Seattube/Toptube Junction reinforcement

I am designing an MTB frame where I want to add a bit of sloping by lowering the point where the top tube and seat stays braze with the seattube. The distance from the center of this junction to the top edge of the seat tube is around 90mm. I plan to have a short 19mm OD reinforcement tube joining the seat tube and top tube (this is same design as you can see in Surly Ogre, ECR and so on).

However my thiсk external butted section of the seat tube is 100mm (1.2mm wall thickness), then 25mm tapered section, then 0.6mm middle section. With such setup top tube will be brazed partly to the thick section of the seat tube, partly to the tapered/thin one, which I’m not really comfortable with.

Should I abandon the sloping idea altogether and reduce the seat tube stickout to ±40mm to make sure the top tube is brazed to the thick section of the seat tube? Or should I keep the original design and add some lug-type reinforcement of the whole junction incl the seatstays? Or am I paranoid?


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What prevents you from making the seat tube shorter and omitting the extra tube? Is it the min insertion of the 27.2 post?

I usually have 40mm of seat tube above the top tube in order to make a roughly 28mm slot in the front of the seat tube for the Engin dual bolt collar and room for brazing.

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Yep, I’m 6’5" so I try to keep the seattube as long as possible.

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If you want the extra standover I reckon you’d be better findng a longer 27.2 seatpost to work with the lower TT and ditch the brace. Would be good to know the current distance from top of TT to saddle rails too.

Personally I couldn’t imagine riding a MTB with a rigid seatpost anymore. Why not a 30.9/31.6 ID seat tube in a lower position to max out dropper post length?

If you feel you need extra support there, couldn’t you just use a different style of gusset? Eg: a plate style on each side which is welded along the entire edge (so it spreads the load across the thicker and tapered part of the tube) or a folded sheet style at a steeper angle? A couple of random images from a quick Google search to try and illustrate what I mean:

images
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Form follows function

I want to keep the seattube longer for aesthetics. Is this kind of reinforcement enough?

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IMG_3316

to rephrase the question: how safe is it to braze the top tube to the thin section of the seattube using such reinforcement?

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  • 6’5" + your weight + mtb + not a huge amount of bike fab experience = don’t simply braze w/out reinforcement.

I would take a straight seat tube (28.6mm) and braze a 1.25 x 0.058" (aka 31.8 x 1.47mm) section of 4130 that’s as long as you need it to be – maybe 6" (150mm)

(@elysianbikeco) • Instagram photos and videos
(@elysianbikeco) • Instagram photos and videos

If you go that route, make sure you drill a few holes in the sleeve to ensure that you’re flowing brass (brass, not silver) through the entire thing.

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@buffet Welcome!

To help you make this seat tube decision, I have a few questions:

  • What is the design intent of this mountain bike?
    • fire roads?
    • flowy singletrack?
    • steep rocky terrain?
  • Is it being run with a dropper or rigid post?
  • How much do you weigh?
  • What DT and TT diameters are you planning on running?

6’5 is a few standard deviations outside the norm, so you may need to design your bike in ways that are unconventional!

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I am 6’5” tall. 242ish and I pealed my reinforcement tube off the TT.
Way too much leverage and a crappy weld.
I will use a straight gauge TT when I fix it and hope for a better weld.

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Bike is primarily for bikepacking on forest/fire roads, nothing extreme at all. Rigid post. I’m ~200 pounds.
DT is 38.1mm, TT is 31.8mm.

I gave it some more thought, I think I will dump the sloping idea and raise the point of ST/TT junction to make sure everything is safely brazed to the thick sections of the tubes. Here is my work-in-progress BikeCAD file:


All of the actual final schematics and building will be done by a repitable local framebuilder, but I’d like to dive as deep as I can into this. Hell, I’m going to be the one to ride it :slight_smile:

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This tube has a 250mm external butt.

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Nice! I like the concept. I think for the design intention, the 28.6-seat tube and 27.2 rigid post is fine.

I don’t think the difference between a sloping top tube vs gusseted ST vs horizontal TT has a major impact on the strength of the frame. So long as the tubing is the correct thickness and diameter.

You could consider using a 32mm ST with a paragon topper: Steel 1-1/4" OD x 27.2 mm ID15 mm Step Relieved ID

That would give you a stronger junction because the 32TT will have more room to land on the 32ST (more surface area), especially if it is brazed.

Finally, if you are going through the exercise of a custom frame, I would consider evaluating the fit. The 72.5 STA, swept-back bars, and design intent suggest an upright position. However, your stack is super low (590mm). For your saddle height, I would expect closer to 680mm stack.

Also, for your height, I think 29 plus wheels would be much better. The fork will help your stack problem, and it will allow you to use a deeper BB drop, which lowers your center of gravity between the wheels.

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Thanks Daniel!

  1. I already got my set of main tubes. I wonder if I should add an external topper/sleeve around the top of my ST to strengthen things up and have more mating surface with 32mm TT? something
    ike this:

  1. This is actually the geo that I’m very much used to (still deciding on 72,5 vs 73 STA). I tried 29plus wheels when I had and ECR, too hard for singlespeed :slight_smile:
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I’m having some second thoughts on the top tube thickness. The Tange tube I got is 31.8mm OD, 0.8-0.5-.0.8
As I mentioned earlier, the bike is intended for some lightweight bikepacking. No crazy descents, no jumps whatsoever. I’m 200lbs. The downtube is 38mm, 1.0-0.7-1.0, but it’s the toptube that I worry about. Or am I being paranoid?

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I’m 82kg (180lbs) and have a light touring bike with an 0.8/0.5/0.8 top tube (need to check the dia) that I’ve done many thousands of k’s of loaded riding on. Should be fine in terms of strength. Biggest concern would be dents I reckon. Touring bikes cop a flogging.

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for the peace of mind, I’m considering to use Reynolds 631 OD31.75 1/.7/1 for my top tube. The down tube is Tange OD38.1 1/.7/1

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Just confirming my bike has a 31.8mm 0.8/0.5/0.8 top tube. And the down tube is 35mm and also 0.8/0.5/0.8.

I feel like 1.0/0.7/1.0 is overkill for a bike that isn’t going to be ridden on rough trail. I’d also say the same about a 38mm down tube. Both those are very much MTB spec tubes. I reckon a 0.9/0.6/0.6 top tube will be plenty stiff and a 35mm down tube should do the trick too.

I’m no expert though. Maybe @manzanitacycles can chime in again. I know he’s built a lot of gravel bikes that see similar use.

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Frankly, I think @buffet should rely on his framebuilder to decide which tube profiles to use. So I’m not going to give specific advice on this design. If he really wants to go down the rabbit hole, the best thing to do is take a frame building class.

In general, if a large or XL bike is going to use a rear rack, a large diameter TT, like a 31.8, is a good idea or else the frame can wag the dog. A 34.9 ST can also stiffen up the frame for rear loads. I’ll often spec 16mm seat stays too.

If a bike will carry most of the weight in a frame bag or on the fork, then you can get away with smaller and lighter tubes.

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Ahh apologies. I forgot the part where @buffet mentioned the frame was being built by someone else. Defo the right advcie to let your framebuilder chose tube profiles for sure!

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