Posts tagged #Derailleurs

Adjusting Front Derailleurs.

        Had an email from a guy yesterday telling of his recent success tuning his rear derailleur, following my recent post on the topic. However, his bike also has a front derailleur...
        So, with sincere apologies for neglecting the front end, here is how to whip those doubles and triples into shape.
        The procedures and adjustments are very similar to the rear but, we do have one rather important extra step. Unlike the rear, there is not just one single point of simple attachment. We have to set the height and angle using our own judgment.
        So, what we are looking for is clearance of the cage of the derailleur as it swings over the chain rings, while still keeping it low. A gap of about 1mm between the top of the big ring and the bottom of the outer cage plate is ideal; manually pivot the cage out to get a look at the gap.
        Next we need to set the angle. As you look down onto the chain rings the cage of the derailleur needs to run parallel with the rings.
       Now you have your derailleur mounted correctly we can set the limit screws. We start on the low limit screw first.
Change gear at the rear so the chain is on the lowest gear, biggest cog. Without a cable attached to the front derailleur, adjust the Low limit screw until there is clearance of about 1mm between the chain and the inner cage plate.
       Change gear at the rear to the high gear, small cog. Here is where it gets a little tricky. You now have to manually swing the front derailleur out while turning the crank to shift into the high gear, big ring. It might take you a couple of tries, the spring on the derailleur is pretty tough. Once you get it pushed out to its stop, adjust the High limit screw to get clearance between the chain and outer cage plate of 1mm, when you are happy, release hold of the derailleur and cycle the chain back to the small ring. Attach the cable making sure that the shifter is set on the low gear or number 1.
       Try it out. If you have a slack in the cable, just take it out using quarter turns on the barrel adjuster, until it works perfectly.     
Posted on August 9, 2012 .

Tip for the Day.

     There are lots of choices when it comes to picking new cables for your bike. Galvanized, stainless, slick, we mainly use the Stainless Steel slick from Jagwire. These guys are pre-stretched and perform very well. Some time ago it became popular to coat the cables with a 'Teflon Coating'. While on the face of it this sounds like a terrific idea, like many things it falls way short in reality. Here's why: 
    The Teflon is a coating, it is put on the finished cable, it is nice and slick and I'm sure that if you wanted to, you could fry a rasher of bacon on it, all be it a very skinny one, but that coating does not like to be pulled and rubbed through housing and it starts to peel off, now we have strips of coating clogging up inside the housing and binding up the cables. 
     If someone brings in a bike with shifting problems and it has coated cables we clip them and run slick stainless with new housing. Problem Solved.
Posted on August 8, 2012 .

Rear Derailleur Capacity.

     Any derailleur will work back there, right? Wrong... I have lost count how many times someone has come into the store after swapping out or, upgrading a rear mech., only to find that it now does not work properly. When I ask if the derailleur capacity matches their setup, I usually am greeted with a vacant stare.
      So, before yanking off chains and clipping cables let us take a moment to work out what you need to put back there.
      Before we do the math, (yes, there is always math), it might be helpful to clarify exactly what a rear derailleur does. I imagine most people can see how it operates to change gear, you make a gear change at the shifter and the cable that connects to the derailleur pulls it side to side and derails the chain from one cog to another, simple. The other thing it does, that is equally important but possibly not as readily understood, is to take up the slack in the chain and keep it tensioned correctly.
       Why the hell is there any slack in the chain you ask? Because each gear combination requires a specific chain length to wrap around the selected cog and chain-ring. When you select the big ring at the front and the big cog at the rear, the chain wrap is at its longest. Conversely, when on the small ring and small cog it is at its shortest. The difference between these two extremes is the amount that the rear derailleur has to be able to handle. The Chain Wrap Capacity.
The math;

(No. of teeth on big cog - no. of teeth on small cog) + (No. of teeth on big ring - no. of teeth on small ring) =

For example;
cassette 12-25 and crank-set 53-39
25 - 12 = 13
53 - 39 = 14
add them together 13 + 14 = 27.
So 27 is your chain wrap requirement that the derailleur needs to be able to handle.

      The way that the derailleurs are made to handle larger capacities is primarily by increasing the distance between the jockey wheels. This is often referred to as cage length and you will see terms like, short cage and long cage used to describe different models. However do not just assume a mid cage will handle what you have, always refer to the actual chain wrap capacity figure that is published with every model and make out there to be sure

       Happy shifting..
Posted on July 31, 2012 .

Adjusting Rear Derailleurs.

It seems that the temptation is great to mess with those little screws on the back of your derailleur when the shifting has gone off the boil. While I'm not suggesting for a minute that you should not attempt this adjustment yourselves, trust me when I tell you that you will not get a satisfactory result by just screwing with the, err screws. You need to follow the steps which I shall outline for you below.

Before we get to messing with limit screw adjustment I will assume that you have checked to see if your problem lies with just a slack cable, which is easily taken care of with a 1/4 turn or two of the barrel adjuster.

The picture on the right shows you the three adjustment screws that reside on your rear mech. From the top we have the B limit screw then the High limit screw and on the bottom the Low limit screw.

The B limit screw adjusts the height of the derailleur another way to look at it is the clearance between the cogs of your cassette and the top jockey wheel. Often the clearance is good until you change into the large low gear cogs and then you start to get a rumble as the jockey and chain is bumping along the underside of the cassette cog. If this is happening on your setup just turn that screw in clockwise to increase clearance. ideally we are looking for a gap of about 4-6mm. Sram works best at around the 6mm mark. Anyone using a Campagnolo system may have to search for the B limit screw, they have a few models with the screw mounted on the pulley cage behind the body and hidden from view.

If you have clearance Clarence on the jockey wheel and your problem is more on the indexing, then you need to check the adjustment of the H and L limit screws.

Firstly loosen the pinch bolt on the cable to release it. Turn the pedal to bring the derailleur to its relaxed position which should position the jockey wheel under the small cog, the high gear on the cassette. If the derailleur shoots inward and comes to rest under the big cog, low gear, then you have a low normal derailleur. These are sometimes found on mountain bike setups. Whatever you have the principles are the same and  I will cover them both. If the derailleur has come to rest under the small cog place a screw driver on the H screw, if it rests under the big cog then the Low limit screw. Stand behind the bike and adjust the screw in and out to center the jockey wheel perfectly under the above cog. As you turn the screw you will see the mech. move from side to side, use that movement to center the jockey wheel perfectly.

Now it gets a bit tricky. You have to cycle the chain through while shifting the derailleur to its other stop. The picture to the right shows it centered under the big cog or Low gear. Again stand behind the bike and use the low limit screw, for a normal derailleur, to center the jockey wheel perfectly under the cog. All the while keeping the derailleur pushed to the stop by hand, as you turn the screw in and out you will feel the pressure in your hand and see the derailleur move from side to side. Once you are happy with the adjustment cycle the chain and let the derailleur come to rest.  Now turn your barrel adjusters in and make sure that you change gear on your shifters to let all the cable out. Pull the cable taught and attach with the pinch bolt. Now try it out. If you shift at the handlebars and nothing happens at the rear then take a\ little slack out with the barrel adjuster. Try again. Keep doing this until you get crisp shifts all the way up and down.

If you are sure of your adjustments and still the shifting is not as it should be then you may have some other problems. Most common is a bent derailleur hanger, which can be aligned, but takes a special tool to do so. Come see us and we can take care of that for you. Another possible cause is a bad cable or contamination in the housing. Easily rectified with a nice slick new cable and a couple of feet of Jagwire housing. Again we can hook you up.
Posted on July 29, 2012 .