914 Suspension motion ratios, Comparing notes Options

[redshift]

First off, all you guys have lost your lids.. (IMG:style_emoticons/default/laugh.gif)

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

[Bigbohr]

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

[Brett W]

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. (IMG:style_emoticons/default/smile.gif)

[groot]

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.

[smdubovsky]

Interesting thread.

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

[groot]

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.

[groot]

And now with a simulated lowered 914....

[Jeroen]

Kevin's excell sheet can be downloaded here:

Front Roll Center Calculation

[groot]

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.

[Nick]

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.

[groot]

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....

[jwalters]

(IMG:style_emoticons/default/smilie_pokal.gif) 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 (IMG:style_emoticons/default/beer.gif) (IMG:style_emoticons/default/beer.gif) (IMG:style_emoticons/default/beer.gif)

[TimT]

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

that the 917/30 had a CG of 15"

[Brett W]

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.

[groot]

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.

[smrz914]

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

[jwalters]

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.

(IMG:style_emoticons/default/wink.gif) 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!!! (IMG:style_emoticons/default/beer.gif) (IMG:style_emoticons/default/beer.gif)

[groot]

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.

[slivel]

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 (IMG:style_emoticons/default/wink.gif) 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.

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 (IMG:style_emoticons/default/confused24.gif)

[groot]

Steve,

I made some minor modifications......

Link to pics of front suspension