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1980 CX500C (cafe in progress)
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Discussion Starter #1
I've been planning my cafe racer build and was hoping to modify the rear portion of the frame to remove/straighten the dip/curve in the seat rails. This mod is near the top of the list of importance for me as it'll really tidy the lines of the bike and produce a much cleaner, finished look. Ideally I want to chop the whole rear section of the frame and replace it with a simple, straight tube frame like this:

202313


This is a rough mockup of what I hope to accomplish, but, being in NSW there are some sticking points when it comes to registration. My biggest concerns are getting structural changes engineered, and how much that will cost me. I know cost is going to vary between engineers, and I'm pretty sure it's going to be high, I just want a ballpark figure.

So has anyone done anything similar in Australia, more specifically in NSW, and how much did it cost you? What specifically did you have done? What was the process like working with the engineer? Did you consult the engineer before modifying or after?

I don't want this build done half assed or illegally, I want it to be something I can be proud of and not something I'll have to hide from the cops. Hopefully I'm not aiming too high.

Thanks guys.
 

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I've been planning my cafe racer build and was hoping to modify the rear portion of the frame to remove/straighten the dip/curve in the seat rails. This mod is near the top of the list of importance for me as it'll really tidy the lines of the bike and produce a much cleaner, finished look. Ideally I want to chop the whole rear section of the frame and replace it with a simple, straight tube frame like this:

View attachment 202313

This is a rough mockup of what I hope to accomplish, but, being in NSW there are some sticking points when it comes to registration. My biggest concerns are getting structural changes engineered, and how much that will cost me. I know cost is going to vary between engineers, and I'm pretty sure it's going to be high, I just want a ballpark figure.

So has anyone done anything similar in Australia, more specifically in NSW, and how much did it cost you? What specifically did you have done? What was the process like working with the engineer? Did you consult the engineer before modifying or after?

I don't want this build done half assed or illegally, I want it to be something I can be proud of and not something I'll have to hide from the cops. Hopefully I'm not aiming too high.

Thanks guys.
you may also want to repost here:
 

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The shock mounts on your example are poorly supported.
You'll have a hard time finding an engineer willing to accept the liability of signing off on such a modification.
You'd do better with an experienced frame fabricator. Weeding the good from the bad will be the challenge there.
 

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1980 CX500C (cafe in progress)
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Discussion Starter #5 (Edited)
Is this example any better or closer to getting registered in NSW?
0A3E3919-1992-4354-BB89-598E954E3806.jpeg
To my untrained eye it looks like the shock mounts would be stronger, as would the bottom of the frame thanks to the steel plate sections welded in. The number plate on the rear is also pretty much what I had in mind for covering the rear wheel, although it'll probably need to be a bit longer to cover the whole wheel. What do we think?
 

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Ah.....we head down the path of FORM before function.

I would be surprised if any engineer would approve, review or sign-off on such a modification.
 

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Discussion Starter #7
I would be surprised if any engineer would approve, review or sign-off on such a modification.
Well I have actually been talking to a local engineer who has recently helped with similar modifications for Deus customs in Sydney, so I don’t think he’s entirely opposed to the idea. I haven’t made any concrete decisions yet, I’m going to talk it through with this engineer first to see what’s allowed and what he’d be willing to sign off before I decide anything. I’m willing to make any changes to the design to make it legal, all I want is straight, level seat rails.
And yes I’m going for looks, but it’s not like I want a hard tail bobber or something crazy, I’m just trying to neaten up the lines and make something I like. I’m not neglecting function.
 

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Consider using double walled gussets with a through bung for the shock mount. The gussets would be welded to the inner and outer sides of the frame tubes and the bottom of the gussets sealed as well. Doing it that way transfers load over a larger surface area of the tube and minimizes stress risers. Look at the stock gussets where the shock mounts.
 
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Yes. Now I know the proper name!
 
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Re form vs function: If you design it to work right it will naturally look right. If you design for style alone it will neither work nor look right.

If you take the doglegs out of the rear end of the frame be very careful to check that the frame doesn't contact the tire when the suspension is fully collapsed...
 

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Discussion Starter #12
Re form vs function: If you design it to work right it will naturally look right. If you design for style alone it will neither work not look right.

If you take the doglegs out of the rear end of the frame be very careful to check that the frame doesn't contact the tire when the suspension is fully collapsed...
Yes that’s true, functionality should always come first, but that doesn’t mean you can’t build something that looks good too. This is why I’m planning everything out first, talking to the engineer, talking to you guys. I’m gonna make sure it all works well and is up to spec before I green light anything. But I do appreciate all these pointers I’m getting. I’m not super experienced with this stuff so I wanna make sure I do it right.
 

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Yes that’s true, functionality should always come first, but that doesn’t mean you can’t build something that looks good too. This is why I’m planning everything out first, talking to the engineer, talking to you guys. I’m gonna make sure it all works well and is up to spec before I green light anything. But I do appreciate all these pointers I’m getting. I’m not super experienced with this stuff so I wanna make sure I do it right.
You seem to have researched this-have you considered a change of forum name?-maybe "not some doofus" lol
 

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Discussion Starter #14
You seem to have researched this-have you considered a change of forum name?-maybe "not some doofus" lol
Haha everyone loves a bit of self deprecating humour. I’m already known by this name in other communities so I might as well stick with it :p
 

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1980 CX500C (cafe in progress)
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Discussion Starter #16
Had a local engineer over today to look at the bike and discuss modifications, and interestingly he seemed less concerned about rebuilding the whole rear frame than things like blinker widths and mudguard lengths. He was very easy going about the whole process and gave me a few booklets with checklists to make sure the bike complies with all the regulations and such. He had lots of handy tips, such as if your bike is pre ‘85 the exhaust can be 100db instead of 90, and pod filters actually aren’t really an issue when it comes to rego. As for the frame, he basically said that as long as it’s made strong, with tubing a similar width to the original, the shock mounts close to the original location and the welds not all duckshits then it should be fine. In his own words, “it’s all pretty common sense really”.

It cost me 350 for the initial inspection, but I think it’s worth it just to get an idea of what you should look out for and what you’re allowed to do, and he said he’ll help me with any questions I have throughout the build.

All in all, if you’re happy to spend around a grand for the paper and can weld half decent or know someone who can then frame modifications in NSW aren’t as far fetched as you might think.
 

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That's pretty much what the fact sheet above says. I went through same process last year and can confirm what you said, but it's important to do your own research I believe. Engineer doesn't look at frame closely so it'll be up to you to ensure rear tyre clears rear subrame at full compression of shocks. Mudguards, handlebars, blinkers, seat length, and stopping distance test to mention a few items. I have pods with straight pipes and bloody loud stainless exhausts without baffles that rang at 95db at 4000 RPM. They passed. They are loud and I love'em, but I've taken to wearing earplugs on long rides. It's a logarithmic scale so 100 may be unpleasant. Not gonna buy into form and function discussion, but many builds one sees from USA and other places wouldn't get passed here on lack of proper mudguards alone.
Good luck with build. I'm sure it's going to look great and work even better.
 

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Discussion Starter #18
Little update on the progress I’ve made. Still working on the frame but I’ve got the basic shape and structure down. Next chance I get we will be adding shock mounts, brackets and struts for strength. Overall I’m very happy with the look so far. I’ve made the seat rails long enough to potentially fit a passenger if needed (still working on a design for the seat). All the welds are as good or better than stock and the vertical section of the frame is welded into the stamped section same as stock (the original tubing here was badly rusted underneath and needed replacing. We’ve also welded the whole way around this section to seal it off and stop water getting in there again). The new seat position will be a few inches higher than it was, but before it felt like a chopper/cruiser so I’m happy to have a sportier seating position. After the frame work is done I’ll get cracking making the electrics tray and a seat with a removable cowl.

203914

203915

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I suggest that you add a brace from the shock mount to the seat rail above, inline with the shock. Then, the load on the shock will transfer to a rigid frame, rather than to an individual member at 1/3 span.
 

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Discussion Starter #20
I suggest that you add a brace from the shock mount to the seat rail above, inline with the shock. Then, the load on the shock will transfer to a rigid frame, rather than to an individual member at 1/3 span.
The shock mounts aren’t going where the shocks are resting in the pics, they will be in the obtuse angle below the seat rails so the shocks are more vertical. The mounts should be in basically the same position as stock so the suspension geometry stays the same.
 
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