bicycle mechanic help please.

[HomeBrewerDude]

I am about to take the bike in and get it fixed, but i have some expereince and tools working on bikes. However, I don't really know where to start with this problem

Problem Description:

When I stand up to accelerate hard, or hit a hill and apply moderate amounts of torque to the petals the derailure in the back appears to move on me, causing the gears to momentarily shift down into lower gears.


This is really frustrating and disconcerting at times. Is my shifter cable too lose? Is there something on the derailure that might allow me to adjust the sensitivity?

http://pics.bbzzdd.com/users/homebrewerdude/LeMond.jpg

the bike is relatively new, and has shimano tiagra rear derailers.

UPDATE:

I decide to let the shop look at it when they told me they wouldn't charge me for a quick check up.

As some people guessed, the derailleur cable had lost a bit of tension. This resulted in the derailleur pulleys being slightly out of aligned with the free cog. More specifically, the derailleur was shifted to the outside by about 2-3 mm. My bike had a quick adjust tension nut on the cable, so the solution was to give that nut a few turns. The mechanic did not need to mess with the set screws.



Bike is riding great now. thanks again

 

[HomeBrewerDude]

anyone?

 

[Koing]

I'd like to know also as my bike has started to do this about 6 months ago also!

I now ride in a gear which doesn't splip

SUCH a pain when I could be hauling a big gear and getting more speed on the flats...

Koing

 

[alkemyst]

you cant use all gears. That's common.

long story short, you have alot of overlap in a 21 - 28 speed. Not all are the best to use due to chain angles.

Outside of that, there could be a problem with the hardware.

 

[Nomada]

You post this just to brag about that sweet ride? It's simply a matter of alignment. The rear derailleur is out of line with the freewheel cogs. Adjust those two little screws on the derailleur (+ & -) til it's aligned. Do it sober or you'll be tryin' for hours.

 

[DaWhim]

shame on you being a lifer

just pm our official bike guru Brutuskend.

 

[DaWhim]

Originally posted by: alkemyst
you cant use all gears. That's common.

long story short, you have alot of overlap in a 21 - 28 speed. Not all are the best to use due to chain angles.

Outside of that, there could be a problem with the hardware.

you can't, i can.

 

[HomeBrewerDude]

Originally posted by: alkemyst
you cant use all gears. That's common.

long story short, you have alot of overlap in a 21 - 28 speed. Not all are the best to use due to chain angles.

Outside of that, there could be a problem with the hardware.

no. thats not the problem. this occurs anytime i put torque on the pedals, regardless of front/back combinations.

 

[Koing]

Originally posted by: Nomada
You post this just to brag about that sweet ride? It's simply a matter of alignment. The rear derailleur is out of line with the freewheel cogs. Adjust those two little screws on the derailleur (+ & -) til it's aligned. Do it sober or you'll be tryin' for hours.

Cool I'll give that a go sometime

Koing

 

[HomeBrewerDude]

Originally posted by: Nomada
You post this just to brag about that sweet ride? It's simply a matter of alignment. The rear derailleur is out of line with the freewheel cogs. Adjust those two little screws on the derailleur (+ & -) til it's aligned. Do it sober or you'll be tryin' for hours.

thanks, but why does the slippage only occur under torque?

 

[DaWhim]

Originally posted by: HomeBrewerDude

Originally posted by: Nomada
You post this just to brag about that sweet ride? It's simply a matter of alignment. The rear derailleur is out of line with the freewheel cogs. Adjust those two little screws on the derailleur (+ & -) til it's aligned. Do it sober or you'll be tryin' for hours.

thanks, but why does the slippage only occur under torque?

bad derailure? change the whole thing?

 

[Koing]

Originally posted by: HomeBrewerDude

Originally posted by: Nomada
You post this just to brag about that sweet ride? It's simply a matter of alignment. The rear derailleur is out of line with the freewheel cogs. Adjust those two little screws on the derailleur (+ & -) til it's aligned. Do it sober or you'll be tryin' for hours.

thanks, but why does the slippage only occur under torque?

More pressure on the chain and the off alignment makes it slip easier when under torgue. This is what I'm thinking. The gear on my bike is the 2nd smallest one slips to the bottom one if I put a lot of power to it. But if I ease up the power it is fine. The 3rd from bottom is fine with whatever I put through it.

Koing

 

[Nomada]

Originally posted by: HomeBrewerDude

Originally posted by: Nomada
You post this just to brag about that sweet ride? It's simply a matter of alignment. The rear derailleur is out of line with the freewheel cogs. Adjust those two little screws on the derailleur (+ & -) til it's aligned. Do it sober or you'll be tryin' for hours.

thanks, but why does the slippage only occur under torque?

Because torque is governed by gravity. The chain naturally pulls down. The torque emphasizes that.

 

[notfred]

The part of the chain that's under stress when torque is applied is the top, btween the rear wheel and the chainring. The derailleur sits on the bottom, out of the way, taking up slack in the chain. There is no more tension applied to the derailleur under heavy torque than when you're not pedaling.

If the derailleur is working properly when you're not cranking really hard, then there's nothing wrong with it. You need to look somewhere else. The first place to look as at the rear quick release: Is the wheel properly seated, and tight? If the quick release is loose you can actually pull the wheel out of the dropouts, and that would certainly cause the chain to skip. If that's ok, then make sure that the cranks are on tight and not wobbling, and make sure the rear axle isn't bent.

Also, a bent or worn casette can result in problems like this, where the chain doesn't line up correctly with the gear until you really crank on it, and then it will jump onto the gear, and you'll feel it as skipping.

 

[angedinoir]

Edit:

no. thats not the problem. this occurs anytime i put torque on the pedals, regardless of front/back combinations.

According to that the cable length is out of adjustment, there should be a screw to adjust your derailer or you can adjust the cable length until it lines up.

You'll want to adjust it so that when it's in neutral gear for both the front and the back, the chain should be evenly spaced between the derailer.

If you have pictures of your shifter and your derailer then...

=============

The problem is that it's in between gears, when you put tension on the chain it causes it to contrict, favoring the smaller of the gears. What you can do is slightly adjust the cable, there should be a screw, to more accurately fit where the gears are. It's not rocket science, but will ultimately depend on the type of shifter you have (clicker or friction).

It's probably just slightly out of adjustment or you have a cheap shifter. My suggestion? Don't buy a bike with more speeds than you need.

 

[HomeBrewerDude]

thanks for the replies

 

[HomeBrewerDude]

update up top.