Framebuilders - 73.5 degree S.T with 19mm setback post or 72 degree S.T. with zero setback?

Hi,

Just like to get some opinions from frame builders/designers on this forum. For a 55cm touring bike with 45cm chainstays, which seat tube angle would you choose given that both arrive at the same saddle position; 73.5 degree with a 19mm setback seatpost or 72 degree with a zero setback seatpost? Touring bike will carry rear panniers with about 30 to 40 pounds and use 700c wheels. I'm wondering which design will be more comfortable and stronger for an extended 1 year plus bike tour.

If this matters, front low rider racks carrying 25 pounds will be used, saddle and handlebars roughly level and Trail is 60mm.

Thanks.

I went through that analysis, and concluded that the straight post and lower angle made more sense. As far as I can tell the offset seat post is mostly a mater of tucking a wheel in more, like a bent seat post. But a tight fit is not an issue with a touring bike. Still you need to look at a few issues:

- The 72 degree post will reduce the clearance for tires and fenders. Not likely to be a problem as long as you look ahead, and the builder accounts for any differences.

- If you were planing on lugs then you are altering the lug angle. I think this actually worked out on my design. I wanted the lower angle head tube, and a slight slope in the top tube was acceptable. Of course lugs can be adjusted.

- You shorten the room for the rear panier, which is normally far too short to start with. Of course it only slides back a slight bit, but that moves the load to the rear. This was of no concequence to me as I wanted far longer stays than the 45cm standard.

- Also, you need to look at more than just the theoretical on this. You need to start with your saddle. Some like the brooks have a relatively narrow range of adjustment, so you want to hit the sweat spot with the specific saddle then choose you post, then choose your tube angle, relative to your position.

- Finally, anything you do out of the ordinary will mean some comunication with the builder, or subsequent owners.

Overall I don't think it is a big deal, particularly as regards long term durability with a good post, but it's the fun of a custom bike to pure all the details.

Peterpan1, thanks for your analysis and I concur 100%. Having to perhaps move the rear panniers back a bit was something I overlooked. Without wanting to sound like I'm trying to split hairs, I once read/heard that some layback seatposts (i.e. titanium) provide a kind of passive suspension effect because of the offset. 2 to 3cm doesn't seem like much but do you think this has any merit as it would certainly add to riding comfort and increase design options?

I imagine a fist full of seatpost exposed doesn't apply but for sloping top tubes and several inches of post exposed, would the above point be valid?

It's mostly aesthetics. I would generally go for a straight seat post as it demonstrates good design. I think that panniers usually are placed where they miss the rider's heels anyway.

Panniers and heels are an important consideration, though with luck your 45 CM stays will take care of that. My concern is just that with 2 bikes with 45 cm stays, and one has a seat tube set further back, they will both have the rack in the same position, but the top of the rack with the (in this case) 72 degree post will have less room for stuff like the tent or the sleeping bag, than the 73 degree seat post. There often isn't enough room regardless, so the real solution is a custom rack. Also the difference between 72 and 73 degrees is about a centimeter, not enough to worry about, but something that one would want to consider on a custom.

As far as suspension effect of a post is concerned, I think it would be negligible, and if highly effective as on the Bike Friday with the cantilevered post, it is a long term fatigue risk that should certainly worry you. With a post set at 70+ degrees I don't see much chance for deflection, even with setback. (possibly with massive extention as on a ATB, but I don't do touring bikes that way) As I mentioned I think the real reason is to tuck the seat post for the kinds of racing that uses a tight, tucked wheel postion. I think it looks a little odd when they put that kind of detail on touring bikes.

I do think that some people probably need a seat post or seat with suspension. But if that is the goal then the design should probably accomodate a sprung seat or one of the quality suspension posts. When I got back into touring, I decided that with my lower back problems I would need a suspension post and stem, but in the rush didn't manage to get either. In the event my back was fine and I haven't considered it since, but for someone who needs it, it should be factored into the frame design at an early stage in the design.

Some of the best modern posts, namely Thomsons, have thicker wall sections front and back than at the sides. This is one key to their good support with light weight. I doubt you'd feel any major difference between a setback Thomson and a straight one.