Please help me understand implications of STA

[SlackMan]

I've written before about my difficulty in finding a bike that fits. In short, given my body dimensions, I ride at about 80cm saddle height, but I need a top tube of about 56cm. Given a back injury, I can't tolerate too much saddle to bar drop (~4cm). Almost all frames with 56cm top tubes have head tubes that are too short and put the bars too low. Almost all frames with sufficiently tall head tubes have top tubes that are too long.

I am currently looking at two different bikes with shallower seat tube angles (STAs), one listed here by a fellow forumite and one a Pegoretti listed on CL, neither of which I can test ride. My understanding is that shallower STAs effectively "shorten" the top tube. Using the Pegoretti as an example, it has a 58cm top tube and a 72d STA. At my saddle height, I think the 72 STA angle "shortens" the top tube by about 1.33cm compared to a 73d STA (calculation at bottom of post). This would mean all else equal, I could push my saddle forward 1.33cm compared to what it is now, which effectively makes it as if the top tube is 56.67cm (58cm minus 1.33cm). The head tube on the Peg is 20cm, which is very close to what I need to achieve a reasonable saddle to bar drop.

The other bike has a 59cm top tube, but a 71d STA. Compared to a 73 STA, the slacker STA effectively shortens the top tube by 2.67cm, which brings it to 56.34cm (59cm minus 2.66cm). It has an 18.5cm head tube, but a quill stem so it could conceivably work as well.

Am I making any logic errors in the above, or does it sound like the frames should fit? Does the shallower STA on the Peg actually make it like a bike with a 20cm head tube but with a 56.67cm top tube? Does it matter if my current saddle setback (nose to BB) is about 8cm? Something just doesn't seem right to me. What I missing about the shallower STAs? Thanks so much in advance for any replies.

Calculation for Peg: 80cm saddle height * (cosine(72 degrees) minus cosine (73 degrees)) = 1.33 cm.

[Mark McM]

Your calculations regarding the relationship between STA and 'effective top tube' appear to be correct. However, figuring the head tube length required for a particular saddle to handlebar drop may not be as easy as it seems. You can't use head tube length alone to figure out the frame stack (BB to top of head tube, as required for saddle/handlebar drop). In addition to the head tube, frame stack is affected by BB drop. Furthermore, for otherwise identical geometries and frame stack, a frame with an external headset will have a shorter head tube than a frame with an internal head tube.

I'd recommend that you start with figuring out the stack and reach that you require in a frame, and then getting the stack and reach numbers of the two frames you are looking at. That will provide an easier comparison to see if either frame will fit. (If you're not familiar with stack and reach, a quick search on this forum or else where on the 'net should provide the information you need.)

[christian]

You understand the implications of STA better than most people on this forum.

[echelon_john]

You've clearly thought things through. One very important aspect seemed to be missing. The real advantage of a slack st angle in your situation is that you can use it to shift your center of gravity backwards to take load off your back. If you're just sliding your seat forward to hit a top tube length number, this piece is missing. Given your description of your back pain and fit needs, you want to rotate back further over the BB. This comes first; everything else in a case like yours follows from this.

Rotation is much more important than TT length or even saddle to bar drop for dealing with back issues.

Google Steve Hogg's approach to saddle setback and do the exercise he recommends where you take your hands off the bars and see if you can comfortably support your upper body without changing your position. You'll immediately feel the effect if even a small (1-2cm) change in setback.

[pdmtong]

agree with your calcs.

[SlackMan]

Your calculations regarding the relationship between STA and 'effective top tube' appear to be correct. However, figuring the head tube length required for a particular saddle to handlebar drop may not be as easy as it seems. You can't use head tube length alone to figure out the frame stack (BB to top of head tube, as required for saddle/handlebar drop). In addition to the head tube, frame stack is affected by BB drop. Furthermore, for otherwise identical geometries and frame stack, a frame with an external headset will have a shorter head tube than a frame with an internal head tube.

I'd recommend that you start with figuring out the stack and reach that you require in a frame, and then getting the stack and reach numbers of the two frames you are looking at. That will provide an easier comparison to see if either frame will fit. (If you're not familiar with stack and reach, a quick search on this forum or else where on the 'net should provide the information you need.)

Mark, I'm glad you brought up stack and reach because while I think I might understand them, the figures often leave me very confused. For example, see the numbers for Moots at the link below. Focus on the fourth column from the left, which is reach for the Moots Vamoots. The reach for the size 56 is 381mm. The corresponding reaches for the size 58, 60, and 62 are 385mm, 385mm, and 389mm. Does that mean that holding saddle to BB position constant, the 62 would only "feel" 8mm longer (389 minus 381) in reach to the handlebars? I understand that the bars up coming up AND back with the larger frames because the head tubes are taller. But, I still find that confusing because it would seem that larger frames should have proportionately longer reaches. In this example, you go from 6cm in firm size from 56 to 62 and reach only lengthens by 0.8cm--is that a correct interpretation?

http://moots.com/wp-content/uploads/ROAD-STACK-REACH.pdf

[jtakeda]

My friend owns a shop that has an Ed Litton for sale right now.

Its a 60cm ST with a 56 or 57cm top tube. I think it has a long headtube too.

Let me know if you want more info.

[pdmtong]

http://freeemoticonsandsmileys.com/animated%20emoticons/Activity%20Animated%20Emoticons/guitar2.gif http://www.messentools.com/images/emoticones/frutas/www.MessenTools.com-Frutas-explode1.gif http://www.messentools.com/images/emoticones/frutas/www.MessenTools.com-Frutas-explode1.gif

[SlackMan]

My friend owns a shop that has an Ed Litton for sale right now.

Its a 60cm ST with a 56 or 57cm top tube. I think it has a long headtube too.

Let me know if you want more info.

Sure, feel free to PM me any more details you have. Thanks.

[don compton]

Check out a rivendell roadeo in 61 or 63cm.

[cmg]

get a custom frame. You can have one built for the cost of the Peg. otherwise you'l buy a used and sell it to go custom.

[Mark McM]

Mark, I'm glad you brought up stack and reach because while I think I might understand them, the figures often leave me very confused. For example, see the numbers for Moots at the link below. Focus on the fourth column from the left, which is reach for the Moots Vamoots. The reach for the size 56 is 381mm. The corresponding reaches for the size 58, 60, and 62 are 385mm, 385mm, and 389mm. Does that mean that holding saddle to BB position constant, the 62 would only "feel" 8mm longer (389 minus 381) in reach to the handlebars? I understand that the bars up coming up AND back with the larger frames because the head tubes are taller. But, I still find that confusing because it would seem that larger frames should have proportionately longer reaches. In this example, you go from 6cm in firm size from 56 to 62 and reach only lengthens by 0.8cm--is that a correct interpretation?

http://moots.com/wp-content/uploads/ROAD-STACK-REACH.pdf

Well, you have to keep in mind that the total reach is a combination of the saddle setback, the frame reach, and the 'tiller' length (stem length plus handlebar reach). In the interest of keeping the front center from growing too large, frame reach typically grows at a proportion than stack. This is compensated for by the fact that both saddle setback and tiller length typically increases at a larger proportion than stack. In other words, total reach may vary in direct proportion to rider size, but this is accomplished by larger variations in saddle setback and tiller length and smaller variation in frame reach.

[pdmtong]

...

[SlackMan]

Well, you have to keep in mind that the total reach is a combination of the saddle setback, the frame reach, and the 'tiller' length (stem length plus handlebar reach). In the interest of keeping the front center from growing too large, frame reach typically grows at a proportion than stack. This is compensated for by the fact that both saddle setback and tiller length typically increases at a larger proportion than stack. In other words, total reach may vary in direct proportion to rider size, but this is accomplished by larger variations in saddle setback and tiller length and smaller variation in frame reach.

So, just to be sure: If one held constant saddle setback and "tiller length," would the overall reach would just differ by increments to the frame reach? e.g., with the Moots figures above, if one took a 56 size frame and a 62 size frame, put identical saddle setback and tillers on them, would the overall reach be only 0.8cm longer on the 62?

[Mark McM]

So, just to be sure: If one held constant saddle setback and "tiller length," would the overall reach would just differ by increments to the frame reach? e.g., with the Moots figures above, if one took a 56 size frame and a 62 size frame, put identical saddle setback and tillers on them, would the overall reach be only 0.8cm longer on the 62?

Yes - if you used the same number of spacers under the stem on both frames. But that would mean that the smaller frame, with its smaller stack height, would have a larger drop from saddle to handlebar.

If you raised the handlebar on the smaller frame to achieve the same saddle to handlebar drop as the larger frame, the reach on the smaller frame would grow even shorter, since the stem moves backwards (at the angle of the head tube) as it moves upward. In the case of the Moots frame, if you put 4.4cm of spacers under the stem on the 56cm frame to match the stack of the 62cm frame, you'd shorter the effective frame reach by 1.3cm

[jamesutiopia]

BikeCAD (http://www.bikecad.ca) is really useful for this type of comparison. The statistics you are looking for are basically "x" and "y" (in BikeCAD parlance), which is the distance from BB to bars on vertical and horizontal planes.

Once you get used to the app, it's pretty easy to model most geometry sheets and compare fit possibilities, and it's free to do what you want to do with it.

[SlackMan]

BikeCAD (http://www.bikecad.ca) is really useful for this type of comparison. The statistics you are looking for are basically "x" and "y" (in BikeCAD parlance), which is the distance from BB to bars on vertical and horizontal planes.

Once you get used to the app, it's pretty easy to model most geometry sheets and compare fit possibilities, and it's free to do what you want to do with it.

Thanks, James. I really wish it had the saddle tip to handlebar distance in the free version, but of course, I should not complain about a free product!

[pbarry]

One can do full sized mechanical drawings of various frame designs. Done on tracing paper, they can be overlaid. It's easy to add the seatpost, saddle, bars and stem.