bloom87

Hi !
I'm a seasoned bike mechanic but I know nothing about frame building.

There a TIG welding co-op that popped up near where I lived, they teach courses and you can use the shop if you're a member.

I would really love to build my own frame, but I think I should probably start a less ambitious first project.

Basically, I'm wondering if it's possible to TIG weld some tubes on my old bianchi CX frame to reinforce it ?
If I have a rear rack load, the bike becomes pretty unstable and a bit scary to be honest.
The reasons for this is that the frame is quite tall (long tubes) and that it only has a slightly OS downtube.

I've seen double top tubes in Mexico... would that help ?
Or a tube that connects the DT and the ST ?
What about the rear triangle ?

Thanks for any ideas !

guy153

Your Bianchi looks like it's brazed with lugs so the first thing to check is that it's actually a weldable alloy.

Not sure what the best modification would be to make the frame stiffer. A double top tube might need a sleeve around the ST where you're attaching it because you might be past the end of the butting at that point.

If it's your first time with the TIG I would anticipate needing a bit of practice. Make a few non bike projects first using cheap 1.6mm wall mild steel tubing, probably square to start with. Then move on to round tubes and figure out how to mitre them and get used to welding around the curved shape. When you can do nice joints on those find an old bike frame, cut it up and weld it back together again to get used to the really thin-wall tubes. If you just start attacking the Bianchi right off the hop there's a risk of blowing a lot of holes in it. You really only get one shot at a nice TIG weld so have to have the practice.

alo

If you have a lot of the weight down the sides of the back wheel, it will be easier to balance. It needs to be back far enough so it doesn't interfere with peddling. Then if you carry too much weight back there, the front wheel will come off the ground when you are not on the bike. You can carry some things on the front to counterbalance this.

Ensure the wheels are strong enough.

The wider the tires, the more weight they can carry.

If it was me, I would start with a frame that is strong to begin with.

Or is this a reason to practice welding. If it is, I could think of a lot of ideas.

bloom87

Thanks for the comments so far
The tubing is Tange Prestige ! Indeed there lugs and it must be brazed. So I would need to check if TIG is possible at all... thanks for the tip ! And maybe add a sleeve because it's past butting (metal too thin).
I would practice before doing this, yeah for sure. This is has been my daily ride for years now, there's no way I'll practice on it.

I guess my question is more of a physics question : if I were to add a tube to make the bike more stable with rear rack loads, what would that tube be ? double top tube maybe ? like Mexican/3rd world type bikes ?

Thanks for the comments :-D

alo

Having a double top tube would not make the bike significantly stronger for carrying heavy loads.

How heavy do you you expect the loads to be?

Why don't you make your own rack. You could make it larger than one you buy off the shelf. Keep weight in mind. If it is really good, but too heavy, you wont want to use it long.

Andrew R Stewart 

Agreed that load placement can make a huge difference. Good solid racks and having loads shared between the front and rear really help maintain the balance of the bike. But do know that this CX bike is not intended to carry any more then a water bottle, it's for racing not touring or basic transportation. The Bb height and steering geometry are the two bigger aspects that you will never change.

Having said that I do enjoy a senseless mod sometimes So what would I do? If the loaded issues are steering shimmy then stiffening the frame's torsional flex is a big goal. Double TTs don't do much for that as the extra tube isn't in the line of twist. tandems use a internal lateral tube, from the HT to the second BB, to achieve this. For a single think mixte external laterals. Mid way down the back side of the HT to the seat stays just above the drop outs is about the most direct line you can add tubes and avoid the brass lugs and drop outs. (TIG and brass are not friendly to each other).Like a mixte attaching these external laterals to the ST mid way on it's length to further reduce the un supported lengths. This has been done by really smart builders long ago before larger diameter tubing was widely available. But it does significantly change the image of the frame. So many these days would scoff at the results.

Further comments- I note the seat forward position. On a racing inspired with usually not slack STs this means there's more forward weight as well as less upper body sup[port from your core muscles (and less gluts involved in pedaling). Then there's the fairly high and close to the seat bar grip locations. I see a bike set up to achieve a short reach to the bars with a very upright upper body. Not at all what I would call a well balanced position that reduced effective power and increases handling issues. That with a racing handling intention it's no wonder that your loaded experiences are less then ideal.

So play with the frame mods all you want but I doubt you'll be happy after them. Are there reasons why this positioning is the way it is? have you tried other fitting positions? Do you have flexibility or medical limitations we don't yet know about? Have you tried other bike sand if so do they feel better? So many questions and little data to help you choose a path. Andy

unterhausen

The old French guys would put diagonal tubes on large touring bikes. Not sure how effective that really is. I think putting most of the load down low on the fork probably does a lot. People carry too much while touring. Cut back on the weight. If you really need to carry a lot of weight, front lowrider rack plus rear rack is the way to go.

wsteve464

TIG weld a bike trailer together and leave the frame alone. Unless you are really good with a tig torch you will most likely just screw up the frame.

Another option, buy a second bike and modify it into a cargo bike.

bloom87

Thanks for all the comments.

@Andrew R Stewart
The seatpost height was a bit too high on the pic -- I had just switch saddle and there was a diff. in rail height, now it's back a bit lower.
I say "CX frame" but it's really an hybrid frame. There are eyelets for mudguards and for a rear rack so it's not racing only. Current position setup is for the city. I would make adjustments for touring.

I guess my question was really about double top tube effectiveness and/or diagonal tube effectiveness and it was answered : not really effective.

I'm going to balance the load. It's just that my partner has a knee problem and I must take more load than in the past. I mean, I enjoy the extra load because I can exercise more, it's just the wiggly frame that's a bit problematic and even unsafe when I need to steer a bit at higher speeds on descents for instance. Going to look into bigger tires and switch my front rim to a Rigida DP18 36h... we'll see how that goes.

Andrew R Stewart 

The suggestion of a trailer is the best one IMO. Good job Steve!

Eric- I very much agree with the weight comments. But this can be a do as I say, not as I did. (On my part cross country tour in 2017 my bike and load weighed in at 111 lbs in Missoula, at Adventure Cycling's offices). So I very well know this weight issue

bloom87- Hybrid? Really? That frame has a really short chain stay for a recreationally intended bike. Also the chainstay/seat tube angle is pretty open, suggesting a fairly high BB. What brand and model is it? I could be easily wrong but my eyes don't lead me to think my opinion is off. When you "miss label" a bike it's hard for us to assume different, especially when we see visual aspects supporting your term. I never made reference to the seat height. I did comment about it's far forward position on the post clamp. Was that confusing? if so I apologize for not being more descriptive. There are many wanna be CX bikes with eyelets and rack bosses out there. (Surley and Specialized come to mind). Having accessory options does not make a performance geometry work any better for the loads and use the mounts might allow.

I doubt that larger tires or a different wheel will make a significant difference. I have built myself 4 touring intended frames and the earlier three suffered from shimmy, even at low speeds. I have spent hundreds (more like thousands) of miles with the bike wiggling along at 12+ mph. The first three had in common classic road bike tubing diameters but varying geometries (and trails) as well as increasing tire/rim sizes. It is number 4 that solved this shimmy. The biggest difference is the OS tubing diameters, including the little mentioned influence of a stiffer steerer (now also larger). I am confident in what I say about touring loads and the bike's design. That is why I went to the external lateral suggestion right away. it offers the most increase of total frame (but not fork!) lateral stiffness for the least work or insult to the frame's existing construction.

But as I qualified my first reply I see this as a somewhat senseless project. get a trailer and be stable. Andy

unterhausen

I think the OP needs to get some good racks and distribute the load front and back. Unless the need for a huge amount of weight is insurmountable, then a bob trailer.

Any bike is going to feel like a noodle with a lot of weight only on the back. I'm pretty sure it's the fact that there is a big hinge in the middle of the frame (headset, in case anyone doesn't see it).