Some naive questions with apologies

[Clancy]

A couple of very naive questions on seat tube angles, seatpost setback, and head tube lengths, fork lengths, and how all interact.

Take a road frame with a seat tube angle of 73.5 degrees. Rider uses a straight seatpost, so setback.

Now, same frame but with a seatpost with a 25mm setback. Doesn't this effectively change the seat tube angle?

This is something I've never understood. If a rider knows he wants "X" amount of set back, and is having a custom frame made, can't the builder design that into the seatpost angle or would it become too exaggerated.

To complicate things, I've noticed manufacturers tend to measure their seatposts differently, making comparisons difficult.

Second naive question

As framebuilders ( and mass manufacturers ) are producing frames with taller head tubes and more relaxed geometry, frames come out with these extremely long head tubes. Why can't that desired effect be designed into the frame by raising the height of the head tube and lengthening the fork? Not sue if I'm explaining it correctly. Angling the top tube up bringing the head tube up rather then simply making the head tube longer. I realize this would require different lengths forks....

Which brings me to my last question. Why are there very few different lengths of forks when frames come in such a variety of sizes, angles, etc. I understand that companies such as Easton or Enve cannot afford to have dozens of molds, but why not a standard that dictates, say, 3 different lengths of forks? And, why do custom framebuilders tend to stay with the standard length of fork? (Steel )

Peter McMahon

[mmoore]

fork questions come down to stiffness. have a really long fork blade and its going to flex much more than a short fork blade with a long head tube.

"effective seat setback" is a whole lot of things from tire clearance (setback post on a steeper frame seat tube) to weight balance of the bike (0 setback -like on some mtbs these days- with steep frame seat tube).

That's my opinion with a bit of background but some more experienced riders might have their own take.

[David Tollefson]

I'll give this a shot. And I'll start with the last questions first.

The head tube thing... Was discussed in relation to Dario's fare just this week. Basically comes down to the fact that some people REALLY prefer the aesthetic of a level top tube.

Fork lengths do vary, but only insomuch as to allow for various tire sizes. But I've put a rigid 29'r fork on my 26'r Spec. MTB frame, and though it looks goofy as all heck from the perspective of tire clearance, I really like where it put the top of the headtube, and how it changed the handling. But yes, the variation will be small because, well, tire sizes (width) within a particular standard (700c vs. 650c or b) are small.

Lastly, there was some discussion a while back about "ideal" saddle position relative to seatpost setback, and where the clamp interfaces with the saddle rails. Let's see if this graphic comes through...

Offset.jpg

[mmoore]

Lastly, there was some discussion a while back about "ideal" saddle position relative to seatpost setback, and where the clamp interfaces with the saddle rails. Let's see if this graphic comes through...

IMG

if you considered the point on the saddle, the BB and the rear axle as fixed points based on frame builder design, the saddle offset should have no effect on the ride. Please correct me if im wrong though; that's how ive been planning some theoretical builds.

[shand]

Which brings me to my last question. Why are there very few different lengths of forks when frames come in such a variety of sizes, angles, etc. I understand that companies such as Easton or Enve cannot afford to have dozens of molds, but why not a standard that dictates, say, 3 different lengths of forks? And, why do custom framebuilders tend to stay with the standard length of fork? (Steel )

Fork length is normally a product of brake calliper positioning (assuming not disk brakes). For steel fork makers (unless you're casting your own crowns), the brake hole to crown race dimension is fixed* and the brake hole to axle is fixed ergo the fork length is fixed.

Another consideration is mudguard (or fender for you weirdos) clearance. A long fork is going to end up with a ton of clearance under the crown.

Of course all this is pretty much only when considering a traditional road fork.

Our standard road fork is actually relatively long (405mm) which means we get lots of people confused by our 'short' headtubes. That problem goes away when you talk about stack but that's another conversation.

*I know you can fill and move the brake hole around (I've done this myself but life's too short for that sort of nonsense) ;-)

[EricKeller]

I think a framebuilder naturally wants to make the fork as short as possible given clearance, fender allowance, brakes, etc. However, there are plenty of suspension corrected forks that are much longer than needed. Anything else is fair game if there is a desire. There was a bike on ebay with 1 foot of head tube over the top tube. A fine balance of standover height and lack of spacers

[e.Gellie]

Take a road frame with a seat tube angle of 73.5 degrees. Rider uses a straight seatpost, so setback.

Now, same frame but with a seatpost with a 25mm setback. Doesn't this effectively change the seat tube angle?

This is something I've never understood. If a rider knows he wants "X" amount of set back, and is having a custom frame made, can't the builder design that into the seatpost angle or would it become too exaggerated.

To complicate things, I've noticed manufacturers tend to measure their seatposts differently, making comparisons difficult.

Peter McMahon

Peter,
I suggest you get yourself some large pieces of paper and just draw these things to scale.

From memory, switching from a setback to 'straight' seatpost for a 6 foot tall rider typically steepens the effective "seat tube angle" by about 1.8 degrees.

So if you are designing a frame for a particular saddle position, work on achieving the saddle height and setback. BikeCAD is great for this. I'm not sure if the free version has all this detail.
You would want the saddle to be in the middle of its rail travel.
You then decide on the seatpost setback to use, which can be from 0 to 25mm, or sometimes more.
The seattube angle is the result of all that. Perhaps 71 degrees for a straight seatpost and around 73 to 73.5 for a setback seatpost is typical.
So your 73.5 ST angle for a setback post would go to ~71.7 for a straight seatpost. That which might generate problems if you planned to use lugs. And you need to check that the tyre still clears the back of the seattube for your preferred chainstay length. And you'll use slightly more toptube material.

So you really need to have a very good idea of the dimensions of the seatpost and saddle.