Modifying a frame
 

Hello All.
I have been asked to shorten a seat tube from 24" to 17-1/2". I have no experience modifying or building bike frames, but do have lots of fabrication and welding experience. I have a few questions I would like input on before I agree to do this work...1) Is it acceptable to miter the top tube (degree to be calculated) just rear of the head tube and forward of the seat tube and perform butt welds to change the angle of the top tube? Or would it be better to remove the existing welds and re-do the original junctions? 2) I would like to remove a chunk at the middle of the seat tube so that I do not have to mess with the seat post clamp or worry about the seat post not fitting. 3) Obviously the seat stay angle must be changed also while maintaining axle center to brake mounting dimensions. This should not be much of a problem. The original frame joints are all butt-tig welded and I believe it is chrome-moly tubing.Thanks in advance for any advice.
P.S. No need to comment on any liability issues.

There was a recent thread about that :thumb:

https://www.bikeforums.net/19899854-post3.html

https://www.bikeforums.net/attachmen...9&d=1268878910

Ok, so can you post a photo of the frame you're working on? Go to the Advanced editor, and click on the paper clip "Attachments" icon, https://www.bikeforums.net/images/editor/attach.gif then upload.

Anything is possible. I suppose the first question is WHY? If it is a quality frame, then there must be better alternatives, and if it is a cheap frame, then they're a dime a dozen.

If you wish to take a horizontal top tube and make a sloping top tube, then you'd have to be mighty lucky with length as the angles should make the tubes longer, although perhaps the angled seat tube will compensate slightly. Still, if you're cutting miters into the tubes, you still may have a length problem.

I've also found it difficult to do precision cuts around welded joints.

One thing to also consider is that a smaller frame isn't just shorter in height, but is usually also shorter in length as the short rider's arms aren't quite as long as the tall rider's arms. And many of the angles are typically changed slightly. So just cutting a frame shorter in height could create an unwieldy frame size that isn't built proportional to the rider.

Smaller frames tend to have a steeper seat tube, and a greater angle for the head tube. Larger frames are the opposite with more of an angle for the seat tube, and a steeper head tube.

Are your tubes "double butted", or "straight"?

The Double Butted tubes are thicker at the ends, and thin in the middle. One end often has a little longer but than the other, but you might not know until you cut it open. Away from the butted end, the frame tubing is mighty thin.

TIG welding is the preferred welding method due to working with the thinwall tubing.

One other note, many of the vintage frames (60's to early 80's) were brazed, so technically the frame should be able to be completely disassembled, cut down, then reassembled (and aligned).

the best advice I can give you is don't take the work. If it's a good frame, you will be cutting away all the butted sections. If it's not a good frame, you can't charge enough

Frames are, to a degree, just Erector Sets with welding, brazing or bonding holding the links together. So any tube is replaceable, sort of/kind of. Where you get tricky is with joints with more then two connecting tubes. Since most top tubes are joined at either end to only one tube (exceptions exist like Hellinec stay attachment) playing with one is pretty straight foreward.


First thing is to ditermine if the resulting frame geometry/fit will work for the next rider. Since lowering the top tube won't shorten the reach to the bars (by its self) this aspect should be considered, will a smaller frame also need a shorter reach to the bars? Finding the answer will potentially avoid a lot of work which might not work out after all. As mentioned above the rest of the frame's geometry also might not be ideal for a rider needing a smaller stand over.


Only then will the questions of the frame's construction be considered. Welded frames will have issues with localized distortion about the welded joint. If this area is maintained in the rebuilt frame then how the old weld area effects the new set up and joining is up to the skill of the "builder" to a great degree. Brazed frames have more leeway here as there's usually less local distortion/surface remnents of the OEM joint. Of course there's no rule that a modified frame has to have the same joinging methode as the OEM was done with.


To do this and retain the tube joints (with the head tube and seat tube) would result in an odd top tube path. First starting up high on the head tube (at the OEM location), then angling down to a stand over equating with what a 17.5" frame might have, then returning to the height that the seat tube attachment stub is at. Not sure if this really gains much in real life. There are reasons to retain the OEM joint and only replace a center section of a tube, but not when the tube's location is so radically different as the OP stated his goal was. I have done this a few times for various reasons but never to make a frame a different size or geometry.


Next up Are the seat stays. Where are they attached to the seat tube? Will the new lower size require to replace them too? If so then the job just got much larger in scope. Just removing a secion of the seat tube won't retain the stay attachment angles, like the top tube joints also will shift in there angles. Along the lines of a bigger job is the question of how well aligned is the OEM frame? Don't assume here as once you've changed the frame you own any issues whether they are of your doing or not. If you have to replace the seat stays then all tubes attaches to the Bb are free for alignment without effecting other tubes directly. If you don't understand this then I suggest you start your frame repair career on more simple jobs. If you have no ability to control this alignment you need to figure this out before commiting to the work.


I have lowered a frame only once. I was building a frame for a "friend" years ago (long before I became convinced of the need for business insurance) and he needed a very short reach yet a tall stack. In my youth I cracked the seat tube when brazing the stays onto it. I well overheated the joint, it's third heat cycle as I had done a special binder barrel after the top tube/seat tube was done. The crack was right below the seat lug bottom. I put this frame aside for the time, returned the deposit to my "friend" (and that was the best decision proven by hind sight later) and later returned to the cracked frame. I cut off the top tube by sectioning through the headtube and seat tube just below the lugs. A new top tube, both lugs and seat stays were installed. I ended up with a mid sized frame with a long reach.


The few top tube replacements I've done while maintaining the OEM geometry/fit have all been larger jobs then I first thought. Nothing went wrong just took a lot longer then I had estimated for. Repairing frames is a great way to learn a lot and to get stuck in job with expanding time/effort. There's a reason that experienced builders often only offer to repair their frames, not others' work.


Given the scope of the possible job and the likely lack of experience in the OP (and he wouldn't post here if he was sure of the job's issues) I suggest to pass. Andy

It is a half-recumbent bike. The owner is determined to have it modified but I will not attempt it if they are able to find someone more qualified than myself. I am not in this for the money and have no intention of making this a career.

Here is a quick and dirty practice weld on a scrap piece of down tube. A little more build up or a cover pass would be nice, and some solar flux on the inside would eliminate the crystals.

Now that is a unique bike.

Bilenky semi-recumbent tandem

And it helps one to better understand the point of the project.

If it is a low volume mostly custom build, it may well have straight tubing which will simplify things a bit.

Do you have access to new chromoly tubing for splices & etc?

The idea of butt joint welds is probably ok, although alignment and mitering will be critical. Another option is to add an internal sleeve, and braze.

A couple of things I'd suggest. First would be to take the owner's favorite bike, and see what it will take to match the seat position and the handlebar position with respect to the bottom bracket. That may be simply moving the seat forward/back on the seat post and getting a different stem, or it could mean some radical revisions.

Is that a water bottle cage mount on the head tube? One on each side? A most unique place to put it, but it does get it out of your way. They are also commonly put on the seat tubes.

Oddly, your chainstays and seatstays appear to be nearly the same size. That could mean the seatstays carry more weight than other bikes. Looking at the seatstays. It appears as if you have some kind of hydraulic rim brake. That means you have to more or less maintain the distance between the rear dropout and the brake bridge, which brings you back to rebuilding your seat cluster. Doing so may not be as bad as it looks, although you may need to ream the seat tube afterwards. Another alternative might be to use bent seat stays, and change to disc brakes, but that is a different project to make the mounts.

Fortunately there is no need to worry about toe overlap, so you could shorten both the top tube and bottom/down tube if necessary.

Whatever you do, I'd build it so it would take a mighty long seat post. I'm seeing some about 410mm long, 27.2mm diameter seatposts. And you need to provide room to drop it down 100% to the bottom, or raise it as high as it will go (thus giving flexibility for rider configuration).

Well! This is a different beast. Before I would proceed I would want to know far more. Like the rear rider's fit needs. Is the saddle at a "correct" height in the photos? If so then I suspect the real issue is stand over and the seat stays and seat tube can be left as is. This would significantly lessen the work. In fact if just a lower top tube is all that's needed then one could join a new tube just below the current and still in place one. Then after the welding/brazing cut out the old top tube, finish off the raw surfaces as the budget allows for.


My hesitation still exists but the project might be less complex then I first thought. It all hinges on the rider's needs. Just a lower stand over or other changes too. Andy.

No, the seat is not at the correct height in the pic. She has since assembled the bike, ridden it, and mentioned that the stand-over height was the biggest issue. If lowering the top tube is all that is needed it should not be difficult. Regardless, if I decide to do it, I want to do it properly. She has also given me the builder's contact info, so I will try to find out what type of tube was used and get his input.

I don't quite understand this. Can you elaborate a bit in layman's terms? Also what is meant by "toe overlap"? Forgive me if I sound ignorant, but I am not familiar with a lot of the terminology used in the bike world.
Thanks, Greg

Toe overlap is the dimension between the tip of your shoe (with the foot on the pedal at the most forward point) and the front tire's back side. Front center is the distance between the BB center and the front axle center. The two relate to each other. Some riders have more issues if the tip of their feet contact the front tire while turning then others do. Just one of the questions a builder will probe with their customer.


" If you have to replace the seat stays then all tubes attaches to the Bb are free for alignment without effecting other tubes directly." means that if the tubes that are joined to the BB shell are not constrained by their other ends (by not having a closed loop or complete triangle) they will be easier to move about to effect alignment. If the seat stays were left joined to the chain stays and also left joined together (at the bridge or seat cluster) then no independent vertical movement of the chain stays will be possible.


This comes from doing enough repairs and such on frames to know that many are not all that well aligned from the factory. If I do a repair I assume the job of frame alignment being right after, regardless of if the frame was straight before.


Back to the rider's seat height. Can it be accommodated with the seat tube left as is? I still am not sure if this is the case. It would simplify the job a lot if this were the case. Andy.

Bilenky does all kinds of goofy mods, the owner should ask them.

Your bike does derive some strength from having the top tube meet the seatstays in the same place, although perhaps it wouldn't make a big difference. Some bikes are designed with the seatstays hitting the seat tube much lower than the top tube.

That seat clamp looks unique. Does it have some kind of internal wedge to catch the seat post? You still should be able to safely cut the mast down to the top of the clamp without hurting anything, giving you another 3/4" or so of seat post adjustment.

Anyway, if the seat can be positioned properly, then I would encourage the owner to get some more experience riding the bike before making any changes, then just chop the top tube if needed.

There are a lot of options. For example, you could replace the top tube with a new tube with a curve to lower the tube in the middle, but still attach it in the same place on both ends.

The "Mixte" style is to have two tubes that extend all the way from the head tube to the rear dropouts to give some extra strength with a lower top tube.

And, many women's bikes have a low top tube, although I'd worry about extra flex being introduced into the tandem. But you could also reinforce it.

OK, so before I dismantle anything I need to measure and record everything including the offset of the dropouts to frame centerline. When it goes back together the rear axle must be parallel to and level in relationship to the bottom bracket.
The seat tube does need to be shortened, but she now says only 2"-4". My main concern will be re-attaching the seat stays to the seat post. I do not have a seat tube reamer and that is a lot of welding that the builder did originally. Perhaps a die grinder could do the cleanup?
I am not sure what's up with that seat post clamp. Normally there would be a slot in the seat tube correct?

Yes, normally a slot in the seat tube. But, there are a number of external clamp handlebar stems without a slot, but using a wedge instead. I assume something similar is being used. There are also some "quill" type seat posts that have a wedge as part of the seat post.

Actually, you could gain about 2" by simply cutting off the seatpost mast, and adding a quill seatpost.

Brazing (fillet brazing) would give you less penetration and distortion of your joints.

Sounds like cropping the seat tube may be good enough for ride height. I have told the owner to look for a quill type seat post that will fit. If all I have to do is lower the top tube for stand over height, I can do that fairly easy.


I am told that the builder likely used 4130 straight tube. From the Miller Welds site I see ER-70S-2 filler is preferred, which I have.
I have not brazed anything since welding school 35 years ago!

A couple more practice welds!

To me, your welds look good with good penetration. You can get tubing to route argon into the fame tubing if desired, but I'm not sure it is necessary.

What size is the seat post? I could probably locate a 27.2 quill post for a reasonable price.

Attachment 584096

Oops, it looks like the retention mechanism will limit how far you can drop the post :(

So, your actual gains may not be much more than simply trimming the top off of the mast.

Assuming that is some kind of a cam lock that you have, you could probably pretty easily move the cam lock to the under side of the top tube, or seat stays, and braze it into place.

In the end, you'll have a most unique looking bike :thumb:

You can make a quill seat post out of a tube seat post as well .. Quill bolt head in the top,

Brompton's saddle clips are superb..

Quill post - if i made a shorter quill post would it clear the retention mechanism?
 

Really appreciate everyone's feedback,.
Quill post - if i found a shorter quill post would it clear the retention mechanism? What is the retention mechanism you're referring to? I've been using the bike wearing thicker soled hiking boots to get a little bit higher over the top tube. Definitely riding the bike every day has helped get a better idea of what we could compromise on - so its not a perfect fit but it still rides beautifully and i can reach the peddles, would be alot easier on my knees if i could lower the seat - is there a clamping system other than the quill post that takes up less space than the super clamp that is currently used - like a snail clamp? but still allows the post to clear the retention mechanism?

[MENTION=391685]Andrew R Stewart[/MENTION] [MENTION=392454]CliffordK[/MENTION] , some really awesome information you guys are posting here. I'm sure the OP is getting more sound advice that he could have hoped for and I'm subscribed so that I can stay tuned.

You are not getting you mentions. I tried to mention you in the thread in post #21 but due to the extra spaces in your user name the "Stewart" was dropped just mentioning "Andrew R". I clicked on that user name and it took me here https://www.bikeforums.net/members/andrew-r-391685.html an inactive member with 3 post back in 2014 but 14 mentions (that I suspect all was meant for you). I then tried to manually move the [mention** to the other side of Stewart but it still on took the Andrew R. I don't know how you would fix it but I thought I'd let you know that you've been missing you mentions.

Oakesje, welcome to the forum. Is the reach to the handlebars ok?

Now that I have seen the bike, I think I would cut out the top tube and the seat stays and replace with new parts at the desired standover height. I don't think you can move the top tube down much at the head tube, maybe a little, so the top tube would be slanted. Modify the seat tube as necessary. If it were me, I would sleeve the seat tube to reinforce the joint at the seat stays and top tube.

I would not splice a tube in the middle. Just cut the old one out and get a new one in there. I don't think we understand the loading of this bike well enough to be possibly compromising fatigue strength

A few suggestions for welding thin wall chromoly. It's prone to contamination from being exposed to air while it's still hot enough to be reactive. Use a gas lens with a least a #8 cup and a longer post flow. Don't try to weld all the way around the joint. It will build up too much heat. Weld small segments and the move on to another joint to allow it to cool. ER 70S2 is fine, ER312 makes a nicer looking weld.

I've mentioned a few people without using the @ symbol. I hadn't thought to look at the profile to see if the mention shows up. I didn't realize it was an option.

Oddly, the dksix profile seems to have all the information blanked out including mentions.

I usually copy and past the user name, which automatically creates a link, but may not make it show up as a mention. @Andrew R Stewart

Edit:
Hmmm, the format comes out different. Perhaps one could force a "mention".
Andrew R Stewart