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Bicyclochondriac.
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Hi, I'm looking at getting a new frame, and as I look at the various top tube measurement, I also stated looking at the seat angle, and I realize that you have to look at both of these together to get an idea of how streched out a bike will feel.

For example, lets say bike A and bike B both have (effective) tt measurements of 23", but Bike A has a 73.5 degree seat tube angle, and bike B has a 71.5 degree Seat angle. In order for the TT measurement to be the same, bike A's bottom bracket would have to be farther back relative to the head tube than on bike B. If you were to slide the saddle to be in the same position relative to the pedals on both bikes, bike A would be more stretched out than bike B. according to my calculations it would be somewhere around 3/4" depending on the seat tube length. That's a notable difference.

For example, my Heckler and my Zion ss have a 2 degree difference in seat tube angle (Heckler = 71.5, Zion = 73.5) I noticed that when I positioned the seat on the Zion to be the same as the Heckler relative to the pedals, the seat was slid farther back on the post, so effectively I increase the measurement from the nose of the saddle to the head tube while retaining the same "effective" seat angle. After all, the REAL seat angle must take into account the fore-aft position of the saddle.

When I looked at a large 2003 enduro frame I saw it had a 24" TT, but a 69.5 degree seat angle. At first it looked like the bike was more stretched out than my Heckler (23.3" TT), but when I took the seat angle into consideration, it put the bb in about the exact same place relative to the head tube as the Heckler. The main difference is that I would slide the seat quite a ways forward on the Enduro to get the same seat position, but it would be about the same excact fit, at least in terms of how long the cockpit felt.

Anyone else notice this?

Kapusta
 

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www.derbyrims.com
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6,766 Posts
Seat tube angle centers the adjustment range

kapusta said:
Hi, I'm looking at getting a new frame, and as I look at the various top tube measurement, I also stated looking at the seat angle, and I realize that you have to look at both of these together to get an idea of how streched out a bike will feel.

For example, lets say bike A and bike B both have (effective) tt measurements of 23", but Bike A has a 73.5 degree seat tube angle, and bike B has a 71.5 degree Seat angle. In order for the TT measurement to be the same, bike A's bottom bracket would have to be farther back relative to the head tube than on bike B. If you were to slide the saddle to be in the same position relative to the pedals on both bikes, bike A would be more stretched out than bike B. according to my calculations it would be somewhere around 3/4" depending on the seat tube length. That's a notable difference.

For example, my Heckler and my Zion ss have a 2 degree difference in seat tube angle (Heckler = 71.5, Zion = 73.5) I noticed that when I positioned the seat on the Zion to be the same as the Heckler relative to the pedals, the seat was slid farther back on the post, so effectively I increase the measurement from the nose of the saddle to the head tube while retaining the same "effective" seat angle. After all, the REAL seat angle must take into account the fore-aft position of the saddle.

When I looked at a large 2003 enduro frame I saw it had a 24" TT, but a 69.5 degree seat angle. At first it looked like the bike was more stretched out than my Heckler (23.3" TT), but when I took the seat angle into consideration, it put the bb in about the exact same place relative to the head tube as the Heckler. The main difference is that I would slide the seat quite a ways forward on the Enduro to get the same seat position, but it would be about the same excact fit, at least in terms of how long the cockpit felt.

Anyone else notice this?

Kapusta
Real long legged riders have even more variation in reach due to seat tube angle and available seat adjustment.

If comparing one bike to another, find a seat tube angle that provides the seat adjustment range to get you centered over the pedals as you like for your riding interest. Steeper seat tube dimensions center you over the pedals better for climbing, and tend to have shorter chain-stays for quicker lower speed handling and some climbing traction advantage if short travel or hardtail, but reduce higher speed seated cornering stability.

Get an effective top tube length that keeps you behind the front wheel enough for downhill. Quicker steering (steeper head tube) than what you are used to moves the front axle back significantly, so you'd need a shorter stem and longer TT in proportion to stay in similar position behind the front wheel while braking downhill. If going to slacker steering, a longer stem slows steering more, but a shorter effective TT and shorter reach than you are used to is quicker handling with a slight loss in very steep climbing balance and postion (bend your elbos more), overall trail and rock climbing and drops handling is better.

The Specialized trail bike interrupted seat tube angles are unusually slack, more DH like. Maybe is to help center weight more forward with seat lowered for better seated smooth downhill cornering.

- ray
 

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here's a link..

I was absent when they taught this in geometry class. Now I get dizzy just reading it. :D

"So the frame dimension that really influences the cockpit length is the reach. This dimension is affected by the toptube length and the seattube angle (since the toptube starts at the seattube, a steeper seattube moves the whole toptube forward and hence a larger portion of the toptube is in front of the bb, increasing the reach. So what we do is we keep the seattube angle constant, and shorten the toptube consistently (not equally given the distribution of people riding the bike, but consistently) as we shrink the sizes. You could achieve the same decrease of cockpit length by shortening the toptube, steepening the seattube and then shortening the toptube an extra bit to compensate for that, but that's just a different way to achieve the same thing."

Here's the link about the subject, scroll down to geometry -

http://www.cervelo.com/bikes.aspx?bike=SLC2006#G
 

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Bicyclochondriac.
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Discussion Starter · #4 ·
older guy said:
I was absent when they taught this in geometry class. Now I get dizzy just reading it. :D

"So the frame dimension that really influences the cockpit length is the reach. This dimension is affected by the toptube length and the seattube angle (since the toptube starts at the seattube, a steeper seattube moves the whole toptube forward and hence a larger portion of the toptube is in front of the bb, increasing the reach. So what we do is we keep the seattube angle constant, and shorten the toptube consistently (not equally given the distribution of people riding the bike, but consistently) as we shrink the sizes. You could achieve the same decrease of cockpit length by shortening the toptube, steepening the seattube and then shortening the toptube an extra bit to compensate for that, but that's just a different way to achieve the same thing."

Here's the link about the subject, scroll down to geometry -

http://www.cervelo.com/bikes.aspx?bike=SLC2006#G
Yes, I followed your link and that is exactly what I am talking about. I had never heard the term "reach" but I like it. I am surprised it is not used more often as it is what I am ultimately interested in, as you can't change it with your seat or stem. Moving your seat forward is essentially (an effectively) shortening the TT and steepening the seat angle and sliding it back does the the opposite. But neither changes the "reach" as described in that link.
 
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