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STA and frame size effect on reach?(11 posts)

STA and frame size effect on reach?castrello
Dec 10, 2002 9:18 AM
Reading the rivendell cycles homepage I came to think of exactly how much reach is affected by frame size and seat tube angle. For example, according to rivendell, by raising your bars 4 cm brings it back 1.5 cm.

What effect does seat angle have on effective reach? I know that a slacker seat angle will shorten reach, but I have forgotten by how much. I know it´s all simple trigonometry, but my calculator (ie my head) is tired and you guys/girls all seem to have this figured out already so...

My qustion is: what would be the difference in reach between these two frame sizes (all things except frame size being ecual; stem, amount of spacers etc)?

Frame 1

58 cm center-top
73.0 sta
57.5 cm top tube

Frame 2

60 cm top tube
72.5 sta
59 top tube

//mattias
re: STA and frame size effect on reach?tarwheel
Dec 10, 2002 9:48 AM
Assuming you keep your saddle in the same position relative to the crank/BB, a slacker (lower) seat tube angle will effectively shorten your top tube or reach by about 1.2 cm/degree. So, with the examples you provided, Frame #2 would have an effective top tube length about 58.4 cm relative to Frame #1. In other words, Frame #2 would fit about 1 cm longer than #1. (I am assuming that Frame #2 has a 59 top tube, and that you meant to type 60 c-t seat tube.)
re: STA and frame size effect on reach?castrello
Dec 10, 2002 10:07 AM
Sorry, typo... It should say

Frame 2

60 cm center-top
72.5 sta
59 top tube

And, adding to that, I suppose the higher frame would also make the effective reach shorter by 0.75 cm...(according to rivendell). This would in other words mean that the 1.5 cm difference in top tube length would be close to no difference at all when I compare effective reach, am I right?
re: STA and frame size effect on reach?Fez
Dec 10, 2002 10:02 AM
Different STAs should not affect your KOPS position.

Therefore, steeper seat angles may require the saddle to be positioned further back or require a post with more setback.

Just make sure you compensate for that when comparing reaches between the 2 frames.

I heard that each degree of seat angle change requires about 1cm of saddle fore/aft adjustment to keep the KOPS the same.
re: STA and frame size effect on reach?geeker
Dec 10, 2002 11:24 AM
1cm per degree is a good rule of thumb. It's slightly more for me (tall guy). Depends on length of st and how far up the saddle is.

Trigonometry: Make a right triangle whose hypotenuse (length L) goes from the center of the bb along the seat tube to the top of the saddle. The horizontal leg is parallel to the tt, the vertical leg goes straight up from the bb, and the relevant angle is 90 degrees minus st angle. The change in horizontal position to maintain KOPS is L times [Sine(90-st angle 1) - Sine(90-st angle 2)]. Or L times [Cosine(st angle 1) - Cosine(st angle 2)]. Too lazy to calculate this for your examples...
KOPS cops everywhere.the other Tim
Dec 10, 2002 4:53 PM
Not to pick on you or your comments in particular (everyone is saying the same thing), but if different STAs should not affect your position relative to the BB, why are they different?

Rivendell puts you on a frame with a slack STA specifically to move your butt (and consequently your knee) back. If you're going to compensate for this by sliding your saddle forward to achieve the almighty KOPS, and then compensate for your now shorter 'effective top tube length' with a longer stem, why not just get a frame with the geometry that suits your KOPS requirements in the first place?

If I buy a bike from a shop, I'll insist on careful sizing. But if they come at me with a plumb line, and a freakishly set-back seatpost to get my knee over the pedal on a frame with a 76 degree STA, the session is over.
You miss the pointKerry
Dec 10, 2002 5:26 PM
KOPS is a starting point. If you need/want to be way behind KOPS, then a slack STA or a seat post with significant setback is required. But that has nothing to do with this discussion. This discussion assumes that you have your position dialed in, and then you get a bike with a different STA. Given this situation, the question is posed about reach. You don't move your position back and forth depending on your STA, you find a bike with the right reach given the TT length and STA combined with your position relative to the BB.
No, I don't.the other Tim
Dec 10, 2002 5:48 PM
"This discussion assumes that you have your position dialed in, and then you get a bike with a different STA."

I read that assumption in another post. It doesn't make a lot of sense to me to make a geometry change only to compensate for it.

My question was: If different STAs should not affect your position relative to the BB, then why are they different?
No, I don't.Fez
Dec 10, 2002 10:00 PM
If you read a Lemond catalogue, it says that Greg believes you should be behing the pedals. Hence the angles are a little slacker than on competing brands. Since the angle is slacker, the top tube measurement is also a little longer. As a rider, you can choose whether you buy into Greg's KOPS belief or keep whatever position you are used to.

Other possible reasons why STAs are different:

1. Comfort? I think I heard a slacker angle may be more comfy, but this could be myth.

2. Sometimes the angles are different depending on the size of the frame. Look at a catalogue and see that the angles do change over the range of sizes.

3. Design? Maybe the frame builder believes that tweaking with the angles and lengths of certain tubes may result in a desired ride or handling characteristic.
Exactly (may get long before I'm through).the other Tim
Dec 11, 2002 9:45 AM
All of the things you listed involved using the STA to adjust the position of the rider relative to the BB, whether it's for comfort, or power, or handling.

I ride a bike with a 73 deg STA (a LeMond, BTW) and another with a 78 deg STA. Within a cm, they have the same TT length, and each fits me very well for its intended purpose (I'm sure Greg would agree). If I tried to achieve KOPS on either of them, I would throw the fit off. More importantly, if I used the same saddle position relative to the BB, I would need a 1 cm stem on one of them.

I realize that I'm giving extreme examples (it's just to make a point), and I realize that I'm mixing two issues (the silliness of KOPS, and the function of STA).

I usually don't even comment on fit, but I see the issue presented this way so often that people assume that you want to keep your knee in exactly the same place when choosing a frame with a different STA. That's certainly not the intention of the frame builder. I don't know what was in the original poster's mind, but Rivendell would not expect you to buy one of their frames to get the same position as a Litespeed Blade (extreme; I know).

I'm not missing the point about transferring a dialed in position to a different geometry. Probably the most valid reason for doing so would be that you've found a perfect riding position, but it's on the wrong frame. Of course, a half a degree change in STA from one frame to another won't change the bike's function, and adjusting the saddle position accordingly is important. Of course, that affects the 'effective top tube length', making all comments in this thread valuable. But STA SHOULD affect your position.

I probably shouldn't start on KOPS. Oh, what the hell. We all know people who have experienced discomfort and pain from riding a bike they got a great deal on from a salesman that never even looked at them. I know just as many people who aren't comfortable on a bike after someone measured every bone in their body, then velcro'd a plumb line to their knee, and worked from there.

OK. I'm tired now. Sorry.
once again...C-40
Dec 10, 2002 11:43 AM
When comparing frames with different STA, add TT length to the frame with the steeper (73) angle or subtract length from the frame with the shallower (72.5) angle, using one of the following formulas: 1.32 x (cosA-cosB) x frame size, or an alternate formula, saddle height x (cosA-cosB). An average amount is 1.2cm per degree for a midsize frame.

As for your question about the head tube length and reach, a larger frame will not necessarily create a shorter reach, unless the stem is positioned all the way down on both frames. The handlebar height would normally be adjusted to the same position relative to the saddle, regradless of frame size. The only difference would be the amount of spacers used or quill showing.

Bottom bracket height can also have an effect on the saddle to bar height difference. A lower BB height will lower the saddle and reduce the height difference between the saddle and the bars.