Jim’s Tech Talk
By Jim Langley
When you work as a quality control engineer for a company that specializes in making cranksets, as I do – you get a lot of front derailleur adjustment practice. It comes from testing new product and also from maintaining bikes built elsewhere and already on the road.
I’ve been seeing two issues with modern Shimano “braze-on” front derailleurs. This type of front derailleur has no clamp wrapping around the seat tube of the frame. Instead it bolts onto a metal piece designed for it that’s bolted, riveted or brazed onto the frame.
To me, the issues fall under the category of assembly or adjustment mistakes. Because, Shimano’s instructions tell how to mount and adjust the derailleur. However, in the assembler’s defense, it’s possible they never received the directions and/or couldn’t find them searching Shimano online.
I should point out that front derailleurs are one of the more difficult bicycle components to adjust correctly. It can’t be done well unless the mechanic understands how front derailleurs, shift levers and changing gears work. And, front derailleurs are getting more difficult to adjust, so mechanics have to keep up with the changes as new designs hit the road.
So, before attempting to fix yours, be honest with yourself and if there’s any doubt that you know enough, hand the job off to a mechanic who does.
To help you experienced mechanics, here’s an illustration from Shimano’s online instructions, slightly revised to focus on the first common mistake. And, some tips for fixing the mistakes follow.
Mistake 1: Shimano Support Bolt not tight against the frame
The symptoms of this mistake can include anything from the chain rubbing on the front derailleur cage, to sloppy shifting, to throwing the chain. And no amount of adjusting the derailleur – whether it’s an electric or cable shifter will fix the problem.
What’s happening is that what Shimano calls the Support Bolt is not tight against the frame. It has to be that way in order for the adjustments made to the limit screws and for the throw of the derailleur to work correctly.
The Support Bolt is hidden inside the body of the front derailleur. You might need a flashlight to find it. It is turned with a 2mm hex wrench to bottom it out against metal. Do NOT tighten it against a carbon frame. For this, Shimano provides a small peel-and-stick Backup Plate that is stuck to the carbon frame to give the Support Screw a solid and safe surface to rest against.
Notice in the illustration that the Backup Plate needs to be mounted so that the Support Bolt does not rest against the adhesive side of the plate.
I’ve seen the pin resting on the adhesive side; carbon bicycles with missing Backup Plates; the flat Backup Plate incorrectly on curved frame tubes; the pin not tightened against anything; and the pin missing the Backup Plate entirely. These mistakes let the front derailleur cage move out of alignment with the seat tube of the frame during shifting and that’s what causes the rubbing and shifting glitches.
Once you spot the problem, the fix is easy. But, if the Backup Plate is missing, you will need to find one and install it. Bike shops that stock Shimano should have them. They recently appeared on Amazon, too: https://amzn.to/2E8zzE5
If you buy the set you will get two plates (one curved, one flat – use whichever fits your frame best) and a Support Pin.
Positioning the Backup Plate
To locate where to put the Backup Plate, temporarily put a piece of tape on your frame where you expect the Support Bolt to touch. Then install the front derailleur, snug the 5mm fixing bolt and turn in the Support Bolt with the 2mm hex until it makes a mark in the tape. When you remove the front derailleur you will have a mark where to place the Backup Plate (remove the tape first).
Once the Backup Plate is attached, you can install the front derailleur. Be sure the the bottom edge of the outside cage plate clears the large chainring all around – about 2mm clearance. And, as you tighten the fixing nut, press in gently on the cage to hold the Support Bolt against the Backup Plate or the braze on if on your bike a Backup Plate wasn’t needed.
What you’re looking for is a derailleur cage outer plate that’s parallel with the large chainring when the fixing and Support Bolts are tight. Note that if the cage is canted slightly towards the chain after tightening the bolts, you can usually simply tighten the Support Bolt very slightly (turn it clockwise) to get the cage to come parallel.
Check that the front derailleur fixing bolt is tight and now when you make the final derailleur adjustments, they should hold and chain rub, drops and bad shifting will be things of the past.
Mistake 2: Cable Isn’t Tight Enough
Since this issue has to do with the cable, it’s obvious it happens on conventional Shimano front derailleurs, not electric models. And, it happens with the type of cable setups with and without inline barrel adjusters.
An inline barrel adjuster is a small plastic piece found on some Shimano front shift housings (also called “casing”) where they run beneath the handlebars. By turning the barrel adjuster it’s possible to make the shift cable tighter or looser, which changes how far the derailleur moves when shifted.
I did not have a bicycle handy with an inline barrel adjuster to photograph so I found this photo of an adjuster on Shimano’s site. It will probably look different than what’s on your bike, but it gives you an idea what to look for.
Note that it’s possible to purchase and install inline barrel adjusters if you would like to add them to cables. But, it is a bit of work since it means carefully cutting the housing section. And, with older cables, you will probably want to start with a new housing and cable.
Chain Rub And Noise
The symptom of a front derailleur cable that isn’t tight enough is chain rub when it’s on the largest chainring and smaller rear cogs. Because these gears are usually used when riders are traveling at a high rate of speed and pedaling with great effort, it can make a lot of noise and become annoying fast.
The reason the chain rubs is because the cable is too loose. And when it’s too loose, shifting onto the large chainring results in only a partial shift. With sufficient tension on the derailleur cable, the derailleur cage will move to its full outside limit and there will be clearance for the chain – and no rubbing. Yay.
Using The Inline Adjuster
If there’s an inline barrel adjuster, you can shift onto the large chainring and then operate the adjuster watching to see if the cage moves outward and clears the chain. If so, you just solved the problem by tightening the cable sufficiently.
However, be careful. The barrel adjuster only has so many turns. If you go too far you can unscrew the barrel from its holder. At that point you’ve used up all available adjustment and you might even damage the cable or adjuster if you attempt to shift – since the two parts are no longer connected correctly.
Tightening The Cable
If the adjuster doesn’t have enough travel to move the derailleur cage far enough, you’ll need to tighten the cable another way. This is also true for setups without inline adjusters.
Start by shifting the chain onto the smallest front chainring. The shift lever and front derailleur need to be fully returned to their starting positions in order to take all tension off the shift cable so you can remove any slack.
To tighten the cable, if you have an adjuster, start by screwing the inline adjuster all the way “in” so that you will have its full travel should you need it in the future.
Then, to tighten the cable, loosen the cable anchor bolt on the front derailleur. Just loosen it, don’t remove it. And, don’t let the cable slip out. Keep it trapped where it was beneath the bolt and washer. Now, hold the end of the cable above the anchor bolt (sometimes called “pinch bolt”), with pliers. The cable probably has a little cap on the end. Grab the bare cable not the cap because the cap will come right off.
Holding the cable tightly so it can’t slip, pull up with the pliers to remove all slack in the cable and hold it like this as you tighten the anchor bolt to secure the cable in place. It should now be tight enough so that when shifted onto the large chainring, there’s no rubbing. If it still rubs, you should have ample adjustment in your inline barrel to tighten the cable a little more and get the cage clearance you need. If you don’t have an adjuster, you will need to get the cable even tighter.
Front derailleurs can be tricky to get right. I hope these tips help. For more of our adjustment tips, simply search RoadBikeRider for “front derailleur.”
Ride total: 9,108