Bruce’s advice on adjusting the Avid BB7 brakes on Catrikes:
Avid BB7 PROPER brake alignment and adjustment, and the physics behind it.
For the sake of simplicity I will restrict this explanation to the Avid BB7 brakes found on Roads, Speeds, Expeditions and 700′s. The BB5′s are similar, but just harder to adjust and maintain.
Be aware that what I describe is for those who want the last degree of fine tuning of their brakes. It involves a lot more work than a bike shop would do for you and is best done with a helper of similar weight to the trike’s normal rider.
The point of brake caliper alignment is to position the caliper properly around the rotor so that the pads contact the rotor squarely and evenly as the brakes are applied. The spacer packs on each side of the caliper mounting ears are made up of ball-and-socket washers that allow the caliper to “wobble” around the mounting bolts to align with the rotor. The caliper’s mounting ears are also slotted to allow the caliper to slide side-to-side as needed to position the caliper evenly over the rotor.
Avid’s instruction sheet recommends that the rotor be positioned in the caliper with a 1/3-2/3 clearance to the caliper body casting. After talking on the phone with Calvin Jones of Park Tools, it is my belief that this is not necessary, and that the rotor should be positioned as evenly centered as possible. Even the techs at Avid that I talked to couldn’t give me a reason for the 1/3-2/3 setting. I set all of mine centered and have had no issues with this. Calvin said that all other manufacturers recommend centering the rotor and that he does so on the BB7′s, too.
Before we get into the actual alignment process, there is some understanding and setup that must be done.
First, it should be understood that these brakes are borrowed from bikes, and their use on trikes is a compromise in application, requiring extra effort in alignment and adjustment for truely proper function. A bike’s front axle is “double hung”; that is, it is attached at both ends to the fork dropouts. There is virtually no bending action of the axle at either end. On a trike, the axle is “single hung” and there is considerable flexing going on compared to a bike, even with our 20mm axle tubes. The caliper is mounted on the spindle steering arm and the rotor is mounted on the wheel’s hub. The point at which the axle flexes is between these two mounting points, so the caliper and rotor move in relation to each other as the axle flexes. This movement disturbs the proper alignment of the rotor to the caliper. As you can easily imagine, the weight of the rider and cornering forces, as well as other factors, will cause the axle to flex. We can’t anticipate and compensate for the constantly changing cornering forces, but we can do something about the flexing caused by the rider’s weight. This is what this “story” is all about, and how to do it.
Second, there is a constantly overlooked situation that affects rotor-to-caliper alignment. It’s the clearance between the 20mm axle tube and the spindle. To accomodate removal of the wheel while on the trail for tire repairs and other reasons, the axle should be an easy slip-fit into the spindle, which means there is some clearance, although a very tiny amount. This tiny amount of clearance allows the axle to “wiggle” a tiny bit in the spindle with no weight on the trike, which translates into movement of the rotor in relation to the caliper body. This tiny bit of wiggle is at the same point as the flexing mentioned earlier, and has the same effect as flexing. However, the quick release skewer removes this wiggle when it is tightened properly, leaving only the flexing to affect the alignment when the trike is loaded and being ridden. Again however, when the trike is in the repair stand, unloaded, that tiny bit of wiggle allows the axle to “droop” slightly from the weight of the wheel as it is mounted on the trike, then the skewer is tightened. All of that tiny bit of play is shifted downward at this point. When the trike is taken out of the work stand and set on the ground, then the rider sits on it, the axle actually shifts upward from the rider’s weight, again shifting the alignment of the rotor to caliper. It is this position of the axle that should be established before doing a caliper alignment. It should also be noted that when the caliper has been previously aligned and the wheel is removed for any reason, then put back on the trike, the caliper will appear to be misaligned until the trike has had the rider’s weight on it and the trike ridden a short distance to help the axle “settle” into it’s “natural”, operational position. It should then be back in alignment again!
It should be clear by now that to do an accurate caliper/rotor alignment, the trike must have the rider seated, and the trike should be rolled a few feet to allow it’s front wheels to “squat” as much as they are going to. Once this is done, the calipers are ready to be aligned by a second person while the rider sits on the trike.
To align the calipers, begin by making sure that the rotor is absolutely straight and doesn’t wobble (this is done ahead of time with the trike in the repair stand), then loosen the caliper mounting bolts (2) just enough to let the caliper body wiggle on the bolts. Now, adjust the pad adjusters (the read knobs on each side of the caliper) until the pads are tight against the rotor, holding it solidly, and the rotor is centered in the caliper body. Use the pad adjusters to center the rotor in the caliper body. Once the rotor is centered and the pads are very tight against it, tighten up the caliper mounting bolts. Now, back off each pad adjuster about 3 or 4 clicks and check for rotor clearance by looking for a gap between the rotor and the pads. It helps to hold a piece of paper behind the brake caliper to see this gap. If there is a gap (and, there SHOULD be!), make sure the rotor looks parallel to each pad, and that neither of the pads is “cocked” at an angle to the rotor. It is best to set the gap to the fixed pad as small as possible without any drag, then set the gap to the moving pad to suit the rider’s feel for lever movement. In any case, be sure that the rotor doesn’t drag on either pad. The fixed pad is the one nearest the wheel with the largest adjuster knob. Some older calipers had a T-25 torx fitting in the center of this larger adjuster knob to make it easier to access and turn. If the gap to the fixed pad is too much, the rotor has to bend too much to contact it during application of the brakes and it may warp under hard use. This is why the gap to this pad must be kept as small as possible without creating any drag on the rotor.
Of course, there are thousands of trikes running around out there with brakes adjusted and aligned while still in the work stand. This is what you will get if you have a bike shop do the work for you. If the wheel hasn’t been recently removed before the alignment process, this will work reasonably well for most folks. But, if you’ve read this far, it is because you want your trike to run it’s very best, and are willing to do the extra work to be sure your brakes don’t drag at all.
As a final note, some riders have reported hearing their brakes squealing in hard turns even when not being applied. This is dirrect evidence that the caliper and rotor are moving in relation to each other from axle flexing, and that movement is sufficient to cause the rotor to contact one of the pads, causing drag and wear of the pad. This is especially true of heavier riders, so if you are a heavier rider who takes corners fast, then try backing off the pads another click or two to give them a little bit of extra clearance for the rotor to move more from flexing. On all Catrikes, the right brake would get those extra clicks on the moveable pad, and the left brake would get them on the fixed pad. Don’t ask why, because that is another story that would take just as long to tell !