jcinnb

Yeah, I'm about to fold um...two final questions.

1) Can I use a piece of pvc pipe (the heavy stuff) for a spacer (short term) until the real deal gets here?

2) I routinely adjust my chain and rear derailleur and get very smooth, precise shifting as I start out, but over time 8 - 10 mile mark, shifting gets mushy, and degrades to the dreaded "auto-shift", randomly. I am gravitationally challenged. Pretty severe. Is my weight causing "stuff" to flex back and forth to the extreme that the adjustments are negated. I have a Jamis Coda Elite, a pretty good quality steel frame, but this is getting tiring. Is there a solution, if flexing the frame or flexing something, due to my weight, is messing up my derailleur?

Help..

cny-bikeman

1. ?*******pacer for what, where?

2. I don't see how your weight would have an adverse effect. Might be very hard to diagnose, especially remotely. I really would advise taking to a good mechanic. Others are of course welcome to put in their guesses, but likely it's something unusual that takes a hands-on approach.

cyccommute 

If the diameter is right, I don't see why it wouldn't work. The PVC is soft so go easy on any compression you might put on it.

You say that you adjust the chain and rear derailer but did you adjust the cable tension? Cable tension is the most important part of the equation. If you haven't do so already, check the Park Tool website on derailer adjustment. Ignore the first half down to "Checking the Rear Indexing Adjustment" unless you are having overshifting problems.

If you have loosened the anchor bolts to adjust cable tension, I'd check them too. If you don't tighten them sufficiently, the cable can slide under the bolt and result in reduced cable tension.

jcinnb

Great site...thank you very much.
jcinnb

fosmith

do you use gatorade (or similar) in your bottles? if you have an under the BB cable guide, that sticky crap collects there when it leaks out of your bottles and gums up your shifting. pretty common.

dabac

The compressionless housing used for shifters is a bit special. If they aren't endcapped properly, the steel "rebar" can squeeze through the cable stops, causing the shifting to go out of tune.

wrk101

+1 I would bet on a cable housing issue as well.

dnuzzomueller

Definitely something to do with either a lack of tightness on a bolt or something with your cable housing. If it is loosening after 8-10 miles I would think that it is something to do with the fixing bolt being loose on the RD.

Suburban Grind

Is it possible the guy is not using indexed shifting, and the friction shifter is simply slipping the chain of the sprocket's centerline? Either from the derailleur tension or from feedback from bumps? I seem to remember a phenomenon like that waaaay back when I was in college, when ten speed meant 2 chain rings x 5 sprockets on the cassette.

jcinnb

My cable tension adjuster was loose as a goose. I guess when they assembled the bike before shipping it was not set, or just got out of adjustment when I assembled upon receipt. I am SURE I can fix this. Thank you all so much. I got the gun back in the case, Bike thanks you too!!!

jcinnb

dstrong 

In my head, I keep hearing Jeff Foxworthy...

"IF you use PVC for headset spacers and can't get your shifting to work right...you might be a redneck!"

dvs cycles

You're no redneck. Can't even spell 12 GAUGE.

cyccommute 

Cables stretch with use. New cables stretch more. If the bike is fairly new, expect some degradation of the shifting in the first few hundred miles.

BikeWise1

Myth Busted.

Cables are pre-stretched these days. They do not get any longer with use unless they are fraying. But new housings do seat in and compress a little, and eventually the housing wires start to extrude through the cable hole, and shift precision flies out the window. This happens quickest with riders who forget they are either in the big ring up front, or big cog in the back and regularly hork on the lever to try and shift into a non-existent gear. The housing wasn't designed for that sort of abuse.

This is easy to test. Put a brand new cable in used housing. If you did your job well, you shouldn't have to touch the adjusting barrel for a long time. Now put an old cable in new housing....it'll need a few tweaks before it's happy.

jcinnb

Well, I went 15 plus this afternoon. Shifted like a charm, after adjustments and my PVC headset spacers got my handlebars up about 2 1/2 inches (with adjustable stem). Looked a little funny, and I wish I could have worked some DUCK tape and stripped wire in, but it worked. No way to compare the ride. Hope the real spacers get in tomorrow, I figure I am pushing my luck.

cyccommute 

Myth not busted. Put any new cable in and watch the shifting deteriorate over the first few miles. I don't change housing as many times as I change cables. New cables always require adjustment shortly after installation. It's probably the number one complaint on the forums about new bikes. We get dozens and dozens of these kinds of post every week. I'll bet it's even the number one complaint you get as a mechanic.

Modern shift cable (not spiral wound) can't compress either. There is nothing to compress. Twisted cable, on the other hand, can untwist and lengthen quite easily. The lengthening process isn't very much but then it shifting is so precise now that it doesn't take much to throw it off. A millimeter would be hugh.

jgedwa

I agree. "Pre-stretched"cables might "stretch" (or more accurately, the strands bed in against each other and the whole cable gets a smidge longer) less, but they still do it.

jim

Kimmo 

I'd be surprised if gear cable stretches more than 0.1%, but housings can bed in a great deal more than that 0.15mm or so - particularly if ferrules haven't been properly seated. Also, the cable develops bends where it wraps onto the shifter and where it runs under the BB; until this happens the cable is a bit springy and slightly less accurate.

jgedwa

In spirit, I agree with you: housing/ends/stops settling in seem to make a much bigger difference than cables settling in.

jim

cyccommute 

The cable housing can't 'bed', i.e. shorten in length. Ferrules might move but that's not a cable housing issue. The housing itself can't be compressed at with the kinds of forces we're talking about on a bicycle. The inner cable, on the other hand, can easily untwist as you put tension on it and lengthen. Occam's razor and all that. An untwisting inner cable lengthening under tension can explain shifting problems much more easily than compressing the metal rods in the housing.

BikeWise1

Nope. I have personally put 300lbs of tension on various derailleur cables with a test jig here in the shop. The old SunTour braided cables would stretch a tiny bit, but that was 20 years ago. Current pre-stretched cables do not stretch until they near failure-which takes far, far more force than anyone can put on them with a shift lever.

Don't think housing can squish a little? We've tested that, too. Since it is virtually impossible to cut housing so that the end is completely square (without using a saw, or dremel tool, that is), housing tends to squish a little as the wire lengths equilibrate, and if you're using plastic ferrules, bed into the plastic.

Number one problem in my shop? Hardly. Since we actually understand what is taking place with cables, we know how to set up bikes so that they require little, if any, derailleur cable adjustments after the bike leaves. On custom bike builds, we dremel cut the housings and often use aluminum ferrules. Set 'em that way, and once it's done, it's done. We're the same way with our wheel builds. Tension relieved and equilibrated when they leave the shop, they don't need "little break-in tweaks".

If you have, say, a mountain bike with 3 lengths of rear derailleur housing, that's 6 ferrules that all have to go through the bedding process. If every one moves only half a millimeter, you have 3 mm of slop! But most people will assume it was just "cable stretch" because everyone "knows" housings don't compress. Many shops with young employees don't understand this, and we're seeing here how these assumptions die hard.

You can believe what ever you want, though.....

cyccommute 

Sorry but I'm dubious that you could put 300 lb of force on a thin twisted metal cable of 5+ feet of length and not see any elongation at all, even if the material is 'prestretched'. The cable and the strands of the cable are just too thin to not stretch at all. A 1% stretch of the cable over that length, is 1.5mm, which is more than enough to mess up the shifting.

I'm also dubious that you see much compression of the housing...especially more than you'll see of elongation of the inner cable. Metal is far easier to stretch than it is to compress. If the housing is going to change length, it will happen on the first shift and should be able to be taken care of before the bike leaves the work stand.

A small stretch of the inner cable which is under tension and more plastic does a better job of explaining cabling/shift issues than compression of the outer cable does.

A 5' foot long, 1.1mm diameter, 60% prestretched, stainless steel 7x1 wire rope stretches 29mm when placed under 300 lb of force according to this calculator

Dr. Banzai

booyah.

BikeWise1

The data do not support your suppositions.

And what does a cable manufacturer say about it? Oddly, their information echoes my own:


Q: Do cables really stretch?

A: Yes and no… Every Jagwire cable made is intentionally stretched during the manufacturing process. By the time any cable reaches the end customer there is little to no stretch left in the cable. What is commonly referred to as “cable stretch” is almost always the housings settling into the bottom of the end caps. It’s critical that housings are cut as straight as possible and then fully seated into the bottom of the end caps during initial installation. A good mechanic will typically grind the end of a housing to ensure it’s as straight as possible. They will also repeatedly compress the housings before making thier final adjustments to the brakes or derailleurs. A little extra work during installation will result in fewer adjustments down the road.

Makeitso

Maybe they will start compressing the housing so we won't have to deal with either issue.