2

I recently bought a 2nd hand kids bike for my 4yr old. Prev owner replaced rear wheel with one having a freewheel and installed rear brake.

I've noticed the brake does not have any stopping power when my kiddo is riding however the brake stops the wheel when on a bike stand w/out any prob. I replaced the cable, pads, lever with new ones .. problem remains. I can't install V-Brake at the rear as the frame does not support it. I can't find a brake (better quality) that would be able to clear the tyre - seems like I need one that would have around 8 to 9cm reach.

Thanks for any suggestions,

enter image description here

Julius

Improve this question
1
  • 1
    It's possible that the brake lever is a poor choice in terms of its "pull", and the associated mechanical advantage. Some levers are designed to pull more cable than others, as some brakes need more cable pulled than others. But if you mate a lever with a lot of "pull" with a brake not designed for so much "pull", you must squeeze a lot harder on the lever to get effective braking.
    – Daniel R Hicks
    Apr 25, 2021 at 13:40

2 Answers 2

Reset to default
2

Sadly braking at the rear wheel is relatively poor because of a combination of factors.

  1. Kid's hand strength - your average 4 year old can't squeeze the lever as hard as a larger rider. This affects braking at both wheels.
  2. The brake lever's pull ratio is also often sized for a full sized brake so it has to move through a certain angle to pull enough cable to actuate the brake. For a little hand the lever can be scaled down, but it still has to pull enough cable, and the brake calipers are not scaled down. Notice you didn't have to fit a "kid sized" caliper.
  3. Mass - the rear wheel's brake is never as effective as the front wheel's brake because the system's center of mass is higher than the axles. As the brake bites, the rear wheel becomes unweighted, so is more likely to skid.
  4. Cable friction and a longer run of outer - You've already addressed these.

You will generally find the small 12" wheel bikes to have a backpedal or coaster brake because kids have better leg power than hand power.

As such, kids can do "mad-skidz" with a coaster brake but not a hand brake. Part of this is the body motion required to stomp on the rear pedal naturally weights the rear of the bike, while I've never seen a kid consciously do the same while hand braking.

You might try roughing the braking surface of the rim very lightly with sandpaper.

As an adult you should be able lock the wheels easily while wheeling the bike around while you walk beside.

Another test option is for you to try riding the bike and test the brake function. If its really way small, you can sit over it and keep your feet on the ground rather than pedalling.

Do basic checks, are the brake levers bottoming out on the handgrips? Are the wheels true ? Can you take some of the gap out so the brake pad sits closer to the rim, but does not rub.

Good luck ! And remember, they grow up fast and will be on a bigger bike in short order.

Improve this answer
5
  • I am not convinced that the low hand strength of a kid and the small brake lever that would pull less cable with equal mechanical advantage (and thus they are made with lower mechanical advantage to pull enough cable) would be greater factors than the low weight of a kid. The less weight (or actually mass to be specific) there is to brake, the easier braking is. Single pivot sidepulls, in general, are rather poor brakes.
    – juhist
    Apr 25, 2021 at 11:42
  • @juhist I agree - single pivot calipers are not ideal brake systems. They're just cheap and relatively simple to make. Remember few kid's bike will go 100 km in its entire life, and I bet many of them don't travel 1 km let alone 10.
    – Criggie ♦
    Apr 25, 2021 at 12:02
  • 1
    @juhist on the other hand kids' bikes are heavy for their size (even the good ones). This bike could be 40-50% of the kid's weight; as an adult you'd have to have a heavily loaded tourer to get to that proportion.
    – Chris H
    Apr 26, 2021 at 10:32
  • 1
    Also kids' levers tend to give up some of their travel to account for short fingers.
    – Chris H
    Apr 26, 2021 at 10:33
  • 1
    That's why it's important to make sure the wheel is very true and the brake pad is as close as possible to the rim. You can adjust side-pull brakes to make sure both pads hit at the same time to get maximum braking power. Make sure the spring is level with the ground by adjusting the nut on the under side of the brake, then make "fine" adjustments to the spring by popping it out and bending it up on the side that hits the rim first. There might be a limit screw on the brake lever that moves it closer to the handlebar to compensate for smaller hands.
    – Rich Moss
    Apr 26, 2021 at 21:34
-1

This is a single pivot sidepull brake.

They have a flaw in the centering spring that makes the brakes center unevenly. Because of this flaw, single pivot sidepull brakes are made with low mechanical advantage so that the brake pads have enough room to retract, in an uneven manner.

The best fix for your problem would be to find a single pivot sidepull brake that doesn't have the flaw in the centering spring. An optimal centering spring would have the loop not in mid-air but rather around the brake pivot bolt. Presumably such a single pivot sidepull brake would have high enough mechanical advantage that reasonable finger force would stop the bicycle.

Unfortunately, such brakes are not available. The manufacturers of sidepull brakes do not realize the problem and instead came up with a different, poor solution: forced centering. This forced centering is present in dual pivot sidepull brakes. They have several problems:

  • They cannot track a wobbly wheel
  • The brake pad on the offset pivot arm goes up as the pad wears, eventually touching the tyre and destroying it, unless the pad is very thin it wears away before this happens, or unless the pad position is adjusted all the time

However, dual pivot sidepulls have reasonable mechanical advantage so they can stop the bicycle just fine.

Your brake looks like a very long reach brake so I'm not sure if dual pivot sidepulls are available with such a huge reach. You may be stuck with the poor brakes unless you replace the entire bicycle. If selecting a bicycle that has room for large tires, you should prefer V brakes and cantilever brakes over sidepulls as V brakes and cantilever brakes don't have the flaw in the centering springs so they can have high mechanical advantage and reliable centering at the same time.

Improve this answer
1
  • 1
    -1: Answer suggest solutions that are not available, so its not really an answer.
    – mattnz
    Apr 25, 2021 at 21:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.