4 link?

i have a 1980 c10 and want to use the stock front leaf spring mounts and was thinking of wishbone. i keep hearing you must make that bottom bars parrallel to the ground at ride height, i dont see this working out since i dont really have a notched frame? and also the wishbone would need to be shorter then the bottom bars i heard something about pointing it down a bit compared to the bottom bar if its like that, if so how much of an angle? do i still try to make my instant centre near the grill?

the thought behind keeping bars parallel to the ground and each other is to keep pinion angle change minimal over a large range of motion.

The thought behind instant center is to provide anti squat properties under acceleration and suspension deflection.

Obviously one is in direct opposition to the other.

Figure out what your suspension is intended for. If you are bagging your truck and want a wide range of ride heights with bags mounted on bars for super lift, the parallel parallel setup will give you the least greif. If you are static dropping your truck with coilovers and want the absolute fastest quarter mile times then you should "pinch" the forward mounts together a bit and mount the lower bars at a slight angle.

I'm not saying you should ignore one or the other, or that one way is better than the other (although taking into account the anti squat properties of a rear suspension on a truck you are building is very thoughtful and probably the "best" solution), just saying REMEMBER THE APPLICATION. The most important thing about installing a 4 link is to make sure it doesnt bind up anywhere in travel or cram the driveshaft into the trans, a lot of guys forget even that.

do the back mounts on the rearend need in a straight line with the centre of the reaerend and perpendicular to the ground at ride height or could i keep them in a straight line but have the top further back and the bottom further forward allowing me to keep the same length bars parrellel to eachother but only parrellal to the ground when im almost laid right out?

if i was to do an actual 4 link instead i would be able to make the true top bar lengths the same as the bottom bars but only at 25* would this work?

I'm not sure the point has been relayed with what I had in mind.

There is nothing that states that the upper and lower bars need to be even close to the same length. On a system that does not have room for a long bar, more care needs to be taken to keep the pinion angle "in check." Nor is there any rule that says that the lower bars need to be parallel to the ground at ride height. The uppers bars can be parallel and the lower would follow the set instant center, or maybe neither bar is parallel and they both point towards the instant center with the lowers pointing slightly up and the uppers pointing slightly down. The big issue is roll-steer. If you do not take it into consideration it can cause issues. If you set up the bars that are (relatively) parallel to the ground when laid out, when it is at ride height the vehicle will start to have a small amount of roll-over steer and as the suspension is lifted higher the roll-over steer get progressively worse. This situation also causes the rear end to move forward in the frame.

The idea of a forward facing instant center has very little to do with pinion bind (sorry Russ.) The drive shaft can be made to follow, with minimal trouble, just about any suspension design. What the IC out front does is properly transfer energy to the chassis. It is very hard to explain why, but it works. If you have a parallel 4-link there is no dynamic energy transfer, it will create anti-squat and roll steer, but no actual energy transfer. With a rear facing IC the energy is transferred in reverse, so when accelerating the suspension tries to lift the rear-end off of the ground. This is not any good for traction, it will still anti-squat (sort of) and roll steer but no quality dynamic function.

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exactly max, I get frustrated when people say "it worked for me" its like the battle cry of the half-assed.

and I didnt say the forward facing instant center caused pinion bind, at least intentionally, only that with a very large lift range it is harder to engineer the proper pinion angle throughout and that acceleration is likely not the primary focus of the suspension anyway. Like building equal length upper and lower front control arms to minimize camber change, obviously the trade off is the huge turning radius, but when guys are trying to stuff the hugemongest (my new word) wheel under the smallest truck, it is more important to keep the tire from tilting even a small amount which would make the overall height any taller and hit the hood.

Thats a poor example, because I dont recommend equal length upper and lower control arms on a front suspension, at least in the sense of eliminating camber to fit tires, but I will usually be found to recommend parallel bars in the rear 4 link to minimize pinion change. To keep driveshaft movement fore/aft minimized AND keep a reasonable pinion angle in 12 inches of lift/drop AND keep IC up under the oil pan, would require more books and calculations than most guys would have the patience for. So keep preaching it Max, you have a great reputation and do some amazing work that makes most experienced builders look like they are banging rocks together, your advice WILL stick.

I know exactly what you are saying about the parallel a-arm/no camber suspensions that are being built, they're my favorite.

It is hard to explain the importance of geometry when many people don't even realize that it makes a difference.

I am quite surprised that I haven't been chased off the site yet and even more surprised that I am encouraged to answer technical questions. I will continue helping as much as I can.

you two guys are the reasons why I sit back and just listen, ultimate wealth of knowledge, max I sent you a pm about a question on my front setup

Max & Russ

What do you guys mean? Parallel arm setups are great for ruining your tires and bad handling characteristics.