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Discussion Starter · #1 ·
I have a brand new bike, with brand new Magura disc brakes. When I brake on a steep downhill, the bike judders in the same way a road bike feels when the headset is loose, sort of a rattly feeling. But the headset doesn't seem to be loose, although it feels like something is. Could it be the brakes? If so, what should I do to fix this?

This is my first bike with disc brakes, so I'm not familiar with how to adjust them.
 

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First up: ensure the headset was tensioned properly. Follow the mfg's istructions for setting tension, then ride again. Even a decent shop can overlook an item as simple as this if they are in a hurry (or are a really bad shop). I presume you have threadless.. simple process of loosening the stem bolts, tightening the stem cap bolt to the correct torque, then re-tightening the stem bolts

Next, check that the calipers are aligned properly. I like to have the two caliper bolts barely loose enough for the caliper to move when I clamp down the brake lever, then re-tighten the two bolts a little at a time so I don't put enough torque on the caliper to move it.

Check that the axles are set properly if they are open cone design. The axle should hardly move againt the hub, but spin freely.

Forks... Cheaper ones are going to have some unwanted lateral movement, but shoudn't be clunking when you push the bike back and forth with the front brake lever depressed. If it is, have the shop inspect it.

Once you have all of this done, you might find the choice of rotor Vs pad can be a source of this type of problem. The biggest issue is large holes in the rotor face. So many rotors have them, but not enough calipers have long enough pads to mitigate the cycling of braking torque they create. For example, Avid Elixr 1's dont like Shimano Icetech rotors, but are ok Formula Race 1 rotors. Both have approximately the same amount of rotor contact surface area, but the Formula rotor has a lot more, much smaller holes.

Everything else solved, you can mitigate juddering by increasing the length of your pads and/or decreasing the size of the holes in your rotors.

There is also an issue of having the wrong type of pad for the material in your rotor that will cause juddering no matter what pad and rotor design. Always check with the mfg for the type of recommended pad when purchasing a new rotor design that you have not previously used. Being a new bike, you should not have this issue.

Good luck!
 

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Why would 140 mm rotors be bad on 29+ wheels?
Its not specifically "bad". Theres a theory that larger rotors help due to the rotational weight and leverage added with bigger hoops. If they stop you without fading/overheating... no need to upgrade. I've even seen a number of ppl run 140 rotors in the rear to add modulation and reduce weight. Most of the power comes from the front anyways.
 

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Discussion Starter · #12 ·
My bike has 180 front, 160 rear. Did you look at the picture of the bike I posted? If the rotors look small, it's not that the rotors are small, but that the wheels are HUGE. I put my Krampus next to my Rivendell touring bike, which has 26" wheels, and they don't even look like they are bikes for the same people.
 

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I was just answering your question. The "if they stop YOU without fading/overheating" was using YOU as a generalization. Not, you as in you. Sorry for the confusion.

And, no... i didn't see a picture you posted. At least not in this thread. I probably wouldn't be able to tell you the rotor size regardless if you had. Most rotors look like a thimble on my monitor. Judging 1-2cm difference is difficult enough without it being a shrinky dink.
 
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