A question about disc brakes...- Mtbr.com
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  1. #1
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    A question about disc brakes...

    Hello Everyone, I just registered and this is my first post here! I've been lurking for a long time though.

    Anyhow, I need some information regarding disc brakes. I recently purchased a new/old stock 2015 Kona Unit. It has Avid BB7 disc brakes with a 180mm front rotor and a 160mm rear rotor. I had this exact same setup on my previous bike (brakes and rotors), a 2008 Surly Instigator set up as a single speed.

    While riding my old bike at very slow speeds, if I mashed down on the brakes, the wheels would lock up and come to an immediate stop. When I do the same thing on my new Kona Unit, it doesn't immediately lock up. When I mash down the brakes, it slows down and rolls for 2-3 ft. before locking up.

    Is this happening because the 29" wheels of the Kona Unit have more rotational mass that the 26" wheels that my Surly Instigator had? To get a similar braking feel on the Kona Unit, do I need to get larger diameter brake rotors?

  2. #2
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    How well cable brakes work is dependent on the quality of the cables, particularly the outer.

    It sounds to me like you have a mushy outer cable. Ideally you want a non-compressible one.
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  3. #3
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    Quote Originally Posted by Velobike View Post
    How well cable brakes work is dependent on the quality of the cables, particularly the outer.

    It sounds to me like you have a mushy outer cable. Ideally you want a non-compressible one.
    I just installed the BB7's and instead of the cable that came with the bike, I used brand new Jagwire Ripcord brake cable. When the bike is raised up in my work stand, the brakes lock the tires up right away. While riding, the bike still stops fast enough, it just doesn't feel as "instant" compared to my old 26" bike.

  4. #4
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    You either have something on the brake pads or they are not broken in. Take them out and lightly scuff the pad faces with some sandpaper (I like 600 grit) then clean them off with some rubbing alcohol. You will need to re-break them in a little but this usually helps.
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  5. #5
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    Could be the levers. Are they the same as on the old bike? I have Speed Dials with my BB7s, I can get over the bars or smooth controlled stopping power.

  6. #6
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    I'm going to go with bedding process here. The brakes need to be bedded in to work properly, and should easily have enough power to send you over the bars.

  7. #7
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    ^ agreed, you need to bed in the pads. you have to get the brakes good and hot. you can do repeated high speed stops on a long downhill, or hit the pads with a blow torch (not too much, your not trying to cook them, just heat them up). the pads will de-gas and your braking will instantly improve.

    if this doesn't instantly fix it, I'd go with new cable housing.
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  8. #8
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    I love BB7s but they have a learning curve to setting them up. are the pads and rotors brand new? if they are new, start with bedding in the pads. make sure the rotors are super clean so you don't contaminate the pads.

    otherwise, it's just a matter of getting the settings and adjustments just right on the calipers- static pad distance, pull, cable tension, caliper alignment, etc.

  9. #9
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    Definitely a set up issue. As others have said, bed them in properly. Also, be sure that the calipers are set up correctly. If you are too far away or too close to the wrong pad you can decrease braking power significantly. Check out the threads here on BB7 set up.

  10. #10
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    Thanks guys, I just got the bike less than a week ago and haven't had a chance to take it out yet. It's only seen some slow speed testing in my backyard. Regarding the brakes, everything is brand new. As mentioned above, I replaced the stock cables/housing with Jagwire Ripcords. I also replaced the stock brake levers with Avid Speed Dial 7's.

    That being said, even after the brake pads are bedded, can I expect this setup to feel different being on 29" wheels compared to my old 26" wheeled bike? Or is there no difference regarding the feel of the same size rotors on different size wheels?

  11. #11
    eri
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    Quote Originally Posted by treeguy2016 View Post
    Thanks guys, I just got the bike less than a week ago and haven't had a chance to take it out yet. It's only seen some slow speed testing in my backyard. Regarding the brakes, everything is brand new. As mentioned above, I replaced the stock cables/housing with Jagwire Ripcords. I also replaced the stock brake levers with Avid Speed Dial 7's.

    That being said, even after the brake pads are bedded, can I expect this setup to feel different being on 29" wheels compared to my old 26" wheeled bike? Or is there no difference regarding the feel of the same size rotors on different size wheels?
    Rotating energy is usually higher on 29er so in theory stops slower but if brakes are working they should feel close: you should still be able to lock front and back (though front will take some squeezing!)

    I had a similar issue with my sons bike, turns out the levers were for vbrake and had the wrong pull.

    I know I shouldn't say it but... Someday you'll get some m785 brakes and never miss those bb7s.
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  12. #12
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    Those brake components should work perfectly. It's just a matter of setup at this point. The difference between 26 and 29-inch wheels would be minuscule.

  13. #13
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    Quote Originally Posted by eri View Post

    I had a similar issue with my sons bike, turns out the levers were for vbrake and had the wrong pull.
    Avid SD7's are V brake levers, and that's what BB7's were designed to work with (V brake levers, I mean).
    They need to be correctly adjusted and bedded in - get them good and hot with repeated hard stops, preferably downhill, and then, while they're still really hot throw some water over them.
    You should be able to lock up a 29" wheel on asphalt, if you feel the need to, and without excessive lever pressures either. Properly set up they are plenty powerful.

  14. #14
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    Quote Originally Posted by Andy R View Post
    Avid SD7's are V brake levers, and that's what BB7's were designed to work with (V brake levers, I mean).
    They need to be correctly adjusted and bedded in - get them good and hot with repeated hard stops, preferably downhill, and then, while they're still really hot throw some water over them.
    You should be able to lock up a 29" wheel on asphalt, if you feel the need to, and without excessive lever pressures either. Properly set up they are plenty powerful.
    Ehhh, SD7s may not have exactly the same leverage as a generic v-brake lever and a generic v-brake lever does not have the speed dialer for adjusting modulation which I found was key for really making the BB7s come alive. A mistake I have seen in the past is people pairing the BB7 with some v-brake lever and then *****ing about the modulation.

  15. #15
    eri
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    Quote Originally Posted by socal_jack View Post
    Ehhh, SD7s may not have exactly the same leverage as a generic v-brake lever and a generic v-brake lever does not have the speed dialer for adjusting modulation which I found was key for really making the BB7s come alive. A mistake I have seen in the past is people pairing the BB7 with some v-brake lever and then *****ing about the modulation.
    My sons bike started with generic v-brake levers and bb7s and the braking was as described above. I replaced with some different, inexpensive, generic levers recommended by Poison Spider bikes in moab and suddenly the brakes were easy to setup and very powerful. The difference was the old levers had inadequate cable motion (neither of the levers had adjustable leverage like the sd7 does.)

    I'm sorry for the potential misinformation above, my point is that I suspect you have a problem with inadequate cable pull and likely is a matter of adjustment. Either your levers are set fine and the pads are just too far from the rotors (easy to adjust on the bb7 caliper), or your levers aren't generating enough cable motion. The sd7 has adjustable leverage so try adding more.

    It could also be that your housings aren't seated well? Check the brakes by repeatedly squeezing and slightly releasing the lever and checking for motion along the cable from lever to brake.
    the truth is always a gift because it offers the recipient of that information the chance to change the outcome - Grace Choi

  16. #16
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    Quote Originally Posted by eri View Post
    My sons bike started with generic v-brake levers and bb7s and the braking was as described above. I replaced with some different, inexpensive, generic levers recommended by Poison Spider bikes in moab and suddenly the brakes were easy to setup and very powerful. The difference was the old levers had inadequate cable motion (neither of the levers had adjustable leverage like the sd7 does.)

    I'm sorry for the potential misinformation above, my point is that I suspect you have a problem with inadequate cable pull and likely is a matter of adjustment. Either your levers are set fine and the pads are just too far from the rotors (easy to adjust on the bb7 caliper), or your levers aren't generating enough cable motion. The sd7 has adjustable leverage so try adding more.

    It could also be that your housings aren't seated well? Check the brakes by repeatedly squeezing and slightly releasing the lever and checking for motion along the cable from lever to brake.
    Yeah, didn't mean to imply that only SD7s would work, just that not all v-brake levers have the same leverage ratio starting out.

  17. #17
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    So when I started this post the other night, for the heck of it, I ordered some larger rotors and brake adapters from Jenson. The rotors are Shimano SLX RT66 203mm front and 180mm rear. They came in today while I was at work and I just finished installing them.

    Same test as the other night, rolling very slowly and mashing the brake, I now have instant lockup! I'm thinking the 180/160mm combo wasn't having instant lockup due to a combo of my weight and the extra rolling mass of the 29" tires? Currently I'm around 200 lbs. and stand to lose a good 25-30 lbs. Other than some slight changes to the pad orientation for the new rotors, I didn't touch anything else. Pretty much a direct swap to larger rotors.

  18. #18
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    203 rotors are totally excessive. It's a fluke that the new rotors are working better. When you reinstalled them, you did a better job adjusting the calipers, or the old rotors were contaminated.

  19. #19
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    Quote Originally Posted by mack_turtle View Post
    203 rotors are totally excessive. It's a fluke that the new rotors are working better. When you reinstalled them, you did a better job adjusting the calipers, or the old rotors were contaminated.
    I didn't really have to adjust the calipers that much with the Shimano rotors, just a few clicks. Other than that it was a direct swap. Didn't mess with the levers or cable. Regarding the other rotors, they are new Avid G2 Cleansweeps that came with the bike. I don't see anything sticky and/or oil residue on them, maybe something else is wrong with them? The new rotors are working so I'm happy!

  20. #20
    Downcountry AF
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    Quote Originally Posted by treeguy2016 View Post
    I didn't really have to adjust the calipers that much with the Shimano rotors, just a few clicks. Other than that it was a direct swap. Didn't mess with the levers or cable. Regarding the other rotors, they are new Avid G2 Cleansweeps that came with the bike. I don't see anything sticky and/or oil residue on them, maybe something else is wrong with them? The new rotors are working so I'm happy!
    I agree, 203mm rotors are excessive and you just got them set up better the second time.

    brakes don't always work perfectly right out of the box. test rides in the driveway is not a good measure of performance before they're broken in/bedded in. get them out on the trail, once you ride them a couple times they'll improve.

    lastly, if you get some shudder from the front fork with the big rotor, go back to the 180. I've found the SLX 203 rotor caused severe shudder with certain rigid forks.
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  21. #21
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    As a 225lb rider I can definitely tell you that the new rotors are not the reason your brakes feel better. I've run BB7s, and even cheaper Nashbar (Tektro really), and Promax mechanical calipers with 160mm rotors front and rear on 29ers and had no trouble at all locking up both wheels.

    In my experience, new pads definitely need to be bedded in a bit, two or three really hard stops usually gets you most of the way there and then a few rides later you're good to go.

  22. #22
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    Yeah, the 203mm does look a bit big, lol. I'm going to run it with 180mm front and back. I'm pretty sure that I had the brakes setup properly the first time. The bike was stopping quickly, just not as "instant" as I was used to on my 26" bike. I have some pretty chunky thorn proof tubes installed in the tires, so maybe the extra rotational mass is why the smaller stock rotors felt a bit more sluggish. That or I just need to get used to the way it feels, as this is my first 29er.

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