Seat clamp slot help

[duanedr]

seattube_clamp.jpg
Ok, so, frame #2 has wishbone seatstays and I wanted a certain curve to the top of the ST and the sleeve is only so long so, the real estate is a bit limited. I've heard (read or made up) that the seattube clamp slot should be 30mm long. I only have about 25mm. Is this long enough or should I do a sort of slotted 'T' with a horizontal (radial?) slot like the marker in the attached picture? And if I go that route, how far around should the slot go?

Also, the picture makes it look like everything is out of alignment. It's not in real life.

As always, thanks for any help, ridicule or shame!!

[steve garro]

I do 2.5cm

I don't like the "T" = crack propagation

- Garro.

[duanedr]

Ok. The 'T' slot was my way of trying to avoid the cracks. Thanks I'll go with 2.5cm.

[Crowemagnon]

I've only built a handful of frames, but on one I cut a very short slot around the 25mm you mention.i was very carefull reaming to ensure a very snug seatpost fit. It has been no issue after serious service off-road. +1 short slot.

Pat Crowe Rishworth

[Zimmermanbicycle]

[QUOTE=steve garro;651633]I do 2.5cm

I don't like the "T" = crack propagation

This should work fine and yeah,no to the T
-Eric

[steve garro]

Thanks I'll go with 2.5cm.

It's one of my most "historic" measurements, when I started I measured a Bontrager MTB and it was 2.5cm

- Garro.

[Doug Fattic]

My experience with a 25 mm slot has not been as successful as others. I always made my customer slots longer but one time when I made a frame for myself using Henry James lugs with Hellenic style (sometimes referred to as a “triple triangle” like old GTs) seat stays, I thought a shorter slot might look nicer on this more exposed lug. I was very careful to ream and hone the seat tube so it was the exact size as the seat post. However, no matter how tight I tightened the seat binder bolt, my seat post could occasionally move. There was too much leverage when I bumped the nose of the saddle and it would twist. It didn’t take all that much force to move it. When I recut the slot longer to 29mm all my problems stopped and seat post no longer moved under any circumstances. I never cut a slot shorter than 29mm ever again.

The difference of my experience may be related to what kind of binder is used and where it is located and where we are measuring. All of mine are either brazed on or cast as part of a lug.

Doug Fattic
Niles, Michigan

[Curt Goodrich]

Just one other data point, I've NEVER measured a slot before. But that's how I roll.

[duanedr]

I didn't measure #1 but I know I was slightly more than 30mm. On this frame (#2) as I was fitting it up and making sure everything was landing where I wanted it, I noticed it was getting a bit tight. He only weighs about 70lbs so there won't be a lot of force on it. I'm brazing on the clamp tabs to the back over the top of a sleeve so, i'll get it all finished up and then cut as far as I can without cutting into the wishbone - which should be just about 25mm. From there, i'll hope for the best.

thanks again

[clam]

I'd go 25.4mm

Caleb Lambert

[duanedr]

Well, at that point, I'd just round up to 26mm.

[crumpton]

nobody ever cut a seat tube slot to a .Xmm tolerance. you guys are funny.

[steve garro]

nobody ever cut a seat tube slot to a .Xmm tolerance. you guys are funny.

Well it's the distance from the top of the ST to the center of the stress relief hole, how's that?

I was very careful to ream and hone the seat tube so it was the exact size as the seat post.


Doug Fattic

Isn't that called a press fit?

Hell you even need an extra few on the right side of the decimal for a press fit….

- Garro.

[Michael Gordon]

nobody ever cut a seat tube slot to a .Xmm tolerance. you guys are funny.

According to my cad file my slot is drawn to an accuracy of X.XXXXmm. My hack saw, files and hands however, disagree with my computer.

[Jamie Medeiros]

I like to use the formula s=r*phi where s is the slot length in mm and r is the radius of the seat tube. Using the golden ratio makes the slot the most visually pleasing length.

[steve garro]

I like to use the formula s=r*phi where s is the slot length in mm and r is the radius of the seat tube. Using the golden ratio makes the slot the most visually pleasing length.

Euclid carved his slot hole in the shape of a nautilus and the slot spiraled around the tube.

- Garro.

[Doug Fattic]

Isn't that called a press fit?

I see I have been caught by the wording police. What I could have said more clearly was that the seat post was the correct size to fit the seat tube so that it could slide easily without play in a consistently round hole. In other words I eliminated seat post fit as the source of the slippage problem. For the impressionable that may search the archives to find out how long a slot should really be (a common question rookies ask), I wanted to warn them that under some circumstances a slot that is only 25mm long may not work well enough. At least it didn’t with me. Back in the 70’s when I started building frames, I never thought about slot length much and just cut them till they looked right. When one time I deliberately made my frame slot a bit shorter and it didn’t hold my seat post securely, I went to investigate what was the problem. I checked the fit of the post and the width of the slot and that the binder and bolt threads were okay, etc. I measured the slot length and it was 25mm from the top of the lug to the bottom U of the slot. When I added a few millimeters to the length of the slot my slippage problem was solved with no other variables having changed. I was amazed that such a little extra length made a big difference. Since cutting a slot longer involves more effort when doing it by hand, it is normal for a beginner to quit as soon as possible. My recommendation is that for a 27.2 seat post, the slot be 30mm long. It can be annoying when your own seat post slips but not funny at all if it is a customer’s.

[steve garro]

I see I have been caught by the wording police.

You were a teacher - I'm sure you understand the importance of clarity.

- Garro.

[Doug Fattic]

You were a teacher - I'm sure you understand the importance of clarity.

You’re right, a teacher needs to be held to a high standard of communication. It didn’t cross my mind that what I wrote could be interpreted some other way then that I prepared the seat tube to be just the right size for the the seat post until you pointed it out. The challenge in communication is not to surround the primary nugget of information with so many descriptive words its meaning is less clear in the forrest. Especially in internet posts. My mind tends to be very specific and detailed and to write everything out can make a point lengthy and difficult to want to read. Pruning the unnecessary words out but still leaving the core clear is a skill we can all practice but not always obtain.

[duanedr]

You were a teacher
- Garro.

To be clear, he still is.

Sorry, I had to.