Technical Front Shackles Question

Maybe a dumb question but how are these three-hole shackles set up? Appears the top holes act as the lower shock mount. If so, are there pros and cons to this? If not shock mounts, what are the top holes bolted to?
Can't remember where or who posted this one but it's a perfect RPU, IMO!

D

 

It appears you are correct.

 

That setup looks easy to build but mechanically iffy.

 

I built a car like that with the top hole as the shock mount. Drove and road great. If you think about it it dampens the up and down movement of the axle, plus it dampens the spring getting longer and shorter with bounce.

 

Speedway sells something along the same concept, I have them on my t-bucket

https://www.speedwaymotors.com/Shackle-Plate-with-Shock-Mount-Hole,35384.html

 

I'm no engineer but at first I thought the arm would be reduced making the ride more firm or making the shock work more or something. Now I'm thinking the shock might more quickly dampen smaller movements caused by the spring expanding. Then, continue absorption when the axle moves upward.
Thanks for the quick responses.
D

 

Why is that ?

 

I see no reason NOT use it. Have you ever jumped up and down on a reversed eye short cross spring straight with no shocks? There may be an 1” or so of travel and probably not even needed.

 

If you don't understand the geometry involved there's not much point trying to explain it

 

Why not try? We all have open minds and can always learn something new.

 

I would like to hear the reasoning for this not working well. I can't see how the setup would have a negative effect on the ability of the shocks to work. Only thing I am not sure about is if the shock absorbing motion causes any reversed reaction back through the shackle to the spring/axle ???

One advantage I see is that as the axle moves up, the spring lengthens and moves the bottom shock mount inwards. This would move the shock more towards vertical. So, in theory, the farther the axle moves upwards (bounce), the stiffer the damping would become, making it a somewhat "progressive" setup.

I see that @TCTND is a retired mechanical engineer, and I am genuinely interested to hear his thoughts.

 

Andy Southard's T pickup ...

 

A bit condescending don't you think ?

 

If someone doesn't understand the geometry involved, there is every point in trying to explain it.

 

OK, here's my reasoning. When one wheel hits a bump the axle rises, the spring straightens, and the shackle swings to allow for the increased spring length. With this arrangement the shock is opposing that motion and the resultant force tries to shove the axle sideways. I described it as "iffy" not fatal. Can you drive a car like this? Sure, It's just not a very good way to do it. Apologies to anyone I've offended, none intended.

 

What I don't like is that it reverses the action of the shock, many shocks have proportional valving. As in 40/60, they have 60% down and 40% up. Look at gas charged shocks, which was discussed at length in a previous thread, they "push" up against the weight of the vehicle, in this case installed they would do the opposite.

I too am not saying it won't work, just saying it will reverse the operation of the shock. Also if this design is that good way doesn't auto makers still use it?

There are many, perhaps millions of designs auto makers used since the beginning of the industry only to come along with a better design, safer operation and designs that did not work out as planned.

Also just because a show car built a suspension comparable to this setup doesn't mean it was a good idea or even a workable one. But I have seen guys see something in a magazine on a vehicle that looked cool or different and copy it only to be disappointed especially in the 4x4 arena....

Like I said there is a reason NO auto maker uses this design....

...

 

You can compress/ extend a shock with your hands , I don't think that's enough resistance to shift the axle in the chassis , especially if the spring is properly installed in tension and or the car is equipped with a panhard bar . Plus the shock is supposed to control the action of the spring/ axle . The biggest problem with most shock mounts on straight axles is they are mounted too far inboard , away from the unsprung mass and the front of the car is not rigid or heavy enough to oppose the movement , similar to holding a sledge hammer by the end of the handle with one hand & expecting completely to control the movement of the head. Feel free to hash this out on your own , I'll watch .

 

I can't see how this would reverse the action of the shock. During spring compression the whole axle assembly is forced upwards as a complete unit (axle, perch and shackles). The shock will still get compressed in a jounce situation.

 

No auto maker still uses a transverse front spring either.

 

Exactly!! And there is a good reason why.


..

 

If you've never used something a factory didn't produce or an engineer didn't approve of , you aren't much of a hotrodder IMO .

 

I don't like that it forces the lower shock bushing into a far greater degree of rotation than conventional mounting.

This will shorten bushing life.

How short is hard to say

 

This thread poses valid questions about the shackle-mounted shock setup, and has the potential to be an interesting and educational discussion.......... unless some can't get to the end of the very first page without getting confrontational because of differing opinions. Can you give us some relevent insight into the actual mechanics of the setup, and any real world pros or cons it may have? It's ok to learn new things, and that comes from discussion, not dick swinging.

 

I saw this setup . I liked the looks & simplicity . I fabbed my version of it . I have been very pleased with the way it looks & works . I have put in the neighborhood of 40,000 miles on the car without any hint of a problem .

 

Very interesting thread. Keep it friendly and upbeat everyone.

 

I remember when the cry babies got my thread locked about my t-bucket having no front shocks.
Even with 2 different driving videos people couldn't see past their own opinion on how it couldn't possibly work.
This thread probably has the same people replying to it.

 

Gimpy beat me to it, it would shorten the life in the shackle bushings if the urethane ones were in place. That’s the only negative this non trained mind can see.

 

It would be very cool if someone could create an animation of this in action. Way above my skillset / pay grade!

See what I did there? Idea for YouTube content for someone. We've had cowl steering, why not this?

Chris

 

In other words, the lower shock bushing fastening detail needs to be designed as a hinge, rather than a rotationally compliant solid attachment?

 

Have not shown any appreciable wear of the lower shock bushings in 15+years & 40k miles , I believe this is a non issue .