Adjust a rim brake by setting the pads flat on the rim and clear of the tire, taking up cable slack at the anchor bolt, and checking that the lever stops well short of the handlebar. With the illustrative figures worked here, a 100 N squeeze becomes about 459 N at each pad and stops a 90 kg rider at 0.37 g from one brake dry, but wet pads cut that to 0.14 g, so the stop from 6 m/s lengthens from about 5.0 m to about 13.3 m, and cable stretch of 2 mm costs about 7 mm of lever travel.
Part of Bicycle Repair & Fabrication
Brake adjustment sets the pad position, cable tension and lever reach of a bicycle’s brakes, which are its safety system.
What The Brake Does, In Numbers
Every figure below is an assumed, illustrative input, to show how the pieces combine; measure your own bicycle before relying on them. The friction values are recalled orders of magnitude, not measurements.
Step 1: the lever. Hand force F = 100 N, applied 90 mm from the lever pivot, with the cable attached 25 mm from it. The lever multiplies force by 90 / 25 = 3.6, giving a cable tension of 360 N before losses. Allow 15% for cable friction, so 360 × 0.85 = 306 N reaches the caliper. (Basic mechanics of levers: Lever Systems.)
Step 2: the caliper. Assume each pad is pressed with 1.5 times the cable tension: N = 1.5 × 306 = 459 N per pad. The overall multiplication from hand to each pad is 3.6 × 0.85 × 1.5 = 4.6.
Step 3: friction. With a rubber pad on a dry aluminium rim, assume μ = 0.4. Friction per pad = 0.4 × 459 = 184 N; two pads = 367 N, acting at a rim radius of 0.30 m, a torque of 367 × 0.30 = 110 N·m.
Step 4: at the tire. The tire contact is at radius 0.34 m (assumed), so the retarding force at the road is 110 / 0.34 = 324 N.
Step 5: deceleration. For a rider and bicycle of 90 kg: a = 324 / 90 = 3.6 m/s², or 0.37 g. From 6 m/s (21.6 km/h) the stopping distance is v² / 2a = 36 / (2 × 3.6) = 5.0 m.
Wet pads. Rubber on a wet rim loses much of its grip. With μ = 0.15 (assumed): friction per pad 69 N, torque 41 N·m, force at the road 122 N, a = 1.35 m/s² (0.14 g), and the stop from 6 m/s is 36 / (2 × 1.35) = 13.3 m, more than 2.5 times as far. Wet braking needs early, gentle application.
The tipping limit. Hard front braking moves weight forward. If the combined centre of gravity is 0.6 m behind the front axle and 1.0 m high (assumed), the rear wheel lifts at a deceleration of g × 0.6 / 1.0 = 0.6 g (5.9 m/s²). One dry brake at 0.37 g is well inside that. Both dry brakes at full squeeze would want about 0.73 g, which is past the tipping limit, so the rider would lift the rear wheel and could go over the bars: at maximum, brake progressively, favouring the rear at high speed.
Cable travel. For each pad to move 2 mm to close its clearance, the pair of pads moves 4 mm in total. With the pad force ratio of 1.5 per pad, the cable moves 2 × 1.5 × 2 mm = 6 mm (energy balance: cable tension × cable travel equals the sum of pad force × pad travel), and the lever end at the hand moves 6 × 3.6 = 21.6 mm. A cable that has stretched 2 mm needs 2 × 3.6 = 7 mm more lever travel, which is why a loose cable lets the lever reach the handlebar before the pads bite.
What You Need
- Wrenches or a hex key for the anchor bolt and pad fixings
- Pliers to pull the cable
- Sandpaper to rough up glazed pads
- Replacement pads if worn to the metal
- Light grease for the cable, and a rag with solvent to clean the rim
Method: Rim Brakes
Step 1: Clean the rim and pads. Any oil or grease on the braking surface ruins the grip.
Step 2: Check pad alignment. The pads should strike the rim flat, not touching the tire.
Step 3: Adjust cable tension. Loosen the anchor bolt, pull the cable tighter, and retighten.
Step 4: Check the lever. The lever should stop well short of the handlebar when squeezed hard.
Step 5: Roughen glazed pads. If pads are worn smooth, rub them with sandpaper for better grip.
Step 6: Replace worn pads. A pad worn to its metal backing must be replaced, because it will destroy the rim.
Step 7: Test. With the bicycle held, squeeze each lever hard. Then roll at walking pace and brake each one.
Coaster (backpedal) brakes. The mechanism is internal and rarely fails. If the brake drags, the hub needs repacking with fresh grease; if it does not engage, the internal brake shoes are worn and the hub needs disassembly (Wheel Hub Bearings).
Common Mistakes
- A cable left loose — each millimetre of stretch costs 3.6 mm at the lever.
- Oily pads or rim — the friction falls toward the wet value.
- Pads touching the tire — the tire wears through at the sidewall.
- Both brakes grabbed hard at speed — the rear lifts and the rider can go over the bars.
Safety
The brake is the safety system
A failed brake at speed is a serious injury. Pads worn to their backing destroy the rim wall, and a thin rim can fail suddenly under tire pressure (Tire & Wheel Care). Test brakes at walking pace before every ride, keep oil and grease off the rim and pads, allow far longer stopping distances in the wet, and ride slowly after any adjustment or improvised part (Improvised Parts & Fabrication).
Illustrative inputs
The hand force, lever and caliper ratios, cable efficiency, friction coefficients, rider mass and geometry are assumed values, chosen to be plausible; the friction values in particular are recalled orders of magnitude. The method of combining them is standard mechanics, but the results are not measurements of any real bicycle.
What’s Next
- Learn next: Cable & Brake Substitutes — when the cable fails
- Combine with: Tire & Wheel Care — the rim the pads act on