Sidepull Brake Adjustment

[2LeftCleats]

I just read through the Park Tool "Big Blue Book of Bicycle Repair" and found that I've been adjusting my brakes incorrectly.

It says that the right side pad should be adjusted to touch the lower part of the braking surface and the left side to the upper part (direction viewed from mechanic's viewpoint). I've always tried to make them symmetric. I realize that there's not a lot of vertical difference but wondered why this is recommended.

[rustychisel]

Having worked on sidepulls for over 40 years, and struggled to get them effectively centred, working most efficiently, etc I feel qualified to say "kerphoooie". I've never heard this idea expressed before, ever.

Mind you, I can kind of see the idea, but is the lower pad to be the one on the side of the cable stop arm, or the other? It's not clear, but neither is the benefit, in terms of the physics?

I suspect any benefit - perceived or real - would be quickly negated by the drawbacks, one of which is keeping them optimally adjusted.

[Mark McM]

I understand why they recommend this, but I'm not sure that it makes a big difference in practice. Also, if you look in the book again, I think you'll see that this recommendation is only aimed at traditional dual pivot brakes.

When the brake is actuated, the pads do not move in straight line to the rim. Because they are mounted on pivoting arms, the pads actually move in an arc. The relative motion between the rim and the pad will depend on the position of the pivot. On the right arm, the pivot is offset from the center bolt, so it is above and to the right of the rim contact point. Therefore, the travel arc of the pad will inward and upward. As the pad wears, it will swing further inward and upward before it contacts the rim. If the pad was adjusted at the top of the brake track when it was new, then as the pad wears it's swing may move upward enough to touch the tire (always bad news).

On the left arm, the pivot point is right at the center bolt, which is above and to the right of the rim contact point so the arc of the left pad will inward and downward. As the pad wears, the swing will move further inward and downward. The pad is unlikely to dive below the rim, but if is adjusted too low, than as it wears the contact area with the rim will decrease as the pad swings further downward, off the brake track.

Will this make a big a difference? Only if the rim brake tracks are very narrow, or if the pads are adjusted too far in the direction of their vertical motion.

By the way, using the above logic, both pads on single pivot calipers should be adjusted to the bottoms of the brake tracks, and both pads on the new Shimano direct mount brakes (both pivots are offset from center) should be adjusted to the tops of the brake tracks.

[oldpotatoe]

Having worked on sidepulls for over 40 years, and struggled to get them effectively centred, working most efficiently, etc I feel qualified to say "kerphoooie". I've never heard this idea expressed before, ever.

Mind you, I can kind of see the idea, but is the lower pad to be the one on the side of the cable stop arm, or the other? It's not clear, but neither is the benefit, in terms of the physics?

I suspect any benefit - perceived or real - would be quickly negated by the drawbacks, one of which is keeping them optimally adjusted.

Agree..bike brake. hauling a 200 pound 'speeding' package from 15-18 MPH..don't think it makes any difference plus hellovalot easier to just make them symmetrical and hit the rim evenly. Think this is in the category of 'over thinking'...

[2LeftCleats]

Thanks for the wisdom. It didn't seem likely to make much practical difference, but thought maybe I was putting my rims or brakes in jeopardy. Or that maybe I could stop a foot quicker.