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Auto shifting mountain bike

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Auto shifting mountain bike

Old 06-20-10, 06:37 PM
  #1  
ill_switch
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Auto shifting mountain bike

Hello all,

Would like some help troubleshooting a problem. I'm a roadie and triathlete at heart, so please forgive my lack of mountain bike knowledge.

I have a 4 - 5 year old Diamondback mountain bike. It's a hardtail, Shimano Deore components.

When applying LOTS of torque to the pedals (i.e. standing and powering up a hill), the chain tends to jump off the front chainring it's supposed to be on. It jumps "down" i.e. towards the frame. Usually after a few pedal revolutions it'll jump back on the chainring, but it's pretty abrupt and disturbing. It almost acts as if the rider tried to shift to a smaller chainring.

Any thoughts? I've had "auto shifting" problems on road bikes, and it was usually because the cable was sticking where it passed under the bottom bracket, which was easy to fix with a drop of lube. But this is a top-pull derailer, so there's no cable running under the bottom bracket! the cables are probably original and probably need changing, but the bike has very low miles and is in very good shape overall - it shifts smoothly, chain and other components don't seem worn.

Thoughts? I'm hoping this is a common enough problem that someone can suggest a direction to troubleshooting this.
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Old 06-20-10, 06:58 PM
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Get it into a corner where two walls meet, jam the front wheel into one and hold yourself up with the other against your shoulder and torque hard on the cranks in various positions. See if you can see any signs of the cranks or big ring flexing. And at the same time look at the big ring to derrailleru cage to ensure that the seat tube isn't flexing enough to make a change. I'm also going to assume that you know enough to already have made sure that your BB isn't toast and is wobbly.

I've never noticed anything like this happening but then at my age yours and my definitions of "LOTS of torque" likely varys by at least two orders of magnitude....
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Old 06-20-10, 07:20 PM
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i'd guess slightly bent chainring/maybe maybe bent crank/bb.
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Old 06-21-10, 05:41 AM
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Thanks for the suggestions, I'll check those items. Hadn't even thought to check the bb for play, but of course that seems like an obvious choice. Really hoping it's something simple though, like a sticky cable, vs. a bent chainring or hosed bb.

BCRider, "LOTS" of torque is a 180-lb, reasonably fit guy, standing and mashing the pedals as hard as he can.
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Old 06-21-10, 06:15 AM
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Well, the bottom bracket looks OK. Chainrings have some wear but look straight.

I noticed that the front derailer seems "sloppy." Like, I can grab the back of the cage and wiggle it 1/8" or so. Again, I'm totally 100% new to mountain bikes, but I've never seen this on a road bike and I'm guessing it's not normal.

Also, the cable routing is not very clean. Up by the seatpost, there's a short piece of housing where the front shift cable turns down from the top tube to the seat tube. It looks way longer than it needs to be.

I'll probably try fiddling with the cable, then I guess looking for another derailer to swap on, unless people have other suggestions. Might post photos of the chainrings so you guys can tell me if they look overly worn.
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Old 06-21-10, 09:47 AM
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I'm beginning to think it's chainring wear, or something of that nature:





I just cleaned up the cable routing, and adjusted things on a trainer, and the cables seem fine. But when I go out and really stomp on it, it still happens. Despite the play in the derailer it doesn't SEEM to be causing this. Once or twice, it seemed that the chain was just "skipping" on the ring it was on (jumping teeth), i.e. it didn't downshift and then pop back up, the chain just jumped and slid on the ring it was on.

Is that typical behavior for a worn chainring?

FWIW this is my brother's bike. He casually mentioned the problem to a bike shop employee recently and the employee suggested that the chain was too long and he should remove a few links. Said bike shop employee hadn't actually seen the bike, he was just guessing. Doesn't seem likely to me - if that were the cause, I'd imagine the problem would have been there all along, but according to my brother, it just sort of slowly developed over time.
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Old 06-21-10, 10:23 AM
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Sounds like a wear problem. This often happens with the middle chainring since it is the most often used on most bikes. Check the chain for wear as well... if the bike is 5 years old and has been ridden a lot the chain almost definitely needs replacing and likely the cassette too.

In conclusion, replace the middle chainring, the chain, and the cassette. Change shifter cables and housing and get it properly set up and the thing will be like brand new.

Oh, and it's nice that your local bike shop is hiring mentally handicapped people, but you really shouldn't trust their repair advice.
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Old 06-21-10, 12:30 PM
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The chainrings look pretty good to me from the pictures. On the ones I've seen wear past their service life the sides of the teeth typically have some metal swageing going on that has produced a ridge that you can hook a fingernail on very easily when dragged along the sides of the teeth. I'm not seeing any sign of that happening here in an amount that makes it obvious.

That "jumping" and skipping over teeth sounds more like a totally bagged out chain. When they wear the distance between the pins grows and hopping over the top like that is very typical. You can often see this happening in a small way when you apply pedal torque by hand from the side of the bike. If you do this and see the chain lift up out of the tooth gullets and try to walk up the teeth then your chain is likely shot. And if the chain is cross chained front to rear so there is a good angle I can see where when it hops over the teeth that it could easily drop down to the next ring or even skip right past and land on the BB shell. Get out a ruler and measure the pin to pin distance over the typical dozen links. If it's more than 1/16 longer than the twelve inches that it SHOULD be then you merely need a new chain.

If the problems persist even with a new chain then try the different chain rings. If may be that it's only the most used ring that has worn and isn't compatible with the new chain.

Keep in mind too that MTB parts see a LOT of rapid wear due to the typical muddy and dusty operating conditions. I can't put a number on it but a chain used for muddy and dusty dirt riding is likely going to be very lucky to last half as long as a road bike chain that gets pampered. The pictures of your bike's BB area show that you're no exception. Buying the very best top line gearing and chain for them is just not all that desireable unless you've got money to burn. The best and the mid line stuff are going to be equal for lifespan under these conditions And once you're into the mid line quality level there's little to be gained from a performance standpoint unless you really do have lots of money and truly need to shave off those last 3 to 4 oz in the drive train. But I think it's fair to say that few, if any, of us truly NEED to lose that last 4 oz unless we're racing in the very top levels.
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Old 06-21-10, 12:57 PM
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BCRider, I know what you're talking about with respect to chainring wear - there's no detectable ridge and the "faces" of the rings (i.e. the parts that actually contact the chain) don't look THAT bad, despite all the scratching/marks on the sides of the rings from shifting.

Chain wear measures a little under 1/16". That doesn't strike me as outrageous but it's definitely worn. I may try a new chain and just see what happens. Hopefully the cogs and rings are OK.

Regarding wear - that's something that is throwing me off bigtime here. I'm used to 3k - 4k miles on a chain on my road bikes. But, I'm riding clean roads in nice weather and lubing every hundred miles. My brother bought this used, maybe 2 years ago, and I bet he's put a thousand miles on it, tops. It looked practically new when he got it, so I doubt it has a thousand or two on it, max.
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Old 06-23-10, 06:57 PM
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Well, problem identified.

Put the new chain and cassette on, and was wiping the bike down afterwards, in anticipation of a test ride to see if the problem was solved. I noticed what looked like a scratch on the bottom of one of the chainstays just beyond the bottom bracket. Ran my fingernail over it and it caught as if it was sharp. So I grasped the frame firmly and flexed it, and sure enough - it's not a scratch, it's a crack.
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Old 06-24-10, 05:30 AM
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So that leads to a question. Again, I'm mountain-bike ignorant but familiar with wrenching on road bikes. If we want to swap these parts to another frame, what do we have to look for in terms of compatibility?

Things I'm familiar with from wrenching on road bikes:
-seat post diameter
-steer tube diameter
-bottom bracket size
-front derailer mounting.

Things I'm not sure of:
-Brakes. Is there ONE standard mounting pattern for disk brakes, or do I have to get a frame that matches THESE disk brakes?
-Fork travel. How critical is it that I get a frame that "matches" this fork's travel?
-Anything else I'm missing?
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Old 06-25-10, 12:43 PM
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Anyone?
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Old 06-25-10, 02:52 PM
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Seat post diameter is different from bike to bike. A new frame often necessitates that you buy a new seatpost.
Steer tube diameter is likely to be one of a few standards (I think there are 3 or 4 standards, and 1-1/8" is by far the most common and found on nearly every modern bike that is not a DH racer or exotic road bike)
Bottom bracket size on a standard mountain bike is almost definitely the standard English threading, and the frame will have a bottom bracket shell width of either 68mm or 73mm. Measure the width of the bottom bracket and you will know what you have.
The front derailleur is chosen by the clamp diameter (which matches the seat tube diameter), which direction the cable pulls (up or down), and how the derailleur swings (the cage swings above the clamp or below the clamp). If the frame you buy has the same seat tube diameter as the old frame,and both cables pull form the top then you can use the old derailleur... if the seattube diameter or the cable routing are different you will need a new front derailleur.

edit: You may need an adapter to make your disk brakes fit, but you may not. There are two common mounting standards for disk calipers and adapters are readily available and cheap.

Last edited by LarDasse74; 06-25-10 at 02:55 PM. Reason: added info
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