Technical FAQ: Road disc vibrations, SRAM eTap, and more

Lennard Zinn tackles questions about a perplexing problem involving road disc brakes and a titanium frame, SRAM eTap, and other topics.

Vibrations using road discs

Dear Lennard,
I have a 2009 Motobecane Cross Ti bike that my local bike shop converted from cantilevers to TRP Hy/RD brakes a few months ago. We also installed Stan’s NoTubes Grail wheels with a Spot Brand fork up front; the rear stays already had disc tabs. I ran through the recommended brake wear-in procedure, but after 5-10 miles I started feeling vibrations, most noticeably in the front, but also in the rear. Light braking doesn’t produce any vibes, but with medium or hard pressure I start feeling vibes front and rear at about 12 mph (high frequency, like the rotor cutouts hitting the pads) and getting progressively stronger and lower frequency until I can see the fork vibrating front-to-rear (~approximately 3mm) when the speed is down to 3-5 mph. At low speed it also produces a solid thunk-thunk sound that riders next to me have noticed.

We’ve checked the headset and it’s not loose. Same for the calipers on the fork and rear stay. The rotor bolts are tight and the rotors are true, and not touching the pads. I’m using 160mm rotors front and rear.

I’ve cleaned the rotors and pads several times with alcohol and degreaser. It seems to stop the vibes for the first five miles of the next ride, but then the vibes return.

Last month, the shop tried replacing the OEM rotors with Tektro HZs, and the original metallic pads with organic pads, but it didn’t help. I’ve tried three different QR skewers, and tried reversing the lever from left to right, with no effect. I also tried running a friend’s front wheel with an Avid HS1 rotor. The vibes seemed to disappear for the first 10 miles, but then returned.

I would certainly appreciate any further diagnosis that you can think of, or recommended fixes.

Are TRP Hy/RDs noted for this problem? How about the Spot fork? Should I try some more pad-rotor combos?
— Nick

Dear Nick,
Here is a response from TRP’s technical service manager:

This one is a bit of a head-scratcher. He’s already tried just about everything I can think of that typically fixes any issues. There are two things that come to mind right away, but they’re both pretty extreme.

The first would be that the calipers are aligned slightly askew. Not enough for the rotor to rub on the pads, but enough that they still contact on an angle. We generally only see this with the Spyre brake, because the pad to rotor gap can be adjusted and if left far apart can allow for a greater range of mounting angles. I don’t recall this ever being an issue with the HY/RD. A more thorough explanation can be found in this video.

The second would be both the frame and fork are flexing under the braking load. This is more common with forks than with frames, but both cases are very rare. Although with it being an older Motobecane frame, I wouldn’t completely rule it out. It is strange that the vibrations disappear with a new rotor, pads or after cleaning, but come back after some time.
— Bryce Olsen

I would add that when we’ve seen issues like this, it is due to the brake vibrations being carried up into the frame or fork due to something flexing to the point that it can set up the vibration in frame or fork tube. On the rear, we have fixed this on metal frames by reinforcing the seatstay to deaden the vibration. I believe that yours already has a strut between the seatstay and the chainstay to do that, but it is still a thin seatstay, and it’s titanium, which really likes to vibrate (a titanium tuning fork rings so brightly and loudly it is almost deafening; same thing if we drop some titanium tubes on the concrete floor; everybody covers their ears!).

You can do an experiment to try to deaden the frame vibration. Cut and miter a ¾-inch or so wooden dowel rod to form the third leg of an isosceles triangle near the dropout; have it intersect the left seatstay and chainstay about four inches from the dropout on each of them. Wrap tape around the seatstay and chainstay first where it meets them, and then tape it in really well with duct tape. If that eliminates the vibration from the rear brake, you’ll know the source. On the front, my guess is that a different fork would do the trick. I have seen brake chatter on disc cyclocross bikes go away with a fork change, and yours moving back and forth 3mm sound a lot like that.
― Lennard

SRAM eTap compatibility

Dear Lennard,
With the advent of SRAM’s eTap, I bet a lot of people are thinking like I’m thinking … can I put eTap on my bike with Ultegra cranks and Ultegra cassette and roll? Or do I have to match with a SRAM crankset/cassette?
— Jonathan

Dear Jonathan,
We won’t know for sure until it comes out, but given that you can currently run Ultegra cranks and an Ultegra cassette with a SRAM road drivetrain of the same number of speeds, I’m willing to bet that you’ll also be able to with eTap.
― Lennard

More on shortening cranks so kids can ride a tandem

Dear Lennard,
Sinz cranks go down to 115 mm in square taper. How short does this guy need for his kids? Other BMX brands go down as low as 130 mm.
— Larry

Dear Lennard,
SJScycles in the UK have a range of crank sets that might be useful for accommodating small children. One has dual drillings at 115mm and 140mm, others are at 125mm, 140mm, and 145mm.

Look for the Thorn brand cranksets on or try this link.
— Blair

Dear Lennard,
FYI, there is a company making kid-sized bikes with various age appropriate length cranks. Not sure if they sell just parts. Starter kids Bikes weigh 15 pounds! Islabikes. It’s a company founded by Isla Rowentree, a master’s woman cyclocrosser with many victories from England. In the USA you have to buy them through one shop. Big market though in Britain. We love ours.
— Brian

Dear Lennard,
Based on experience with my own kids and a tandem, I recommend (loose) toe clips for the kids. My daughters preferred clips as they would mitigate their feet sliding off the pedals and getting hit by the pedal. With clips the kids routinely did a cadence of around 90!

Also, for their comfort and safety, the kids should be wearing more substantial shoes than the ones shown in the picture.
— Raymond

Dear Lennard,
Just saw your recommendations on shortening crank arms. When I built up a tandem to take my eight-year-old daughter to school and for weekend breakfast trips, I investigated a number of options. Ultimately I didn’t want to spend a lot on something she would outgrow in a couple of years. I’m a cheapskate, and she’s growing like a weed. So I cut some 2x4s about four inches long, sandwiched the pedal between them, and then attached the 2x4s with 3” screws. It’s not an elegant or lightweight solution, but it’s worked for the past six months. I might eventually add some sandpaper tape for grip, but right now she doesn’t really “pedal” that much anyway.
— Brian

More on wheels for big guys

Dear Lennard,
Just a note to your clydesdales. We’ve found the Campagnolo Khamsin (or Fulcrum Racing 7) to be pretty much bulletproof. Had a guy this year with us blow up his Mavic rear wheel and the shop guy reminded me of the reputation for strength and reliability of these wheels when he suggested them as replacement … and that was far from the first time I’ve heard this. The latest ones are wider as well.

Not expensive, fancy or light, but big guys shouldn’t be too concerned about that anyway.
— Larry