I'm using a 180mm disc on the front with an IS caliper and IS mount forks.
I've just bought a post mount fork.

Can someone confirm that this is the adapter I want to keep the 180mm disc.
http://www.wiggle.co.uk/p/cycle/7/A2Z_Adapter_PM_To_IS/5360033034/

And if I use this one I will need to change to a 200mm disc.
http://www.wiggle.co.uk/p/cycle/7/Hope_74mm_Post_Mount/4000000720/

The A2Z one looks like it's folded out of flat sheet. Is it as strong as the Hope one ?
Is the caliper likely to move in the slotted holes ?

95kg rider on 29" wheels, so probably putting a bit more load on the brakes than most, although I've never had any significant fade with my current 180mm front, 160mm rear set up, so I don't want to go bigger on the front if I don't have to.
But then, I don't want to have the adapter bend and jam under heavy load either.

this one is a bit cheaper
IS mount front calipers must be pretty rare

Edit: the hope one is plus 20mm so will take your 180mm rotor- with no adapter at all the standard PM calipers fit 160mm discs. Go with that one I think

Thanks docrobster.
I don't know how I missed that.
I looked on CRC and couldn't find what I wanted, now I can see 160mm and 203mm adapters as well as the one I want. 😕

Are you sure 160mm is the "standard" for front discs ?
An IS caliper on an IS fork, with no adapter, fits a 180mm disc.
I assumed it would be the same for post mount.
It doesn't help when Hope simply describe it as "+ 20mm" without saying what the default measurement is.

An IS caliper on an IS fork, with no adapter, fits a 180mm disc.

depends on the caliper – my Magura IS calipers are 160mm.

the problem of going from 180IS to 180PM is that most of the adapters add 200mm (as you've seen).

Giant also do a 180IS to 180PM that doesn't push the caliper further from the axle.

my Magura IS calipers are 160mm

Now I'm even more confused.
I've mixed Hope and Magura with Fox and Rock Shox. All IS and all 180mm.

I just realised that after finding the Hope adapter on CRC, I searched for "IS to Post", which is why I didn't find the "IS to PM" BA ones. 🙄

Some pm forks are set up for 180 rotors as standard, magura thors are 180 pm fitting without Any adapter with a pm caliper

I find it all very confusing too, especially when you add in the 183s and 185s. I suffered rub for ages on a front disk till I realised I'd got a 183mm avid rotor with a 180mm hayes adapter!
Re the 160mm rotor- I'm really just thinking about PM as this is what most forks and calipers are now. A standard PM Caliper (to be fair I've only looked at hayes and avid) bolted straight to a PM fork fits a 160mm disk, so you need an adapter that adds 20mm and converts to fit an IS caliper.
I'd go for the hope one as it's the cheapest and probably best quality.
In fact your LBS might have a selection of Hope adapters you can look at to try to work out which one fits. I had to get some longer than standard bolts for my brakes and the LBS had a little box with one of each adapter and a selection of bolts so I could see exactly what I needed. I would imagine anywhere that stocked hope stuff had this as it had come from hope.
"hope" that helps! 😀

Some pm forks are set up for 180 rotors as standard…

Does that mean that most are not ?

Looking again at the list of Hope post to post adapters, they do a 180mm and a 203mm adapter, but not a 160mm adapter, which suggests that 160mm is the default size.

I thought with PM that you had a standard caliper shape, and then the rotor size was then defined by the fork. So, for a Thor or a Pike/Rev the post mounts are setup for a minimum 180mm front disc, whereas other forks will have the post mounts set for a 160mm front disc.

To then mount a larger disc requires an adaptor.

This is not the same as how it was done with the IS stanard, which had the disc tabs placed in a specific place, and depending on the brake manufacturer you had different implementations of the caliper. So, for example, some Hope brakes are 180mm IS, while others are 160mm IS. Some other manufacturers just made PM brakes and supplied with an IS adaptor, eg Magura, Shimano.

So, in the end it really will depend on what brakes you currently have, how they are currently fitted and what fork you buy.

In general, it's a bit of a mare!

IS calipers vary in size – you can / could get IS calipers to fit 160 / 180 / 200 mm discs without adaptors at all – it depended on the mounting on the caliper body.

I use this hope adaptor to combine an XTR IS brake caliper with a Post-mount SID Race fork using a 180mm disk.

Works just fine, though I should paint it black because the bare metal finish stands out quite a bit…

Slight highjack here, but when a mount is described as IS – PM is the first 2 letters (IS in this case) the caliper or the mount?

So if I had an IS caliper (Standard 160mm Shimano size) and wanted to fit a 203mm disk to a Post Mount fork would I needed +43mm IS-PM or +43mm PM-IS?

Thanks in advance!

Thanks again for all the help.
I've ordered the Hope +20mm adapter. I've just got to wait for it and the forks to turn up and I'll see if it all matches up with my calipers.

With so many different manufacturers and standards, it's a shame there isn't a definitive list somewhere of what fits what. Even Sheldon Brown wasn't much help in this case.

Lunge, I can't be bothered to go back and find the links from all the websites I have looked at over the past few hours trying to sort this out, but I am sure I saw them listed both ways.
An "IS to PM" adapter could mean either IS caliper to PM forks or PM caliper to IS forks. Best to find one with a picture to make sure it's what you want.

Probably a bit late, but if you're running a 180mm front rotor IS to IS with no adapters(If it's a Hope caliper it'll have a no.3 stamped on it's inner face just above the lower mounting hole), and you've ordered a +20mm mount(part no.HBSM20H)then you'll also need a 200mm disc to go with it(183/203mm are for post mount).

Again, probably too late: I believe coatesy is correct. If your IS caliper is for 180mm disc you don't need +20mm. I recently did similar with a 160mm IS caliper and disc to PM fork. I used this KCNC one
It worked fine.

Worth remembering that not all calipers will work with a +0 IS to post mount due to clearance around the caliper body, a 180mm set up is unlikely to cause any problems though.

Good point – my 160mm IS Hope mini mono worked with the KCNC adapter but there was only just enough clearance, other calipers could be bigger.

Well the forks are on the bike, the brakes are connected and everything works. 🙂

Things I've learned over the last couple of days…

Hope and Magura make (or made) two different sizes of IS caliper.
The smaller one bolted on directly fits a 160mm front or 140mm rear.
The larger one bolted on directly fits a 180mm front or 160mm rear.

A PM caliper bolted on directly fits a 160mm front or 140mm rear.

Once you've got those measurements and know which calipers you've got, it's fairly straightforward to work out which IS to IS or IS to PM adapter is needed.

In my case I was replacing Magura brakes with Hope at the same time as fitting the forks.
It was only when I started looking deeper in to it all that I realised I had the larger Magura calipers and the smaller Hope ones, so the larger IS Maguara caliper worked with a 180mm disc on my old IS forks with no adapter, but the smaller IS Hope caliper needed a +20mm adapter to work with the new PM forks.