sjanzeir

Hi everyone,

So... Help me scratch the surface (pun) as to how much the quality of a brake rotor's design and material contribute to stopping power and consistency.

I mean, there are rotors for a mere $6, and then there are ones for a whopping $60, and even those are priced far below the most expensive floating types.

So, would I really be six times better off running a $60 rotor than the $10 factory rotors I'm running now? Is the grade of stainless steel that makes up an expensive rotor really that much higher than the bargain basement variety? How much of a difference do the shape and design of an expensive rotor really make as opposed to a cheap one?

Andrew R Stewart 

A few comments- Ranking bike parts by their cost to the consumer is not the best way to compare between them. These days there are various channels that products take, from the manufacturer to the consumer. Each channel adds it's cost. But each channel has differing overheads and different business models that the transfer of said product must pay for. Additionally short term "loss leaders" are a common method to gain consumer purchases of other products. So that $6 rotor might cost 2x or more if bought from a different seller or even at a different point in the calendar.

The bike industry is well known for chasing diminishing returns. (The auto racing world is similar). So to assume there's a linear relationship between cost and performance is wrong. To gain (or with weight, loose) 5% might cost 50% more at the source. More reasonable is a geometric growth of cost for gains.

Real life settings are not those in the testing lab. Dirt, contamination, set up, all contribute their aspects to real life performance and, IMO, are vastly greater in effect then changing out a rotor is. It is easy to think of a rotor shaping (solid VS cut outs as example) which might work better in one condition but less so in another environment.

Lastly I know of no one (not that I research this stuff) that has done a true and properly controlled test of the various rotors, with all other factors being constant (outside of the larger manufacturers of course. And good luck prying out that data from their grasps). So most likely any claims will be from end users who are well known for the lack of scientific methods and instead see how their claims relate to their on line presence. Double blind testing run by third parties it not the usual process we read about here and in other forums.

So I will turn this back to the OP and suggest that they do this extensive and scientific testing and report back... Andy

hokiefyd 

I can offer some first-hand experience with this. Up front: I'm not a bike racer and do not buy "racing" parts and I do not demand "racing" performance. The stuff I'm into is basic recreational rider level parts, but I do appreciate quality. Okay, with that said -- I've recently purchased a pair of genuine Shimano SM-RT56 brake rotors and also a pair of knock-off RT56 brake rotors (eBay link). I have each pair on a different bike and I can give you my observations after a relatively short amount of time with these.

The genuine Shimano units have a very smooth surface and all edges (even the "spokes" and each of the holes) seems to be softened a little bit, either with radiusing or stoning or something. Or maybe it's how they're stamped. There are no machine marks on them at all and the pads bedded in pretty quickly. The knock-offs were obviously manufactured differently -- each rotor has a very directional machining pattern across its face (like it was flat ground on a very large coarse disc). It's going to take a much longer time to properly bed and wear in these rotors. The machined pattern is relatively rough, and you have two spots on the rotor (180 degrees apart) where the machining pattern is more or less parallel with the rotor's rotation through the brake pads and you have two spots on the rotor, both 90 degrees offset from the others just mentioned where the machining pattern is pretty much perpendicular. This has created a very visible pattern of pad material -- where the machining is parallel, the pads have deposited relatively little material on the rotor...and where the machining is perpendicular, the pads have deposited a lot of material. This creates a bit of a pulsing feeling when slowing down. The braking surface is polishing up, but it's taking quite a while to do it. I wouldn't buy them again to save the small amount of money I saved.

"You get what you pay for" seems to apply here.

cpach

Cheaper rotors use a different temper of steel and are only compatible with resin pads, which isn't a big deal if you only use resin pads. More expensive rotors also often have the steel braking surface mounted to an aluminum spider which can reduce weight and which should help some with heat dissipation. Really expensive Shimano rotors have cooling fins on the aluminum spider to have greater yet heat dissipation.

If you're overheating rotors going up in rotor size (which will also increase braking force) and/or going with fancier rotors can help.

sjanzeir

All great points, thanks. Yet another layer of complexity on top of what Andy laid bare is that, more often than not, I find myself needing to order most everything online, right down to things as mundane as inner tubes with longer Schrader valves and BB5-compatible brake pad. All of this poses its own problems in this COVID world, what with increased lead times, significantly greater shipping costs than 2019 levels, and little or no guarantee that I'm getting a genuine article, let alone the right one. Unlike any country with a highly developed cycle culture, I can't just trundle my way to the nearest LBS and grab anything and everything I might need right off the shelves.

Which is why I have to ask seemingly dumb, uninformed questions like these - I need to be extra careful as to what it is exactly that I'm ordering and how much I must pay for it. I've bought my fair share of wrong, bogus, or downright no-show parts and it stings.

The reason I brought up the rotor quality issue is that I'm looking to mix up brake pads (see the other thread I started on the matter earlier today, which no one picked up yet.) As cpach pointed out, I might have issues running types of pads other than resin on my decidedly cheap, stock rotors.