Friday, January 01, 2010

Vacation post: Sarah's FS bike

I don't usually post build lists, but I thought Sarah's new FS bike was worth a little bit of a writeup, because it's got a lot of interesting tweaks both partswise and in terms of geometry. Also, it's VERY similar to what a couple of my friends have been pondering (Eszter? Hassan?) so I thought I'd save some effort and do a writeup. Also, I've got some kind of weird insomnia going on tonight. Too much red wine?

First off, the geometry. Couple of things worth noting, first of which is that the effective chainstay length (with the rider onboard) is a little under 445mm (so about 17.5") That's about as short as I can really conceive of going using the Ventana rear triangle, simply because you run into tire/seat tube clearance problems if you go much shorter. Honestly, if you need shorter chainstays than this, on a full suspension bike, IMO you probably should not be on a 29er. I generally also don't go longer than 470mm, though I guess I could for a very tall rider if needed.

Second is that this bike uses a custom built lightweight rear end. Sherwood is great about doing these (as long as I give him a few months warning), and for smaller riders, it's a great way to save some weight (at a pretty reasonable cost). The standard Ventana rear end is SUPER burly, more than sufficient for 250# people, and hence also overkill for a lot of smaller folk. In this case, we've bumped down the diameters and wall thicknesses of the seat and chainstays, and gone to double sets of bearings everywhere except the main pivot, which stays with quad bearings (yes, you could go to doubles here too, but Sarah is pretty aggro and will appreciate the extra stiffness, I think). Total weight savings over a standard rear end (including the rockers) is about 275 grams. Going to double bearings at the main pivot would save another 25 or 30, but would sacrifice some major stiffness in the rear end.

For those considering this option, be aware that if you're over 150 pounds or so, we have to start adding back in some significant weight to make everything strong/stiff enough. For those over 175 or 180, the standard rear end is the best option. Extra cost is $150, and the lightweight setup is available for 650b and 26" setups as well. Just remember: I need advance warning if you want one of these, as it takes a couple months for the Ventana guys to get it done.

This is fairly close to the lightest FS frame I'd ever do (at least in 29er form) at about 5.8# with the RP23. Some 8/5/8 tubes (I used 9/6/9, since it'll be going on the plane in a suitcase sometimes) would take it down to 5.6 or maybe 5.5, but that's about as light as it could get without being fragile. I have not weighed the entire bike, but I'm guessing it's about 26 pounds with Crank Bros pedals. Some silly light tires might take it down below 25, but Sarah likes her Ignitors and Weirwolves. And of course you could go nuts and put carbon doodads and ti bolts everywhere. Not interested. She's fast enough as it is, I don't need to make myself work even harder to keep up. For a budget around $4k-4.5k (for frame/fork/parts), I could see building a bike like this up in the ~24 pound range if you were clever about parts choices and not a clyde.

Otherwise, it's a pretty straightforward design - 95mm rear travel, 90mm front, 22" effective toptube (that's equivalent to more like 22.6" if you correct for the steep seat tube angle and corresponding setback post that isn't shown in this shot, though), 13.25" BB height unsprung, 70.5HTA with a 46mm rake Reba.

Partswise, the main notable things are the fact that it's got a cool direct-mount front derailleur (first impression: it shifts freaking awesome), and Avid Elixir 5 brakes. So what, you say? Well, for once I'm not in the mood to endlessly gripe about parts...

I've talked about the derailleur in a previous post and don't want to repeat myself, but the brakes are pretty cool - inexpensive, light, super powerful and nice modulation, and so far (I've got a set on my bike as well that I installed in the fall) very reliable. I'm a brake power snob and had a 180 rotor on the front with my XTRs - actually had to replace it with a 160 because the front brake was disconcertingly powerful. I could easily see running a 140 in the rear and not having any issues with these. And given the price (I believe I sell them for about $80-90 a wheel for frame customers) they're darn hard to beat. Their more expensive brothers (the Elixir CR and CR Mag, or whatever) just seem to have lots of weird unnecessary adjustment features (the 5s only have reach adjust - nothing else) and weigh/work the same. But they cost 2x as much, if not more.

But damn does that white look ugly.


steve-o said...

Avids work great, but they are a royal pain in the ass to bleed. Pure seething red-hot hatred is what i see when i pull a ticket with an avid brake bleed.

Anonymous said...

I assume you meant Single Bearings on all the pivots (Ala Ventana's Fuego) and then quad bearings at the main pivot?

And cool idea on the tiny rear tri.

Boone said...

whats a Clide?

Boone said...

what is a "Clyde"