I am on the hunt for new rotors for my mechanical disc brakes, as the old are too twisted now and the brakes can't be adjusted without the disc rubbing the pads.
What is the difference between the cheapest possible rotors and several times more expensive "series and branded" ones?
One of the most advanced designs is the "floating rotor", made of two different materials connected together. They look like this:
These are exceptional brake rotors. They are very rigid and won't bend sideways easily. Overheating is much less of a problem for these. If they are getting overheated, they don't bend that much because of the material expansion. Usually the spider in the middle is made of aluminium alloy, but you can get rotors with carbon core. Carbon ones weigh less, but cost a lot more than with alloy spider.
Also you can get a full carbon rotor:
These are very expensive. Marketing materials for these say they are perfect, but you can easily spend a lot of money only on one rotor. I have never tried a carbon rotor or even seen one in flesh, only on pictures. You can also get a combination of carbon with ceramic spider and other space-tech materials. kettlecycles.com provide with examples of these. (Thanks @J-unior for the link)
One of the extremes I've seen is combination of carbon and ceramic in a disk brake rotor. Don't really think these are available in shops, but interesting to know.
If you go for single piece cheaper rotor, then you don't have such a big variation in materials: they are generally made of stainless steel. And here the shape is making a big difference.
If you get one of these:
You'd be able to stop on a dime, but you don't get a lot of modulation. These things will be like an on/off switch if combined with good pads. These are Hope trials rotors and I would not recommend using them for anything other than trials. (If you are doing trials, you should already know this)
Then you have middle ground rotors with a lot of variations on patterns:
These are the most common and most affordable with prices from £12 to £25. I do believe that in this category there is really not much difference in quality. But make sure that the pattern has overlapping holes: when the disk is dragged through the pads, there should not be a continuous line of solid material. Pads should have a way to clean themselves and if in a section of a rotor there is no gap, the pads will not self-clean, decreasing the overall performance. In this category choose the ones you like most.
And the last in the category are cheap and nasty rotors you get on bikes from Wal-Mart or Asda. They are usually heavy, easy to bend, with poor pattern. They look like this:
Apart from the material and type of construction don't forget that you have different diameters of the rotors: 145mm, 160mm, 180mm, 203mm and 205mm. The bigger the rotor, the more braking power you get and better heat distribution, but also the greater the weight. Usually downhillers go for 205mm rotors on the front and 180 on the rear. For more calm disciplines like XC 160mm rotors are more common.
The size of the rotor must match your current one, but you can buy an adapter to place you disk brake caliper in the right position for the size of the rotor.
And you can get different types of mounts to the hub. The most popular is 6 hole international standard - on all of the rotors above. But you also get Shimano Centre Lock:
For this type of mount you need to have a hub with the same mount or use adapters from 6-bolt to Centre lock. 8-10 years ago there used to be rotors with 5 and 4 holes for mounting, but you can't get them now.
There are differences in disc rotors: These are typically:
Material - Stainless steel is the most commonly used rotor, but you can get carbon and other exotic materials, and probably (although I have not seen it myself) at the other extreme, mild steel.
Material Quality - There is stainless and then there's stainless. As with carbon. This can affect the rust resistance and strength of the disc
Amount of material - thickness of disc as well as the amount in the center section (spokes)
Design. A disc needs to be round and flat, but within these bounds, most have holes (how much, how many and what shape varies) that allow better cooling and water sheading. There are also variations of the outer rim that are supposed to clear mud from the caliper better. The other aspect is the discs response to vibrations and harmonics, a good design is more resistant to squealing.
So how does this all add up: saying a disc is "just" as stamped piece of stainless and they are all the same is like saying a bicycle has 2 wheels and pedals, its correct and misleading at the same time.
There is no doubt that there is difference between the discs on a K-mart kids bike compared to a $10K Downhill racer. The question then is "how much difference". Weight is important one for many of us - but at some point the $/gram savings are not worth it. Noise is a problem that is impossible for an individual to test for and impossible to live with squealing brakes. I don't know if price/quality makes any difference, but I do know changing to different discs can. Ability to clear mud is important to me, but not so much, if at all for others. That said, I can't be sure the shaped profiles make a difference. The cooling issue is probably moot, as I am not aware of problems with brakes overheating (except maybe in extreme competitive environs)
Unfortunately I have no idea how to tell if higher price is a "Brand tax" or a better rotor.
Well, I've been using 180 Magura Storm with Shimanno 775 hydraulic brake with sintered pad. All I can say is, the brake is a monster, it modulates well and can easily lock the front wheel with 2 fingers.
About quality, before these Maguras, I had the "cheap" nasty rotor, it used to overheat after 10 seconds holding hard on descends. I knew it because I could feel a lever loss of pressure and after about 15 seconds without braking, the brake came back to normal. After replacing to 180 Magura, I can brake descends nice. I weight 210 lbs, and do touring with my bike, so, final weight is about 250 lbs. The secret basically is not the "disc", it's the disc + pads + caliper. A cheap caliper with expensive discs brakes ok, so expensive calipers with cheap disc.
