sjanzeir

I came across this while looking at 203mm rotors and wondered just how safe and reliable this kind of setup could be. I'm not a formally trained engineer, but I see a few too many possible fault points here. Wouldn't you agree?

CliffordK

Fault points like missing the upper mounting bolt?

Likely it only has moderate side to side force, but does experience a significant sheer force in tangent with the disc.

sjanzeir

I actually missed that. But still, as you said, that's putting a lot of faith in an extra pair of 6mm bolts.

Barry2 

JENGA !

Barry

cobba

Spend a few $ and buy the correct adapter.

https://www.jensonusa.com/Shimano-F2...-Brake-Adapter

hokiefyd 

Additionally, for what it's worth, those calipers should not have the ball-and-cup washers as shown in the image -- they're designed to sit square with the mounting surface, with only a flat washer under the M6 bolt head. It appears that the bolt head is not square with the caliper, indicating some sort of misalignment somewhere. It's not clear whether that really is a long bolt that does thread into the posts on the fork or if it's just set in the hole for the photo.

andrewclaus

That's a good catch. In addition, it looks like there's no way to thread a longer bolt through the upper holes in the adapters, they're so misaligned.

Yeah, that's a bad installation. I've seen a few (maybe 5 mm) stacks of conical washers between the frame and the caliper, and that didn't bother me much, but this one does.

The guy who mentored me on all sorts of mechanical work stressed to me the importance of brakes. If you mess up on a drive train, the vehicle stops working. If you mess up on the brakes, the vehicle doesn't stop. He insisted on checking my first couple of solo brake jobs. That was for cars, but I use it for bikes, too.

base2 

The ball & cup washer set up there is actually pretty common. I've built hundreds of bikes with the ball/cup on top of the caliper (as shown) The reason is to account for an off-square drilled hole evenly loading the bolt/hole threads & bolt head; Or both above & below the caliper for the same reason, but to also allow caliper rotation to better square the frame mounting face to be perpindicular with the plane of the rotor. Townie Go! OEM single piston mechanical disc brakes for example, spring immediatly to mind.

The rear caliper mount of Giant branded frames are the worst offenders, IME. I think they heat treat the frames after facing the mounting surface & invariably there is some movement in mount surface perpindicularity. The mounting surface becomes not perpindicular to the rotor, & indeed some times not even in the same plane as itself as the frame stresses equalize during treatment.

This appears to be Rockshox fork. Older Avid/Sram post mount adaptors come with the ball/cup washers & instructions to install the cup/ball washers on both sides of the caliper (2 each per bolt.) For obvious reasons, the instructions instruct the ball/cup not be installed at the post interface to the frame. The lever action there would be insane.

https://www.worldwidecyclery.com/pro...mm-rear-rotors

Installed per instructions by Richard Mozzarella, on Flickr
Note the ball washer under the caliper nested in the mating cup of the post.

cobba

With some adapters you're supposed to use those spherical washers above the caliper.

+20mm adapter
https://si.shimano.com/pdfs/si/SI-8170A-001-ENG.pdf

+23mm adapter
https://si.shimano.com/pdfs/si/SI-0146A-000-00-ENG.pdf

cobba

When you double stack those types of adapters, you won't have a straight hole going through both adapters.

hokiefyd 

Thanks! I learned something. I've only ever seen them used above and below the caliper ears (as in the Avid BB7). I've not seen them used only above the caliper ears before. But I can understand from your description how they'd be useful.

base2 

No worries. Learning is one of the reasons I this forum.

cobba

That setup looks like it might be wrong for that caliper, the pads aren't connecting with the inner part of the brake track on that rotor and they're probably overhanging the rotor.

The top CPS washer stack is about 5mm thick, that will give you an idea of how thick the lower spacers are.

With the SRAM/Avid spacers, there is spacer setups for CPS type calipers (BB7's and older Avid/SRAM hydraulics) and there is spacer setups for standard type caliper (anything that isn't a CPS caliper).

SRAM adapters: https://www.servicearchive.sram.com/..._mtb_rev_e.pdf


* 20mm (160-180mm & 180-200mm) spacer stack for a CPS caliper


* 20mm (160-180mm & 180-200mm) spacer stack for a standard caliper.



If you use a CPS spacer stack with a standard caliper, the pads will overhang the rotor by about 5mm, this is something that shouldn't happen.
Overhanging pads will get a step worn in them, this could lead to brake failure.

base2 

Indeed. That is why it is a 185mm rotor. I had the issue you describe with a 180mm rotor.

I could find a mm or so facing the powdercoat off fork post mounting face. But it seems to work well enough & has full pad contact.

I'm not worried.

cobba

Why didn't you just buy an adapter for the 180mm rotor?

If you ever want to replace the rotor, there won't be much of a choice in what you can get.



Do the pads overhang the rotor by a couple of mm or do they have full contact?

base2 

Because I was at the mercy of what the bike shops had in stock. I went to 4 bike shops and this was the best that the 4th bike shop could come up with. They assured me it was the correct one. It turns out it was the correct one for a 185mm rotor, but I didn't realize that for a few years. A the 2.5 mm larger radius of a 185mm rotor was just the easier fix, since I needed a rotor anyway. So that's what I did. I do see what you are saying. A 17.5 upper/10 lower should be proper for a SRAM/Avid caliper on a 180mm rotor. But this caliper is a TRP caliper and a 185 rotor, hence the 12.5 lower post to compensate for difference in my installation. If the proper 10 mm post was included in the package the bike shop handed me, I surely would have used that & saved all the trouble. Page 8 illustration 25S of the pdf you linked to is the closest illustration the installation I have. I got here by way of experimentation & empirical evidence to derive the best fit. I believe I have succeeded.

I'll cross that bridge when I get there. There is hardly anything more common than a post mount adapter & now that I have access to QBP, JBI and the other usual suppliers, none if this is likely to be an issue.

Nope. the peak of the shark fin edge is about 1/4 to 1/2 mm taller than the top of the pad. Full contact. It's not unusual to have a few mm of unworn area stabilizing rotor edge in the area you seem concerned about. I set up 6 bikes this way today. It is done so that if the rotor wears too thin it doesn't shred to Chantilly lace & self destruct leaving the bike operator with no brakes at all. Something has to connect the rotor spokes to each other to share the braking load.