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Seat angle 73 to 75(18 posts)

Seat angle 73 to 75luapmichna
Mar 12, 2003 2:07 PM
does any one know how the seat angle affects the top tube?? Right now i ride a 73 seat angle and a 59 TT. If i have a bike built with a 75 seat angle how will it affect the size of the top tube ? Will it have to be longer or shorter to make up for the seat angle????
re: Seat angle 73 to 75chriscpa
Mar 12, 2003 2:19 PM
I finally understand this after drawing alot of pictures. You effective top tube will be shorter. i.e. you bottom backet will move backward.

Am I correct on this?

I still coundn't figure out what is the benefit.

Mar 12, 2003 2:31 PM
In general, a simple seat angle change to the steeper 75 would shorten the top tube because the seat tube becomes closer to perpendicular. The bottom bracket would remain the same.

To confirm that the bottom bracket position is the same, check the chainstay length and the front center dimensions. Also check the overall wheelbase and fork rake measurements to further confirm.

Remember that the saddle fore/aft position has to be adjusted to keep same KOPS when you change the seat tube angle.
Everyone is wrong !!!C-40
Mar 12, 2003 4:20 PM
If you were to build a new frame with a 59cm TT length and a 75 degree STA, the reach to the handlebars would increase by about 2.5cm with the saddle in the same position relative to the bottom bracket.

You would need to use a 2.5cm shorter stem, UNLESS your desire is acutally to move your body position forward by 2.5cm, in which case there would be no change to the TT length.

The center of the bottom bracket is the reference point for frame measurement. The BB is never "moved" by alterations to any other other dimensions. In this example the front-center dimension and the wheelbase would also increase by 2.5cm, if no other geometry alterations were made.
Not really, read againFez
Mar 12, 2003 5:48 PM
The original poster said his current 73 seat angle frame has a 59TT length.

He simply asked what the effect of a 75 seat angle would be. He didn't specify that he wanted particular top tube length.

This question comes up a lot and people answer it from different perspectives or assume that the poster has a particular objective in mind, so there is plenty of disagreement in the repsonses.
re: Seat angle 73 to 75zooog
Mar 12, 2003 3:06 PM
What??? I always wanted to ask this question but never did. Great explanations but now I need the moron translation....
I'll try...Frith
Mar 12, 2003 4:18 PM
your considering two different bikes... both have equal length top 'a' has a 73deg S.T. bike 'b' has a 75deg S.T.
bike 'a' requires you to move your seat further forward to be in the right position with respect to the bottom bracket. So now even though the top tubes are the same you have shortened your reach on bike 'a' and will require a shorter stem. Exactly how much shorter depends on your seat height but someone mentioned 1 cm for every degree as a general rule.
half right...C-40
Mar 12, 2003 4:30 PM
You're right that the frame with the 73 STA would need the saddle moved forward, but that would reqire a longer stem to maintain the same reach.

Just turn your answer around and you'd have it. The new bike (with 75 STA) would require a shorter stem because the saddle must be moved back to place the saddle in the same position as the old one.

The formula for the difference is: saddle height x (cosA-cosB) where A and B are the two ST angles. The range is 1.0 to 1.4cm per degree, depending on saddle height.

Just remember when comparing two frames to add length to the TT of the frame with the steeper STA.
That's dead wrong.elviento
Mar 12, 2003 5:01 PM
Sorry for the blunt comment, but it's definitely wrong.

You and C-40 both assumed that after the seat angle change, the rider will still keep the saddle in the same position relative to the BB.

This is a dangerous assumption because 99% chance is that the seat angle change is INDEED to move the seat forward a little. If he wants to keep the saddle in the same position relative to the BB, there would be little reason for the seat angle change at all, unless you are thinking about aesthetics, which is too tricky a topic for me to touch right now.

So the net effect, suppose the same seatpost and clamping position is used, a bigger seat angle will shorten the effective toptube a little.
That's dead wrong.chriscpa
Mar 12, 2003 5:57 PM
LOL, so after all I am correct. If effective top tube mean the distance between your seat tube and BB.
Try an extreme example to prove my point. If you have a 55 cm top tube and a 90 degree (right angle) seat tube angle. You will have 0 effective top tube lenght. ie the effective top tube length will decrease when angle increases.
now you're really mixed up...C-40
Mar 12, 2003 7:02 PM
There is no such thing at "effective" top tube length without two frames with different seat tube angles to compare. The term "effective" TT length corrects for the difference in the two angles, ASSUMING that the rider wants to maintain the same positione relative to the bottom bracket. It will tell you how much longer or shorter stem would be required to achieve the same reach to the bars on the two frames being compared.

Look at the formula:

saddle height x (cosA-CosB)

A and B are the two seat tube angles of the frames under comparison. A single frame has no "effective" TT length.
normal assumptions...C-40
Mar 12, 2003 6:51 PM
Actually I covered both bases, noting that if the riders' position was moved forward by 2.5cm due to the change in seat tube angle, then there is no change to the TT length.
If the saddle remains in the same relative position to the BB then the TT length is lengthened. Perhaps you should read my anwers again.

If you don't specify a specific TT length, an answer may not be wrong but it is meaningless.
State what objective you are trying to achieve by changingFez
Mar 12, 2003 5:53 PM
the seat angle?

Assuming you have your KOPS dialed in, is your saddle too far forward presently?

Are you a road rider and want an aero/tri bike?

Do you want to maintain the exact same reach and you are simply changing from 73 to 75 degrees?
yes everybody is right (and wrong)Frith
Mar 12, 2003 6:15 PM
Everyone is approaching it from a different perspective. Depending on what is constant and what is variable in your perception.
In my case I'm seeing two bikes with different seat tube angles where actual top tube length and KOPS remain the same. I then went ahead and described how the two different angles change effective reach (although admittedly getting the stem lengthening and shortening thing mixed up).
Someone may also look at one frame that starts with a 73 deg STA. Changing the angle to 75 deg the actual top tube becomes shorter, and if the same KOPS is maintained the stem length will not need to change.
Someone may also see it as an attempt to make a difference in KOPS and in this case actual top tube is also shortened but the stem will need to change in order to compensate for the difference relative to the BB.
So three different viewpoints with three different outcomes... I will say though that understanding how two bikes with the same top tube and different STAs affect overall reach is probably the most important because people often use the TT measurement to determine fit without taking into different STA's into consideration.
State what objective you are trying to achieve by changingluapmichna
Mar 13, 2003 8:45 AM
Question #1 move the seat back on the post. right now I ride a thompsom post with zero set back and the seat pushed as far forward as it will go to get into the right spot on the bike and this is correct. I was fitted by andy pruitt in colorado.

#2 no

#2 Road Racer
#3 yes.
then I was correct...C-40
Mar 13, 2003 9:15 AM
Then my assumptions were correct. If you don't want to change your position relative to the BB, and you want to use the same stem length, then a frame with a 75 STA and a 56.5cm TT would produce the same fit as a 73 STA and 59cm TT.

With the saddle moved so far forward, don't you have to use a very long stem? If you also want a shorter stem, the TT length could be lengthened (from 56.5cm) by whatever amount that you want to change the stem length, in addition to changing the STA to 75.
then I was correct...luapmichna
Mar 13, 2003 10:20 AM
Now what if I ride a 61 cm bike and need the 59 top tube??
Don't understand....C-40
Mar 13, 2003 12:48 PM
You will have to get a custom frame is you want a 61cm frame with a 75 degree STA. You can get any TT length you want in a custom, but as I noted in my very first response, a 59cm TT with a 75 STA will make the total reach 2.5cm longer unless you use a shorter stem.

I would select a TT length that would produce the desired total reach to the bars with a 120 stem.