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Technical FAQ: More on disc rotors in soupy mud

  • By Lennard Zinn
  • Published Feb. 5, 2013
Solid rotors from SRAM and Magura are one solution to reduce disc pad wear in soupy conditions. Photo: Lennard Zinn | VeloNews.com

Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

In my recent column on dealing with disc-brake problems in soupy mud, one suggestion for preventing rapid pad wear was solid rotors, as they have worked well for motorcycle racing in those conditions. Here are some suggestions and more discussion on that.

SRAM’s Solid Sweep rotors

Dear Lennard,
Avid used to make the Solid Sweep rotor (they still may). It looks like SRAM released them for “resonance” issues, but they appear as if they would help premature pad wear as well.
—Nathaniel

Dear Lennard,
I did a search on the SRAM website and found Solid Sweep rotors listed in their “spare parts” catalog.
—David

Dear Lennard,
Avid makes a rotor called the Solid Sweep: BTI part number AV-7171. It only comes in 140mm, though.
—Ian

Magura alternative

Dear Lennard,
I’m sure you’ll get a few responses, but there are non-vented rotors available on the market. I’ve got Magura Marta SL rotors that are wavy but not drilled.

May not be widely available any more but shouldn’t be too hard to find still either.
—Andrew

Going carbon

Dear Lennard,
I read your article on the failures of disc brake systems and the response of Wyatt Seals with his suggestion of using solid rotors. I’ve found some solid rotors with the advantage that they’ll be lighter than your existing rotors as well. They’re carbon rotors from Kettle Cycles and are to be available soon. I almost supported their Kickstarter campaign, but decided to let others test them out to see if their claims are true before I used my hard-earned cash to “carbon bling” my bike. You should probably get your hands on some so I can have a trusted source to report on their performance.
—Colin

Dear Lennard,
I’ve been an early adopter of discs for ’cross and everyday commuting use — I ordered a XCR-tubed ’cross frame w/ an Enve disc fork not long after the fork was released. We do end up having our share of mud days as well, especially in XC but also CX. I’ve consistently found the advantage in disc brakes in ’cross to be coming into a sharp u-turn or series of turns after a long, high-speed straight. I’ve seen guys on cantilever brakes pop after having to repeatedly close the small gap due to the braking differences.

I have ordered a couple sets of the new kettle carbon discs to try. One is for some Enve tubulars that see daily use for commuting & training (Challenge 27mm rubber), CX racing, and also XC MTB racing. The other set is for a mountain bike. But after reading your article, I may try them on my dedicated mud CX wheelset as they don’t seem to have many if any holes in them. They are pricey for sure, but they may fit the bill for those using discs and racing often in the wet if they do in fact hold up.
—James

Solid rotor trade-offs

Dear Lennard,
After reading your column about how to avoid the issues people were seeing at nationals I wanted to chime in about one of the suggestions.

Solid rotors may be a good option for the kind of mud that was causing folks problems, but they have their own trade-offs. One of the reasons for drilling or slotting disc rotors, aside from weight savings and cooling (which might on its own be the reason no one seems to sell solid rotors for bikes, since thin rotors are probably more prone to warping due to heat) is that the drilling helps preserve the rotors themselves. Look at any solid rotor, on a car or motorcycle, that has been in service for a while and you’ll see deep gouges parallel to the circumference of the rotor. This is because small metal shavings and debris can get embedded in the pads and each time the pads engage, it drags against and gouges the rotor.

Drilling and slotting allow the pads to “clear” debris and generally, drilled holes are arranged so that debris at any position on the pad will pass over one hole in each “constellation” and have a chance to dislodge from the pad. Car and motorcycle rotors are thick enough that they can be turned on a lathe once or twice if the gouges get too deep and start affecting braking, but I imagine that bike rotors are already so thin that any significant gouging would make the rotors structurally unsafe in a hurry.
—Seth

A brief history of wavy rotors

Dear Lennard and Wyatt,
To address your issues with muddy/dirty races wearing through brake pads at an accelerated rate, Wyatt, you are right that one solution (and the most common one) to solving this problem is to go with a solid rotor. When a rotor has small, circular holes, the muck/mud gets trapped and drastically speeds up the rate of wear on the pad and creates and uneven wear pattern on both the pad and rotor.

The true “wave” rotor was originally designed and patented by Galfer brakes in the early 1990s with the performance improvement being that the rotor is self-cleaning. Because the leading edge contact point of the pad is constantly being moved in a consistent up and down pattern, dirt is expelled much quicker than on a solid rotor. The secondary advantage to a true wave design is that the rotor is lighter and runs substantially cooler.

The original application for Galfer’s wave design was observed trials motorcycles (makes sense as Galfer is a Spain-based company and the Spanish are, and have always been, nuts about observed trials on both motorcycles and bicycles!). When Galfer’s wave rotors started to really get popular about 10 years ago, a bunch of companies (like Kawasaki for motorcycles and Avid, Hayes and Shimano for bicycles) started doing cutouts on the outside diameter of the blade to make people think that they were getting real wave rotors and the increase in performance that comes with them. In reality, when the leading edge contact of the pad is not met with a consistent amount of blade surface (as is the case with the Galfer copiers, where the cutouts are only on the outer diameter), there is no advantage in cleaning and the uneven heat distribution as well as uneven pad and rotor wear happens because the top edge of the rotor doesn’t have the same amount of surface contact as the middle or bottom edge.

You will also notice that Galfer rotors never have round holes. The holes are always in a semi oval shape to facilitate centrifugal force removing any debris from the holes. To answer your concerns about rotor strength, Galfer does Finite Element Analysis on all of its rotor designs (and on many of its competitors’ rotors as well), and a Galfer Wave rotor has just as much strength as a solid round rotor, despite the fact that it has less mass. At this time, the only company making a real, licensed “Wave” rotor other than Galfer is Magura, which has been doing so for about 10 years.
—Andy Schwartz, former bicycle division manager of Galfer USA

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Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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