Brake pads don't cover 203mm rotor fully (using 203mm Shimano adapter)

[Manc33]

Caliper: Avid BB7 (MTB)
Rotor: Cheap one, 203mm
Brake adapter: Shimano SM-MA-F203P/S "⌀203 Rotor Only" (PM brake to IS fork)



I ran this setup for a while and it worked OK but I noticed the shiny worn area of the 203mm rotor looks like the pad wasn't covering it properly, like this:



The line you can see around it is where the bottom of the brake pad reached to.

I measured this and it seems to have a depth of just under 16mm...



Does anyone know what the height of BB7 pads is?

The pad went 16mm into the rotor, but aren't pads bigger than that?

The narrowest points on this rotor are just 12.5mm!

Measuring it, the pads could be 2mm further into the rotor and there would be 0.5mm spare to the inner edge of the rotor.

To be honest I never liked how this rotor has a wavy edge and would prefer it to just be circular.

I can't find the height of these BB7 pads listed anywhere (they are stock pads) and I have put my front wheel back on again, I would have thought it would be listed somewhere but I can't find out. I'll have to take my wheel off at some point and try to measure the pad.

Why doesn't the pad fully cover the rotor? Is it because I'm using a Shimano adapter on an Avid BB7 caliper?

A lot of listings on eBay selling adapters say they work on Avid and Shimano, for the same adapter - so even if I got one of those, isn't it just going to end up exactly the same again?

It's been running OK setup like that but on the last few rides, I noticed a slight pulsing when braking, along with hearing it (sounds like really quiet cassette pawls) as the wavy edge of the rotor slices past the disc.

Slices... not good is it! So I swapped back to my old 160mm G2 rotor, where I know the pad fully covers the rotor because it's a stock setup.

I want to run a 203mm but I don't know why the pads aren't fully covering it and if they aren't, what's the point, it's just going to gouge a groove in my pads and it's not going to be as powerful as it should be.

Maybe I just need an Avid BB7 specific 203mm IS to Post Mount adapter?

EDIT: Looking around, the cheapest Avid one (it has "40mm IS" on the side) is £16.50 for the adapter then I'd need a 200mm rotor and the Avid ones of those are nearly £40. So that's about £55 to run a 200mm rotor with a proper Avid adapter... it's not even worth doing it.

Gone are the days of rotors costing £9.99 (not £37.99) and adapters costing £5.99 (not £16.50). What used to cost around £16 two years ago now costs £55.

I was never impressed with the stopping capabilities of the 203mm rotor while I was using it, but if only 2/3 or 3/4 of my pads were covering it, that would be why the braking wasn't up to scratch.

[mattsccm]

Because it doesn't need to? All rotors are different so that spare metal at the inner is wiggle room. I have several Avid rotors that look the same but measure differently.
At thew outer edge the wavey bit is to clean the pads with an angled edge. Or it was on motorcycle discs when they became common on off road bikes 30/40 years ago.

[531colin]

BB& pad friction material measures 15.4mm on my vernier, on a sample of 1!
Because the BB7 caliper is mounted on "dome and cup" alignment washers, is there any reason not to file a bit off the adapter to get the caliper a bit closer to the axle?
On any caliper not mounted on those alignment washers, filing the mount would mess with the caliper alignment, and so is a no-no.

[Manc33]

Because it doesn't need to?

It does. You're also gouging a line into your pads like I was. It's not meant to be like that.

It needs to fully overlap otherwise there's going to be a notch worn into the pad eventually, like this:



^ That's not my own pad and I doubt mine is that extreme, but it's just showing what it shouldn't look like. The above pad is only getting 80% of the braking it should be getting - not to mention what happens when it wears enough for the wider top parts to meet.

The following image shows how the rotor should look, with a thin strip of untouched rotor around the edge:

BB& pad friction material measures 15.4mm on my vernier, on a sample of 1!

OK cheers. Since the narrow parts of my 203mm rotor are 12.5mm then I know it can't ever be covering it, even if the pads were set in more. I wonder why they make the edges wavy so it's metal-nothing-metal-nothing-metal-nothing as you're braking. That completely defeats the object of having a bigger rotor because sometimes as it rotates, there's no pad touching it, then there is, isn't, is, isn't... as it rotates. This explains the slight pulsing I could feel when braking (then when not braking I could hear it, really quiet, maybe due to the disc expanding slightly from heat so it was able to lightly rub the pad).

Because the BB7 caliper is mounted on "dome and cup" alignment washers, is there any reason not to file a bit off the adapter to get the caliper a bit closer to the axle?

I was wondering that, or just try to take out a flat washer if one exists there, maybe I did add one by mistake way back, when first setting it up, or they came in the kit so I just added them there. EDIT: No it's just got the 1 dome and 1 cup washer there. If I file off a bit of the adapter I'm probably going to mess up the threads trying it.

On any caliper not mounted on those alignment washers, filing the mount would mess with the caliper alignment, and so is a no-no.

I think with the rotor being wavy I'm not trying it back on again anyway. If I can find one that's got a circular edge, I might swap back again.

I was reading about bigger rotors putting more force on the forks (I'm on 28mm stanchions and apparently it's 32mm or more for a 203mm rotor) but ended up reading that bigger rotors don't put more force on the forks (or QR) if you're slowing down on the bike the same amount as with a 160mm rotor. I had a long think about it and agree, if you're slowing down the same amount then the force exerted on the forks and QR must also be the same amount, regardless of rotor size - otherwise we'd have rim brakes snapping forks and popping wheels out of the QR dropouts all the time Think about it, a rim brake is just a massive rotor compared to a disc rotor.

The problem can come when you pull the brake too much on a 203mm rotor, but then you're doing an "endo" as the kids call it and... is that too much force on the forks and QR, maybe, maybe not, but the rotor size seems irrelevant after what I was reading last night.

The only reason I swapped the 203mm off was because I was worried about the braking forces, but maybe it's nothing to worry about, I mean you can't pull the brake hard enough because you'd be going over the handlebars or losing traction.

What is going to keep the 203mm off though, is that lack of overlap I have got.

[offroader]

So are you gouging a line into your brake pads?
It should be quite obvious if you are and it looks to me like the very outer tips of the rotor might be untouched.

All interrupted brake rotors produce that noise in my experience. Certainly my Avid and hope bike setups along with multiple motorbike and car setups of varying manufacturers

In theory wavey rotors have no impact on performance as the decreased surface area is balanced by increased surface pressure. In practice I doubt it works like that at all. They were trendy for a while as they offer (minimal I expect) improved cooling but most mainstream manufacturers seem to have settled on mostly round rotors now

A quick survey of my old "one or two rides left in em if you're desperate" avid BB/Juicy pad collection measure at 15mm +- a bit . Digital vernier calipers have a flat battery. What did you measure your pad width at out of interest?

Whilst I sort of see your reasoning on stanchion size I'd like to point out you're quite wrong about how rim brakes work. Rim brakes in the normal in front of the fork crown position will force the axle into the dropouts. Disc brakes in the normal behind the fork location will force the axle out of the dropouts. Think of the wheel trying to pivot around the clamping point and it should make sense. This brake pivot point is also the reason why disc brakes produce more fork stress than rim brakes but that's quite hard to get your head around. Or my head at least!

[Manc33]

So are you gouging a line into your brake pads?

Sorry I put my 160mm rotor back on again and, since the braking is the same as it always was with this rotor, I don't think much (if any) gouging was happening.

It should be quite obvious if you are and it looks to me like the very outer tips of the rotor might be untouched.

OK but even if that's the case, what about the other parts where the rotor goes narrower? Yes I think you're right, each peak has a little bit of untouched rotor there, but that's only on the peaks, so there's no braking surface (or half or 3/4) on the dips. This is the main reason I took it off, it's not a circle at the edge.

What did you measure your pad width at out of interest?

I measured the shiny part on my 160mm rotor and it's about 14mm. This correlates with the height of the shiny part on the 203mm rotor too. I wish I had a spare set of new BB7 pads then I could just measure them. It's hard to find the info for some reason.

Whilst I sort of see your reasoning on stanchion size I'd like to point out you're quite wrong about how rim brakes work. Rim brakes in the normal in front of the fork crown position will force the axle into the dropouts. Disc brakes in the normal behind the fork location will force the axle out of the dropouts. Think of the wheel trying to pivot around the clamping point and it should make sense. This brake pivot point is also the reason why disc brakes produce more fork stress than rim brakes but that's quite hard to get your head around. Or my head at least!

Oh yeah, I wasn't even thinking about that.

Well I just bombed it down a 13% decline tonight with my old 160mm on and I honestly couldn't tell much (or any) difference in how hard I had to pull the brakes, I'm just going to keep the 160mm on because it's a miles better rotor than the cheap 203mm one and, it's not got those peaks and dips, it's a G2, just a circle.