SOHC.co.uk Forums > Project Board
1975 CB750 P2 Police Bike
Lobo:
…how is the siren engaged James? Could this be achieved at speed? What min speed is necessary for it to work?
Honestly it seems a bit Heath-Robinson given the era, I’d have expected a power off-take from the engine …. eg the SOHC, or perhaps electrical + uprated ALT. Intriguing! (Tho’ it’s apparent a very high siren drive RPM was necessary given the massive ‘gearing’…. bet the top speed bled off pretty quickly 😂)
McCabe-Thiele (Ted):
You have a brilliant looking build there it's looking factory fresh!
Makes mine feel a tad Heath Robinson by comparison - never realised how good a white bike could look.
MrDavo:
Having seen it mentioned in two consecutive posts, there will be young people out there wondering what or who the hell was Heath-Robinson?
Lovely build as expected, most of us don't have a load of NOS parts stashed away for future use, especially a genuine exhaust set, or a Police speedo for that matter.
JamesH:
Thanks for the encouraging words / comments / feedback.
With the words ‘Heath Robinson’ ringing in my ears, I had a spare hour over lunch so decided to mock-up a replacement for the siren drive to test-fit before committing the design to metal.
I’m lucky to have access to a ‘fablab’ in the studios where my business is now based, including an array of ‘Prusa’ 3D printers. I was trained how to use the printers last week, and this was a fairly ambitious ‘second print’ so I was fully expecting it to go ‘tits up’.
So working from the existing siren bracket and some ‘guesstimated’ dimensions from photos found online of the ‘correct’ 750 bracket (turns out mine is for a 500/4 police bike) I designed up the unit in 3D (using Fusion360) before exporting an STL file, ‘slicing’ it up in preparation for 3D printing, outputting the G-code and then 3D printing in PLA.
Having removed the support material, have to say I’m pretty chuffed with the result.
However, the rule of ‘measure twice, cut once’ applies here (well actually check your design twice, 3D print once lol).In my haste I managed to place the design for the return spring hole on the wrong side (realised after printing). So I’ve adjusted the design and set one to print overnight.
Will test fit tomorrow evening to see if it’s a better solution than the 500/4 ‘bent arm’ design, before manufacturing it.
JamesH:
Thanks for the encouraging words / comments / feedback.
With the words ‘Heath Robinson’ ringing in my ears, I had a spare hour over lunch so decided to mock-up a replacement arm for the siren roller-drive to test-fit before committing the design to metal.
I’m lucky to have access to a ‘fablab’ in the studios where my business is now based, including an array of ‘Prusa’ 3D printers. I was trained how to use the printers last week, and this was a fairly ambitious ‘second print’ so I was fully expecting it to go ‘tits up’.
So working from the existing siren bracket and some ‘guesstimated’ dimensions from photos found online of the ‘correct’ 750 bracket (turns out mine is for a 500/4 police bike) I designed up the unit in 3D (using Fusion360) before exporting an STL file, ‘slicing’ it up in preparation for 3D printing, outputting the G-code and then 3D printing in PLA.
There were actually 2 machines free at lunchtime, so I ‘hedged my bets’ and set off two of the machines printing (one in black PLA and one in Grey PLA). That’s why the ‘finished’ part is black - that was one of the two prints..both successful (apart from the design cock-up - see below).
Having removed the support material, have to say I’m pretty chuffed with the result.
However, the rule of ‘measure twice, cut once’ applies here (well actually check your design twice, 3D print once lol).In my haste I managed to place the design for the return spring hole on the wrong side (realised after printing). So I’ve adjusted the design and set one to print overnight.
Will test fit tomorrow evening to see if it’s a better solution than the 500/4 ‘bent arm’ design, before manufacturing it.
Dismantled 500/4 ‘bent arm’ unit:
Reference photo of correct 750 unit..
Designed up in Fusion360 (note the wrong spring hole position ha ha)
Mid-way through the print:
The printed part - even the M8 x 1.25 thread was a decent fit for the nut.
The ‘printed’ prototype, with the original roller test-fitted/assembled:
The ‘revised’ arm design, with correct spring hole position (still can’t believe I missed it first time ha ha).
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