Knocking sound ? bottom bracket area

[summerdays]

My youngest kids bike which we were given second hand, is generally fine apart from a sound/feeling that I'm going to find difficult to explain. Its a Giant bike with 5 gears.

Its almost if the first inch of turning the pedals is slack then it becomes tensioned. If you carry on rotating the pedals in a very contant way you don't hear any noise and feel like its under normal tension. However if you are more like a 7 yo (or even me when trying to ride it), you vary the pressure you get this slack feeling for a second followed by a knock sound. So when he's cycling along you generally get a knock knock knock sound.

There if anyone has the foggiest what I'm describing can they let me know. (I feel guilty taking it to the LBS as I didn't get it from there).

 

[GilesM]

Can you try the following, it may help shed some more light on the problem.

1. if you hold both left and right cranks, is there any side to side movement in the BB.

2. When you first start pedaling, does the free wheel move forward slightly before the bike starts moving, maybe easy to checking by moving the pedal with your hand.

The first thought from what you have written is that the BB is very lose, or the freewheel is slippy slightly.

Giles

 

[alecstilleyedye]

last time i had anything like that, it turned out to be a worn cassette hub. probably a freewheel with a child's 5 speed, but it might be worth checking that it's tight enough.

you would feel it through the pedals, so it's understandable to assume it's a bottom bracket problem. as a kid's bike, it's probably a one-piece crankset, which if lose would be quite noticeable as the pedals would move around a lot.

 

[summerdays]

Can you try the following, it may help shed some more light on the problem.

1. if you hold both left and right cranks, is there any side to side movement in the BB.

2. When you first start pedaling, does the free wheel move forward slightly before the bike starts moving, maybe easy to checking by moving the pedal with your hand.

The first thought from what you have written is that the BB is very lose, or the freewheel is slippy slightly.

Giles

I have already tried wobbling the cranks and they seem firm (sorry I should have said that in the post).

When you say free wheel... what exactly is meant by that .. is that one of the 2 what I thought were jockey wheels? (terminology isn't always my best subject). I thought the freewheel was very close to the cassette. The knocking sound may come from back there ... its hard to be precise.

Could you me more about freewheels please... in simple terms as you have both mentioned it. Is it a sort of adjust problem or replace bits problem?

 

[summerdays]

OK I've gone and had a look at the bike... when I turn the pedals, the chain goes round for half an inch before the wheel starts to turn. Does that describe the freewheel slipping?

 

[GilesM]

When you say free wheel... what exactly is meant by that .. is that one of the 2 what I thought were jockey wheels? (terminology isn't always my best subject). I thought the freewheel was very close to the cassette. The knocking sound may come from back there ... its hard to be precise.

Could you me more about freewheels please... in simple terms as you have both mentioned it. Is it a sort of adjust problem or replace bits problem?

Just to clarify, the freewheel is the unit with the 5 cogs on the rear wheel, if working correctly it should allow the pedals to turn backwards without the bike moving, amd as soon as you turn the pedals forward then the bike should start to move forwards, if there is alot of movement before the bike starts moving forwards (the freewheel ratchet is taking a long time to engage) it does sound like a freewheel problem, however I would also recommend checking that the chain is not too slack, just look at the chain with the bike stationary on a flat surface, the chain from the freewheel to the front chainring should be a straight line, not with a large sag (it should not look like a telegraph line on a still day). If the problem is the freewheel it maybe possible to fix it, there are usually three reasons for the freewheel to stop working correctly:
1. Too much heavy oil or grease in the ratchet
2. Ratchet is rusty
3. Ratchet is just worn from too much wear.

In the case of 3. you have no choice but to buy a new freewheel.

For 1. remove the back wheel and use a fine spray oil, WD40 or 3 in 1 spray with PTFE, these are very fine oils which will help remove the thicker oil or grease which is clogging the freewheel ratchet.

For 2. do the same as above for one, you are just trying to lubricate the rusty ratchet. Also if it's rusty. lying the wheel on the ground and giving the spindle a few light taps with a hammer can help free the ratchet.

The are basically two types of freewheel, either freewheel or cassette hub, I expect you have a freewheel not cassette hub as it is a 5 speed, but have a look at sheldons website, this will help you identify things and give you a little background reading. Stuff abount ratchets and pawls may also help.


http://www.sheldonbrown.com/free-k7.html

I hope this is some use.

Giles

 

[GilesM]

OK I've gone and had a look at the bike... when I turn the pedals, the chain goes round for half an inch before the wheel starts to turn. Does that describe the freewheel slipping?

This could be a freewheel problem, have a read of my previous post and see if I am making sense, apologies if it seems like gibberish, please ask again.

Giles

 

[summerdays]

I think your post makes sense but of course the bike is at school at the moment. The bike isn't generally rusty (just normal amounts for a bike that gets riden in the rain) so I don't think its rust but I will check when its home again.

 

[GilesM]

I think your post makes sense but of course the bike is at school at the moment. The bike isn't generally rusty (just normal amounts for a bike that gets riden in the rain) so I don't think its rust but I will check when its home again.

Apologies, I don't mean to be harsh about the way the bike is looked after, some freewheels do not have the best weather protection, (more recent cassettes are much better, mainly due to the designs for the off road world) and when the chain and normal gear parts are oiled the freewheel workings are often missed as they are not in anyway visable. Years ago when I had a Saturday/evening job in a Bike Shop we would often see a bike come in with a freewheel problem, the bike would be well maintained and look immaculate, however the freewheel workings would be nicely rusted up, a tap with a hammer and a good amount of a light oil usually solved the problem.

IHTH

Giles

 

[summerdays]

No offence was taken ... I'd already said it was second hand so its a fair assumption to make about it being rusty. And I certainly wouldn't realise that it could look OK on the outside and be rusty in the inside.

I've never lubricated either of MY bikes there so I better look at that too. How often would you suggest it ... once a year sort of job, monthly or weekly? Well is it an everytime I clean the chain cogs thing or would that be too much.

I'm still learning loads about the bike - I'm slow at mechanical things so I don't take it all in a once. This gradual process of learning a bit more all the time works for me. I usually read other peoples problems too to see what information I can pick up.

 

[GilesM]

I've never lubricated either of MY bikes there so I better look at that too. How often would you suggest it ... once a year sort of job, monthly or weekly? Well is it an everytime I clean the chain cogs thing or would that be too much.

This really does depend on whether it's the freewheel or cassette hub, for cassette hubs, every six months is fine, but once a year would probably be okay, but for freewheels I would do every couple of months.

 

[summerdays]

Thanks for that - I'm presuming an LBS ought to do that as part of a service?

Anyway I'm back, he's back, but I'm too tired cycling against the wind to do anything other than a quick look without taking the wheel off - no obvious rust on the outside. Chain may be a tiny bit slack....

I think I may leave the full investigation until the weekend ... I will see how I feel later.

 

[dantheman]

my bike has started making a slight knocking noise as i put power onto the peadl on each stroke, too. the BB doesnt seem to have any play, and as the freewheel doesnt disengage (i dont think) as im constantly spinning with some pressure on it. its i suppose a ticking noise, and gets really annoying now........................... is there anything else worth checking???

 

[GilesM]

my bike has started making a slight knocking noise as i put power onto the peadl on each stroke, too. the BB doesnt seem to have any play, and as the freewheel doesnt disengage (i dont think) as im constantly spinning with some pressure on it. its i suppose a ticking noise, and gets really annoying now........................... is there anything else worth checking???

The back wheel bearings could be worn or badly adjusted, take the back wheel out and see how smoothly the spindle rotates, also check if there is any play.

 

[summerdays]

Right the rear wheel is off and the noise definately comes from area of the cassette/freewheel (which says index freewheel something about long dnp on it).

I can move the cassette from side to side which seems to be making a knocking sound ... would that indicate that the freewheel is worn out? or loose?