My Back Gears Switch By Themselves to Higher One

My back gears switch by themselves from first to second and second to third. The gear switch lever moves up by itself.

How can I troubleshoot and/or correct this problem?

 

What type shifter(friction, index/click shifting?), where is it, on the downtube, barend, integrated brake lever/shifter?

Or take a trip to a competent bike shop and ask them.

 

ALSO:

Is your bike, by chance, a MTB with a rear suspension?

Has this always happened?

Have your recently done some "maintenance" on the bike and the problem subseqently manifested itself?

How old is your bike and/or the shifters?

 

Hi.

Bike: is a Peugeot 10 speed racer.

Age: Unknown. My rough guess based on scratches/paint/dirt before I cleaned it: 10 years.

Shifter: Friction

Shifter Mount: Handlebar stem

Problem started: Since I started riding it. Seller moved and I have no way of finding out about the bikes history.

Maintenance Done (before 1st ride Cleaned off sprocket, crank, chain, and rear shifter mechanism with WD40 and other grease removing spray products. Cleaned rear axle bearings.

 

Start by tightening the screw on the friction shifter. Then make sure the freewheel isn't worn out.

I've got an old low end peugeot too. They're pretty nice but the paint and chrome aren't so durable.

I am sure you know that the way you tighten rear axle bearings is absolutely critical for longevity and safety.

 

You need to tighten the bolt that holds the shifter in place ...

If the "washer" (which provides the friction) is worn, you can make a new one from a plastic MILK cap -- punch a hole in the middle, trim to fit, install.

 

Nice fix alfeng......pitty ergo shifters aren't that cheap to repare!

 

Check your chain for wear - this started happening to me once and a few days later the chain broke. A bad link can cause the chain to stretch under tension and skip to a different gear.