Roll Center Correction on McPherson Struts

[ChadH]

This is more of an open-ended thinking-out-loud kind of question - or an opportunity for open discussion.

In picking through some suspension geometry web sites, it looks like lowering the front of a McPherson Strut car may not be complete win-win for handling. From what I understand, substantial lowering will lower the center of gravity, but has two drawbacks.

1) The instantaneous roll center will also be lowered, even more than the CG. This may create a condition where body roll is actually worsened.

2) If the static ride height control arm angle become flat, or is angled upward (from the center pivot), camber gain when the car leans is reduced, or would even lead to negative camber gain.

Here's a pretty brief example discussion of the issue:

http://www.motoiq.com/MagazineArticles/ ... r-Car.aspx

On my 1302 autocross build, I plan on substantial lowering - limited to the point where there's no mechanical bottoming while cornering, or fender clearance issues. I'm not sure how much yet, since I haven't progressed that far.

So, to add another element of complexity to my build, it sounds like I need to think of ways to correct this. It looks like there's two potential solutions:

1) Rise the inner control arm pivot points.
2) Lower the outside control arm pivot point (perhaps spacers between the control arm and ball joint.)

So, has anyone had any experience with this? I'm not feeling too enthused about DIY hackery on highly stressed front suspension parts, but if there's an off-the shelf solution, I'd be interested.

..or, maybe I'd actually be better off not lowereing the car as much as I had planned.

[ChadH]

So here's where I'm going with this. I found this from our very own STF - extended ball joints for a 1303. This is a similar method used for roll center correction in other cars too.

viewtopic.php?t=141698





Now - I don't want to swap over from my 3-bolt setup to the 1303 2-bolt, since I already have too much invested in goodies. However, it would be possible to make a triangular spacer between the 1302 ball joint and the bottom of the spindle to accomplish the same thing. I'm at work now, but when I get home, I'll see if I can sketch something up that makes more sense.

[H2OSB]

Obviously if you read through the short thread you referenced above, you saw I posted I want a set of those longer ball joints. I think you're on the right track with a triangular spacer.

It's well known lowering a car below the roll center will cause adverse effects on the handling, however, I can tell you a moderate amount of lowering on a Super Beetle makes it stick like glue. I would recommend no more than 3-ish inches of lowering for the early suspension and 2.5-ish inches of lowering for the later suspension. Try that first, I think you'll be happy.

H2OSB

[ChadH]

I know maybe I'm diving too deep into what's practical - but then again, here's the other XP car in our region:



So yah...I'm going to need every bit of help I can get. Being that his car is broken 2 out of every 3 races, my best bet is going to be finishing every race and getting him in yearly points.

Thinking more about the spacers. I have a neighbor with a water jet. I'm not sure how thick it can cut, but it might be easy to have him make several sets of 1/2" spacers, and I could stack them and use different stack heights to fine tune.

[Jadewombat]

You're on the right track, no pun intended, to consider all of these factors. I wouldn't lower it more than 2" in the front for a starting point. I don't have a picture of my old 1303, but that is the best layman's way to measure what is too much of a good thing--if the front lower control arms are absolutely flat or negative, then you've gone too far. Mine had a hair of positive left in them and the front was lowered just under 2", meaning if I wanted to go more I would have had to do what you're considering.

I can't explain it that well, but I think of it in terms of the flattest point would be neutral, when the suspension goes negative the ability for the springs and arms is suddenly fighting a much bigger load. I know, I know the mass of the car wouldn't change, etc. but it is true that suspension is designed to work in a very specific range to make the car react optimally to different g-forces. I guess I'm thinking out loud also.

I don't know how the Austrian guys did it with their beetles, kinda hard to tell from the picture but the lower control arms still look slightly positive to me and the car sits really low meaning the control arms were probably modified OR the bulkhead up under the car. Wouldn't hurt to ask those guys though.

http://www.aichlseder.info/bilder/proje ... 303_20.jpg

[ChadH]

Hmm...It doesn't look to me like there's anything too special going on up front. I think a lot of their "low" is from 13" wheels. Unless maybe they have lowered spindles. I wish I could read German, and they had more pictures on their site. (pictures of that engine and transmission make me want to cry.)

I guess my spacer idea is something I can keep in my back pocket for later, it certainly wouldn't hold up my build. I'm just getting sick of bodywork, so I start trolling for ideas for when the car is running.

[H2OSB]

Well, after reading a couple of posts on this thread last night, I took a flashlight out and looked under the front end of both my car and my son's. Both cars have 2.5 inch drops on the front. My son's car is an early 73 but I converted it to the later suspension and my car is a 74. My car, at current height, has the control arms pointing slightly down below horizontal. On my son's car I had no options for the inner control arm bolt. The only bolt hole is in the same place as the the lower hole on the later suspension. His control arms are sitting perfectly level.

I may have to rethink the inner mount for his car...mine too, but I think his may be more imparitiv. I think I could benefit from the longer ball joints, but I don't think they would be enough for his car. I will definitely keeps my eyes on it.

H2OSB

[H2OSB]

Well, after reading a couple of posts on this thread last night, I took a flashlight out and looked under the front end of both my car and my son's. Both cars have 2.5 inch drops on the front. My son's car is an early 73 but I converted it to the later suspension and my car is a 74. My car, at current height, has the control arms pointing slightly down below horizontal. On my son's car I had no options for the inner control arm bolt. The only bolt hole is in the same place as the the lower hole on the later suspension. His control arms are sitting perfectly level.

I may have to rethink the inner mount for his car...mine too, but I think his may be more imparitiv. I think I could benefit from the longer ball joints, but I don't think they would be enough for his car. I will definitely keeps my eyes on it.

H2OSB

[ChadH]

Ha, it looks like this disease I have of overthinking things is contagious.

It would be nice to have detailed CAD drawings of the front suspension, to analyze the geometric changes from lowering and suspension movement. Maybe if I find the time, I'll crawl around the car with my trusty tape measure and calipers, and try to make a rudimentary (2D) drawing (that would be the hard part). After that, it would be pretty easy to trace out roll center, various arcs and changes to camber, etc. as the suspension moves.

[Jadewombat]

OK, not a real strong argument, but I was behind one of these today and couldn't help but think how slightly positive the rear a-arms are and how small the suspension travel must be. I'm sure there are electronically controlled sway bars and magnetic struts, but still...

http://st.motortrend.ca/uploads/sites/4 ... ar-end.jpg

[Piledriver]

Are those struts or a dual wishbone or multi-link?
The rules/limitations change.

Hell, I'm still considering adapting a (probably shortened) 928 rear setup to my 914.
Looks easier than adapting a Boxter rear setup, I want no non-reversible mods, and the shocks are more or less in the right place.(for a 914)

I suspect you are overthinking it, dropped supers can still be on rails, althogh theres a great deal to be said for the 13" wheel trick, including cheap used racing slicks. Hand Of God brakes are not required for AX, and the itty-bitty tires amplify what brakes you have. A good, simple set of drums are also lighter than almost any disc setup.

[ChadH]

I need to buy this book again:



It was my "bathroom reading" for years, but I don't know what happened too it. It goes into good detail on roll center calculation for various suspension types, along effects of camber, caster, ackerman, alignment etc. etc.. It has enough detail to understand the theory, without being an all-out engineering reference.