Increasing bending stiffness of a steel tube

[jcw]

Hello,
Looking for some advice on increasing bending stiffness of a mild steel 1" x 0.065" round tube.
This is for a motorcycle frame and clearance is an issue. There is a fuel tank above and the engine below this top tube (two running parallel) that I wanted to reinforce against bending.

Tried internet searches on stiffness and learned a little about beams and elastic modulus.

So I came up with this...

The segment that needs to be braced is about 12" long from the rear cross brace in red to where the boxed section ends on the top tubes.

Thanks much in advance!

 

[SteamKing]

It's not clear why you think these particular tubes need reinforcement.

With such thin walls in these tubes, I would be very careful about trying to weld anything to the outside along the length of the tube. You might weaken the existing tube if the welding isn't done correctly.

 

[jcw]

Thanks for the reply.

the welding is the easy part. the physics is the stuff I need help with. :)

i've already tig welded the rear crossmember in place.

and I feel comfortable making a safe weld. Your caution however I do take very seriously. Thanks.


I also built this to help visualize the stress through the frame.

Looks like to me the deficiency in the frame design is that forces through the head stock are transmitted only part way down the top tubes.

Triangulating the frame isn't really possible with the tank and engine in place. (BTW, the bike is a yamaha xs750)

The idea is to make the path from the head tube to the swingarm mounts more stiff. (sorry if this is the wrong terminology)

 

[AlephZero]

I think the OP means the tubes are strong enough already, but not stiff enough (and the required stiffness depends how hard you want to ride the bike).

I agree with SteamKing. At a theoretical level you have designed an I-beam which is a good idea to increase the stiffness, but fabricating it from a thin walled tube and a couple of flat plates might cause you more problems than it solves.

 

[jcw]

I think the OP means the tubes are strong enough already, but not stiff enough (and the required stiffness depends how hard you want to ride the bike).

I agree with SteamKing. At a theoretical level you have designed an I-beam which is a good idea to increase the stiffness, but fabricating it from a thin walled tube and a couple of flat plates might cause you more problems than it solves.

I should test it on some practice pieces. I'll do it and post it up. same size tube 1" x 0.065" with 1/8" plate. I've practiced this weld "a million" times. It's a piece of cake. ;)

Maybe with a bending test set up?

My question I guess is it worth the extra weight. Am I looking at a minimal stiffness increase or better. I guess maybe I could go thinner on the web and 1/8" on the flange?

EDIT-If mods want to move topic to automotive subforum, sorry, i should have put it there in the first place.

Thanks,
John

 

[SteamKing]

Well, you're the one riding the bike (I hope).

 

[jcw]

Alright. If it is generally considered unwise, I guess I will stop.

Thinking about it, any undercutting of the tube when welding the plate to it would be pretty disaterous for the integrity and strength of the tube.

Just a thought. I'll skip it. Saves me a ton of work anyway.


But theoretically, this design could be used on things like shelves to increase bending stiffness. Just thinking out loud. Although why would you want to go to the trouble if you could just design it with a i beam in the first place. LOL.

 

[AlephZero]

Although why would you want to go to the trouble if you could just design it with a i beam in the first place. LOL.

I-beams are a good idea if you know the bending loads will be in one direction (e.g. to support the loads on the floor of a building). They are not so good at resisting bending in any direction, or torsion. A hollow circular tube is a pretty good choice if you need to deal with all three load conditions.