Originally Posted by
The Golden Boy
I've always labored under the understanding that a higher angle is a steeper angle, and therefore more "aggressive."
I understand that steering with a more aggressive angle will be more precise and more "twitchy."
What are the other characteristics of shallower or steeper angles?
For example- my 1984 Schwinn Voyageur SP is stated to have a head tube angle of 72 but a seat tube angle of 75 (which seems really steep to me- even compared to most "race" bikes). I've always had a hard time getting any other saddle to work on that bike, other than what came with it. I eventually swapped out the seat post for a more adjustable Superbe Pro. I'm not sure what else that 75 STA does for the ride of the bike, or my pedal stroke other than making it difficult to find a saddle/post combo that works.
For the record, the seat angles on my Miyata 1000 is 72, my 620 is 73.5 and my 720 is 73.
The following uses examples from road racing bikes, but they should illustrate the issue - especially since "aggressive handling" more often comes up with road bikes.
Really, the angles themselves aren't aggressive, it's their effect on wheelbase that is. And this is mainly true head tube, not the seat tube.
Supposing the same seat tube angle and same top tube length, a bike with a 73 HTA will have a front center or wheelbase about 1cm shorter than a bike with a 72 HTA. Both head tubes originate at the top, and angle down from the top, so the net effect is seen at the front dropouts.
Short wheel base bikes will turn quicker than longer wheel base bikes for a given lean angle - the contact patches two wheels being closer together sit on a smaller circumference circle than a long wheel base, and that reduces the turn radius of that circle. So when you lean modestly you get more change in direction with a short wheelbase.
Steep seat tube angles were at one time used to maintain tire clearance on extremely short chain stay bicycles, but this can be accomplished other ways, like by curving the seat tube around the tire. These days, steep seat tube angles are generally found on smaller sized bikes, and are mostly used to artificially shrink the top tube, because steeper STAs lean toward the head tube. Some companies have called BS on this practice at various times - Cannondale in the '80s and Cervelo today just use 73° for all road seat tubes regardless of size.
Regardless of HTA, steering feel is largely controlled by rake, which can make up for the actual steering angle by keeping the trail numbers consistent across size ranges. This used to be easier with metal forks, but molded carbon generally forks require a different mold for each fork rake, so many brands simple have 1 to 3 fork rakes that have to serve the range of HTAs for that model, making trail less controlled than it used to be.
The reason we use the HTA angle range you see on road bikes - about 70 to 74.5° - is largely because of toe overlap preventing the angle from getting too steep, and wheel flop making shallow angles feel less controlled. More wheel flop makes steering accelerate away from center, and at some point that starts to feel bad. If you were building someone 7 feet tall a bike, you could make the TT long enough to allow for an 81° HTA, and then the fork would have zero rake to produce normal trail.