# What's the point of 2 piece rotors?



## Lucky13 (Jan 14, 2004)




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## RobsterCraw (Oct 19, 2004)

Maybe for better lateral rigidity


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## SHIVER ME TIMBERS (Jan 12, 2004)

Lucky13 said:


>


lame in my opinion


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## JBsoxB (May 18, 2004)

looks cool?


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## The Kadvang (Jul 25, 2004)

they run em on alot of superbikes... allows the rotor a little bit of play so it will self center and not rub


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## cdub (Feb 27, 2005)

i got em but dont really know what the do? came with my brakes


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## Lucky13 (Jan 14, 2004)

The Kadvang said:


> they run em on alot of superbikes... allows the rotor a little bit of play so it will self center and not rub


I other words black part is always locked while rotor itself floats?


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## skitrev33 (Mar 30, 2005)

Correct. The black spider mounts to the hub, and then the rotor itself is attached with bolts/rivet type fasteners in a way that is similar to chainrings, except that they have a small amount of play in them. These fasteners are called "float buttons." The basic idea of this play in the rotor is to counter any warp that the rotor develops as a result of braking forces or by being hit. This is important because when braking, any wobble in the rotor transfers directly to the suspension and with todays more sensitive suspension technology, manufacturers want as little input from non-trail related forces as possible. With this small amount of play, the rotor can wobble through the caliper to a point while having as little effect as possible on the suspension. I have also been told that they help compensate for expansion caused by the heat created by braking, but I would think that this would not be as big an issue.


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## JBsoxB (May 18, 2004)

skitrev33 said:


> Correct. The black spider mounts to the hub, and then the rotor itself is attached with bolts/rivet type fasteners in a way that is similar to chainrings, except that they have a small amount of play in them. These fasteners are called "float buttons." The basic idea of this play in the rotor is to counter any warp that the rotor develops as a result of braking forces or by being hit. This is important because when braking, any wobble in the rotor transfers directly to the suspension and with todays more sensitive suspension technology, manufacturers want as little input from non-trail related forces as possible. With this small amount of play, the rotor can wobble through the caliper to a point while having as little effect as possible on the suspension. I have also been told that they help compensate for expansion caused by the heat created by braking, but I would think that this would not be as big an issue.


wow.... thanks... i want one! too bad.


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## velco (Jun 25, 2004)

What's the central part? Composite, like, e.g. Shimano RT96? They ought to be lighter.


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## flymybike (Jan 6, 2004)

Those roters are the largest that Hope makes. They use a aluminum spider to keep the weight down and help transfer heat before it reaches the hub and mounting screws. Keeps the rotor stiff at the outer edge as well. If it was all stainless it would be quite heavy.


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## Mike B. (Feb 21, 2004)

Lucky13 said:


>


It's all done for weight and looks. skitrev did a nice job of describing the floating rotor system as seen on some motorcycles but the Hope rotors are not real floaters.


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## zedro (Jan 12, 2004)

Mike B. said:


> It's all done for weight and looks. skitrev did a nice job of describing the floating rotor system as seen on some motorcycles but the Hope rotors are not real floaters.


 yeah, only Pro-Stop made floating rotors i think for bikes, bike brake makers do it for the reasons flymybike said


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## Andrewpalooza (Dec 7, 2004)

zedro said:


> yeah, only Pro-Stop made floating rotors i think for bikes, bike brake makers do it for the reasons flymybike said


So, on a mountain bike the two-piece rotors are for thermal and weight-saving reasons only? No floating...


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## Hot Butter Topping (May 5, 2005)

Disclaimer... I have no clue why you would need these on a bike other than insane amounts of downhill brake dragging.

Anyhow, on pricey automobil racing brakes such as Brembo or StopTech, the two piece arrangement is for heat. However, these are not like that for the reason that the outter disc (the "swept area") is not, if I'm understanding the descriptiong of the mounting bolts correctly, allowing movement towards the center of the hub. Heated brake discs expand in ALL directions, and what causes warping on cars is the expansion of the disc near the hub (where the spider meets the disc). Because the spiderless hub (on a car) can't expand vertically it causes rotors to buckle in the lateral direction, causing wavy warped rotors.

So what have we learned? Because the bolts only allow for left-right movement/expansion they really won't protect against any warping because warping comes from expansion of metal towards the interior of the disc resulting in torsional warping because that's the only way the metal can expand.


That probably makes zero sense to anyone. Oh well.


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## [email protected] (Jan 14, 2004)

As Lance and Mike have pointed out, it's to save weight. Aluminum weighs less than steel. BTW Señor Canfield y todos, all of the brakes in the Mono line come with two-piece rotors this year.

But in all reality, it's mostly it's because it looks totally neat-o.


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## Jm. (Jan 12, 2004)

old magura gustavs had floating caliper-mounts. That was pretty cool, when the disc was warped the caliper would actually slide back and forth so the brake wouldn't drag bad. 

if it only it didn't leak it would have been awesome.


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## Lucky13 (Jan 14, 2004)

Jm. said:


> old magura gustavs had floating caliper-mounts. That was pretty cool, when the disc was warped the caliper would actually slide back and forth so the brake wouldn't drag bad.
> 
> if it only it didn't leak it would have been awesome.


Had one of those. HUGE mistake! ALWAYS dragged, never got centered no matter what I did. 2 pistons on one side just isn't cutting it.


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## skitrev33 (Mar 30, 2005)

Definitely agreed with all who said it that it is mostly for the bling factor. What I described is no where near as important for mountain biking as it is for F1 and croch-rockets which is where these have trickled down from, kinda like stable platform shocks... Thank god for things that go fast...


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## knollybikes.com (Jan 8, 2004)

Jm. said:


> old magura gustavs had floating caliper-mounts. That was pretty cool, when the disc was warped the caliper would actually slide back and forth so the brake wouldn't drag bad.
> 
> if it only it didn't leak it would have been awesome.


They still have floating caliper mounts and haven't leaked since 2003!

However, they are NOT lightweight brakes...

But their modulation AND power is awesome.


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## like_the_wheel (Aug 26, 2004)

The Hope 2-piece rotors make cool ticking noises when they are cooling down after a run and the 1-piece rotors don't that's the only real difference worth mentioning.


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## 702Biff (Dec 10, 2012)

This is an old thread but I had the same question and found this in a product description:

Rotors are usually cut from one piece of steel, which has two disadvantages. Firstly, steel is heavy. Secondly, as the outer section heats under the pads (and so expands) more than the inner, very high temperatures can cause them to warp. Two-piece rotors solve both problems with a lightweight alloy carrier and ‘floating’ dowels that act like the expansion joints in bridges. 

This would explain the cool ticking noise punkassean described. As the rotor outer ring cools it slips in the rivets, the metal to metal contact causing a ping or ticking. Sorta like an earthquake...the pressure builds and slips all at once causing disruption.


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## Gman086 (Jul 4, 2004)

I've seen several warped one-piece rotors before from riders dragging their rear brakes (especially for park riding) so the whole 2-piece thing has a lot of merit and is here to stay.

Have FUN!

G MAN


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## Tim F. (May 22, 2006)

Because you can get them in black, so they'll spin faster, fly higher, and enables you to pick up more chicks.


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## dth656 (Feb 12, 2009)

a few thoughts, although would want someone w/ a thermodynamics or materials science background to chime in as well:

-the coefficient of thermal expansion is different for aluminium and steel, so you may have improved resistance to warpage (since the outer portion can expand freely, and is only restricted by the 6 fixture points)
-the two metals also have different coefficients of thermal conductivity, so the two piece system may be more efficient at dissipating heat

some useful discussion on calculating the temperature in an idealized steel braking rotor and pad combination:

http://www.fem.unicamp.br/~phoenics... 2 SEM-11/TURMA A/G7 OK/artigo_referencia.pdf


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