Slight Bump in Crank

[Enargins]

I have a Rad Power Bikes 2018 RadCity and I noticed a slight "bump" in my left crank arm when it's at the 2:00 position. Only in the left crank arm, not the right. And it happens in all gears.

At one point I felt several "bumps" with each cycle of the crank arm. But almost all the time it's just one slight bump at the 2:00 position.

My thought is that the crank arm needs to be tightened. (Seems that if it was a bad bearing, it would be felt on the right as well, but maybe not.)

I noticed that my bike uses a large hex wrench to tighten the crank arm. I don't have anything that large. Plus, I should probably use a torque wrench for something like the crank arm.

1) Does this sound like the crank arm is loose? If not, then what else could it be?

2) Should I get a torque wrench to tighten it, or is it OK to just use a regular hex wrench until it's tight by feel?

3) Anyone know what size hex wrench is needed?

Thanks!

 

[Sierratim]

I don't believe it's a loose crank arm. I'm guessing here but your bike probably has the widely used tapered square shafts that the cranks slide over. When their securing bolt is tightened it jams the softer aluminum cranks onto the steel shafts. This connection is so tight that it requires a crank puller to get the cranks off.

At 2 years old I'm going to suggest that it's more likely the bottom bracket, the bearing set that connects the cranks to the frame. Feeling a 'click' in one crank is often a symptom of an issue with these bearings. You can verify this by removing the cranks to eliminate their flywheel effect and then manually rotate the bottom bracket shaft where the cranks were connected. This should rotate smoothly with no feeling of friction or rubbing. You'll probably feel a slight roughness. In this case you'll need to replace this bearing set.

This Park Tool video illustrates how to remove many styles of cranks.

In many modern bikes the bottom bracket is usually a sealed set of bearings that are replaced as a unit often termed a cartridge style bottom bracket. You can order a replacement online by matching the diameter and length specs usually stamped on the side of the original. My mech bike uses this bottom bracket.

This page has a video that demonstrates how to remove the sealed cartridge style bottom bracket.

 

[Enargins]

Thanks for the reply! That's very useful.

In another forum someone said that sealed bottom brackets usually last about 50,000 miles (my bike has 2,000 miles). But I guess you're saying that because of the type of crank shaft, it may have applied too much pressure to the bearings.

Someone else suggested it could be a pedal bearing. Do you think there's any possibility of that?

Thanks.

 

[Sierratim]

Thanks for the reply! That's very useful.

In another forum someone said that sealed bottom brackets usually last about 50,000 miles (my bike has 2,000 miles). But I guess you're saying that because of the type of crank shaft, it may have applied too much pressure to the bearings.

Someone else suggested it could be a pedal bearing. Do you think there's any possibility of that?

Thanks.

I can't see even the highest quality sealed bottom brackets lasting anything like 50,000 miles. Mine usually get ~5,000 miles. 2,000 miles is not unheard of, depending on original component quality levels.

Maybe a pedal bearing? Perhaps. You can check this by removing the pedal and rotating the threaded shaft by hand being sensitive to roughness. If there's roughness you can disassemble most pedals, clean the bearings and sleeves, add quality grease and reassemble. If the click persists, go for the bottom bracket. BTW - many pedals have loose bearings. Be sure to disassemble over a clean cloth so they don't bounce away from you.

 

[Enargins]

I can't see even the highest quality sealed bottom brackets lasting anything like 50,000 miles. Mine usually get ~5,000 miles. 2,000 miles is not unheard of, depending on original component quality levels.

Maybe a pedal bearing? Perhaps. You can check this by removing the pedal and rotating the threaded shaft by hand being sensitive to roughness. If there's roughness you can disassemble most pedals, clean the bearings and sleeves, add quality grease and reassemble. If the click persists, go for the bottom bracket. BTW - many pedals have loose bearings. Be sure to disassemble over a clean cloth so they don't bounce away from you.

So, if I take the pedal off and it's perfectly smooth, should I just assume it's the bottom bracket, then?

Also, I'm not very mechanical (though I'll do minor things). I was planning on having my bike tuned up soon by Velofix, doing the level 2 tune up. If it turns out it's not the pedals, do you think I should just have them replace the bottom bracket cartridge? (I'd rather let them do it than deal with it myself.)

 

[Sierratim]

So, if I take the pedal off and it's perfectly smooth, should I just assume it's the bottom bracket, then?

Also, I'm not very mechanical (though I'll do minor things). I was planning on having my bike tuned up soon by Velofix, doing the level 2 tune up. If it turns out it's not the pedals, do you think I should just have them replace the bottom bracket cartridge? (I'd rather let them do it than deal with it myself.)

There are other possible causes, like a loose handlebar clamp, loose spokes that are rubbing, etc., but it does sound like a bottom bracket issue to me. I do my own bike work so checking it out just takes a few minutes of my time. If I'm wrong I move on to another less likely candidate until I get it fixed.

In your case I'd suggest checking the pedals. If that's not it, share my suggestions with the Velofix tech and let him make a hands on analysis and proceed from there.

 

[Enargins]

There are other possible causes, like a loose handlebar clamp, loose spokes that are rubbing, etc., but it does sound like a bottom bracket issue to me. I do my own bike work so checking it out just takes a few minutes of my time. If I'm wrong I move on to another less likely candidate until I get it fixed.

In your case I'd suggest checking the pedals. If that's not it, share my suggestions with the Velofix tech and let him make a hands on analysis and proceed from there.

Sounds good.

Just curious, though: how would a handlebar clamp affect the crank shaft? They seem unrelated to me.

Also, since you mention spokes, I'll just note that I noticed the other day that I had a disconnected rear spoke. Wasn't broken; just came out off the socket. Tried putting it back in and tightening, but the nipple threading must've been stripped. So I ordered a new spoke from Rad. Should be here in a couple of days.

Right now the disconnected spoke is zip-tied to the one next to it. I didn't think that that's what it might be, so I didn't mention it previously. But since you mention that it could be a spoke, then that seems like it would be a likely candidate.

While I don't know how long the spoke was disconnected before I noticed it, I do know that the slight bump I'm feeling only started after I zip-tied to the spoke to the one next to it. Is it possible that that's what's causing it?

But if it were the spoke, would it necessarily always happen in the same position of the crank arm? Seems that in different gears, the crank arm would turn a different number of times for each cycle of the wheel, so the "bump" would be felt in different places in different gears.

BTW, I did tighten all the other spokes, so none of them are loose now.

 

[Sierratim]

You shift your weight with each pedal stroke putting changing pressure on the handlebar. A loose clamp can squeak/click in sync your pedal stroke. The clamp might actually seem tight but if not torqued to spec you can get a bit of rubbing between the clamp and the handlebar causing a squeak, etc. Other parts such as the saddle and headset can also squeak or click in sync with your weight shift/pedal stroke.

I did comment on your spoke issue on another thread. The spokes are usually stainless steel, the nipples are often softer brass. The nipple usually strips well before the spoke hence my sugestion to get a replacement nipple at your LBS. The Velofix tech should be able to address this as well though he may need a heads up just to be sure he has time to true the wheel, etc.

Can a spoke cause a squeak that syncs with your pedal stroke? As you noted, a single spoke is not a likely candidate, at least not in every gear. It is way down on my list of possibles, but you've got a rear wheel that's out of wack with the missing spoke. If the more likely candidates didn't resolve the isssue, I would take a look at the wheel since it has a whole chorus of spokes that are out of adjustment, not to mention a wheel that is likely in need of trueing.

 

[Enargins]

You shift your weight with each pedal stroke putting changing pressure on the handlebar. A loose clamp can squeak/click in sync your pedal stroke. The clamp might actually seem tight but if not torqued to spec you can get a bit of rubbing between the clamp and the handlebar causing a squeak, etc. Other parts such as the saddle and headset can also squeak or click in sync with your weight shift/pedal stroke.

I did comment on your spoke issue on another thread. The spokes are usually stainless steel, the nipples are often softer brass. The nipple usually strips well before the spoke hence my sugestion to get a replacement nipple at your LBS. The Velofix tech should be able to address this as well though he may need a heads up just to be sure he has time to true the wheel, etc.

Can a spoke cause a squeak that syncs with your pedal stroke? As you noted, a single spoke is not a likely candidate, at least not in every gear. It is way down on my list of possibles, but you've got a rear wheel that's out of wack with the missing spoke. If the more likely candidates didn't resolve the isssue, I would take a look at the wheel since it has a whole chorus of spokes that are out of adjustment, not to mention a wheel that is likely in need of trueing.

I should note that there is no sound -- no squeaking or clicking. I just feel a slight "bump" with each crank rotation, but no noise.

As for the spoke, I was planning on just replacing the nipple myself. I figure I can just deflate the tube, take the tire out of the rim, and then reach in and remove the old nipple and replace it with the new nipple and reconnect the spoke -- all without taking the wheel off again. Unless I'm underestimating how difficult it would be. Thus, it's something I'm planning on doing before I call Velofix out.

As for Velofix, the level 2 tune up includes wheel truing. So that'll be done either way.

Oh, and BTW, I did go to my LBS to get a replacement nipple, but they didn't have a 12 gauge (14 gauge was their largest), and I called a couple of others as well. Thus, I just ordered one from Rad.

Anyway, sounds like the first step is to fix the spoke. While I agree it doesn't seem likely to be the cause, the fact that this "bump" started happening right after the spoke issue makes me suspect it might be related. So that, and checking the pedal, will be the first steps.

Thanks for your assistance!

 

[Handlebars]

I had that and bought a set of hex wrenches, $15 on sale. Solved. It started up again but this time both sides needed tightening so I tightened them both up, a lot harder.

 

[Enargins]

I had that and bought a set of hex wrenches, $15 on sale. Solved. It started up again but this time both sides needed tightening so I tightened them both up, a lot harder.

You tightened the cranks or the pedals?

And, did you have any noise, or just feel a slight bump at each crank rotation?

Thanks.

 

[Handlebars]

Tightened the crank. No noise, just a feeling of a slight "skip" on every rotation.
I wasn't sure I had the right largest size hex key when I bought the hex key set, so I tried it out for fit right in the store parking lot and it tightened the crank bolt, and that fixed it.
I do remember though very early on with the bike first suspecting the pedal and trying to investigate that for a very small feeling of bumping on rotation and I settled for spraying lube into the pedal parts. Now, later, I think that was just the crank a tiny bit loose and hard to tell because it was so slight of a bump.
The cheap hex set cost about the same as one wrench at home depot so I'm happy with it and I really tightened the crank bolts up hard this time.
So just go try a cheap hex and solve the puzzle "yes or no". Probably you can walk your bike right into a store and check for fit and give it a try right there and then you buy it, if it turned the bolt.
CanadianTire, infamous for cheap junk, $15 set on sale. Works fine.

 

[Enargins]

Great. Thanks for the reply. Very helpful!

One thing to be careful about with cheap tools is that they sometimes break if too much pressure is applied to them, and it could result in injury. So be careful!

 

[Sierratim]

I've never had a ctank come loose, but apparently it's happened to others. Easy enough to check. For reference, the cranks on my mech & electric bikes are secured with bolts that accept an 8mm hex wrench.

The bolts pressing the cranks onto their tapered shafts should have a threadlock compound on them to prevent loosening. Even if they did originally I always reapply this product to help prevent loosening in the future.

 

[Handlebars]

Rize/Spark had an issue with loose crank bolts previously. They sent out an advisory a while back.