Me again with question about wheel hop

[Jim Zimmerman]

The problem has been solved, mostly. In this first photo from the rear of the upright bike, you can see the entire brake caliper with the rotor on edge. There is a gap on the right side of the rotor which is more obvious in the close-up.





The adjustment for the right side brake pad is the large red "button" looking screw pictured below.



I turned it tighter, until the rotor began to rub on the brake pad. That reduced the amount of "bend" in the rotor when the caliper squeezed on it, but it made the grabbing even worse. I followed the instructions posted by Super Slim above, loosening the caliper mounting bolts, squeezing and holding the brake lever while tightening the caliper mounting bolts. That centered the caliper on the rotor but I had to back out the red adjusting screw and re-center the caliper so the rotor didn't rub on one side or the other. The rotor was obviously not straight with the wheel.

That's when I went to the LBS and asked for advice. The mechanic told me the hub bolts will self-center the rotor so there isn't anything I can do about the rotor being non-concentric. Regarding the wobble from side to side, he offered two options. 1. He can try to straighten out the rotor. 2. He can sell me some new rotors. The cost was about the same either way but the non-concentric problem would not be cured by option #1.

I bought a pair of Shimano 160 mm rotors and brought them home to assemble.

After installing the new rotor (this is on the rear wheel only) I examined it closely both for wobble and whether it was concentric. It was concentric but the wobble was just as bad as before. I am therefore concluding that the rotor mounting base is not straight to the rest of the wheel.

Fortunately the kit for the new rotor came with some shims. I'm not sure why they were there but I decided to use them to "straighten" the rotor. Each shim spans two bolts. Multiple assemblies later I came up with the combination pictured below. Two shims are overlapping and another is isolated.

I re-centered the caliper and then set the red adjustment screw to just barely touch the rotor, to determine that it contacts on and off around about 3/4 revolution of the wheel. Then I backed it out until it doesn't contact at all and left it that way.

After testing on the street I have decided it is acceptable. Braking is not perfect but it doesn't lock up the rear wheel any more. The photo below shows the odd location of the shims, and BTW you can see the wear patch on the rotor, how it doesn't quite align with the intended friction rim. I checked the original Clarks rotor and its wear pattern is also in the wrong place.



Now that I have concluded my adjustment to the rear brake I took a look at the front brake. While sitting on the bicycle seat I could see right away that one of the rotor mounting bolts was unscrewed about four turns. I tightened up all six (all were loose) and set the torque to 5 newton-meters (nearly as hard as I can turn my T25 Torx screwdriver). This rotor was also non-concentric but it didn't seem to have as much wobble as I had seen on the rear rotor. I tested the front brake on the street and concluded that it was acceptable as well.

SUMMARY
My problem with the rear brake is the rotor mounting base on the wheel's hub is not aligned with the wheel, requiring shims between the mounting base and the rotor. I did not need to buy a new rotor but I may want a new wheel. My problem with the front brake was loose rotor mounting bolts. I should have checked them when assembling the bicycle in the beginning.

 

[super slim]

From the last photo, the wear pattern on the disk is too low, as below the main part of the disc, hitting the arms of the disc!
The wear pattern should JUST stop before the top of the disc, and JUST before the bottom of the main part of the disc.
Washers are required under the Caliper, and use a permanent marker on the disc, then apply the brake gently and rotate the wheel, to see where the marker is rubbed off, and add or remove washers under the caliper.

 

[Jim Zimmerman]

Ahah! Yes, that makes perfect sense! I will get some washers and try moving the calipers up a bit so they contact the disc in the correct place.

Thanks for the additional tip. BTW those videos you uploaded earlier were very helpful too.

 

[Jim Zimmerman]

Brake caliper update for Suz and the other 8 owners of the T50 Custom Black Cruiser: The brakes are Clarks model CMD 17. The retail price of these brakes are $21.50 including a rotor. As a comparison I paid $22.00 each for the Shimano rotors I bought the other day. The Clarks web site has lots of video guides for their other models but nothing for this CMD 17. Yesterday I completely disassembled and re-assembled my rear caliper in an effort to understand how to correct the problems with grabbing, rubbing, and apparent misalignment with the rotor.

I tried putting washers under the caliper mounts but that didn't help. It turns out the pads are aligned with the rotor but when they contact the rotor they do it unevenly and that uneven contact is what is causing the wear pattern I see on the rotor. The calipers appear to be pressing the pads against the rotor at an angle that cannot be adjusted.

In my posting above I pointed out that the red "button" looking thing is actually the depth adjustment screw for the inboard brake pad. Now I have learned that the set screw (in the photo below) is intended to "lock" the red "button" in place. There is a tiny plastic cushion under the set screw so you don't have metal-to-metal contact with the red button's threads. I recommend loosening the set screw a turn or two before adjusting the red "button" then when you're happy with the adjustment, don't forget to tighten the set screw again.

If you want to take out the brake pads, you need to first remove the wheel to get the rotor out of the way. Then you need to pull the separator spring out from between the pads. Don't pinch it with pliers, but instead put the tip of a screwdriver between the legs and pry out the spring. After the spring has been removed you can wriggle each pad, one at a time, into the slot where the rotor normally sits, and slide the pad out through that rotor slot. When the pads are new this is a tight fit. The pads are round rather than rectangular like most disc brake pads. I found them for sale at the LBS but I intend to follow Jeff Parker's advice above and replace these brakes with a better quality set of calipers when the original pads wear out.

After reassembling everything I had to re-install the caliper to reduce the amount of rotor bending when the brakes are applied and simultaneously reduce the amount of rubbing on the rotor when they are not engaged. The inboard pad can be backed out by unscrewing the red "button" but the outside pad is set in position only by re-positioning the caliper. Ignore the crooked alignment when the brakes are not engaged as that is merely how they land when the separator spring pushes them back out.