disc/brakepad alignment? (using all the disc)

been having a discussion with my friend.

its sparked a few questions i would like some clarification on.

say, if your not using all of your brakes pad surface due to mis-alignment (forming a lip on the pad material)

1. is the percentage of pad area not used proportional to braking force lost? or is this exponential?

2. is the percentage of pad area not used proportional to increased pad wear? or is this exponential?

then say, if your pads are aligned correctly and you are not forming a lip on your pad surface.....but the outside track surface area of your disc is narrower than disc pad surface area.

1. same kinda question as above. ( effect on brake force lost) ?
2. same kinda question as above (effect on extra pad wear) ?

here a pic of my brake wear/alignment. do yo think itd be worth finding a disc with a wider track?

In your case, the pad running slightly off the edge of the braking surface is effectively the same thing as the pad running over the slots cut into the braking surface: not great for braking, but not a serious problem either.

The fact that bicycle disc rotors are always slotted or perforated in a decorative pattern that reduces their braking power and increases wear is tangible evidence that disc brakes on bicycles are made for how they are perceived, not for how they perform. Their actual performance is irrelevant, because those who prefer them believe they're superior to other kinds of brakes even when that isn't remotely true. The wanky ventilated rotor is totally in character with the wanky brake; it's a symbol to appeal to a market more than it is a tool to do a job.

Fortunately, most bicycle disc brakes work adequately enough (until they get contaminated, anyway) that silly frilly rotors don't constitute a functional problem by themselves.

jimmyhackers said:

been having a discussion with my friend.
its sparked a few questions i would like some clarification on.
say, if your not using all of your brakes pad surface due to mis-alignment (forming a lip on the pad material)
1. is the percentage of pad area not used proportional to braking force lost? or is this exponential?
2. is the percentage of pad area not used proportional to increased pad wear? or is this exponential?
then say, if your pads are aligned correctly and you are not forming a lip on your pad surface.....but the outside track surface area of your disc is narrower than disc pad surface area.
1. same kinda question as above. ( effect on brake force lost) ?
2. same kinda question as above (effect on extra pad wear) ?
here a pic of my brake wear/alignment. do yo think itd be worth finding a disc with a wider track?

Click to expand...

Hard to tell without seeing the whole picture ...
Wear pattern suggests that alignment washers are missing between the caliper and frame mount.

yeah the first situation is my friend....hes not using all of his brake pads. as his spacers are too big or his brackest wrong or both...
hes always complaining his pads dont last very long, although he does ride his bike harder than me.

i wondered, say if hes using %90 of the pad surface. does that means 10% less braking force and 10% less pad lifespan?

the 2nd situation is mine....which works fine....i havnt gone through a set of pads yet. i made custom spacers to get the pads just to the edge of the disc without it forming a lip on the pad material surface.

all this talk with my mate made wonder how much braking potential i was loosing due to these fancy pants rotor designs, i picked it because it was the cheapest, not for looks

The formula for calculating friction is:

Friction force= Clamping force x coefficient of friction between the two materials.

The size of the contact area does not effect the friction force.
However, smaller area does mean higher wear.

Avner.

ferret said:

The formula for calculating friction is:

Friction force= Clamping force x coefficient of friction between the two materials.

The size of the contact area does not effect the friction force.
However, smaller area does mean higher wear.

Click to expand...

In theory, this is true. In practice, the surface subjected to friction often has other factors at work that cause changes in area to cause changes in friction. The larger the area, the more asperities there are to engage one another on the surface, which would tend to increase friction. But there's also more opportunity to have particles on the surface which may function as a lubricant and reduce friction until they're cleared out.

We use bigger tires with larger contact patches to increase available traction, because coefficient of friction isn't the only relevant factor. Relatively hard disc brake pads on a stainless steel rotor don't exhibit as pronounced a change in frictional force with change in area as rubber tires on concrete, but some of the same principles apply.

Chalo said:

ferret said:

The formula for calculating friction is:

Friction force= Clamping force x coefficient of friction between the two materials.

The size of the contact area does not effect the friction force.
However, smaller area does mean higher wear.

Click to expand...

In theory, this is true. In practice, the surface subjected to friction often has other factors at work that cause changes in area to cause changes in friction. The larger the area, the more asperities there are to engage one another on the surface, which would tend to increase friction. But there's also more opportunity to have particles on the surface which may function as a lubricant and reduce friction until they're cleared out.

We use bigger tires with larger contact patches to increase available traction, because coefficient of friction isn't the only relevant factor. Relatively hard disc brake pads on a stainless steel rotor don't exhibit as pronounced a change in frictional force with change in area as rubber tires on concrete, but some of the same principles apply.

Click to expand...

In other words, jimmyhackers, max out the contact area so most of your your pads sweep the disk. Everyone agrees, no matter what they think about the equations, is the pads will wear faster with less contact. In an extreme case, with really minimal contact, you can wear a deep enough groove in the pads, that the unused parts of the pad touch each other.

i guess its as much as i assumed.

ive had a look for "better" discs. if u look at my wear pattern, at best id be gaining another 1-2mm of OD and some unused disc space if i got "rounder" discs.

seeing as my brakes work, feel good, dont wear out quickly, dont form a lip on the pad, and i can lock up either wheel with ease.
i figure i dont need to upgrade.

little update.....needless to say my friend was running his calipers too high, not using all the pads and had a badish time, the results to the disc were pretty catastrophic. luckily everything else including him was ok, he didnt even come off the bike.

here are the pics of his disc after the fact.

here's the bit of his disc that came off.

i guess bad verticle/radial caliper alignment can be detrimental

I guess they never inspected it in quite a while.

By the way, I went thru all my boxes and tool ases looking for those doggone concave and convex washer spacers. I was just going to buy some when I saw they were $4 each online. Maybe less at the bike shop, but don't know. Finally found two pair to install a caliper,

docw009 said:

By the way, I went thru all my boxes and tool ases looking for those doggone concave and convex washer spacers. I was just going to buy some when I saw they were $4 each online. Maybe less at the bike shop, but don't know. Finally found two pair to install a caliper,

Click to expand...

Those are the same as the thinner kind of spherical washers on V-brake pads. They're plentiful in the waste stream of any bike shop.

Wow, that’s scary. I’d stay away from cheap bike rotors. This reminds me, I need to change the rotors on one of my bikes

i would say that wasnt a result of cheap rotors.....my friend was running his calipers too high and not using all the brake pad surface.

i would imagine the extra wear biased to the outside of the disc and resulting excessive differential heating lead to an early failure.

I think I finally got full engagement on my brake pads, after three fine tuning attempts. Oddly, even with almost full engagement, and very slight misalignment, the pads would wear quickly, lasting a month at most, after a few long descents. This as with organic and with sintered metal. Since my third/last attempt, I've gone a couple of months and about 800 miles and there's still a lot left on the semi-metallic pads. There's still fading getting near the bottom when descending 800 feet or so, and although I'm grabbing the levers hard, there's still stopping power. It's surprising how much just a little bit of misalignment can affect pad wear and performance.

glad to hear proper alignment helped....turns out its quite important.

it would appear the brake efficiency lost by not using all the surface area isnt exactly linear.
in respects to % pad surface used vs pad wear increase and brakeing efficiency.
i.e. 90& of pad surface used equates more so to 70% braking force and 70% pad life.

you could always go for larger rotors if your experiencing brake fade, unless youve already gone to a 200 or a 220mm.
pad material can make a difference to heat fade...but i havnt seen any ceramic versions for my calipers, you might be luckier

"bedding them in" properly may have some merits in pad longevity also.
its possible that a brake thats not using all the pad, wouldnt bed-in properly.
potentially not being able to deposit a resonable/even amount of pad material onto the disc rotors surface as they would always be running hot/out of spec.

another potential "heat fade" fix is trying copper grease on the rear of the pads.....it will help conduct heat away from the pads and into the calipers better (like thermal paste on a heatsink)....but you could increase the risk of boiling your brake fluid this way.

failing all that....you could passively or actively duct air to them. a little 30mm server fan can shift some air.