Hi again
Ooohh... suspension.... Love the stuff: ready for another "far too long" post?
I just had to google "Torsion Stub Axles" to see if you were talking about the thing that I know as "Indespension" units. ... I think you are.
I have considered them, but have a few demands of my setup that would make them miss the mark. I want to have a 3pt hitch front and rear on my machine. My main use for it will be clearing snow, and getting off our land onto the roads I can see from here to help people when they get stuck - (a regular occurrence). At the moment with the tractor I need to clear our driveway in order to get out, meaning that a lot of people go without help. Being able to drive *over* the snow to get there means that I'd do it far more often. Hanging a 150Kg snowblower on the front will mean I want to stiffen the front end's suspension... so I want something a bit more adjustable than a torsion system.
As far as I see it, there are 4 ways to go.
1. No suspension at all (if you're only on snow, then that should really be fine)
2. Independently sprung wheels / Full suspension
3. Linked pairs of wheels (allowing the track to follow the contours of the ground to some extent, but offering no "spring"
4. A hybrid mix of 2 and 3. (this is what I intend to do)
Take Nr 3... I can't find the technical term for the linked pairs. It's a common feature of heavy 4-wheeled tractor trailers, and after a brief google, I've found this.
http://www.freepatentsonline.com/7048281-0-large.jpg which pretty much sums up concept 4.
You'll see the same concept daily on the windscreen wiper blades of your car(s)
This seems to be the basis of most bogey-wheel systems on tracked machines. Google "Tank suspension" for some more ideas.
When it comes to calculation, bare in mind that your front wheel is going to take the "pull" from the track that goes round it. This is why a lot of tracked machines have the two end wheels off the ground: it's easier to deal with track tension if they're not sprung.
Your weight distribution is also important. Pointless to have tracks if all the weight is at one end!
Assuming a 1000Kg vehicle and 5 bogey-wheels per side, I reckon you're wanting a 100Kg torsion unit per wheel... but it's an expensive trial and error route to go it you're going to be buying 10 torsion units.
... which brings me to one of the ideas I've played with.
Instead of springs that you have to buy, and instead of suspension units that'll end up costing you a total in the "thousands" of dollars region, Why not try a system that has proven itself on the budget motorsport scene?
Bungee chord.
longer length = lower spring-rate
More length
s = stronger spring.
It'd be really easy to play with simple trailing/leading arm suspension, using multiple wraps of chord to vary spring-rates. The temperature might be an issue, as might buildup of ice/snow, but a good silicone grease should fix the latter problem, and the setup will be completely tuneable and
seriously cheap
While it sounds silly... think about it: If you need 100Kg for force per wheel, We can both agree that a single thickness of bungee can give around 10 kg of force... That's only 5 circuits/loops of chord to give you the 100Kg per wheel you want. (assuming you've got a 1:1 ratio on your trailing arm wheel:spring geometry)
Also... lets say stretching a length of chord takes from 0-10Kg depending on the length, if you set it up so the vehicle is suspended when the chord is stretched to 5Kg, and you know (from the tests you'll do in the next 2 minutes) that extending it another 4" will take it to 10Kg, you can easily calculate the loadings/travel you need/want to get the results you want.
To keep the crap out of your bungee springs, you could also make some tubes from neoprene or old inner-tube to cover them....
If it's good enough for aeroplanes....
http://www.bydanjohnson.com/articleart/272_11.jpg
