Toe Curve for Semi-trailing arm, From my 914 Options

[Brett W]

If you are building a competetive EP car you should be shooting for 10-15K per degree for chassis stiffness. Most production chassises will have a tough time reaching these numbers with out a properly engineered chassis. Just adding a cage won't do it. From teh cages I have seen many are severyly lacking.

[ChrisFoley]

QUOTE (914_Guy @ Nov 18 2005, 06:16 AM)

In theory, should the toe and camber change at all for the rear? Maybe you were measuring the change in rear track?

In theory, the toe can change a maximum of 12.5 degrees, (if you could rotate the trailing arm through a full 90 degrees). (IMG:http://www.914world.com/bbs2/html/emoticons/rolleyes.gif)

[Jeroen]

this pic claims that the angle of the pivot is 25deg
maybe that info is usefull making calculations (not me... this is still way over my head (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif))

[ChrisFoley]

QUOTE (Jeroen @ Nov 18 2005, 07:09 AM)

this pic claims that the angle of the pivot is 25deg

I think it's only half that.

[Jeroen]

hmmm... looks like your right
just did a crude measurement off the pic in graphix software and it looks like 14deg (IMG:http://www.914world.com/bbs2/html/emoticons/rolleyes.gif)
nevermind... I'll just shut up now and listen (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

[Brett W]

The factory angle is 12 degrees. In my research it looks like the best compromise of all the available angles. BMW offered two different options for the M3 on was 12 deg and the other was 15 deg. They kept there race stuff at 15 degrees, but the toe curve is not as nice for the 15 deg angle. It does gain more camber.

[MattR]

QUOTE (914_Guy @ Nov 18 2005, 09:43 AM)

Now the x-axis will correspond to the trailing arm’s pivot axis and as the arm rotates it moves only in a plane corresponding to the y-z plane and therefore its camber angle never changes.

I dont agree with that statement. I dont think the axis of the trailing arm is parallel to anything (either the x, y, or z axis).

Remember, camber change is a fuction of instant center. The instant center is fixed on a semi-trailing arm, but that just means the camber change is linear and not exponential.

[MattR]

It has nothing to do with orientation and everything to do with Instant Center. Its a geometric condition. Where you orient your origin is arbitrary.

[Thorshammer]

This is a good site to understand what is happening, although not exactly the numbers we are dealing with, very close.

http://e30m3performance.com/tech_articles/...nter/page-2.htm

What we are considering is obtaining a little bit more camber gain, and lessen or eliminating the toe change. In some of our cars, and everyone is running something a little bit different, Roll center MAY be TOO high. In others there is a discussion on how we can change the trailing arm to deal with some of this. There are options, but they need to be researched, not just guessed. This is what Kevin (primariliy) is doing. others are as well.

Brett,

I was talking about Finches spring stiffness in the actual springs. He is running the car alot stifffer than many of us with regard to spring rate. I would say your numbers for platform stiffness are very close. I don't think I am quite there yet. Hows the Ultimate 914 coming??

Erik

[jhadler]

Can you try that link again? It doesn't work...

Thanks!

-Josh2

[Brett W]

Its kinda on hold until I find some more shop space.

[ChrisFoley]

QUOTE (jhadler @ Nov 18 2005, 06:25 PM)

Can you try that link again? It doesn't work...

Here

[ChrisFoley]

QUOTE (Thorshammer @ Nov 18 2005, 06:19 PM)

He is running the car alot stifffer than many of us with regard to spring rate.

He also has several more bars in the roll cage than we do.

[ChrisFoley]

The math is given at the website link I provided.

[Brett W]

QUOTE

Does anyone have a book on suspensions that states trailing arms like the 914’s do change camber and toe as the arm pivots?

You don't need math you just need a tape measure and some string, and maybe a camber gauge. Supoort the body on jack stands and pull the shock loose. Set the trailing arm at a level position. Then take a really big square and stand it up next to the wheel. Measure the distance from the front of the wheel to the square and measure the top of the wheel to the square. Now cycle the trailing arm as far as it will go into bump and re-measure. You will see the toe and camber change. You don't really need math to see that.

But if you must see the math.
(IMG:http://e30m3performance.com/tech_articles/susp-tech/rear_curves/toe_equ.gif)

(IMG:http://e30m3performance.com/tech_articles/susp-tech/rear_curves/camber_toe_calculations2.gif)

[ChrisFoley]

If you plot toe or camber as a function of rotation you get a sine wave, assuming you could rotate about the pivot axis through 360 degrees.

[ChrisFoley]

The pivot axis isn't parallel to the direction of travel. Your graph does not take this into account.

[groot]

Here's an interesting article... that contradicts Milliken's book.

Basically, it says that toe is a complex curve that can toe out (proven by my chart)... Milliken states that a semi-trailing arm never toes out. Camber is linear over our range.

BTW... BMW uses 12 degrees on their early race M3s and used 15 degrees on their street cars.

Article has been removed from this post... its text is in a later post.

[groot]

Eric.... yes, I'm finally on my home computer to share this file. Sorry it took so long.

Table to go with text.

[groot]

Pretty charts