Thursday, January 14, 2010

SRAM PressFit30: Initial thoughts

So the SRAM guys sent me out a PF30 BB to play with (see the first post on this topic here) and I have to say: so far, I don't see that there's much point (at least for mountain bikes).

Note that I am decidedly NOT just a hater of all things new. I think the new direct mount front derailleurs kick ass. I like through axles even for XC bikes and am working on getting set up to build around tapered steerer tubes. I like it when stuff gets better, because it means that mountain biking can be even more fun for all of us (the question of stuff always getting more expensive is another story).

To whit: the claimed advantages of the system are 1) Weight and 2) Stiffness. There's also some stuff about improved bearing life, but given that you can buy an external cup King BB with a 10 year warranty for around $120 retail, I don't see the point in worrying about longevity comparisons. Existing systems last quite well.

Now I can't speak to the stiffness claim, but I submit that *crank/BB systems are plenty stiff already*. Sure, you can make something deflect less in the lab by making it bigger, but we reached the point long ago where no human being can really detect crank flex unless they're a champion track sprinter or something. And heck, some of the track champs probably don't notice or care either. I can't personally even detect significant flex in a square taper setup, for heaven's sake, and I'm a (supposedly) pro cyclist.

As an aside, I'd like to see stiffness reported in absolute numbers - ie, we attach the cranks to some rigid test structure, subject them to typical cycling forces, and measure how much the crankarms and chainrings deflect - in millimeters, or fractions thereof, from their intended positions. It's not that useful to say something is "50% stiffer!" when in fact that means it defects .001mm instead of .002mm - there's a threshold at which nobody cares anymore. But instead every year we hear how much stiffer everything has gotten - without any concrete deflection numbers to make the information meaningful.

There's also a claimed improvement in ankle clearance for the PF30/BB30 (though not stance width, which they call "q factor" in their literature), but as a *massively* duckfooted cyclist (I wear the paint off chainstays) I don't find that ankle/crank clearance has ever been a big problem, so I can't imagine it matters to more than a percent or two of the cycling population, and even to them not a great deal.

So really, since the PF30 system is significantly more expensive and a moderate pain the butt to build with (though certainly not as bad as BB30, which is just ridiculous), there had better be a significant weight advantage. But I just don't see it, after inspecting and weighing a few bits and pieces.

The BB itself is 88g. Not too shabby, but a generic SLX-level Shimano external bearing BB is around 90g (it depends a bit on how many spacers you need, but it's basically 90g). So 2g. But the PF30 shell weighs 125g, as opposed to the 100g of a plain-jane english threaded shell from Paragon. So thus far, the PF30 setup is 23g behind. And that's not to mention that if we used a nice King BB, we'd save another 20g for the traditional setup, albeit at significant cost.

Yes, you'd miter all the tubes a tiny bit (about 1/4") shorter with the PF30 shell, which helps a bit, but it's not that significant. About 15-20g worth of tubing, if you're building with steel, and depending a bit on what kind of tubes you use. So call it dead even so far.

I don't have a set of PF30/BB30 cranks in my grubby paws to weigh against their standard counterparts, and the best info I can find is this article in Bicycling, which seems to indicate that the BB30 cranks are a bit *heavier*. I'd be hard pressed to figure out a way that the PF30 crank (assuming the chainrings and bolts are the same) could shave off more than a few grams, since the only difference is in the spindle, though SRAM claims their XX crankset is 60g lighter (including BB) for PF30.

So there's a bit of confusion, but my impression is that when all is said and done, the weight is a wash. At best, you might save a couple of grams. At worst, you've actually got a heavier system than a conventional one.

And of course the PF30 is a bunch of extra work - you've got to have a setup to adapt it to the fixture, you've got to have a 2" hole saw (turned down a tiny bit on the lathe), and you've got to get set up to machine the inside of the shell to size after you're done building the frame. It doesn't require the same crazy tolerances as the BB30 standard, but it's still a lot of extra work.

Bottom line: I don't see the point. So I will probably not even bother to get a shell and build anything. Memo to SRAM: if it ain't broke, don't fix it. You guys have done lots of kickass stuff over the years, this BB30 and PF30 stuff isn't, at least so far.

Of course, I could certainly be wrong about a lot of what I've said. Maybe the stiffness improvement is night and day, and when I finally ride a PF30 bike, I'll refuse to go back. But I doubt it.

19 comments:

Anonymous said...

It is posts like this that are why I read your blog. It is very interesting to see all the technology and thought that goes into a bike.

-Mike.

Anonymous said...

Thanks for the no-nonsense review, Walt. Too bad the bike mags can't/won't do reviews like this. New does not necessarily mean better!

steve-o said...

it seems like other than slightly more slack setup woes, its not terribly different than the bb30 post you made. Too bad too, i was a little excited.

Mike said...

walt,
great post. you blog like a pro. how is the track bike coming along?
Mike in SD

Walt said...

Anon - If mags did reviews like that, they wouldn't get any more ad money or more free crap to review. I expect SRAM will never give me anything for free again - but that's ok.

Steve - I'll do a post on the new "whip" soon. Gotta get some tires and find some handlebars, and she'll be ready to go ride around in circles.

steve-o said...
This comment has been removed by the author.
Dave said...

One thing that also comes to mind after reading this: Is it possible for a ride to isolate BB stiffness from frame stiffness in the BB? Of course, the importance of BB stiffness is another topic, but I'd have a hard time believing a human can detect the difference between the two.

Feldy said...

>I can't personally even detect significant flex in a square taper setup, for heaven's sake, and I'm a (supposedly) pro cyclist.

Then again, size small shorts fall off of you and you get beaten by girls in sprints, so... ;-D

(If I don't get a, "FU, Feldman" as a response, I'll be a bit disappointed)

Walt said...

FU, Feldman!

Heh, just kidding. And sadly, I'm not even the skinniest person on the team...

Anonymous said...

"Memo to Sram: If it ain't broke, don't fix it." It is broke and that is why they are fixing it. Their standard english bottom brackets are garbage and don't last at all. This is just an attempt to fix that with a new standard instead of developing a better bb like the one King did. The ISIS people pulled the same crap. They make a bb that can't have bearings with any kind of reliability and then try to get the framebuilders to change to the "overdrive" standard to cover up their mistake. But then Shimano came up with the current outboard bearing system and made the ISIS guys look like the tools that they are. And yes, I know that King was part of the original collaborators on ISIS but once he saw that the bearing problem was inescapable he distanced himself from the rest of those knuckleheads.

Walt said...

Anon - Good point. I always had bad luck with all the Truvativ BBs, regardless of standard. If I were them, I'd have hired King to make a BB (ie, I'd have used the Shimano outboard design).

Then again, Shimano is doing a bunch of weird pressfit BB designs too.

dougyfresh said...

Do you think Cannondale has any data showing crankset deflections for the BB30 -vs- traditional style? I thought they were one of the key players pushing and developing BB30. I wonder if SRAM can add to that?

I, too, would like to see that information as opposed to a percent gain (or loss if we're talking 'deflection' in the case of BB30/PF30).

Do you also have any information regarding the intricacies of building a BB30 frameset? You elude to it in this entry but I have not searched your blog for previous posts about it. Sounds like a ton of tolerances and stuff that makes your life more of a PITA.

This is the kind of stuff I like. Guess it is the Engineer in me.

Keep on keeping on! I enjoy reading this blog.

Ennio said...

Moots for their TI roadie are enthusing over the PressFit30, check here: Moots blog
"The Press Fit 30 unit is easy to install, easy to remove and accomplished exactly what we wanted; lighter crank component, stiffer BB area, better power transfer, an outside diameter that gave a larger landing area for the down tube, seat tube and chain stays."
I'm not much buying the claim of a stiffer crankset because of the larger axle (30 vs 24), but I'd like to know your view on the claim of stiffer frame because of the larger shell. Indeed a 50mm shell is way larger than the 39mm thread BSA; does it translate into a more rigid BB area as far as the frame (considering steel) is concerned?

Walt said...

Hi Ennio -

No, I don't see this making a different for frame stiffness, at least not a noticeable/measureable one. That's mostly determined by the diameters of the downtube and chainstays (mostly) plus all the other tubes. And of course the geometry of the frame overall will make a difference.

The larger shell is great for ti builders because it makes things easier - attaching a 44mm downtube (which is pretty common in ti) to a 38mm BB shell is a huge pain. With the larger shell, it is much easier for them to do. But I'd eat my hat if it makes the frame stiffer.

Ennio said...

Thanks Walt for your informed reply. Let me ask you on the subject of tapered steerers, you said you're in favor of it. On a road steel bike I can have a tapered fork either 1.1/4" or 1.5". The 1.5", which is common for mountain bikes, makes a bulky build on a rodie and 1.1/4" seems enough for the kind of abuse, or lack thereof. Anyway I'm still debating since 1.5" forks are more ready available and also because 1.1/4 is just 3mm more than a straight 1.1/8 and so I wonder if it makes any sense. Thanks.

Anonymous said...

One of the weak point of external bearing systems is that they are more exposed to water & dirt. Is this design any better inthis respect?

Anonymous said...

I think the biggest advantage of BB30 (and thereby PF30) is the lower Q-factor. For some it's not important, but others prefer a narrower stance.

Walt said...

AFAIK, the 156q is the narrowest (mountain) crank that SRAM makes, and it's available as a standard GXP or PF/BB30. No difference in Q between the setups.

Geotrailing said...

Great article. I think its just easier for the bike companies to use, but, offer few benefits, except a wonderful creaking noise