Chapman Strut geometry Questions.

Ok.. Long time listener, first time caller.

The Project is yet another mid engine conversion involving a 1985 Shelby Charger and the front clip of a 1988 Daytona.

The question I have is this, the front suspension of the daytona (and of course the SC) have a decent amount of Anti dive geometry dialed in, and the basics of that make perfect sense to me. Now that this suspension system is moved to the rear this same geometry becomes what looks to me like an excessive amount of "pro squat" geometry, something I am pretty certain I want to avoid.

It would seem that the strut needs to move forward into what would be a dangerous wobbly pro caster position if it were still in the front, and the Lower control arm needs rotated a bit to end up with a "anti-squat" design.. I kinda fear moving things this far because clearly I don't have enough knowledge to understand how that might effect a number of other variables of the suspension design..

It seems that since it is not as stylish to run this type of rear suspension the information is more limited. So any answers would be great, or steer me to the right place learn. I am not afraid of reading or libraries. I think if I felt really had a better grasp on the side view instant center and how all that works with the McPherson/Chapman strut rear suspension I would be more comfortable, but then again, maybe that wouldn't be a good thing.

Am I just worrying about something that isn't that big of a deal? Over thinking this maybe? Is this something the Shogun guys and others just don't worry about? I can't figure out if I am just adding more crazy to a crazy project

I am wondering how you determined that the suspension has anti-dive. anti dive is determine by the angle of the mounting of the control arm. for and aft. The rear mount would be higher than the front. In other words if you drew a line throught the control arm mounting points it would slope upward toward the rear.

Yes that is correct the rear mount is higher than the front.

I apologize for not have exact numbers.. and kinda exposing the budget I have to work with (car is on the lift and the rest is my redneck chassis leveling and construction system.

But that red post in the background is perfectly plumb. You can see the rod run through the K frame mounting points to get a general idea of the angle of the LCA. and it should be noted that the subframe is currently tacked in sitting just VERY slightly tilted up in the front.

Alright. After running a tape measure and an checking the angles of everything I could measure under a Boxster and a 88 Fiero.... I think I figured it out.

It didn't make sense before simply because the geometry as it was engineered in the FWD setup is so far off what at least I think I need to use this same stuff in a RWD setup.

Basically if I am understanding everything correctly if I left it the way it is I would have figuratively an 86 Fiero when I am looking for a 88.

Sometime down the road I may add the forward strut rod like the MR2, 88 fiero, and Boxster............. but for now I figure the least I can do is get things pointed in the right direction.

Right now the angle of the LCA slopes towards the ground 3.2 ish degrees... and what I am pretty sure what will at least get me in the ballpark is about 2 degrees the other direction. (combined with around a degree or maybe just less of 'caster') I am not really looking for a lot of anti squat.. I just don't want "pro squat"

So now the fun part continues on... More drilling, welding and grinding..

Then making sure I haven't totally jacked the camber curve.

All of this stuff was helpful to me late REALLY last night.
http://lek.holy.se/mark_ortiz_2000-2007.pdf

http://www.eviltwinmotorsports.com/wp/wp-content/uploads/2010/03/Chassis-Newsletter-2007.4.pdf

http://www.fiero.nl/forum/Forum3/HTML/000116-9.html

I'm sorry I don't have anything more helpful to add with respect to getting to the result of what seems like a reasonable set of assumptions, but I recall reading in Neil Roberts' Think Fast that a little pro squat can actually help drive off corners by counteracting the effects of rear sway bar stiffness' tendency to pick up the inside rear. Apologies for not providing a snippet, but I don't have to go grab the book right now, and my recollection of the principle is too fuzzy to be worth spelling out...

Maybe worth looking into, either for best performance, or just as a rationalization for leaving it all as-is

You could always use a spacer to level the control arm.

Thanks for the input and letting me think outloud... I think I have a plan now.. If I decide I don't like the changes.. I can always spacer the front of the K frame back down....

I am not just dropping the back simply because I like the current ride height, along with the subframe placement... and want to keep the rear roll center as high as possible.. Moving the front like this will allow me to do that without moving the engine and everything up to maintain roll center height and improving the roll couple over what I would have doing it the other way... I just need to double check and triple check to make sure I am not creating any axle clearance issues...

Theoretically.. It looks like I am on the right path to as little sway bar as possible in the rear.. But then again.. I still am not totally sure I know what the heck I am talking about..

Anyway... I will post some pictures of the progress.. so PLEASE let me know if I am making any mistakes...

Now.. Off to the shop I go..

ronholm wrote: I am not just dropping the back simply because I like the current ride height, along with the subframe placement... and want to keep the rear roll center as high as possible.

Speaking as someone who's never done this sort of thing, only done a bunch of reading: Why do you want to keep the rear roll center as high as possible? I'm under the impression that close to ground level is preferable, at least in part to avoid unnecessary scrub and jacking effects, before getting into the pros/cons of whether you want to try to get a little jacking to provide dynamic resistance to roll (at the cost of having roll stiffness change as the car moves around).

Again: Grain of salt, no practical experience, if you're going to ignore anybody, I'd be near the top of the list. I'm just going off of what I think I've learned from reading Carroll Smith and company...

You don't get to be the first to be ignored.. that should be me..

With MacPherson /Chapman struts running close to they way they were setup in the car the roll center is already going to be low... I have no intentions of tying to jack it way up... Just put it very slightly above where it used to be in the daytona for the rear and just below where it used to live in the front..

Long story short I could BS my way pretty good through an explanation of why I think the rear should be higher(but not to high) But the truth on this end is that it seems to be a very common starting point for just about everyone.. Only a very few production cars have the front center higher or even equal to the rear.. and near as I can tell this remains pretty much true once they hit the track.. Not to high of course.. But no silly point the control arm for the sky nonsense either.. In my strut suspension this is also a big part of the camber curve...

I am just basically trying to avoid messing up the stock geometry as much as possible while lowering the whole car... (easy enough thing to do in the rear right now) and really trying to change as little as needed to really make things that used to work alright (that might be up for debate on this one) in the front, at the very least acceptable as a rear suspension setup...

Also instead of extending ball joints.. or anything like that.. Once I really start dialing the car in I have the option to pick from 2 different knuckles that basically bolt right in.. one is the 88 daytona... and I have the later minivan knuckle... In 91 Chrysler decided to raise the roll center via the knuckle... and even added some caster through a control arm redesign... So If I get to carried away with any of my "anti's" they can be dialed back...

This is more than I know about it..

http://www.auto-ware.com/ortiz/ChassisNewsletter--May2007.htm

Oh... and the K frame mounts got popped off tonight.. and moved to the point where the rear control arm pivots are now level (well just above) that is where I run out of feasible room with the axle.. Should I decide I need more anti squat it is going to have to come from a spacer under the rear of the k frame...

Hopefully I didn't mess things up to bad..

Since you are re-purposing a front suspension as a rear suspension, what is your approach to the steering link? The location of the inner pivot can be very critical to minimizing bump steer, toe changes, etc. If you exactly mimic the relative position of the steering rack ends, then you will at least have duplicated the factory geometry. Changes to the location of this pivot point will then either: 1) Allow you to fine tune and dial-in the behavior to extract the best possible handling, while exhibiting flawless road manners and driveability. 2) Condemn you to an endless and thankless cycle of tears and frustration while you chase the elusive 'handle'.

I would invest $100 in some suspension modeling software, and make as many of your mistakes as possible ahead of time in cyberland before committing to metal.

Carter

ronholm wrote: The Project is yet another mid engine conversion involving a 1985 Shelby Charger and the front clip of a 1988 Daytona.

Just so you know, they've already invented the Fiero.

Shawn

"They have already invented the fiero"

I know.. but no thanks.. Heck.. I had a 1992 MR2 turbo sitting at my place for the longest time.. But they just are not a Shelby Charger.. And I have the perfect powerplant..

Which 100 dollar suspension program do you recommend?

And bumpsteer?? Heck... I am still working out where to mount the seat for the rear driver...

Just kidding... Yes.... My tie rod mounts will be thought out carefully. For now the steering rack is just tacked solid... (that will be fixed before the car moves under its own power) Factory geometry in this case is pretty terrible though.. Loads of bumpsteer... But when I get to that point I plan on rigging it up so that I can adjust bumpsteer until I find that happy place. After taking measurements from a boxster and a MR2 the initial goal will be a very slight amount of toe in under droop, but not much (if any) at all, and only really thinking about a tiny bit simply to counter the little bit of camber loss I am going to have to deal with... I am really trying to avoid Trailbraking/throttle lift "snap oversteer" issues that are kinda a problem for shorter wheelbase MR cars..... I am trying my level best to put myself in a place where I can work up to that point instead of trying to work away from it...

Here is what I have done so far...

Started here...

And ended up here... This is where making room for the axles will start to be a problem.. So I stopped... (the back of the K frame can come down if need be)

How did this ever turn out?

i went sailing all summer work should progress again soon.

ronholm wrote: Thanks for the input and letting me think outloud... I think I have a plan now.. If I decide I don't like the changes.. I can always spacer the front of the K frame back down.... I am not just dropping the back simply because I like the current ride height, along with the subframe placement... and want to keep the rear roll center as high as possible.. Moving the front like this will allow me to do that without moving the engine and everything up to maintain roll center height and improving the roll couple over what I would have doing it the other way... I just need to double check and triple check to make sure I am not creating any axle clearance issues... Theoretically.. It looks like I am on the right path to as little sway bar as possible in the rear.. But then again.. I still am not totally sure I know what the heck I am talking about.. Anyway... I will post some pictures of the progress.. so PLEASE let me know if I am making any mistakes... Now.. Off to the shop I go..

Why do you want a high roll center in the back? Sounds like a really good recipe for a ton of oversteer.

There's a really good article over on mr2oc.com on setting up a rear engine chassis. Roll centers are pretty key.

The Chapman strut is a McPherson strut that uses a fixed length driveshaft as the lateral suspension link. AFAIK, no current production car uses it. The effective pivot centers of the lateral link are thus the U-Joints, meaning the outer pivot is always located at height of the hub center, and the link is effectively horizontal. This fixes the roll center height at hub center height. These factors all make achieving effective suspension geometry an issue.

In your case, since you have McPherson struts, none of the above applies.