Brakes - 160mm or 178mm?

[bartspedden]

I've been thinking about brakes for the CTR. 160mm front and rear has served me well, but never to perfection. Maybe I ride my brakes too hard, but I always ended up with modulation problems at some point after lighting things up on downhill. I've used Avid Juicy 7s (with avid rotors) and Hayes Stroker Grams (with shimano rotors - had to modify the inner pad tab so that the tab wouldn't hit the rotor spider). The Strokers had a much smoother feel to them and I generally felt safe with them.  But sometimes the levers would sink in just a little too far for my comfort.  A quick pump or two and the pressure was built back up.  I've had this experience with magura Louises on my DH bike too so I guess you could say I'm "comfortable" doing this even on fast descents. The Avids on the other hand would boil and the levers would stick out! I got used to that too, but it was much scarier for some reason.

Either way, I'm left to assume that over heating is cause of my issues regardless of brakes and my only option is too try a 170mm front rotor.

Any thoughts?  Does anyone else have issues like this?

[JCarr13]

Hey Bart.
 No such thing as a 170mm rotor. I used 185mm front and rear for the CTR. They worked well and dissipated the heat well on most descents. Stony Pass down to Silverton was the worst of it though.

[bartspedden]

Ahh - I gotcha about no 170mm.  What I meant was 178mm (7 inches) like the ones here:

http://www.jensonusa.com/store/product/BR307C09-Hayes+V7+Rotor.aspx

I'll update the title so others won't be confused.

I figure I would need an adapter too to make it work.  Glad to know that the 185s work well though.  I did the Stony-to-Silverton descent in the dark. Pretty freaky! Went very slowly.

I'm a light guy (started at 145lbs) and had another 18lbs of food/gear plus 100oz of water. So that's roughly170 lbs rolling on the bike at any given point in time. I can certainly move to the 8 inch rotors front and rear, but am pretty curious to see if anyone else has tried just beefing up the front rotor a little?  Hell, now-a-days I'm seeing DH pros run 6 inch rotors??? Different goals apparently 

[Area54]

I would go a 7" on the front and 6" on the rear, try metal pads as well. Braking is all about heat dissipation, disc systems are best run within the temperature parameters of the manufacturer and conditions/terrain. In short, too 'cold' (big rotors, light rider, low speeds) and rotors glaze, lots of noise, poor braking performance. Too 'hot' (small rotors, long descents, heavy loads, organic pad material) brake fade, rotors can warp/discolour, boil fluid (moreso when contaminated) poor braking performance overall. Match the temp range to your application and you're sweet.

Your fade issue could be due to contamination in the fluid, recommend a bleed to eliminate this as a fault.

Shimano have also downsized their rotor  sizing, however this is as a package to include cooling fins on pads and other improvements (alloy/stainless steel sandwiched rotors) to improve heat dissipation.

[bartspedden]

Thanks Area54, pretty insightful info about going too big.  I don't think I would of thought of that.  Looks like it's time for a bleed too.  I think I'll order a 7inch rotor and give it a try.

[Slim]

It's a pretty easy and cheap thing to try out indeed so just give it a shot.

From a heat standpoint, organic pads run cooler than metallic ones.

[wheelmanron]

Another thing to consider. The problems you have stated are caused by the fluid boiling and causing problems. What about mechanical brakes? No fluid, very easy adjustments, and worst case, you can carry an extra cable. 

[bartspedden]

I use mechanicals on my commuter and they do just fine. But for big DH (i.e. 3000+ foot descents) I'm just too used to the additional power from hydro's. I guess my riding style from racing DH is kinda set in my head for now. I'm not certain because I 've never tried, but wouldn't heavy/hard braking also affect mechanicals? It's interesting from a physics perspective if nothing else. I wonder if you have to increase the amount of pressure applied to the brake lever as the rotor/pads heat up on the mechanicals, and if so, how much? From what I've read, temps of 300F are common on rotors after a DH run.  I would suspect that our fully loaded bikepacking rigs would require similar braking forces.  -- side note: I once hit my calf on a rotor during a DH crash got a burn welt from it - OUCH   

And here's some more intel - like a dumy I didn't check my brake pads during the CTR because I wasn't experiencing anything too drastic, just the symptoms mentioned above that appeared more psychological then anything else.  If I would of checked them I would of noticed that I was nearly through all of the braking pad material on one of the front pads. Hayes recommends changing them with 4mm of material left.  That means I should of changed them probably on something like day 5 of the race. That was a big mistake that could of cost me.  Luckily I didn't have any issues and have learned from it.

Thanks for the idea wellmanron.  I do like the idea of mechanicals, but at this point I'm not seeing any problems with the hydros that would make me want to jump ship. I'm just searching for the perfect day of riding my bike in the mountains and the ultra smooth modulation combined with light brake lever forces are what I'm seeking. 

I'll upgrade the rotor over the winter in preparation for the AZT 300 and let folks now how things turn out.

[Slim]

Mechanicals can suffer from heat issues too. In a worst case scenario the plastic internal components can melt, as noticed by Santana testing them for tandem use.

The other more likely issue with using them on longer sustained descents is that as pads wear you have to manually adjust them closer to the rotor, perhaps mid run.

[Area54]

I'm running Avid BB7R s on teh Fargo, with drop bars etc and about to upspec my rotors for next season. Only had 6" rotors in the spares bin when I built the bike, but yep really need to upsize. Same predicament, working through the options of 7" or 8". I haven't been happy with the braking on the 6" rotors.

Another thing to consider is the max rotor size recommended by the frame or fork manufacturer - ie 7" max for a SID, 8" for a REBA, and also clearance on the chainstay for rear rotor. I had to modify the non-drive side of our tandem for more clearance to fit a 7" rotor.

[Area54]

Some more thoughts on this subject. Leaning more towards keeping the same sized rotors front and rear, biggest reason is if a rotor gets bent up, you can interchange them to suit the best braking option, without an issue.

Decision made, 7" front and rear.

[bartspedden]

Interesting thought.  If I had to choose between a front or rear rotor, I would choose front because of the greater braking force. So, if the front rotor got mangled, I guess I could always remove the required fork adapter for the 178mm rotor and run the 160mm up front.  I'm not sure I would of ever throughout about that if you didn't bring it up Area54, thanks!

[gregclimbs]

...I guess I could always remove the required fork adapter for the 178mm rotor and run the 160mm up front.

Except that you can't (unless on a fatbike with same fore/aft brake spacing)...

The rear and front spacing are different - there is ~5mm difference in where the rotor mounts relative to the dropout face.

I'd run the same rotor size like area45 mentions...

[JReeves]

While we're talking about brakes...  I'm on the fence between hydraulic and cable on the new build. 
Custom titanium rig is being made as we speak, with a main emphasis on bikepacking.  I have a full set of Avid Elixir CR's in the box that I got from Avid as a warranty replacement for the Juicy 3's I was having a problem with.  I have the same Elixir's on my race bike, but I'm wondering if it's worth selling them and running BB7's on the new build.  I love the feel and performance of the hydraulic, but with plans of riding the CTR this year, I'm curious as to how realistic problems arising are with hydraulic.  I've dug though threads on the forum and it seems that people are pretty split down the middle. 
I know it's neurotic, seeing as either way you could finish the trip on one brake if needed.  The simplicity of the BB7's have an appeal, as trail side repair isn't out of the question.  But, is the extra power and "feel" of the hydraulics enough to outweigh the drawback of a more problem "prone" system?

[Area54]

What may also be different is the frame may be IS mount, and the fork may be post mount - two very different mounts and specific adaptors required to mount the caliper. Need to carry adaptors too, not feasible.

Hydraulic versus cable, hmm, people will have polarised experiences. Important to note the strengths and weaknesses of both systems, them make a decision based on the facts. One thing I see in the shop a LOT is hydro calipers with imbalances in piston stiction, mainly to do with poor maintenance (clean/lube pistons and seals, bleeding), poor alignment when replacing pads (then causing irregular pad wear and piston misalignment and excess stiction during retraction), contamination (dirt, air, water) - and this is just on bikes that tootle around local trails. Ive seen calipers do strange things once the pads are worn to 20% or so of material left. Add in the longer duty cycles and potential contamination of a long back-country tour with all manner of terrains, ups the potential for problems with a system that is performing marginally to start with. But in saying that, plenty people ultra race on hydro with no problem, certainly can't say anything against the solid history of hydro in that field.

Cables aren't all hunky either, the cables are subject to contamination with mud/water/dirt/corrosion, but can be reduced with a sealed kit from Gore or similar, but at least with many calipers you can adjust the pad reach independently (BB7 eg). Cables break. Not self adjusting like hydro. Differing modulation compared to hydro (with servo-wave or taper bore levers), But able to incorporate inline CX levers for drop bars.

These are just a few things I can think of during lunchbreak.

[bartspedden]

Except that you can't (unless on a fatbike with same fore/aft brake spacing)...

I'm confused here. My front and rear rotors are exactly the same. Of course I can switch them if needed. So that means that if I switch to a larger front rotor and bend it, I can remove the adapter use the rear rotor.

[chrisx]

http://www.universalcycles.com/shopping/product_details.php?id=178&category=13
http://www.universalcycles.com/shopping/product_details.php?id=16651&category=13
http://www.universalcycles.com/shopping/product_details.php?id=46833&category=13
The adaptor on front and back are not the same.  There seems to be a 20mm difference.  More than likely the box included adaptors for front and back.  Good thing I read this thread, I did not think to put a 160mm front adaptor in my tool bag. The larger front rotor is nice to have.  I went with the 200mm on the front,  because that is what they had in stock that day I decided to put a larger rotor on the front.  The BB7 brakes are reliable, and trouble free so far.    They are also heavy, and less precise than hydraulic brakes.  More like bus brakes instead of sports car brakes.  BB7 pads are easy to change, no tools needed. 

[Pawel]

JReeves, if I were you I would stay with Avid elixirs. CTR was very muddy in 2010 and I was happy with my hydraulic brakes. But my Rohloff cables got clogged. Hence I would use BB7 only with good, sealed cables. Nevertheless I prefer hydraulic brakes over mechanical because of better modulation and lighter lever action.

Regarding rotors 180 mm in front is definitely helpful on CTR downhills.