Looking for advice on adapting frame
 

First post here but I think you all will like the project I am starting.

I have a Vroom electric bike that I want to adapt for my SO to use. Because of a recently developed permanent nerve issue the motion of leaning forward and sweeping a leg over the seat isn't an option so I want to make this a step through frame. Basically I want to remove the cross bar and move it as low as possible then add reinforcing tubes where needed. I added pictures of the frame and what I plan to change to my gallery but can't post links to the picture until I have made 10 posts myself. I would rather not just post smileys on a bunch of posts to get the count so I will post this and hope for the best.

My initial thoughts on how I would modify the frame would be to remove the red tubes and add in the yellow. I think I would use 1.5" C channel that would be welded directly to the remaining tubes.

Would the planned additions give the strength needed? What changes would you make?

Should also add that appearance isn't at all a priority getting something functional for as low cost as possible in most important.

Thanks for the help!

Is the frame steel? I can't find any info on this bike on the interewebs.

Yes steel.

The down tube (tube from headset to bottom bracket) is 1.75" OD while the seat post tube is 1.25" OD.

Its an older and now defunct electric bike company. Batteries are impossible to find but I am going to power it with Makita 18v rechargeable batteries.

You are referring to a diagram on a picture no one can see... because you just got here & 2 posts are 8 post too short of the minimum

to offer any pictures..

Pretty sure he knows that, Bob.

My recommendation is to get some tubing. Possibly the easiest way is to get some 1/2" tubes for laterals, like the old fashioned mixtes. Go from the head tube to the rear dropouts. Some of those bikes had a bend in the laterals to make the step lower.

Otherwise, get a 1 1/8" tube long enough to go from the head tube to the seat tube. Then 1/2" tube to go from the seat tube to the dropouts. Probably need to sleeve the seat tube to properly attach tubes to it without failing.

@arizonabuckeye's Album:

https://www.bikeforums.net/g/user/523275

https://cimg4.ibsrv.net/gimg/bikefor...226a35838.jpeg

https://cimg4.ibsrv.net/gimg/bikefor...2853479f4.jpeg

Whew, all those curves will make mirroring the original tube shape to add a second tube complex.

If you do this, don't just use pipe... find a source for some quality chromoly 4130 steel.

A lot of modern step-through bikes use some sort of monotube. That would mean rebuilding the entire front half of your bike.

https://images.giant-bicycles.com/b_...LDS_ColorA.jpg
https://image.made-in-china.com/2f0j...-City-Bike.jpg

Of course, a few vintage bikes have used the parallel tube design, sometimes with "ladder rungs" between the two tubes.

https://i.pinimg.com/originals/7a/35...8afec5ab6e.jpg

I have seen step-through frames (two parallel tubes) with bent seat tubes. You are significantly changing the stress on the seat tube.

Ideally the design would be to transfer most of the strength of the bike down the downtube, through the bottom bracket, and down along the chainstays. And, the seatstays would simply support the seat tube.

The battery mount in your E-Bike looks like it would create a weak spot and put a lot of stress on the chain stays.

I know. Like I said I don't want to just spam random topics with :popcorn or :injured: just to hit the minimum. I get the need for having minimums just makes it difficult for someone like me. Up to 3 now!

So What kind of shop tools can you access? , a tubing bender that will hold up forcing 4130 to bend is a substantial cost...

a frame builder I knew in Eugene has one Its anchored to the concrete floor an d the leverage on the bending arm is about 6 feet long..

so where will you source the materials? one of those high end custom chopper motorbike shops?





:foo:

I have access to slightly more then basic tools. My background is mainly woodworking but I have access to a MIG and a metal band saw along with grinder etc.

I don’t plan to bend anything. It’s gonna be ugly as sin but I think I will add some extra pieces to make the whole thing look more Mad Max style.

Planning on sourcing from a local metal supplier and doing all the fab work myself.

I paid like $75 for the whole thing so if it fails it’s no big deal.

I wouldn't use plain steel. Most of the vintage frames used some kind of a "High-Tensile" steel. But most of the new stuff is chromoly (4130). There are online sources for chromoly including Shapiro's in St. Louis.

My cargo bike uses some of your "Mad Max" styling. I got a lot better welds using TIG than MIG. Although, a lot of car sheet metal is done with short bursts of MIG.

https://www.bikeforums.net/attachmen...hmentid=596135

Is there a way to get to that by clicking on a link or did you just reverse engineer the format?

:foo:

I haven't really found it...

What works for me is to click on the user's profile:
https://www.bikeforums.net/members/u...en-136384.html
As part of the URL, it gives the user's ID# 136384

Then I open up my own gallery (which by now the name is cached in Firefox).
https://www.bikeforums.net/g/user/392454

Now replace my user number with the target user number:
https://www.bikeforums.net/g/user/136384

And I get the album:

I don't know if there is a link or a search function, but this method is fast enough for me.

I do like to post the link back in case more photos are added. I like to upload most of the relevant photos, but sometimes there are more photos than I want to upload.

Also, of course, Bike Forums changed the method of uploading photos a few years ago. One can still access the old photos through one's own control panel (attachments), but I don't think one can see other people's old format attachments. Just pick out the attachment and use the [img]...[/img] to link it into the message.

Thanks, I thought that might be what you were doing once I saw the url

I quoted your post in the user assistance forum

Mig will burn holes... TIG is the Industry standard for welding thin wall steel,

A Mig ..
https://upload.wikimedia.org/wikiped...VVS_museum.jpg

I think what you will be doing is adding weight and very little strength or stiffness. I don't think you can re-use the top tube, but I would check. I would take a tube and go straight from the top of the head tube to the seat tube. Or do laterals, like I suggested before.

Another idea is to take a tube diagonally from the top of the head tube to the down tube and then from the down tube to the seat tube. There is no point to the way you have it drawn, needs to be triangulated.