Why two ladders?

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BillyShope
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Why two ladders?

Post by BillyShope » Wed Jun 27, 2018 9:07 am

This is just something to consider: After Ford and Chevrolet built millions of cars with a single ladder bar (better known as a "torque tube drive"), why do the builders of ladder bar cars use two ladders? Yes, I'm aware that Ford used a Panhard and Chevrolet used leaf springs to prevent rotation in the "Z" axis. And, yes, I know that the torque tube attached to the rear of the transmission and not to the chassis. The principle...and question...remains. (I put an Olds in a '37 Ford in 1955.)
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Re: Why two ladders?

Post by Brian P » Wed Jun 27, 2018 5:54 pm

Probably because the designers don't actually understand suspension geometry, or they don't care, or in the specific application that they are using it in doesn't matter so much.

A ladder-bar on each side of an axle means the suspension will bind in one-wheel bump and basically act like an infinite-stiffness antiroll bar ... unless the discrepancy is taken up somewhere.

There are some kinda-similar situations to consider.

Twist-beam axles. Common in front-drive cars. The trailing arm on each side is basically the "ladder bar". The axle is intentionally flexible in torsion, which allows the left wheel to go over a bump without forcing the right wheel to go up together with it.

Semi-truck air suspension. The axle is essentially solid, but in this case the "trailing arms" or "ladder bars", if you wish, are more-or-less leaf springs. Left/right differences are taking up by bending the "trailing arms" and the bushings that attach them to the axle.

Ford "radius rod" solid-axle truck front suspension on the heavy-duty F450, F550, etc. Yeah it's turned around front to back from what you're describing but it has the same issue. Binding. I know they started using this design in the 1960s Bronco, and I know those used a really flexible bushing between the radius rods and the axle. But the radius rods nowadays look bolted pretty solidly to the axle. I don't know how they handle one-wheel bump.

The last couple generations of GM F-bodies used a single central ladder bar with a flexible mount at the chassis end, and a trailing arm to locate the axle on each side. The flexible mount on the ladder bar was necessary because the trailing arms swung in a different arc from the ladder bar, and also the trailing arms "steer" the axle a little bit and so the ladder bar can't be rigidly attached.

There was another car that used this general design ... the Chevette! (Along with all of the other rear-drive GM T-bodies.) In that case, the "torque tube" was really short, probably shorter than it ideally should have been, but then, it didn't have much power to contend with ...

Drag racers get away with stuff that you couldn't do if you want the car to turn corners decently.

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Re: Why two ladders?

Post by BillyShope » Thu Jun 28, 2018 6:10 am

All very interesting, Brian, but not to the point, which is that racers are needlessly complicating their cars. With a single ladder, offset to the right, equal rear tire loading on launch...and all the way down the strip...is easily accomplished.
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Re: Why two ladders?

Post by MadBill » Sat Jun 30, 2018 10:04 pm

I guess if the front wheels are off the ground, the L/R rear tire loading percentages would be the same as at rest, regardless of the axle location method? :-k
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Re: Why two ladders?

Post by Brian P » Sun Jul 01, 2018 9:12 am

If you have functional compliance in roll ... there will be some transient effects because of one side unloading and the other side loading up, thus rotating the entire bodyshell with respect to the axle. You can see that in some drag-race cars (the ones that have functional suspension) doing a hard launch.

I suppose using two ladders (thus removing all roll compliance by forcing the whole axle to go up and down together relative to the bodyshell) kindasorta halfarse removes this, and thus kindasorta works for drag racing, although the lack of functional roll compliance has its share of bad side effects that the drag racers evidently don't care about (or don't know about).

Certainly BillyShope is right about this. A single ladder offset to the right should allow hard and straight launches without making the body rotate on launch and evenly loading both rear tires, AND still allow a suspension design that allows one-wheel bump and has compliance in roll without binding.

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Re: Why two ladders?

Post by MadBill » Sun Jul 01, 2018 10:14 am

Among other effects, theoretically infinite rear suspension roll stiffness with huge floppy slicks can make it interesting when the car gets sideways...
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Re: Why two ladders?

Post by peejay » Tue Jul 03, 2018 3:35 am

Brian P wrote:
Wed Jun 27, 2018 5:54 pm
Ford "radius rod" solid-axle truck front suspension on the heavy-duty F450, F550, etc. Yeah it's turned around front to back from what you're describing but it has the same issue. Binding. I know they started using this design in the 1960s Bronco, and I know those used a really flexible bushing between the radius rods and the axle. But the radius rods nowadays look bolted pretty solidly to the axle. I don't know how they handle one-wheel bump.
They have large bushings on the axle. I think Ford gets away with it because they spring them so stuff that the suspension hardly moves.

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Re: Why two ladders?

Post by peejay » Tue Jul 03, 2018 3:40 am

I think the main reason for two ladders is simplicity. No need for a swaybar, no need for any more suspension links. Just two mounts on the chassis for the ladders, and you run a diagonal from a chassis mount on one side to the axle on the other side for lateral location, and you're done.

Given that every ladder bar setup I have seen used clevises and not rod ends, street drivability is not a concern.

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Re: Why two ladders?

Post by af2 » Fri Jul 13, 2018 9:25 pm

BillyShope wrote:
Thu Jun 28, 2018 6:10 am
All very interesting, Brian, but not to the point, which is that racers are needlessly complicating their cars. With a single ladder, offset to the right, equal rear tire loading on launch...and all the way down the strip...is easily accomplished.
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Or the fact the right will over come the left and steer the car. More if you are sideways...
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Re: Why two ladders?

Post by Brian P » Fri Jul 13, 2018 10:43 pm

Adequate toe control would require a trailing link on either side and some "give" in how the torque arm attaches to the axle and the chassis ... which is just like the 1982-on Camaro except with the torque arm offset to the side.

I know "in theory" a rigid or triangulated torque tube doesn't require any additional locating links, but the necessary triangulation would be heavier than just having a trailing link on each side, and slice diagonally through parts of the car currently occupied by other stuff.

For some reason I can't find a diagram of the stock setup, but this will do (same layout, just different parts, conveniently painted red)

Image

Obviously GM didn't offset the torque arm because they wanted back seat passengers someplace to put their feet.

That might not matter so much for a dedicated race car ...

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Re: Why two ladders?

Post by Nefario » Wed Aug 01, 2018 12:31 pm

BillyShope wrote:
Wed Jun 27, 2018 9:07 am
Why do the builders of ladder bar cars use two ladders?
I'll suggest that most builders have never heard of 6 degrees of freedom or understand what that means. You also need to be able to visual the 6 motions and how they are controlled. Or just use rubber bushings and let parts bend...which is fine in many applications.

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Re: Why two ladders?

Post by shiftbyear » Thu Aug 02, 2018 2:58 am

Is the nascar setup only good for the type of racing they do, or is their setup good for other types of racing. Sorry I'm suspension stupid.http://speedtalk.com/forum/download/fil ... w&id=19050
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nascar rollover.jpg

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Re: Why two ladders?

Post by Brian P » Thu Aug 02, 2018 2:07 pm

That suspension design is referred to as "truck arm" suspension because an early production application was the '63 (I think) - '72 Chevrolet half-ton pickup trucks.

There is a subtle but extremely important difference between that, and what is commonly referred to as "ladder-bar" suspension: The chassis-end pivot points are brought in closely to the center of the vehicle. Ideally, they would be brought to an actual single point in the center of the vehicle, but you can't quite do that because the drive shaft slices through the same spot.

Let's analyse the ideal case - with the two trailing arms brought forward to a single central pivot. With that arrangement, the entire axle and both trailing arms form a simple planar triangle that doesn't have to change its shape. It can pivot around that central pivot in both side view and front view and top view (the latter generally being restrained by a panhard rod or watts linkage). This gives sufficient "degrees of freedom" to the mechanism to allow left and right suspension movement both up and down together or moving separately in one-wheel bump or body roll, without having to bend anything. The entire axle does not have to twist to accommodate this - the whole thing pivots around the single central pivot as a unit.

Contrast that to the usual two-ladder arrangement with one on each side. Each trailing arm pivots around the chassis-end joint to accommodate suspension motion ... but the left and right arms have to rotate separately to account for roll or one-wheel bump. The only way that can be accommodated, is by bending something ... or binding up, acting like infinite roll stiffness.

Obviously the actual "truck arm" suspension does have separate pivots for each trailing arm which are separated a short distance apart for drive shaft clearance ... but this small separation is easily accommodated by deflection of bushings, or by the trailing arms themselves - which are quite long, and can deflect by bending a little bit.

Functionally, "truck arm" suspension can be made to work just fine, as NASCAR shows. The main problem with it, and why it isn't seen in production vehicles any more, is that it eats up a ton of valuable space underneath the vehicle. The original pickup trucks where this was used, had their fuel tanks inside the cab, which is obviously a non-starter today. Nowadays, the preferred location is between the frame rails, on either side of the drive shaft ... right where those truck arms want to be. The arms are also quite heavy because of their length.

Modern pickup trucks and SUVs that use beam-axle suspension with coil springs use a multi-link design with more but shorter links, avoiding the "truck arm" packaging headaches; look underneath a late model Ram 1500 for an example. The other choice is full independent multi-link with the diff fixed to the subframe and driving the wheels through CV-jointed shafts ... see Honda Ridgeline (which is the same underneath as a Honda Pilot). Or a bazillion other SUVs that use 4 wheel independent suspension.

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Re: Why two ladders?

Post by Brian P » Thu Aug 02, 2018 2:11 pm

Gotta go, but to understand why the "ladder on each side" arrangement is a problem, look up a diagram of a typical twist-beam-axle on any of a multitude of front wheel drive cars. There's a single trailing arm ("ladder") on each side and an axle between them ... but this axle is deliberately designed to be flexible in torsion. It twists, by design, in one-wheel bump or in roll. Now imagine what happens if you beefed that up so that the axle can't twist. If you do that, the whole thing can go up and down together, but the left and right sides can't move separately. A one-wheel bump on the right would lever the left wheel off the ground (and/or send a big jolt through the whole bodyshell). Body roll could not be accommodated - infinite roll stiffness - which causes major handling problems.

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Post by dwilliams » Fri Aug 03, 2018 6:38 pm

BillyShope wrote:
Wed Jun 27, 2018 9:07 am
why do the builders of ladder bar cars use two ladders?
For street or street/strip use, most (unibody) cars have nothing in particular to attach a single central bar to; if you add a suitably stout crossmember, you now have problems running the exhaust.

For a track only car, the main reasons are probably "don't know any better", "you can buy the bits already made", and "works good enough, as far as we know."

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