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Suspension Idea


simonr
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My indi suspension is coming along slowly -

post-74-1179357190_thumb.jpg

Thing is, this has only delivered 13" of travel - limited both by the ball joints and UJ's - and that's after:

post-74-1179357203_thumb.jpg

Tickling the chassis a bit! (it will look like it was always meant to be come MOT time - lots of filler ;) )

Now, I could go back to square one with the joints, make new drive shafts etc - but that will delay the project and cost even more!

Alternatively, I could settle for 13" front and rear, but on the rear......

Make up a sub-frame which contains the diff, drive shafts, wishbones, springs and shocks in a single self contained unit (much like Jaguars have). Then mount this whole assembly on a pivot above the diff such that it can articulate like a beam axle. use a steering damper (same rate in both directions) and a couple of springs to hold the pivot in the middle.

This way, I should get some of the behavior of indi (low unsprung weight etc) but with less of the grounding out associated with indi, but most importantly 13" of travel added to say 30 degrees of articulation.

This seems to tick most of the boxes, but there is a stubborn mark in the wacko box. Since I trust you lot to flame things that are daft (as well as give constructive criticism of course!), I wondered if you can see any major pitfalls in my thinking.

What would be a good height for the pivot above the diff? This will effectively form the roll centre off road.

I have a picture of the idea - but it's only in my head at the mo and it comes out all blurry when I try to attach it!

Si

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My indi suspension is coming along slowly -

post-74-1179357190_thumb.jpg

Thing is, this has only delivered 13" of travel - limited both by the ball joints and UJ's - and that's after:

post-74-1179357203_thumb.jpg

Tickling the chassis a bit! (it will look like it was always meant to be come MOT time - lots of filler ;) )

Now, I could go back to square one with the joints, make new drive shafts etc - but that will delay the project and cost even more!

Alternatively, I could settle for 13" front and rear, but on the rear......

Make up a sub-frame which contains the diff, drive shafts, wishbones, springs and shocks in a single self contained unit (much like Jaguars have). Then mount this whole assembly on a pivot above the diff such that it can articulate like a beam axle. use a steering damper (same rate in both directions) and a couple of springs to hold the pivot in the middle.

This way, I should get some of the behavior of indi (low unsprung weight etc) but with less of the grounding out associated with indi, but most importantly 13" of travel added to say 30 degrees of articulation.

This seems to tick most of the boxes, but there is a stubborn mark in the wacko box. Since I trust you lot to flame things that are daft (as well as give constructive criticism of course!), I wondered if you can see any major pitfalls in my thinking.

What would be a good height for the pivot above the diff? This will effectively form the roll centre off road.

I have a picture of the idea - but it's only in my head at the mo and it comes out all blurry when I try to attach it!

Si

Do it Si! Very interested to know how it works out. I once was building up a 4x4 Volkswagon Beetle, with a Suburu rear diff fitted to the front VW twin transverse torsion bar beams which were abapted to Austin 1800 FWD stub axles, early Kombi portals etc. I centrally pivoted the complete assembly on a Landy 110 front radius arm bushing and located for/aft with a shortened 110 rear lower control arm on each side. My thinking at the time was that this system if made lockable for road work would give the advantages of low unsprung mass that independant susp provides , with the articulation that only a beam type axle can give . Unfortuntely I met the woman who became my wife and spent far too much time overseas before I could complete the project. Yes do it Si, I'll come over and give you a hand each night after work if you like. :)

Bill

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Blue Sky... I like the idea a lot. How about suspending the Jag-like subframe on an A frame and a pair of radius arms? 13" on the indi plus maybe 11 (harder) inches on the subframe - 25" ought to be enough for anybody!

Assuming this is intended for use in speed events I think that a lot of development work would be needed to 'manage' the stability. Like you I 'feel' that the roll centre of the subframe (and it's relationship to the roll centre of the indi setup) is critical, but have no relevant experience to draw on for a first guess. No reasoned argument then, but I feel that as a first shot it should be relatively conservative - say a couple of inches above the propshaft?

Everyone: Feel free to shout 'bull***t!' I won't be offended :)

TS

p.s. where is the roll centre with just the indi setup?

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EFI - They are jag Diffs which are a D44 centre, same as a Sallisbury.

They are kind of heavy for my app, but I was swayed by Najw's argument that difflocks are available for these, but not for my original choice (BMW diffs & shafts).

There are one or two photos here. It's not very 'Land Rover' I'm afraid, although it does have an increasing number of LR bits!

TwoSheds - I'm worried about making the roll centre close to the centre of rotation of the prop because it will increase the drive torque induced roll.

I'm still a bit undecided whether it's better to have a high or low roll centre. The IRS centre is very low (roughly level with the road surface). Will having two roll centres make it unstable as the effective height will change depending on circumstance?

Bill, I like the idea of locking it off on the road - although LR bushes are a bit sloppy for my liking. I'm using polymer bearings for the wishbone ends from Igus which (apparently) are designed to run dry in abrasive environments and should be OK in mud. These should be OK for the sub-frame mounts as well. Flexible joints (such as elastomer bushes) just lead to vague handling. Do pop round for a cup of tea any time! ;)

Si

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wierd = cool

is the trailing arm/diff carrier assembly going to be moving in just the vertical plane or will it also offer the diff the option of rotating as well in a mog torque tube stylee?

:)

I'd have thought that just allowing the subframe to articulate would be enough, as the rest of the suspension (13") will give you enough for shock absorbtion, and the increae in articulation will allow you to keep wheels on the ground when cross-axled. What it will do is make the car very unstable on side slopes and under cornering. Perhaps you could mount the sub-frame so that it ability to roll is controlled by a hydraulic ram, which could fulfil 2 roles - resistance to roll by forcing the oil to flow through a small hole (i.e. as a shock absorber) in conjunction with some springs (or with 2 rams mounted vertically with a gas compensator like a Citroen has to give some spring effect to self right), and the ability to force articulation to give stability on side slopes.

The only thing is though, if you're doing it on the back, you also want to do it on the front to give a balanced vehicle. :lol:

Toby

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as long as we're in whiteboard scribbles and hobnob fuelled randomness hows about having a ram thats drawn down by the travel of the swingarm (providing the swingarm is a mover?) forcing fluid into a slave ram that moves the upper spring mounts downwards? starting to sound a lot like something you could adapt a Boge strut for? (mmm plentiful and cheap :) )

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I like the idea od using a ram with a small bypass to restrict the articulation - plus using hydraulic pressure to force articulation where necessary.

The setup I'm proposing only allows articulation, I don't think there is a need for additional up & down movement. That also makes it easier to lock out for road use.

Sadly there is no room to do this on the front - there's kind of an engine in the way!

Maybe the answer is to use a pair of short, adjustable coil-overs. Then the amount of force holding the frame central is adjustable. Hopefully there will be a happy medium for side slopes and general use.

If I were to make the lock-out electric and controlled from the cab, It could maybe be locked out on side-slopes?

A couple of scribbles:

post-74-1179396546_thumb.jpg

Si

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It looks like you would be adding a HEAP of weight? I would try and keep the weight down. Also, there is sometimes no need for huge amounts of articulation. I prefer low weight, and picking up wheels over holes and to keep on driving rather than having heaps of travel and getting bound up in holes. If you cant get enough traction to the ground, install lockers.

My 2c anyway

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Just going back to basics, to find a possible solution from the assembled 'whiteboard scribblers', what part of the current arrangement limits the travel, is it the wishbones, the coilovers, the driveshafts?

Is there a 'hidden' solution within the current setup?

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Just going back to basics, to find a possible solution from the assembled 'whiteboard scribblers', what part of the current arrangement limits the travel, is it the wishbones, the coilovers, the driveshafts?

The limit is imposed by the max operating angle of the UJ's in the drive shafts (36 degrees), but this is further restricted by the need to 'steer'. Steering lock is 25 degrees, and that combined with max rise or droop must be less than 36 degrees.

The shafts are forged so difficult to change their length (there is no spline / plunge joint in them). I could 'add' a splined expansion joint in the middle - but I'm not convinced about welding the drive shaft and it's effect on strength.

The ball joints have about 32 degrees of movement - and they are the widest I could find for a regular TRE style joint. I don't want rose (or similar) joints - they just do not last in mud.

One of the biggest limitations is the length of the wishbones (410mm between centres). These could be lengthened, with a different drive shaft. But I wanted to keep the track about the same (running rims with 2" negative offset). OK, this is not fixed in stone.

It looks like you would be adding a HEAP of weight?

A little. Hopefully not too much though. I have already lost 350kg from the start weight of 1040kg. Although there appears to be a lot of tube in this, it is replacing the last three feet of chassis. I reckon this will gain about 30kg over the same thing without the additional articulation. It makes it tighter - but it's still on course for a total weight under 1 ton.

It will get lockers in due course (if it performs OK) and it has 4 wheel fiddle brakes. I have a theory about how to build a better traction control system than is currently used on 4x4's.

Before everyone leaps in with "?? has spent millions on R&D, how can you come up with something better" - they are bound by a different set of compromises such as pad life, on road stability and reliability. I'm using separate calipers for fiddles and TC so these are less of a worry.

My idea is this. Normally TC locks the spinning wheel, then releases it allowing it's speed to increase. There will be a point where it's speed matches the ground speed where it will re-gain traction. It will continue to speed up until there is sufficient torque to break traction and spin. The whole process then repeats.

I reckon that it only has traction for about 30 to 50% of the cycle. This is one of the reasons diff-locks generally outperform TC.

My idea is for more progressive pressure control. Still using PWM, but with an accumulator to smooth out the ripples. Calculate the maximum differential speed achievable by steering. Outside this differential speed window, the TC algorithm increases or decreases the duty cycle to bring the differential speed within those limits. Thus, the wheels should not lock up on each PWM cycle and should not break traction as often. Because it allows for the difference in speed while turning, it should behave more like an intelligent limited slip combined with a solid locker.

One of the reasons this could not work on a production car is that it is effectively riding the brakes all the time traction is limited. This may lead to fade but certainly to accelerated pad wear. Since the pads are separate from the service brakes - who cares!

Question is - would this count as none, one or two lockers in a competition?

Si

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use Mog type back to back UJs for a 50 degree turning angle? if its a comp only vehicle they shouldnt be a major nightmare to build

would all four channels run independantly for the TC? tied into steering?

Worth a look!

All four channels would work together. There does not need to be any tie-in to steering as such, only a knowledge of the maximum difference in speed between the wheels on a given axle that can possibly result from steering alone. Inside this window, it does nothing, outside, it works to try to bring the difference inside by slowing down the faster wheel.

Si

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Si,

As you have a 'compact' vehicle as your base, is there, other than preference, a practical issue for not increasing the width?

Will this not additionally aid stability?

Also with regards to the driveshafts, would a sliding joint similar to a propshafts have any significant issues regarding the strength?

In other applications, driveshafts and axle casings are adjusted?

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Simon,

I dont think there is a mechanical way of extending beyond 13" that you have achieved so far, unless you do all mods as you stated like sleeving the driveshaft, or lengthing the wishbones.

Are the wishbones equal length??

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I dont think there is a mechanical way of extending beyond 13" that you have achieved so far, unless you do all mods as you stated like sleeving the driveshaft, or lengthing the wishbones.

It is possible, but it's not really what I'm after.

I was hoping for some thoughts on whether the pivoting rear axle was a good or bad plan?

Are the wishbones equal length??

Yup. You cannot really combine unequal lengths with adjustable height because your camber will only be correct at one height.

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It is possible, but it's not really what I'm after.

I was hoping for some thoughts on whether the pivoting rear axle was a good or bad plan?

Yup. You cannot really combine unequal lengths with adjustable height because your camber will only be correct at one height.

Please correct me if I am wrong Si. Just like the trailing arm front suspension on a VW or the sliding pillar suspension on a Morgan, don't equal length wishbones cause the wheels to remain parrallel with the chassis rather than with the road with body roll ?

Bill.

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Please correct me if I am wrong Si. Just like the trailing arm front suspension on a VW or the sliding pillar suspension on a Morgan, don't equal length wishbones cause the wheels to remain parrallel with the chassis rather than with the road with body roll ?

Bill.

You're not wrong.

Will - I have some recollection of that - but couldn't remember the actual geometry of it (or if it worked!). Thanks for the lead though.

Si

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