Bike pedal arm keeps falling off.

RimBlock · 16 Jul 2010 at 10:00

I have been meaning to post on this for a while but do not believe I ever got round to doing so. Apologies if I did...

I have a cheap as chips mountain bike mainly on my wives insistence as bikes get stolen regularly here if you have nowhere indoors to store them.

The bike is fine for the most part but after cycling for 5 minutes or so I will notice a wobble on the pedal (non crank side IIRC) and within another couple of minutes the pedal arm and pedal will fall off.

I have screwed up the nut as tight as possible to no effect, same thing still keeps happening.

I am around 100Kg (15st 10lbs), like to cycle in a high gear with slow steady powerful cycles. As I am now doing 200Kg on the leg press in the gym, I imagine there is a fair bit of pressure being applied to the pedals.

So, the question. What do I need to replace to stop my pedals falling off

, and are there any recommendations (parts rather than a whole new bike

) ?.

Cheers
RB

Inquisitor · 16 Jul 2010 at 10:09

You could try some thread locker I suppose. Not sure how much it would help though. Also, the optimal rate for pedalling is somewhere around 90rpm!

Jumblejug · 16 Jul 2010 at 10:15

I had something like this. The crank arm had come loose causing it to wobble slightly, this rounded the edges of the hole that fits onto the bike (for want of a better description). Ended up having to replace the crank arm as it would come loose after a couple of minutes of pedalling. Thread lock didn't work for me.

Gaijin · 16 Jul 2010 at 10:55

It depends on the bike but mine has a spline that the crank arm slips onto, which then you can bolt it place on the end to secure the arm into to the bike. You can then do up two allen bolts which grip the crank arm onto the spline more.

This is obviously for my bike and so totally depends on what you have.

weescott · 16 Jul 2010 at 11:33

Take everything apart, clean and use blue loctite on the crank bolt. Tighten. If it doesn't work then tell your wife that it's a common problem with cheap mountain bikes and you need a better one for your safety.

PardonTheWait · 16 Jul 2010 at 12:54

RimBlock said:
I have been meaning to post on this for a while but do not believe I ever got round to doing so. Apologies if I did...

I have a cheap as chips mountain bike mainly on my wives insistence as bikes get stolen regularly here if you have nowhere indoors to store them.

The bike is fine for the most part but after cycling for 5 minutes or so I will notice a wobble on the pedal (non crank side IIRC) and within another couple of minutes the pedal arm and pedal will fall off.

I have screwed up the nut as tight as possible to no effect, same thing still keeps happening.

I am around 100Kg (15st 10lbs), like to cycle in a high gear with slow steady powerful cycles. As I am now doing 200Kg on the leg press in the gym, I imagine there is a fair bit of pressure being applied to the pedals.

So, the question. What do I need to replace to stop my pedals falling off , and are there any recommendations (parts rather than a whole new bike ) ?.

Cheers
RB

You're quite heavy, and riding in such a high gear all the time you're putting tons of torque through a cheap crank, you've probably just knackered it.

IMO either ride in a more normal gear or get better cranks! I think the same thing will happen with the next cheap crank if you're putting lots of power into it.

As said above, it's better for you and your knees to be a bit more spinny anyway, I presume the bike's for cardio anyway?

Lakeland · 16 Jul 2010 at 13:04

weescott said:
Take everything apart, clean and use blue loctite on the crank bolt. Tighten. If it doesn't work then tell your wife that it's a common problem with cheap mountain bikes and you need a better one for your safety.

This

Lakeland · 16 Jul 2010 at 13:05

Jumblejug said:
I had something like this. The crank arm had come loose causing it to wobble slightly, this rounded the edges of the hole that fits onto the bike (for want of a better description). Ended up having to replace the crank arm as it would come loose after a couple of minutes of pedalling. Thread lock didn't work for me.

or this

I had to do this seen as I don't have a wife. I let some idiot get some beers on my old MTB and the crank wobbled loose everytime i rode it after that, dont know what he did. replaced the crank and it was fine after that

Ian · 16 Jul 2010 at 13:25

Jumblejug said:
I had something like this. The crank arm had come loose causing it to wobble slightly, this rounded the edges of the hole that fits onto the bike (for want of a better description). Ended up having to replace the crank arm as it would come loose after a couple of minutes of pedalling. Thread lock didn't work for me.

Exactly the same thing happened to mine. It fell off while i was 10 miles or so from home lol

New crank arm and all has been well since

fez · 16 Jul 2010 at 13:58

Yeah, the crank or the bottom bracket will have rounded off and the torque going through them will just undo any tightening you may have done. The crank arm and BB spindle need to be as tight as possible at all times or they will round and become useless.

RimBlock · 17 Jul 2010 at 03:33

PardonTheWait said:
You're quite heavy,

Yep and no longer all fat

.

PardonTheWait said:
and riding in such a high gear all the time you're putting tons of torque through a cheap crank, you've probably just knackered it.

Possible. I have ridden the bike about 3 times and only for around 15 minutes a time so far.

PardonTheWait said:
IMO either ride in a more normal gear or get better cranks! I think the same thing will happen with the next cheap crank if you're putting lots of power into it.

The high (not highest) gear is reasonable for me especially as I have only ridden on road and flat road at that. The torque starting off is bound to be quite strong but once going it is just to keep it going. For such short distances I usually pedal standing as well.

PardonTheWait said:
As said above, it's better for you and your knees to be a bit more spinny anyway, I presume the bike's for cardio anyway?

The bike is for cardio as we have a good mountain bike track near us which has just been upgraded for the Asian Youth Olympic Games. I thought I would give the track a go so got a bike. As the pedal on one side is falling off I thought it wise to wait and get it sorted out before going on the track

.

weescott said:
...tell your wife that it's a common problem with cheap mountain bikes and you need a better one for your safety.

Tried that after the second time it happened as we had a 'heated debate' of which bike to buy in the first place and settled on a cheap one due to the bike theft problem here. Unfortunately she didn't fall for it

. I wonder if she will change her mind when I take it to bits in the kitchen

.

The bike has a spindle going through the hub on the frame where the ball bearings presumably reside. The spindle has a square cut section showing that leads to a cylindrical section with the thread on the end. The crank/penal arm is square cut at the top back with a recessed hole on the front for the nut to be hidden in. From memory there was some rounding on the square section of the spindle and the entry of the square cut on the crank/pedal arm.

I would imagine I would have to change both crank arms and not just one (left and right) and are there any recommendations for spindles for someone of my stature (weight

).

Cheers
RB

hux · 17 Jul 2010 at 10:55

Just say to her, do you want me to get fatter, if that doesn't work next time you're having nookie go on top, im sure that'll change her mind.

With regards to pedalling, just keep an nice steady rate, not too fast nor too slow. Too fast will knacker you and too slow will break bits on your bike.

"andy" · 17 Jul 2010 at 17:03

can you take some photos of the crank arm and spindle ? (while apart of course)

will make it much easier for us to tell you how to fix/replace it as there are various different spindle/arm designs

weescott · 17 Jul 2010 at 17:13

Ah Square taper BB and cranks. They are old tech and a POS. It's common for them to round off. That's why companies moved to ISIS/Octalink/Hollowtech 2 and external BB's. Get a Shimano Deore crankset and never to deal with rounded crank arms or BB spindles ever again.

SkodaMart · 17 Jul 2010 at 17:15

Sounds like you have a wafty crank, a common problem amongst married men!

"andy" · 17 Jul 2010 at 17:16

if you can find an arm though then making sure it is fully tightened will stop the rounding

weescott · 17 Jul 2010 at 17:27

No point in putting a new crank arm on a rounded spindle though.

MikeWild74 · 18 Jul 2010 at 18:08

i had the same problem myself, tried everything thing to keep the arm on and the the best i could do was to mig weld the arm.

it was a cheap bike to so i did'nt give a **** and it worked

RimBlock · 19 Jul 2010 at 04:26

"andy" said:
can you take some photos of the crank arm and spindle ? (while apart of course)

will make it much easier for us to tell you how to fix/replace it as there are various different spindle/arm designs

Have been meaning to do just that but unfortunately didn't manage to get round to it over the weekend. Will try to do so.

What I did notice today though was that the arm on the crank side connects only in the centre of the crank rather than having the X type connection to the crank.

weescott said:
Ah Square taper BB and cranks. They are old tech and a POS. It's common for them to round off. That's why companies moved to ISIS/Octalink/Hollowtech 2 and external BB's. Get a Shimano Deore crankset and never to deal with rounded crank arms or BB spindles ever again.

Thanks, I will take a look. Cheap bike so cheap parts to be expected but I was more than a bit surprised to be seeing these problems so soon.

SkodaMart said:
Sounds like you have a wafty crank, a common problem amongst married men!

I think you may have misread there SkodaMart. My crank is fine, it's the arm that starts it moving that tends to fall off :eek:

.

MikeWild74 said:
i had the same problem myself, tried everything thing to keep the arm on and the the best i could do was to mig weld the arm.

it was a cheap bike to so i did'nt give a **** and it worked

Ahh, how I miss not having a garage. Workshop solutions. Part of what makes the UK ....

I am sure my father would have suggested the same think if I was still in the UK. That or Araldite

.

Thanks for the continuing replies
RB