1. Preface
This document deals with cable pull: long pull and short pull. In both disc brakes and also caliper or cantilever brakes. This stuff is design theory, whereas the "Interfaces" document is just about making parts fit. Eg: In this document we talk about cable tension which is irrelevant to actually fitting the bike together.
2. Cable Pull Theory
There are two types of calipers and levers: short pull and long pull. Easiest way to understand it is to start at the break caliper end. The brake caliper has to move the pads a certain distance. On an Avid BB7 pad clearance is 0.5 mm per side. So we need to move the pads that far. They are moved by an arm which pivots. You can easily see that if you can attach the cable way out on the arm, you have more leverage than if it was in close. When it is attached way out, that is long pull. Now think of the cable tension. The way leverage works, the further out the cable, the less force needs to be in the cable. So long pull results in less force in the cable. But it needs to pull the cable farther. Long pull is about 15mm, whereas short pull is only 7 mm.
Now at the lever end, our job is to pull the cable 15mm. The lever also has a pivot. A long lever needs to rotate less to move the cable 15mm. First consider a lever where the pivot is right in the center, and the point of force is at the end. Such a lever must then move 15mm to get the desired cable travel. If the lever was twice as long from pivot to handle as from pivot to cable attachment, the handle has to move twice as far, but will take half the force to get the same tension as we previously had.
So a complete example, suppose the pad clearance is 15mm and we need 500 N pad force. Therefore the long pull cable tension must be half that, say 250 N. At the lever end witha 2 to 1 lever, we need 125 N force. And the travel has to be 30mm.
EXPLANATION TWO: To understand the things you read, I found it necessary to envision a really "super long pull" system, which has very low tension in the cable.
Theoretically, you could have a "super long pull" system whereby the cable moved a huge distance with low tension. Envision a caliper arm where the cable attaches way out from the pivot point,such that it has a lot of leverage on the arm. On the other end is the pad. You would have twice as much leverage if the cable is 20 cm out from the pivot as opposed to 10 cm. If the pad was also 10 cm from the pivot,then for a given force at 20 cm, you get twice as much force (and half the travel) of the pad. The caliper arm would be really long, such that With very little force in the cable, the pad could be squeezed really hard.
Now envision a handlebar lever which also had a lot of leverage. Eg: you apply a force 10 cm from the pivot point, and the cable is attached on the other side 2 cm. So you've got 5 to 1 leverage. So a force of 1 newton (N) on the lever results in 5 N tension in the cable, and 10 N force on the pad.
However, such a system would require the lever to also move a big distance. (Super long pull). Where it gets confusing is that your hand only can move so far, so they might want to decrease the length of the lever to 5 cm, so then you'd only need to move it half as far, but with double the force.
Draw a diagram with two arms: lever and caliper, each pivoting. You can see that by moving the pivot away from the cable attachment at both ends, you need to have more movement in the lever.
Now envision different lengths of lever. If you have a really short lever, short travel in lever handle, you have to grip it pretty hard because you don't have a mechanical advantage in the lever. So depending on how long the lever, one tends to cancel the other out.
3. Measurements for Avid BB7, TRP Spyre
In general, you set the pad clearance to a certain amount say 1mm each side. The caliper design has a certain "pad travel ratio" such as 20:1. (exact numbers not available). So if the cable moves 10 mm, the pad moves 0.5 mm. The "arm" (torque arm or actuation arm) has a certain maximum travel such as 40 degrees or some distance. This could be measured by distance or by degrees. So in a long pull system, you pull the lever, it pulls the cable max 15mm, that pulls the arm 15mm and moves the pad 1/20 of that which is 0.75mm.
Mechanical disk brake calipers have a fixed pad clearance. Eg: Avid BB7 is 0.5-0.6 mm per side. (So it takes 10 mm of cable movement to go from fully off to fully on.
Long-pull levers (V-brake, MTB disc): cable anchor is farther from the pivot
- Pulls more cable per degree of lever swing,
- But with lower cable tension (lower mechanical advantage).
3. Why this matters for the BB7
Avid made two versions of disk calliper:
- BB7 MTB (long-pull): needs ~3–4 mm of cable travel at the arm to move the pad properly.
- BB7 Road (short-pull): cammed differently inside, so it multiplies the cable tension from a short-pull lever into the right pad motion.
That’s why you must match the lever type to the caliper version.
DIMENSIONS
Short-pull levers (road/cantilever) pull approximately 7 mm of cable.
Long-pull levers (MTB/V-brakes, mechanical disc) pull roughly 15 mm—about double the travel of short-pull.
Additionally, the lever's "cable to pivot" distance contributes directly to this difference:
Short-pull levers: ~21 mm radius from pivot to cable.
Long-pull levers: ~42 mm radius—roughly twice that, which explains the doubling in cable pulled.
"Mechanical Advantage" means more movement for a given movement. The opposite of "leverage"
Summary of Measurements:
Short Pull Long Pull -------------------------------------------------
Cable Pull 7 15
Movement at arm 7 15
Pad Clearance: 2 5
Arm to Pad Ratio 15:1
Lever Cable to pivot: 21 42
Max Arm Sweep 30 30 //same on both models, does not vary
Dimension Definitions:
Example (your numbers): If the arm rotates and pulls 7 mm of cable and that translates into 1 mm of pad travel, the ratio is ~7:1. The torque arm (actuation arm) moves maximum ~30 mm, equating to perhaps 2 to 2.5 mm of outer pad movement. (So that would be 15 to 1)
Chat says: The amount of cable pulled (or pushed) at the brake’s actuation arm when the lever is fully squeezed. It’s often listed in mm of travel. For example: A road (short-pull) lever might generate ~8–10 mm movement at the arm. An MTB (long-pull) lever might generate ~14–16 mm movement at the arm.
So it seems to me that it is normal that the cable pull is only a fraction of the potential arm movement. To move the arm the full distance, you'd need 30mm of cable pull, not just 15mm. It is not sufficient to move the arm the full distance.
3. Cantilever Brakes
I have a 1993 Rocky Mountain steel frame mountain bike with 26" wheels and Cantilever brakes. They are the classic center-pull style with straddle cables, before V-brakes came out in the mid-1990s) They use short-pull levers and require quite a bit of force on the levers to put the brakes on hard.
Why? Cantilever brakes (and side-pull calipers on road bikes) don’t need much cable movement, but they do need higher cable tension ; short-pull levers match this.
When Shimano introduced V-brakes in the mid-90s, they needed much more cable travel to close the arms and that’s when “long-pull” levers came along. The cable tension is half as much but has to move twice as far.
Quick Summary Table:
Brake Type Pull ---------------------------------------------
Cantilever short-pull
Side-pull/caliper brakes (road) short-pull
V-brakes / MTB mechanical discs long-pull
Road bike mechanical disk long-pull
5. Cable Travel Adapters
In order to use long pull brake with a short pull lever, there are adapters. For example,
one of our bikes has drop bars and V Brakes. The drop bar brake levers are short pull, but the V-Brake requires long pull. So to do that, the cable goes thru a pulley-like converter that turns short pull into long pull. The cable is attached to a small diameter pulley which is attached to a larger diameter pulley.
Those little pulley gadgets were popular in the late ’90s/early 2000s when people wanted to run V-brakes with drop-bar levers (road levers are short-pull, V-brakes are long-pull).
That device is commonly called a Travel Agent. Brand name: "Travel Agent" was the original version made by Problem Solvers. Generic name: cable pull converter or cable travel adapter.
How it works: it uses an eccentric pulley (two different radii) so that a short-pull lever’s small cable movement is translated into a longer cable pull suitable for V-brakes (or MTB mechanical discs).
Alternatives today
Instead of a Travel Agent, you can also solve it with:
Mini-V brakes (shorter arm V-brakes designed for road pull).
Long-pull drop-bar levers (e.g. from Tektro or Cane Creek).
Road-specific mechanical discs (Avid BB7 Road, TRP Spyre, etc.).