How important is the rotor to braking?
Trying to sort out an xt brake that just won’t play ball.
System bled , rotor dulled and bled. Normally that sorts everything.
Should I be investing in posh rotors?
I have no idea of the pedigree of my current ones.

How important is the rotor to braking?

Not very. If you’re racing DH in the Alps, sure. For most people, not much. Obviously, shoddy crap bought online might vary, but any properly made rotor will be fine unless you’re really pushing things hard.

I’ve used loads of different rotors including lots of cheap ones. Only ever had two that were dodgy. Both were Hope. That said, when sorting out a dodgy brake I’ll always swap the rotor anyway in case it is contaminated.

Have you checked the pads arent contaminated?

Once you have cleaned everything have you followed the correct bedding in procedure?

You will need to do some progressive braking from speed but not actually stopping to build up some pad material on the rotors and the brakes will improve

when sorting out a dodgy brake I’ll always swap the rotor anyway in case it is contaminated.

If you sand and then clean the rotor with isopro alcohol then it wont be contaminated, seems a bit extreme to throw away a contaminated rotor when they are easily cleaned, brake pads once contaminated then fine throw them away but not the rotors

expensive rotors or cheap rotors (which are the same size) dont make any real difference in braking performance in my experince, especailly for UK riding

If you sand and then clean the rotor with isopro alcohol then it wont be contaminated, seems a bit extreme to throw away a contaminated rotor when they are easily cleaned

I wouldn’t throw it away. Just swap it to eliminate it. I always have a few spares around. Normal detergent to properly clean them.

The rotors are very important, as are the pads. Not that you need fancy ones but you do need to ensure they are doing their job as they are the parts creating friction so you can come to a halt. Yours may be contaminated beyond repair. Fyi I have cheap Clarks rotors and they work as well as any high end rotor I’ve used, you don’t need to spend a lot.

the only problem ive ever had with rotors was flutter, caused by daft patterns, resulting in variable braking around the rotors circumference.
Oh, and some really noisey ones that i put down to being cheapy stamped ones. You could see rounded edges on one side of the rotor, rather than a crisp jet/laser cut edge.

Is the brake lacking power, or does it have poor lever feel?

I found going from the cheapest Shimano rotors to the Ice tech made huge difference in braking
Just made better with better materials
Makes sense they will operate better

How important is the rotor to braking?

There are some hopeless XC lightweight rotors out there, more air than metal which is stupid when you rely on friction and heat transfer for braking. I prefer round, simple shape rotors with even cut outs – or at least a wavy rotor designed by someone who understands how the surface area and shape needs to work.

Very! Decent rotors make a big difference, especially to the wandering bite point on Shimano. Galfer for the win I reckon, if not the cheap solid Shimano ones. Stay away from the fancy Shimano ones if you have long descents as they heat up and can warp slightly, resetting the pads and making the bite point wander.

just won’t play ball.

Can you elaborate? You say that you’ve bled the brakes, but a bad brake bleed and a lack of rotor friction will have very different symptoms.

Just made better with better materials
Makes sense they will operate better

The friction between the pad and rotor won’t vary by much, if at all, with different alloys of steel. The difference will be with heat dissipation for fancy rotors. If you’re descending in the Alps, that will be a big consideration. If the pads aren’t overheating, it won’t make any difference.

Are they old worn rotors?

My SLX brakes went through a bad spell where bleeds and new pads didn’t help, worked out eventually rotors had got too thin. I think they start 1.8mm, with recommended replacement at 1.5mm. I think mine were down at 1.2mm.

I think the rapid wear was down to some non OEM pads that were made of hardenned kyrptonite. They lasted ages but clearly the wear was happening elsewhere…

I had formula rotors (I think) which in one week in Sierra Nevada had to be binned. The metal became dangerously thin in places and the whole rotor went from new to below limits in 6 days riding.

Another subject where there is a lot of mis-information floating around.

All the discs i’ve had are made of stainless steel (yes, i know 2-piece ones have ally hubs – but i’m talking about the friction surface)

Stainless is available in lots of different grades – but the most common are 304 and 316 – which are broadly similar to A2/A4 (which you’ll see stamped onto SS bolt heads)

304/A2 is much more common (and cheaper) it’ll be used for most industrial/architectural applications where it will not come into contact with any chemicals.

316/A4 (also sometimes referred to as ‘food’ grade) has a little more Nickel and a small amount of Molybdenum in it to improve corrosion resistance – this is used where SS might come into contract with food, or seawater.

Brake discs are almost certainly made from 304 – the cheap ones will be stamped, whilst the better ones will be either laser or waterjet cut.

The notion that they can become ‘contaminated’ and need replacing is not right – the material is not porous, and any ‘contamination’ can be removed by washing in detergent, lightly sanding with a fine grit paper and wiping with isopropyl alcohol.

Despite the name stainless steel does stain – particularly if it gets really hot (although heating to the point of discolouration has also altered its chemical structure slightly) or exposed to certain chemicals.

Stainless also corrodes – 304 has pretty poor resistance to salt/saline so the discs on your winter road commuter may exhibit a light uniform corrosion after a winter on salted roads.

Prolonged exposure to salt water will also cause pitting corrosion, but this doesn’t really apply to bikes, unless you’re parking them in the sea.

From a material science/chemistry perspective – there will be no difference between a cheap 304 brake disc and a top-end item.
What makes a difference is design – particularly the shape/profile of the friction surface and its ability to shed heat – the posh Shimano discs with heat sinks/etc will work better due to the design, but the braking surface is almost certainly the same material.

Hope this helps.