I am returning to riding a MTB after a 20-something year hiatus. To give context, when I last purchased a MTB Mavic 517 Citron rims and the Sobe-Headshock team were still popular.
Anyway, last year I built a “contemporary hardtail” and have been riding in the Santa Monica mountains for the past 6 months or so. (67-deg HTA, 75-deg STA, 420mm chainstays, 2.4" tires and a 120mm SID). There’s definitely a learning curve but I’m getting the hang of things and am getting a sense of what I’d like to change for MTB no. 2.
As I work on V2’s CAD I’m left wondering about seat tubes:
My understanding is that the bigger tires, shorter chainstay lengths and rear suspension drove the adoption of bent seat tubes and steeper STA. I get it and it makes sense. (I’m sure there’s more nuance.)
For a hardtail with shorter CS couldn’t I eliminate the bend in the seat tube by welding it forward of the BB (e.g. onto the down tube itself)? Obviously DT size/thickness would be an issue (my assumption is that 38 x 1.2mm is the minimum) but are there other gremlins hiding that I’m unaware of?
It’s basically the reverse of Electra’s Flat Foot Tech. As a MTB newb, how much trouble am I in for?
I think the seat tube angle has gotten steeper over the years due to the dropper post, it works great for winching up steep climbs and since you can drop the seat on the down hills it’s not in the way for descending.
I just did a mtb frame for myself with a 73° effective STA and 425mm stays. The center of the seat tube is set 1/2” forward of the bb center. As far as thickness goes I don’t have too much experience or insight. I used a 35mm OD seatpost and 35mm down tube that is butted .8/.5/.8. It’s works so far, only 100 miles or so on it as of right now but it’s not an area that I’m concerned about failing. I could see this down tube being too small or thin depending on the rider and riding style though.
I have seen bikes with the seattube on the downtube without any reinforcements break so I always brazed on a piece of 0.9 or 1mm tube. Since then I found that short stainstays or only good for wheelies and longer chainstays just add to the ride of any modern mountainbike, so I didn’t make any seattubes on downtubes lately.
Actual STA: 70.8°
Effective STA: 74.2°
Offset: 40mm
Unsagged geo
The drawing shows how effective STA is measured. The larger the offset you use, the bigger discrepancy you have between the actual and effective STA.
The bent seat tube is purely to achieve clearance. The steeper seat tube is about fit, and is a product of the “longer slacker lower” trend of the 2010’s. It solved two issues:
STA’s were notoriously slack on full suspensions (they sag backwards, hardtails sag forwards)
The steeper STA allowed you to have a longer reach
Of course, some people just bump the STA because it conveniently clears the rear tire.
I am curious why you are avoiding the bent seat tube? I have found that offsetting the seat tube on the BB does not give you as much design freedom and can lead to complications: a much deeper ST-DT notch or strength problems.
Some companies/builders do steepen the seat tube just to clear the rear tire, which is a bit of a cop out. Enter bent/offset posts. Like @project12 i have seen and heard of frames breaking that have the seat tube attached directly to the down tube instead of the bb. I much prefer the bent seat tube for that reason - plus, it’s aesthetically pleasing. You can clear the tire and put the saddle in the right place. The only trick is you have to start thinking in terms of effective seat tube angle - and that depends on a specific saddle height. Not a big deal for a custom frame, but if you are a bigger company designing for a general rider it gets a little fuzzy around the edges.
My two cents: get a bent seat tube from @BikeFabSupply unless you want to bend your own.
You can see here how a small, sharp bend placed low on the seat tube creates the necessary clearance for the tire and also allows plenty of straight tube for a dropper post. At least a 185 mm dropper is possible in this size M frame. 425mm stays - could be shorter even.
Or get busy in Fusion and get yourself a bendy lug. At least for my full suspensions this is the way to go, as it also gives me the option to put a slighty heavier tube at the bottom where the main pivot is located and integrate the dropper port.
Thanks ya’ll for the responses. I’ll reply to various questions/thoughts below:
Ah! I’m not avoiding the bent seat tube I’m trying to wrap my head around the decisions/styes/improvements that have occurred over the past 20-some years. I just don’t have knowledge base to wrap my head fully around the concepts yet nor do I have the technical skills (and youth) to push things to a conclusion.
My thought was really just: "What if I keep the top of the seat tube where it is and, instead of bending it, let it end up wherever it wants to (on the DT in my case).
Thanks for the links. Definitely helps solidify some of my thoughts. Interestingly, it seems that Peter V. has/had a similar idea.
If you want to keep it as simple as possible, just use part of the cutoff from your downtube as a gusset on top/almost around the lower end of the downtube and then put your seattube on it. I had zero issues with this solution. Another “trick” can be to use a T47 bracketshell and put the seattube all the way to the front. Just make sure you have some room for the downtube then.
I did similar thing with my Victor fully, but with a BSA shell and a reinforced sleeve. If you don’t that much of an offset this might just work.
