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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 03-16-06, 04:38 PM   #1
joshua rheaume
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chainwheel

hey what's up my name is joshua rheaume. i was wondering how you can adjust your chainwheel so there are no tight spots when you pedal.
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Old 03-16-06, 04:46 PM   #2
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hey what's up your post is rather uninformative.

The first thing I can think of off the top of my head is that you're wanting to use non-round (e.g., Shimano Biopace) chainrings on a singlespeed of fixed-gear bike. If that's the case, yes you can. But you can't adjust the chainring itself - the adjustment comes at the rear wheel.

My second guess is that your bottom bracket axle (on which the crankset turns; the chainrings are connected to the crankset) is not turning well, either because it is poorly adjusted or because the sealed bearings are shot. If it's the former, then yes you (or your bike shop, because if this is indeed your problem then you know very little about how to describe it) can properly adjust the BB for you. If you've got a modern sealed-cartridge bottom bracket, whose bearings are ruined to the point that you feel noticeable resistance while pedaling, then you need to replace it (or have your bike shop replace it).

If one of these sounds like your issue, and you want to work on it yourself, fill us in so we can give you a more detailed answer.
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Old 03-16-06, 08:16 PM   #3
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Hey Tim,

Thank you for your reply. I am sorry for being so uninformative, but the bottom line is I am very naive when it comes to bikes. here is all the clarity I can give you about my problem.

When I was about 8 years old all my friends were into bmx bikes, so I went to the bike gallery with my dad and bought a 1998 diamondback assault bmx bike. I was never really hard on my bike, riding it off and on through the years until about a year ago when I started riding much harder on my bike, and thus Iím having many problems with it.

I know my bike is just a low end beginner bike, but I would like to know if certain things could be fixed without costing me money.

A few months ago my sprocket went out, so I took the back wheel off of my brothers redline BMX bike and put it on mine. The new sprocket works great but now one of my problems is that the back wheel is tweaking out of alignment and the wheel is rubbing on part of the frame. I can re-align my wheel, but it just slips back out of alignment again.

The other problem is my chain will tighten and loosen when you pedal making it imposable for me to tighten the chain enough for it not to fall off all the time. A worker at a bike shop told me that the chain wheel needs to be adjusted. my friend had is chain wheel adjusted and he said the guy just found the tight spot when he turned the pedal, and then he hit the chain wheel with a mallet to adjust it. My friendís bike is fine now, and has no problems.
Do you know how I could do this myself?

Thank you for your help.

-Joshua.
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Old 03-17-06, 07:56 AM   #4
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Check out Sheldon Brown's website.
He has a whole chapter on how to adjust the chainring so there are no tight spots when you pedal.
PS: I had no problen understanding what you were asking for.

Enjoy
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Old 03-17-06, 08:36 AM   #5
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Okay, makes sense as you're asking about a BMX (my first guess: singlespeed of fixed-gear bike, with not-perfectly-round chainrings). I realize you don't know much about bikes, but that info (and what you explained in your second post) adds a lot to your question.

First, you could buy a new sprocket for your bike, or switch the sprocket from your brother's rear wheel onto yours. To do this you'd need a BMX freewheel remover, available at most bike shops. Probably 4-prong. Here's the sort of thing.

With the rear wheel slipping, I suspect you've just not tightened the bolts on the axle hard enough. If you tighten these down correctly, there should be no slipping.

When the wheel does slip out of alignment, this could be connected to the apparent non-roundness of the chainwheel. But since you didn't have a problem with thise with your own wheel, perhaps the sprocket on the rear wheel is out of round. But then your brother should have had some problems with it when it was on his bike.

I don't see how a hammer treatment could get your chainring back into round if it's out of round.

My best guess is that your bottom bracket is loose, which allows play in the chainring's position. The Bottom Bracket is the bearings that the crank rotates on. If you take your crankarm (on either side), line it up with the seat-tube (or any other tube on the bike), and try to push it toward, then pull it away from the tube, and check for play. If there's play, your bottom bracket is loose. A lower-end BMX bike probably has a one-piece crank, and so you can adjust the bottom bracket by using an adjustable wrench to loosen the lockring on the non-chainring side of the bottom bracket, where the crankarm exits. Then tighten the cone (further inward along those threads), and re-tighten the lockring. You'll have to mess with this a few times to get it right, where the crank spins freely but without play.

Also, Sheldon Brown's article page is at
http://www.sheldonbrown.com/articles.html
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Old 03-17-06, 08:36 AM   #6
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try here
http://sheldonbrown.com/straighten-chw.html
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Old 03-17-06, 11:53 AM   #7
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Quote:
Originally Posted by timcupery
I don't see how a hammer treatment could get your chainring back into round if it's out of round.
It's usually not that the chainring is out of round. It's slightly off center. If you slightly loosen the chainring bolts, you can usually tap it just a bit one way or another to better center the chainring on the crank spider.
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Old 03-17-06, 11:57 AM   #8
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Quote:
Originally Posted by supcom
It's usually not that the chainring is out of round. It's slightly off center. If you slightly loosen the chainring bolts, you can usually tap it just a bit one way or another to better center the chainring on the crank spider.
My guess is that this bike has a one-piece crank, so there's no chainring bolts involved. These sorts of chainrings can be loosened with a nut, however, I think.

But if you're loosening a chainring's connection to the crank, probably no need to use a hammer - just shift the thing by hand when it's loose enough.
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Old 03-17-06, 05:19 PM   #9
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I have an old DB Formula One BMX bike that has one-piece cranks, but a removable chainring. The chainring and the spider are two seperate pieces. They are joined by the "BMX-style chainring bolts" that the FG/SS crowd often mentions, which are slightly shorter than those used on a double or triple crankset.

Joshua might get lucky and solve his problem with the help of Shelon Brown's webiste.
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Old 03-17-06, 05:37 PM   #10
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One might think that a bike with only one gear would be easier to work on than one with 20 gears, but it ain't necessarily that way. It sounds to me like your bike needs at least a couple of adjustments.

It sounds to me like your rear wheel is slipping in the dropouts. The cheap way to get it to stay put is to use a pair of star washers under the axle nuts. A neater but more expensive way is to buy a pair of chain tighteners. Don't expect every kind of chain tightener to work with your bike. You might have to look around a bit to find the right ones.

How does your chainring attach to the crank? If you have a 1 piece crank and the chainring attaches at the center, you're pretty much limited. If the chainring attaches to a spider with 5 bolts, loosen them just a touch. You want the chainring to be loose enough to slide a little but still have enough friction to hold it's adjustment. Then crank the pedals a few times. The chainring should center itself more concentrically with the crank. Retighten the bolts and you'll be good-to-go.

Good luck!
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Old 03-18-06, 02:29 PM   #11
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yes it is a one piece crank.
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