Usable frames for BTR?

What a great pair of loop frames! If they're not bent or concealing terminal rust I think you have two board track frames. Are the frames large enough to fit you? Straight top tube problem? More explanation required, but I'm guessing that you might have a certain fuel tank shape in mind that requires a bar change? Adding a lower straddle bar is one of the easiest fabrications to make on a steel frame bike (two bar straddle) to support a tank from the bottom and for attaching a top engine motor mount, if going 4 stroke motor. Motor size matters and a photo doesn't suffice, but should be plenty of room for a 2 stroke China girl, or depending on fuel tank location etc. and actual interior frame dimensions, larger 4 stroke motors could be a tight fit. I'd suggest taking measurements of your frame first, give us an idea of what engine and at least the style tank you want to fabricate and go from there. At any rate I encourage you to forge ahead with the project after a bit of pre-planning. Rick C

*drools over the size of an engine I could fit in that frame*

What is Briton Bees?

This article has links to the manufacturer as well as a photo that might answer the OP.

http://www.pipeburn.com/home/2011/06/25/briton-bees-motocyclettes.html

If you had in mind a more traditional frame shape and configuration, I would say that those frames probably aren't a good starting point. Extensive modification would be required. On the other hand, the finished examples shown in the link Harold provided are really cool.

I followed up Harold's link to Pipeburn and quickly discovered the Briton Bee was the creation of a bike designer whose custom motorcycle work I'm quite fond of. I'd think the frames to be constructed of top shelf materials and built using both good design and proper technique. I really like the frame shape, though Wret makes a valid point about "classic" style. I think it in the ball park for a nice track bike, either board or flat.

Cutting the top bar & dropping at an angle as shown is a visual improvement, but would require some thought turned to strengthening the seat tube at the point where the seat stays connect. I'd probably insert & plug weld a permanent saddle tube inside the seat tube, full length, to increase the strength & rigidity of this high stress component. Small grommets could also be added below the added straddle tube both front and rear.

Also if you aren't using a frame jig make sure to weld in place the new lower straddle tube before cutting loose the top tube to make the angle. This insures keeping the frame in perfect alignment. It's all up to you & I think it would look good either way.

Engine choice is really about what will fit or how much work you are willing to put in to make what you select for power to fit. One off builds aren't easy but they can be great fun! I'm looking forward to seeing what you want it to Bee. Rick C.

A couple points to consider regarding your drawing:

Adding a slope to the back of your top tube leaves your seat tube high and dry. Likely not the look you want. Cutting it down would also mean modifying the length and angle of the seat stays.

The overall scale of the frame seems pretty compact. After adding the lower straddle tube, there might not be much space left for the motor you want.

It really goes to knowing what you really want and then decide how to get there. The saddle position that Wret mentions will be quite high compared to a traditional track bike unless the seat stays are lowered to a position at or below the bottom straddle tube. If you go in that direction weld the new seat stays in place prior to cutting the old ones loose. Relocating the stays will restore some strength & rigidity to the frame as well as facilitating the lower ride height of the saddle. Change seems to begat change.

Selecting an engine and designing a drive train that will work and also allow an exhaust and intake system requires a lot of space with certain engine designs. Just because a bare motor can fit in an area doesn't mean it will operate in that space. Using a jack shaft to position your chain lines requires space as does front mounted exhaust manifolds. Routine servicing dictates room and positioning of the motor for ease of: exhaust manifold, valve cover/head, removal as well as spark plug and oil change service.

This is just the start of planning a build...lots to consider and probably change during the rest of the process. You'll figure it out! Rick C.