sunburst

Had my first experience with disc brakes today. I mostly do vintage.
Anyway, I was putting together an "mail order" bike for someone. I think it was this one: bikesonline
The marketing said it was a 25 minute job It was straight forward until I mounted the front wheel. The outer brake pad rubbed the disc.

So, after going online I tried a few things. I verified that I could move the inner pad with it's 5mm adjustment bolt. There did not appear to be a way to do this for the outer pad, unless that outer Torx bolt does the same thing (a mirrored location to the inner pad adjustment). I tried that but it didn't seem to move the outer pad.
Then I loosened the caliper mounting bolts, squeezed the lever and tightened the bolts. Still rubbed.
I loosened the mounting bolts again and hand-placed the caliper to the far left/outer position, then tightened. Outer pad still rubbed.

The rotor seemed fairly true. I just can't get the caliper moved over far enough. And btw, if you are not familiar with the Mira, it is a cable/mechanical disc.
Since returning home I carefully reviewed Tektro's data sheet, and watched 4 youtube videos, but can't identify what if anything I was doing wrong. I thought it was maybe the front wheel was not torqued down enough (solid axle with end-hex bolt, the nut is embedded in the left/non-driveside fork blade), but that doesn't seem to be it.

Questions:
1) are both pads adjustable or just the inner?
2) if the outer is adjustable, is it with that Torx bolt?
3) just for my general understanding - does the inner pad stay stationary while the caliper moves only the outer pad? I saw that in one of the videos - it said the outer pad pushes against the rotor, pushing the rotor into a stationary inner pad.
4) any suggestions?

some pix:

1) pad clearance problem looking straight down at it


2) Mira mounting bolts



3) Mira caliper

4) Mira calipers - The top pic is the front caliper, and the Torx (pad adjusting?) bolt I'm asking about is the lower right of that front caliper

cobba

With that brake, only the outer pad moves and only the inner pad can be adjusted.

Would loosening the brake cable allow the outer pad to retract and give more clearance?

cxwrench

Screw the barrel adjust out a couple turns. Squeeze the lever...if the amount of lever travel is close to what you want turn the adjustable (inboard) pad in a couple clicks. Squeeze the lever again and snug the bolts down. Turn the barrel adjuster back in and back the inboard pad out that click or 2. Check and tighten. You'll have to adjust for pad wear during the life of the pads, I normally do that by eye as it's just quicker. The quality of the mounting hardware makes a difference in ease of adjustment, as well as the post or flat mounts being properly machined.

cobba

That will advance the outer pad onto the rotor more, the OP wants more clearance with the outer pad as it's currently rubbing on the rotor and there isn't anymore adjustment.

cxwrench

Please read my post again. If you don't understand it that's ok. I know what the OP is trying to do and I'm explaining how to get there.

cobba

Would I be right in saying it's a constant rub and not a intermittent one.

I'd go about it differently.

First I'd loosen the pinch bolt on the cable and see if the lever arm can be pushed back to make the pad retract to stop it rubbing.
If it can and it does, I'd then re-tighten the cable when it's in that position.
After that I'd play around with the inner pad adjuster and maybe the barrel adjuster to see if it could be fine tuned any better.
As the brake is currently attached on the outer most position that the mounting bolts will allow and the rotor looks to be close to the outer side of the caliper, I wouldn't make any adjustment with the mounting bolts unless it's needed, the only way the caliper can move from it's current position is inwards and moving the caliper inwards will move the rotor closer to the outer pad and outer side of the caliper.

If the outer pad can't be moved any further from the rotor or moved enough to stop it rubbing, there must be something else going on and I can't see any adjustments of the caliper mounting bolts, the barrel adjuster, the inner pad or the cable changing that.

sunburst

I loosened the brake cable at some point, but not sure if I did all the right things at the same time. For example, when I loosened the mounting bolts to slide the caliper over, I don't remember if the cable was still loosened or whether I had tightened it again. The bike is at the buyer's currently, so I won't get to try anything until Mon or Tues.

But thx everyone, I learned a lot in a short period of time, and have a few things to try. My usual heuristic approach to wrenching did not work this time, but between Tekro's install notes, youtube and bikeforums, I think I've got a much better handle on things.

Btw, these 25 min bikes do NOT come with assembly instructions (there's very little to go wrong), just a generic bike manual that does not even cover disc brakes.

sunburst

Yes, constant rub. The rotor does not seem bent.

cxwrench

It's my fault for thinking some things should be obvious. Do NOT use the barrel adjuster to make adjustments for brake pad wear. As you tighten the barrel adjuster and move the actuator arm forward you start to kill the leverage/power of the brake. I was explaining how to get the caliper in the right place and adjust the brake properly by using the barrel adjuster to help hold the caliper in the right spot while tightening the mounting bolts. Any adjustment for pad wear should be made by moving the adjustable piston in and then recentering the caliper. NOT by using the barrel adjuster(s) for the previously explained reason.

sunburst

I tried to fix this at someone else's house. Decided to bring it home to my shop/garage. Within a minute or so I realized the wheel needed a spacer between the non-driveside of the hub and the fork. DUH!!!

Now, bikesonline provides this spacer and it is somewhere in the wheel packaging (it was already unpacked when I arrived), not in the box of tools, pedals and accessories where we assumed everything we needed was. After talking to bikesonline the owner looked again and found it. With the spacer in place I was able to adjust the brakes quickly using the mounting bolts only, the cable already being in a neutral position. The owner wanted me to true the wheel just because, and that was fairly quick too, even without a rim brake to guide me.

Bikesonline claims a 25 minute build, no experience necessary. Yeah maybe, but:
1) no instructions, just a generic and very dated "bicycle manual". We really needed no instructions except "find the spacer and use it!!!"
2) stuff goes wrong: rear rotor ever-so-slightly rubs the pads and can't be adjusted away (bikesonline says it will "seat", iirc, and resolve itself), the chain rubs the FD and can't be adjusted for a wide range of gearing/cross-chaining (chain angle looks strange to me for most of the cogs), front wheel not true (not a functional problem but the owner wants it true!)

I may be back to talk about that FD. Spent 40 minutes trying to eliminate the chain rub, and my take at this point is that they did something funky to get around a design problem. They pre-tensioned the cable to be able to reach the big chainring. The limit screws would NOT position the FD correctly, with the cable slack.

Thx again everyone.

hokiefyd 

In complete agreement with the above, I've found the best way to adjust these types of brakes (in general, wheel spacer issues noted in this thread notwithstanding) is to mount the caliper so that the outboard pad (the moving pad) rides as closely to the rotor as possible without rubbing. Don't worry about the inboard pad yet, and don't even have the cable clamped yet. Set the caliper so that the outboard pad rides nice and close, then adjust the inboard pad inward with the threaded "piston" backing the pad. As with the outboard pad, you want to get the inboard pad as close to the rotor as possible without rubbing. This little bit can take some fiddling, and likely some truing of the rotor to get it perfect, etc.

Only after the caliper and pads are adjusted, would I then connect the cable. Pull the cable tight through the caliper and cinch it with the bolt. Where you have the threaded barrel adjuster is largely immaterial, since you probably won't want or need to touch it anyway. I'd screw the barrel adjuster out only to take up any remaining slack in the cable that may still exist (but there really shouldn't be much if you pulled it taut to begin with).

As cxwrench stated above, using the barrel adjuster to adjust for wear or sloppy initial cable setup will preload the return spring(s) in the caliper and you'll have a high effort pull for no legitimate reason. In the worst case, you'll run out of lever on the bar before fully actuating the brake. That threaded barrel adjuster is very tempting to use, but don't turn it to adjust pad clearance.

sunburst

thx again everybody. I literally took me a minute or so to dial in the clearance, with the cable slack, doing pretty much what you describe above. The brake has a good lever feel now, with no rubbing. I learned a lot in a short amount of time and will not be reluctant to face a disc brake in the future. I even bought an inexpensive tool over the weekend to tweak the rotor if necessary. Their other bike needs this so I thought I'd get the tool if they want me to improve that bike's brakes as well.

Winfried

I'll add a question here instead of creating a new thread just tor that.

Considering the Tektro Mira MD-C400 is a mechanical brake, do I need to insert a pad spacer when travelling (to prevent the two pads from locking tight), or is it only really needed for hydraulic brakes?

Thank you.

Arrowana

The pad spacer is not necessary for a mechanical brake.

Winfried

One less thing to worry about when travelling. Thank you.

veganbikes

I would have created a new question when needed then bringing up zombies. This is an unrelated question but also easy to answer with the googles. You wouldn't need one though only hydraulic brakes.

grumpus

That's an annoying habit.