Notes on TRP Spyre and Avid BB7

1. Preface
2. Initial Caliper Alignment
3. Adjustment
4. Basic Facts - Cable Pull and Pad Movement
5. TRP Spyre versus Avid BB7

1. Preface
This document is all about adjusting cable disk brakes. In particular, the TRP Spyre or Avid BB7. I wrote it when we were adjusting a typical road bike. Road bikes have drop handlebars and short pull brake levers (as opposed to Long Pull brake levers). The first two chapters are just "how to" instructions. The last chapters are more the theory as to how the cable pull translates into arm rotation and brake pad movement.

2. Initial Caliper Alignment
The first step is to get the caliper aligned and pad clearance adjusted. This is not done by the cable, it is done by the pad adjustments on the caliper. So loosen the cable clamp. Then loosen the caliper mounting bolts. Then turn both pads fully out, then adjust both pads the same amount inward until they grip the disc tightly. That pulls the loose caliper into alignment. Then tighten the caliper mounting bolts. Then back each pad off equal amounts so there is clearance on both sides. At this point, the caliper is aligned and pads have the proper gap.

Now the cable. You want the lever and cable to have maximum travel, such that the brakes can move from fully off to fully on. You want the brakes to be fully on long while there is still lots of lever travel available. So tighten the pinch bolt on the cable clamp such that there is zero slack in the cable when the brakes are fully released. That ensures you have maximum cable travel, taking the brakes from fully released to fully on. This assumes the pad clearance you have set is small enough that only part of the cable travel is necessary to put the brakes on full. If you have set too much pad clearance, even if you move the lever it's full travel, the brake won't be tightly on.

3. Adjustment
When doing later adjustments, do that with the pads, not with the cable. The cable will still be such that the handlebar lever immediately starts to move the brake arm. No slack. So all you do is move the pads closer to the disk, and the setup will be exactly the same as initially.

The other thing that people do when there is too much pad gap is tighten the cable, such that it is already pulling the brake arm part of it's travel. This works as long as there is still enough travel in the arm to close the gap. But eventually with successive adjustments via the arm there will not be enough travel left in the arm. Then you must move the pads in.

Cable slack: There should never be cable slack. The cable should be clamped such that any cable pull immediately starts to move the arm. If somehow the cable has stretched or the clamp has slipped such that there is cable slack, of course you take up the slack with the pinch bolt.

4. Basic Facts - Cable Pull and Pad Movement
In this chapter is a general discussion of how the cable pull translates into pad movement. For this, I used Grok AI to get the actual measurements. There are two types of lever depending on how much they pull the cable: short pull and long pull. Road bikes are short pull. "Pull" means the distance the cable moves for the full travel of the handlebar lever. When the brake arm is pulled, that results in a certain distance of pad movement.

 - Short Pull Lever: assume 2.5mm of cable
 - Pad Movement 1:2 is 1.25mm both sides, or 0.625 per pad. The Spyre’s actuation arm can handle up to ~5mm of cable pull, translating to ~2.5mm of pad movement per side (5mm total, based on the 1:2 ratio). This is the caliper’s mechanical limit, determined by the internal cam and linkage stopping further travel.

A common ratio for dual-actuation systems like the Spyre is approximately 1:2 (cable pull to total pad movement). This means a 2.5mm cable pull results in 1.25mm total pad movement (0.625mm per pad). Say 0.6 per pad. So if you set the actual pad clearance smaller, say 0.2 to 0.5, then it will only take part of the lever travel to put the brake fully on. Eg: If you set the pad clearance wide to 0.5, the lever will have to pull almost all of it's 0.6 range.

For this example, let’s fix the pad movement at 0.6mm per side (1.2mm total) for a 2.5mm cable pull, which aligns with the Spyre’s design for precise, short-travel braking and the 0.2–0.5mm recommended pad clearance per side.

With Short Pull, the lever is the limit. It can only pull the pad 0.6mm. If you had long pull lever, then it could pull the lever more and thus the pad more. At some point the lever can't rotate enough to take advantage of all the cable pull.

5. TRP Spyre versus Avid BB7
The most important reason to use the TRP Spyre rather than the Avid BB7 is ease of pad replacment. With the Avid BB7 it is really difficult to assemble the flimsy little spring in place. So you end up with the pads not fully seated. They still seem to work with them not seated, but it is difficult to get rid of wheel drag. We used to have Avid BB7 brakes but eventually replaced them with TRP Spyre just to make it easy to replace pads.

The difference between a TRP Spyre and the Avid BB7 is that with the TRP Spyre, both pads move toward the disk, squeezing it, whereas with the Avid BB7 only one side moves. If you visualize what happens with the BB7 is that the outside pad moves in toward the disk, and then deflects the disk slightly to push it against the inner pad. Whereas with the TRP Spyre both sides move in equal distance. So you might like the TRP Spyre better because the disk doesn't have to deflect, and you might think the pad wear would be more even.

To adjust the pads on a TRP Spyre, you stick a 3 mm allan key into the hole on each side, then wiggle it around until you feel it seat properly inside the 3 mm inner socket. And note that it is 3 mm hex key, not a 2.5 mm hex key. The 3 and 2.5 look almost identical, but the 2.5 will fit too loosely in the inner hole.