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Old 11-24-09, 10:19 PM   #1
bigpaul652002
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rotor size for clydesdale

rotor size for clydesdale

I am putting together a surly km rigid and trying to decide on rotor size. Me 6'5" 300lb so i was thinking a little bigger 185mm . My mtb know is a 96 trek 930 that i changed into a 69er.

I ordered some avid bb7 185mm with xt v levers

Last edited by bigpaul652002; 11-27-09 at 01:49 PM.
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Old 11-25-09, 07:03 AM   #2
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Just remember that if you go to a bigger rotor size, you'll probably need the adapter plate from your brake manufacturer, as most are sized for 160mm.
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Old 11-25-09, 08:33 AM   #3
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Bigger never hurts. The weight penalty is negligible, all things considered. What calipers/levers are you thinking of running?
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Old 11-25-09, 08:48 AM   #4
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The penalty for mine was noise. 160/170 was noisy...140/160 is quiet.
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Old 11-25-09, 09:07 AM   #5
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Originally Posted by bigpaul652002 View Post
rotor size for clydesdale

I am putting together a surly km rigid and trying to decide on rotor size. Me 6'5" 300lb so i was thinking a little bigger 185mm . My mtb know is a 96 trek 930 that i changed into a 69er.
I'm up there in your weight range. And ... I ride a Karate Monkey. I have a 185mm on the front and a 160mm on the rear. You do want a smaller rotor on the rear than on the front as it will reduce the incidence of rear wheel lockup. Most everywhere you brake you will have the front wheel loaded WAY more than the rear brake.

If buy a brake kit at 185mm or 203mm it will come with the appropriate adapters. But do beware that your fork must be rated for a 200+ mm rotor. I'm pretty sure you can't put a 200+mm rotor on the stock rigid fork.
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Old 11-25-09, 10:48 AM   #6
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Sure ya can....just use some dang good skewers!!
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Old 11-25-09, 01:10 PM   #7
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Originally Posted by BearSquirrel View Post
I'm up there in your weight range. And ... I ride a Karate Monkey. I have a 185mm on the front and a 160mm on the rear. You do want a smaller rotor on the rear than on the front as it will reduce the incidence of rear wheel lockup. Most everywhere you brake you will have the front wheel loaded WAY more than the rear brake.

If buy a brake kit at 185mm or 203mm it will come with the appropriate adapters. But do beware that your fork must be rated for a 200+ mm rotor. I'm pretty sure you can't put a 200+mm rotor on the stock rigid fork.
Ok.......So why do freeride and Downhill rigs come with 8" rotors front and rear? I've been running 8" rotors on the front with QR forks for a long time with no issues.
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Old 11-27-09, 01:47 PM   #8
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Originally Posted by Grimlock View Post
Bigger never hurts. The weight penalty is negligible, all things considered. What calipers/levers are you thinking of running?
avid bb7 185mm with xt v levers
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Old 11-27-09, 02:28 PM   #9
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Quote:
Originally Posted by BearSquirrel View Post
I'm up there in your weight range. And ... I ride a Karate Monkey. I have a 185mm on the front and a 160mm on the rear. You do want a smaller rotor on the rear than on the front as it will reduce the incidence of rear wheel lockup....
Same set-up here, but I'm a svelte 230#.
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Old 11-27-09, 02:36 PM   #10
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There was an old rule that 205 mm rotors should be used only with 20mm hubs, since there was supposedly added risk of pulling the wheel out of the dropouts under hard braking.

However, as noted, many riders have used this setup with no ill effects.

I would not, however, then proceed to grind the lawyer-lips off the dropouts, or fit an open-cam titanium skewer.
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Old 11-27-09, 03:04 PM   #11
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I'm 190-200 pounds depending on how many beers I have. I have 160mm front and rear on my 29er and they work great. I wouldn't mind 185 on the front, but I'm having a hard time finding an XT rotor in centerlock for a decent price
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Old 11-27-09, 04:02 PM   #12
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I am in a similar class for size. I only run 8" rotors. I find 6's adequate, but not good enough.

Never had a problem with QR vs 20mm...been running 8's for years in almost any condition of riding.
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Old 11-27-09, 09:52 PM   #13
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At 310, I noticed a huge difference when I swapped my stock 6" front for hydraulic 7". The sixes never failed to stop me, but the sevens are waaay better. Go ahead and put them on the rear also, but do try and learn good braking technique to avoid skidding. If you happen to be in a situation where your weight is to the back and you need those holders, you will appreciate them back there too.
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Old 11-30-09, 10:38 AM   #14
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Originally Posted by mtnbiker66 View Post
Ok.......So why do freeride and Downhill rigs come with 8" rotors front and rear? I've been running 8" rotors on the front with QR forks for a long time with no issues.
There is an engineering department, a manufacturing manager, a marketing department and a finance department. The engineer doesn't always get their way. Specing different size parts on the front and back is likely more of a hassle and increases the cost of the bike (I doubt that bike companies pay significantly more for larger rotors and spacers like we do). For an example of my point, look at inexpensive cars that typically come with disc brakes in the front and drum brakes in the rear. You really don't sacrifice anything since under heavy braking only the front wheels have enough traction to slow the car.

I have noticed recently that more and more bikes ship with assymetric rotor sizes. This decreases the overall weight of the bike and doesn't sacrifice any performance. On a 26er assymetric XC setups are 160/140 mm. On 29er XC bikes its a 185/160mm. Look for this to become the pattern as time goes bike.

For the person who suggested just using 203mm on a non-rated fork ... DON'T. You risk a catastrophic failure. And it will only fail at the WORST possible moment under emergency braking. If you're putting that much energy into the system, that fork probably would not survive anyway. At 280 pounds, I've bent two of them in crashes. OP, if this is your original KM, you may wish to consider going straight to a 20mm or 15mm maxel. I switched my KM to a suspension fork and ended up being stuck as I have three wheelsets, each of which is quick release. It would have been VERY costly to switch. At the very least consider a convertible 20mm front hub with an QR adapter.

Two parting thoughts:
1) Surly capabilities - the rear end of the Karate monkey has track ends with a rotating disc mount. You cannot get the wheel off using a round rotor. It is in your best interest to put a smaller rotor here. With a 160mm serrated rotor, you can get the wheel off without undoing the brake mount. I"ve heard limited success with larger rotor sizes.

2) I went riding this weekend in slightly muddy conditions. With a 185mm/160mm Avid SD7 setup for maximum leverage on the front, minimum leverage on the back ... I was STILL locking up my rear wheel too often for my comfort. The rear wheel IS just a helper under maximum braking conditions (downhill).

Last edited by BearSquirrel; 11-30-09 at 10:41 AM.
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