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Bike Bounces When Applying Front Brake

  #1  
Old 03-13-2016, 06:10 PM
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Default Bike Bounces When Applying Front Brake

Hey guys! Got the old girl on the road and she's been a blast, save for the sporadic turn signals (seems to be a delay of 2 seconds before they turn on, but it works). I'm having a problem where applying the front brake even gently at low speed bounces the front end a bit, and it's kind of upsetting.

I think it's the rotors, based on what I've read. What's the course of action here? Sanding them a bit and using brake cleaner helped, but it's not perfect.

(I also have an issue where after riding for a while, my idle sits at around 15-1600 instead of the 1300 I have it set to. Blipping the throttle quick seems to fix this, so it's a relatively minor issue. Curious if the lack of O2 sensor on the bike is affecting it, or something)
 

Last edited by Zealot; 03-13-2016 at 08:03 PM.
  #2  
Old 03-14-2016, 08:39 AM
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You might have warped rotors. When you push the bike forward do you get a dragging sound and increased resistance which then eases off and comes back when a certain point of the brake rotor enters the caliper?

If it's more of a judder you might have worn steering head bearings. Pull the front brake on and rock the bike back and forth. Are you hearing and/or feeling "clicking" from the steering? That would be a telltale sign of that. Another sign is if the bike tends to "wander" left and right instead of wanting to go straight.

How old/worn is your front tire?

If your brake discs are in fact warped you need to replage them. Sanding will do nothing to the problem and bike rotors are not thick enough to be turned straight in a lathe. A new set would probly be cheaper anyway.

If you end up replacing your discs I'd advice to stay OEM. they're more expensive than those aftermarket snowflakes, or "gingerbread discs" as we call them but be adviced many cheap aftermarket rotors may be already warped coming from the box.
 
  #3  
Old 03-14-2016, 04:12 PM
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Originally Posted by Mattson View Post
You might have warped rotors. When you push the bike forward do you get a dragging sound and increased resistance which then eases off and comes back when a certain point of the brake rotor enters the caliper?

If it's more of a judder you might have worn steering head bearings. Pull the front brake on and rock the bike back and forth. Are you hearing and/or feeling "clicking" from the steering? That would be a telltale sign of that. Another sign is if the bike tends to "wander" left and right instead of wanting to go straight.

How old/worn is your front tire?

If your brake discs are in fact warped you need to replage them. Sanding will do nothing to the problem and bike rotors are not thick enough to be turned straight in a lathe. A new set would probly be cheaper anyway.

If you end up replacing your discs I'd advice to stay OEM. they're more expensive than those aftermarket snowflakes, or "gingerbread discs" as we call them but be adviced many cheap aftermarket rotors may be already warped coming from the box.
Heya, Mattson! I sat on the bike and rolled it around, and had a hard time discerning if anything was up. I also tested the head bearing by clamping the brake hard and trying to rock the bike, but aside from a teensy tick which I'm pretty sure was a line of some sort moving slightly, there was nothing abnormal or clunky. Nothing could be felt through the handlebars that really concerned me, but I'm not well versed in this stuff. Turning them side to side didn't have any clicking or crunching either. What I did notice however was that up on the stand, I slowly spun the front wheel to see how it went, and it seems something is up.

My dad half heartedly confirmed he thought there might be a little something. I kept the pressure and method of spinning the wheel even, and would watch the little red "Diablo" text on the totally new front tire. Every time it reached a certain area in the rotation, the familiar "Phwoaaahh" of the brakes rubbing on the rotors would turn into "Phww" and it seemed like there was more friction, since it would neither spin as much or as easily as before. I'm always open to more in detail explanations or analysis, but it seems I'm getting close to the culprit.
 

Last edited by Zealot; 03-14-2016 at 04:18 PM.
  #4  
Old 03-14-2016, 05:10 PM
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Yuppers. Seems pretty much like a warped rotor(s) to me. If you want to be sure bolt the rotor off from the wheel and place it on something you know to be level like a glass pane, I`ll bet a section of it is elevated. Notice that the different is probly miniscule but that usually is enough. I`ve seen rotors so bent you can notice the wobble by just spinning the wheel fast on the stand.
 

Last edited by Mattson; 03-14-2016 at 05:14 PM.
  #5  
Old 03-14-2016, 08:44 PM
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Originally Posted by Mattson View Post
Yuppers. Seems pretty much like a warped rotor(s) to me. If you want to be sure bolt the rotor off from the wheel and place it on something you know to be level like a glass pane, I`ll bet a section of it is elevated. Notice that the different is probly miniscule but that usually is enough. I`ve seen rotors so bent you can notice the wobble by just spinning the wheel fast on the stand.
The rotors themselves look straight enough. That being said, I'm rather confident there's something up since it was always the same spot where I noticed I had trouble turning the wheel. Looks like it's time to find some OEM rotors floating around!
 
  #6  
Old 03-15-2016, 12:04 AM
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Obviously goes without saying, before ordering any expensive rotors do verify that indeed is the culprit. I haven't seen the bike in person so I can only throw (somewhat) educated guesses at what could be the issue. But if the rotor/rotors are grabbing at one spesific spot then something has to be out of alignment.
 
  #7  
Old 03-15-2016, 09:24 AM
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Originally Posted by Mattson View Post
Obviously goes without saying, before ordering any expensive rotors do verify that indeed is the culprit. I haven't seen the bike in person so I can only throw (somewhat) educated guesses at what could be the issue. But if the rotor/rotors are grabbing at one specific spot then something has to be out of alignment.
Just had my mom check, letting her know what to look for, and she seems to have found a binding spot in the same location I did. I'm going to order a set of rotors now.

Thanks for the help!
 
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