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Using A Smaller Disc Rotor?


Jolfa

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Right, I've just bought an old Hope C2, the rotor it came with is 180mm, the rotor I want to use is 165mm.

The 165 is too small for the caliper, what can I do about it?

I can't seem to find any kind of adaptor to go down a size anywhere?

I absolutely won't (and can't anyway) use the 180 rotor. I won't be buying new rotors.

Ta!

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I don't even think you could make an adaptor really. The 'arms' on the caliper are too long, sticking it out from the fork too much. Only way would be to try and drill a new hole in the arms then cut them down, which may or may not work?

Exactly. I'm assuming as it's an old C2 that it'll be an IS mount, as opposed to a post mount. If so it'll be set to work with a 180mm rotor meaning you won't be able to use anything smaller.

Why are you so against using a 180mm rotor? Also, how do you imagine an adaptor would work to go down a size?

You have two options:

1. Run the 180 rotor that the brake you've bought was designed to be run with.

2. Buy a calliper thats intended to work with a smaller rotor.

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a rotor is probably the same price as an adaptor?

just buy a rotor?!

I already said I won't be buying a new rotor.

Why are you so against using a 180mm rotor? Also, how do you imagine an adaptor would work to go down a size?

Because I spent the best part of 10 years (half arsedly) looking for an original set of wavey discs (not the kind you get with the mono/mini) so I will definitely be using them one way or another!

Also, off the top of my head an adaptor would essentially just mount to my forks and step it down, rather than up/out :)

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I already said I won't be buying a new rotor.

Because I spent the best part of 10 years (half arsedly) looking for an original set of wavey discs (not the kind you get with the mono/mini) so I will definitely be using them one way or another!

Also, off the top of my head an adaptor would essentially just mount to my forks and step it down, rather than up/out :)

How on earth would it step it down though? You just couldn't do it. Essentially you need to move the piston part of the calliper 7.5mm closer to the axle, meaning your current bolt-holes in the calliper will need to be 7.5mm further in than they currently are. Where are they going to go? Don't suggest moving it inboard as you'd either foul on the spokes/hub, and irrespective of that there'd be no chance of any form of central alignment. Can't move it outboard either.

As I said either get a 180 rotor (trial zones are available in 180 and are wavy) or get a suitable calliper.

I don't see how you think it's feasible? Surely the fact that you can't track down an existing adaptor (based on the fact it's impossible) says it all.

Edited by Matthew62
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I have a wavy rotor sat in my garage, lol.

Stepping the caliper down using an adaptor is not possible unless its so offset from the rotor it makes it useless anyway.

I've just been looking, and if that's impossible I'll eat my own face! If I can make a twin engined car then I can step a bike disc down 7.5mm :lol:

Sod you all, I'll report back when I've done it :P:lol:

....Or with a video of me eating my own face :D

Edited by Jolfa
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As I said either get a 180 rotor (trial zones are available in 180 and are wavy) or get a suitable calliper.

You miss the point of these discs, it's not just the fact that they have a wavey edge that I like :P

The caliper will just be moved down a touch, using a plate with 4 holes in it; 2 threaded holes to mount to the forks, and 2 to mount the caliper to a touch lower down.

Nothing about this sounds like rocket science to me :blink:

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Just buy the correct number calliper half.

http://www.ebay.co.u...=item337d04358d - Post mount = ideal.

Don't want post mount brakes, or anything at all 'modern' for that matter, taken me long enough to find a whole bikesworth of 15 year old stuff as it is! I could just go on TartyBikes and buy a whole new bike, but I'm using very specific old bits, to build a very specific bike, you seem to be missing this point.

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I'm obviously not missing the point, which is why I linked you to an old C2 calliper. I get it you want a retro ride - that's great. But there's no reason you can't have something old and compatible. The beauty of those old Hope callipers is that they are 2 piece, so as your 180 sized calliper isn't going to fit, why not source the correct (equally old) part that will. If you're being so specific, i'd have thought it would be worth doing it right.

If you don't want post mount that's fine - but that ebay link wasn't for some new-fangled Hope part. Just think it's weird to create something clearly niche and specific then get stuff which is incompatible to which your solution is some size-reducing plate which won't work very well (if at all possible).

If you've spent 10 years tracking the parts down - why not source the right ones?

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If it makes you feel any better, I understand what you mean about your bracket idea, and yes it'll work but it'll look fugly and surely be pretty stressed due to being pretty small and receving lots of leverage??

Fugly maybe, but it'd barely be stepped off the forks so wouldn't suffer any huge amounts of additional stress :)

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I would be genuinely interested to see a step down adapter, and would be chuffed for you if you pulled it off. From my perspective without drawing anything out I can only see it being possible to run the calliper inboard so my reservations are:

1. How does the cycling body not foul on the fork leg?

2. When bolting the adapter to the fork tabs how do the bolts not go straight in to the calliper body?

3. The intended holes on the calliper....where do these end up? Where does the adapter go that they'd need to bolt on to?

4. How would you align the brake as the adapter needs to be of reasonable thickness? If the adapter is thin (like washer thin) it'll brake. If it's anything over 3mm surely the brake won't align?

5. Is the adapter going to be threaded? If not, where will the nuts go without hitting either the disc, fork or calliper?

As I say i'm genuinely interested and I guess to an extent it depends on your set-up. But with a sue-fork I can't see how a step-down would fit?

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