RocThrower

I'm building up a bike with IS tabs and my only experience with disc brakes is on post mount frames. I know most post mount frames are built for 160 rotors, and 140 on road bikes. Are the IS tabs built for a specific rotor size, or is it as simple as installing the adapter that fits the rotor size I want to use? I will be using RS785 brakes with 160 rotors front and rear, so I believe I just need the correct front and rear P/S adapters for 160 rotors? The bike is a Macho Man Disc, and the specs only list max rotor size of 160.

I'm asking because I ran into an issue before on a road bike with post mounts. I thought I needed 160 P/P adapters, but it turned out I actually needed 180 adapters as the adapters were based off a 160 post mount standard, and I the 180 gave me the 20 mm difference I needed to go from 140 to 160.

smoothness 

try to fit your IS tabs with the brake calipers that you want to use. if it doesn't fit, then you need to get an adapter. make sure you get an IS -> [insert size you need], and you should be good to go.

cobba

Usually there's 20mm difference between front and rear IS mounts, front is 160mm and rear is 140mm.

Shimano adapters.
Front: SM-MA-F160P/S
Rear: SM-MA-R160P/S

F = front, R = rear
160 = size (160mm)
P/S = Post Mount to IS Mount

The 20mm difference between front and rear IS mounts means that a rear 160mm adapter put on the front would space the caliper out for a 180mm rotor, a front 160mm adapter put on the rear would space the caliper out for a 140mm rotor.

Tech info: https://si.shimano.com/

fietsbob

The adapters are marked , to run same disc size , front and rear, you need 2 different adapters .

Example : Front 160/140, rear 180 /160 .

RocThrower

Thank you for the helpful explanation, and those are the two adapters that I actually purchased. I just want to make sure that with the Post/IS adapters, 160 means it will work with 160 rotors. I know it sounds trivial, but I wanted to make sure I wasn't going to run into situation like I mentioned with the road bike where I had to buy 180 P/P adapters even though I was only running 160 rotors.

cobba

Those adapters are for 160mm rotors providing the IS mounts are 160mm front & 140mm rear.

1) contact the frame manufacturer to confirm the IS mount sizes
or
2) wait till the adapters arrive to see if the brakes fit properly over the rotor

There can be exceptions to brake mount sizes.

A 180mm PM to PM adapter is only a 180mm adapter when it's used on 160mm post mounts, if the adapter is used on 140mm post mounts it's a 160mm adapter and if it's used on 180mm post mounts it's a 200mm adapter. It would be better to call this type of adapter a 20mm adapter like Avid does

Avid has sensibly changed the way they name their adapters and don't make them front or rear specific, they have 0mm IS, 20mm IS, 40mm IS, 60mm IS, 20mm PM, 40mm IS.

Front IS mounts are usually 160mm and Rear IS mounts are usually 140mm, there is a few exceptions to this standard.
0mm IS adapter + 160mm IS mount = 160mm rotor
0mm IS adapter + 140mm IS mount = 140mm rotor
20mm IS adapter + 160mm IS mount = 180mm rotor
20mm IS adapter + 140mm IS mount = 160mm rotor
40mm IS adapter + 160mm IS mount = 200mm rotor
40mm IS adapter + 140mm IS mount = 180mm rotor
60mm IS adapter + 140mm IS mount = 200mm rotor

Post mount size varies, they cam be 140mm, 160mm, 180mm, 200mm & 203mm
20mm PM adapter + 140mm PM mount = 160mm rotor
20mm PM adapter + 160mm PM mount = 180mm rotor
20mm PM adapter + 180mm PM mount = 200mm rotor
40mm PM adapter + 140mm PM mount = 180mm rotor
40mm PM adapter + 160mm PM mount = 200mm rotor