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I'm new to disc brakes and trying to figure out what I need to buy to buy them for a wide fork (135 mm dropout spacing) with IS disc brake mounts and an electric front wheel. I've researched, but I haven't found any information that specifies a standard distance between the I.S. front disc brake mount (i.e. where the disc brake caliper attaches to the fork of the bike) and the disc brake rotor.

Given that getting this distance correct is crucial, since a few millimeters distance is all it takes for the rotor to rub up against the caliper pads, I would think there would be a clear standard distance, but I haven't found one.

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    You're right, there should be, but while I've been looking at drawings of IS brake mounts today for a question about the other dimension I haven't seen anything. I do know it's fine tuned with spacer washers.
    – Chris H
    Commented May 21, 2019 at 18:42

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The standards for disc mounting specifies a distance between the rotor and the inner face of the dropout. This distance is different for front and rear brakes.

A 135mm spaced fork sounds like a fatbike fork. Early fatbikes used 135mm front and rear, often specified to allow two "rear" wheels to be used, and swapped in case of freewheel failure or to change singlespeed ratios. Hence, some fatbike front hubs are labelled "RDS", meaning rear disc spacing.

Nowadays, fatbikes with 4" rear tyres tend to use 170mm QR hubs or 177mm thruaxle hubs, and 5" tyres usually have 190mm QR or 197mm TA hubs. The front hub will generally be 135mm QR or 150mm TA, but both would normally be spaced like a normal front hub.

I think front spacing requires the caliper to be mounted 4mm wider than rear spacing, I'll try to find a spec document later.

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I installed the front disc brake and found that a distance of 15mm was needed between the rotor and the mount surface. I was able to find a forum where someone said the same thing, but haven't found any specified standards, so for now, I guess 15 mm is the best answer the internet has.

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    The typical rotor spacing for road/mountain bikes puts the rotor-mounting surface of the hub at 15mm from the inner dropout (rear brakes). This is the case regardless of ISO/Post/flat mount. It is 5mm from the inner dropout for front brakes. You can measure this or you can find documents from Shimano, Avid etc. consistent with this. In special cases (like Fatbikes) some bikes use a 15mm offset on the front fork so that they can use a standard rear hub on the front. There are also other custom situations like folding bikes, cargo bikes etc. that may also be different.
    – BetterSense
    Commented Jan 12, 2021 at 17:41

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