Fleetdog

Basically, is it normal for new pads to get a bit more grip after a small amount of wear?

I'm sure there will be plenty of folks wanting to tell me to just switch to salmon pads, but I bought some adjustable brake pad holders form my cross bike and I'd like to at least give the pads that came with them a fair shake. I replaced the front pads a week or so back and spent a few days riding then tweaking, etc and I'm pretty pleased with the front now. On the back, I just put the pads on last night and set them up like I now have the front and they seem to have very little grip on the rim.

I'm curious if maybe it takes a bit of breaking in new pads and that I should expect slightly better performance after a couple of rides on these rear pads as a few days use is the only real difference I can tell between my setup on the front and rear right now.

operator

I hope you cleaned those rims before replacing those pads.

SJX426 

As the pads are used, the material will be removed and the shape will conform to the rim and will improve bracking somewhat. But you should plan on getting the Salmon from KoolStop! there are few substitutes. I put a set on a 1974 Raleigh GP mixte that would not stop to save your soul. Totally redeamed!

AEO

if the pads don't hit flat on the rim, then yes.
If the pads do hit flat on the rim, then check for dirt, grease or soap on your rims and pads.

Yan 

Frictional force is a function of the coefficient of friction as well as force applied along the normal axis, and has nothing to do with contact area. Perhaps your straddle cable is too long.

mkane77g

Bit of toe in

LeeG

A little break-in makes sense but if they aren't working well from day one and still are substandard after a few long braking descents something isn't right. Do you have a sense how much of an imprint or wear area the pad is making? If it's on edge it's possible there isn't enough pad touching the rim.

I had a heck of a time getting the front brake on a CrossCheck to not squeal. For awhile I was using regular Shimano cantilever pads on Oryx brakes with Alex rims, the toe in was perfect but the Alex rims with their striated finish (machined lines in braking surface) and the hard black shimano pads resulted in the worst braking in rainy conditions as bits of grit got embedded in the pad and bits of aluminum grit piled up on the rim. It didn't squeal but it was scouring the rims something awful. I sanded the rims with 400-600 grit paper to remove the piled up aluminum bumps and replaced the pads with,,,yes, Salmon pads. All is well now.

RaleighComp

+1 on cleaning the rims and a bit of toe-in (assuming these are cantis). Also, if you don't feel like you're getting enough braking power you might have your straddle wires set to high. The levers should feel spongy on contact, not stiff (the opposite of car brakes).

Fleetdog

I do have some toe in and the straddle cable is set pretty low. There is also a fair amount of give on the brake levers after pad contact a little more would be nice, but that will have to wait until I get around to buying a new brake cable since the yoke is already pretty much at the end of my main cable.

I have not tried cleaning the rims yet. I'll give that a shot. I'll also check the pad alignment again. to make sure the rear pads aren't hitting on their edges. If none of that works and they don't get better after a little riding I suppose I'll have to start replacing brand new parts (get longer cables and salmon pads).

Jamesw2

Do you use auto brake cleaner on bicycle rims?

AEO

except with rubbery objects like rim brake pads.

The force of friction is independent of the apparent area of contact. (Amontons' 2nd Law) (Amontons' 2nd Law does not work for elastic, deformable materials. For example, wider tires on cars provide more traction than narrow tires for a given vehicle mass because of surface deformation of the tire)

Yan 

Link.

The above article, pertaining to motorcycle tire physics, states the reason performance motor vehicle tires are wider than standard tires as being that performance tires use softer rubber with a high coefficient of friction than standard tire rubber, and therefore need a larger tire size to compensate for wear, heat, and structure.

Section 7.5 of this book ("Fundamentals of Surface Mechanics with Applications"), viewable via Google books, states that Amontons' Second Law holds for both plastic and elastic contact: Link.

operator

Yeah the fact that braking power decreases substantially with increasing toe-in to the point where only half the pads touch the rim is just my imagination.

DieselDan

I'm just surprised the words "break" and "brake" have been used properly in this thread.

Yan 

Mechanical advantage decreases as the lever pull progresses. Toe in delays the application of full braking. This has nothing to do with area of contact. If it weren't for the complicating factors of structural rigidity, wear rate, and heat, you could use brake pads the size of pencil erasers.

This topic has come up on these forums before.

Fleetdog

While I don't have enough materials physics training to argue with Yan, I did take a look this morning and found that the pads aren't aligned all that well in the back and they are only making contact with their top edge. This seems like a problem to me, so I will adjust them vertically flat (still with toe in) and see if that helps before doing anything else.

operator

Except at the point where toe-in delays application of full braking, indefinitley. Don't repair or build many bikes do you? This is the typical response to fork chatter on canti equipped caron fork bikes or to fix squeal where replacement of parts is not possible.

If the brakes don't squeal, they don't need toe-in. It would be rare to see a modern road bike with dual pivots that requires any sort of toe-in at all. Theory is great until you apply it to the real world where practical results matter.

onbike 1939

You might find that it helps to give the pad surfaces a rub with a rough file.

DieselDan

Case in point: my old RSX brakes needed some toe in to prevent squealing, but the new Ultegra brakes the pads hit flat and brake much better and don't make a sound.

helicomatic

The next time I'm trying to start from a standing stop while holding my brakes, I'll keep in mind that it doesn't matter how big the contact area between pad and rim is.

You're taking this out of context. Amontons' second law concerns static friction, like your tires on the road, not dynamic friction, like your brake pads applied to a moving wheel.

Yan 

You misunderstood my post. I meant that because toe-in introduces additional flex to the pad before it is fully clamped down onto the rim, the brake lever is at a more severely depressed position than on a setup with no toe-in when the brakes are fully engaged, all other conditions being equal. Since levers have lower mechanical advantage the further they are depressed, a lever used on a setup with toe-in cannot achieve the same mechanical advantage as in a setup with no toe-in.

1. If a theory doesn't match the real world it's because the theory is incorrect or inadequately refined, not because it is a theory. We're discussing Newtonian physics here: it's hardly cutting edge stuff.
2. I doubt I've built or repaired as many bicycles as you, because you do that for a living; though I don't see how that's relevant. If one of us is correct it's because that person has the facts straight, not because he has repaired more bicycles.

helicomatic

You are still wrong.

RaleighComp

When we're talking cantis, the angle of the straddle wire at any level of brake contact has more to do with mechanical advantage than the distance the levers have been pulled, with maximum mechanical advantage being obtained when the included angle of the straddle wire is at 90 degrees.