[Damo]

A few weeks back I was looking for something completely unrelated on a speedway parts site of all places, when I came across an alternate for shackles in a leaf spring setup. They look like this:


After doing a bit of Googling it appears that the concept is not a new one and that it's been used pretty successfully on leaf sprung circle track and drag cars. A bit more digging turned them up in use off road, in this thread the people who comment are generally positive toward them, where as this thread details problems with them. My summary of that is in slow speed offroad (rockcrawling) they work and work well, but for high speed (Baja) they had reliability issues. Incidentally, that Pirate thread is not a bad read and has a fair few pics of them setup.

So why bother? initially I thought it would be a good way to help reduce the overall height of the suspension so the CoG can be kept as low as possible. After doing some reading there are other advantages:

Consistant spring rate - this is because the shackle angle changes the spring's effective rate. That means that throughout the range of wheel travel the spring's overall rate changes due to the varying shackle angle. What does that mean overall? Dunno, maybe the feel of it off camber will be more predictable.

Lateral stability - without the shackle that end of the spring will be contained a lot better. More precise steering on road, and offroad the diff will have less tendency to be pushed from side to side under steering.

Constant roll centre height - if the slider is located properly the roll centre height wont change as the suspension cycles, unlike a leaf spring with shackles. Real world affect? probably SFA on a zook, I doubt you'd notice the difference, onroad handling might be better, I can't imagine it would make things any worse.

The measurements I have been taking and mocking up are based on a hybrid Corolla leaf pack with 1200mm long main leaf. Same length springs front & rear but higher rate (more leaves) pack in front with shackle reverse. Spring camber would be minimal, about same as stock. The goal is to get something that's compliant without being so soft as to scrape the door handles on the ground when you round a corner. Oh, and get some good wheel travel.

The sliders themselves would be pretty straightforward to fab up. Use some 75mm RHS for the main bracket, 10mm square bar for the slide rails and instead of bearings UHMW polyethylene blocks.

[Fatzook]

After our discussion ysterday, I though of another potential issue.

A shackle can; to a point; twist. During articulation the spring will want to twist.

A. How well do you think that slider design will hold up to a twisting force?

B. do you think it would limit travel due to the springs reduced ability to twist?

[Damo]

Fatzook wrote:

After our discussion ysterday, I though of another potential issue.

A shackle can; to a point; twist. During articulation the spring will want to twist.

A. How well do you think that slider design will hold up to a twisting force?

B. do you think it would limit travel due to the springs reduced ability to twist?

The spring will still be able to twist as it will still have rubber bushes just like it would if it were connected to a shackle. The slider should hold up to the force, the force would be transferred to the UHMW block which is then transferred to the square bar welded to the outside of the bracket. Even if there was a failure of the UHMW block etc the spring end would still be contained by the bolt going through the whole assembly, so there is some redundancy built in.

There may be some limit to the wheel travel, i'm not really sure. The thread on Pirate indicates that travel is not a problem though, so it must be possible to make it work.

[Gwagensteve]

Lots of ideas like this were tried in the late 90's in the US. I have heaps of Petersens, Four Wheeler and 4WD&SU mags from that period with lots of alternative ideas for shackles.

They've all pretty much petered out now. Mostly, they seemed to be very high maintenance items, and became quite complex once you are trying to chase the issues out of the design.

There's a couple of things you need to consider first up. There's lots of force and friction on these slides. It will be a good test of your ability to design a good part to make it reliable and "fail safe" My guess is that UHMW will be overloaded if used as a slide - it will wear VERY fast. Genuine teflon might be a solution.


Go for your life, but the reasons they work in speedway are exactly the reasons you don't want them in a 4WD. You have lost 33% of the potential flex in the system by eliminating one set of bushes. That's good for handling, but bad for capability. The spring can't flex enough to give you that flex back.

You're also going to loose droop, for the same reason the roll centre stays consistent. The arc of the shackle adds droop as it moves from compressed (flat) to drooped (vertical)

You're going to loose lift and mess up your caster unless you drop the slide down to the position of the bottom of the shackle, effectively reducing clearance in a way equivalent to running a shackle reversal, except you're now putting some moving parts down there.

As an aside, I agree that the roll centre change is an issue with conventional shackles, however, that's dependant on your setup. If the car settles onto the bumpstop on hard cornering, then the roll centre becomes the bump stop.

I'm going to try and explain something that I think is another issue and I'm not sure it's easy, but here goes.

As leaf suspension flexes, the shackled ends of the springs effectively "spread" or narrow relative to each other. Have a look at a leaf car flexed up and you'll see that the shackles aren't parallel viewed from the front (or rear) That's one of the reasons that shackles with centre bars reduce flex (or rather, work the springs and bushes harder, which might reduce flex)

By running slides, this action is being locked out, and it will be being transferred into side loads on the slide, and binding in the suspension.

As I said, go for your life (prove me wrong!) but theory, and the experience of others, would indicate it's a bit of a dead end.

PS for a low travel/handling focussed application like a speedway car, where roll stiffness is really important, I'm sure it works a treat.

Steve.

[mnemonix]

Gwagensteve wrote:

If the car settles onto the bumpstop on hard cornering, then the roll centre becomes the bump stop.

Does this mean there may be stability/side angle benefits if you relocate your bumpstops inboard/outboard?

[Gwagensteve]

I just skimmed through that pirate thread. Interesting- there wasn't a lot of info from guys who have been running these since the 90's. Lots of guys in the shed.

Interesting too that one of the guys commented about the friction of "teflon" (The friction coefficient of teflon is roughly equivalent to wet ice) so at a guess, there's some serious binding/overloading going on with some of these designs. (and/or they aren't using actual teflon, which is dearer than smack)

Steve.

[MrRocky]

it looks to be an interesting concept but i agree that where flex is concerned these would probably not be better than a well worked shackle setup, however i can see the merits of such a thing where handling at higher speeds is the primary concern but i suppose you would still want to retain a certain amount of droop as well.

[kenn]

Gwagensteve wrote:

I just skimmed through that pirate thread. Interesting- there wasn't a lot of info from guys who have been running these since the 90's. Lots of guys in the shed.

Interesting too that one of the guys commented about the friction of "teflon" (The friction coefficient of teflon is roughly equivalent to wet ice) so at a guess, there's some serious binding/overloading going on with some of these designs. (and/or they aren't using actual teflon, which is dearer than smack)Steve.

Have you become a smack head Steve

[Fatzook]

kenn wrote:

Have you become a smack head Steve

Only cause snorting Teflon has become too costly

[Teracis]

What about these things with shackles attached then springs? Or is that the worst of both worlds?

[LAY80N]

Fatzook wrote:

kenn wrote:

Have you become a smack head Steve

Only cause snorting Teflon has become too costly

Damn you, i just went and scratched the good teflon coated cookware thinking Steve was on to something. Now i'm in the shit and im not high


Tech content of this post: Sliders are not that great on a car, but on a lighweight trailer they work fine.


Layton

[atari4x4]

LAY80N wrote:

Damn you, i just went and scratched the good teflon coated cookware thinking Steve was on to something. Now i'm in the shit and im not high

but you shit will just slide out & there will be no evidence of ever taking a dump.

[Damo]

I actually agreed with pretty much everything you said Steve. And for that reason anyone who follows in my footsteps should have their head examined

Gwagensteve wrote:

Go for your life, but the reasons they work in speedway are exactly the reasons you don't want them in a 4WD. You have lost 33% of the potential flex in the system by eliminating one set of bushes. That's good for handling, but bad for capability. The spring can't flex enough to give you that flex back.

You're also going to loose droop, for the same reason the roll centre stays consistent. The arc of the shackle adds droop as it moves from compressed (flat) to drooped (vertical) [/quote]

The way I see it is the last 1/3 of the wheel travel doesn't give you much traction anyway, since you have so little weight on the tyre. The idea of using very long springs is to get some of the travel back, but at the end of the day wheel travel is not my primary concern. Same deal with losing droop, it will be reduced but there will still be enough t keep me happy.

Gwagensteve wrote:

You're going to loose lift and mess up your caster unless you drop the slide down to the position of the bottom of the shackle, effectively reducing clearance in a way equivalent to running a shackle reversal, except you're now putting some moving parts down there.

Losing lift is actually part of the reason of doing it this way in the first place. It will take some messing around to get the caster right

Gwagensteve wrote:

As an aside, I agree that the roll centre change is an issue with conventional shackles, however, that's dependant on your setup. If the car settles onto the bumpstop on hard cornering, then the roll centre becomes the bump stop.

One thing i'm hoping to do is have the cornering a bit flatter. It sort of flies in the face of conventional offroad suspension but with lockers i dont think you really need huge wheel travel, and the advantage of having something super low and stable would offset the lack of wheel travel. That's my theory at least.

Gwagensteve wrote:

I'm going to try and explain something that I think is another issue and I'm not sure it's easy, but here goes.

As leaf suspension flexes, the shackled ends of the springs effectively "spread" or narrow relative to each other. Have a look at a leaf car flexed up and you'll see that the shackles aren't parallel viewed from the front (or rear) That's one of the reasons that shackles with centre bars reduce flex (or rather, work the springs and bushes harder, which might reduce flex)

By running slides, this action is being locked out, and it will be being transferred into side loads on the slide, and binding in the suspension.

As I said, go for your life (prove me wrong!) but theory, and the experience of others, would indicate it's a bit of a dead end.

PS for a low travel/handling focussed application like a speedway car, where roll stiffness is really important, I'm sure it works a treat.

Steve.

Thanks for all your input Steve, gives me more to think about.