[MMcG]

I've seen some with 73 degree ST angles, some with 73.5, and some with 74 and even 74.5 degree ST angles (primarily smaller sizes).

Makes me wonder - why? Why so steep? Well I guess 73 is normal, but I'd love to learn more about why the use of other steeper angles like 74 and 74.5 on smaller sizes.

Oh and I'm right in my assumption that a steeper seat tube angle actually makes for a longer effective top tube given the same head tube angle through size ranges correct? It sorta seems counterintuitive, but this is what I remember from other posts/threads. Slacker seat tube angles actually cause a shrinking of ETT as the seatpost and saddle raise to higher and higher lengths above the TT, thus actually "cramping" the cockpit more as the saddle raises - right?

So for example - how much real difference in ETT would there be for two frames with the following geometry numbers.

74.5 ST angle, 71 degree HT angle and a 22.5 inch Actual Top Tube length and a 16" Center to top seat tube length

vs

73.5 ST angle, 71 degree HT angle and a 22.24 inch Actual Top Tube length and a 18" Center to top seat tube length

Would the difference in ETT be minimal between these two bikes? If so, the 16" will provide more standover and possibly more exposed seat tube (possibly actually helping with ride quality in terms of lessening some hartdail backend harshness).

Are there any production frames utilizing a slacker angle than 73? If so, who are they?

Who's got a theory? Time for GT and jn and bcd and others to chime in here. Thanks.

 

[ÆX]

seems most co's run 73 or more for better climbing.
i like 72's b/c i have long feamers. have to throw into
account setback posts too that eff make slacker seat angles.
so a 73 is a central area where people can go both ways.

IMO "they" should make 13, 17 and 21 in long and short.
i'll just run my post high for flex like you said.

 

[el-cid]

Makes me wonder - why? Why so steep?

First thing first; the first and main reason that the seat tubes are spec'd steep is for tire/derailleur clearance. The manufacturers can try to keep the chainstay length in check (at least relatively short) and still have room for the rear tire to play nice with the front derailleur.

Oh and I'm right in my assumption that a steeper seat tube angle actually makes for a longer effective top tube given the same head tube angle through size ranges correct? It sorta seems counterintuitive, but this is what I remember from other posts/threads. Slacker seat tube angles actually cause a shrinking of ETT as the seatpost and saddle raise to higher and higher lengths above the TT, thus actually "cramping" the cockpit more as the saddle raises - right?

The issue here is the fact that most (anal retentive) people keep their saddle position relative to the bottom bracket position on their bikes. If the seat tube is steeper on one frame than another then they would shift their seat back on the seat rails, thus lengthening the ett./cockpit measurement. The actual ett. measurement from the center of the head tube to the center of the seatpost will be what it is but once the saddle is slid back on the rails then the cockpit stretches out.

Would the difference in ETT be minimal between these two bikes? If so, the 16" will provide more standover and possibly more exposed seat tube (possibly actually helping with ride quality in terms of lessening some hartdail backend harshness).

The cockpit, once the seat position relative to the bb is standardized, should be pretty close between the two. Frankly, I haven't ever been that hung up on KOPS or my fore and aft saddle position so I've had a lot of variation on the same bike. I've been thinking of this lately and come to the realization that I could actually be pretty comfortable on bikes with up to a 1.5" difference in ett. length with some simple adjustments. Maybe I'm just numb though.

 

[MMcG]

Thanks for the response el cid! Much appreciated.

 

[Lumberjack]

You got me to wondering what the actual seat tube angle on the Kona Kula 29'r I am currently riding is. Being an old metal fabricator I grabed my angle finder and found a level place in the shop. I have an 18" frame with right at a 73 degree seat tube angle with an 85 mm travel Reba installed

 

[ONTHELEVEL]

seems most co's run 73 or more for better climbing.
i like 72's b/c i have long feamers. have to throw into
account setback posts too that eff make slacker seat angles.
so a 73 is a central area where people can go both ways.

IMO "they" should make 13, 17 and 21 in long and short.
i'll just run my post high for flex like you said.

I agree 73 is a far better angle for climbing.