[Closed] 180, 183 & 185mm discs. Does it matter ?

Does 1.5 or 2.5mm on the radius make a difference ?

You could probably get away with it, but I wouldn't recommend it.

Will any disc work with any caliper ?

Yes.

It depends on the fork - I had trouble with a 203mm disc rubbing on the caliper but it was fine with a 200mm (Maverick SC32 fork) but using when using Pikes the 203mm was fine.

OK, thanks.
Looking at it again, a Hope Mount H is for a 183mm front, so I guess if I'm using that I should be OK with a 180 or 183mm disc.
I was just worried about using too big a disc and it contacting the caliper.

This is all a bit complicated as I've got two bikes and two pairs of wheels for each and I'm trying to make as much as possible interchangeable between the two so I can swap wheels easily if I want to and I'm just confusing myself.

A 185mm calliper doesn't work very well with a 180mm rotor, there's a noticaable lack of brake pad contact, I've tried it and it's not too good (Avid 185mm calliper, Magura 180mm disc)

Oh, OK, that shouldn't be a problem for me as I'm using all Hope calipers, although a mixture of post and IS mount.

Everything was OK when I had two bikes with 180mm front and 160mm rear, then I bought a new (to me) single speed bike with 160mm front and 140mm rear.
Rather than change my spare SS wheels to 160/140, I thought I'd change the new bike to 180/160 to keep as much as possible interchangeable with the other bike.
I need to buy a 180 or 183 front disc and a pair of step up adapters.

Yes it does matter - smaller rotor = less pad contact.

Ooh!

[img] [/img]

So, prove that less pad contact = less braking force. That should spark off a geek-fight! 🙂

FWIW, I've used 180 and 183mm rotors with 185 adaptors and I reckon it makes sweet FA difference.

I don't ride fast enough or steep enough for that much difference in pad contact area to matter much to me.
I just wanted to know if it will work.

A 185mm rotor definitely doesn't fit with a 180 caliper. Got some dodgy spacers in at the moment while I sort it out, but the squealing is horrific.

Odly I ran a 180mm shimano rotor, a shimano IS calliper and a "+20mm" addapter from discobrakes.

The addapter had oval holes in it to fine tune between 20mm and 25mm presumably.

However even at the minimum it was still ~3mm too high to line up with the rotor properly. Didn't bother me and didn't affect braking, and being in Spain pad wear wasn't an issue, but in the UK a couple of muddy rides and the pads would have met over the rotor.

Generaly IME bigger rotors usualy fit as theres always ~1-2mm of space above it in the calliper.

i run a 185 rotor and a 180 caliper with dodgy spacers and its perfectly fine.

I recently swapped a Mono Mini front brake for an x2 because of a change in fork mounts. The x2 required a 183mm rotor so assumed the old 180mm rotor would work fine, which it did until the pad started to get very worn down.

Because the rotor is slightly smaller you end up with a small lip of extra brake pad at the very back which means that you don't get full 'bite' when you only have 1mm or so of pad left and you have very little braking power.

So most of the time its Ok, the only noticeable issues are when the pads are wearing down, however you may get slightly more power all the time with the correct size rotor as a small amount of extra pad is in contact with the disc.

friction is independent of the contact area

http://www.stevemunden.com/friction.html

So, in conclusion, are we saying (assuming for this case the use of Hope brakes) that a 183mm rotor is designed for use on post caliper to post fork mounting and a 180mm rotor is used on IS caliper to IS fork ?

idiots.

There will be increases in power of 3.36% and 5.63% respectively in using 183 or 185mm discs over 180mm, this could make all teh difference.

😉 etc.

as long as the pads run on the correct track (not on the rotor arms)
and the disk doesnt rub the caliper body!

if the pads dont sit on the track, the pistons can skew in the cylinders which ruins them over time.