Bicycle Mechanics - Bicycle automatically up-shifting when going up steep hills
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06-08-09, 08:55 PM
Hi, I'm relatively new to road biking. I have an '80s era Peugeot road bike. I've replaced some of the components, most notably the freewheel. It has the older gear shift levers on the down tube and I believe the deraillers are '80s era too. Lately, I've been finding that when I'm going up steep hills (in the lowest gear) and exerting a lot of pressure on the pedals, the chain will shift from the largest rear cog to the second largest by itself. Sometimes, it will go from the largest to the second largest and then to the third largest. This is very annoying when I'm trying to make it up a hill. If any of you out there might know what the problem is and how to fix it it would be greatly appreciated. Thanks.
06-08-09, 09:03 PM
Could be that you are hitting the shifter with your knee ;)
Try slightly tightening the friction adjuster on the right shifter - it is probably a screw through the turning axis of the shifter... there is often a little steel loop on the shifter so you can adjust it by hand. It usually only takes a tiny tiny adjustment to make a world of difference.
Check the cables to make sure there isn't any friction. Also, make sure your shifters are tightened properly. Friction shifters can slip sometimes.
Another common problem is the loop of housing that runs into the rear derailer. The frame can flex under load, which changes the effective length of that housing. That can be enough to shift gears. There are a few products made to resist that behavior.
I'd still check cable friction first. Every time I or a friend has had that problem, new cables and housing solved it.
06-08-09, 09:25 PM
Check the friction setting, certainly, but also inspect all teh cable guides to see if they're gritty, especially the ones at or under the BB. The BB moves back and forth as you pedal, especially hard, and the grit grabs the cable and pulls it, hence a ghost shift. Keep it clean down there!
I think the new cable and housing fixed it because new ones are clean.
06-08-09, 09:44 PM
When you put on a new freewheel, did you also replace the chain? If not - you should right away. This can cause what you're describing. As well as the gear-cables.
06-08-09, 10:22 PM
Sheldon Brown has something to say about Automatic Upshifting (http://www.sheldonbrown.com/autoshift.html). Heed the words of our late master!
06-09-09, 12:56 PM
When mine does this I have to tighten my lever. The screw on the side. I have a kind of wing nut and all it takes is a quick twist to keep it tight and that keeps it in place. I should change the shift cables on this bike, and will soon.
06-09-09, 07:18 PM
i also had the same problem on my Peugeot, i thought the sprockets were worn and were skipping, but all i had to do was tighten the screw on the shifters.
06-09-09, 09:49 PM
I had this problem with my bike and it ended up requiring loctite on the threads of the shifter to hold the proper friction setting. I'm assuming you have the old plastic Simplex shifters, in which case all I can say is, may the deity of your choice have mercy on your soul.
06-15-09, 06:52 PM
I think it was the friction setting afterall. I have them set much tighter now and haven't had the problem since. Thanks for all your help.
06-16-09, 07:31 AM
I carried a stubby screwdriver in my pocket after I fitted new simplex gear levers to my Viking in the 80s.
horrible shifters that loosened off all the time and flexed a bit to much
My late '80s bike with Shimano 600 rear derailleur and SIS shifters started doing this also, although only to the next higher gear, not more than one. Adjusting the cable tensioner on the derailleur out one revolution solved the problem for me. I've only been at this a couple months and have almost no idea what I'm talking about, but I suspect that there was enough cable stretch that the indexing was positioning the derailleur at the lower limit of what was necessary to engage the next larger gear. The extra stress of climbing hills was enough to cause it to drop to the next smaller sprocket.
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