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My left pedal keeps falling off, I think it's loose, or I'm not tightening it properly, but every time I ride my bicycle it gets loose.

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    Welcome to Bicycles Stack Exchange. Some more information will help us with you problem. Please edit your question to add the make and model of bicycle. Adding a pictures of the pedal and crank threads will also help (You have an upvote so you should be able to add pictures. If not, provide links and someone will edit your question for you.) – Argenti Apparatus Apr 17 '18 at 16:22
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    If this bike has been ridden with a loose pedal for a long time, then it could have damaged the threads on the pedal or inside the crank arm. Loctite/threadlocker may help if its not too far gone. – Criggie Apr 17 '18 at 22:44
  • Do note that the left side pedal is reverse-threadded, so you "tighten" it to loosen it. – Criggie Apr 17 '18 at 22:45
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    Note that if the bearings are bad in the pedal itself then this will cause the pedal to unscrew from the crank arm. If the pedal doesn't spin freely either rebuild the bearings in it or get a new pedal. – Daniel R Hicks Apr 17 '18 at 23:40
  • Not even a long time, in my experience. The left crank arm is toast. – whatsisname Apr 18 '18 at 23:06
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There’s a best case scenario and a worst case scenario. Neither is too terrible.

Best case scenario: Your pedal is just loose. You can tighten it with a pedal wrench (some pedals can use a hex wrench) — however note that the left pedal is left-hand threaded so it’ll be the reverse of what you are used to. Thus, you have to screw it counter-clockwise to tighten it.

Worst case scenario: is that the crank threads themselves are badly damaged. This can happen if you pedaled on it while it was loose — because the crank is usually soft aluminum and the pedal is hardened steel. If the crank threads are worn, then no amount of tightening will work. Instead, you’ll have to take the pedal to your local bike shop where they will drill out the threads, tap slightly larger ones, and insert a helicoil. This will allow your pedal to go back on. It shouldn’t be too expensive, usually one-half hour of shop labor plus the cost of the helicoil (5-10 eurodollars).

So neither scenario is too terrible. Good luck!

P.s. don’t ever use thread lock to try to retain a loose pedal. Pedals by design are always tightening themselves when pedaled normally. If a pedal is loosening itself, it’s not because of screw loosening forces, it’s because the crank threads are bad, and threadlock will not help. However, if the threads are still good then threadlock can cause the pedal to bind terribly such that the pedal won’t ever come off. Instead, you should grease the threads of pedals rather than ever use threadlock.

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  • Pedals will UNTIGHTEN when riding. It is a safety measure, meaning that if the pedal bearing locks while you're riding it doesn't lock any further. In the other case it will just snap your ankle and throw you over the bars. So if a pedal is loose when you start your ride it will loosen further en route thus the need for a correct tightening torque from the beginning. – Carel Apr 18 '18 at 15:47
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    That’s an urban myth @Carel - see the Sheldon Brown link I put in. – RoboKaren Apr 18 '18 at 15:50
  • Have you ever experienced that? – Carel Apr 18 '18 at 15:51
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    Experienced what? That you posted less than 1 minute after I posted the link seems to suggest you didn’t bother reading why Sheldon Brown would think you were wrong. – RoboKaren Apr 18 '18 at 15:53
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    @Carel Just to set the record straight, if the bearings work properly epicyclic fretting precession should tighten the pedal threads as you pedal. That said, if the bearings cease, or intermittently cease, the pedal can become un-threaded. I have personally experienced this in a muddy and wet 24 hour race. The bearings became contaminated, ceased, and the pedal un-threaded itself. After freeing the bearings, re-threading, I was able to continue and finish the lap. At the end of the lap they started to cease and un-thread again. In this case ceasing force > precession = un-thread. – Rider_X Apr 18 '18 at 17:32
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In my experience it is the pedal arm that is the problem. When crank bolt loosens it wears away what should be a snug fit to become slightly rounded hence however much you tighten will always come loose

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  • You've got an excellent point there - While OP did say their "left pedal falls off" it may be a difference in terminology and they may refer to the pedal and crank-arm as one unit. Welcome to SE and thank you for contributing. Do take a moment to browse the tour – Criggie Nov 26 '19 at 7:57
  • Thanks will do the tour 😊 – Dazza Nov 27 '19 at 8:43
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The bearings on the pedal are stiff. if the pedal is sealed then you will probably need a new one. If it is not sealed and is serviceable then you might attempt to fix or replace the bearings.

What happens is that when the bearings on the pedal become stiff, the whole pedal with its axle will spin in the crank threads instead of the pedal spinning around the axle, causing it to unscrew and eventually fall out. Pedals are self-tightening by design only in the normal condition of good bearings, where the main acting force on the threads is precession. When the bearings stop moving freely, your force gets transferred to the threads, thus defeating precession and unscrewing the pedal.

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This might sound dumb but use j-b weld, a new bearing, and duck tape. Trust me I've had that happen to me alot.

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    "a new bearing" ? How does a new bearing help with a damaged thread in the pedal/crank ? I can see how JB Weld might work to rebuild the missing parts of the thread, but what does the tape do ? Please use edit to improve your answer. If you still have access to a bike with this repair, then a photo may be an excellent addition. I'm genuinely curious about your suggestion. – Criggie Dec 1 '19 at 4:10
  • "I've had that happen a lot" ? Is it always the same bike that you repair often ? Or are you strong enough to damage the pedal/crank on multiple bikes ? :) – Criggie Dec 1 '19 at 4:11
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Wal Mart has been causing lots of pedals to come loose because they don't follow the manufacaturer's instructions and use a torque wrench when they install them. They don't follow instructions by the manufacturers. This has caused many pedals to come loose and many people to be badly injured. Huffy continues to use Wal Mart because Huffy is making money, and it puts up with the shoddy work of Wal-Mart.

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    I'm not sure what this has to do with the OP's case. It's not apparent that he's riding a Huffy from Walmart – Paul H Sep 10 '19 at 23:08
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    @PaulH is correct, and I'm also not sure how this would happen. Assuming we are talking about threading pedals into the crankarms, pedals with standard threading will tighten themselves as you cycle. Consider that looking at your right pedal, you pedal in a clockwise direction. That should tighten the pedal, not loosen it. If the left pedal is reverse threaded as all the ones I know are, then you also tighten it as you pedal. – Weiwen Ng Sep 11 '19 at 17:58

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