gthomson

Hopefully I'm explaining this correctly, but recently I upgraded my 80's Peugeot with new 700c wheels and a 7 speed cassette. The bike originally has 27' wheels with a 6 speed flywheel. All works great with the new set up but the rear derailleur will only go up to the 6th gear. Is there any way I can adjust the derailleur so it will jump up to the 7th cog? or would I need to replace the gear system entirely to do that?

ofajen

Assuming it’s friction shift and also assuming the rear shift lever still has room to move further and pull more cable, it could be as simple as loosening the low position “L” limit screw on the rear derailleur.. if you turn that screw out, say a 1/4 turn at a time and check to see if the shifter can pull the derailleur a bit further, then there is a good chance you will be able to get it to go the extra couple of millimeters to cover the slightly wider cassette.

If the shift lever lever runs out of travel by hitting a hard stop, that may be an issue. If that happens, take it back down to the first rear cog and remove any slack in the cable, so you are using all of the lever’s travel to actually move the derailleur. Then take it back up and try to get to the 7th cog.

Hope me that helps.

Otto

cudak888 

^
What Otto said. Adjust the limit screws.

Keep in mind that though it's unlikely, the Shimano Light Action rear derailer may be fighting the final shift given that it has a spring in the pinch bolt assembly to facilitate extra-smooth shifting.

-Kurt

gthomson

Would you be able to point out which screw on the derailleur this would be? sorry a bit of a newbie to bike repairs.

uncle uncle

see if this helps....

masi61

Based on your picture , it appears as if your chain is not long enough. Even though the “big big” combo is not recommended (cross chaining), you should still be able to safely shift into it.

dddd

Of the two rear-facing limit screws going directly into the top knuckle of the derailer, the lower-most screw controls the travel limiter up to the largest-cog position.

The screw just above that controls the travel limit going to the smallest cog.

The third screw, up at the mounting bolt pivot, controls the mounting bolt pivot spring and so can adjust for clearance between the teeth on the pulley and the teeth on the largest cog (lways set this "B"-pivot screw with the chain on the smaller front chainring).

noobinsf 

A couple of helpful images -- not mine, found through Google:

On this one, the "limit adjusting screws":




One this one, this is the rear view of the derailleur, and it would be the "L-screw," as dddd describes above.

gthomson

Great video and image above, I am learning something and this is all good! I have another bike that's not going up to the big cog as well so can fix both. This will also help me understand why a road bike I have on a trainer is skipping gears when I'm Zwifting.

thanks all!

Chombi1 

Yup, if that was your bike on the pics, it does look like you are maxed out with your chain legnth. Which will make it hard for the derailleur to move the chain over to your biggest cog, if it's even possible. Get a proper legnth chain first then fine tune with the limit screws. And be careful you do not get too close to the spokes with the derailleur on the lowest speed cog.

ofajen

I was looking at that. I also notice the rear axle is bolted all the way at the back of the dropout. Typically the rear axle is bolted further forward, closer to being in line with the seat stays.

Also, rear derailleurs usually have some recommended spec for how many millimeters forward of the rear derailleur pivot the rear axle should be located, though that varies.

moving the axle forward would also give some slack and possibly allow the derailleur to work on the largest cog. OTOH, this seems like the perfect time to replace the chain.

Otto

Steel Charlie

I must admit that I've always been dubious of those claims vis-a-vis axle position. What with all the available motion in the rear der I seriously doubt that the bike would feel the difference. AFAIC what I saw was that bikes, even really good ones, often did not have the rear of the dropouts in the same place and a bit of adjustment was necessary to get the wheel square to the BB and parallel to the front. If all the precise measurements are so important in rear der/axle placement how is it that vertical dropouts can use about any der out there?
Back in the day all the kids pushed the axle to the limit forward cos it was "racy!" - - what BS. Seems to me that Old Wives write about as much BS for bikes as they do for guitar amplifiers and shotguns

be safe
Charlie

dddd

I usually put the axle forward, not so much for being "racy" but to avoid shifting problems caused by the axle being so far back.

The chain gap gets pretty large on many vintage bikes when using the smaller cogs, and moving the axle forward seems to help a great deal vs. being all the way back.

Having the axle rearward does put more weight on the front wheel, increasing steering stability, though at some cost to responsive steering.

All of this depends to some degree on the particular bike, and on how the particular rider is fitted to same bike.