Building a road bike with disc brakes

mamil_man

Firstly I was going to post this in 'Workshop' but whenever I do that I get told I am not logged in. Stupid message board!!!

I’ve been having so much fun cycling to work I’ve decided to build another bike.
Today I’ve ordered a radially mounted disc-ready cyclocross frame with the intention of making it road focussed. The frame has a 135mm rear axle.

My budget doesn’t stretch to hydraulic road discs - even if they were either available or had not been recalled.

So… I’m going to try some mechanical discs.
Only the frame, seat post, bars and crank/BB have been ordered up to now & I was intending to use Shimano 5700 for the gearset.

I am obviously keen to ‘crowd source’ experience (as opposed to opinion) on road discs brakes before laying down any more cash on parts.

As I see it I have 2 choices.
Avid BB7-SL’s or Shimano BR-CX77. I’d prefer the Shimano but I’m damned if I can find any info on line about them bar links to people selling them or Bikeradar’s own press release reprint.

So any idea’s are welcome.
What size rotors should I be using? - looks like 140mm or 160mm. My commute has a steep hill with a ’T’ junction at the bottom where I have to slam all on.
Floating discs? On Mechanical set up do they offer any advantage over fixed?
My largest concern is cable pull from a 5700 lever to whatever I fit, be it too much or not enough?

Cheers all.
R.

ugo.santalucia

I've had BB7 and Hayes CX 5. They are both very good. If you get the BB7, get the road ones and you won't have lever pull issues. I use 160 mm discs, which seem adequate, don't feel the need to go smaller to save 10 grams or so
I really don't understand the rationale behind floating discs... maybe someone can enlighten us

poppit

Floating rotors don't really 'float' it's just a term to discriminate them from 'standard' rotors. The outer steel braking surface is mounted onto an aluminium inner spyder, this makes the the rotor as a whole lighter than a solid steel one and dissipates the heat build-up better. It saves on heat being conducted into the hub, which can cause bearing problems, and, as they don't get so hot, minimises warping in the rotor.

I've never used them on my MTBs but have looked into them a bit as I can easily smoke my brakes up on a long descent.

ugo.santalucia

poppit wrote:

Floating rotors don't really 'float' it's just a term to discriminate them from 'standard' rotors. The outer steel braking surface is mounted onto an aluminium inner spyder, this makes the the rotor as a whole lighter than a solid steel one and dissipates the heat build-up better. It saves on heat being conducted into the hub, which can cause bearing problems, and, as they don't get so hot, minimises warping in the rotor.

I've never used them on my MTBs but have looked into them a bit as I can easily smoke my brakes up on a long descent.

Not clear how they dissipate heat better, yet don't heat up the hub... shouldn't be one or the other?

poppit

Not sure, think its just due to the different metals and the rotors having a slight disconnect due to the join between the two. I wasn't convinced which is why I don't use them, there will be more people with greater knowledge in the MTB forums.

desweller

Weren't there some cases of tourer or tandem cyclists softening the aluminium in those rotors enough to squeeze it out on descents?

rafletcher

The better heat dissipation is because aluminium conducts heat better that steel can. Melting the spiders can happen if you "drag" the brakes - keep them on for extended periods on a descent. Doesn't affect racers of course.

Third option - TRP Hy/Rd mechanical/hydraulic calipers. £100 per wheel.

andy_s_t

I have TRP Hy/Rd calipers on my Croix de Fer, love them! definitely a big improvement on mechanical discs and work perfectly with the Shimano 5600 Shifters, I have 160mm front and rear. If anything they can be a litte too powerful and you have to get used to how easily you can lock the wheels up. With a full backpack on your back going downhill in the pouring rain they give you a lot more confidence than normal rim brakes, I have 105 brakes with swiss tops on my other road bike, and the Hy/Rds blow them away.

veronese68

Happy with the BB7s on my bike, not tried the other options though so can't really comment. Regarding disc size if the frame is post mount that may dictate the size of rotor you can fit. If the frame is made for a 160mm disc you won't be able to fit a 140mm. My Kinesis is made for 160mm and I'm more than happy with the stopping power on that.

Ouija

Whatever you do don't put Hope floating style rotors on a BB7 style mechanical disk if the rotors are 160mm or smaller.

The bolts that connect the inner aluminium spider to the steel outer ring are wider than the rotor itself and on 160mm rotors the top of that circular bolt goes into the aperture of the caliper as it rotates. It's just slightly narrower. On a hydraulic system, the pads both move inwards and clamp on to the rotor in the center of the calipers aperture but on a mechanical like the BB7 one pad flexes the rotor slightly towards a static pad on the other side. As the rotor flexes slightly that bolt on the rotor then catches the edge of the calipers aperture causing all sorts of mayhem (noise being the least of it).

Not a problem on larger 180+ rotors as the distance from the rotors bolt to the outer steel ring is further, so the bolt swings past the underside of your caliper but on the shorter 160mm version it goes into the caliper (at least the top half of it does). I use my Hope floating rotors on some clarks mechanicals now as they have a wider aperture and the caliper has a different shape that allows it to work with them but Avid BB5's and BB7's don't.

mamil_man

Cheers guys. Some interesting info there.

I hadn't considered the issue with the floating disc join bolt fowling the caliper but a quick look at a few high res pics of rotors shows exactly how it might be a problem.

I like the look of those TRP HyRd... and seeing as the Whyte Suffolk is fitted with them paired to 5700 shifters it looks like compatibility isn't a problem... just getting my hands on them might slow the build down.

Thanks again for the advice... lessons taken from this.
R