Gears change under pressure

mog812

I have a new-ish bike, which I have taken out for a few little runs, however, when I change down a gear (or is it up, am always confused on this; smaller rear cog), to pick up a little more speed on approach of a hill, or to try and power over the top of it, the pressure causes the gears to drop another 1 or 2 gears which then throws me right off and makes life all the more difficult, the same can also be said when moving to easier gears, which then jump to even easier gears and end up peddling air.
I have read somewhere on here, that chain/cogs being worn can cause this; however with the bike being new, I was wondering if there might be any different problems I might look for?

many thanks for your time.

Mog.

wishitwasallflat

Could be a few things - if you post what type of shifters gears you have that would help - e.g. SRAM Shifters have a facility to downshift (move from smaller to bigger sprockets) up to three gears at a time if you push the lever over further. Moving between sprockets is controlled pretty much by how much tension there is in the wire that runs from shifter to deraileur so that may be your problem. On newish bikes the cable very often stretches a bit and needs tightened up. If you search there are loads of video and web guides on how to adjust derailuers - try here
http://www.parktool.com/blog/repair-help
for a good start to all bike mechanical skills - this is porbably the one you want to start with

http://www.parktool.com/blog/repair-hel ... derailleur

If its a new bike though the place that sold you it should check it out for you free - most places do a free check and service on new bikes after the bedding in period.

mog812

the following is just copied and pasted, for Carrera Vanquish (red/black) 2011:
Gears: 16-20
Chainset: Truvativ Tour Compact
Front Brake: Shimano Tiagra Lever, Tektro R720 dual pivot caliper
Front Mech: Shimano Sora
Gear Shifters: Shimano Tiagra 18 Speed
Number of Gears: 18
naturally I have no idea what most this stuff means of course
hope this helps
in mean time, guess I will have to get it checked over...
thanks again.

Mog.

TommyEss

Ok - so this is happening when you press the small black lever, the one behind the main brake lever? You press it once expecting the chain to shift one cog (smaller cog = harder gear) and you get it jumping two or three instead? And you're also getting it going the other way, using the main brake lever to change to easier gears?

Sounds like the rear derailleur isn't set up very well at all - but there are plenty of tutorials online to help - park tools are excellent as mentioned above. I'd start from scratch to be honest.

mog812

no, the gear changes, as it is suppose to, but when the pressure is loaded it, then it jumps gears...
however, I am checking out tutorials (youtube helpful) and will be making some tweaks, if any are needed.

thanks.

Mog.

TommyEss

Ok - maybe give the cables a clean, and check the condition of the teeth and chain while you're at it. Fingers crossed.

andrewjoseph

ghost shifting under pressure could possibly be caused by a flexible mech hanger. it's always a good idea to have a spare. BETD do a lot of models for about £15. try a new hanger, if it solves the problem, throw away the original and buy another spare. if it does not you may have frame issues. you may be twisting the bike frame with the phenomenal power you produce. ;-)

onbike 1939

mog812 wrote:

no, the gear changes, as it is suppose to, but when the pressure is loaded it, then it jumps gears...
however, I am checking out tutorials (youtube helpful) and will be making some tweaks, if any are needed.

thanks.

Mog.

If the RD is jumping towards the outside i.e. the smaller cogs then the tension on the cable may not be sufficient to hold it in the chosen gear against the return spring tension. Try increasing the cable tension by turning the rear adjuster a quarter to a half turn Anti-clockwise.

mog812

think I might have sorted it now, but how I'm not entirely sure, I did mess with the cable tension a little, took it out for a run, and seemed to be ok, but then perhaps I wasn't putting the pressure on as much, as was a little worse for wear that day, but am hoping it's sorted, as for the twisting of the frame, I did read that somewhere too...
oh yes... Sheldon Brown!

Thanks to you all for your help and support