jsdavis

I changed my gf's brake pads over the weekend and she said that the brakes don't work as well on long descents. Said they don't work as well after a while and that while it happened in the past, it happened less frequently. The only way I can interpret this is brake fade.

My gf loves to climb but hates descending anything steeper than about 4% so she drags the brakes and goes less than 9 MPH the entire way down. Even if it means descending 2500ft over 12 miles though that is a rather extreme case.

The bike is a clobbered Diamondback steel MTB, Avid Single 1 V brake, Alivio levers, Alex rims on Shimano hub.

We bought a set of Shimano M70R2 inserts from the LBS and they appear to be soft so I do not think they are aged. It's a high volume shop too, so I have no reason to suspect old rubber pads. When I removed the old pads, there was a lip of brake pad material towards the bottom of the old pads where it did not contact the bottom of the brake track. I inserted the new pads and found that I could no longer close the V brake arms sufficiently to insert the noodle. I did not install the old pads.

I saw the big washer was inside the arm and the small washer is outside and then switched their places. I then reinstalled the holder with new pad inserts aligning the top of the pads about 1mm from the top of the brake track. There were maybe 2-3 mm below the bottom of the brake pad to the bottom of the brake track. I figured the pads would move down the track as the pads wore out and this is what I've always done with my own bike.

The question now is why the brakes are fading more. They brakes seemed to work ok when I took it up and down the block and she said the brakes work fine on the flats. There is not nearly as much heat generated in those situations though.

One thing I can think of is the brake tracks have worn sufficiently so that when I aligned the pads at the top of the brake track that I now have a large portion of the pad not contacting the brake track. In other words only the top and bottom of the new pad is touching the rim due to wear of the rim brake track.

She told me about the brake problem via text this afternoon when she was in the Oakland hills and she lives 30 miles away so I have not had a chance to inspect the rims to see now the pads sit against the brake track.

canklecat

Check with Alex on the specs for those rims. The older Araya rims on my 1992 mountain bike have concave rims. This was a design feature on several Araya rims back then. Seems peculiar and perhaps a fad. I got better braking power after switching to narrower Kool Stop Eagle 2 pads that make full contact within the slightly concave center of those Araya rims.

jsdavis

Is Alex and Araya the same thing?

Bike Gremlin

Dragging brakes for miles - it hurts just reading it. Really don't know how to help. I'd say get disc brakes, but she'd warp those as well probably with such riding style.

For making brakes better:

Reinstall the big washer on the inside of the brake arm - so that pads get closer to the rim, and brake arms have a better angle.

Loosen the bolt holding brake cable, so that you can set the brakes with the new pads to the appropriate distance. You might need to change the cable to do that.

The way you set it up now makes the brake pads move downward as the braking force is increased, with brake arms going past the vertical point toward each other, and in practice it seems to lessen the braking force applied to the rim. Not sure if I explained it right.

jsdavis

Should I still align the pad towards the top of the brake track as I have it now or more towards the center where I suspect it may have been aligned in the past.

Bike Gremlin

for v brakes- 1 mm below the top of the rim (where the tyre starts). just make sure that pads don't rub the tyre both when pressed and when released.

Andrew R Stewart 

Pad glazing is the first issue I think of. The pads will soon enough wear in to the shape of the rim's brake track, if concave then the pads will become convex. I wonder if the pads were toed in much. While I hate squeal as much as most heavily toed in pads reduce the surface contact and therefore the stopping "power". I also wonder if the rider was using both brakes equally and/or alternately.


I would first look at the pad surfaces for that condition. A light sanding of any glaze will reset the friction capability. I would then revisit the pad/rim relationship. Both the pad's location WRT the brake track and any toe in, with less toe in being the goal. I like to have the arms being approximately vertical. The pad mounting washer thickness choices are for achieving best set up, not for avoiding cable adjustments. While I was doing these steps I would lube the pivots and the cable noodle. If the cable was getting frayed or had wiggle/kinks in it I would consider replacing it too.


The last thing I would do would be with a very careful approach. I would talk with the rider about getting best performance from her brakes. This might be easily improved with a changed use style. But this is dealing with human nature and the "battle of the sexes" which has a cost far greater then merely trying pad after pad brand/type until the magic one is found. (which ia another option too). Andy.

Leisesturm

I stopped reading the o.p. two paragraphs short of the end. IMO the o.p. should not be working on his GF's brakes! No, it's not rocket science, but the questions being asked, and the situation being experienced would not be happening to an experienced mechanic. Take the bike (now) to a qualified authority, they may not have any more talent or expertise, but they have accountability. I would hate to be the one responsible for someone close, anyone really, having an accident because I worked on their brakes without enough experience. I am fairly confident in my wrenching ability, but I would be reluctant to work on the brakes of friends. Gears, accessories, electronics, but if they have brake issues, I would refer them to a shop I trust.

I don't think they are magic, but, Kool Stop pads would be my go to pad replacement. I could at least then be certain that any performance degradation after replacement is due to poor adjustment. It appeared to surprise the o.p. that installing new (thicker!) pads made it impossible to re-install the noodle. The proper response should have been to loosen the pinch bolt on the caliper, re-seat the noodle, and find the new spot on the cable to secure the pinch bolt. I adjust this with the return springs unclipped from their caliper arms so I am not fighting spring tension.

jsdavis

I had 15 minutes to look at the brakes this evening and the brakes are functioning except that they are making some scratching noise. Sounds similar to the sound when I hold a green nylon scrub pad against the brake surface and spin the wheel. There is no actual fading as she describes. I took the bike down a 300m/1000ft hill with about 5% gradient 6 times. I did both quick stops from about 20 MPH/30kph and dragged the brakes all the way down. The brakes stop and slow the bike with no issue other than scratching noise. There is no problem with the pad and rim interface that prevents the brakes from functioning.

The scratch noise makes her think the brake is not functioning. I am unable to convince her the scratch noise does not affect capability to stop and slow the bike.

I will have more time to look at the brakes again this weekend. I think maybe it needs some cleaning. I think the pads picked up some material from the rim or dirt when she went in a rain storm.

The V brakes were toed in with a business card inserted under the rear 1/3 of the pad. Not very much toe at all.

gsa103

Pads are probably glazed. Take a file or coarse sanding block and remove the outer layer of material, then clean the rims with isopropanol, they should quiet right down.

fietsbob

They're Both Aluminum..

Andrew R Stewart 

scratching pad noises often mean that the pads have picked up grit that then gauges the rims, leaving AL chips imbedded in the pads. Andy.

Bike Gremlin

I find brakes work better without toeing them in. Especially V-brakes. I like them to grip as soon as possible, that's why I keep the pads close to the rim, and without toeing them in.

jsdavis

I took the pads out and found one pad picked up a small piece of aluminum about the size of a grain of table salt. I suspect that was the cause of the scratching noise.

I then cleaned the pads and brake track with 180 grit sand paper. Then cleaned rim with nylon scrub pad.

Issue is now resolved.