Author Topic: Head & Seat Tube Angle affect on Frame Behavior  (Read 12635 times)

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Offline Steven

Head & Seat Tube Angle affect on Frame Behavior
« on: August 27, 2008, 11:37:21 pm »
Four touring bike frames made by various bike manufacturers for heavy loaded touring. All frames are the same in every way. Same top tube length, same stand over height, same seat tube height, chain stay etc. The only differences between them are the head and seat tube angles. Bike 1 has a head tube angle of 71 deg. and a seat tube angle of 73.5 deg. Bike 2 has head/seat tube angles of 72/72.5 degrees respectively. Bike 3 has head/seat tube angles of 72.5/74 degrees respectively. Finally Bike 4 has head/seat tube angles of 70/73 degrees respectively. How do these angle differences affect the overall behavior and comfort of these bikes?

Offline whittierider

Head & Seat Tube Angle affect on Frame Behavior
« Reply #1 on: August 28, 2008, 01:53:47 am »
The seat-tube angle doesn't really affect handling directly-- just your position.

The head-tube angle works together with the fork rake and the wheel size to produce "trail."  These factors plus things like stem length make for a trade-off between high-speed stability, low-speed stability, maneuverability, and other things involved in the behavior of the bike.  I'm no expert at it, but this should get you going on a productive web search.

Offline RussellSeaton

Head & Seat Tube Angle affect on Frame Behavior
« Reply #2 on: August 28, 2008, 09:19:38 am »
As already mentioned, seat angle affects where you end up sitting on the bike.  You usually try to get your knee in a certain position over the bottom bracket.  You will slide the saddle forward or backward on its rails to achieve this.  A steep seat angle such as 74 degrees may require you to slide the saddle all the way back to get the knee over the bottom bracket.  And thus increasing the length of the reach to the bars.  And a 72.5 seat angle may require you to push the seat forward on its rails, thus shortening the reach to the bars.  So in reality the four bikes you mention do not have the same top tube length when you actually set the bikes up the same.

In the range you mention for seat angle, there is no comfort difference or behavior difference.

Offline staehpj1

Head & Seat Tube Angle affect on Frame Behavior
« Reply #3 on: August 28, 2008, 10:19:28 am »
From Sheldon Browns website:

    The usual angles that are referred to in frame design are the head-tube (or fork) angle and the seat-tube angle. These angles are usually measured with reference to the horizontal. The typical range is from 68 to 75 degrees.

    In general, bicycles with shallower, "slack", "relaxed" angles (lower numbers) tend to be more stable and comfortable. Bicycles with steeper, more upright angles (higher numbers) tend to be manuverable, but less comfortable on rough surfaces. Shallower frames tend to have longer wheel bases than more upright frames; bicycles with shallower head angles normally have more fork rake. All of these factors contribute to the riding characteristics cited.

Offline wanderingwheel

Head & Seat Tube Angle affect on Frame Behavior
« Reply #4 on: August 28, 2008, 01:42:01 pm »
Seat tube angle has already been well adressed, so here's my take on head tube angles.

Head tube angle does two things, together with rake it changes the amount of steering trail, and it affects the weight displacement between the front and rear wheels.  

Trail is fairly well understood, in brief less trail will require less force to change directions, and more trail will require more force.  High trail and low trail is not better or worse, it's just a personal preference, and can change based on speed, terrain, and load.  All else being equal, and as long as the head tube angles are close (as they are here) bikes with the same trail will will handle the same.

Weight distribution I think is as important to handling as trail and is almost unrecognized.  This is especially important to those of us carrying heavy loads on racks and in panniers.  Assuming the rear wheel remains in the same place relative to the bottom bracket, moving the front wheel further out will reduce the amount of weight on it.  This will make the steering feel light and vague regardless of the trail.  Moving the wheel back in will make it feel very responsive, but also may require a significant amount of effort to turn.  We have an advantge over other riders that we can change our weight distribution by moving our gear around.  For me, I like to put as much weight in the front panniers as possible on most touring bikes in order to get the handling that I like.  Others happily throw everything on the rear rack.  Just like trail, it's not a better or worse thing, just a personal preference.


Offline Steven

Head & Seat Tube Angle affect on Frame Behavior
« Reply #5 on: August 29, 2008, 12:57:20 am »
I very much appreciate all of your replies. They helped clear up a lot.
Thanks - Steven