A (not so) quick Watts overhaul

[festy]

The other day I went to swap out the springs in my perpetual project car, but as I jacked up the rear I noticed a bit of movement in the Watts linkage.
I thought I should do something about that before going any further, so I rummaged around for some suitable rod ends and made up a pair of replacement arms.


But I had some doubts about whether the aluminium tube would be up to the job. It had roughly 4mm walls, so would probably be fine for "normal" use but I didn't like it's chances of surviving a decent hit (like sliding the rear wheel into a curb) so was about to start turning up another set from 25mm solid 6061 bar when I realised that they'd be lighter (and cheaper) if I used thick wall steel tube instead - and 1" thick alloy watts arms would probably look pretty funny 

So I made the new ones as close to the factory length as I could, they're adjustable but they're strongest when closed right up. I went to put it all back together, and noticed that most of the movement I'd spotted was in the pivot arm bush, not the arm end bushes
I better make a new one of those too then...

I didn't have a chunk of billet big enough and really wanted it to be one piece, so pulled out my hot wire cutter and cut a centre arm out of foam 


Obviously polystyrene isn't quite tough enough for use in suspension components, I used it as the pattern for 'lost foam' casting process. It's called lost foam because you pack the foam pattern in greensand then pour molten metal into it - which evaporates the polystyrene and fills the space where the pattern was.
It makes for pretty quick parts casting, but the downside is that you can't re-use the pattern

This is just after pouring molten aluminium into the flask. It doesn't look hot, but it's about 600 degrees.
 

I'm a bit out of practice with sand casting, so made a few mistakes.

One was a bad sprue placement which meant an extra hour or so of machining was needed, and the other was not removing the excess foam from the pattern - which meant I had to remove the excess metal from the part which added another hour or two of machining effort.
It's much easier to cut foam than cast alloy 

After tracking down some rather overpriced bearings and a bit of fiddling, it was finished so I finally swapped the springs over and put everything back together.

And that's when I discovered that these new lowered springs need shorter than standard arms to keep the centre pivot vertical, and my new ones were already at their minimum length 


Luckily I got away without making yet another set of arms, by turning the locknuts down a bit with a lathe that gave me enough adjustment.

This is why I never seem to make much progress with this car, something as simple as swapping springs leads to long and stupid modifications like this one 
At least there's no play in the new one, and a cast watts link is pretty unique on an alfetta I suppose 

[AikenDrum105]

Need to have a chat to you about casting like that - a bunch of ideas just popped into my head,  as they are wont to do...    Very cool.

[scott.venables]

That's fantastic!  What do you use to melt the aluminium? And was the donor aluminum an old casting or other?  What sort of sand did you use?

I plan to make a some links but I was planning to either turn up some threaded inserts to weld into the original tubes after the ends have been cut off or keep the original tubes as they are and turn an aluminium sleeve to adapt a spherical bearing in the end instead of the rubber bush.

[festy]

My "foundry" is a tub from an old washing machine.
it's lined with 6-8 inches of home-made refractory (a mix of bentonite, sand and perlite) with half a dozen vertical columns of packed perlite (for extra insulation) around the edges.
There's an air pipe near the bottom, which is fed by a vacuum cleaner motor running in reverse, on a variable speed controller.
My crucible is an old nord liner with a steel plate welded on the top (which is now the bottom) and a couple of tabs to handle it by (using a 2 pronged fork sort of tool, not by hand!)
I can take some pics of the setup if you're interested. It currently burns wood (cheap and plentiful) but takes over an hour to get up to temperature - so I usually start it up, then go and make the patterns and prepare the mould while I wait.
I was going to build a waste oil feed for it so I could use up my endless supply of sump oil, but I'm about to move into a populated area so can't do that, I'll instead make a gas burner for it which should reduce the melt times by half.
The other advantages to gas is I won't have to constantly stop and add more fuel, it won't sound like a jet engine when the air pump is running hard, and my castings won't end up with ash flecks in them

The sand is known as "greensand", it's a mix of very fine sand, bentonite (a sort of clay that's used in stock feeds) and water. You get better results with an oil based sand mix, but the ingredients for greensand are cheap and easy to find.
I would have got much better results if I'd taken a bit more time in preparing the pattern, and packed the sand better but I was in a rush. I wasn't too worried about the appearance this time, but it was threatening to rain which is not a good mix with molten metal...

The aluminium scrap I used was some alloy compressor housings I found somewhere.
Generally speaking, if the scrap is cast then it must be a suitable alloy for casting
Don't use anything with magnesium in it, very very bad things will happen!

I was going to chop up the original arms too, but didn't want to do anything irreversible so I didn't touch the original bits in case something didn't work out and I needed to put them back on

[Duk]

You are a braver man than I am..................... 

Machined out of rolled billet or plate (6061), yes!

Made from a backyard casting, no thanks!

[festy]

I have no doubt at all that it's strong enough. I've done enough casting to be confident of that.
Considering the gauge of the sheet metal brackets that each arm bolts to, this is not the weakest link.
And remember the factory version is 2 piece 3mm pressed steel with a rubber bushing, the thinnest point in this casting is still around 10-15mm thick.
I'd put money on the body mounts or arms giving up way before the casting was damaged.

[alfa duk]

 Festy, you remind me of Burt Munro, casting his own stuff in the garage, primitive, but Im with Duk, seems a bit risky considering what that part does.
Quality of the ally, quality of casting, not tested.  Sorry about the neg feed back but well done for your effort

[Duk]

I have no doubt at all that it's strong enough. I've done enough casting to be confident of that.
Considering the gauge of the sheet metal brackets that each arm bolts to, this is not the weakest link.
And remember the factory version is 2 piece 3mm pressed steel with a rubber bushing, the thinnest point in this casting is still around 10-15mm thick.
I'd put money on the body mounts or arms giving up way before the casting was damaged.

Stamped sheet metal parts parts are made from a much more consistent quality, forged (rolled) metal.
Rolled steel is significantly stronger than cast steel.
I did a small job at work ages ago where I needed to drill and tap some cast slabs (I work at a steel plant) to be used as a test weight. It was like drilling into an engineering plastic like Nylon, it was that soft. This was the exact same steel that gets rolled (forged) into structural sections (big H and I beam and column, channel and big angle) for large building construction.

I'm all for innovation, but how do you know it's strong enough?

[Neil Choi]

Cast metal does not allow the alignment of the metal crystals.

Forging metal ie rolling allows the crystals to align and hence is stronger, ie better crystal stacking.

Think of a Japanese sword being forged and folded 20 times, over a million layers and see how strong it is.

Compare forged pistons vs cast pistons.

Likewise, cast exhaust manifolds how often they break, ie Alfetta or 105.

[festy]

Well this isn't a road car, so really i'm the only person who needs to be satisfied in it's abillity to do the job.
But anyway, I tested it and the factory arm bolts gave up first, with the original pivot arm being very deformed and the bolt holes starting to tear out. No sign of damage to the casting at that point. That was somewhere around 4-6 tonne static force. The bolts have a shear rating of 3 tonnes from memory, so they did well
I didn't bother testing the sheetmetal mounts or the factory arms, because both are weaker than the bolts so there was no point...

[Duk]

Because by the sounds of it, he has been doing this for a long time and knows what he is doing!

Choder, go and check out factory aluminium suspension arms from various performance cars (Series 6 onwards Mazda RX7, JZA80 Supra's or, at some point in their history, the Evo Lancers).
They are definitely strong enough!
They are probably forged.
They are definitely tested by the manufacturer to know when and how they fail.
And they are definitely made from known metal type and quality.

I'll give Festy the praise and recommendation he deserves. He is doing some absolutely awesome work with the factory Motronic system and is sharing what he is developing and learning. But I think that a critical suspension component should only be made from consistent quality material of known strength. As he said himself, it's 'just' made from previously cast material. Pretty sure Mazda weren't casting their aluminium control arm blanks out of old inlet manifolds or water pump housings.
I once had a starter motor mount crack and fail. Part of the starter motor body got chewed up by the flywheel, made horrible noises and I shat myself (I didn't know what had happened at the time).
Imagine if a critical suspension component did the same.