I measured my front and rear motion ratios yesterday and wanted to compare notes with any that have done the same.

For the rear suspension I took ~20 data points and did a best fit analysis to come up with 1.00 to 1.00 damper travel to wheel travel with strong statistical correlation.

My front geometry is a bit different than a standard 914, so the motion ratio should be different, too. I measure ~10 points and found 0.83 damper travel to wheel travel, again with strong statistical correlation.

I'm about to order my rear shocks and this is one of the bits of data they need to build the shocks.

And....

If anyone had determined the height of their Cg, that would be useful, too.

I can't do mine now until I reinstall the powertrain......

Your making me think..

I have all of this info on a laptop that we use for data acq on a FP 914.

I'm at home this weekend doing NOTHING.. but the laptop is at the shop (not in the trailer) so... I might have some info for you by Monday evening if you still need it.



B

Kevin,
I realize that I sent the wrong numbers to you in a PM. Sorry. My measured front MR is .909 and rear is 1.296. But this has to be squared to get Wheel Rate. Apparently I can't attach an Excel file. Let me know if you want me to mail you the file.

Steve

Kevin
Who are you having do your shocks?

Steve
What is your setup? Stock suspension points and components? Could you email me the file as well?

Brad
Please email me that info as well. I would really be interested in comparing it to my own notes and theories. On that FP car what was the setup? Relocated mounting points, custom trailing arms and struts, etc?

Kevin, Brett,

I would love to compare notes with you guys about setup and will gladly send you the Excel file via email but can someone tell me how to do that. I spent hours studying formulas and measuring suspension components and creating spreadsheet with imbedded formulas. I think Kevin sent me his address but some filtering software seems to have removed it on my end or the Board's end. There is no way via reply to attach the file. Am I missing something?

Really want to maximize my car's handling and give more of those 911 drivers fits. Sharing information makes sense.

My car uses stock suspension mounting points except that front shock towers have modified camber caster plates. I am running RSR front struts and no torsion bars. Alll suspension mounting points have been strengthened through plating and tubing to the cage. Bilstein shocks front and rear.

Steve

Nice to have a Brother in the shock business...

Kevin,


What do you plan on using for shocks????
I like the Ohlins

Slivel,
Y
ou may find the RC will be high, especially in the rear causing the instant axis from front to rear to be much higher in the rear. This causes a number of issues, Brad may feel different, but I have found 2.5-3.0 inches of rear suspension pick up point movement (up) to fix this and restore the rear trailing arm angularity. This will make the rear of the car much more compliant while enabling you to lower the rear RC to an acceptable point. This will make the IA similar front to rear althought the MA will change due to the difference between Struts and trailing arms.


Erik

If you really want something good, add 4.5 inches to the front a-arms and relocate those points 2.0-2.5 inches up. Then add 2.5 degrees to the strut knuckle angle and reposition the front camber plate. This will give you almost 0 scrub and a better camber curve, and allow for the proper amount of front track, well at least in my class. This is my plan for next year when the car gets a rebuild. Kevin is on the right track with his front suspension. But, he may find the rear pick up point change will give him immense benefits.

I just uploaded Steve's excell sheets.
You can find em here:

Suspension worksheet

Spring rates

I too spent hours measuring the points for all the suspension mounts and marked them on the floor and measure to each point but none of my point measurements were relative to the program I was using so I get to do it all over again. (for the third time) To bad I don't have any of my old suspension components. As soon as I figure out this program I will be runnning numbers on the rear. I feel that lengthing the trailing arm and moving the rear pickup points are steps in the right direction but I will not argue why until I have the numbers to back up my theories. Same goes for the front. Simply moving the pickup points only fixes part of the problem.

Steve, I'll send you an email with my address in it.

I'm going with Koni 30s in the rear. They have an incredible range of adjustment.

I know we've beat the RC discussion to death a few times, but let me add a few statements from what I've learned.

The front roll center on a lowered 914 front stock suspension is below the ground. Bad, very bad. There are several approaches to fix this. Eric mentioned one method.

The rear roll center is higher than the front...that's okay, it's supposed to be that way. The rear roll center, from what I've read, is determined by the angle of the mounting points to the chassis in the horizontal plane, in our case 12 degrees. Another line is drawn from the intersection of the 12 degree line with the wheel centerline to the center of the tire patch. The roll center is where this line intersects the center line of the car. So, the rear roll center is between the ground and the center of the wheels and it's height is determined by the angle of the trailing arm points relative to the wheel. BTW.... thanks to Brett W for helping me figure this stuff out. We traded many e-mails working through this discussion.

So, raising the points in the chassis does nothing for the rear roll center. But, raising the points does change the angle of the trailing arm relative to the body, which is where there can be a benefit.

Gillespie says that if a semi-trailing arm is angled up front the rear wheel, it contributes to roll oversteer, if it's level, neutral steer, and angled down from the rear wheel, roll understeer. And this is due to the track change at the outside wheel. However, Gillespie does not quantify this.

Now, we disagree about the severity of this effect.

So how low is TOO low on the front of a stock 914? I have heard a rule of thumb for this is not to let the front lower A arms go below parallel to the ground. Comments for us stock guys?

Thanks-

Angled down towards the wheel (better than parallel) to the ground is a decent rule of thumb, but .... see my signature.....

You can always just drive it and have fun and not stress about it. If the roll center is too low, you just need more roll stiffness to counter it. I think that's why there are so many big ARBs for the 914.

Since I know I'm not Montoya I need to build a car that is easier to drive and I like the engineering exercise behind it all.

That would reduce actual body roll but you'd still get weight transfer and loss of front traction. Or did I understand that wrong?

Also, how do you increase the spindle to strut angle by 2.5 degrees? Customized knuckle I assume. Or do some strut assy's, like RSR, have a larger strut to spindle angle?

That's how I see it.

I didn't increase the angle between the strut and knuckle, but I do have custom (read home modified) knuckle/strut assemblies. Eric would have to enlighten us on how he's going to do that.

I worried less about the scrub radius and more about roll center, right or wrong. I did raise the steering rack to reduce bump steer.

Well, I re-measured and still get 1:1 on the rear, but.... since I'm going to use spherical bearing mounts, I think I'll move the upper mount to get the loads straight into the cage. So, I'll lean the damper more forward in-car than factory, which results in a .92:1 motion ratio (damper travel:wheel travel).

BTW...... Koni has expanded their 30 series line to include a lot longer dampers than before. So, now they make 30 series dampers up to 24.5" long extended (the longest 30 series they used to make was only 17.4" long extended).


Does anybody have any Cg height information? Preferably for an open 914.

I was told by someone I trust to figure about 16 inches.

That sounds like it's in the ballpark.

For reference a Ford GT has a Cg of 17.7"

I guess the DOHC V8 moves some of the weigh a little higher.

I like this thread

First off, all you guys have lost your lids..

I never imagined the roll center on a lower car being below 0.. this is very interesting. That explains some of the twitchy stuff you get at medium speeds with a very low street car.

I need to go back.. and read it again... keep talking...


M

If somebody wants some info on front Mac suspension and semi-trailing arm suspension, I thought this site has some good stuff. This link opens a 2 page article and there are links to more info embedded. Explains effect of lowering suspension, and how to get rear RC lower. Enjoy ...


E30 suspension

The factory front suspension geometry moves all over the planet when it goes through its travel. One of the draw backs of really short arms. I have yet to see, but a few people that got that one right (Kevin). Even Sheridan's cars has arms that are a little short, but his geometry may actually work with that. Most people just lower stock suspension and create a bunch of problems that have to be band-aided with heavy springs and huge sway bars.


Damn I am sharing all my secrets. Not really.

Here's a simple front roll center calculation spreadsheet I wrote.

The geometry represented is not necessarily a stock layout, I guessed at it.

Anything in orange can be changed. Play with cell C5 (which is the height on the inboard lower control arm mount) and watch what happens to the roll center location (rows 34 and 35). In the current configuration, the inboard point of the lower control arm is 1 inch higher than the outboard point, resulting in some fairly reasonable roll center locations.

Now change C5 to 6.5 (1/2" lower than the LCA point, simulating a lower 914).

Attachment removed..... since it was gibberish.

Interesting thread.

Kevin, When I d/l your spreadsheet it comes out as mostly garbage. What version of excell are you using?

Excel for XP.... don't know why it does that.

Maybe this will work.

The hollow diamonds are the roll center locations with no roll (large one is static condition, smaller one is 2" jounce condition). The large hollow circle is the roll center under roll condition, in this case 3 degrees.

And now with a simulated lowered 914....

Kevin's excell sheet can be downloaded here:

Front Roll Center Calculation

Maybe some more explanation would help...

The chart shows a head-on view of the critical points of the front suspension. Starting from the left and working to the right.

Lower left corner-center of the tire patch
Lower left corner, a little higher-bottom of strut/attachment point of the lower control arm to the strut
Upper left-top of strut
Middle left-inboard point of lower control arm
Everything is repeated on the other side of the zero line

You see multiple points at each spot because I plotted the static and roll condition.

I'm reading this thread and wondering if anyone has a book recomendation for suspension 101 type stuff. I don't even have enough suspension vocab to completely follow this thread.

The Carrol Smith books are a great start.

From there you can go to:
-Gillespie's Vehicle Dyanamics
-Milliken's Race Car Vehicle Dynamics
-Paul Haney's The Racing and High Performance Tire

There's plenty more....

You know what somebody needs to do--is design new pick-up points for the arms that use the same type of adjuster as seen on many crotch rockets, you know, the round one which you turn in a desired direction for chain tension and ride height---if someone were to design these into the pick-up points with a bearing action--when the arm moves thru its travel, the geometry would be locked in place solid

FWIW i just read in Paul Freres book "Porsche race cars of the 1970's"

that the 917/30 had a CG of 15"

Those books that Kevin listed are considered by most in the industry as the emminent sources for this type of info.

For the rest of you guys that aren't interested in the formulas and major engineering text. Get:

How to Make Your Car Handle by Fred Puhn
Chassis Engineering by Herb Adams
Race Car Engineering by Paul Vanvalkenburg

Although a little older in material, they are easily understood and won't put you to sleep right away. Once you read these and decide that they can no longer explain the finer points of this complex study. Then pick up the books that Kevin listed. They will help with the true nuts and bolts of what really happens.

The offset busings for the rear are an idea that was proposed before. Tough to manufacture for our situation.

The moveable suspension pickup points is in theory a great idea, but difficult to implement. Not impossible, just difficult. Those adjusters you mentioned would be great on the rear suspension to adjust toe.

I wanted to make my front lower control arm points height adjustable, so I could change the roll center as conditions ditacted (like for rain, to soften the weight transfer). But, in the end I decided to get the car on track and maybe change it later. I made it all so incredibly beefy an wee little adjustor like that would be the weakest point.

For those of us that like engineering, cars, but are visual learners can somone make up some simple diagrams that show what you guys are talking about? maybe even have 914 parts in the drawings. Thanks

All depends on the material of choice---A one inch thick by 2.5 inch diameter chunk of stainless will be strong enough to lift a 40 ton cat bulldozer welded to it---would easily take on the teener.

These have actually been implemented in years of past, ever hear of a motorbike called a "BIMOTA". Early eighties they made a model called the " TESI" it had a front and rear swingarm, no forks as conventional even unto this day.

Bimota solved the problem of toe changes in the front swingarm by using those same eccentric adjusters--not to boot that after they invented it first, almost every bike manufacturer went with them for adjusting the rear wheel!!!

Too bad the technology did not take hold, people freaked at the sight of a front swingarm motorbike---but when Bimota raced it, even down on power, NOTHING could touch it in the twisties!!!

Oh, I know what you're talking about now. Those eccentrics are used quite often on cars these days, when they care about adjustment. My Focus had them on the rear suspension links. They are very common on Mazda cars, since Mazda designs in adjustability.

With longer lower control arms, you actually have to move the mount a fair bit (at least an inch) to get the desired effect.

I thought you were talking about the screw adjustors that are used to adjust chain tension. My bad.

BTW... BMW has some morphed version of a swing arm on their bikes. It's quite intriguing.

Kevin,
Right now my front arms are roughly parallel to the ground. To adjust the roll center by angling the front up and down in the rear (towards the wheel) don't you wind up causing a binding in the pivot points. The crossmenber doesn't seem to allow much in the way of latitude here. Do you make some mod to either end of the suspension arm to allow the repositioning without binding? Additionally this may change the roll center but doesn't this now cause the arc of the wheel to be other than vertical under suspension travel?

Steve

Steve,

I made some minor modifications......

Link to pics of front suspension

The wheel travel is never strictly vertical because the lower control arm pivots about a point. The shorter the control arm, the more track change you get during the suspension travel.

Yea, that makes sense. In my case using stock arms I am probably better off not mucking with trying to change the roll center without changing the pickup points.

From the pics in the earlier thread - you certainly did make some "minor modifications". I don't think that I can go that radical with the rules in the clubs that I run. I am a fairly experienced club racer (10 years) but only recently have started to seriously think about chassis and suspension tweaks. Thank you for your informative posts.

Steve

I've been using MR SEARCH to try to find out what the stock motion ratios are. I found a post by "slivel" stating the stock rear was 1.68. I haven't been able to find a stock front motion ratio. Would like this to help figure out spring rates for a DP autocross car. Anyone know what that stock front motion ratio would be, or could give me a rough guestimate?

Typically a strut front suspension has a motion ratio of ~.92-.95 : 1... meaning the strut moves .9x and the wheel moves 1.

Great, exactally what we were looking for. thanks!

QUOTE (groot @ Dec 2 2004, 08:03 AM)

The wheel travel is never strictly vertical because the lower control arm pivots about a point. The shorter the control arm, the more track change you get during the suspension travel.

You guys are way ahead of me when it comes to the technical theory of suspension design, but this is an interesting thread.

For an example of short versus long a-arms, look at any modern open wheel or sports racing car. A-arms are as long as possible. In F-1 the lower a-arms pivot at the centerline of the chassis.

Longer arms reduce the camber, toe and roll center changes. Better in every way (if you get the geometry right).... except package.

Kevin's right packaging is a big problem with long a-arms. I am having to run pretty short upper and lower arms in the rear to get around the tranny. Up front I could run as long as necessary but found that the 21in length will work fine for my application. With as little wheel travel as I am planning for the camber change is minimal.