Eli5: Why do bicycle disk breaks have “%” and “V”s stamped through them. Is this the most efficient way of displacing heat / reducing cost ( materials ) or does it just look cool?
In: Engineering
Makes the disk break lighter and aerodynamic from cross winds.
Also improves breaking in wet conditions as the water gets pushed through instead of maintaining surface tension and impressing breaking
Those disc brakes are likely going on mountain bikes. Dirt, mud and water are likely to get on the brake rotor surface. The cross drilling and slots serve 2 purposes:
1. Allow the debris to get wiped off as the rotor rotates
2. Allow the hot gasses that build up from heavy braking to escape
>Why do bicycle disk breaks have “%” and “V”s stamped through them.
Clearly it is the “%” and “V”s that caused the disk to break. It works like the perforations on a ticket stub. The paper of the ticket tears at the perforations. The disk of the bicycle is going to break at the perforations too.
Former bicycle mechanic and current engineer here. The question isn’t really straightforward to answer. There’s some element to this that’s simply, “they’re built that way because that’s how they’ve always been built.” The first ever bicycle disc brake, the Shimano B700, used a slotted rotor, and ever since then every single bicycle disc brake rotor I’m personally aware of has been slotted in some way.
Why did Shimano use a slotted rotor? Well, most likely, because high-performance cars did.
Now, that’s not to say slotting or cross drilling the rotor doesn’t provide advantages – the generally accepted knowledge is that it indeed does. In no particular order, the theories I’ve seen are:
* It reduces weight. In a world where cyclists obsess over single grams, the 50g you could save by drilling out your rotor becomes *very* significant to some people. Especially since it’s rolling weight.
* It provides a channel for water and mud to escape the brake system into those channels and eventually out of the rotor, rather than becoming stuck against the edge of the pad.
* It provides more surface area and agitates airflow around the rotor to increase cooling.
* It can affect pad bite.
* It looks cool.
The thing is, though, my own experience leads me to believe that the slotting pattern doesn’t make as much of a difference to these factors as you might think, so long as it *is* slotted in some way. Bike brakes aren’t under anywhere near as much stress as car brakes, where slotted rotors can noticeably perform better. Rather, almost every bicycle rotor I’ve ever ridden has performed essentially identically in wet weather and braking power, regardless of whether they used simple slotting where a few circular holes were drilled in the brake track or extremely complex slotting with a bunch of different shapes.
Rotors of different construction *can* perform differently in cooling capability, but even that appears to have very little to do with slotting pattern and much more to do with rotor construction. E.G. The Shimano RT-MT900 and the RT-CL900 both use Ice-tech Freeza construction but have *entirely* different slotting patterns, and both perform essentially identically in all aspects (save for tendency to warp, which is a problem that RT-CL900 was specifically designed to solve). Both of them are noticeably better in cooling than the SM-RT64, which uses simple stamped steel construction but has a superficially similar slotting pattern to RT-MT900. (Yes, I have overheated an SM-RT64 before. I’ve never managed to overheat any Freeza rotor.)
I’ve also had the chance to speak with a certain industry professional in a smaller company that manufactures rotors (among other things), and he seemed to suggest that the slotting pattern his company puts on their rotors are primarily designed to look cool, rather than being intentional shapes to accomplish certain effects. He seemed to echo my sentiment: as long as it’s slotted in *some* way, the difference between different slotting patterns is practically nonexistent. Frankly, I could see even the larger companies like Shimano or SRAM primarily designing their slotting patterns to look cool as well.
So given that the slotting pattern makes much less of a difference than many other aspects of the rotor, I’d personally be quite curious to try a solid rotor to see if, indeed, there *is* a difference we can notice. My guess? For most riders, in most situations, they’d probably never see a difference. But in the end, it’s likely that nobody will ever make a solid rotor, so we’ll never know.
Mechanical engineer here. Similar to the design in many circular saws, they are for “absorbing “ thermal expansion when the rotor heats up thereby preventing warping.
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