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  1. #1
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    New question here. Are bigger discs better?

    I just bought a used Trek 4900 (hardtail) that the previous owner had put Hayes discs on. It came with an extra set of discs because he was switching to 8" from the stock discs. He had the rear mounted up but the front is still stock.

    I'm just wondering if there is any real advantage to the bigger disc or if it (and the required adapter) adds weight.

    Another thing that I wonder about...it seems like the extra leverage of having the caliper held further from the mounting points would increase the likelihood for tweaking a seatstay. Is this something to be concerned about? I ask because I've seen some threads on Treks with cracked or broken seatstays near the disk mounts.

    By the way, I'm migrating the brakes/wheels over to a 2001 Trek 8000 hardtail. The mounting setup is the same between the two bikes.

    Thanks!

  2. #2
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    The weight is negligible and it is a good idea in my opinion to have larger disks unless you weigh 160 lbs.

  3. #3
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    i'm just building a new bike, went over to the lbs and weighed every brake on the shelf.

    for example the hayes nine rear caliper with lever and hose weighs 430g with 160mm rotor and 480g with a 180mm rotor.

  4. #4
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    Quote Originally Posted by syl3
    i'm just building a new bike, went over to the lbs and weighed every brake on the shelf.

    for example the hayes nine rear caliper with lever and hose weighs 430g with 160mm rotor and 480g with a 180mm rotor.
    Don't forget the required adapter and bolts. The one on my bike is pretty meaty. I don't mean to nit-pick but if you are concerned with weight...it's something to factor in.

  5. #5
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    Ok...couple things. You will pay a minor (60-90g) weight penalty for the adaptors and larger rotors and you'll lose a bit of modulation. On the other hand, you will gain more power and heat resistance/dissipation.

    My bike came stock with Hayes 9's (6" rotor F/R) and some of the local trails are moderately downward pointed stuff (I typically ride a river valley...not super steep or long, but some decent runs that will want to push you to the 45 mph point if you don't work the brakes). I'm a bigger rider (235+gear) and I was able to routinely fade (boil) the front. I upgraded to a 8" rotor on the front and never had the problem again. I noticed the power but also the slight loss of modulation. On a ~30 lb bike, the 3 oz of extra weight for the adaptor and rotor was not noticeable.

    I ended up getting a better brake and going 8" front and 7" rear...overkill for a long travel XC bike, but the brakes have far better modulation for larger rotors than the stock brakes did. Heck, I even went to the local bike park and wasn't able to fade the brakes on a lot more demanding terrain (and I even had the rotors pinging as they were getting nicely heated up).
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  6. #6
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    Quote Originally Posted by gotboostedvr6
    The weight is negligible and it is a good idea in my opinion to have larger disks unless you weigh 160 lbs.
    Before dispensing such advice, it's adviseable to sort of know what you're saying, as in putting larger than approved rotors on a fork.

  7. #7
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    A larger disk may have negatives too: too much torque for frame and fork (check what the manufacturer says).

    Some difference in weight.

    A larger disk may be more likely to get bent out of true, or develop irritating noices from vibration.

  8. #8
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    A larger disc can also present modulation issues.

  9. #9
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    Quote Originally Posted by perttime
    A larger disk may have negatives too: too much torque for frame and fork (check what the manufacturer says).
    That's what I'm mainly concerned with. It's an older frame (2003 Trek 8000 SLR) and I'm not sure if I can even find that information.

  10. #10
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    I'm really surprised the rear of that frame can clear a 203. However, I wouldn't recommend it. There are even people on FR rigs that don't run 203's. I'm heavier, on a mid-weight FR rig and I actually do 3000+ foot descents that take close to a couple hours and I don't use, nor need a 203 in the back.

  11. #11
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    Quote Originally Posted by Jerk_Chicken
    I'm really surprised the rear of that frame can clear a 203. However, I wouldn't recommend it. There are even people on FR rigs that don't run 203's. I'm heavier, on a mid-weight FR rig and I actually do 3000+ foot descents that take close to a couple hours and I don't use, nor need a 203 in the back.
    +1. If you NEED a 203 in the back, you need to reevaluate either your braking technique or your diet. The big rotor makes the rear end lock up very easily, even with xc brakes. This 230lb fella ditched his 185 rotor for a 160 for that reason, haven't had a problem with fading.
    .

  12. #12
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    I think the reason the previous owner may have swapped out the disk was because of damage to the original. I looked at the spare smaller disk and it has some wear on it. I laid it on a glass tabletop and it doesn't lie perfectly flat. I suspect it is warped or bent. So, he probably just opted to go big when he got new rotors.

    Maybe I'll just buy a new smaller rotor. I'll ride this one for a while first to see how it goes. I haven't even been able to ride this bike yet. The weather has sucked and I need to get tires.

    Thanks for all the feedback.

  13. #13
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    I am going to say you most likely don't need 203mm, I just put a 203 on my bb7's, and if the trail had been dry, they would have been an amazing brake, but with the wet leaves and roots, they were just way to grabby and I was barely able to save the front end on numerous occasions. so if you have great traction, go for the biggest, but if you are riding in any sort of adverse conditions, they are way too much power.
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  14. #14
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    Coming off my experience with cars- but the general principles should still apply...

    Simply having a bigger disc won't provide any more stopping power unless the brake pad has a larger contact area with the disc. What it WILL do is provide more surface area to absorb and dissipate heat to avoid brake fade during prolonged braking. Since any decent bike brake setup should have no problem locking up the wheels, you should only be concerned with fade and not stopping power.

    Therefore, if you are doing prolonged downhills, a big rotor will help keep the brakes stopping well as you descend. If your downhills are short, all the big rotor will do is add unnecessary weight.

  15. #15
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    Since when does larger rotors not produce more stopping power>?

  16. #16
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    If the larger rotor has more pad contact, it will produce more stopping power. Whether that is the case will depend on the specifics of the setup. However, stopping power alone is almost never an issue on a bike unless the brakes are fading from a long downhill. If you can lock up the tires, you have already used all the available stopping power. This is simple physics. Short of having a drag chute, you can't get more stopping power than the friction of the tires on the ground provides.

    Stopping power as opposed to the effort needed to squeeze the levers is a separate issue. As long as modulation is o.k., most would prefer less effort needed to produce a given amount of stopping power.

  17. #17
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    While I'm not saying your 100% wrong, we can agree to disagree.

  18. #18
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    I can speak for bigger rotors and same pads = more braking power. I think it is not because of more friction between pads and rotors, but because of the longer lever arm the disc now has on the hub.
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  19. #19
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    While I'm not saying your 100% wrong, we can agree to disagree.
    I think I am being a bit misunderstood...

    I'm not saying that a bigger rotor setup will always not have more stopping power, but to the extent that is does have more stopping power it is due to ancillary factors other than the rotor size.

    More brake pad contact or more leverage due to a longer arm would both be reasons why the larger disc setup could have more stopping power, but it's not directly due to the bigger disc itself. It would be theoretically possible to design a brake setup with a smaller rotor with more instant stopping power by changing leverage characteristics, brake pad shape/design, etc.

    However, what I am saying is that stopping power by itself is not the biggest challenge in a brake setup on a bike. It does not take much force to lock up a bike wheel because there is not that much weight involved. Because of the precision necessary in mountain biking, modulation is far more important. By resisting fade, a bigger rotor could help keep modulation consistent over the course of a big downhill. Fade, if it was bad enough, could also cause enough of a loss in stopping power that it would be no longer possible to create enough clamping force to stop the wheel from turning. This is why downhill bikes need big rotors- they need to be very fade resistant because a long downhill will create a serious amount of heat.

    More braking force can also be nice in that it will take less finger strength to produce the force necessary to lock up the wheels- that is good up to a point- but it could go too far and cause modulation issues and oversensitivity.

    For the OP: The correct choice in braking depends on the use. If you are doing 1000 ft+ vertical descents, you probably are going to generate enough heat to cause brake fade over a long descent. In that case, a big rotor is going to be very useful. However, if your descents are short and quick (say more like 50 vertical feet) there is no way enough heat is going to be generated to cause fade. A bigger rotor will just slow you down by adding weight. This is why downhill bikes have huge rotors and XC bikes have small ones.

    As long as you are generating enough force to lock the tires up, a bigger brake setup won't stop you one bit sooner. That much is not really debatable. What is debatable is the effect of overall stopping power on brake feel, etc., which is often a matter of preference and subject to debate.
    Last edited by nealric; 11-10-2008 at 06:52 PM.

  20. #20
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    Quote Originally Posted by nealric
    I think I am being a bit misunderstood...

    I'm not saying that a bigger rotor setup will always not have more stopping power, but to the extent that is does have more stopping power it is due to ancillary factors other than the rotor size.
    No, you arn't understanding at all.
    The greater braking power from larger discs is ALL about the size. Due to the moment of force being radially further out in space the leverage is multiplied for the given force applied to the disc. Its a pure application of mechanical advantage, in a rotational format. Just think of it as a second-class lever. Increase the lever arm, and you increase the moment of force.

    Also, if you think its easy to lock up a front wheel at speed, then think again. I have Avid Codes on my DH bike, and i regularly struggle to slow on the local trails, and thats with a 203mm front rotor, and a 185mm rear.

  21. #21
    Nightmare on Lyrik st. VI
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    Quote Originally Posted by nealric
    Simply having a bigger disc won't provide any more stopping power unless the brake pad has a larger contact area with the disc.

    I'm not saying that a bigger rotor setup will always not have more stopping power, but to the extent that is does have more stopping power it is due to ancillary factors other than the rotor size.
    You're missing one thing. The larger the disc the larger its circumference. Therefore, for every wheel revolution the pad drags over a larger area of disc, which is what gives you more stopping power.

  22. #22
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    Quote Originally Posted by nealric
    I think I am being a bit misunderstood...

    I am following you but in this context your theory is a bit off.

    With all else being the same a larger rotor will produce more stopping power then a smaller rotor with the same input pressure at the handle.

  23. #23
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    With all else being the same a larger rotor will produce more stopping power then a smaller rotor with the same input pressure at the handle.
    You are both correct on the bigger rotor. My bad now that I think of it. Part of the problem is I was coming from a car paradigm. Installing a big-brake kit on a car will not change the stopping distance for a one time braking maneuver.

    However, I do stand by that the real issue is not maximising one-off stopping power. If you can't lock up the tires, either your brakes are fading from heat or are not setup correctly. I suspect that is what is happening in takai's situation. I guess it could also be the case that DH tires just provide a ton more traction than the XC tires I am used to. My puny 160mm rotors with Bb7s have no problems locking up my tires at any speed when they are cool. After heating up, that's a different story.

  24. #24
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    Quote Originally Posted by nealric
    You are both correct on the bigger rotor. My bad now that I think of it. Part of the problem is I was coming from a car paradigm. Installing a big-brake kit on a car will not change the stopping distance for a one time braking maneuver.
    cars don't really compare that well to bikes. With cars the center of mass and brake balance is fixed. You add a larger rotor and caliper bracket to a car and the front biased brake system will simply lock the front end up even sooner- braking distance will be worse because the rear brakes are under used. Stick a big brake kit on and it gets even worse, the fluid displaced in the new calipers may more or less what the rest of the system is designed around. It's silly to add a big brake kit to a car without an adjustable brake proportioning valve, and on a street car even then it's stupid because you can't set the balance from light to heavy braking.

    On a bike, none of that is true. Bigger rotor = more power = shorter stopping distance, given infinite traction.
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