Larger rotor in the front (?)- Mtbr.com
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  1. #1
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    Larger rotor in the front (?)

    I see a lot of bikes with a larger disk brake rotor on the front tire and a smaller one in the rear. I know that it's done for the best stopping power, but on a bike, you (I) would think that a larger one on the rear would be better. (?)

    I know on race cars, most of the stopping power is done with the front brakes. The front rotors are much larger than the rears. This is on a race car though, it has 4 wheels.

    On a bike, if there is too much braking with the front tire, you know what can happen....OTB's.

    I only ride XC and do not go that fast, but I was thinking of upgrading to a bit larger rotor for a bit better stopping power. Thoughts?

  2. #2
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    more stopping power with front brakes, it's true of all vehicles. I've been mtbing for 18 years, and can honestly say I've never had a brake-induced endo that wasn't deliberate - if that's happening to you it's not the brake, it's poor technique. Too much power in the rear can cause the rear wheel to skid.

    getting a bigger rotor will increase stopping power, the front moreso than the back.

  3. #3
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    Motorcycles are the same way, two large rotors up front, one small one in the rear, the reason being that the rear wheel will lock up with almost no braking force applied due to weight transfer.

    Weight "transfers" to the front wheel as you increase your braking, giving you more traction to brake even harder, at the same time, the rear wheel loses traction because of the torque about the front axle, in the most extreme scenario, an endo/stoppie, therear wheel is in the air so it doesn't matter how big a rotor you have on it, it's not going to be doing anything.

    As joules said, going over the bars by braking too hard is simply poor technique.

    If you snatch the front brakes you'll just lock up the front wheel and crash, the trick is to progressively squeeze the front brakes, and drive the front wheel into the ground, move your bodyweight rearwards to stop the bike pivoting around the front axle.
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  4. #4
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    Thanks guys, I knew about weight transfer with cars (I have a Vtec prelude) and have gone to a few clinics which talked about weight transfer. I guess I never applied it to a bike because I'm on 2 wheels instead of 4. (But physics applies to them both)

    How do you go about upgrading to a larger rotor? (I know you buy it and put it on) But what about the spacing on the front shock? Is there a spacer that is applied as the brake pads will be further away from the disk. (Avid Juicy 7's)

  5. #5
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    There is a spacer, but it varies from bike to bike. The best option is to talk to your LBS.
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  6. #6
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    as stated

    all vehicles will do the majority of slowing/stopping with the front wheels. Larger rotors help with a better mechanical advantage for the braking system and dissapates heat better. (theres a myriad of discussions on MTBR on this). Anyway, as weight shifts forward, I think, if I remember correctly, the ratio becomes 80% - 20% front - rear braking upon (contolled) panic stopping on a motorcycle. That's what I remember from my motorcycle racing days. I assume the same applies to bikes.

    Supposedly smaller rotors also give better modulation compared to larger rotors. Use the front for stopping and rear for slowing and control. I think the new trend is 7 inch front rotors for trail riding. I've been using 8 inch front rotors since 98 and anything less just doesn't seem right to me.

    I believe theres separate adapters for parallel as well as post mount disc brakes for 7 and 8 inch rotors.

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  7. #7
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    OTBs...

    Quote Originally Posted by Bail_Monkey
    I see a lot of bikes with a larger disk brake rotor on the front tire and a smaller one in the rear. I know that it's done for the best stopping power, but on a bike, you (I) would think that a larger one on the rear would be better. (?)

    I know on race cars, most of the stopping power is done with the front brakes. The front rotors are much larger than the rears. This is on a race car though, it has 4 wheels.

    On a bike, if there is too much braking with the front tire, you know what can happen....OTB's.

    I only ride XC and do not go that fast, but I was thinking of upgrading to a bit larger rotor for a bit better stopping power. Thoughts?
    ... don't happen from too much braking power. OTBs (you mean an endo?) happen because the rider does not lean back when hard braking.... or probably leaning forward while hard braking. I can't imagine anybody who has been riding a bike for any length of time not knowing to shift his/her weight back when braking really hard.

    I guess OTBs can also happen from hard braking while turning, if the front wheel washes out. A bigger front disc brake may decrease modulation (fine control) and that might cause more crashes until the rider leans to control it.

    Unless you weigh over 250 and ride steep stuff, you probably don't need a bigger rotor unless you like to hammer down the descents. I'm 200# and ride steep stuff all the time, and only occasionally get the front 160mm brake really smoking hot.

  8. #8
    local trails rider
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    First, you check if your fork manufacturer approves of using a larger rotor. If yes, you go find the larger rotor and an adapter that fits fork, rotor size and caliper.

  9. #9
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    With a pushbike, the percentage of front braking is even higher than on a motorbike because the CoG is higher. Using the rear brakes when slowing quickly is all but pointless, as the weight transferred onto the front wheel will be very close to 100% on most surfaces. I usually only use the rear brake to induce slides or save too-vertical wheelies.

    The larger disc is far more about cooling than added stopping power. If you use sustained braking on downhills you'll cook a 160mm disc faster than a 180, but you'd rarely need the extra torque available (how far back can you get your weight?!). I don't see why a larger disc would have worse modulation though, while i could understand how 4 pot calipers may have some pad distortion issues.

  10. #10
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    I recently upgraded from a 160 to 185mm front rotor, I did that though once it was habit to primarily use my front brake during rides.

    It's pretty easy to upgrade though - get the right adapater and a larger rotor, good to go.
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  11. #11
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    Both of my wrecks when I first started riding where from too much back brake in the wet.

    On certain wet surfaces you won't find a faster way to dump yourself of your bike.
    They both happened so fast.
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  12. #12
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    With a pushbike, the percentage of front braking is even higher than on a motorbike because the CoG is higher. Using the rear brakes when slowing quickly is all but pointless, as the weight transferred onto the front wheel will be very close to 100% on most surfaces. I usually only use the rear brake to induce slides or save too-vertical wheelies.

    I disagree that the rear brake is pointless. So long as I can get my weight far enough rearward, it's useful for that much more contact patch on the ground.

    The rear brake is also useful:

    1) for steep downhill turns where you need to manage your speed, but want the front wheel to turn somewhat freely for maximum traction.

    2) to 'weight' the front wheel in high speed turns and maximize front wheel traction.

    3) to scrub speed in high speed turns and maintain maximum front wheel traction.

    4) to stop your bike on a steep climb and hold it in place.

    The larger disc is far more about cooling than added stopping power. If you use sustained braking on downhills you'll cook a 160mm disc faster than a 180, but you'd rarely need the extra torque available (how far back can you get your weight?!).

    I would have to disagree on this point too. The fact that an 8" rotor is one inch further away from the axle compared to a 6" rotor gives it a 33% increase in leverage. As someone that has cooked more than one set of 6" hydros (that's cooking the rotors AND calipers), I can tell you that the stopping power of the 8" rotor is far superior to a 6" rotor. The 8" rotors stop much better AND I have yet to cook anything with this setup.

    I don't see why a larger disc would have worse modulation though, while i could understand how 4 pot calipers may have some pad distortion issues.

    A larger rotor (remember the part about being 33% further from the axle) requires less pressure to go from free spinning to full lockup, so it's especially easy to lock an 8" rear rotor with less pressure than that needed with a 6" rotor. So a smaller range of lever input (pressure) is needed to go from free spin to lockup with the 8" rotor = 'worse' modulation or not as easy to modulate would be more accurate.

    Think of it as having a "hair trigger" on a pistol.

    I will say that I probably could have still used a 6" rotor on the rear if I have an 8" rotor up front, but feel a 7" would still be the most balanced with the 8" rotor.

  13. #13
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    How did you "cook" the pads and rotor if the issue wasn't heat buildup? You didn't use the additional torque available on the 8", you merely reduced the heating of the disc and pads. Have you ever pulled a 6" as hard as you can to stop and though "damn, if only i had 33% more torque available"?

    Using the rear brake "for that bit of contact patch" forces the weight to shift further forward, which is why i indicated that for merely stopping on most surfaces, the front brake does all of the work. I did make sure to say "for just stopping" as i'm well aware of the uses for the rear brake. On grass with the right tyres, the rear brake will actually pull the tyre into the ground a little, but if we're just talking the physics of stopping in general i prefer a more "imagine the hypothetical bike on perfect tarmac" approach. If exceptions to the rule didn't exist, neither would the rear brake!

    Also, when using a larger front disc, you can maintain "modulation" by changing the cylinder sizes or leverage, beefing up the hand-power required to pull the brake. You still get the efficiency of the larger rotor without the snatchiness. I don't know how adjustable that value is on pushbikes, as my experience of hydraulic discs is largely on cars.

  14. #14
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    [QUOTE=Cubits] Have you ever pulled a 6" as hard as you can to stop and though "damn, if only i had 33% more torque available"?[QUOTE]

    I have. After cooking a rear brake i was pulling hard and saying 'damn i wish i have more torque available!' Would that count?
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  15. #15
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    How did you "cook" the pads and rotor if the issue wasn't heat buildup?

    The pads and caliper, of course, cooked from heat buildup. So, why did the heat build up?

    The 6" rotors build up more heat because you have to apply more pressure (ie pinch them tighter together) because they have less leverage. The extra material in an 8" rotor also dissipates heat better due to more surface area available to cool the brakes, but main reason the 8" rotors stop me better is due to increased leverage otherwise they wouldn't stop better when they are cold (as well as hot).

    You didn't use the additional torque available on the 8", you merely reduced the heating of the disc and pads.

    No, actually I did use the additional torque available, otherwise you could not explain why an 8" rotor stops me easier even when it's cold.

    Have you ever pulled a 6" as hard as you can to stop and though "damn, if only i had 33% more torque available"?

    No, but I don't want to have to pull "as hard as I can". That makes for painful forearm pain pump-up on extended descents. Also, I have had those 6" disc brakes fade significantly when they get cooked which does make you have to pull harder, just never "as hard as I can" not to mention could lead to failure of the pads. I've gone down steeper and longer descents with 8" rotors with no issues at all.

    Using the rear brake "for that bit of contact patch" forces the weight to shift further forward, which is why i indicated that for merely stopping on most surfaces, the front brake does all of the work. I did make sure to say "for just stopping" as i'm well aware of the uses for the rear brake. On grass with the right tyres, the rear brake will actually pull the tyre into the ground a little, but if we're just talking the physics of stopping in general i prefer a more "imagine the hypothetical bike on perfect tarmac" approach. If exceptions to the rule didn't exist, neither would the rear brake!

    Maybe your technique needs a little refining or your bike's design is different, but the rear brake definitely helps me stop even on level asphalt surfaces, not just the odd or hypothetical situation.

    Also, when using a larger front disc, you can maintain "modulation" by changing the cylinder sizes or leverage, beefing up the hand-power required to pull the brake. You still get the efficiency of the larger rotor without the snatchiness. I don't know how adjustable that value is on pushbikes, as my experience of hydraulic discs is largely on cars.

    If you have less leverage (as with a 6" rotor), you need more pressure (in this case friction) to stop. More friction means more heat. Use more leverage (as with an 8" rotor) and you reduce the heat. Use an 8" rotor and you buildup less heat because you have more leverage, AND you have more surface area available for cooling.

    If you want to relate this to a automobile's brakes, when a vehicle like a truck needs more stopping power, they use larger diameter drums (they are many times wider too, but diameter is the number they quote you) as the leverage they provide is what makes them stop better.

    As for modulation, a larger disc will feel more "off" or "on" because it takes less range of pressure to go from full off to full on. You don't have to be as precise with smaller rotors. You can grab a big handfull and still not be as likely to lockup...... hot or cold.

  16. #16
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    I recently read an article in MT Bike Action that said there was a study done and it determinid the larger rotors don't offer any better stopping power. I don't know if it's true or not, but I thought I'd let ya know.
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