Krakatos

I'm trying to repair my parent's Nakamura mountain bike with V-brakes. While the front brake has good stopping power (although there's some shudder), the rear is very weak. It's a new bike but I don't think it's properly adjusted.

I've gone through a bunch of books but I can't figure it out. I had the local bike shop take a crack it, and they replaced the bike pads, thinking the brake pad material was bad, but it hasn't helped. The pads are adjusted 1 mm from the rim. But pushing hard on the right brake lever has almost no braking effect on the rear. The only way to create any braking force is to pull the brake cable very tight across the fixing bolt (and the bike shop did this too), but then the pads are jammed against the rim, resulting in constant rubbing and slowing of the wheel. Any looser, and the brake won't work. I've read that v-brakes are supposed to be very powerful, so it doesn't make sense to me. The cable is new and looks ok except its a tiny bit mangled where it's been adjusted.

Another problem is that once the lever is released, the brake arms don't tent to come apart on their own, distancing the pads away from the rim as they should. Perhaps the pivot bolts have to be greased, I don't know. But it doesn't explain the lack of brake force.

Any suggestions would be appreciated. I know it's a cheap bike but I'm hoping it's just a mistake in adjustment that can be remedied.

surreal

Pictures, please. Otherwise, we'll probably be taking shots in the dark

FBinNY 

Step one, determine whether the issue is in the brake, cable or lever.

Disconnect the noodle as you would to remove the wheel, and work the brake manually. Is it smooth, and does it open crisply? If so, you know the brake is OK, but the cable is binding. If not, you know it's the brake. Odds are only one arm sticks, so start by loosening that mounting bolt and see if it improves. If so, the brake may be thick compared to the length of the post, but the fix depends on the details. Otherwise, it's likely a lube issue at the post, or possibly a weak spring.

Take it step by step, and come back when you've narrowed the possibilities.

techsensei

I think a large part of the problem is that the cable friction is too high and is preventing the brake from operating properly. Not only does it keep the brake from releasing from the rim, it also prevents the brake from fully engaging when you depress the lever. I see it a lot on cheap bikes, especially those that have women's frames, where the cable routing often has tight radius bends. A lot of cheap bikes have poorly manufactured brakes too, where the springs have a lot of slop and don't work as intended. First thing I would do is see if the brake has any stiction with the cable disengaged. If it moves freely, then you know it is being caused by the cable.

Leif1313

Cheap brakes have more flex in them, does the frame flex? Can you see either flexing? I'm guessing that the brake housings go to stops on the frame with open cable across the top tube? Are the housings in the ferules seated fully? Does the piece of cable going round the seat post go fully from stop to stop?Are the ferules seated properly? I once messed up cutting the housing a bit short, it made things spongy.

leob1

Take the cable out of all of the housing, I'll bet you fid a rusted spot. If you do, replace the housing and cable. And find a different shop.

miamijim

Too much cable friction as previously mentioned.

kr4k4

Are you sure those are v brakes ?
cause that sounds like those center pull type
and the pads are all the way in
if you pull pad out some and readjust cable
will give it better braking
also there's a screw to tighten that tension
as mentioned prob only one side is weak

Zef

Along with your cable friction issue...if your brake housing extends from lever to rear v-brake caliper that could contribute to the weak response of the brake. The long run of housing tends to flex as the lever is pulled which translates to weak and/or spongy feeling at the lever as brakes are applied often times it is hard to adjust so that the lever does not bottom out against the bar.

The options to fix this are to replace your housing with the stiffest you can find and throw a couple of zipties around the housing and frame tubes at key areas that wont cause kinks in the housing...such as the headtube, top tube near where it joins the seat tube, and possibly the handlebar as well. The zipties will reduce some of the flex of the housing and will give you better brake response at the lever.

-j

Turtle Speed

I suggest you do what FBinNY said first. Isolating the problem is the fastest way to get a proper diagnosis. Find out if each brake arm can move freely on its own. Disconnect the brake's quick release and simply push each brake arm towards the rim with your hand, feeling for the resistance from the brake's spring, and then let go of the arm. See if they spring back sharply and strongly (good) vs. feeling weak, mushy, or sticky (bad). If you need more help getting the quick release off (Sheldon's pics confused me when I was a newbie), let me know and I'll find you a video.

Given that it's a brand new bike, I'm going to assume there isn't some complicating factor, like gummed-up brake levers or grease on the rim. But those can be checked later, when appropriate.

The reason some of the readers in this thread are suspicious of your bike shop is that moving the pads closer to the rim isn't normally going to make the braking action any stronger. It simply reduces the amount of lever travel before the the pads are touching, e.g. the lever might only have to be pulled 1/4 of the way instead of 1/2 before the pads touch and braking begins. That's somewhat of a personal preference, but the point is that it's sort of concerning that they tried that as a solution, because unless there was some sort of weird other factor going on (e.g. cable slack at the pinch bolt was so loose that the levers were totally bottoming out), it kinda suggests a lack of understanding.

FastJake

Even the cheapest V-brakes with marginal pads can be made to work amazing, especially for the rear brake. You don't even need to do a super job with the housing.

So something is horribly wrong. Any competent mechanic should be able to diagnose the problem in about a minute. That's what I'd suggest.

TiBikeGuy

Unhook the brake cables, then check if there is any spring tension. If there is no spring tension, you can increase the spring tension by screwing in the set screw. This is usually sticking out of the side of the v-brakes. But if it is all the way in and still have not spring tension. Then reset this out again.

Next remove the v-brakes from the bike. On the brake boss, you will find 3 holes for the sping to be inserted. The lowest is the weakest, the middle is normal and the top hole is the strongest spring tension. Use the next higher setting to increase the spring tension.

Fix these back and you may need to give more cable slack to allow the brakes to move apart. Next step is to make sure that both sides move apart and hit the rims evenly. Depending on which side contacts the rims first, screw in the set screw to increase the spring tension and screw out the other side to decrease the spring tension. Do this until both left and right brakes contact the rims at the same time.