My '71 510 Wagon - A long and slow project

[iceman510]

5 hours ago, MrQuiet said:

 

My understanding is a bit different. And my comments relate to the illustration below.

 

The distance A-B never changes, as these are fixed points on the chassis. And distance A-C is fixed, as this is the length of the control arm. The caster is adjusted by changing the length of the TC rod, distance B-C.

 

Once everything is torqued down, the distances within the complete suspension assembly never changes. The A-B, A-C and B-C are fixed distances. Hence the angle at C, does not change either, as the suspension goes through its motion. The entire suspension assembly rotates around an axis passing through points A and B (The green line).

 

If one look at the B interface alone, as the entire suspension assembly goes through the motion, the TC rod will experience a slight rotation, in relation to the TC rod anchor on the chassis. If the bushings at B are too stiff, and not being supple enough, I would think some unfortunate torque and/or bending forces are transferred to the chassis and the TC rod. With enough cycles the weakest link breaks. And from what I can gather, people experience that the TC rod breaks, and not the chassis anchor.

 

It would be a good practice to torque down all the bolts in this suspension assembly (At A, B & C) with the suspension at its nominal position. As it would be positioned when the car is complete, sitting on the ground. To ensure the bushings are as "stress neutral" as possible in the nominal position of the suspension.

 

 

Great explanation.  Only a couple things I would add.

 

Location A should definitely have the fasteners tightened at ride height, as that avoids the bushing being in a bind and potentially tearing.

 

Location C should not matter as it is a hard metal to metal joint and really should not move.

 

Location B I believe has special instructions as to tightening.  You do not just crank it down because some compliance there is needed.  The ball and socket joint kit shown above contains the U-shaped gauge that is used to set the proper tightening location.  I have one of those around here somewhere.

 

As the control arm AC sweeps through its arc (picture a bow), it would shorten/lengthen the dimension in a single plane between B and C a bit, requiring essentially a little bit of left/right movement at joint B, but much less than the up/down range of motion required there.  The aftermarket replacements use a rod end to allow both of those ranges of motion.

 

As fiveoneO mentioned above regarding bolt hole elongation at location C, I believe that is only for variation in mounting pitch. Those joints should not be able to move. 

[MrQuiet]

The inner bushing on the control arm, at A, is quite compliant. It has to be, to be able to adjust caster. So the arc of C is fully driven by the TC. So I would assume the spindle moves slightly fore-aft as the suspension goes through its motion. 

 

I assume the elongated hole at C on some aftermarket TCs is to accommodate a wide caster adjustment range. 

[fiveoneO]

You are correct that all the A-B-C lengths, that once set never change, but unfortunately, suspension doesn't travel in a perfect up and down motion. Suspension travels in an arc. Think about the bushing mounting point of the control arm as the center of a circle. As the control arm moves up and down that bushing stays in the same place and the outside of the control arm makes an arcing path. If you could move the control arm all they way around, the outside of the arm would make a circle. If the control arm is perfectly parallel to the ground and the suspension moves, the outside of the control arm moves along this circle path and actually gets shorter, that's why we get camber change when we lower our cars. Since the control arm and T/C rods are different lengths, they travel on different arcing paths therefor they change lengths at different rates.

[MrQuiet]

I might be dense. But once caster is set and everything is torqued down, the suspension module (arm + TC) is effectively is acting like solid plate type control arm (Blue). Rotating on an axis between A & B (green). Point C is following a circular arc on a plane that is perpendicular to the axis A-B.

 

[fiveoneO]

It does try to act like the blue, a solid control arm plate, but typically in that set up the bushing B is perfectly inline with the thick green line and only has to rotate. On our cars it has to rotate and move up and down, it puts more load on it than a more modern day control arm. Cusco makes really nice stuff and I don't think your T/C rod will fail, but there will be some load on the B bearing. 

[banzai510(hainz)]

thanks Rosso for the photo.

I got it.

Does the urathane ball get grease or just as it is?

How much are these?

[rosso]

The metal sockets are aluminum and the spherical pivot balls are white plastic. No grease necessary. The holes in the plastic pivots are tapered to allow for tc rod movement.

 

[datzenmike]

Hmmmm dirt/water is going to get in there. Really need a boot like a CV joint.

[iceman510]

I don't know about now, but my kit bought back in the 80's had boot covers over the front.  Rustbin's photo above suggests to me (if I am interpreting it correctly) that his boot may have been in the wrong location.  It should cover the ball and socket joint on the front end of the body pocket.

Edited July 29 by iceman510

[MrQuiet]

These pop'ed up on the local classifieds site. I picked them up.

 

[WAGON JON]

Build it! Has a great start. 

[MrQuiet]

2 hours ago, WAGON JON said:

Build it! Has a great start. 

I will! I just need to get this project race ready, before I get up to speed on the Goon >>> 944 spec racer & road car in one - Project thread - Rennlist - Porsche Discussion Forums