Author Topic: Bike repair help/advice  (Read 4927 times)

face-punched

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Bike repair help/advice
« on: March 26, 2014, 10:53:12 AM »
I thought you guys would be some good people to ask exactly what I need to do with this issue. For some reason my front gears won't shift from 2 down to 1. It does everything else just fine, but for some reason it intermittently will not go to my lowest set of gears. It did it a while back and then started working, now it is sticking again. I was wondering if it is something I am doing wrong or a simple fix? I have only had the bike for less than two months. Thanks in advance guys and gals!

Edit: Thank you for the info, I will check into it as soon as I get off of work today!

« Last Edit: March 26, 2014, 11:58:31 AM by face-punched »

FunkyStickman

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Re: Bike repair help/advice
« Reply #1 on: March 26, 2014, 11:31:07 AM »
Well, could be 1. the derailleur needs some oil on the pivot points, or 2. cables need adjusting.

MDM

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Re: Bike repair help/advice
« Reply #2 on: March 26, 2014, 11:35:47 AM »
The short but unhelpful answer is "probably because the derailleur isn't moving far enough to cause the chain to move."  Is that what you see also?

Assuming so, the real question then is "why isn't the derailleur moving far enough?"  Couldn't see details in the pictures, but two reasons come to mind:
1. The adjustment set screw is too restrictive. 
      - There are (often) two set screws that limit derailleur motion: one to prevent the chain from slipping beyond the high gear, and the other to prevent losing it inside the low gear.  When you try to downshift, does the derailleur hit the low side set screw?  If so, a screwdriver is all you need to loosen that and allow more travel.
2.  The cable is sticky.
      - If you move the shifter but the derailleur doesn't move, and it isn't inhibited by a set screw, then your cable may need lubrication or replacement.  With the bike stationary, shift to low gear, then try to push the derailleur by hand.  If it moves (and you see the cable move too), it's a good bet this is the problem.
FunkyStickman's ideas look good too.

Good luck!

TrMama

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Re: Bike repair help/advice
« Reply #3 on: March 26, 2014, 12:14:18 PM »
If it's a brand new bike, or has had new cables put on recently, then it could be because the cables have stretched. This is normal and they just need to be tightened a bit. Once cables have initially stretched, they don't stretch any further so you'll only need to do this once.

dfrei

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Re: Bike repair help/advice
« Reply #4 on: March 26, 2014, 12:19:44 PM »
You might also need a new chain (you'll want to replace the rear cassette as well if you replace the chain)

face-punched

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Re: Bike repair help/advice
« Reply #5 on: March 26, 2014, 12:51:35 PM »
If it's a brand new bike, or has had new cables put on recently, then it could be because the cables have stretched. This is normal and they just need to be tightened a bit. Once cables have initially stretched, they don't stretch any further so you'll only need to do this once.

Interesting, that sounds really likely. Thank you for the input and I will look into that as well.

enigmaT120

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Re: Bike repair help/advice
« Reply #6 on: March 26, 2014, 01:47:58 PM »
If it's a brand new bike, or has had new cables put on recently, then it could be because the cables have stretched. This is normal and they just need to be tightened a bit. Once cables have initially stretched, they don't stretch any further so you'll only need to do this once.

On both of my derailer bikes, cable stretch would keep it from shifting up to the larger sprockets, not prevent it from going the other way. 

m8547

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Re: Bike repair help/advice
« Reply #7 on: March 26, 2014, 05:36:09 PM »
Front derailleurs are tricky. I have a good procedure for the rear, despite it being more complex than the front, but I don't have a good method for the front.

First thing to check is the limit screw. Shift to 1, and make sure the chain is on the first ring. Check if there is tension on the cable (pull on the exposed part of the cable with your hand). If there is, loosen the low limit stop, which is usually the screw closest to you, often labeled L. If you can't tell which screw it is, just look at it closely and you should be able to tell which screw does what. Loosen the screw until it shifts reliably but not so much that the chain falls off.

If there is no slack in the cable, it may be more difficult. You will probably have to let out some cable tension with the barrel adjuster on the shifter. Turning the barrel as if you are tightening a screw will reduce the tension on the cable. Adjust until it shifts reliably from 2-1. If you hit the limit of the barrel adjuster, turn it back out, loosen the pinch bolt that holds the cable and let a bit through. Maybe 1/8" to 1/4". Now adjust the barrel adjuster until it shifts from 2-1 reliably.  The problem here is that it will likely be more difficult to shift up now, since there is more slack in the cable. I don't know of a good way to fix this except that you need to "overshift" every time you shift up. Basically you have to push the shift lever (or turn the knob) past where it clicks and hold it there until the chain catches on the next gear. Then release it.  It's kind of a hassle but just about every front derailleur system I've used needs that.

Other, more advanced things to adjust:
-See if the derailleur cage is bent or twisted. Sometimes you can bend it or rotate the entire derailleur to tweak it. I don't recommend it.
-The height is adjustable by loosening the derailleur where it clamps to the tube and sliding it up or down. When in the 3rd gear position I think it's supposed to be a few mm above the 3rd chainring. I don't recommend adjusting this either.
-The cables or housing could be sticky. It's easy to pull the housing out of the frame by creating slack in the cable, then you can slide it on the cable and add lube. The shifter could be sticky, but I don't recommend trying to lube it unless you are sure everything else is good.

I think the standard procedure for adjusting a front derailleur it to set both of the limits then adjust cable tension so that you can shift into and out of 2 from 1 and 3.

Finally, note that cables don't actually stretch. The ferrules compress into the housing, and the cable might crease where it's clamped, and if the cable wears into the lining in the housing that could add slack too. Stretch is just a convenient thing to call it.

Vilx-

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Re: Bike repair help/advice
« Reply #8 on: March 29, 2014, 02:59:04 AM »
Here's another possibility which I didn't know about until a bike store told it to me once. My bike, for example, has 3 gears in front (at the pedal) and 6 gears at the rear (at the wheel). I can use most combinations fine, but trying to get it to work at both extremes at the same time (like front 1, rear 6) is not healthy for the bike, and indeed sometimes doesn't work. That's not a design flaw or poor adjustment, but simply the chain being forced in a very diagonal position.

db_cooper

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Re: Bike repair help/advice
« Reply #9 on: March 29, 2014, 05:04:49 PM »
I had the same issue on my mt. bike today, and it was a crink in the cable above the derailleur.  It effectively shortens the cable, so it won't go all the way to the lowest gear reliably.  Make sure your cable is in good shape before you start adjusting the limit screws ,  like I did.

peppermint

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Re: Bike repair help/advice
« Reply #10 on: April 06, 2014, 09:11:25 PM »
I have another bike question that is probably stupid, but I can't seem to figure it out.

I recently inherited a road bike from a friend -- the tires are flat so I was trying to pump it up with my regular bike pump. The road bike tires have Presta valves, whereas my normal bike's tires have the larger Schrader valves. It seemed to attach and was pumping (my pump has a single head not double, I don't actually know if it's intended for both types of valves) but the tire pressure wasn't changing, and then when I took the pump off and played with the valve again it seems weird -- it's not hissing anymore, which it was doing before I pumped it up (checking to make sure it was open), even with the thingy unscrewed.

Did I do something to the valve with my pump? I'm also not sure if the pump is compatible with the valve, but I'm more concerned I did something to the tire for now. Thanks for any advice...

MDM

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Re: Bike repair help/advice
« Reply #11 on: April 06, 2014, 09:38:13 PM »
From your description "...the tire pressure wasn't changing, and then when I took the pump off and played with the valve again...it's not hissing anymore, which it was doing before I pumped it up" it appears there was "some" air in the tire, but now it is completely flat.

Assuming(?) the pump itself is working, here are 3 things that can happen:
1.  Air goes through the pump hose and out the head but goes around the tire valve and into the atmosphere.  You should hear/feel air flow around the valve if so.  This implies the pump head does not fit Presta valves.
2.  Air goes through the pump hose but neither into nor around the valve.  It should quickly become hard to pump in this case, as you are compressing air in only the small volume of the pump hose.  This implies the pump head does fit Presta valves but you haven't opened the valve enough.
3.  Air goes into the tire.  But it seems that is not happening....

Some bike pumps have multiple heads, some have single heads that work with either valve type, some have single heads with "some assembly required" (e.g. unscrew a head cap and flip an internal part) to adjust for valve type, and some are valve-specific.  Maybe check the internet for more information if you have a brand name and model number?

But to your last concern: based on your description you likely did not damage the tire.