What would you do? TT to ST tube diameters
 

Hi everyone! I'm new to the forums and also new to framebuilding.

I was wondering if someone could help me in regards to joining Top Tube and Seat Tube. For some reason I've been designing the frame with a 31.7mm TT and a 29.6mm ST but that doesn't feel like a great joint to me. What would you do? Would you get a smaller TT or a wider ST or maybe not do anything about it? Even tubes with the same diameter feels like it won't be ideal right?

It's a gravel type of bike and I've attached what it currently looks like this in RattleCad. Lowered the seat stays so you could see the current issue. Oh, and it will be fillet brazed.

Would love some feedback for a newbe like me!

Noticed I wasn't allowed to post images yet. Doh!

Thanks guys!

Welcome to the forum. You can post a picture in your gallery and someone will rescue it for you.

I would either use a bigger seat tube or a smaller top tube. Your tubes are strange sizes, standard is 28.6 and 31.7. The issue with 31.7 is availability of seat posts, although you can shim them.

I often will ovalize the top tube vertically to give more area for a fillet. Doesn't take much.

Hi and thanks! I've uploaded an image to my album if anyone is interested in taking a look.

OK, yeah the seat tube is double butted with an outside butt on the top end so that's why it's 29.6. It's 28.6 at the BB shell. Smaller top tube is what I'm leaning towards actually.

How do you ovallize them evenly? Do you have a mold you put them in or do you do it by hand? Also, before of after mitering? =)

Appreciate the help!

I ovalize using my vise and soft jaws. Just at the end. Sometimes I'll do a downtube vertically at the HT and horizontally at the BB. Anyway, you only need to do a few inches worth of the tube, BEFORE mitering.

Great. Thanks David! I’ll give that a try since I already have all the tubes :)

link to Aldatroid's pic
https://www.bikeforums.net/g/picture/18723578

https://cimg5.ibsrv.net/gimg/bikefor...9a9a1e4ad6.jpg

One option is to leave the tube diameters as planned but have the TT miter's ears cut flat/vertical and then have the seat stays tops butt/flow into the TT. If done well the stays flow into the sides of the TT with no edge or step. Of course this means that the stays will not butt into the ST below the TT as the imaged plan shows. Andy (who never really likes the fast back stay style even after owning a couple of early 1970s Raleigh Pros)

Ah yes, good point, thanks for the input. I'm planning on having the seat stays join where the TT intersects with the ST, i just move the stays down to illustrate the issue =)

Or you could just fill the gap with filler.

Have you checked for side tire clearance with fastback stays? Are you using s-bend seat stays? Side tacked seat stays offer significantly more clearance if they are straight.

Working on a full scale drawing now so will know soon enough but I have s-bent stays I'm planning on using so should be fine.

I might be over thinking it, this being my first frame but I'd love for it to look decent at least =)

If you are referencing my suggestion just to fill the gap, it all depends on how you finish the ends. If you file them neatly with a scallop, they will look like a design choice. I can imagine other ways of finishing it that wouldn't look quite as polished. The ears of the miter are going to be fairly long and thin, so just bending them in after mitering is also something I would consider.

A 31.7 TT seems oversized for such a small frame. Are you around 5'8"? Unless you're a really powerful rider or pretty heavy or both, I'd suggest moving to a 28.6 TT and 31.7 DT. Then you don't have to worry about ovalizing the TT to fit the ST.

Also, with a front center of 606 if you're using large 700c tires you'll likely be flirting with toe overlap. Some riders don't mind it, but I hate it. One solution is to push out the front end and shorten the stem. If you're running 650b then you'll probably be fine, but it's something to consider nonetheless.

I've done several frames with those same tubes and slightly ovalizing works well. I've also just rounded the ears with a file and then flattened them slightly to make good contact with the seat tube for brazing/welding. As was already mentioned, you can line up the seat stay joint with the ears of the top tube and flow the fillets into each other to cover the ears of the top tube. Otherwise, it looks just fine with an external fillet that transitions into an internal fillet at the ears, as long as you round the ears nicely before you braze it.

This issue is much easier to deal with if you are welding. This picture is a 35mm top tube welded to a 32mm seat tube.
https://cimg1.ibsrv.net/gimg/bikefor...7d5786e98f.jpg

I'll second the suggestion to push the front end out and run a shorter stem. I'm 5'7" and I design my personal gravel bikes to use a 50mm stem for several reasons, the least of which is toe overlap.

Great points. I mainly choose the tubes because I like the look of slightly larger tubes but then again the ST is only 28.6-29.6 so a 28.6 TT might look more aesthetically pleasing. Not sure.

Yeah around 5'8" and I’ll be putting a pair of 650b 47c tires on there. Last time I checked the drawings it looked ok with the toe overlap but I’ll double check that because it’s also something I’d like to avoid.

I haven't gotten used to the look of a short stem yet, but I like to run randoneur bags on all my bikes and a short stem makes that work a lot better.

I like the idea of a little less weight on the front wheel so it will float better and possibly reducing the feeling of not going over the handlebars. Although I don't think that is a big factor in my riding. Anything else recommending it?

Mostly its to get the front wheel out further to better deal with obstacles and to reduce the feeling of going over the bars on steep rocky/rooty descents. I like to be able to ride my gravel bike on singletrack, as well as gravel roads and having less weight on the front wheel makes it easier to lift it over obstacles like downed trees. I prefer to accomplish that by keeping the same HTA and trail and pushing the head tube out, rather than going to a slack HTA which will result in more wheel flop on low speed climbs.

Short stems seem to offend people who are accustomed to seeing "proper" drop bar bikes as having a long stem. A lot of people also mistakenly believe that a short stem will negatively affect the handling of the bike.

Thanks, I'll have to see if I feel more confident on steep rocky descents on my new bike, if I ever manage to finish it. Mostly I worry about not being able to see because the rocks have shaken my helmet over my eyes. I definitely don't like wheel flop.

Yeah, it's something I figure I will get used to. Someone asked me to build a zero length stem for a recumbent. At first I said it was a bad idea and then did a free body diagram. It turns out that the stem has near zero influence at reasonably small steerer angles. It's one of the things I have decided to ignore for mental health reasons.

Pushed the front center a bit and change the HT angle to 71 instead of 70.5, now I have a trail of almost 64, before it was closer to 70. I've been going over all the number again and some of them have changed quite a bit actually. Should be closer to what I'm looking for.

I've uploaded a second picture to my album and inching closer to those 10 posts as well so I can post pictures =)

Trail of 64mm is pretty much what I aim for if possible with 700c wheels. Andy

new design

https://cimg3.ibsrv.net/gimg/bikefor...83d81dfd2e.jpg

Thanks unterhausen!

The front end looks good. The saddle height looks low for a 5'8" rider and the standover looks too high, if you have input the correct value for that. I would check on that standover(769mm on the drawing) and change the top tube angle to get some standover clearance.

Hey! Thanks for the feedback. Hmm ok. So, I'm 5'8½" with an inseam of roughly 77.5 cm(measured at home =) ). Going for crank shafts that are 170mm. I measured a couple of my bikes and yes, the saddle height is too low on the drawing, it should be around 660mm. Stand over height on those other bikes are between 770 and 780mm which could be lowered a little of course. What would you have expected those measurements to be?

Thanks!

I have a similar inseam and my saddle height (680.5 on your drawing) is over 700mm. I won't say how much over, because my position is a bit higher than the standard saddle height formulas would suggest. On RattleCad this is important, because the seat tube angle changes as a result of placing your saddle at the desired height and setback. In other words, as you raise the saddle, it also goes further back and RattleCad will make the seat tube angle steeper to keep the desired amount of setback.

As for the standover, it just stood out to me as the drawing shows the frame being 10.26mm over your set value. If that height works on your other frames, its not a problem.