freetors said:
ChasH said:
Fitzauto said:
In reply to ChasH :
Has a tendency to bind up. Options are either equal length upper arms or 3 link, which I’ve been told works a little bit better when designed right.
Yup, the 4 links need to be equal length if the ends are "Heim" joints. You can help that by using urethane bushings in an upper link.
This is just wrongity wrong.
Tell Ford that - when they engineered the "perfect" solid axle rally suspension ("perfect" in that everyone else copied it, because it worked well) they used Heims (err, rose joints) at the axle end and rubber bushings at the chassis end, even though it was an equal length parallel bar setup. Even if everything is perfect, the axle will distort and the tub will distort and you need to be able to accomodate that.
In reply to Knurled. :
Shouldn’t be too bad. I can cut into the floor if needed.
In reply to Knurled. :
Can you post a picture of said suspension setup?
I don't know why everyone that talks about four links are so focused on having everything being "equal length" and "parallel". That's just so square. And limiting. And about the only special case where actual bind comes into play. Four links need some triangulation!
Look at purpose built rock crawlers and off roaders. They will typically use fully triangulated four links with massive rod ends, massive links, massively overbuilt frames, and massively overbuilt axles, yet their axles will move totally uninhibited in bump, droop, and roll.
In reply to freetors :
If you do a Google Image Search for "Escort 4 link" you will probably find many many examples.
If you do a Google search for "Group 4 Escort rally preparation manual" you will probably find links to the Boreham Bible where they describe, in detail, what to do and how to do it.
The idea was to eliminate the axle tramp and rol lsteer characteristics of the leaf sprung Escort. It turned out to work so well that other manufacturers copied it, as did privateers, like this well turned out stage rally BMW E30:
(which has a 3 liter evolution of the Volvo "Redblock" engine...)
Personally, I like the absurd thrust-grip you can get from a 3 link that contains a hefty dose of anti-squat, but then my goals are more geared toward getting max grip under acceleration from 25mph to 50mph on dirt, not predictable handling under varied conditions at 30mph to 100mph+ on all kinds of surfaces including tarmac.
In reply to Knurled. :
Funnily enough I kind of see where you're coming from now. That is the exact case of a four link configuration that would bind in roll without some form of flexible member. There's no convergent instant center between any of the links (in top view) and therefore the axle or the frame would have to deform to accommodate if all the joints were rod ends. Move a pair of those links together at one end and voila, no binding, and no need for a supplemental lateral constraint!
In reply to Knurled. :
Tell me more about this thrust grip and how I can have it.
Fitzauto said:
In reply to Knurled. :
Tell me more about this thrust grip and how I can have it.
If you read the Billy Shope website you will Have your Answer, And An Immence amout of Useful Knowledge The pictures are hand drawn cartoon like and easy to Comprenend, the Methed's for Testing are SIMPLE.
A double-triangulated 4-link is bind-free if you use some kind of spherical bearing at all ends. The less free the links are to move and rotate in any direction, the more bind you'll have, with cylindrical urethane bushings at all ends being the worst-case scenario. Land Rover uses a torus-and-hourglass bushing setup as a decent compromise.
In reply to GameboyRMH :
I think you are confusing bushing bind with geometrical bind. If a suspension can move freely with rod ends it will work just fine with bushings.
Geometrical bind is the situation you get with some suspensions where the suspension cannot articulate unless there are compliant bushings. Old RX-7s are like this, if you articulate the suspension with one upper link disconnected you'd see that it would need to change in length +- 1.5 to 2 inches if it had rod ends! (A good chunk of the reason why they are known to tear the upper link mounts off the body. And why they are prone to get snap oversteer if you lower them, as the bind is worse at lower ride heights. They also snap if you remove the rear sway bar, since a suspension that cannot move can also do no wrong)
^The trouble is that cylindrical bushings only allow free movement on one axis - call it up and down (with very limited left/right and rotation movement), and the links in a double-triangulated 4-link setup need to be able to move freely up/down and left/right plus they need to be able to rotate, this is where the bushing can introduce bind.
Think of it as the difference between a door hinge (more extreme but basically the same problem as a cylindrical bushing) and a person's hip socket. If your hip socket were replaced with a door hinge swinging forward and back you wouldn't be able to walk very well...this is the same problem that would affect any link in a double-triangulated 4-link setup.
In reply to GameboyRMH :
Oh, I hear what you are saying. This is why the bushes in my rear suspension are in the forward end of the lower links, and the Watts linkage. These are the only ones that move in a nearly single-axis motion. The upper link is high misalignment rod ends and the lower links have UMI joints in the rear.
This is probably also why Ford specified them only in the forward ends of the four-link. With slots that narrow, it ain't articulating much.
