Crank arm keeps breaking off

[ryerye]

I bought my 15 year old a Schwinn Ider Mountain Bike. The left pedals broke (I think he installed them incorrectly) and we had to order new parts, the crank arm and pedals. My ex husband used a torque wrench to put the new crank arm on. It fell off when my son was riding it. I took it to a bike shop and the guy put it back on with all his body weight. It seemed fine and when my son rode it and the bolt stripped and the crank arm fell off. I just ordered the same parts again sigh..... Does anyone know the correct torque for a bike like this? Should I use Loctite thread repair? I don't want to have to pay to get the bike fixed it is only a month old.

Thanks,
Jen

[Jesper]

(11-10-2020, 10:31 PM)ryerye Wrote:  I bought my 15 year old a Schwinn Ider Mountain Bike. The left pedals broke (I think he installed them incorrectly) and we had to order new parts, the crank arm and pedals. My ex husband used a torque wrench to put the new crank arm on. It fell off when my son was riding it. I took it to a bike shop and the guy put it back on with all his body weight. It seemed fine and when my son rode it and the bolt stripped and the crank arm fell off. I just ordered the same parts again sigh..... Does anyone know the correct torque for a bike like this? Should I use Loctite thread repair? I don't want to have to pay to get the bike fixed it is only a month old.

Thanks,
Jen

Hi Jen,
Welcome!
I don't know what configuration of crankset those bikes have. There certainly shouldn't be a problem with both the pedals or the crank arms. Remember, the pedal on the left side uses reverse threads (left to tight, right to loose). If this was a store bought bike and bought partially assembled then there is a good chance that it was not put together properly at the store in the first place. If you provide photos of the affected area (include threading if possible), we may be able to determine why this is happening. If you took it to a bike shop and all they did was to reinstall and tighten the crank arm up, then there should not have been another incident since the mechanic (after hearing your history with the bike) should have properly inspected the bike for any issues that may have caused the problem. Telling me that the mechanic "put all his body weight on it" is a sign that the mechanic may not be sufficiently experienced at his job. Did he research the torque specs and did he use a torque wrench himself? There should be a recommended torque spec for the crank arm bolt, but with proper tightening (with or without a torque wrench) you should not be having those problems. I've never used a torque wrench for the crank arms; never had one loosen up or fall off during over 40 years cycling. Also, I have never used thread-lock on those bolts either, but nothing wrong with doing it. If the threads on the bottom bracket spindle are stripped (I assume they are) then you will have to replace that part as well as the bolt. Does the crank arm fit snugly on the spindle before tightening? If loose then a new crank arm is in order. It seems that these issues would be covered under warranty (except maybe the pedal). I don't know how long it takes the crank to "fall off", but the conditions are certainly not normal and should not be reoccurring at the frequency that you describe. There are some mechanics on this site so hopefully one of them will give you their opinion of your problem, and possibly a solution. I don't really ride or work on mtb's, nor am I a mechanic except to primarily work on my own bikes. I speak only with the experience I have working on my own bikes, cars, and other mechanical equipment.