Brake Discs: What Goes Right Must Go Wrong.

Attractive though anodised multi-piston calipers are, it’s still the humble brake disc that’s at the heart of effective long-term braking performance. Considering their extreme replacement cost compared with a set of pads, it’s good practice to keep a weather eye on the condition of these boring but high precision rotating parts.

The usual problems bikers face fit neatly into four categories:

1. Out Of Tolerance Discs. The result of straightforward wear and tear, with the discs eventually reaching the minimum thickness limit allowed by the manufacturer. This is normal – even if you think they should last forever like I do, don’t forget discs are still classed as service parts.

You’ll find the important measurement stamped on the inner part of the rotor in mm and if you haven’t got a measuring tool, a quick check with your fingers is a good first stab at maintenance (plus it’s free).

minimum disc thickness limit
Minimum thickness is usually stamped here

To test, wait ‘til the disc is cool then see if there’s a noticeable unworn lip at the extreme outer edge. If so, more serious investigation is required: beg, borrow or even buy an external dial caliper gauge or micrometer and compare your readings across several points of the wear surface to the minimum allowable limit.

You’ll find wear prevalent on 90’s and onward bikes, whose discs (for performance reasons) are thinner, typically drilled and larger in diameter than earlier Classics whose discs are thicker, smaller in diameter and (for many other reasons) largely crap.

Top Tip: Seems obvious but you’d be surprised: always change worn out discs in pairs on a twin disc front end!

2. Unevenly Worn Discs. All things being equal, discs should wear evenly across the braking surface. Heavy grooving or scoring of the disc signifies problems.

What goes wrong? Poorly fitted, jammed, bad quality or incorrect compound pads are all potential causes of uneven disc wear along with seized calipers or lazy pistons in multi-piston calipers. If you ride on dirt or gravel, stones caught between the pad and disc are a perennial problem and if you ride a bike with cast iron discs and leave it too long without using it, apathy-corrosion can take hold and damage the disc surface, especially when the static brake pads ‘weld’ themselves in place. Nasty.

Perhaps the most common real-world problem is when the metal backing of worn-through brake pads contacts the disc. This is usually self-evident even for the laziest riders: the brakes stop working and there’s a terrible grinding noise as an accompaniment. Simple diagnosis as always the best.

worn disc pads
Avoid trashing your discs by changing pads before they look like this

3. Warped Discs. This catch-all misnomer is applied to the time the rider feels a kind of rapid ABS-like pulsing during regular braking through the lever or foot pedal. It’s an especially common term and frequently misunderstood, for not all warping is, in fact, warping. Actual disc warping, where the disc run-out (‘buckling’ if you like) increases to such an extent that it’s felt as either pulsing or vibration through the machine under medium to heavy braking is rare.

Actual Disc Warping

So it does happen, but how? The first issue is poor manufacture or quality of the discs themselves. As discs cycle rapidly from hot to cold and must remain true through their thermal cycle, it’s vital that their metallurgical make up through manufacture is as uniform and predictable as possible. It’s here that super-cheap aftermarket replacement discs are a false economy. Buying well-known aftermarket parts from the likes of EBC or similar, or biting the bullet and using Original Equipment discs mitigates manufacturing defects.

The second is down to poor rider technique. If a rider brakes from high speed and overheats the brakes, then continues to absent-mindedly apply the brakes once they’ve come to a complete stop while they think of what to get for dinner, the pads in contact with the disc have an effect of hot-spotting the precision rotating part, leading to uneven cooling. Warping occurs as the disc cools unevenly to its neutral state. As motorcycle discs are extremely thin in comparison with car discs, they are much more likely to suffer damage in this scenario.

Top Tip: Always decide on dinner pre-ride. 

Phantom Disc Warping.

No matter what your MOT fail sheet says, unless you saw the tester using a Run-Out Gauge, don’t assume their diagnosis of warped discs is always correct.  Pulsing brake levers can also be a sign of uneven brake pad deposits on the discs themselves. The real issue in this case is not a warped disc but uneven comparative thickness of the disc around its circumference or ‘Disc Thickness Variation’ (DTV).

Boring Technical Bit (skip if under 50 years old): 

As you probably know, your motorcycle brakes are simply a mechanism to convert kinetic energy into heat. You might not know that they do this through two distinct mechanisms: abrasive and adhesive friction.

Abrasive Friction is the process where chemical bonds in the pad material are permanently broken as a result of friction between brake pad and the disc, producing heat as a result. Pad material is worn in this process and your bike’s front wheel gets covered in brake dust.

To demonstrate the breaking of these chemical bonds in an understandable way, try turning a bicycle upside down, spin the rear wheel up to a good speed then place your hand over the tyre and quickly try to slow it down. In seconds, you burn your hand, skin comes off and you say “Aiee!”: Abrasive Friction in action.*  (*blogger does not advocate burning your own hand – get someone else to do it)

ebc brake pads
Modern pads: abrasive and adhesive friction

Adhesive Friction (sometimes referred to as Cohesive Friction) is something else. Adhesive Friction relies on the pads depositing a microscopic film of friction material onto the surface of the disc. As the disc and pads contact each other and the temperature rises, the film and pad surface become ‘sticky’. As the heat rises further under braking, the pads and friction film continually chemically bond to each other then break apart, absorbing energy in the process. Advantage? A cohesive friction material (an asset of modern organic brake pads among others) reduces wear on the disc and (depending on the abrasive properties of the pad) reduces brake dust. Less cleaning of wire-spoke wheel hubs you say? That’s the definition of progress right there.

Disc Thickness Variation

DTV occurs when some of the brake pad’s friction material is microscopically bonded to the disc surface in an uneven or random manner. If this uneven build up is allowed to start, the process will continue, leading to irregular disc thickness and so irregular frictional qualities in the disc – and that pulsing feeling as its logical result.

How can you help avoid this? By bedding in new brake pads carefully for one. This is largely the reason pad manufacturers warn you to gradually bed in the brakes as they advise – it’s not simply a case of wearing in the pad surface to make it fit your worn discs. The manufacturer wants you to make sure your disc doesn’t load up with randomly applied uneven deposits of abrasive material or fresh bonding resins from potentially overheated new pads. Bedding in has got to happen in an even, unhurried fashion – just like your braking should be throughout the pad’s life. If not, it’ll be new disc time.

Top Tip: Always follow the manufacturer’s instructions on bedding in new brake pads. They write this on the packet for a reason. 

4. Cracked Discs. Much more unusual but it’s worth understanding why it can happen.

broken disc
An absolute corker

First off, if you spot even teeny, tiny cracks, don’t ignore them! A potentially shattered disc is safety critical. You’ll probably notice them either during your regular scientific pre-ride visual inspections (undertaken by no one, ever) or more likely as a consistent and mysterious ‘ding’ or ringing sound as the wheel turns and the crack hits the brake pads. It’ll be especially noticeable at low speed, changing to a rhythmical chafing or tapping sound as your speed increases until becoming inaudible at higher speeds, usually at the same point where you decide ‘nah, it’s nothing’ and happily ignore it.

Cracks can be evidence of anything from a compromised microscopic grain structure in the metal (rare but possible, sometimes seen in poor quality cast discs found on older European machines) to the much more common overheating / uneven cooling issue.

blueing disc

In these images we’re looking at the death of a 1990 Honda VFR750 front brake rotor that in technical terms now resides in the Cracked To Buggery category. Heat damage is immediately evident through those burnt pad marks and the slight bluing of the disc’s steel through the sustained build up of heat. It’s likely heat was a prime factor in its failure rather than poor manufacture.

There’s nothing wrong with hot brakes before you ask. They’re designed to get hot, performance friction material being more efficient at elevated temperatures. As motorcycles are lightweight machines, brake discs are intentionally made quite thin so heat can rise rapidly in the friction material and disc rotor. This provides brakes with that almost instantaneous ‘feel’ you get in modern machines – vital when reacting to differing road conditions at a fraction of a second’s notice.

The logical disadvantage of this lightweight design is that modern motorcycle brakes have an extremely low engineering tolerance for too much heat. Especially when they’re drilled.

Top Tip: Get into the habit of checking your discs and brake calipers for unusually high (or if you have twin discs, uneven) temperatures at the end of a ride. Raised temperatures when you know you haven’t been braking hard could signify a sticking brake caliper or piston – and a costly disc failure. Uneven temperatures between discs sharing a wheel signify the same. A stitch in time…

Now as any engineer worth their salt will tell you, geometrically uniform metallic shapes like plain, boring old un-drilled discs like you get on cars are much less liable to fracture or fail than ones full of sharp edged holes – or ‘stress risers’ as they’ll gleefully call them.

Oh crap, what’s a ‘stress riser’?

Good question. Imagine you’re trying to open one of those sachets of ketchup in McDonalds. If you don’t try and tear it at one of those little serrated nicks in the top of the packet, you’ll be there all day pulling, but you sure won’t get any sauce for your fries. That little nick is a designed-in weakness in the plastic which you can exploit by concentrating all of your effort in that one part.

mcdonalds-ketchup-packet
Food packet engineering. One of the few positive uses of stress risers

This, in effect, is what a stress riser is. A place where stress is concentrated, and failure more likely. In a motorcycle brake disc subject to the stresses of rapid expansion and contraction through heating and cooling, a sharp-edged hole is classic weak spot, just like in the ketchup sachet.

stress risers forming cracks
A crack begins to form at a sharp-edged stress riser

If the metal is going to fatigue and break from repeated cycles of hot and cold, it’ll almost always give way here.

stress risers forming cracks 2
The cracks spread…

Engineers who know their stuff can introduce stress-relieving techniques in any area where failure is perceived as either likely or particularly undesirable. A simple smoothly chamfered edge on a hole usually does the trick. But discs wear down as is their nature, chamfered edges are impossible to maintain and sharp edged holes are in the end unavoidable.

stress risers forming cracks 3
…and it’s time for another overdraft letter

Why then, the proliferation of drilled discs in the safety-critical braking systems of road-going motorcycles you ask. It’s a good question. Brake nerds will tell you holes are a good thing. They clear away water efficiently, release gases built up by friction material under heavy braking, shorten heat cycle times in the disc, allow pads to wear more evenly and even reduce un-sprung weight.

The science is sound, but the performance argument is hard to sustain when you see drilled discs appearing on the front of this year’s awesome 28mph Honda Vision 50.

Whether you like them or not, make sure you religiously check those discs for signs of failure. It’ll definitely save you money, and quite possibly your life too.

3 Comments

    • Trev, good call – have to say I’ve never experienced it. Then again, every bike I’ve ever owned has had out of tolerance (aka ‘shagged’) bobbins, giving fully floating discs an entirely new meaning..!

  1. i think you may have solved a mystery problem with one of my bikes, i have two the same the old xj 600 pre diversions, one doesn’t feel good under braking, especially on uneven road surfaces downhill. had the head stock bearing checked out, which was ok. great article, well written in just the right tone as usual.

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