In this edition of Ask a Wrench, pulled from the members-only segment of the Geek Warning podcast (May 22, 2025 episode), tech editor Dave Rome and pro mechanic Brad Copeland take on four listener questions. They cover disc brake rub for heavier riders, rotor damage from desert dust, bike washing with disc brakes, and the ongoing tire-versus-suspension debate for Unbound.
Geek Warning is tailored for riders who love to tweak, tune, and push their setups. Questions and answers have been edited for length and clarity, so if you want the full nuance and discussion, you can listen to the complete members-only episode embedded below the paywall down at the bottom of this post.
Question #1: Rotor rub
From Kyle:
“I’m 6’3” (190 cm) and close to 240 pounds (108 kg). On my last bike, a Trek Checkpoint, I had flex issues that caused rear rotor rub. I’ve since built up a BMC Roadmachine 02 with Shimano 105 12-speed hydraulic, making sure calipers are faced, brakes are bled, and everything is aligned. Still, during hard efforts or after a long descent, I sometimes hear rotor rub – especially when leaning the bike hard or sprinting. What gives? How do I achieve perfect disc brake alignment?”
Brad Copeland:
This is a can of worms we’re just kicking off with here. A lot of things are working against you, Kyle – not least of which is your size, and in some cases, the length of the tubes on your bike that might exacerbate frame flex.
I’ve certainly seen this problem, including on Treks. I’m a bigger rider too – not quite your size, but enough to make things flex – and I’ve experienced this. There’s a certain inevitability with disc brakes: under side loading on climbs or during sprints, even when everything is set up perfectly, you may still get some contact.
If you really heat things up, there’s lingering drag at the bottom of descents – fluid expands, the rotor may warp slightly, pads expand. Shimano has good heat dissipation tech, but there are limits.
You mentioned wider caliper spacing – that’s helpful. Shimano’s newer stuff has a slightly bigger gap between pad and rotor. But even with that, because the tolerance is so small, even slightly bent rotors or “shelving” on worn pads (caused when the caliper sits slightly high on the rotor) can cause contact.
I don’t have a silver bullet, but some of this is just part of the system.
Dave Rome: Yeah, a little ‘ting-ting-ting’ at the bottom of a descent isn’t uncommon – and sometimes just unavoidable.
I have found some hubs perform better than others. Cheaper hubs or those with floating end caps may allow more lateral movement under load. Something like a Chris King hub tends to be more stable versus something like a cheaper and lighter DT Swiss-copy used on many OE wheels.
And rotors matter too – Shimano’s newer CL series (like CL800 or CL900) are more rigid and tend not to warp as much. That could be money well spent.
Question #2: Rotor grooves in the desert
From David in Tucson:
"I ride a Number 22 gravel bike with SRAM Red and organic pads. One of my front pads is worn evenly, but the other has visible gouges that match the curvature of the rotor. The rotors themselves have shallow circular grooves. I ride mostly off-road in sandy conditions – could this be caused by dirt or sand?"
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