Jacob W asked in SportsCycling · 1 decade ago

Why does my mountain bike randomly change gears when I am riding?

When I am riding, especially in gears 6, 7, and 8, they randomly change gears. So when I am standing up and riding, it changes gears and the pedals smack into my leg and I fall farward and crush my chest on my handlebars! It hurts! Does anyone know why it is doing this? I THINK it might have something to do with the barrel adjusters but I am not sure what to do with them, or what they are. So if anyone knows, please can you tell me why and explain what to do? Thanks.

5 Answers

  • Anonymous
    1 decade ago
    Favorite Answer

    Sound like they are auto shifting. If you shift gears and it wont go into the next one or drops back to the previous gear it is usually the cable needs adjusted. This video will explain it better then I can. Make sure that the derailleur is firmly attached to the drop out.


  • 1 decade ago

    Your right, it's the adjusters of your derailleurs

    Here's a quick guide I wrote:

    1. With the cable tension cable loosened, move the chain to the largest freewheel sprocket and smallest sprocket on the crankarm.

    (Now is also a good time to bring the small adjustment barrel back to normal - turn clockwise until it bottoms out and then counterclockwise 1/2-1 full turn. Also do the same with the Top and Low adjustment screws, but back them out 1-2 full turns.)

    2. Pinch the derailleur arm between your thumb and forefinger and push the derailleur towards the largest sprocket. Turn the Low adjustment screw so that the the derailleur sprocket is in line with the largest freewheel sprocket. CCW will move it towards the wheel while CW will bring it towards the smallest sprocket of the freewheel.

    3. By placing your thumb on the bolt which attaches the derailleur to the frame, use your index to appy pressure to the side of the derailleur arm facing the wheel and pull it towards the smallest sprocket of the freewheel while turning the crank until the chain is on the smallest sprocket.

    Keep the pressure in the derailleur arm by pulling towards you and turn the Top adjustment screw until the derailleaur cog is inline with the smallest sprocket of the freewheel. CCW moves the derailler cog towards you while CW will move it towards the largest freewheel sprocket.

    4. Bring the chain to the largest sproket (lowest gear) by turning the crank. Set the rear derailleur shifter to the lowest number and reattach the tension cable by pulling any slack out and tightening the cable screw.

    5. Place the chain on the smallest chainring of the crank arm and the largest sproket of the freewheel and turn the crank backwards. Adjust the B screw (facing the back of the bike and closest to the frame) until the derailleur gets close enough to the largest sprocket withou interfearring with the movement of the chain.

    Now move the chain to the smallest sprocket on the freewheel and largest on the crank and readjust the B-screw.

    6. Finally move to the largest sprocket of the freewheel and the middle sprocket of the crank. Then back down to the second largest sprocket of the freewheel. Place some pressure on the rear shift (like you were going to shift to the third largest sprocket of the freewheel) and the chain should begin to make noise as it attempts to make the chain jump.

    If it doesn't then tighten the cable by turning the barrel adjuster CCW. If it jumps to readily with out much play on the shifter then loosen the barrel adjuster.

    Once these have been done then release the pressure on the shifter and turn the barrel adjuster CW until noise is heard. Then turn CCW until the noise stops.

    Shift through all the gears... your done!

  • 4 years ago

    you will desire to adjust the swing of the front mech (or derailleur). it incredibly is complete via turning screws on the derailleur unit to aline the unit so as that the chain differences gears easily. you do no longer say how previous the motorbike is, yet whilst its long previous that a ways i could think of its getting on a for some years, so it incredibly is beneficial to get a clean mech, or possibly even some thing so common as a clean cable, which if crammed with gunk in simple terms won't pass interior of its casing, and must be the clarification the gears are caught. a save should assist you with this (yet please do no longer bypass to Halfords - down with grasping companies). As for the fork, the main significant adjuster you're possibly to have (on the perfect of one or the two legs) is a preload adjuster. This adjusts the sag (the quantity the fork drops once you take a seat on the motorbike) and could be set to around 20% or so of the full commute. As for the bouncing around - it incredibly is a damping situation, and you're able to do nicely to get the fork serviced if its a respectable one, or replaced with a clean one if its no longer incredibly genuinely worth the cost.

  • Anonymous
    1 decade ago

    could be that the teeth on your gears are worn out. Check them and compare the teeth to other gears you rarely use. Also gunk built up between gears can cause the chain to skip and shift upward. Its always important to keep gunk away from between the gears. You may also need to readjust your derailer. Its easy to do, just turn the adjusting knob thats connect to the derailer and spin the chain backwards and adjust untill any clicking sound is gone.

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  • Jacob
    Lv 4
    1 decade ago


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