Ask Boris: Cable Stretch
Cable stretch…that infamous cable stretch. What is it? What effect does it have? How can I avoid it? There probably isn’t a mechanic that hasn’t been asked these questions before. I know I certainly have been!
Before we can really talk about how to avoid and fix “cable stretch” when it happens, it is important to understand what that phrase really means…there is more to it than meets the eye.
Cable stretch tends to occur shortly after a new bike has been ridden a few times OR when your existing bike has had any of its cables changed (brakes or gears). A rear derailleur that had been tuned to hit every gear, nicely and quietly, may now not be making each shift causing that sensation and sound of “being in between gears”. Generally you may have to shift up, or shift down a couple times to quiet it down and settle the derailleur into a “happy gear”. A front derailleur may no longer want to shift all the way onto the big ring OR will require a much firmer push to get it there.
Your brakes will also be affected by cable stretch (provided of course that they are NOT hydraulic in which case this does not apply). The symptom here is that they usually start to feel quite loose. I.e. you have to pull the lever much further back or harder in order to get the same stopping power you once had.
So what’s the culprit?
It’s what we broadly refer to as “cable stretch” – your derailleurs and brakes are connected to your shift/brake levers by thin, braided steel cables. These shift or brake cables are amazingly thin for what they do! Shift cables are usually 1.1-1.2mm, while brake cables are thicker at 1.5mm. The action of repeatedly shifting up and down, or braking and releasing, will tension and release these cables many times. This action is thought to cause the cables to “stretch” or in essence elongate. It is this stretching that causes the looser brake feel, and deteriorated shifting.
BUT WAIT…there’s more!! So exciting right?
Many will argue that actual shift and brake cables DO NOT stretch. Wait a minute…doesn’t that just void everything I said above? Not really and here’s why:
“Cable stretch” is actually a broader term used to describe the action of the WHOLE cabling system settling into place. Your brake and shift cables pass through your brake and shift housing-the other key components in the WHOLE cabling system. When housing is cut and capped with ferules, it may not be fully seated into those ferrules, or fully seated into the frame stops, derailleur, brakes, shifters etc. As you shift and brake, a huge portion of the initial settling that occurs is actually the housing fully seating itself into place. Once settled, you can be left with a considerable amount of slack, causing the symptoms above.
How do I prevent it?
Cable stretch is difficult to prevent entirely. The absolute best thing that your mechanic can do is “pre-stretch” your cables. What this means is that we apply a lot of force (by pulling on the actual cables or shifting/braking) in an attempt to settle the whole system before we even begin to tune it. This can save a lot of hassle and re-adjustments after the installation of cabling components.
How do I fix it?
Thankfully this is a pretty simple one! Cable stretch is remedied by ADDING tension the lines affected. This is what those barrel adjusters on your bike are for! You or your mechanic can simple wind those up in order to add tension and restore braking feel and shifting performance. Voila!
A little food for thought-My $0.02 on cable stretch
Contrary to popular belief, I personally BELIEVE that the actual cables themselves do stretch. I agree that the bulk of the settling that occurs is in the housing BUT I also really do believe that some stretch occurs in the cables too, and heres why:
Take a rear derailleur cable install for example. This is the longest line on the bike, and the thinner of the two types of cable. Let’s say you are just installing the inner cable and leaving all the existing and already SETTLED housing in place. How come when a NEW inner cable is fitted into an EXISTING housing system, you can take it, load it heavily, and end up with a considerable amount of slack? If the whole housing system is already settled, where is this slack coming from? Maybe some additional settling, but I think that cable gives a bit too! 🙂