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i'm trying to spec out a bike and want your impressions...

i'd like to build a disc brake bike that's less than 20lb and in the $2500 range.

the Specialized E5 OSBB frame can be had for $880 or so. but i think  the build is going to come in over 21 lb.

asik the carbon disc bikes are all over 3K, but it's hard to put all the pieces together and come up with a weight.

what would you guys do within those parameters or am i should i stay with canti's.

dave

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I am building an aluminum Co-motion CX-Rex 'cross frame with disc's (BB7's), a mix of SRAM Red and Force, and Mavic Crossmax 29er wheels. I am confident that it will be under twenty pounds. I think the consistent braking will save more time than a bike that is a pound or two lighter, especially in poor conditions, or if you tweak a wheel while on course.

I really hope the recent disc craze results in some better mechanical cable-actuated disc brakes. The BB7 is a 10 year old design and are a pain to work with.

I'd rather not mess with hydraulic brakes, and I think a mechanical brake could be as good for cyclocross if hydraulic concepts were reused in a better cable-actuated design (top load pads, dual pad movement, greater clearance).

Personally, I just retrofitted my disc bike with TRP mini-vs and I couldn't be happier. Two races last weekend through mud and sand, and they performed beautifully.

Earlier this year BikeRumor reported that Scrub Components, a company noted for very light and very pricey disc brake rotors, was working on bringing Alchemy mechanical disc brakes - 'a true dual actuating mechanical caliper Ultra-light system for cyclocross, road or mountain' design to market. Several months later and cross season is in full flight, there has been no further information nor mention of it at either EuroBike or Interbike and yet the company still has the same generic product page on its site.

I recently got a Specialized Crux Carbon Expert Disc, size XL (58cm). Above your price target (retail $3800), but useful for some weight comparisons. It weighs 19.5lbs with XTR pedals in the stock build. It was 2 lbs lighter than the alloy version, as I recall, which had some heavier components (Rival v. Force), but the same wheels and cockpit. The stock wheels are really heavy. I dropped a pound just swapping them out for my Enve 29er mountain wheels. You could get close to the same impact for much less cash (though much less stiffness) with some of the lighter aluminum wheelsets out there, like the Stan's Iron Cross or the new Mavic Crossmax. I dropped another 0.2 lbs with a Thomson seatpost. Going to carbon bars would save some more weight at a decent $/g ratio, but with potential durability issues.

I think the canti versions were a pound lighter with roughly the same spec. Where I ride (NorCal), I could have done linear pull brakes and been fine, but I decided that discs are the future, and I have nice mtb wheels to use. The mechanical discs (BB7s) are a bit fiddly, but with organic pads and some patience, you can get them to work well. I think you need to run tubeless or tubulars, though, because if you flat, the spare wheel is unlikely to have the disc in exactly the same place, and so it will rub unless to take another 15 seconds in the pit to adjust your brakes. Pros have all their wheels set up with identical hubs, so they don't have to think about that. Hydraulic brakes will make this all better, when they are available.

If weight is your primary concern and you ride in a drier area, it seems like linear pulls will be lighter and a bit cheaper. Put the $ savings into a light wheelset.

well,

i pulled the trigger on the crux alloy frame (49 cm - short legs!), iron cross tubeless.

For the most weight savings vs. price i  upgraded seat post and bars, went to rival, and Hayes disc brakes. it is wet and muddy much of the time here in upstate ny, so discs it is.

although in my last two mudfests, TRP eurox's were great.

As i dropped off beer for the mechs,  i peeked at the frame. It  is gorgeous!!

The welds are so good i thought it was carbon. i've never seen them so perfect.

When spec'ed out , it should be 18-19 lbs.

i'll post pics and weight when the build is further along.

dave

I just finished my build-up of a Co-Motion CX Rex.  19.5 lbs in a size 58 with SRAM Force and Mavic Cross-Max 29 tubeless wheelset.  It rides beautifully but came in too late to race this season.  I went with Avid BB7's and had very little trouble with set up.  Going to organic pads seems to give a nice progressive feel with no squeeling.   I should be able to drop another pound by next season by switching over to tubular tires/ wheels.  In the meantime, this bike makes an awesome commuter.  

18.5 lbs

great ride, feels like it has a front shock, and that's at 33 psi.

brakes are smooth and well modulated. alas, i too missed the season.

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