Disc brake help

[seanchad]

Hi

I have a giant talon with a cyc motor fitted, the stock tektros that came with the bike weren't great so I upgraded to Shimano br-mt420's. The bike came with 180 rotor up front and 160 in the rear and I went with a 203 up front and 180 rear so I bought a 160 to 203 post mount for the front and moved the adapter which was on the front to the rear.

All working fine but it looks like the adapter is too big and the discs don't fully contact the rotor, any ideas?

 

[docw009]

Buy a shorter adapter, or take off enough metal from the adapter until it fits.

 

[Chalo]

If you're using spherical adjusting washers on an IS mount that's designed to be used without them, then your caliper will sit too high over the rotor.

 

[seanchad]

The front and rear both look like they need to move in by about 10mm but I don't get why this would be the case? The rear had 160 stock on rear I've used a 160 - 180 post mount, I do have these like two part washers which I guess are the spherical ones being referred to.but there are under the bolt head so not spacing out further. Is it just down to poor machining of the bike frame?

 

[Chalo]

The front and rear both look like they need to move in by about 10mm but I don't get why this would be the case? The rear had 160 stock on rear I've used a 160 - 180 post mount, I do have these like two part washers which I guess are the spherical ones being referred to.but there are under the bolt head so not spacing out further. Is it just down to poor machining of the bike frame?

Understand that IS mounts work on front or rear, but in the rear they serve a rotor 20mm smaller than the one in front. Brackets are rated such as 160F/140R, 180F/160R, etc.

 

[seanchad]

Sorry I forgot to mention that the bike has post mounts so all the adapters I have are post mount to post mount. Cheers

 

[Chalo]

Every post mount is dedicated to a single rotor size, not necessarily all the same size. So any post mount adapter will be +XX mm over whatever the posts were designed to support. You just need a thinner one, or no adapter, depending on what you have going on.

 

[seanchad]

I had another look today and I think it's just the design of the rotor, with the adapter removed completely I can only move the caliper in by about 5mm before it starts hitting the rotor anyway.

I think it's just down to rotor design, the wave pattern on the outside is too much and you lose out on about 5mm of contact area at certain points. I can see the high points making full pad contact so shouldnt cause an issue it just looks odd. They are cheapo rotors anyway.

 

[Chalo]

I think it's just down to rotor design, the wave pattern on the outside is too much and you lose out on about 5mm of contact area at certain points.

Well, the high points of the wave should be close to touching the calipers, or else you're losing potential pad contact area. Whatever it takes to get to that point is what you should do.

 

[seanchad]

Cheers, I'll order a smaller adapter and use washers or something to get as close as possible.