Thread: Tandem Suspension Concept

First off Brian, I looked into your two suggestions and found pictures with 8 or so links for each design. Cant really see the simplicity in it.

I mucked around with a tape measure this arvo looking into utilising a camel back inverted leaf setup. Easy enough to fabricate a pivot point in double shear and still clear the hull. Width would be okay to run the springs inboard of the chassis to minimise height. Obviously this will increase leverage on the stub axle but that can obviously be accounted for. Tandem 14" light trucks will need a minimum of 750mm long spring centrally pivoted. I have some 75 series rear which are up around 1100mm total length that could be modified to work - even if only in a test bed capacity.

The only sticking point I see at this moment which Bill should be able to help me on is each axle connection to outer edge of leaf. I guess ideally the axle should only be firmly attached to the longest leaf allowing sliding of the smaller length leaves with spring flex... But this would then surely be a point of weakness attaching the axle to a single 6,7,8mm piece of flat? Or am I mistaken.

Obviously if I went with parabolics this would be less of an issue.

And on much red contemplation the powered concept seems a little far fetched at this stage... assuming this goes ahead I will run it with tandem 14s LT tyres and then up on the Aframe run a pair of spare tyres on swing down hubs.
When it gets bogged which wil be regularly I am sure. Swing down spares = 6 tyred trailer with fronts having some serious tow in for tracking stability drive 130 through then drag the rig on the winch. It would be much easier to setup a rear mounted winch than driving the trailer axle.

And serg,
thanks for thoughts on downhill issues, hopefully that wont be a huge issue and luckily this thing wont be going down into the Howqua Valley What is a concern with a tandem is manauverability in the tight stuff. Main travelling companion at the moment is running a dual cab Canter towing a 435 tinnie on offroad trailer. They slow me down heaps compared to the 130 and 4m tinnie so I suppose its time I put the handbrake on!

S

Interesting idea Steve. I assume this was what you sent me in the .odt file I haven't yet had a chance to look at???

As mentioned, the angles of springs/shocks look a bit flat. Also, I am not sure that the response of both axles would be the same to the same input???

yeah similar enough.
It really is me just thinking of ways to get the good stuff up out of the salt.

Im still not 100% why others have said that the struts wont operate well?
Conceptually surely it should work with the right dampers. Irrespective of the angle to earth they still travel parrallel to lines of force. Gassed dampers dont care what is up or down, just what is stroke. Flat dampers dont work when an axle is going up and down but on the posted setup strut travel is linear with stroke = ~100% efficiency.
But having said that it would be easy enough to run springs/airsprings in the place of the struts in drawing and then damper from axle to chassis as per Dougals comment.

The pivot arm power reduction/multiplication is a real issue.

I also dont think that front and rear axles would respond the same... but was curious to see opinions on the concept.

Anyways, it is all still drawing board stuff.

I have however decided that the risk of hull damage is outweighed by awesomeness of having a 5m glass boat up the cape. So it will just be a matter of time and design

Also this trailer will definately NOT be gal dipped as I cant be bothered transporting it 700km each way to the pot (Cairns Pollards now closed). So I could very easily run with a few different tandem setups over a few seasons and see how they go.

S

Originally Posted by roverrescue

...
Im still not 100% why others have said that the struts wont operate well?
Conceptually surely it should work with the right dampers. Irrespective of the angle to earth they still travel parrallel to lines of force. Gassed dampers dont care what is up or down, just what is stroke. Flat dampers dont work when an axle is going up and down but on the posted setup strut travel is linear with stroke = ~100% efficiency. ...

They will work, but you need to compensate for the angle. E.g. at 45o to the vertical, the spring rates and damping rates need to be 2x the rate of vertical springs/shocks. Plus on top of that you have the leverage of the arms. You would need to calculate the moments accurately to allow you to have a reasonable first guess at spring and damper rates.

Ben, My understanding of the concept was with say conventional setup, axle is travelling vertically. If damper is canted at 45 degrees to vertical it is half as efficient as stroke of damper is almost double the stroke of axle.

In the posted diagram, stroke of link is the same as stroke of damper. As in eye of link is moving horizontally, damper is moving horizontally = no efficiency loss.

The efficiency loss in the posted design IS in the leverage applied by the central pivot point.

A gas charged damper shouldnt care which way is up.

S

I don't think it is quite that simple Steve.

Consider the front axle only for a moment. If the wheel goes over a 1' high obstacle, it moves 1' vertically. The other side of the swingarm moves down less than 1' - at a rough guess it looks like 6". If the shock/spring were vertical, it would need to cope with 2x the load that the wheel sees. So ballpark numbers, 1000N force at the axle would mean 2000N force at the shock/spring. The shock/spring is however inclined at an angle ~30o. So instead, they will need to exert ~6000N (rough estimate) to counteract the 1000N at the wheel. However it is still not that simple, as the arc that the end of the beam is travelling at is nontrivial, so will change the rate the load is applied (i.e. serious nonlinearity).


Now adding in the 2nd axle. the load looks like it is halfway between the wheel and the pivot. So again, 2000N (or half of = 1000N) if vertical. Instead you need half of 6000N = 3000N. However that is again not accounting for the nonlinearity, which will create a rising or falling rate depending on the part of the arc.

***caveat*** posting this after some very good red, so don't assume any of the above is correct...

Steve, I don't have the pic up ATM but from what I remember your motion ratio was roughly 2:1.

Damper efficiency is reduced as it's moved away from 90* to it's lever, (axle, bellcrank, etc) not necessarily the vertical.

Twin tubes are limited to a tilt of 45* maximum from the vertical, but if you use a mono-tube you can mount it at any angle you want, and the other benefit of a mono-tube is that a larger piston diameter is more sensitive to small shaft movements which is a plus as you are getting less than 1/2 the movement at the piston rod/piston that the wheel sees.

Originally Posted by isuzurover

[snip]

***caveat*** posting this after some very good red, so don't assume any of the above is correct...

It's nice to see the Australian wine industry is being well supported in this thread, although I limited myself to one glass last night