Scooped or fastback lugs?

[rnhood]

Can someone tell me why some lugged steel bikes have the beautiful scooped style seat tube lug while some have the almost as beautiful fastback style lug? Is there an advantage, or disadvantage in one over the other?

[dave thompson]

I don't think there is any advantage one way or the other except for personal esthetics. I have the 'scooped' or side-tack style seat stays on my Kirk and am getting the rear triangle changed shortly. The new seat stays will be in the fastback style.

[Smiley]

Dave is Kirk going to funk out the rear end of your bike frame? What's this like a make over ? Would like to know what the stays are all about, thanks

[dave thompson]

Smiley: I wanted a new bike this year so why not a makeover of the old? That bike is so damn near perfect for me I would hate to change it, or relegate it to a secondary role. But in talking to Dave this afternoon (I sent him my bike for a repaint) everything he told me about his curved stays convinced me that they will do pretty much what the DKS and ST rearends will do, just not quite as much travel. He's done a lot of prototyping and testing using his personal bike as a mule, and likes what the finished product does.

Now you know how much I've lusted for a Hors Cat and couldn't find one in my size, so I bought another brand Ti bike with curved chainstays. So just yesterday this 'new' Kirk popped up on the radar and I committed to the change. Personally I think the curved rear triangle has a certain elegance and beauty if nothing else, and it will certainly be different. Everything Dave Kirk has done for me, and for others including Serotta, has been well thought out, well researched and executed, so I see no reason not to go with his latest direction. Plus the 'makeover' is far less than the cost of a new bike, so that means my bike budget is still pretty intact. More toys!

[Smiley]

Dave you and Dr Kirk whom I have fondly called the Absent Minded Professor make a great team. I for one don't mind the looks of Dave's new idea, Heck I liked the looks of the DKS and I am all ears my friend when you tell me what this make over bike will ride like. It will be especially interesting since its the exact same bike you owned, which will make the comments more precise. Is Dave Kirk going to chime in here or just wait till you get the product and tell us about it. Has he used a softer stay or what has he done to get the vertical compliance he needs without the pivot. Would his work be better served if he had access to the use of a pivot in his tinkering . I guess if he can make the chain stays flex without a pivot , what would he do with a pivot , interesting that he's doing it with steel too.

By the way I've seen his bike and I think he'd take offense to you refering to it as a mule. Maybe a skunkworks :)

[dave thompson]

Smiley:

I should be able to tell you how the 'new' rear works in about three weeks or so, after the frame comes back from paint. New colors too.

I don't think Dave could use a pivot with his set-up, it would probably infringe on the Serotta-held DKS patent. But he seems pretty satisfied with the results. Here's a quote from his e-mail to me "...And yes they do a very nice job of keeping the wheel stuck down. They are also very durable. I rode a test mule over some of the nastiest singletrack dirt this fall and they hold up very well."

I don't think the curved stays will offer the same plush ride as the DKS, but it does keep the rear wheel on the ground and that's what I'm after. A lot of the riding I do here in Eastern Washington/Idaho/Montana involves crossing mountain passes and some pretty interesting descents. Being the old-guy wuss that I am, I want the feeling of control at speed, like the DKS. The Kirk is an able descender, but on not great surfaces any bike can become sketchy and I need all the help I can get.

And I gotta tell ya', I'm really drawn to the curvy esthetics of the rear end. If you look at the "What's new at the Frameworks" link on his homepage, you can see how the seatstays compliment the curve and taper in the front fork. Maybe the curvy lines remind me of my wife(!)

[rnhood]

After reading this thread I had to surf over to DK's website and look at those curvy seat-stays that seem to be garnering so much comotion. Well, I'm impressed! Not too much curve so as to make it gaudy. And, not too little. Just the right amount of bend to give the bike a unique character of it's own. Very nice. Interesting how just minor touches can sometimes project an entirely new character. Part of art I guess. Anyway, I like the looks of this better than the dks system and, if it improves the ride then it is doubly beneficial.

[MadRocketSci]

From what i remember taking an aerospace structures class 12 years ago, if you want to make something stiff, strong and light, put your elements (think bridge trusses) in compression, minimize bending. Thus, the straight stay is most weight efficient at carrying axial loads. The curve causes the stay to carry bending moments, have more stress in the stay, and thus more deflection. I wonder how big an issue fatigue is here...

ie, push axially into a straight piece of metal. can you get it to compress appreciably? probably not. but it's a lot easier to bend it...