Shackle Angle Question

I have an under-slung suspension that utilizes torsion bars, torsion arms, and adjustable spring perches run upside down. After getting it all put togeather it looks like the shackles may run close to vertical. It seems to me this could cause binding.

Does anyone have practical knowledge of if this is true?

If it is,what are opinions on removing the adjustable part of the spring perch, turning the spring perch perpindicular to the axle, and replacing it with a bolt that directly hooked to the shackles ?

I would think this may actually make it stronger as the moment associated with the cantelevered adjustable piece would be eliminated and it would put the spring perch bolt in double shear.

I will attach a picture of what I am talking about. The masking tape in the picture is holding the torsion bars off the ground while I can have the shackles drilled out. I understand that the axle may look pushed to the passenger side, but that can be adjusted some later.

Thanks
John

 

In your design you should take into consideration the relative arc travel of the axle under bump and drop condition of about 5 " from horizontal to have a good idea if there will be any bind. Remember the the axle pivots off the center line of the opposite tire and your torsion arms pivot off the center line of the torsion bar. These two arc must have a similar parallel swing to eliminate any bind. While the shackle will take up some misalignment, if the swings of the respective components are two dissimilar they will result in a bind regardless of the shackle angle.

To test this out you can just plot it actual size on the garage floor illustrating a front view position. Draw a central horizontal line to represent your axle measured from the center of tire to the center of the opposite tire. Then measure the height of your tire and divide by 2 to get the radius of the tire. Produce another horizontal line below the axle line at that measurement distance, this represents ground level. Drop a line down from the axle tire center line measure square to the ground level line. This will produce the point of rotation of the axle on the ground plane. To determine the arc travel of the axle you can now swing an arc thru the shackle axle bushing pivot center line using the center of the tire ground point as the radius distance.

Now to determine the swing of the torsion arm, find the vertical center of the axle and measure down towards the ground line the distance the horizontal centerline of the torsion bar is below the axle, draw a horizontal line at that distance and on that line measure from the vertical centerline of the axle over to the axial center of the torsion bar. This produces the torsion bar rotation point. Now to represent the rotation of the torsion arm swing an arc from the torsion bar center thru the distance of the center of heim end on the torsion arm . By comparing the difference in arcs of the axle movement and the torsion bar rotation you can determine if the shackle will rotate smoothly or at what travel distance it will lock into a bind. If the distance decreases slightly the shackle will pivot, if the distance increases the shackle will lock up.

 

Dick,

I have not had a chance to reply but have looked at your post several times, from work. Thank you very much.

 

Looking at your picture, is that a panard rod that we can see behind the axle? With your set up you will definitely need a panard rod or some sort of locating mechanism to locate the axle laterally. If you do something to locate the axle laterally then I would think that running the shackles vertical would be preferred, this would allow you the most swing in both directions. I hope that the rod ends on the ends of your torsion bar arms are not the $5 specials from Speedway but are real aircraft quality, teflon lined, $40 rod ends as you are putting the threaded section of the rod end in bending and it will not like it. At least with the real aircraft quality units they are made from good steel. What size are they and how much do you think the front of your car will weigh?

Rex

 

Yes that is a panhard bar you see behind the axle. The rod ends are chromoly and were supplied by Diversified Racing. They built the torsion arms and supplied the stops and bars. I would say the shanks are 1/2" diameter. Won't be much thread sticking out (actually none) so pretty much just shear stress on the rod end.

 

Soap stone is your friend or popsycle sticks. As Mr Spardo said and I agree the shackles will bind. I think that the very short panhard bar is going to cause problems. Before panicking how sure are you that the torsion arms are the correct length to give you the spring rate that you want? Long story coming to give some suggestions. If you block the frame up so there is no preload on the bars. Place a bathroom scale under the torsion arm and place a jack on the scale have a tape measure behind and at the shackle end of the arm. Jack up the jack to touch the arm. Now if you continue to jack up the arm one (1) inch the scale will give you a pretty good rating or spring rate.The reason for this exorcise is to see if the torsion arm needs to be shorter or longer which will affect your shackle angle. Second thought is eliminating the shackles completely. Yes I can hear the rumblings of you can not eliminate the shackles. Modern day sprint cars and midgets run the torsion arm directly on the axle, no direct connection. Your front bars are parall race cars are usually transverse. You can make a carrier for lack of a better word that the arms would rest on. Think of a rectangular tube with one side cut off to make a U channel, the channel would have a bolt 3/8 " or larger going across the U channel with a roller that the torsion arm would rest on, make the arm a little longer so it can not slip off or out of the carrier during axle movement.
If any of this makes sense to you have fun but build it safe. Check out my albums for pics .
Good Luck
DT

 

The torsion bars are the correct length. While I had Diversified Racing build the set-up, I researched it a lot before going to them.

I ended up using Dick's advise about the different archs of travel. I put togeather a Pro-engineer sketch and constrained everything so the shackle could rotate as the axle was rotated up and down (nothing could change length). It looks like I get roughly 4" of travel up. The shackle will be 90 degrees at static.