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New rotors installed; Mounting bracket issue resolved thanks to home shop.

Saturday, April 28, 2012 by  
Filed under Daily Blog

New Magura Storm Rotor mounted on my Trek Fuel EX 8.

New Magura Storm Rotor mounted on my Trek Fuel EX 8.

The new brake pads that I installed on my Trek Fuel EX 8 highlighted an issue that I was aware of, but had not been a problem worth addressing until the new pads were in place: out of true rotors.

I knew my rotors (both of them) had a few very minor bends in them, but it wasn’t enough to cause rubbing on the pads. Well, once I got the new pads installed the tolerances between the pads and the rotors were much tighter, and there was some rubbing.

Truing up a bent rotor is a essentially a rather archaic process: it involves physically bending the rotor until it’s back in true.

Some people perform rotor truing using nothing more than an adjustable wrench along with visual and audible cues. This process basically involves watching and listening for rub as the rotor is spun between the brake pads and then bending the rotor at those points. This procedure is very trial-and-error, and can be a bit frustrating.

There are, of course, several specialty tools that can make rotor truing considerably less difficult and far more precise: the Park Tool Rotor Truing Fork, the Park Tool Rotor Truing Gauge and the Park Tool Dial Indicator Kit.

Rotors are not expensive, and so I decided to forgo the truing process and simply replace my Avid rotors with Magura Storm rotors. The new rotors are perfectly true, and there is absolutely no variance as I spin them between the new brake pads.

Every self-respecting homeowner should have a vise.

Every self-respecting homeowner should have a vise.

Of course, as is almost always the case, there was an unforeseen problem. The front Avid rotor was 185mm, which deviates from the current standard of 180mm. When the replacement 180mm rotor arrived and I pulled the Avid rotor, I realized the problem. The solution is pretty simple: install a new caliper mounting bracket appropriate for 180mm rotors. The problem is I didn’t have that mounting bracket, and I wanted to ride this morning. Of course calls to all my local bike shops looking for the part were fruitless (“We can order that for you“). I decided there was no choice but to go the MacGyver route…

The radius of 180mm rotor is 90mm, while the radius of a 185mm rotor is 92.5mm. The solution was pretty simple: I needed to remove precisely 2.5mm of metal from the 185mm mounting bracket. Situations like this are precisely why I invested the time and money into a well-equiped home bike shop. I simply measured the bracket on each side with my digital caliper (which has a resolution down to .001mm), put a metal file in my vise, and shaved 2.5mm of metal from the bracket.

When I bolted up the reworked bracket the result was absolutely perfect: the edge of the brake pad was aligned to the outside edge of the rotor down to 1/100th of millimeter.

I literally have not had any of my bikes in the shop since I built my home bike shop. Not only have I saved a huge amount of time and money, I’ve learned a tremendous amount about bike maintenance and repair along the way. Of course when I’m out riding and my bike is running perfectly, knowing that I’ve done all of my own maintenance and repair work is quite satisfying. Bike repair is like an awesome hobby within a sport.

Now it’s time to ride and bed in these new pads and rotors! 🙂

John Stone Fitness Comments

5 Responses to “New rotors installed; Mounting bracket issue resolved thanks to home shop.”
    • I do have a 180mm bracket on order, but I’ll probably just keep it as a spare because my reworked bracket is perfectly fine. Not only is 2.5mm a very small amount of material to remove, this is not a part subject to any serious forces (it just mounts the brake caliper to the front fork).

      GD Star Rating
    • Ah, I didn’t know that. I saw some people complaining about Avid’s deviation from the 180mm “standard”, and just took that at face value.

      As you indicated in our email exchange yesterday, it sounds like Avid has switched to 180mm now, too?

      GD Star Rating
      • Yeah all the 2012 Avid stuff is 180mm or 200mm vs. 185mm & 203mm from last year. I think Magura and Shimano switched in for 2011 and Avid followed in 2012. All about standardization, now if they would just do it with bottom brackets, headsets, and rear hub spacing.

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