The point that a pro-lift/pro-dive front not only "softens" the springs for compliance, but also reduces the front roll resistance was new to me. I had not connected those dots. Thanks for posting that, Golgo.

 

Pro-lift/dive shouldn't have any effect on spring rate or effective stiffness. That's purely an effect of mass and rate. Loads change do to acceleration but hit a bump and the stiffness is the same.

 

I just wrote a long reply and then deleted it because I realized that I was just babbling BWBS [i.e., big-words bull-pucky] in an attempt to (a) justify what I wrote this morning and (b) not end up arguing against the piece quoted by Golgo.

I'm back to thinking that the piece quoted by Golgo is actually close to nonsense. Yes, anti-dive etc does shift some of the work from the springs to the control arms, which has lots of consequences, but it wouldn't do anything to vertical movement due to things other than braking and, therefore, wouldn't do anything to roll resistance. (I'm assuming linear springs or, at least, springs that are linear in the ranges of force that we're talking about.)

So why do so many websites claim that adding pro-dive effectively lowers the spring-rate?

 

I'll admit, my source wasn't an authority on the subject. I just tried to find something that broke it down into layman's terms (so I could understand it ) and for everyone else that might read this thread two years from now...

Really, I just need to know what moving the LCA up or down does to the car so I can decide where the hell to put my spacer.

 

I'd like to think we all know the actual spring rate isn't changed, but expressing it that way helped me understand how these parts work.
I'd imagine "effective spring rate" is determined my a lot of things such as friction, leverage, gravity, etc. Maybe a different term could be used that is more accurate such as "effectiveness of spring rate"? Either way, we get it.
I'd assume this all has the effect on the car that running lighter front springs would have.
I put my spacer on top to get more compliance up front. We'll see if understeer is lessened.

 

Yeah, I'm sorry if that appeared to be a criticism of you, as you clearly didn't endorse it.

I understand why changing the angle of the front lower LCA changes the pitching of the car during braking and power (i.e., standard anti-dive stuff) and I understand why a pro-dive geometry is used on rough surfaces (e.g., rally on gravel), but I'm completely lost on the claims about "effective spring rate" and roll resistance. Thus, I don't understand why folks who run on smooth surfaces would do this.

I also don't understand if (and, also, why) angling the strut, itself, across the top of the front axle can provide a form of anti-dive via torque on the hub, but I went on to list all the things that I don't understand I'd soon hit the character limit for posts.

 

Yep, we can assume that we agree that the spring isn't changed. It has to be something else, such as a change in motion ratio.

Not to put you on the spot, but what kind of compliance are you referring to? When I say that pro-dive adds compliance, I am referring to a reduction in the amount of longitudinal impact (and force) that is transmitted to the chassis when a longitudinal impact (or force) is applied to a wheel. In simpler terms: a reduction in the wham! that occurs when you hit a pot-hole. Is that what you mean?

 

No worries. I'm still learning the engineering behind it, but if it results in less understeer I'm in. And If it's the less "wham" that keeps my contact patch where I want it then I'm a happy camper.
I just replaced all front bushings, installed stiffer springs (now 8k front, 9k rear), and added the PSRS. I'm hoping to like the result.

 

This^^

 

Totally concur with the bushing comment above.

With the offset PSRS you decrease the need for grinding and hammering if you put the bushing on top.

Most people would likely prefer the spacer on top as it has some advantages for handling in that it increases traction and decreases understeer.

But you do loose some roll center height when you put the spacer on top which is less important if you put an RCK in at the same time. And there will be a bit more pitch in the car with the same front springs.

Here are some comments on the issue from this website:http://www.fsae.com/forums/showthrea...nti-dive-setup

The pro's of anti-dive are:

1) that you will have less pitch under braking which in many cases will help you to run a weaker front spring. Typically rear engined cars would need to get the best front end grip a soft front end spring which would hurt brake pitching. Anti-dive certainly helps to resolve that compromise.
2) Anti-dive helps to control the roll-center position better. Typically on the front the car would dive, lowering the roll center significantly causing the vehicle balance to become more "oversteer" - and that on top on the effect of load transfer. Same thing rear lift ... increasing anti-dive on the front can solely due to this effect improve corner entry stability.
3) If your car suffers from pitch sensitive aerodynamics a certain percentage anti-dive can help to keep your aerodynamics under control. Nowadays some fancy hydraulic push-rods try to do the same thing....
4) An anti-dive geometry will increase caster angle in jounce motion, this will help braking stability (causing extra understeer in braking) and will also in cornering increase the caster angle on the outside wheel causing better camber gains with steering angle towards less understeer.
5) Since anti-dive is actually describing the "link load transfer" the response on the chassis is immediate with as soon as the "acceleration" applies. A damper reacts with "velocity" and is thus an "integrational loop" lagging being the acceleration and a spring is lagging another "integrational loop" behind being driven by displacement. So in short Anti-Dive is about 3 times quicker than spring reaction ......

Now the con's of anti-dive:

a) Typically anti-dive tends to make the car less dive, which said differently increases the wheelrate. A car with 100% anti-dive will not move a mm under braking which is "technically" the same as an infinitely rigid suspension. We know that these kind of "rigid" suspension setups are bad for grip. So if you are running your car on a pool billiard table no problem ... elsewise reconsider....
b) as a bonus to a) anti-dive makes the contact patch move forward in impact. This makes the suspensions typically more harsh and as said under a) does not help grip under braking.
c) Anti-Dive being a link load transfer does unfortunately "interpret" the longitudinal component of the front tire lateral forces (which are perpendicular to the wheel but not to the car/suspension links) as a braking force and act thus in a similar way as the roll center causing jacking forces.

Also a good Whiteline paper on the issue with regard to Subarus: http://www.whiteline.com.au/articles...WL%20ALK_b.pdf

BTW, Perrin may be discontinuing Mitsubishi parts. The offset PSRS is already no longer on their website.