Friction has three sources, and the most expensive one is the only one you can rule out for free
Noise from a wiper is always friction: the rubber is not sliding, it is sticking and releasing in rapid little jumps. That can only come from three places, and they are the three things on the diagram. The glass — an oily film, wax from a car wash, silicone from a glass treatment, tree sap, traffic film. The rubber edge — hardened by sun and frost, rounded off with use, or notched by grit. The arm — its spring weakened so the blade floats, or its angle twisted so the edge drags instead of trailing.
The expensive one is the arm, and the good news is that almost every arm story starts as a glass story that nobody cleaned. So the order on the right-hand ladder is not arbitrary: the free step is not a cheap alternative to the repair, it is the test that tells you whether a repair is needed at all.
The both-at-once clue, and why it is worth more than any of the others
Two blades do not fail on the same day. They are different length, they sweep different areas, and they were fitted at different times. So if both started squeaking at once, or both started the week the car was washed and waxed, or both started after a glass coating was applied — the blades cannot be the cause, because nothing about them changed. Same for the opposite: one blade that chatters while the other is silent is almost certainly that blade or that arm.
Two more patterns point the same way. It is worse in light rain and better in heavy rain: light rain does not put down enough water to float the blade over the film, so the rubber meets film instead of water. And new blades make it worse, not better, which reads as “poor quality blades” and is really the film asserting itself — a new edge is stiffer and sits flatter, so it transmits the unevenness instead of riding over it. That is the single most useful thing on this page: if you have already replaced the blades and it still chatters, do not buy a third set.
Cleaning glass is a grinding job, not a wiping job
Ordinary glass cleaner removes what water removes. The film that causes judder is oil-based, so it needs either isopropyl alcohol or a dedicated glass polish with a mild abrasive, worked in with a sponge and pressure, then flushed away. Wipe with a clean microfiber and check in the dark with a torch at a low angle — an oily patch shows up as a dull sheen exactly where a wet patch would bead. Do this before you judge the rubber, because a dirty screen wears a new edge fast enough to look like a defective blade, and that is how people end up replacing a part that was fine.
The edge has two ways to die
Hardened: sun, frost and screen wash do this. It feels like plastic rather than rubber, it can leave black dust on your finger, and it cannot conform to the curve of the glass. Rounded or notched: grit and ice do this. The edge should be a fine lip; worn, it becomes a flat band with chips in it, which is why it leaves stripes rather than a smear. Run a thumb along the whole length at both ends, not just the middle — the outermost 100 mm is where wear starts.
Life is six to twelve months, six if the car lives outside in sun or heavy rain, two years at the very most in a garage. When you replace, get both the length and the connector right — hook, pin, side-lock or bayonet — because a blade that is too long will strike its neighbour, and a wiper arm hitting a wiper arm at speed can crack the screen. And clean the glass before the new blades go on, for the reason above.
The arm: pressure and angle, and the adjustment nearly every guide gets wrong
Lift each arm off the glass and let it go: the two should feel the same. One that returns limply has a tired spring, and a blade that is not pressed down cannot wipe. Then park the wipers and sight along both from the front and the side — a twisted arm holds its blade at a different attitude, and you can usually see it before you can measure it. The attack angle should be about 45 degrees, trailing slightly, so the lip flips cleanly on the return stroke.
The adjustment is a gentle twist of the arm with two spanners, a degree at a time, testing between each one. What most guides tell you — bend the arm towards the glass to increase pressure — is the wrong reflex: too much pressure raises friction and can stop the edge flipping at all, so a blade bent down chatters worse than one bent up. And never sand, trim or reshape the rubber lip: the profile is the whole component, and there is no way to put it back.