new 6-wheeler disco plan

"Austin Shackles" wrote in message news: snipped-for-privacy@4ax.com...

One thought........... fit coils! Far less stress.

Not so sure two height sensors in parallel will give you an average, I don't think they're analog in nature. BICBW!

Only if the resistors are equal in value! These by definition are not!

hmm I do assume that the disco 2 does not have active suspension so the sensors are not used to correct the height on the move

so when you set the height with the switch on the dash it just keeps it at that height

so by definition (providing you use the same sensors) they will be

equal on flat level ground with equal loading on each axle

trying to work out the transients in movement is way beyond me and would need lots of testing anyway

i think the best way to do it is to use only sensors on the rear axle and link the two air suspension units together

How the f*ck would it know to pump it up then?

It's not active in the sense that it reacts to bumps etc but it is active in that it maintains the correct ride height in the long term. Put a few boxes of tiles in the back and it sags noticeably, start up and it pumps up to the correct height. Take the load out shortly after switching off and it'll rise about an inch then go phissssstt and sink back down, it may do that sveral times depending on how much load you are taking out.

Thats just a +2" offset from normal ride height. You can also get a fob that will do -2".

I think that is probably the best option unless you make all four rear air bags independant of each other. It certainly saves all the faffing about of combining the sensor outputs into a form acceptable by the existing computer, be they simple potentiometers or digital encoders.

The rear axle bags will have to extend more than the middle axle ones as the car body effectively rotates about the front axle. Something to think about?

I wonder how well the compressor will cope with double the air volume required for a given adjustment. It's not quick as it is, and any adjustment would take roughly twice as long. Will the computer would get upset with the excess time, decide there is a leak, sulk and lower the suspension onto the bump stops as they do.

The compressor has a reputation of not tolerating small leaks very well either, wearing out, and they ain't cheap...

Big leaf spring, pivot in the middle ;-)

Whether you can easily combine the sensor readings depends on the detail of the sensors themselves, but a pair of potentiometers in parallel wouldn't give an average reading, the reading you'd get would always be smaller than the smallest of the two values. You'd have to feed them into another circuit to get an average reading, probably a quite simple off-the-shelf IC from Maplin or sommat.

Bear in mind though that there needs to be some limit on the bag pressure otherwise with one axle up and another down, the average reading would be low enough to trigger the compressor but one axle could end up being over-pressurised. If you can find the right IC, perhaps one that output the maximum of the two sensor readings would be better.

On or around Fri, 29 Jan 2010 19:52:00 +0000, mike hunt enlightened us thusly:

nah, it is active. Provided the height sensors are resistive, they will combine, but might not give readings that the ECU will work with. If so, they would need to be custom built.

The other alternative (which I just now thought of) is to link the 2 axles mechaincally to one height sesnor with a mechanical averaging device. That might be better, in the end.

On or around Fri, 29 Jan 2010 20:48:27 +0000 (GMT), "Dave Liquorice" enlightened us thusly:

Good point, might need an uprated compressor.

One height sensor each side but yes that does seem a better solution but I'm not shure how the mechanics could be arranged, the sensors attached between the body and about half way down the drag link via two short arms pivoted in the middle (and each end...).

I've been following from a distance here, and it seems (IMHO) that you're making it too complicated. Leave the single sensor attached to whichever axle is easiest to get at to repair, then just connect both airbags to the same pressure source, (two pipes, tee-d into one, then fed to the control valve.) This way, both axles will always carry about the same weight, as long as the suspension travel is sufficient. Simple, less to go wrong, and less to get damaged in the rough stuff. If you wish, put another sensor on the other axle, with a switch to change over if there's a fault. Add another switch to override the auto mode, and you've got full pressure when you need to lift it a bit. Add another bit of valvework, and another couple of switches, and you can dump the air in one axle to give maximum weight on the other for those tricky moments when you've really gone that bit too far.....

Unless I'm missing something, and the suspension isn't pure air, which is what I'm used to on larger stuff. Keep it simple, is my motto.

On or around Wed, 03 Feb 2010 14:03:05 +0000, John Williamson enlightened us thusly:

It's pure air with normal dampers.

yeah, that might work as you describe - the only thing that would bother me is the effect of putting weight right at the back. You'd have to put the sensors on the rearmost axle, I think, or you could get silly answers.

Since the sensor connects to the radius arm, in a normal one, I don't think there's be much issue with a link between the 2 axles with the sensor attached to it, and it'd not be that hard to do - then the sesnor can detect the average ride height of the 2 axles, which is what it really needs to do.

I was already going to interconnect the bags via a T piece. I wonder what the wagons with air on the back of tandem or triple-axle trailers do?

As for the questions of over-pressure or over-extension, the system has limits for the latter built-in (in the dampers, I believe) and the bag/pipework has to be able to cope with shock loading from big bumps at speed and off-road loads, without blowing up. The pressures generated by one wheel going up and the other going down when off-road at low speed will be no bigger.

OK, then.

Possibly, it may depend how stiff the chassis is. Still, you're right, the further the sensor is from the centre of the wheelbase, the better. The rear axle's probably easier to get at anyway.

If you're on a flat surface like tarmac, then axle ride height is automatically going to be the same within the limits of chassis flex and the actual smoothness of the surface. On a rough surface, you definitely

*don't* want both rear axles to be at the same physical level as dictated by independent valves, but you do normally want them both to be carrying the same weight. Having them both on the same pressure feed will ensure this. The mechanical averaging may help, but I don't think it'd be worth the extra effort. It's also six more relatively delicate things to go wrong when someone leaves a hundredweight of mud in the wrong place.

As far as I remember, they have an independent set of valves and airbags for each axle, with an override valve to lift any axles that aren't needed, which is usually manual, but can be automatic, controlled by the air pressure in the loaded axles.

See:-

for an example. One set per axle, no interlinking. It's an Australian page, but similar equipment's used worldwide.

Then again, they're generally assumed to be running on more or less flat surfaces.

The non-air ones either have separate springs or a central spring arrangement on a rocking beam with the axles mounted on the ends, but that's more for trailers that need lots of suspension travel, like tippers. That might be an idea for the Disco, if you're up to fabricating such a thing, and you're expecting to do silly things off road with it. I could draw it on a drafting machine,or sketch it on an envelope, but I don't have the Autocad skills yet.

Bear in mind though that with a 6-wheel vehicle you lift wheels much more regularly than with a 4-wheeler so you'll get axles at max end of travel much more frequently (both upper and lower) than with a 4-wheeler. If you're not intending to off-road it then it's not much of an issue.

Basically with a 4x4 you really only reach axle travel limits when cross-axleing, grounding out, jumping or overloading, but on a 6x6 or anything with more than 4 wheels, you regularly lift wheels or compress suspension to the limits when just transitioning from flat to a slope and back, cresting hills, going over deep potholes etc.

"Austin Shackles" wrote in message news: snipped-for-privacy@4ax.com...

Had a Tri axle trailer in last night for service rear four airbags are piped together through a single pneumatic leveling valve, front axle is a lift axle so runs on seperate piping through lift valve but when down is governed by the single levelling valve most trucks running Ecas just have a pair of sensors on the rear axle (assuming its not a lift axle)

andy

I'm assuming that's not for off-road use though ;-)

Not sure if Austin's is or not, would be simpler if not as no need to take extremes of axle travel into account so much.

On or around Fri, 5 Feb 2010 23:31:55 +0000, Ian Rawlings enlightened us thusly:

yeah - this is true. this is one reason for letting the 2 bags on one side of the vehicle equalise their individual pressure, by means of them being on the same air supply - when you do something like going from flat to steep slope, you will compress the rear-most set, which will push air out of that into the middle axle ones, until the pressures are equal - i.e. each axle is carrying the same weight. The only thing that slightly bothers me is the flow rate you can get through the small pipes that are used. However, as a general rule, you're not going to traverse ground that requires axle movements of a foot or more in the space of under a yard of forward motion at any great speed - and if you're doing it slowly, the suspension should have time to react.

As for max/min travel stops, it takes quite extreme movement to hit those on a disco, as there's a lot of travel. nevertheless, the system has stops (bump stop on the chassis, max travel in the damper, IIRC) which are supposed to be able to cope.

On or around Sat, 6 Feb 2010 16:01:41 +0000, Ian Rawlings enlightened us thusly:

It'd have to be capable of off-road use. But I don't intend building a trialler.

A bigger issue in some ways is a credible way of getting drive to the rearmost axle.