17 replies to this topic

[Spider]

If the classic car motoring is what attracts one to a Mini, then I agree 100% with Cooperman's comments, it's pretty darn good right out of the box and even by moderns car standards, isn't too bad at all - when everything's in good order and at standard ride height.

 

I mentioned above that I have my own Air Suspension set up in the back end of the Moke. Cassic Car Motoring isn't what attracts me to the Moke, especially for doing the extended trips, in fact, I gotta say, the whole classic car 'thing' in respect of the Moke I almost find a turn off.

 

The Rear suspension in the Moke, isn't too hevily modified, it is beefed up to cope with the rough tracks. When away on these trips, it's not uncommon for the Moke to tip the scales at between 1200 and 1500 kg and it is for this reason that the Air was designed and added. The Suspension Rate in the back end of a standard Moke is quite woeful.

 

 

This set up doesn't replace the stock suspension, rubber cone or any of that, but supplements it. As the rear travel is somewhat limited and loads (in the back) go from zero to 600 kg, a Rubber Cone and Trumpet just ain't going to cut it. So, while it's possible to sort it so it works with 600 kg in the back, it would be damn awful - locked solid in fact - when empty. So, the Air fills this gap with ease.

 

Just a few comments I'll make to some of the posts above;-

 

Surely the Airride stuff is just a prank isn't it?  The bracketing that attaches on the front end is just waiting to get ripped off and it does really look a poorly engineered concept. The Minivation stuff while attempting to replace the rear cones with Air Bags just won't work, you simply can't get enough air pressure in to any commercially available air bag that will fit to come within a bulls roar of the rate needed.

 

Maybe I'm being a little pedantic, however the stock rubber cones on their own also have a linear rate, just like a coil spring. It is the combination of the shape of the Cone and the Flange on the Trump that gives this set up a 'rising rate'. 

 

The Rubber Cone / Trumpet set up is also very 'tunable' and in ways that a convention spring could only dream of. To move away from this set up is a move backwards. It does have a down side that we all know too well and that is it sags over time. It's also incredibly compact. If we could get any type of conventional spring to replicate what the Rubber Cone / trumped set up does, I'd have it in a heart beat, but it doesn't exist.

 

I 'doff my hat to the late Alex Moulton for the design of it, it really is genius.

 

Just coming back to Air Bags. All the commercially available Air Bags I've looked at do increase in spring rate with an increase in air pressure within them, so in short, adding air increases the rate for the same height. It's all Pressure over Area stuff.

 

<Edit: Just found my numbers on Air Bags. To replicate the same mean rate as the Rubber Cone / Trumpet set up, the Air Bag needs to have a base diameter of 6.2", and allowing for clearances for the bag, this comes in at just under 9" in dia.

 

And that's running at their maximum rated pressure - 100 PSI

 

So, if you wanted the set up 'adjustable' you'd need an even bigger bag. When I looked at it some years back, I needed an 8" Bag, which needed just under 11" to make it sensibly fit. >

 

<2nd Edit:-  If the OP is looking for a set up that can quickly adjust ride height from the driver's set, I have seen a hydraulic set up that uses the Rubber Cones, but has small hydraulic rams in place of the trumpets. I'll see if I can find a link. >

 

< Found it

 

Youtube clip

 

 

http://www.rayvernhydraulics.com/  >

Edited by Moke Spider, 28 January 2017 - 07:16 AM.

[tiger99]

Thanks! You have saved me the trouble of doing the calculation. It was kind of obvious that with the lever arm ratio in the Mini, and the usual air pressure of 100 psi, that the air bag was going to be much too large to be useful. I was going to refer to the book at work for confirmation. In fact I will still do that to see what it says.

 

Citroen hydropneumatics work with lever arm ratios that look roughly similar to a Mini, at least at the rear, and use a much higher pressure of nitrogen. You can't use air at that sort of pressure, because a leaky diaphragm would make the suspension sphere explode as in a diesel engine. You can't use non-inflammable fluid as the valves and pumps need lubrication. So you change the fluid volume,keeping the actual quantity (mass) of gas constant. At high load, the gas is compressed to a smaller volume so the spring rate rises, which is what is wanted. The system has much to commend it. Alec and Alex knew of it but of course it was way over budget. Hydragas is a simplistic substitute.

 

Now here is a thought. I have suggested hydropneumatics before, and a very stripped down version with only the pump and front and rear height correction valves is not unfeasible. But how about using interconnected hydragas components, with a pump to add or reduce the fluid volume? The fluid is not a lubricant so the choice of pump is not easy. but as the base system can work well on a Metro it is not far off what is required in a Mini. The Metro rear subframe, at least one version of it, is not too hard to cut down the middle to reduce its width, and that would give better radius arms. The snag is that there is a very limited supply of Metro parts now.

[Spider]

I'm none too intimately familiar with the Citroen set up (French Engineering - spare me!), though I do understand it's 'in principal' operation.

 

But, Tiger, you've reminded me on another issue with Air and it's none too stable, especially with heat, so Nitrogen, as it seems Citroen use, would be a better filling gas.

 

Also, with the Air Bags, there's nothing in the bag itself to keep the ends aligned, so the non-mounted end must have a guide of some sort or the Bag will move sideways once it comes under any type of load.

 

Here's an old Dunlop Chart that I have (I think from a 3" Bag ??) showing that they are quite linear and that the rate does increase with pressure.

 

 

There are Bag available though, that have a Conical Base that forms an internal piston in the Bag, which - from memory - increase the effective surface area as the bag compresses, and in doing so, gives a increase in rate in a similar way the the Rubber Cone / Trumpet set up on a Mini does..

 

<EDIT: I've just looked again at the Air Ride system offered for Minis and one other 'thing' pokes me in the eye. It works off the Lower Suspension Arm on the front. The Original Mini Suspension was designed to load the Top Ball Joint  and in compression. These type of Ball Joins on a Mini were never designed to work the other way, ie 'pulling' on the Pin of them. Given that the Lower ball Joint is also not set at an idea angle and will reach lock at full extension when steered, this set up could well lead to Pin Breakages if it doesn't pull the Cup open first.

 

Not the best engineered product out there.>

Edited by Moke Spider, 28 January 2017 - 09:03 PM.