CR750 replica build

[Napoleonb]

Nice find on that hub!

[CR21]

Cheers Nap!

[philward]

Great detail Scott - but you know that already! Having gone through a compromise replica build, really appreciate your build

[CR21]

Thanks Phil. Your build is far from a compromise. It is a cracker of a bike.

A few more frame jobs completed today!

A couple of options for mounting the oil cooler on the CR750, a bracket was available for the kit bikes which provided the offset holes for the cooler. I have not seen the frame area on a factory bike, however my suspicion is that they may have mirrored the horn bracket to accommodate the cooler...



Folded up a mirror image bracket:



Welded on some flanged nuts:



Trimmed back and jigged up:



Welded on and ready for final fettling:



Still a fair few things on the frame to-do list!

- Rear hoop and cross brace to accept catch tank
- Steering damper mount
- Lower oil-tank mounts
- Lower fairing mounting plates
- Brake stops, and welding of the cable bushes
- Remove chain guard mountings from the swinging arm
- Many other small details!

Cheers,
CR21

[philward]

Thats exactly what I did Scott

[CR21]

Thats exactly what I did Scott

Hi Phil,

Hope it turned out ok? I think a few people have done it, how accurate it is to an original I don't know but I think it will be in-keeping once it is all painted up.

Cheers,
CR21

[CR21]

A little progress from today:

Oiltank

Oiltank brackets tacked on, ready to check chain-clearance etc:



Exhausts

The CR75O exhausts are a 'big' part of the appearance of the machine, and something that I would like to get looking tidy.

I believe on the original bikes each exhaust pipe was made of two pressings, which were then welded or seam rolled together. This left the appearance of a seam running at the top and bottom of the pipe. The headers were formed separately and were welded or clamped to the pipes.

Although the shape profile could be achieved with rolling or even sand bending rolled cones, the appearance of a seam is hard to achieve



The obvious next step was to give it a go...


So a good chunk of time was spent today trying out hydroforming! Something that I have watched many videos on and read about but never tried.

Quick trip out to pick up various fittings to allow the cheapo pressure washer to be hooked up to the sheetmetal assembly, tapped hole in the blanking plug for bleeding air out of the system:



The hardest part of the CR exhaust shape is the curve, so the first flat pattern was knocked up to test the curve profile section. Complete guesswork for the angle and diameter (scaled off of photographs) at this stage and purely for experimentation:




Seamed together and valve welded in:





Hooked up, with a little safety cage!

Pumped up around 400psi



Happy for a first attempt, but very wavy edges and some buckling 

The pattern next to the product:




Round two!

Stainless sheet, smoothed pattern shape, special attention on matching the two halves and having smoother curves:







The latest attempt above the first attempt and pattern:



What I have learnt about hydroforming today is that the pattern, weld penetration and consistency are key to repeatability.


And that it is with a bit more experimentation I could possibly achieve what I am after...


I think I am going to purchase off the shelf header pipes, however I may experiment with another process I have been wanting to play with for a while which is sand-bending...

Cheers,
CR21

[Laverda Dave]

Wow, fascinating work. I like to have a go at most things buts hats off to you for trying hydroforming with that pressure and being successful.
Great thread, looking forward to the next installment and photos.

[philward]

I think I've mentioned it to you before Scott, but when trying to calculate the visual up sweep on the pipes, don't forget that in the picture of Dick Mann on the bike, his weight is on the bike and hence the swing arm is pushed up. I positioned mine relative to the rear wheel spindle as a reference the the picture- but the bike was on the 'A' frame stand when I welded the exhausts together - with no weight on it (which looked the right up sweep). But when everything was assembled and I sat on the bike, the exhausts are lower than I wanted. Just a thought!

[AshimotoK0]

Looking fantastic... talked to a guy yesterday at Newark who I get parts from and he told me he just recently sold a CR rear hub for £350 to somebody. Thought it might have been you Scott but seems not.
His was from the estate of Alf Briggs who was Honda UK's Service Manager from the very early 1960's and held all of the race parts at one time. Alf was a close friend of Mr. Honda and when they pulled out of racing  Alf was allowed to keep all of the spares I believe.

[CR21]

Hi Laverda, it was pretty simple to be honest but I can't call the result a success yet! I think once I have developed the pattern, along with some very consistent welding it will be more successful. Learnt a lot from the trial parts.

Phil - I remember you mentioning this - thanks for reminding me! So many details on these bikes. 

Hi Ash, I think I know who you are talking about. pm sent for further discussion.

Rear Hub

A little more progress on the replica rear TLS hub.

The CAD assembly is coming along, still a lot of detail to add!





Cheers,
RD17

[CR21]

Hi all,

Small and slow steps on this projects due to other projects consuming heaps of my time!

Anyway a tiny update! The fork bottoms are back from the foundry. How just to figure out the machining techniques and cartridge details 

An original image for reference:



And the cast blanks!








H2 Eric did a wonderful job of his forks, I have asked him for a little more info.

Cheers,
CR21

[Napoleonb]

Looking good!

[CR21]

Hello all!

Sorry it has been a while - other projects have taken over, but I am starting to catch back up with this one. Time has however helped with certain contacts, pieces of information, parts etc...

A few things have happened since my last post:

- Forks converted to '4 bolt', I will add some images on a seperate post.

- I managed to source a replica revcounter (lovely piece!) and an oil pressure guage - will add some images soon.

- Hub patterns are with the casting company ready to pour, these will then need a fair bit of machining - I will post a seperate update.

- Considering what to do with carbs as the 'smoothbore tickler' carbs are extremely hard to find and the new 'CR' carbs don't quite have the right period look for the machine...

- Considering lowering and altering the headstock angle as the 'original' bikes had this from factory...

- Due to start work on the engine shortly.


....

A brief update;


Finally found a complete scale model!



A few interesting finds:

Genuine honda CR750 carbs:











These are new old stock, in terms of rare CR parts it?s about as good as it gets. Owned by a friend this set is earmarked for a project and is not for sale, however luckily another set has come up for sale. It still amazes me how many ?rare? parts are still about and how they often emerge from historic parts stores all at once!

Cr750 revcounter:





My ?replica? unit next to a new old stock one. I purchased the replica from a chap in Australia, and although it?s a great replica this has highlighted some differences. Think I need to do some more work to the mounting bracket and change the finish to ?olive drab? to make it more authentic. Interesting how the bezel has been formed around into a circle and then welded.

I also now have access to the oil pressure gauge mounting bracket so will try to get this detail right as well.

CR750 rear hub brake shoes:

Found on eBay in France. Happy with these, containing the original over cast steel inserts:






Cr750 rear hub:

The original hubs were cast in magnesium, with the used hub I previously owned I have drawn this assembly up in 3D CAD. This gives me the option to reverse engineer the assembly in a few ways:



Machining the hub, brake plate and associated parts from solid aluminium.

This has a number of benefits, it?s very suited to low volumes.
The tolerances, finishes and fits are easy to control.
A cast appearance would be a real challenge, options would be course sand blasting mid machining operation, or even spark eroding or etching the finish to the desired surfaces.

Casting the hub, brake plate and associated parts from aluminium or magnesium.

The benefit of sand casting is that the process is the same as the original, and in some places the cast finish is still visible and is very much part of the appearance.
The post machining set up is difficult to get right with a small batch and the patterns that are required for sand casting are expensive.
The central fins pose a challenge for casting as this will require multiple inserts to  be made for each casting.

Although the solid machining route appeals I would like to challenge myself with the casting route! So this is the plan - update coming soon.

I did however manage to find a new old stock hub assembly in the USA  .









Still searching for the headstock information on the daytona bikes - anyway enough rambing - that?s all for now!

Cheers
CR21

[CR21]

A sneak peak of the hub progress 





Currently in the machine shop shaping up nicely;







Super happy with the quality of the castings, and the contrast between the cast surface finish and the machined.


Cheers,
CR21