quote Originally posted by Yarmouth Fiero: Can anyone offer a layman's interpretation for the "suspension challenged" ie: me

Well, your static roll center is located at the point where the two lines that come up from the center of each tire patch intersect each other. At ride height (with both L&R suspensions level), those lines intersect at the chassis centerline at 9" below the center of gravity.

The next things you want to look at are camber change in roll, and roll center migration in roll. You might be able to do it longhand, but here's where a software program like Lotus Suspension Analyzer comes in handy. Try PM'ing Zac88GT to see if he'll lend a hand... he's helped me and fieroguru in the past.

Your configuration is going to produce some roll center migration, simply because it's unavoidable with a strut design. Whether it's going to be worse than the stock car or not, only an analysis can tell, though I'd say you'll find your roll center migration will be better than the stock Fiero's simply because at ride height, your lower control arms have a long way to go before they ever come close to becoming perpendicular to the strut. To get an appreciation for why it's an issue with a strut design, I'll paraphrase a discussion I had on page 13 of my build thread:

Bear in mind that the roll center location depends on the intersection of two lines, one perpendicular to the strut and the other through the lower control arm axis. Since in a strut-type suspension the strut angle doesn't move appreciably throughout the range of suspension travel, this leaves the change in angle of the control arm as the major variable responsible for the change in roll center location. In double wishbone style suspensions the rigid strut axis is replaced by moveable upper control arm which changes in angle as well as the lower control arm so it can be designed to hold the roll center location relatively constant. On the Chapman strut suspension, as the angle of the lower control arm approaches perpendicularity to the strut in jounce, the instantaneous center for that side of the suspension dives away towards infinity, taking the roll center on a nose dive and sending it out laterally as well. Do this for both sides and where the lines from the tire contact patches to their respective instantaneous centers intersect with each other, that's where the rear roll center will be located.

[This message has been edited by Bloozberry (edited 01-25-2015).]

Thanks for the feedback Blooz. I'll digest what you typed.

Until then, here is the chassis rolled 6 degrees with the corresponding suspension geometry. This was a helluva lotta work.



Edit: to add a dimension x 2

[This message has been edited by Yarmouth Fiero (edited 01-25-2015).]

You're still missing a dimension. You listed the height of the rear roll center (5.182") but not the lateral migration from the centerline. From the looks of things, it should be around 19.25".

[This message has been edited by Bloozberry (edited 01-25-2015).]

Crap........ I may have kept my upper strut bearing in the camber plate rigid when I rolled the chassis. That may be why my outside wheel has such a large angle and my inside wheel has such a small angle. I think allowing the top of the strut to pivot would let the outside wheel pivot down and the inside wheel pivot upwards.

Back to the drawing board.

Edit: Fixed that. You were close Blooz

[This message has been edited by Yarmouth Fiero (edited 01-25-2015).]

OK, so now if you add in the effect of the slight change in strut angles, you'll get the position of the rear roll center in the worst possible state. It'll change the roll center a bit, but not by much so it's looking pretty good. The stock '88 suspension at 6 degrees of body roll places the rear roll center at 82" from the centerline, and drops it 18.5" below static. Currently, your design shows a 23.2" lateral move and a 3.8" drop. Definitely a significant improvement over even an '88!

Thanks Blooz. I've gone back and looked at my struts and I did allow my struts to pivot. Because they are fixed to the spindles, the whole assembly pivoted about the ball joint center. My mistake is that I pivoted the whole camber plate as opposed to just the spherical bearings. I realized this when I turned my strut towers back on and saw that my camber plates were no longer sitting flat on the tower mounting surface.

I'll fix that and repost.

Edit to add: Is it correct to assume that since my struts did pivot as they should, I can glean from the drawing with 6 degree body roll that the camber in this worst case condition would be +3 degrees on the outboard wheel and -1.2 degrees on the inboard wheel. Is it also correct to assume that since both my struts are probably bottomed out at 6 degrees of body roll, that there is no need to calculate jounce and rebound conditions while rolling?

[This message has been edited by Yarmouth Fiero (edited 01-25-2015).]

I have been watching this thread since it started .The drawings amaze me and your work is amazing .But one thing I would like to suggest is that you move the inboard pivot mount for your rear lower tie rod to the bolt that holds the control arm and get it off the arm .Since your outer mount is on the same plane as the ball joint , bolting the inner mount to the arm mount puts it on the same plane .No calculations involved , it eliminates all possibility of toe movements top to bottom .

Thanks wftb. I have considered what you are suggesting as it would make bump steer zero. The only problem is that I would have to move the rear inner control arm mount almost 2" further forward which would essentially have the rear rail of the control arm swept backwards slightly as opposed to swept forward as it is now. So I may have to move the rear inboard control arm bushing farther aft and mounting the tie rod on the forward side of the bushing. I just have to make sure the tie rod doesn't want to pass through the 1" diameter cross brace of the control arm. I am sure before I actually build the control arms, I'll modify the design to improve it as you have suggested.

I think your bracket to keep the toe in place is the best idea I have seen yet for a strut based Fiero suspension .No matter what the final design of arm ends up being , that ball joint to tie rod bracket is an easy solution to a big problem .Keep up the great work .

[This message has been edited by wftb (edited 01-26-2015).]

You got me thinking this morning wftb with your statement regarding the mounting location of the inboard end of the tie rod. I was sure I had it settled in my mind that my current arrangement for the tie rod and control arm would generate zero toe change so I went looking for my old drawing. As you can see, with the tie rod and control arm sharing a common point on the ball joint axis and the two pieces fixed together at the inboard end, the assembly is free to rotate together with no net effect on the ball joint / spindle, ie: no toe change. So the fixed end of the tie rod can mount anywhere on the control arm. Thanks for getting my brain in gear.

You are correct Yarmouth Fiero... your design does not require the toe link to be mounted to the chassis. If it were mounted to the chassis though, then it would have to be located where wtfb suggested to avoid introducing toe in bump.

Well today is a snow day so perfect for concentrating on capturing the mind numbing yet important data of Body Roll vs Roll Center.

Here are the results of my suspension and stock data supplied by Blooz. Again, I am comparing my '85 style suspension against '88 style so take it with a grain of salt. It does show though my my suspension seems to have a much more stable Roll Center which should be a good thing.

This data is based on a CG of 18" but I think my final CG will be slighlty lower. This should further improve the stability of the car in hard cornering.

That's an amazing improvement over the '88! Nice work!

I tried to find a source for the Lotus Suspension Analysis software; a Brit source showed it on sale but (maybe my order was too small?) I couldn't get to the check out page; it just refused. Another that (yeah, yeah, I know...) had it for free - and there are lots of sites showing it for free.. ended up with the inevitable. Spent well over an hour cleaning up my computer from all the trash it installed and even before that, it 'looked like' I had to use the software on a shared server, ... somewhere... no thanks!

So.....
does anyone know where it can be obtained safely? I've sent a note to Lotus but no answer at least yet.
thx
GP

And from the previous page:

quote Originally posted by Yarmouth Fiero: Well I managed to capture the results of Body Roll vs Camber. The results seem to indicate that while there is camber change during body roll, it seems to be significantly less than the stock suspension. Am I interpreting this data correctly?

You have misinterpreted the data. Your camber gain is much better than the stock '88 Fiero. Take for example the extreme end at 6 degrees body roll where your plot says you have +2 degrees camber on the outside tire. That 2 degrees is measured from the angle of the tire patch to the ground. That means it's much closer to being flat than the '88's tire patch which is 3.75 degrees away from being flat on the ground. Of course the flatter the tire patch, the more traction you have, but there's more that comes into play than that because the tire also flexes.

As I mentioned the other day when we were talking, some of the modern mid engine cars like the Ford GT have an average 0.7:1 camber to roll ratio to account for these other factors. You can sample your data at various points and see where your ratio lands (it varies... and don't forget to take into account your -1 degree static camber!) So for example at 3.5 degrees body roll, your camber gain ratio is [3.5 degrees - (0.58 deg + 1.0 deg static)] / 3.5 deg = 0.55. At 6 degrees body roll it's 0.50:1. That decreasing trend is a characteristic of the Chapman strut design... as the lower control arm approaches perpendicularity with the strut, the camber gain will decrease to zero.

Just to show I CAN stay on topic:

what is the best way now to predict, and control, body roll? Shocks, springs and sway/roll bars? Basically I am wondering IF and WHEN you would get to (ie) 6 degrees of roll; if it might be less or it might be even more? Do you have a goal in mind for maximum roll amount?

What software are you using; is it any kind of suspension design software or are you cranking all this thru Autocad in 3D (wow you are quick if so).
Do you have plans on how the rear setup will affect (the design of) your front end setup; ie assuming it will be a narrower track and somewhat lower due to tire/wheel size, how will you thereby 'link' the two together? Just curious as to what's in the back of your mind on this.

GP

quote Originally posted by 85-308: The going rate for the real thing, by the way, is about $3150 per year for Shark alone.

The best way is to find a friend who already has access to it and buy him a beer.

quote Originally posted by 85-308: what is the best way now to predict, and control, body roll? Shocks, springs and sway/roll bars?

I can try to answer this one. There are several spring rate calculators out there that suggest an appropriate spring rate after having entered a bunch of measurements and going through the interim step of calculating your wheel rate. They usually give end up suggesting a much softer riding car than an enthusiast would like, so take it with a grain of salt. Luckily coil springs can be had in any rate you want in 50 lb/in increments.

Controlling body roll then becomes choosing the correct sway bar (since they only stiffen the chassis in roll) usually through trial and error. It would be nice if there were suppliers who could make any physical configuration and torsional stiffness you'd like, but for the most part we have to settle for what we can make fit, or design the suspension in such a way as to minimize the need for sway bars altogether. Not great choices no matter which way you look at it.

Thanks for all the comments guys. I just got back in from a couple hours of snow blowing so I need to let the feeling come back in my fingers. I have a lot of widows on my street needing to be dug out.

Back with more shortly.

quote Originally posted by Yarmouth Fiero: Thanks for all the comments guys. I just got back in from a couple hours of snow blowing so I need to let the feeling come back in my fingers. I have a lot of widows on my street needing to be dug out. Back with more shortly.

My Condolences...don't know how you northern guys do it!

Thanks for the feed back Blooz. I've read not only your thread over and over but a tonne of info online. It can get pretty detailed but there are some good sources out there that the authors have really made an effort to be clear and concise. But the best education has been modeling the suspension geometry and indexing through the range of motion of the various analyses. Its quite eye opening and there were many times I went backward and repeated the motion just so convince myself. It is going to be very interesting to actually drive the car hard one day on a track or open space and get a feel for how it handles.

With regard to Lotus Suspension software 85-308, I am sure it is a powerful tool and would certainly make it easier to make on-the-fly suspension design changes and see the results immediately. Its pricy stuff though. For me, I am just using Rhino for my 3D design and then modeling the linkages as lines and the joints as points in space and then slowly adjusting them through a range of motion. I also have Inventor which can model motion and you define the characteristics of each linkage and joint. Unfortunately, I don't know the software well enough to use it properly. Atleast in Rhino, I put each instance of suspension analysis on different layers so that I can save them and turn them off or on at will so I can scroll through the motion. Its usually pretty clear if you made a mistake somewhere as the geometry suddenly moves in an uncharateristic way.

Overall, I am happy with what I have found in the analysis of my suspension. Just by keeping a few conditions like control arm angle and strut position in a desired range, its allowed the chassis drop and track width increase to add their respective improvements. Now I just need to built is properly so it doesn't fall apart or send me flying ass backwards across the highway one day. Thanks to Blooz for being patient answering my many off the wall questions day and night.

Edit typos.

[This message has been edited by Yarmouth Fiero (edited 01-27-2015).]

quote Originally posted by 85-308: I am wondering IF and WHEN you would get to (ie) 6 degrees of roll; if it might be less or it might be even more? Do you have a goal in mind for maximum roll amount?

I forgot to mention that as a starting point, the stock '84-'87 Fiero had a documented 3.5 degrees of body roll per lateral g, and it was capable of sustaining a steady state 0.82 g on a 100' diameter skid pad. Theoretically that translates into a steady state 2.9 degrees of body roll.

So if you consider the wider, grippier tires, better camber, and roll center location, Yarmouth Fiero's car should be able to generate better lateral grip than the stock 0.82 g. If everything else were equal, then you might expect to sustain up to a max of 3.5 degrees of body roll in steady state. The thing is, everything else isn't equal. He'll probably end up using stiffer springs, different roll bars, etc. which will lessen the degrees/g.

Sustainable lateral g's are less than what a typical car can generate temporarily though, so designing a suspension that still performs decently up to a max of 6 degrees body roll is a sound decision. (full disclosure: that's what I designed mine around too)

Thanks for that interesting factoid regarding body roll Blooze. I agree that it's certainly a good idea to design around the extreme, knowing that the actual operating conditions are well within that envelope. It just so happens that the 6 degrees of body roll put the tire almost perfectly against the notched out upper frame rail. That would also be the maximum travel of my coil over struts. It would certainly be an interesting exercise to push the car to the limits on a skid pad one day. Be sure to keep some contacts at the base Blooz. We may need 100 ft x 100 ft section of their tarmac one day.

[This message has been edited by Yarmouth Fiero (edited 01-28-2015).]

just make the sleeves where they pass thru the frame the bushings as well?

I like that idea.

Thanks 85-308.

Edit to add..... something like this?

[This message has been edited by Yarmouth Fiero (edited 01-28-2015).]

You would need to come up with some sort of retaining caps both ends but it might look a bit cleaner plus 'sorta' keep dirt etc from accumulating in that sleeve anyway. The sway bar bushing is split of course so should be 'doable' - just slide it over the bar and press into the sleeve. And it gives you the widest spread on your bar support points so the arms do all the work (the centre portion will twist regardless). Only thing is being able to grease it.... hmmmm.

With my poly bushings I grooved them and drilled them so grease had a chance to get to the inner parts where the friction surfaces were; I guess you could do that from the inboard side then install a zerk fitting.

85-308........

well isn't that clever...
were you planning on leaving the end open? If you prefer, you can get grease extension lines; they are used on mechanical equipment (fan shafts and bearings etc) all the time when 'far side' grease fittings aren't accessible, or readily accessible. If you wanted to close up that end of the frame, that is.

My plan 85-308, is to make an aluminum cradle and typically we don't weld aluminum structures completely closed. Plus it will be going in the oven for powdercoating so I will leave some strategic drain/ vent holes in the structure. One of those holes will likely be in the end so I can get a socket in to fit the zerk and a grease gun periodically. I'll look at how close the body is going to be to the end of the cradle and if there is an issue, I'll perhaps locate the zerk and drain hole in the bottom of the bushing / cradle. Although 5" of ground clearance doesn't leave much room to get at it.

Here are a few pictures of the rear control arm assembly with sway bar and hardware in place. While I'm waiting for my DOM tubing to arrive, I'll start building a jig to assemble the control arm for welding.

Bit by bit it's coming together... well, in the cybersphere anyways. It'll be nice to see some actual parts! (I remember being at exactly the same stage... it sucks after a while of not actually turning wrenches.)

You got that right Blooz. I've been tweeking the parts for the strut towers and lower frames rails all day while in a holding pattern for my welder. I really need to get better at it so I can do the welding myself. As soon as the DOM tubing is in I'll jump on the control arms. I have everything I need for the aluminum cradle as well. With any luck I'll be able to roll this thing into the sunlight once the snow is gone.

Looking good!

The one suggestion is to add a triangulating link between the lower tab for the toe link and the bolt hole on the wheel bearing. Right now the bracket is double cantilevered and might flex some with excessive loading, adding a small bracket back to the outboard side of the wheel bearing mounting bolt would help stabilize the link more.

[This message has been edited by fieroguru (edited 02-01-2015).]

Thanks Fieroguru. I agree that having that structure hanging out there is certainly a point of concern. Perhaps something like this would help. Just run a tap right through the spindle casting and fit a longer bearing bolt with a nut on the end to help support the toe bracket. The back side of the casting is quite flat so it might be a good place to mount the extra gusset.

YF, I have a question. If you are going through the effort of building a new custom engine cradle and suspension, why try to integrate the stock 84-87 rear knuckle? It seems like a lot of thought and design has gone into just trying to eliminate the bump steer issue with the placement of the tie-rod connection points. Not to mention the extra gussets needed to keep it from flexing too much. If you are going to fabricate your own control arms, why not just use the 88 knuckle or even use one from a completely different IRS setup that doesn't rely on a tie rod end to keep the knuckle steady. You have what is essentially a blank canvas in the custom rear section of our car, and the skills to design and build a custom IRS. Something outside the box would compliment all the other mods you've done to this point and bump up the cool factor a whole bunch. Just my 2¢ on this.....this build is in my favs.

Wow... good question Seajai. I'll try to answer this now otherwise I'm not going to be able to enjoy the game this afternoon.

First, I will be the first to admit there is a big difference between "drawing" suspensions and "designing" suspensions. I certainly will never claim I can do the later. While I have the ability to produce drawings that are both accurate and visually descriptive, I really don't have any suspension experience with regards to different arrangements, setups and analysis. I have been reading all I can on the subject but that isn't a replacement for experience. My plan originally was to stay with stock components. Then I started looking at semi custom control arms because of the change in track width that I wanted to incorporate in order to get the proper fit for the car body. As you may know, Blooz and I plan to use the same plug/ molds for our individual projects so that set some significant constraints. It then became apparent that it would be difficult to purchase control arms that suited my needs exactly so I started designing my own, based on my general engineering knowledge and the basic suspension knowledge I have slowly acquired. This brings me to where I am today. I guess you are correct in that I have missed an opportunity to start with a clean sheet of paper and come up with a unique and effective design. However, that is well outside my comfort zone on a project like this and like everything regarding this project, it's been a slow gradual developement to get where it is today as opposed to starting from scratch and building everything fresh from the ground up. I am hopeful that what I eventually build has reasonable performance and is safe to drive on the roads. Also, because I will eventually need to get this project inspected and certified to drive on the street, I have been conscious to use as many stock or off the shelf parts as possible, hoping to fly under the radar of the inspector when the day comes.

There are many very talented and experienced builders and fabricators on this site and its been a very positive experience to follow their builds and recieve their comments on my project. I have a great deal of respect for their abilities and knowledge.

I hope this answers your original question.

Edit: many typos

Edit : Go Pats

[This message has been edited by Yarmouth Fiero (edited 02-01-2015).]

That makes sense to me, especially if you factor in inspection process. I'm gonna keep watching....so many good ideas...keeps my brain churning. Been thinking about doing a custom cradle for the Chrysler swap. Wish I had your cad skills, I took drafting in high school but that was in the paper and pencil days. Software is so expensive.