So, I’ve heard a theory that a rotor which is larger than it needs to be reduces heat in the pads. That makes sense. However, the theory then goes to suggest that a lack of heat will increase pad wear.

Anyone have any thoughts on how likely this is? Is the temperature difference between rotor sizes that great? It the ideal operating range of pads that small.

Could this be the reason that some people hate a brake that other people love?

No idea. I run rotors that mean my brake don’t overheat. I replace pads when I need to. It all works.

There might be a grain of truth in the theory, but I’d have thought other factors will over ride it. How much you use the brakes, weather conditions and soil type will all affect how fast the pads wear more than rotor size.

I’m curious because I’m riding the same stuff, with the same compound pads in the same way with the same size rotors, give or take 1mm. The only thing that has changed is its now a more powerful brake and the braking track is a little deeper.

However, rear pad consumption has increased dramatically while the fro.t has remained the same.

Bump for the evening crowd.

That’s an interesting thing to think about.

A larger rotor would be moving through the calliper faster than a smaller rotor. I would have thought that when you brake, this means increased friction, and hence heat and wear?

Just realised you’re talking about brake track depth? my bad..

As long as the pads are bedded in properly in the first place, I would imagine the difference in wear probably isn’t that great.

Yes. There’s a range of operating temperatures where the pad works best – a useful bit from here: http://www.topbrakes.com/faq.php

“What makes Brake Pads work?

The simple answer is friction. BUT- that’s only part of the answer!
Brake pads work with a combination of TWO FRICTION TECHNOLOGIES:

1) ABRASIVE FRICTION
This involves the braking of molecular bonds between the pad material and the iron in the brake disc. Pads that function on this basis (typically organic pads) tend to have a high wear-rate and low resistance to high-temperature brake fade.

2) ADHERENT FRICTION
Adherent friction is developed when a transfer-film of the same compound of the pad material is deposited as a very thin ‘film’ on the surface of the rotor.
In this instance, the friction is caused by a breaking of molecular bonds between the two like friction materials amongst themselves (one on the pad and one on the surface of the rotor)

Most performance brake pad manufacturers now manufacture pads that function as a combination of these two technologies. These pads tend to have higher coefficients of friction over a wider range of temperatures.”

Pondering this myself, this explains why my Hope organic pads seem to be lasting forever as they’re working mostly on adherent friction when dry but they’re not so powerful when wet as then they’re more reliant on abrasive friction.

I always thought my pads lasted longer when using 203mm rotors, but that may have been original pads v’s cheaper versions I know use. So many variables I guess.

Interested in chiefgroveguru’s post – may explain why the hope brakes on my commuter are fine in the dry but shocking in the wet, kind of thought is was to do with contamination on the pads with all the road filth, but lower temperature and loss of friction could be the cause – though that doesn’t seem to happen off road in the wet so maybe not?

Cheers guys. The front is fine so I guess that gets hot enough. do you think a smaller rotor on the rear might be the answer?

I think I get a better balance with a smaller rotor on the back. On the one bike I run with same size rotors front and rear (160mm) I find the front does get cooked occasionally, though never blown it totally – will replace with larger front eventually!

I’m running one size smaller on the back now. I’m thinking of going two sizes smaller (203 up front, 160 out back)

Worth a try, think I’d cook mine!

225/205 / Mono6 Ti (DH bike)
205/185 / M4 (AM bike)

Rather be a little over-braked than under…

That’s always been my thinking and it holds true on the front. I’m just wondering if being over braked is killing pads on the back.

If it’s not caused by being over braked on the rear, I’m stuck for ideas as to what it might be.

I would have thought that pad wear is down to two factors.
1) How you brake i.e. short and sharp or drag.
2) The conditions you’ve been riding in. Clearly dry dusty trails are not going to wear your pads like a wet gritty ride will.

I disussed (yes, not an argument!) this with TJ before the mods got all ban-hammer-happy.

I’ve always run tiny brakes on my winter bikes. 160/140 on my 26er SS and 180/160 on the 29er. The 26er regulalry got hot enough to boil the brake fluid even riding in SE England and turned the spiders varying shades of brown/blue, the 29er just wouldn’t stop with those brakes though as the wheels rotate slower so need bigger rotors to get the same braking power.

The other thing to note is I run variations of the windcutter/aries/aligator rotors with very little ‘rotor’. These seem to get seriously hot as there’s very little rotor material to act as a heat sink, but they do cool down very quickly as soon as you stop braking (i.e. on steep switchbacks I could feel them pumping up aproaching a corner but they’d be normal when you started braking for the next one).

I’m pretty convinced that running the smallest rotors that do the job is helping me get several times more life from the pads.

Very interesting tinas, thanks for that. Seems there could be some sense behind this theory.

Bigyinn, the reason for this discussion is that riding/braking style hasn’t changed, it’s still me. Riding location hasn’t changed either. Rotor size is nominally the same as is pad compound. The only thing that’s changed is the power of the brake. It’s only my assumption that there’s a corresponding decrease in pad/rotor heat associated with the power increase.

I would have thought that when you brake, this means increased friction, and hence heat and wear?

The opposite is acutualy true.

energy = force*distance
power = force*speed

If the rotor is moving quicker the force required is less (hence big brakes = more power).

The heat generated is a fucntion of the energy transfered, so regardless of rotor size you’ll generate the same total heat. The bigger rotor will however be reaching a lower peak temperature under the rotor is moving quicker and physicaly moving that heat away from the pad. It’s also bigger so can absorb more enrgy for a given temperature change, and is better at dissipating that energy due to the bigger surface area.

There’s lots of other variables in pad wear tbh… I use the same brakes on 3 bikes with an assortment of rotor sizes, the bikes with the biggest rotors go through pads by far the fastest but then, they’re the ones that get ridden on the nastiest stuff so it’s not surprising.

I suppose also, overheating brakes has other significant effects- risk of glazing pads and needing to replace early, and risk of unexpectedly tree-ing yourself.

Isn’t the larger rotor to do with higher leverage, not ‘moving quicker’ ?

So, someone else who’s noticed that big rotors kill pads (for whatever reason)

Isn’t the larger rotor to do with higher leverage, not ‘moving quicker’ ?

Difference between torque and power.

But yes, it’s two different ways of mathematiclay ending up at the saem conclusion. To ilustrate the difference if your at a standstill on a steep trail you need brakes with a lot of troque, but they won’t heat up as no power is being transfered.

In force terms friction force at the pads * rotor diameter / wheel diameter = force slowing bike down.

In power terms force at the pads * speed of the rotor = braking power = rate at which the bikes kinetic energy is reduced, kinetic energy is e=mv^2, w=e/t, e=fd (so w=fv), f=ma, a=dv/dt and (so f=e/d), keep going through the maths and you’ll end up with the same answer for f as you did in the first part. Unless as noted above v=0 in whichcase f is unsolvable.

TINAS – thanks 🙂

Not touched engineering/maths since GCSE physics (a while ago…)

Never thought of looking at it via energy methods. However, what do we know about the operating temperature of pads?

Have a read of this thread, some good stuff. I think you’re correct, but there’s more too it.

Brake discussion

Scienceofficer, thanks for that. kind of fits with what I’m experiencing. sounds like I’m using friction rather than adhesive braking on the rear due to a lack of heat. think I do need to try something smaller.

How about cutting half the pad material away to make a smaller pad? Would that make it work harder and therefore get hotter? Counter intuitively making it last longer?

useful post chiefgroveguru.

I guesss my understanding of it all would be:

A bigger rotor = more mechanical advantage thus requires less force to be applied at the pad and hence (depending on pad composition, weather and grime conditions) some reduction in the amount of abrasive/adhesive friction required to achieve the same results as with a smaller rotor.
Ultimately this means in less wear and less heat generation for the same rate of deceleration?
Additionally you have a marginally larger body of material in the bigger rotor to act as a bit more of a heat sink (some of the thermal energy has to dissipate via the rotor)…

????

I have been using a 203 F / 160 R setup on my DH bike for a little while based on a sketchy idea that a smaller, less effective rear brake would prevent me from “over-braking”/locking up the rear so easily; a sort of F/R brake balancing by simply undersizing the rear rotor… It sort of works but obviously lacks a bit of “feel” or modulation…

Cookeaa, what you say makes sense to a point but it doesn’t consider the possible negative impact of not generating enough heat.

having said that, I spoke to hope yesterday. they were very helpful but did suggest that the temperatures achieved with disc brakes don’t vary wildly with rotor size.

some reduction in the amount of abrasive/adhesive friction required to achieve the same results as with a smaller rotor.

but offset that against the longer distance the pad is dragged allong the rotor?

Suppose the braking power is a function of breaking the bonds between molecules in the brake pad (as the brake pad is depositing a layer of itself onto the rotor) then pad wear would be a linear function of braking power x time. That’s abrasive friction. But once the rotor is hot and a layer is bonded to the rotor then you’ve got adhesive friction and relatively little pad wear.

How about cutting half the pad material away to make a smaller pad? Would that make it work harder and therefore get hotter? Counter intuitively making it last longer?

Possibly better for adhesive friction and worse for abrasive, same friction coeficient but quicker wear.

they were very helpful but did suggest that the temperatures achieved with disc brakes don’t vary wildly with rotor size.

I suppose it depends on factors like the approptriateness of the rotor size. An 8 stone XC wippet with 203mm rotors at swinley is not going to generate a very small temperature in comparison to a 20stone mincer on a DH course with 140mm disks. A 203mm disk ona DH bike probably is the same temp as a 160mm disk on an XC bike.

Try, but for a given rider weight/style/speed, they were saying the temperature difference between running a 183 and a 160 on the rear is very little.

Not sure I’ve overly convinced though given what we’ve said here around having to convert the same amount of KE into HE.