bblair28

I've beend trying to tune up my own bike. I've got it shifting into all the gears fine, but sometimes the chain wants to slip off of the front sprocket (mostly the larger one). It usually happens when peddling hard in a high gear. Anyone know what could be the problem?

cascade168

Yes, it's almost certain that your "H" limit screw is set too far out. Shift your front derailleur into the high gear (i.e. the largest chainwheel) and shift your rear derailleur into the highest gear (i.e. the smallest cog).

Now, with the bike just sitting, pull on the cable that shifts the F.D. and observe the movement in the F.D. chain cage. It should not move outward from the seat tube more than a tiny amount. If it does, that is why your chain is coming off. Just turn the "H" limit screw CW until you feel resistance and then back it off 1/4 turn. Your chain will not come off the outside again. Now, when you pull on the shift cable, the FD cage should only move a smidgen, but no more.

feethanddooth

or your chain is real dirty, or you may need to replace the chainrings

cny-bikeman

There are a lot of possible causes, but front derailleur adjustment is not one of them. A proper drive train should be able to keep the chain on a particular chainring without the front derailleur on at all. If the chain unships during pedaling and not shifting then something is out of line or flexing too much. Some possible contributors:
-Loose crank bearing
-Weak or loose chainrings, missing chainwheel bolt (on better or older chainwheels).
-Bent chainring teeth
-Damaged chain
-Poor chainline (alignment between front and rear of drivetrain)
-Weak or cracked frame
-Inappropriate use of high gear.

"Peddling hard in high gear" simply should not happen and will accentuate any existing problem. It will neither get you in shape nor help you go fast (in fact more the opposite). High gear is for maintaining speed once you have used the leverage of lower gears to achieve it, or for downhill if you like doing that. Depending on gearing high gear should be used only at 30 mph and above.

cascade168

Nonsense. If the OP overshifts with a friction shifter - enabled by an improperly adjusted "H" limit - then the first thing that's going to happen is that the chain is going to try and come off the outside of the chainring.

While there are certainly other possibilities for the cause of his problem, he was honest enough to admit that he was trying to tune his own bike. The first thing to look at is an improperly adjusted FD.

bblair28

I'm sorry let me clarify myself, when I am pedaling the chain tends to slip off on the inside of the chain ring. This puts it down a sprocket or off completely on the crank. Thanks for the info.

cny-bikeman

As the owner reclarifies below, the problem does not occur during shifting, therefore the shift adjustment is not an issue. No properly adjusted bicycle needs the front derailleur to keep the chain on the chainwheel.

cny-bikeman

Incidental comment on front derailleur adjustment/unshipping:
The chain can come off the front chainwheel during shifting for a variety of reasons, the limit screw setting being only one. Others include the height and angle of the front derailleur in relation to the chainwheel, a bent or worn derailleur cage, and a worn chain.

cascade168

The clarification by the OP definitely sheds a different light on the problem. The point that I was trying to make was that when a customer comes in the shop and says that they are having problems with the chain coming off after they tuned the bike, it is, more often than not, a simple adjustment that will fix the problem and it can often be corrected in a minute or two. I would probably not even charge the customer for a minor adjustment like that, in most cases.

In the OP's case, I would still make sure the FD is correctly adjusted (height, rotation, cable tension, and limits) before proceeding.

RussB

Try this website
https://www.parktool.com/repair/
it has instructions for the complete setup for derailers, brake calipers etc.