So you know how a parallel leaf can increase or decrease (depending on orientation) wheelbase under compression and deflection. Would a buggy spring with ladder bars do the same ? I know the rear end would articulate on an arc without the spring, theoretically shortening the wheelbase at full separation from the frame. But would the spring stop it from articulating ? The topic came up when a friend mentioned he was picking up his completed chassis from storage and was thinking about stopping at the driveshaft shop to get measured for one. He said he had straps to compress the suspension to simulate the weight of the body and I proposed the question if it was needed since he has ladder bars and buggy spring behind, effectively locking it in place.
My first thought is no it would not change wheelbase if it spring is mounted a t a true 90 degree angle to the rear end. Although I always measure with the chassis loaded
The arc of the ladder bar swing changes the wheelbase. The spring, in this case, resists that swing not just by needing to be compressed, but also twisted. On a light vehicle with a this setup, you are talking about fractions of an inch, as there is not much suspension travel. Keep in mind that the driveshaft is also moving at the same time as the ladder bars do, so the extent of the length change will be subsequently minimized.
If the ladder bar front mounts are on the same plane and inline with the front driveshaft uni, and with only a couple of inches of travel in the diff then the slip yoke movement would be negligible.
Parallel Leaf Springs do not alter the wheelbase unless the idiot installing them sets them up like trailer suspension. The springs are mounted at an angle so the arc movement cancels out the spring lengthening when compressed. Crawl under any leaf spring vehicle and you'll notice the rear eye is a lot higher than the front eye.
Have you ever seen the rear higher on a front mount straight axle vehicle ??? So yes, it CAN effect wheelbase, on my Henry J it was 3/4 inch difference when compressed.
Ive seen the wheelbase altered significantly on some of the stupid low stuff I’ve messed with. Friend of mine didn’t listen to me when he installed 5 inch lowering blocks. The drive shaft beat the thrust bearing in the engine enough to affect belt alignment. Short arms like on a x frame gm affect the wheel base without the body. honestly I don’t measure drive shaft length until I got the bulk of the weight and ride height figured out.
They do the opposite on front parallel leaf springs with the shackles at the front. BUT it is not so critical on the front end because any wheelbase variation causes the steering geometry to "Parallelogram itself" so the wheels always point straight ahead [as long as bump steer is sorted out] Whereas on the rear end the wheels are always perpendicular to the axle causing rear steer [or roll steer] I've done the opposite on my trailer. I de-arched all the leaf springs, then raised all the end spring hangers to change the spring angle, now the axles move up and down in a more vertical path. I now have a tandem trailer with rocker equalizer suspension, that tows like it has torsion axles.
The parallel leaf springs will have the rear end wanting to travel a certain path/arc during suspension cycling. The added ladder bars will want the rear end to follow a different arc. This will cause bind unless you can set it up so the springs and the ladder bars cause the same path of travel. Not easy to do, or maybe not even possible to do.
If there is enough arch in the spring [or it has lowering blocks , or rubber front spring eyes] you can simply attach the ladder bars to the rear end. There is enough compliance in the spring so it won't bind with approx 3" of compression travel . The trick is to line up the axle centerline, with the front spring eye center AND the ladder bar pivot center along a common "datum line" at ride height. The axle thrust tries to drive forward via the ladder bar pivot points into the frame [these compression loads will arch the front of the spring and/or compress the front spring eye] The only downside is you are limited to adjusting squat / antisquat , but you can adjust pinion angle via wedges and ladder bar heims
Does the "compliance in the spring" make the ride stiffer? If the compliance is needed to allow it to work it means the two are working against each other to some degree, no? Hence, a stiffer ride due to some bind in the setup? Edit: This question is relevant to parallel and transverse springs. I also need to ask you about a trailer setup. I will PM at some stage if that's ok.
I just re-read the original question and see now that you are asking about ladder bars with a transverse spring. I have sidetracked this with comments regarding parallel leaf springs....... Sorry. Post #3 ( @gimpyshotrods ) is a great explaination of what you are after.
When the suspension compresses with a ladder bar the pinion angle "rotates down on the nose" This increases some leaf spring tension in front of the spring pad, But relaxes some leaf spring tension in behind the spring pad. Soo in theory they cancel each other out ["in theory"] In reality the spring stiffness [actually spring load] increases due to the offset of the centrebolt. Front pivot placement of the ladder bar does have some effect due to arc movements. A 48" long ladder bar will arc 0.090" with 3 inches of travel in one direction The same 48" long ladder bar will only arc 0.020" with 1-1/2 inches of travel in one direction So if the rear suspension has 3" travel [1-1/2 inches of travel in compression and 1-1/2 inches of travel in droop] place the front pivot in a central position If the suspension is set up to extend when the tyres are "hit" you are better off raising the front pivot. If it squats then lower the front pivot. People misunderstand the purpose of ladder bars! We all get this fixation on Pinion Torque reaction Axle thrust is the greatest for due to torque multiplication [gear ratio] This forward thrust will cause compliance in the front of the spring [or front spring eye] only 0.020" is needed with a properly set up ladder bar. [I would not recommend poly bushes in this set up] Also the forward thrust is a "reaction" to axle torque / wheel radius / traction so there is incredible bending loads on the diff housing , which is why the ladder bars need to be mounted as far apart as possible at the axle. The reason leaf springs wrap is a combination of 2 forces ! The major force is compression loads from thrust, and the minor force is bending loads for pinion torque reaction. On vehicles like 3rd /4th gen Camaros with a central ladder bar [torque arm] all the thrust is controlled by the LCA's They alter the LCA angles to hit the tyres harder or softer [Camaro LCA's work on the same principle as Cal-Tracs]
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