Thursday, January 22, 2009

F*^%ing seatstays!

So I spent most of the day making some S-bend stays for Dan's frame. And I learned several nice lessons:

-Making S-bends with enough tire clearance (in this case, we wanted 70mm minimum) for small 29er frames (this one has a 15" seat tube) is a PAIN. In the end, the stays had to end up being only very subtle in their s-bendiness. I might need to make a tighter radius bending mandrel for this sort of situation - the 6" radius wasn't tight enough to allow both bends AND sufficient tire clearance. Maybe I'll do a 5" or 4.5" radius mandrel sometime soon.

-Laying the stays on a flat surface (table) and letting the bent portion hang off is an easy way to assess if the bends are symmetrical. Of course, they're never going to be *exactly* the same, but this method is good enough, as long as you've got a decent eye for detail. Thanks to years of messing with bikes, I do at this point.

-The bender itself needs some work. The mandrels are great (though Jason was right - they have started to warp slightly on the back side - though I don't think this will end up being a problem) but the cradles/spine probably needs to be rebuilt/rethought. They tend to hang up, and the cradle holders are starting to bend from the force of bending the tubes. Not so good, especially if I want to bend beefier stays (these were just 16x.7mm cromoly ones, fairly strong, but nothing crazy). I need to make some cradles that are shorter (they hit the mandrel when trying to do tight bends) and possibly also that fit the radius of the tube better.

Long story short, I spent like 4 or 5 hours on arguably the least important structural portion of the frame today, all in the process of trying to get a spiffy look. What have I become?

The frame is freakin' sweet, though, especially now that it's *finally* done.


Dan Paulson said...

sorry for the hassle on those seatstays!

Looks pimp though...can't wait to have it in hand.

Anonymous said...

Well...thanks for sharing, but from the second image the point of curvature on the chainstay doesn't seem gentle, as it ought to be, but dented. On my "Corporate" bike the this point is clean. Maybe it's just the image?

Walt said...

Hi Anon -

I'm not sure what you see in that second image - it's pretty much impossible to tell there's an S-bend at all there.

There IS some magic marker marking from where I marked to to bend. I'm guessing that's what you're talking about.

Magic marker is one of the most used tools in the shop, believe it or not. And it's how I mark the spots I want to bend so that they're in the same place on both stays.

Does that clear it up?

jkeiffer said...

Wow, what's next? Curvy fancy boy bikes? You are starting to slip. You may have to update your site:

steve garro said...

welcome to my hell.......steve.

Walt said...

J -

It's a good thing hypocrisy doesn't bother me, huh?


MMcG said...

Are the rotor braces the request of the customer or something that you are now incorporating into things on your own Walt?

Walt said...

Mark -

I do them unless the customer requests otherwise. The weight is about the same as using a piece of tubing, and the work is about the same, too. And I think they look neat. So all things being equal, I usually do them. Some folks don't like the look and request not to have them.

MIClyde said...

70mm clearance, for what? my weirwolfs on a gordo or kris holm rim are about 60mm - is there anything wider than that out there

Walt said...

Hey Dave -

With a 60mm tire, and 70mm clearance, you've only got 5mm on each side. That's really the minimum, IMO, if you're going to be pickup up rocks/sticks/mud, and/or knocking wheels out of true during a ride.

Yeah, you can fit a 60mm tire into a 65 or 62mm space, but it gets really touchy. 5mm on each side is a nice amount. I really don't like to have less than 65mm clearance on any of my XC bikes. In this case, we wanted clearance for 2.4ish tires, so I went bigger.

The tough thing, of course, is that in this case we also wanted 44cm (pretty short for a 29er) chainstays. So everything is pretty crammed in there.


Drew said...

What are you measuring for that 6" radius? That die is like a 12" radius. The CLR is measured from what would be the center of the entire radius collar. A 6" radius is rather tight and perfect for seat stays. The bend at the BB of an oval chainstay is around a 4" radius bend and with round it is done on a 2" radius. Bending equipment is always going to find a weakest link and the homebrew stuff is always going to have a fault. I recently pulled the trigger on a DiAcro bender and really glad i did.


Walt said...

Hey Drew -

It's measured from the center of the rotary table to the cutter. It really is 6"radius, dude. Maybe you're thinking diameter?

No room in the shop for a "real" bender, unfortunately, so this is a stopgap until Sarah graduates and we move to somewhere bigger.

It does work really well, at least the mandrel portion.