is a different style brake; the "holding block" style from the 550s, 750 Seca and XJ1100. They're not supposed to be tapered.

All the ones I've dealt with from 550s have been worn straight; the taper may be causing the issue.

So, now I have some questions:

-How many miles in "a season?" (How many miles on the pads?)

-Take a look at the annotated diagram below. Do you have the piston side pad shim (blue arrow) installed, and in the correct orientation?

-Do you have both of the holding block shim plates (red and green arrows) installed? I'm especially concerned about the small one (red arrow) it doesn't quite look like the diagram, it's only like 3/8" by 1/2" and it goes on the "leg" of the holding block.

-Is the "Pad Spring" (anti-rattle spring) worn at all on the inside, where the protrusions on the pads ride against it?

-Are either the sliding pin or the caliper retaining pin obviously worn? The sliding pin should be replaced if it is; the pad retaining pin can be polished up and re-used.

 

Is it possible the wrong pads were installed?

 

OK now I think we are getting somewhere, Bigfitz! To answer your questions:

The bike now has 70,000 km (45,000 miles). The small and large shim plates are there, but the piston side pad shim (blue arrow) IS NOT. According to the diagram on BikeBandit, where I ordered the pads, it was supposed to come with the "brake pad assembly" item I ordered but it did not. I erroneously assumed that if it wasn't in the kit, it wasn't needed (stupid me). I'm currently looking to see if I can locate the old set of pads I may have kicking around somewhere to see if that shim is still stuck on one of them, but my fear is they were tossed long ago.

The anti rattle spring is worn a little, but no surface deformities or noticeable grooving. Ditto for the other two shim plates.

So looks like I need that piston shim. Is it's function to allow more precise piston to pad contact, or does it play a function in allowing the pad to slide a little relative to the piston contact surface? If the latter, it could explain a lot if the pad is "binding" to the piston contact surface when it shouldn't.

So where can I get one? Chacal doesn't sell them, brake pad kits don't seem to have them, and these are stainless steel, correct? Maybe I can get a sheet of stainless the same thickness and make one?

 

I've never been quite sure why it's there, as it only contacts about half the piston, diagonally.

But now that I see how your pads are worn, it appears to be there to counteract whatever forces caused the tapered wear.

You certainly could make one, especially if you had one to use as a pattern. It's an odd shape.

PM me with your address, and I'll mail you my spare as a prototype, provided you promise to send it back once you've duplicated it.

 

4nik8r

Do your pads look exactly like the ones in Bizfitz diagram?

A few details that I can see look different.

You have "notches" on each side of the pad hole. The diagram does not.

If your pads are not identical can you post a pic of the side view?

 

Hi Bigfitz and BillB,

The pads do not look identical to the diagram; you are correct BillB in that there are notches on either side of the pad hole, but they do not seem to contact anything within the caliper housing. I'll post a side view pic tonight.

If I recall correctly, that shim has two "clips" at the top and one at the bottom. The two top clips secure to the metal pad backing on either side of the hole protrusion. This may be a case where an aftermarket pad manufacturer made slightly different pads than stock to universally fit more than one bike model, and as such I got pads with notches to replace pads without them. So, trouble is, the original shim may not fit, so it looks like I'll need to custom build one.

Bigfitz, your offer to loan me your spare is a generous one, but to save time, postage and hopefully avoid the risk of it getting lost in the mail (I'm in Ottawa Canada BTW), how about just tracing around it on a piece of paper, write the dimensions along each straight edge (in mm), and if you have calipers measure the thickness and write that down too. Then, scan and post here. If BillB is correct in suspecting that the one you have might not fit the pads I have, I will have to custom cut the top clips to fit in the recessed notched area anyway. But your template will be a great starting point for the rest of it. Waddya all think ?

 

Way to use your noggin and technology.

 

Just wondering??? So you have the bike up on the center stand and the front wheel off of the ground. With the caliper off do you have any play in your front wheel bearings? How does it sound when you spin the wheel?
You would think the brakes would be draging both directions if there was a calipier issue. Not to say there might be some shims and other things not quite right.

I had a similar problem with my 750. The wheel bearings were toast. They would only rotate in one direction and bind up big time in reverse

MN

 

I know that the replacement of parts can cause all sorts of problems if they are "will fit" and not vehicle specific. Don't be led astray by lots of speculation as to the cause. It can be expensive. I have the same problem with my 1981 Maxim 550. It is all original with only 14000 KM on it. The brakes have never been done. I will be looking at it over the winter to see if I can identify the problem. If I locate the cause I will post it.

 

I think it sounds like a fine idea!




Sorry about the crude draughtsmanship, I used a ball-point pen. Also threw the real thing on the scanner while I was at it.

If my dimensioning of the drawing leaves anything in question, just ask.

0.40mm stainless. Have at it.

 

You may find this hard to believe but bleed your brakes. I opened the bleeder while pushing the bike backwards and some fluid came out as the bars were turned. I flushed the system and refilled with DOT 5 fluid. Added benefit is DOT 5 is silicone based and doesn't hurt your paint. The bike now rolls as easily in reverse as in forward. Who'd of guessed.

 

Is it possible that turning the bars was either tugging or pinching a brake hose? I had this happen with the throttle cable at least, the PO had the cable mis-routed.

 

I checked for that but I have owned the bike since new and it has never had any work performed that would reroute the hoses. When I started to bleed the brakes I got some voids in the steam as though there was air in the system. However the lever had not been spongy. I switched to a vacuum bleeder to remove the old fluid and then flushed the system and refilled with DOT 5. It's a simple procedure and worth a try before stripping or replacing parts. It helped my situation.

 

Wow...lots of good suggestions here. Thanks to all. I was going to post the pic of the pad but I left my camera cable back at the office. I had another look at the situation and I've discovered that while there is no rotation of the caliper housing across the axle of the pin (side to side), there is too much slack between the caliper housing and the mounting bracket, causing the caliper housing to rotate counterclockwise slightly when backing up the bike, as the piston pad rides the rotor surface and pushes the housing downwards or counterclockwise. I think there are three things to fix:

1. Reduce the slack between the housing and the bracket, likely caused by worn shims and worn slider pin and bushings. I may test the theory that this is contributing to the binding by placing very thin secondary shims under the existing bracket shims to reduce the slack, although this would be temporary and just to see what improvement is achieved

2. A pad shim based on Bigfitz' diagram (excellent drawing and scan BTW - very helpful !) to help keep the pad from sliding down with the rotor as the bike is backed up (this may be what it is for to begin with - to keep the pad and piston from shifting relative to each other)

3. Piston chamber cleaning and bleed to ensure piston retracts properly.

I bet the skew in the pads (caused possibly by the missing shim), when reversing, "wedges" the piston side pad between the piston and rotor when the housing rotates that little bit. If the rotation is eliminated by eliminating the slack, the situation will likely improve but I think the key may be that missing pad shim. I'll make one and see.

So, that's my course of action and I'll let everyone know what I find out.

New pads, sliding pin and bushings are likely in order too

 

You've discovered a little-known factoid about the shim plates. In the 1100 fiche, and only the 1100 fiche, Yamaha talks about shims to go between the plates and the body of the holding block.

You are correct in that the caliper body should not "pivot about" or "hang from" the pin; the pin should hold the caliper and block/plate tightly (but not tightly enough to bind) allowing the only movement to be the sliding between the caliper and block.

Once you've replaced the sliding pin, if you still have play, it is certainly acceptable to add a thin shim between the main shim plate and the holding block to take up any excess play.

I just had to shim the plate on one of my bikes to eliminate an irritating rattle caused by excess slop in the very same area. Left un-shimmed, I'm sure some odd pad wear would have developed. In my case, a .005" thick shim did the trick. I took some pics, if you'd like me to post a couple say so.

If you do, you only need shim the large one, because: take a close look at the assembled caliper-- the "arm" on the holding block doesn't actually come in contact with the caliper, just the top corner of the inner PAD. No need to shim the small plate.

However, I think the missing pad/piston shim is the key to your tapered wear. The shim "offsets" the very conditions that caused the pads to wear in a taper at the top and not the bottom, preventing the inner pad from "rocking" under braking load and causing the taper. Seeing your taper-worn pads explains the piston/pad shim.

I also think the tapered pads are the cause of the dragging in reverse issue, as the taper tries to wedge itself in tighter.

 

Excellent analysis Bigfitz....as part of my rebuild I'll be ordering new pads and a speed bleeder screw. I'll make the shim from your diagram once I have the new pads. By eliminating the wiggle between caliper block and holding block, and using the shim, I suspect the new pads will wear evenly and won't bind. The existing pads I don't think will be salvageable.

I may also upgrade to steel braided lines while I'm at it. Bike still has the 1982 original lines.

 

Speed bleeders tend to be more hassle than they're worth. You cannot use them until the system is up and initially bled; so it's kind of a non-plus.

If you have trouble locating thin shim stock, I can mail you a couple of holding block shims; it took me a while to find shim stock so now I have enough to make a couple dozen if I wanted to.

Gotta change those lines. Stainless is a major improvement.