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Bicycle Mechanics - Spongy downshifting -- how do I cure it?

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View Full Version : Spongy downshifting -- how do I cure it?

mswantak

11-09-04, 05:45 AM

Bike in question: A '68 Motobecane Grand Touring, original SunTour V-GT rear derailleur, downtube shifters replaced with bar-mounted friction shifters. I got the bike about three or four months ago and have been riding it daily. Upshifts are okay, but lately I've noticed when downshifting, I have to move the lever quite a distance to get it to change cogs, then back it into alignment. In short, it feels spongy -- a lot more so than it used to. What's gong on here -- a crap cable?

sydney

11-09-04, 06:39 AM

mswantak

11-09-04, 09:55 AM

Yeah, good point. I'm thinking the tight radius on the rear casing (shown below) could be the culprit.

http://home.comcast.net/~mswantak/wsb/media/1477/site1324.jpg

LittleBigMan

11-13-04, 08:00 PM

You might play around a bit with the tension. Fiddle with that screw on the end.

roadfix

11-13-04, 08:14 PM

Weak derailleur spring or too much friction in cable/housing.

mswantak

11-14-04, 12:01 AM

Well, here's what I ended up doing about it: First, I went to the LBS and bought a heavier Shimano SIS-style cable, and new nylon lined casings. I cut the new rear casing long enough to increase the radius you saw in the picture by 30 - 40%. That made things better, but not really 'right'. Let me digress here and explain my shifter situation.

This Motobecane originally had downtube shifters, but when I bought it, the original owner had been in the process of changing over to a set of old Schwinn stem shifters -- the front shifter was still on the downtube. I didn't particularly care for either of these choices, and I had a set of old friction thumb shifters from a 'mart' BMX laying around, so I installed those. I liked the ergonomics, but there was a subtle problem in using them on my upright bars that I didn't grasp until I was trying to solve this shifting issue. The clamps on the thumb shifters were meant to be used on a straight bar, but my bars had a slight concave curve where they needed to go. By the time I had the clamp tight enough on the bar, it would press on the shifter, making it bind. Before I understood this, I was compensating with the adjusting screw on the shifter -- which would continually work loose.

The ultimate solution showed up on a thrift store donor bike I bought for another purpose -- it had a set of Shimano bar-end shifters on it. I'd never seen them used with upright bars, but I couldn't see why not; it couldn't be worse than what I had. Once installed and adjusted, the mushy downshifts are gone -- and they're a helluva lot easier to use than the thumb shifters.