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Mountain bike repair, bottom bracket sprockets

Mountain bike repair, bottom bracket sprockets

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  #1  
Old 08-02-18, 11:02 AM
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Mountain bike repair, bottom bracket sprockets

Hello,

I have an older big box store style (costco - northland) mountain bike.

21 speed, 3 gears on bottom bracket.

The highest gear on the bottom bracket is damaged, one tooth is worn down and it's slightly bent so the chain clicks when peddling.

How do I proceed? Can this be fixed easily. Bike shops charge an arm and an leg and are a pain - plus it's not worth putting more than $100 into this bike.

I can post a picture of needed.

Thanks
 
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  #2  
Old 08-02-18, 11:16 AM
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You can look online at the cost of parts and determine for yourself if it is worth replacing the damage components. Often when I have to repair something I consider it an opportunity to do an upgrade. There are cheap parts and there are gear sets (cassettes) that cost as much as your bike did when new.
 
  #3  
Old 08-02-18, 11:32 AM
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When the front gear set gets changed the rear cartridge typ gets changes as a set along with the chain.

In addition you will need some specialized tools.

It the bike is higher end or you are into tuning then not an issue,if a budget rebuild then maybe better to apply to a new bike,

I would estimate a min cost for parts and tools around $100 if DIY.
 
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Old 08-02-18, 04:14 PM
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Why does the rear and chain need to be replaced?

Is the model required to source the parts or can I use generic?

Would pictures be helpful?

This is a bike bought at costco, canada 17 years ago. I don't like throwing things away but if it's super expensive, won't do it.

It's a northland magnum.

I think the crank is one piece for both sides and it's pre-sealed bearing era.
 
  #5  
Old 08-02-18, 04:26 PM
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I've been out of the business for more than 2 decades but typically parts would be available for 3-piece crank sets, but maybe.

As for replacing chain and rear sprocket I suspect Marq is correct. One test is to measure a section of the chain. Links are 1/2" long so at 12" (24 links) you don't want the stretch to be more than 1/8" . Using a stretched chain will cause rapid wear of the new sprocket or may not work at all, skipping and jumping. Since the rear sprocket has fewer teeth that usually wears out first.

Got a picture?

Bud
 
  #6  
Old 08-02-18, 04:53 PM
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So you're saying the sproket may have gotten damaged due to a bad chain?

The chain is probably cheap to replace. but the rear gears wouldn't be.

The rear bearing makes some noise, more noticeable when passing by cars and going through tunnels and could probably use a repack.

Pictures attached. The last one shows the damaged part.
 
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  #7  
Old 08-02-18, 05:19 PM
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Ya, that's a single piece crank set and possibly the triple ring set would need to come as a set. Looking at the valley on that large ring tells me chain and rear sprockets will need to be replaced as well, assuming you can even find a replacement for the 3 in front.

Bud
 
  #8  
Old 08-02-18, 05:36 PM
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Is there a way to check the rear sprockets and verify they're bad? (other then visual which may not pick up every problem)

Is it possible the gear just got bent from being hit or something and got bent and the teeth got damaged as a result? I routinely carry the bike up and down flights of stairs.

I sometimes use a low gear on the back (largest sprocket) and highest gear at front (largest) - may have something to do with it too.

How would I go about sourcing the parts? Is it possible to use different parts?
 
  #9  
Old 08-02-18, 05:55 PM
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Even though a triple front and 5 to 7 sprockets in back give a lot of combinations it is best to keep the chain lined up front to rear as best possible. Everyone will have their preferred pairings but cross sprocket alignment is not good for sprockets or chains.

Check your triple to see if just the larger ring can be replaced. If you have used that one most of the time the other two should show little wear.

Hard to inspect the rear sprockets but with the chain wrapper around the ring see if you can lift the chain off the mid point. The front is easier, but the rear can be concluded by knowing which sprocket you use the most. Plus, that chain measurement is a product of sprockets and chains wearing together. See if you can get that measurement. at 3/16" or 1/4" stretch in 24 links (12") all sprockets are toast.

In my shop we rarely had single piece crank sets with a triple front set. Have you googled that make and model. Seems like there should be a bicycle parts source out there somewhere.

Bud
 
  #10  
Old 08-02-18, 06:05 PM
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I can't find a model number. There seem to be several different versions of this bike.

There are two serial number stickers though.

The bottom bracket gear set says "prowheel".

Where's the mid point? Is being able to life the chain off good or bad?

For front, I mostly use the biggest one, the smaller gears are only for hills.

For back, I use gear 3 to 7 mostly with 4 and 5 getting the most use.
 

Last edited by user 10; 08-02-18 at 06:24 PM.
  #11  
Old 08-03-18, 03:44 AM
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Good morning, Did a bit of searching and found zero cranksets for a single piece crank. I suspect there might be some old 10 speed parts with 2 rings, but didn't even see those. I think the problem will be, the bike is (sorry) too bottom line for any mfg to be stocking parts. All kinds of 3-piece sets and your single piece crank could be converted but a bigger project and more cost. I wasn't sure how to describe the midpoint but chain leads onto sprocket, wraps partway around and then leaves. Mid point is halfway between where it goes on and then goes off. New chain and sprocket would fit perfectly into the tooth and valleys. A stretched chain and worn sprocket will be loose as the chain wraps around. Personal opinion, time to shop for a new bike or maybe a quality used one. A lot of nice bikes never get used and just decorate a garage. Find someone who doesn't ride but wants to sell. Best, Bud
 
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Old 08-03-18, 09:53 AM
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It it possible to just find a replacement sproket and chain but leave everything else?

Could probably find the sproket much more easily than a new crank/assembly.

The rear sprokets look okay visually so maybe just the chain and biggest gear at the front.

There's a guard screwed into the biggest gear and from there I see allan key screws I believe holding the assembly together.

The trouble with new bikes is they're either big box style crap and I would be in the same boat later or super expensive.

Used bikes are often stolen and sold.
 
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Old 08-03-18, 10:21 AM
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Actually the allan key screws appear to be rivets.
 
  #14  
Old 08-03-18, 10:37 AM
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Rivets is what I expected.

Buying anything used runs the risk that it was stolen, but getting a driver's license and having them sign a statement that they are the owners should sent the crooks packing. On a bike there should also be a serial number, often on the bottom of that bottom bracket. Check all of the "for sale" papers and web listings at least it will give you a better idea as to the price range.

Having sold a few thousand bicycles i would estimate that less than one in ten adults get much use out of them. Mostly garage decoration. If your local bike shop has a bulletin board that might be good to check out. Also, add your wanted specifications on every local cork board you can find.

We have a metal recycling place that our town manages and i do see a lot of bikes there, possibly a source of parts.

Bud
 
  #15  
Old 08-03-18, 12:23 PM
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Big box store bikes are meant to be disposable. Repair parts -- if they exist at all -- will be scarce.

In bike-speak, what you're calling the gears on the bottom bracket are known as "chainrings" or "chain wheels."

Those rings are steel so the bent tooth could be repairable.

Bud9051 refers to the "valley" in the big ring (which is "bike speak" for the largest chainring), but it would be unusual for the big ring to have the most wear because most riders will spend most of their time riding on the middle chainring or the smallest "granny" chainring. So I'm wondering if there might be some distortion in the picture (fish-eye effect) that makes this "valley" look more pronounced than it is.

The dean of online bicycle repair information (and especially for "vintage" hardware) is the late Sheldon Brown. He left behind an unimaginable treasure trove of obscure bike information, and others are keeping his website active since he died. Sheldon's article on bent chainrings is here. He notes that you might be able to straighten a tooth with the chainring still on the bike by clamping on the offending tooth with a crescent wrench.

Sheldon's article on chain wear is here. He relies on what others already have mentioned here, measuring the links. There also is a subjective test where you pinch the chain on the rear sprockets (known in "bike-speak" as the rear "cluster") and pull them out and see how much daylight you can see between chain and sprocket. It should be really minimal.

The gear cluster is always going to wear faster than the chanrings for the simple fact that they have fewer teeth, so if the chainrings show wear, the rear cluster should show a lot of wear.

This shows a new chainring compared to one that is severely worn:

The valley in a new ring is perfectly symmetrical (and I mean perfectly), and the teeth do not come to a point. The valley on the worn ring is lopsided and the teeth come to a point. The wear patterns on the rear cluster will be similar to that on the chainrings.

The chain you have probably is 'endless,' no master link, so its removal requires either that you disassemble the drive train to get it off or use a chain tool/chain breaker. If you intend replacing it you could get it off with bolt cutters but you'd still need to disassemble the drive train or use a chain tool to put the new one on. The cheapest chain tools cost about as much as two sixpacks of cheap beer.

But if your sprockets aren't notably worn then your chain likely isn't notably worn either, since the primary cause of sprocket wear is chain stretch.

If it were me, I wouldn't go but maybe half of your budget because you can buy a new bike of similar quality from Wally World that will cost less than $100. Before I'd paid for chains or sprockets (and a chain tool), I'd save myself a lot of headache by donating that one to the landfill and buying a new one.
 
  #16  
Old 08-03-18, 12:50 PM
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>>"He notes that you might be able to straighten a tooth with the chainring still on the bike by clamping on the offending tooth with a crescent wrench.<<"

This thought crossed my mind as well. If the offending bent tooth isn't too far out of line this could be the easiest fastest cheapest solution.
 
  #17  
Old 08-03-18, 01:06 PM
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The tooth is literally ground down and the chainring is actually bent off. I doubt the part can be repaired.

Have no idea how it happened.

This bike sat in a garage most of the time but now gets only 2 to 4km per week for grocery shopping, etc. So it shouldn't be very worn out.

I use the biggest gear the most.

With the middle gear, I do still hear a click (not from chain) every revolution, so suspect the bearing needs work too.

The rear sprockets look good and shift smoothly as well.

I believe this bike is a bit better quality than the cheapest ones at big box store. I'm willing to put up to $100 canadian to repair it. We have a store chain here that sells bikes under the super-cycle name and they have a horrible reputation. There must be differences in quality even if it comes from a big box store.
 
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Old 08-03-18, 01:35 PM
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If it comes to replacement, any thoughts on something like this: https://www.sportchek.ca/categories/...%5Bcolor%5D=99
 
  #19  
Old 09-24-18, 03:09 AM
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Ok... Sorryfor the delays.
on the larger front sprocket, does the sprocket appear to be riveted on or are there little allen key (torque bit) screws holding the sprocket to the next? Some (even cheaper brands) have replaceable rings.
As for replacing cassette and chain, rule is if you replace the rear cassette, chain goes too. As the others have mentioned, if you measure a length of chain, and 12" is equal to or more than 1/8" away from a link pin, replace.
The replacing the chain and cassette at the same time is critical to keep the teeth meshed with the chain. I use to keep sets of changes and cassettes marked in pairs so if changing gearing for different conditions came up, I'd not skip the chain .

Back to the original post... If you are doing a lot of riding, spend a few dollars on a new front crank and BB. It's been a while since I rode seriously but i do remember my last crank set (decent quality stuff as i road hard) was around $100 USD .
Tools needed is a pipe wrench, crank puller (~$10) and normal hand tools .
I your outter ring is held on with torque bit screws (4-5 of them) order a new ring and simply replace it . If riveted, you are looking at a new crank and bottom bracket .
 
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Old 02-25-20, 04:37 PM
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update years later -> straightened with pliers and has been okay.
 
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