I am building up a fatbike frame that has a twin top tube setup;



I am wanting advice on the material for the plate at the seat tube to top tube junction. The tubes are 4130 and Columbus Thron, and the plate will be fillet brazed in.

I have some AISI type O1 tool steel, but i am extremely reluctant to try and braze this and expect no cracking..

What should i be searching for?

https://pogwardbicycleindustries.blogspot.co.nz/

I would go for a piece of scrap mild steel the right size. I know it's always tempting to use an alloy steel, but I don't see the point here. Save the O1 for a knife.

Why does the joint require a plate? A piece of tubing might serve well here. I'm also curious to why you're using a twin tube top tube.

I also see that your design shows a segmented fork. Do your homework well with it. Search this and other building forums for others' experiences. Andy.

I guess mild steel should be ok, as long as there is enough joint area... it is not a massively stressed area of the frame. This is good, I can get a plate laser profiled to suit...

The twin top tube and plate is because I think it will look cool :-)

Apart from joint strength, what are the issues with that design of fork? A wee bit more info will help me narrow a search down - thanks.

https://www.velocipedesalon.com/forum...archid=4469833

This is a search result list from Velocipedsolan's frame builder forum. There are a couple of fork failures shown and discussed. Lots of info on tube dimensions too.

I was going to suggest mild steel plate too but decided my curiosity was more to the general design. Andy.

If you search on your favorite search engine for "segmented fork," you'll find that there are lots of threads about building them. People have been building segmented forks for decades now, including on crappy department store bikes. You have to be careful and trying to save weight is a really bad idea. Experimenting with new designs might not be a good idea on a first fork. I have to say I'm more comfortable with other construction methods, but segmented forks are a proven design.

My Touring Bike ,Mandrel-bent loop-stay - Twin Tube .75x0.049",4130 , got a gusset plate Joining the 2 top-tubes to the headtube.
fitted to bottom of the parallel tubes.
the Top tubes , passing by the headtube , offering a place between them for nestling the Frame Pump.

(it was a field repair , Stainless .125" plate , Tig, stainless filler wire )