bikenutt

i was just reading about head angle and seat angle, it said seat angle is the smaller of hte 2 angles formed by the seat tube and any horizontal line and head angle is the smaller of the 2 angles formed by the centerline of the head tube or the fork and any horizontal line, i am confused as to exactly what they mean by this can anyone explaiin it in any simpler terms i am just completely baffled on this one

MichaelW

Tie a piece of string, horizontally from your seat-tube to your head tube. At each end, you have 2 angles; measure the smallest one.
Frame angles are not particularly useful to know. I don't know any of mine, but I do know the horizontal offset from bottom bracket to saddle. Offsets [x,y] are much easier to measure than angles.

Al.canoe

Another way; it's the angles from the horizontal of tubes on the left side when the front wheel is on the right. The angles are important to me.

I much prefer a 73-deg seat tube angle over say 72 as it moves me forward for a better seat placement relative to the crank spindle. It also effectively increases the effective top-tube length, especially if you add seat-post set back. In other words, more of the bike is forward of the bottom bracket with 73, so you are sitting further back from the handle bars for the same knee over spindle position, the same top tube length and stem length. Another result of a larger seat tube angle is larger toe clearance with the front wheel.

I also prefer the handling of a 72-degree head tube angle with typical fork rakes. The fork I have now has a rake of about 35 mm best I can tell and more than 72-degrees would be too much for my liking; 71 would be a little too slow.

I guess if "non of the above" matters to a rider, then the angles are not important.

Al

sydney

Handling is about trail,and trail is a function of HT angle,fork rake and other factors, and not defined by HT angle alone.

sydney

Maybe all this would make more sense with a picture.

Retro Grouch 

The really pertinent question is: "How tall are you?"

Through the years, framebuilders have settled on seat and head tube angles of "around" 72 degrees as achieving the optimum balance and steering quickness for average height men. For people at the extremes, it's common to fiddle with the frame angles. A common example is to move the front wheel forward on small frame sizes to minimize toe overlap by extending the head tube angle to 73 or even 74 degrees. In other words, solve one handling problem by creating a different one.

I'm 5'10", right at the mean for American men, so I don't think about frame angles much. If I were 4'11" like my mother, I think that I'd have to investigate a lot more into the angles of any bike that I was interested in buying. The bottom line is that every product involves design compromises. You can research into what compromises were made, but there is simply no substitute for test riding the bike yourself to determine how well it matches your expectations.

sydney

I think 73 is more common.But if you look at every manufacturer,the is still a wide range. A 4'11" person ought to be thing 650 wheels and alot fewer design compromises. And heatdube angle is relatively meaningless without taking into account fork rake and trail.

Retro Grouch 

That's what I think too, but I didn't want to hijack the thread. It sure can be frustrating to try to explain to people that they really won't be any slower and it really isn't a big deal to have to pack along your own inner tube.

Al.canoe

Trail is a function of, or dependant on rake and head tube angle. That's about it, I believe.

Al

Al.canoe

I believe 73 (seat tube) is more common with road bikes, 72 with touring and 74+ for time trial and triathlete bikes. The Time trial/trath. folks like to sit way forward for max power.

Smaller touring& road frames have steeper seat tubes for toe clearance as mentioned. My 5'7" wife has a Trek 520 with a 74 deg seat tube angle. We had to get a seat post with 2" set back to get her back far enough so her knee wasn't forward of the pedal spindle.

Al

sydney

Check the formula for trail. Sheldon Brown has it somewhere.

Al.canoe

I don't think I need too.

Al

sydney

If you did,it would show that tire radius is part of it.

Al.canoe

So is fork length. However, I don't change tire size and fork length when I buy a frame set.

Al

Phatman

triathalon bikes are far more then 74 degrees. try 78 or more. the reason for this, though, is that the position is just rotated clockwise around the bottom bracket if you are looking from the drive-side. this "rotated" position is the same as a relaxed road bike or touring bike, but it allows for a lower bar position.

Al.canoe

I've never ridden a tri bike, But I can't imagine having the hips rotated that far forward at all relaxed . It certainly puts you in a low position with your back horizontal and gets the hips far forward for the aero bars. My road bike puts me at a 45 degree back-angle when I'm on the hoods. Now that's relaxed.

If that tri position was all that relaxed, US Postal/Lance Armstrong would use it for more than just the few time-Trial stages in tours.

Al