2

I'm replacing my mountain bike soon, and have been looking at how to improve it as a work bike. Basically I want to weld some plates onto the frame to use as mounts for things. The stand bent shortly after I got it because it was in the way of the pedal so I got rid of it. So I want to weld a plate further back along the bottom arm for a stand as well.

The other main ideas are to weld on some brackets to hold a dismountable car baby seat at the back, a pannier at the front and maybe a mount for a trailer. I've already made everything except the trailer, but I may not need a mount for that anyway depending how I decide to do it.

Does anyone know how much abuse frames can take doing this sort of thing? It's a pretty thick frame so at this point I'm assuming it's not a close tolerance thing and I can safely do this sort of stuff, just wondering if anyone else has tried.

I'll be arc welding, not gas.

  • 1
    Do you not have frame eyelets for things? all the tasks you've mentioned can be done with those. – Batman Jun 12 '16 at 0:56
  • @Batman no, the frame has only two eyelets at the back for a pannier, none on the front forks at all, and none on the back arm. But thinking on it I might be better off using those eyelets for the babyseat. – Kilisi Jun 12 '16 at 0:59
  • Steel or aluminum frame? – mikes Jun 12 '16 at 1:04
  • 1
    Steel frame, cheap one, but seems pretty solid and it's heavy. – Kilisi Jun 12 '16 at 1:04
  • The problem is that the frame is probably not made of ordinary steel. Assuming it's not aluminum or some sort of composite, the tubes of the frame are (except for very heavy, inexpensive bikes) tempered steel, and if you try to weld on them in the conventional fashion you will destroy the temper and they will fail rapidly. – Daniel R Hicks Jun 12 '16 at 1:06
3

It depends a lot on the fine print of the bike, and how good you are at guessing where forces will go. Broadly, if you make a solid structure and mount it properly the bike will usually be fine, but most parts of a bike are not designed for forces in new directions.

Generally muffler tube is 1.6mm or more and mild steel, where even cheap bikes are 0.9mm or thinner and a harder alloy. If you're confident stick welding muffler pipe you can probably weld the thicker parts of a cheap bike, and if you can somehow magic up some really thin rods it'll be much easier. You want to be down around 30 amps for much of the welding, and most transformer stick welders struggle to maintain an arc at that sort of current. Modern switch mode ones work better, even without HF start, because they can up the voltage to keep the arc active and drop it back when the arc starts, on a millisecond-to-millisecond basis. You can't do that by hand :) If you can get gas a TIG kit for a switch mode welder might be worthwhile, but I suspect you might struggle to do that based on your comments.

Normally I wouldn't spit on a gasless MIG, but for you it might be the least worst option if you get the chance to try one in civilisation on day (FFS don't mail order one sight unseen). Maintaining the flux cored wire is not much more hassle than keeping welding rods dry, but you can weld thinner tubing more easily. Or you just just decide to burn 10 or 20 rods getting the hang of thin wall tubing (or just design so you don't need to weld it very much). Running a bead along something solid so you're just barely melting into the thin tube might be the least awful solution. You can't easily join two thin tubes, but you can still do a lot. Or maybe "pre-set your filler" rod by tying non-galvanised wire along the join before you weld, to bulk it up a bit. I haven't done that with stick, but I do it occasionally with TIG with I have something really tricky to weld (a sudden transition from 6mm dropout to 0.6mm tubing, for example)

Welding a new stand plate on, for example, should be fine as long as you can weld the thin wall bicycle tubing. Look closely at the existing plate, most of them are kind of )o( shaped, with the tapered ends designed to give a gentle transition in the amount of force applied, so that the thin tubing of the bike can better cope with the load. Cheaper bikes sometimes fake this with tack welds, or just don't bother because the cheap tubing is thick enough that it doesn't matter. But then you go and load the bike to twice what it's designed for and it does matter.

Panniers and load platforms can often mount to existing attachment points. The hassle is that welding to the seat tube has to be done very carefully, and if you can back purge with TIG that's even better. If you even roughen the inside surface of the seat tube it can make getting the seatpost in difficult to impossible. Filing that smooth requires a half-round file hand patience (or a rotary tool with a filing bit plus skill and luck). You're trying to file 0.1mm off a tube that's 0.9mm thick at most.

Ideally you'll add vertical loads to or very near the axles. If you're lucky the dropouts will be cut rather than cast, and will take welding well. In that case you can just leave enough space to do up the wheelnuts and weld directly to the dropout. Making a rack out of whatever you have, as one piece, is usually fairly easy. Making it light and strong is the hard part/ So give up on light :)

My limited experience is with chopping up bikes to make prototypes, and they seem to survive that better than I expected. Skim the "I built it" section of my site and you'll see a few hacked-about-with bikes. http://moz.geek.nz/mozbike/ and specifically the "Bin Bike prototype" which was a BMX with extras, including a rack largely made of 10mm reinforcing rod.

  • wish I'd seen your site earlier, I've had a shot at welding plates onto a wreck today, it was finicky but doable. – Kilisi Jun 13 '16 at 16:20
1

You cannot stick weld ("arc weld") on a bicycle frame unless it is some custom build bike with thick tubes. You would have to be so extremely good with a stick to not blow through the tubing. And you would have to use some special rods that have a lower melting point. If you are going to weld on the frame you need to either use a MIG or TIG (preferably TIG).

But a better and easier idea is to go the no welding route. Get bolt on attachments for the rack mounts. These clamp onto round tubes and provide mounting points (called P clips sometimes). This prevents any unwanted side affects from welding and very easy to install.

Here is a seatpost clamp that create your upper mount points for a rack: http://problemsolversbike.com/products/seatpost_clamp_with_rack_mounts

P Clips for seatstays to create your lower mount: https://www.evanscycles.com/tortec-p-clips-00120876

  • Here are even more options: bikebagshop.com/rack-accessories-c-89.html – Brady Jun 12 '16 at 10:28
  • Unfortunately I can't buy the sorts of thing you might take for granted, they just don't exist here unless you can get them off a wreck. Nethertheless you make a good point and I have been looking around searching for wrecks. I've arc welded plenty of rusty old exhaust pipes,are bike tubes harder than those? – Kilisi Jun 12 '16 at 11:02
  • If you are crafty, you can easily build those things in those links. They are essentially straps wrapped around tubes. The difference between a bike tube and the car exhausts is the ability to dissipate heat. Car exhaust tubes are large and long, they will spread the heat out more. Bike tubes will be a little harder. Plus a car exhaust tube is not load bearing, just contains gas - strength is not a real issue except for vibration. My dad has been welding for 45 years professionally and I am a Mechanical Engineer, grew up around welders. – Brady Jun 12 '16 at 15:04
  • 1
    They all function via an arc of electricity, but in the US the term "arc welding" is what people use to refer to a stick welder since it is the most common form of welding. So we may have got some terminology mixed up in this discussion. – Brady Jun 13 '16 at 10:27
  • 1
    I updated my answer to be more clear. Replaced arc welding with stick welding. – Brady Jun 13 '16 at 10:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.