Old 01-25-20, 09:23 AM
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Originally Posted by sweetspot View Post
thank you very much for your answer. If you don't mind I have a few more questions to you:
1. How would you rate the compliance decrease between 430 and 420 mm chainstay of a steel frame? The common notion is that the longer the chainstay the more movement it can provide so shorter, at least in theory, should also mean less comfortable, yes?
2. Why there is so little steel bikes with the seat stays connected somewhere in the middle of the seat tube (Marin Nicasio Ridge is one of the rare examples of that approach)? When carbon is considered, this lower cross-section point is considered as more comfortable than connecting seat stays with the seat tube at the top tube level.
3. Do you use a bended seat stays (like Litespeed Ultimate gravel bike has) for increased rear-end comfort or this is just a gimmick in your opinion?
4. What do you do to increase the stiffness at the bottom bracket area? My steel Jamis Renegade Exploit is very comfy but also flexes a lot when pedaling hard. Is it possible to get a comfy steel bike that will be also very power efficient?
5. Do you use different seat post diameters (for example 27,2 and 30,9) and if so, how would you rate the compliance decrease when going from 27,2 to a wider seat tube diameter?

I'll try to answer your questions, but I feel like your mission of a comfortable, vibration free off-road ride is misguided. Riding a bike off-road is never going to be a comfortable and vibration free experience.

1.Compliance(flex) in a triangulated steel structure is not something that happens. I believe that the reason shorter stays feel harsher is because they place the axle closer to being under the saddle and rear wheel impacts are transferred more directly to the rider.

2.Attaching the seat stays to the middle of the seat tube is a awful design from an engineering standpoint. You see something that will flex to provide some comfort and I see something that is likely to collapse because the load is being transferred to an unsupported tube at its weakest point.

3. I do use S-bend seat stays, but only because I prefer the look of them over straight stays. They do not flex once they are welded into a triangular structure. They will move horizontally, but not vertically.

4. The bottom bracket doesn't flex. It has 4 tubes welded to it and it is not long enough to have any mechanical advantage over those welded joints. The flex you see is actually the top and down tubes twisting over their length and the chainstays flexing horizontally. To make the frame stiffer, use larger diameter tubes.

5. All of my gravel frames use 27.2 seat posts. My mountain bike has a 30.9 seat post, only because I wanted the option to use a dropper post. I can't tell the difference between them, but the MTB has a 2.8 inch rear tire that absorbs most of the hits at 12psi. The flex of a seatpost is going to be depend on the angle of the seat tube and the amount of post that extends out of the frame. It stands to reason that a smaller diameter post will flex more, given that both posts have the same wall thickness.
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