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Discussion Starter · #1 ·
Morning, thought it was about time to try and post a build thread. Sorry if you're reading this in 2022 and it's not finished yet!

Here it is then, a 31 Austin 12.



 

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Discussion Starter · #4 ·
right I have a few photos now;
Forstly, the rules;
No body mods, I want to keep the paint as it is (apart from a clearcoat or oil).

Running gear- For a while I considered Scimitar, but the outriggers would have prevented the body sitting low enough, and the rear of the chassis would have to channel into the body.

Eventually settled on MX5, I can work with the standard track, and use the subframes complete onto the original (modified) chassis.

I then discovered ProLaminates had MNR sportscars build a chassis for their 34 bodies. Same wheelbase and track. After some to and fro, I got hold of one
 

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Discussion Starter · #5 · (Edited)
This isn't my actual chassis, I don't have a picture of mine but it's the same. Nicely built, but as you'll see in a few posts, ultimately not to be used on this project.
 

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Discussion Starter · #6 · (Edited)
So, next thing was to get the Austin sitting where I wanted it. I got hold of some old Rover wheels. They have the same PCD as Mazda and the offsets work so they'll do for mocking up.
Standard;


and dropped down on the wheels;

 

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Discussion Starter · #7 ·
I then got hold of an MX5 and stripped the suspension, engine, and box from it. Lots of measurements on these and the MNR chassis made me rethink.
I'd have to move so much stuff on the MNR chassis (A and B posts, remake the A arms that don't fit properly, etc) that it's going to be easier to make my own chassis.

While the original chassis was in good condition, it was only 40mm by 90mm. I could have boxed it but it's less effort to make new rails as they're flat with just a kink at the end.
I realised that I had so much depth between the floor and the bottom of the running board I could get a pair of parallel rails in there, set 200mm apart.
Here's a confusing 2d drawing;

 

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Discussion Starter · #8 ·
from that drawing I projected out the braces that join the top and bottom rails together, and check they kinda worked with a paper template;



So, off to the cnc mill (yep, I've got a few very useful tools) to cut all the braces;

 

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Discussion Starter · #9 ·
There were some bits I found it quicker and easier to have laser cut. I also had some jigs made that hold the rails in the right place and locate the A arm brackets;



These braces attach to the orginal body mounts. Nice that my drawings match up with the old chassis so far

 

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Discussion Starter · #11 ·
Whilst planning the chassis I'm thinking about seat and belt mounts. As the B pillar is so slim I'm not gonna get a belt mount on it. Not wanting to put a roll cage in it I had a look about and came up with Ford Bmax seats. They have a built in seatbelt and they just about fit.
What's more of a suprise is that the rear ones fit with no modification at all!

 

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Discussion Starter · #12 · (Edited)
So, we're almost up to date. I've been making threaded bushes to weld in to chassis rails, and some other plain bushes that are being used to make the front and rear suspension jigs. Whilst I've measure everything, it's usefull to have jigs to ensure I'm bang on. Oh, forgot to mention, I narrowed the original mazda front suspension subframe by 80mm to give me a narrow front track allowing the wheels to tuck up under the front arches and still turn.

Plan is to get everything cut then assemble a giant jigsaw and hope that it's braced enough not to warp when I start welding it!
 

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Love it!
Im fond of an old British car getting rodded.
Will be watching, keep your progress pics coming!
 

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Discussion Starter · #16 ·
Gathering up some of my thought processes;

This is being built to go through BIVA, so some compromises have to be made. The modern seats are a little out of character, but it removes any seatbelt mount issues. Hopefully I'll be able to remove the side airbag without unstitching anything.

MX5 as a base... surely a V8 would be the thing. Yes, for sound. But power? Standard its 148hp, and they regulary turbo to 250-300. It's got an LSD and 6 speed box, it's double A arm suspension, and if I use enough of it I might get an age related plate. An age related can have a private plate on it, so I can go with something that looks right on the car.
It's a daily cruiser that'll be simple to fix, and won't embarass itself in modern traffic.

Paint. I love it as it is, everything you see is surface rust- bar a little perforation at the bottom rear edge, and the sheet metal between the running boards and the frame. If whatever finish I put on there degrades over time then I'll have to paint it.

Wheels and tyres, I wanted wires of a similar rolling diameter as the originals (about 30 inches). But having the mazda 100mm pcd kinda limits options. Gearing would be a problem as well. I've gone with a 205 45 17 on the front, and a 225 65 17 on the rear. This gives me the tall sidewall look I was after and just about fills the arches. Can't go wider as I'd have to tub it and maybe shorten the driveshafts. I think there's 5mm a side I can save by using the early uprights and shafts. Maybe.
Gearing. The crownwheel and pinion are used in the Kia Sportage, in ratios (4.67 or something like that) that will just about get me back to stock.

Gear shift is way too far into the car- it's a long box, but I think I can shorten the mech about 200mm which might help. It'll have a tall lever on it anyway, probably bent about to clear the seats, which touch in the middle.

Chassis design. I'm making it up as I go along- although the main drawings took me several weeks of messing about. The stock mazda dimensions are not easy to come by- much of the info you can find is just wrong.
The dual rail design came about because I had 200mm depth to play with. The deeper the rail section, the stronger it is. Two lighter rails give a similar benefit, plus they kinda form a triangulated structure as the lower rails sit much closer together than the uppers. Upper rail is 50x50x2.5, lower is 48.3x2.5. I went round for the lower as I thought I'd have to cut each brace to fit and it would be simpler to fishmouth the round one then rotate it into place. It's round. I'm happy with it. I think. Too late now... move on.

External radius compliance. Mmm.
Wings are ok.
Hinges bad.
Bumpers nope (not for the test anyway), will have to be fabricated round tube.
Suicide doors- need latches and warning buzzers. Handles, have to return to within 2mm of the door. I'll have to make something.
Lights, need something similar to standard (10 inch+)?
Wiring and dash- all Mazda sourced and modified to fit.

That's the stuff that's been rolling around in my head for the past few months! There's probably more.

Before Xmas progress plan is to get the chassis tacked together, with the A arms on, so I can drop the body on it and do some head scratching.
 

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Discussion Starter · #19 ·
have you looked through this build mate? built then rebuilt to pass biva properly. https://www.rodsnsods.co.uk/forum/garage/my-pop-165
Morning Neil, as you can see, I've made a start. Couldn't post on here for a while- combination of lost password and my other pc not showing bits of the forum properly.
Gonna have to read the pop thread in the evenings- I can't see my thread being 150 pages long! I'll have given up or finished before that.
Most of my Biva issues will have already been addressed by other people, so I'll be looking at what other people have done. I read the BIVA section on seats and seatbelt mounts last night. It doesn't actually clarify what a 'thin walled tube' might be. I'm planning on M12 bushes though the 50x50x2.5 box section- with some extra braces. I might locally thicken the rails with some 3mm plate where the bushes are welded top and bottom.
 

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Discussion Starter · #20 ·
:tup: Subscribed, as long as you keep the glass visor..
Glass visor is probably a BIVA issue so it won't be appearing on the first incarnation of the car. But something very similar might be a future improvement. Same with the sidelights and the rear luggage rack.
 
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