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Disc Size Advice
http://www.retrobike.co.uk/forum/viewtopic.php?f=41&t=251073

Author:  NeilM [ Sat Apr 06, 2013 7:55 pm ]
Post subject:  Disc Size Advice

I'm going to be building up my first ever disc equipped bike soon.

In preparation I bought a 160 and a 180 rotor, planning (motorcycle style) to have the bigger rotor on the front. I am now advised by my son that he would have used 160 / 160 and that a 180 will be overkill on a lightweight hardtail (Dave Lloyd CATS WISKAS).

Your views please: Do I get a second 160 rotor? Or do I stick to my original plan?

Author:  cyfa2809 [ Sat Apr 06, 2013 8:32 pm ]
Post subject:  Re: Disc Size Advice

If you are setup for the 180 already then its no harm in sticking with it.

Author:  Matthews [ Sat Apr 06, 2013 9:10 pm ]
Post subject:  Re: Disc Size Advice

I run a 203 180 combo on a lightweight HT, you can never have too much braking IMO. I'm not a weight weenie so the extra few grams on the rotors and adapters don't bother me but being able to haul anchor at any speed down any hill makes me happy . :D

Author:  NeilM [ Sat Apr 06, 2013 9:18 pm ]
Post subject:  Re: Disc Size Advice

COOL!

In that case 180 & 160 it is.

Kid's eh? What do they know? :roll:

Author:  theredchili [ Sat Apr 06, 2013 10:43 pm ]
Post subject:  Re: Disc Size Advice

I'm running a 180/160 on my single speed, I do ride it hard though ,as you already brought it,use it, otherwise a 160/160 would be fine with a few grams saved.

Author:  valve bouncer [ Sun Apr 07, 2013 4:37 am ]
Post subject:  Re: Disc Size Advice

That combo will be fine. Having a 203mm on a 9mm hub is not recommended as it can apparently generate enough torque under braking to rip the wheel out the dropouts. (Although I have this set-up atm on my FSR, waiting for a 20mm front wheel)

Author:  NeilM [ Sun Apr 07, 2013 7:20 am ]
Post subject:  Re: Disc Size Advice

valve bouncer wrote:
Having a 203mm on a 9mm hub is not recommended as it can apparently generate enough torque under braking to rip the wheel out the dropouts.


:shock:

Author:  02gf74 [ Sun Apr 07, 2013 10:42 am ]
Post subject:  Re: Disc Size Advice

disc brake rotor sizing depends on rider weight and terrain.

bike manufacturers tend to fit 180/160 for L hardtail and FS frames due to the weight of riders on those frames, and probably the same for M but typically 160/160 for M. For downhill 203 on the front is common.

Also the type of rotor makes a difference - I am heavy rider but try to weightweenie my bikes so all of then are on 160/160 - I find this good enough for the sort of riding I do, namely XC. I have 2 bikes with 160/140 but find the 140 too weak - will be replacing them with 160.

Lightweight discs achieve that by being 0.2 mm thinner plus have metal removed from the braking surface - e.g. KCNC razor - I find they are less grippy than more solid disc e,g, Hope SAW or Avid XX.

Without knowing rider weight and disc rotor models, hard to say but I would be surprised if 160;160 is not good enough.
BTW if you have post callipers and post mount fork, the difference between fitting 180 or 160 on the front is an adapter (spaces the calliper futrher out) and longer M6 bolts.

Author:  Mindmap3 [ Sun Apr 07, 2013 12:58 pm ]
Post subject:  Re: Disc Size Advice

If you've bought the rotor etc, I'd leave it as it is.

I tend to ride with bigger rotors because I like lots of braking power. My Chameleon used to have 203mm front and rear (but that was mainly because the brakes were off my DH bike that I split and sold. I was too lazy / tight to buy new rotors and mounts).

Modern brakes are pretty powerful so you can get away with smaller rotors.

Author:  Barneyballbags [ Mon Apr 08, 2013 11:40 am ]
Post subject:  Re: Disc Size Advice

I'd prefer my brakes to be more than enough for the job, rather than "good enough".

There will always come a situation where you're going to need good, reliable, powerful brakes! A larger rotor means better cooling properties (so less chance of fade) which in turn leads to generally better control over braking and the bike in general.

I always used to fit the biggest rotors I could get away with on all my bikes, whether they were for XC, Enduro or DH duties. Admittedly, you don't need a 203 rotor on the rear, but I ran my Cove Stiffee with a 203/203 Mono M4 combo. There is something very reassuring about knowing that you have enough braking power for any situation, and the small weight penalty was well worth it.

I ran a 238/203 combo on my Intense M1 :shock: - Hope had to send me a custom size rotor for the front brake as I was running an early set of 2003 Monster Ts which had a really odd mount size! I remember sticking the 203 rotor on and it not even coming close to the pads!

Author:  NeilM [ Mon Apr 08, 2013 12:11 pm ]
Post subject:  Re: Disc Size Advice

Thanks for all the information.

For now I am going to stick with my original plan.

The frame is at Argos at present, I'm due to take a look at it next week and give the yes / no on the new rear disc mounts, after that I have to wait for the phone call... and the bill :shock:

Author:  MikeD [ Tue Apr 09, 2013 9:32 pm ]
Post subject:  Re: Disc Size Advice

valve bouncer wrote:
That combo will be fine. Having a 203mm on a 9mm hub is not recommended as it can apparently generate enough torque under braking to rip the wheel out the dropouts.


Nope. All else being equal bigger rotors generate a lower force at the axle, but a higher force on the fork. Not all forks are rated for 203mm rotors, nearly all will take 180 though.

The weight of the bike isn't all that important (assuming that the frame/fork will take your desired rotor sizes), it's the weight of you that you're mainly trying to stop ;)

Or, in a nutshell, 180/160 will be fine.

Author:  valve bouncer [ Wed Apr 10, 2013 6:32 am ]
Post subject:  Re: Disc Size Advice

MikeD wrote:
valve bouncer wrote:
That combo will be fine. Having a 203mm on a 9mm hub is not recommended as it can apparently generate enough torque under braking to rip the wheel out the dropouts.


Nope. All else being equal bigger rotors generate a lower force at the axle, but a higher force on the fork. Not all forks are rated for 203mm rotors, nearly all will take 180 though.

The weight of the bike isn't all that important (assuming that the frame/fork will take your desired rotor sizes), it's the weight of you that you're mainly trying to stop ;)

Or, in a nutshell, 180/160 will be fine.

That's good to know then, was always told don't run 8" rotors on 9mm quick releases. I'm running my 8" rotors on a double crown (Jr T) so I should be OK. Thanks for setting me straight.

Author:  dyna-ti [ Fri Apr 12, 2013 3:00 am ]
Post subject:  Re: Disc Size Advice

180/160 is perfect.

Author:  NeilM [ Fri Apr 12, 2013 7:56 am ]
Post subject:  Re: Disc Size Advice

dyna-ti wrote:
180/160 is perfect.



That's what I thought, but as I've never used them on a bike, as opposed to a motorbike, I was just a little unsure.

Author:  dbmtb [ Fri Apr 12, 2013 4:50 pm ]
Post subject:  Re: Disc Size Advice

I use 160/160 on the Pace twins as the forks stipulate rotors of that size. But these bikes never get ridden down mad stuff....
Tried 180/160 on the mad stuff and found that I still needed a bigger rotor at the back.

But 180/160 rides better for most XC. Ignore your offspring. I make a habit of it!

Author:  dbmtb [ Fri Apr 12, 2013 4:52 pm ]
Post subject:  Re: Disc Size Advice

dbmtb wrote:
I use 160/160 on the Pace twins as the forks stipulate rotors of that size. But these bikes never get ridden down mad stuff....
Tried 180/160 on the mad stuff in Germany and France - and found that I still needed a bigger rotor at the back.

But 180/160 rides better for most XC. Ignore your offspring. I make a habit of it!

Author:  NeilM [ Fri Apr 12, 2013 4:56 pm ]
Post subject:  Re: Disc Size Advice

dbmtb wrote:
Ignore your offspring. I make a habit of it!



Sound advice for every occasion.

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