WK95

A farfetched thought occurred to me.

The shifting system of a bike relies on tension to pull on the derailleurs to shift gears. Too lose and it might not shift up properly. Too tight and it might overshift. Thus, tuning up the shifting system requires adjusting the tension at the cable fixing bolt or the barrel adjusters.

Problems occur over time as the cable stretches which reduces the tension on the cables thus requiring adjustment. Othercourse, I am sure there are other problems that occur in the shifting system with use such as wear but bear with me.

What if I were to measure the tension of the shifting cables after the bike has been perfectly tuned up and then, before every ride thereafter, somehow measure the tension of those same cables and adjust them until they reach that same tension that was had when the bike was in perfect shifting state.

Park tool has a tool to measure the tension of the spokes, a similar or perhaps even teh same tool could perhaps be used to measure the tension of the cable in various positions.

Would this idea actually work or am I missing something?

prathmann

No, you need the effective length of the cable (i.e. from the shifter to the mounting bolt on the derailleur) to be correct. Tension will vary all over the place as you go through the process of shifting up or down.

WK95

Yes, i know that tension of a massed able is not constatnt throughout but what it one were to take the measurement at a consistent position?

cny-bikeman

People call the adjustment tension for convenience sake, but as noted it's actually effective length. The problem with your approach is that the tension will not vary enough when the derailleur is in or out of adjustment. Once you pull against the spring a difference of 1 mm of movement is not going to change the tension significantly. It's also a solution looking for a problem, as it takes all of 10 seconds to adjust the tension if that is the only issue.

WK95

I see. Thanks for the information

dsbrantjr

The cable tension is set by the derailleur return spring and the friction in the system. Measuring it will not yield useful information about the adjustments.

Bill Kapaun

Also, what "might" seem correct when measuring statically, may be different with your butt on the saddle and actually pedaling.
When setting up a RDER, I get things pretty well dialed in on the repair stand, but usually end up doing a minor adjustment for actual riding conditions.
I might do a minor tweek from a cool morning to a hot afternoon- or not-

mconlonx

Try it out. Let us now if it works or not.

FBinNY 

Gear cable tension is nearly constant throughout the range. It's purely a function of the RD return spring, and varies only slightly from the tightest (low gear) to slackest position. The maximum difference is determined by the character of the RD spring, mainly the number of turns in the working section. The tension change involved in a small trim adjustment would be so small as to be immeasurable by anything but the most sensitive of instruments.

BTW- this is why I've always hated the "adjust cable tension" in tutorials. You don't adjust the tension since it's not adjustable, you adjust the length.

jyl

The only time I pay attention to cable tension is when pulling the cable and fastening it with the derailleur's anchor bolt (right term?). When the derailleur is in the smallest cog/chainring, the cable should not be floppy or loose. You don't want the shift lever to have to take up a bunch of cable slack before it can start moving the derailleur.

Question: I usually screw all barrel adjusters all the way "in" then back out 1 or 2 turns, before fastening the cable. Is that correct, or should I initially set barrel adjusters in the middle of their range?

rpenmanparker 

A lot has to do with your particular equipment and technique. Take brakes for example. I am using Zero Gravity Ti calipers, and for some reason I am able to get the pinch bolt tightened up with the pads squeezed really tightly against the rim. So I need to have plenty of adjuster barrel travel available to loosen up the calipers. So I start with the adjusters nearly fully extended. Then after I screw them in a few turns to open the pads up a bit, I have the adjusters just about in the middle of their range. That way I can go either way as needed, tighter if the cables stretch or looser if I need to open up the calipers more than the quick release allows to get the wheel out with a fat tire on it.

With another brand of calipers or using a little different technique you may have to tighten them after clamping the cable with the pinch bolt. So in that case you would want to start with the barrel adjuster all the way screwed in. Then when you were finished with the adjustment it would also be in the middle of its range.

That is the ultimate goal IMO.

leob1

IMHO. It's because it is the most understandable of common terms. If the cable is too slack, or loose, i.e. the effective length is too long, it is most notable by the cable flopping around(in techinical terms). And it is easier to tell someone to adjust the cable tension, by lossening the anchor bolt, and pulling the cable, than it is to explain to them to increase the effective length, by looening the anchor bolt and pulling the cable. Adjust the tenson is just easier for most people to under stand. I've spent lots of time trying to get folks to the starting point of rear delrailer adjustment, by "adjusting the tension so there is no slack", and that is tough enough

rpenmanparker 

Actually the correct cable length at the small chain ring (FD) and cog (RD) positions is the length that gives a taut cable with no slack but with exactly zero tension, the length that if it were just one milli-smidgen shorter would cause tension. The idea is to have the derailleurs in perfect position for the smallest cog (RD) and smallest chain ring (FD) due to the high and low limit screws, respectively, not due to being pulled on by the cable. Then when one unit of shifting is applied to the shift lever, the cable instantly pulls on the derailleur. That is when the tension in the cable is supposed to start. It is an ideal that is hard to accomplish, but that is the goal.

FBinNY 

Both of the above are wrong. You adjust the RD trim in any except the high gear position (usually one or two in form high), since the high gear trim is usually determined by the limit screw. Then the tension or slack of the cable in high is whatever it is, which varies by brand. For example with Campagnolo, there will be noticeable slack in high.

When I teach repair, I have no problem teaching without saying tension. You start by pulling up the slack, then adjust the trim with the adjusting barrel, shortening or lengthening the cable as needed. Of course You're not shortening the wire, but changing the length of the run by lengthening or shortening the rear housing loop, but I explain how the cables work in the preliminary overview, so don't have to be as pedantic throughout the rest of the course.

IME- folks with a clear understanding of how things work fare better in the learning process and are less likely to go astray later.

BTW- this thread is a perfect example of how speaking of tension instead of length/position leads people astray.

JerrySTL

As a practical matter, I don't know of a tensiometer which will work with such thin cables nor at that light of a load. Even the smallest that that I used on aircraft were in one pound increments. You'd think that something as light as a derailleur cable and rear derailleur spring would need to be in ounces.

rpenmanparker 

Regarding your statement that a Campy rear gear system has significant slack in the cable when the chain is on the smallest cog. That would mean the shifter moves the RD less to shift to the second cog than any of the succeeding shifts leftward on the cassette because some of the first shift is lost to taking the slack out of the cable. If not, the shifter would have to have more "throw" built into the first shift than the rest. Is that the case? Otherwise how does that work?

And about adjusting the trim on a lower gear than the smallest cog. Sure, but it is while on the smallest cog that you have to attach the cable. As I said, you won't likely make that connection with the perfect cable "length". The trim on a larger cog fixes that. But if you then shift back to the smallest cog, if the high limit screw is just right, shouldn't the cable now be very close to the zero tension point, just a tiny bit away from slack (except for what you said about Campy)? I understand the correct manipulations that you described. I am just trying to understand the underlying principles so it won't just be a rote process.

FBinNY 

Yes, the lever has extra throw in the "start" position of the cam. They do this to make assembly of the the cam into the spring carrier easier. I invite those curious to try assembling these in any other position. It's a subtle difference to anyone assembling a single lever, but a big difference to the women who assemble these all day.

As to your reference to finding the no slack/zero tension point when stringing a cable, you're making the simple complicated. Basically, pull down the cable pulling out as much slack as you can and fastening. Then bring the lever to the first shift, and turn the barrel adjuster until it completes the corresponding shift.

Experienced mechanics often cheat, manually shifting the RD in a few positions and letting it hang there by the chain, making it easier to string the wire without pulling out all the slack. It takes a bit of practice, but once learned is quick and easy. It's especially handy for setting up front derailleurs, or one can as I do and jam a pencil into the pantograph to move the FD out a bit, and hit the cable length spot on every time.

RaleighSport

OP: Just invent a barrel adjuster for DT mounts that auto corrects for cable stretch, then take all our moneyz.

rpenmanparker 

Quite informative. Thanks.

goldfinch

Can I come to your class?

FBinNY 

You just did.

Jeff Wills

I'll tell you my method for getting the initial rear derailleur cable adjustment close, then y'all can beat me up:

1. Take the cable completely off the rear derailleur. Verify (by manually moving the derailleur cage) that the derailleur stops are set correctly. Leave the rear derailleur in high gear when you're done. Shift the shift lever all the way to its high gear position, letting out all the cable. Hook the cable back to the derailleur.

2. Now, manually shift the rear derailleur to the second cog and stop pedaling so the chain hangs on this cog. Now, readjust the derailleur cable, taking out all the slack without moving the derailleur cage. (This takes some sensitivity to not move the cage.)

3. Now check the index adjustment, shifting only between the top two cogs. Fine-tune the adjustment with the barrel adjuster. Recheck that the shifter hits all the cogs correctly.

This procedure should take up any slack in the cable, housing, and stops and put a tiny amount of preload on the cable. I found that once I started adjusting rear derailleurs this way, I can get the indexing correct on the first try almost every time.

rpenmanparker 

I can't figure out what you mean by "manually shift the rear derailleur" and "hangs on this cog". You haven't shifted the brifter at the time you take the slack out of the cable? Why would that work? Please help me understand this.

Jeff Wills

OK...

2a. Shop adjustment- with the bike on a rack (or similar), move the rear derailleur with your hand while turning the pedals. This shifts the chain to the second cog.
2b. Stop pedaling, and let go. The chain won't shift, and there should be a bit of slack in the cable because the shifter is still in "high" gear.
2c. Take up the slack in the cable with the barrel adjuster.

rpenmanparker 

Thanks.