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Rear hoops: the right way

1274 Views 16 Replies 6 Participants Last post by  CaTacL1sm
46works out of Japan just put up a new video on his latest project... he's a fantastic craftsman and quite the accomplished racer there, so noobs take note of what he does here:

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That sort of joint, if properly welded, is perfectly adequate without a sleeve. The purpose of a sleeve is to move any bending stress away from the weld and into the tubes, but at the end of the sleeve there's a huge increase in stress if it's a straight cut. A seat hoop carries little to no stress despite what we all read online. It does add to frame stiffness between the shocks if that's the only cross tube behind the main cradle.

So add a cross tube to take the stress and leave the hoop as a way to tidy things up and attach a seat. If the join between two frame tubes was in a high stress area, then for sure step the joint or add a slug with tapered ends to spread the load. I thought I was the only one bending stock thickwall frame tubes that way.

On TZ frames, the rear loop was welded on top of the frame rails.
And of that weight, how much of it is exerted behind the seat and under the hump? I'd speculate that it's less than the weight of the tube. The rationale is that if there's no other significant structure between the shock top mounts, then the subframe, such as it is, will tend to flex. In reality, most people are cutting off the ends of two straight, or almost straight tubes.

But the point I think that people here try to make is that newbies need to understand what they are cutting off that the potential impact of the changes they make. And that is totally valid. That video was interesting tough and the fabricator clearly is thinking things through one step at a time to avoid future problems.
Also if you cut the right diameter of a piece of an insert and place in into the frame you will have a 10 times stronger connection/weld. Cafe Racer Loop Las Insert voor Subframes
Could you please provide the calculations to support that comment and let us know how much force is actually applied when locating a light rear fender. I'd like to put that into context.
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