Mr Grinch

Very strange. Rented a Trek 7.2 to do the Coastal Trail in Alaska. When I would stop for traffic or to take a picture, the rear brake would lock the rear wheel. When I would pedal, the brake would release and work normally. When locked, the brake lever was firm and in the release position.

At first I thought it was some sort of tourist bike safety feature, but now I'm thinking it was frame flex. Anytime I was off of that bike, the rear brake locked. If I stood on the pedals, I could feel the rear caliper pulsing, trying to apply the brakes. The quick adjuster did nothing.

Again, the bike seemed to be a stock rental with no mechanic on duty when I brought it back after 27 miles. It was a Trek 7.2 20, and I am 6'3" and 203 lbs with many miles training on a Domane and trail riding on an AWOL.

Any thoughts?

Bezalel

Too much resistance in the pivots or cable. Turning the rim sends vibrations back loosening whatever is stuck.

Mr Grinch

I wonder if the brake cable coming out of the frame keeper on one end would result in that behavior? I didn't notice anything.

This situation had spring pressure closing the brake will the brake lever completed extended and essentially locked at that position because the brakes were fully applied.

FBinNY 

Your problem exactly mimics the effects of brake shoe underhang.

If the brake shoe is mounted so it underhangs the rim (toward the hub), it forms a raised lip of unworn material as the rest wears. Eventually this lip is high enough to grip the belly of the rim and hold the shoes closed against the spring. Th moment you roll the grip is released and the brake springs open.

The fix is easy enough, remove the wheel and use a file, rasp or razor blade to trim off the lip.

cny-bikeman

...except the OP says the brake lever was "firm and in the release position." I don't understand how that is possible if the caliper is compressed at all.

spdracr39

How about too short brake cable and when he is stopped the bars are turned pulling the brake closed.

Mr Grinch

When tracking straight at slow speed I could feel the rear brake cycling, trying to grab. Don't recall if it was speed related or correlated with my standing instead of sitting. When I got off of the bike and lifted the rear wheel to see if I could move the crank, the brake was locked solidly. I think it may have been related to frame flex interacting with the brake system.

I'm gonna be there again next week, I'll see if I can look at the bike again, or find out what they did to repair it. Relatively small touristy rental shop with no mechanic on site- I should be able to find the bike or learn the solution.

Mr Grinch

I looked at the pads but didn't see a lip. Could easily have been one as I didn't look for that specific issue. The wheel was locked when trying to roll forwards or backwards.

FBinNY 

As you describe it now it sounds like it might be a rim blip or wide spot. It pulses as it passes between the shoes, and being the tightest spot, you tend to stop when the shors come to it (or the other way around) so the brake seems locked before starting out.

rmfnla

My first thought as well...

ThermionicScott 

+2.

The cable housing might well be the optimal length, but turning the wheel all the way to one side is enough to yank on the cable and pull the caliper partially closed. One or two of my bikes do this, and since there is never a reason to turn the wheel all the way to one side when actually riding, it's a non-issue as long as it doesn't pull the caliper off-center when it happens.

Mr Grinch

Good thoughts but that isn't it. This was happening with the handlebar straight.

The brakes trying to grab was also happening coasting slowly with the bars straight. The pulsing was due to the typical crappy rental wheels not being true.

Will be in Anchorage next week, and hope to find the bike again. Good chance they haven't fixed it yet, so maybe I'll be able to get a look at the brakes. I was in a hurry when I returned the bike, so didn't spend time looking at it closely. It worked while riding the trail and was only a problem when very slow or stopped.

If I were a betting man, I'd lean towards the pad lip phenomenon. I'd not heard of that, because my own bike pads are adjusted properly.

rmfnla

Can't imagine riding a bike with a problem and not taking the time to figure it out...

Mr Grinch

I was eighteen miles into a 27 mile trail ride before I made it back to "civilization" and had to start stopping for road crossings and pictures around Lake Hood. The bike was rolling fine when I was, well, rolling, so I thought it was some weird rental bike "safety feature".

When I got back, I had to haul arse so all I did was ask about it before I had to leave. Working for a living and all that. The young lady renting the bikes was a sweetie pie, but didn't know a thing about the Trek 7.2 and she initially told me that "they all do that". Then she couldn't roll the bike. It did not do this when I initially fit the bike for rental and hit the trail. Neither of us could we see anything wrong visually, despite looking at the brake pads and rear system.

Not like I had any tools to do anything about it on an Alaskan trail either.

I'll try to do better for you next time, but I typically don't ride such primitive bikes.

rmfnla

Not a criticism, BTW, just me and my OCD...

Mr Grinch

My OCD is why I am curious about what caused this phenomenon, so no worries. I just emailed the rental company to see if they have any feedback.

rmfnla

FBinNY 

Earlier I posited the possibility of a rim blip. I hope they don't take a look and decide you for bottoming out the tire (if it is a rim blip).

Mr Grinch

The owner looked at the bike and replied that the brakes sometimes stick after a long rain, which is apparently what they had the week prior to my ride. Said they sanded the pads a little and solved the problem. I asked if he noticed a little lip on the pads, but no reply yet. This trail is paved with some interesting hills. I didn't come close to bottoming anything, and I ensured that the tires were filled to max pressure before I left. Both rims were out of true when I left, but nothing a typical renter would notice.

And just for reference. The brakes would lock hard onto the rim when I stopped. I could not release the brakes using the lever when this happened, and the brakes would lock in any position on the rim. I could roll a half a wheel turn and stop, causing another lock. The strange part was the behavior of the brake lever. You would think it would be depressed. The pads were pretty worn.

They are a great little shop for a quick ride in the Anchorage area, but they aren't a full service bike shop. There are a few places to rent up there, this one gets you on the road the quickest.

Oh well. Enough of this. If I find anything useful, I'll post it.