Hi,

I have a Mountain Bike with 8 gear shift and 2 chain rings. The gears seem to change smoothly when riding casually however when I apply consistent pressure on the pedals to pick up speed whilst on the highest gear, the chain seems to be in contention. It feels as though the chain is not in gear properly e.g. the chain shifts due to the force applied to the pedals and makes a ' tuck ' noise. When I feel the chain has finally gone into gear and apply pressure to the pedal again, chain shifts again sometimes causing my foot to go off the pedal due to the contention. What could be the problem and how do I fix it?

Thanks

Could be a slight misadjustment that shows up under the pressure and deflection of high pressure pedaling. Though that should be corrected trying to pick up speed in high gear is simply the wrong technique. It's very inefficient, wears your bike and does little or nothing for your fitness. You ride a gear to match your speed, not the speed you want to go, unless you are a racer sprinting.

I agree with cny- you may need to adjust the shift cable at the rear derailleur. If the chain is trying to "auto-shift" from a smaller cog to a larger cog you should try turning the barrel adjuster clockwise. Try a 1/2 first, if that doesn't cure the problem try another 1/2 turn.
If the chain is slipping over the teeth of the cog but not moving laterally then the cog is probably worn out. This often happens when a new chain has replaced a chain that was allowed to stretch too much.
It also sounds as if you are trying to ride in too high of a gear. You would have much to gain by selecting gears that will allow you to spin the pedals at a high rpm.

i really like this web site. seems to have the answers to what your just not sure about, and puts the "oh yeah, that makes sense" into bicycle maintenance. the two pic show my old and new chain, new on the left in the photos. the second picture is about half way down the chains. notice the one on the right is off center from the one on the left. the one on the right only has 1708 mile on it. but still time for a new one.

forgot the web site. and the pictures are sorta backwards. the one on left is the midway down the chain picture. the one on the right is at the top of the chains. new chain on the left in both pictures. here's the web site. https://www.athenscyclepath.com/wconnchainskips2.html

gtrpro, i forgot to mention. exact same thing happened to me. i have 1733 miles on my bike since last may. (09). anyway i changed the chain and then 5th and 6th on the rear would skip like a schoolgirl at recess. didn't change gears. just skipped. look real close at the gears giving you problems. i had to take the whole rear wheel off of the bike before i could see it. the distance between the gears IS GETTING larger. the gaps from one gear to the next is bigger, and if you look close you can see flat spots on the edge of each tooth. get a new freewheel or change the gears that are worn for new ones. either way will work. this is the first chain and freewheel i have ever wore out on a brand new bicycle. think of gears and chains as consumable parts, along with tires and inner tubes. your life will be easier. oh yeah. congrates on riding so much your wearing out your bike parts. this proves what i've always thought. there is freedom in destruction. keep riding.