Irentat

iTrader: (1)

Hey all,

Looking at the strut suspension we have on the front of our FC's. It seems to me if you have coilovers with pillowballs (and I do) that you can modify the lower strut location.

what I mean by this is to move the strut in towards the center of the car making the coilover more straight up and down.

Is this practical? Did a search here and on the web with no success.

I ask because it looks like it would allow for larger tires and maybe change the suspension dynamics somewhat?

Just tell me if this is stupid.

Thanks,

Turbo II Rotor

iTrader: (2)

You would gain a lot more negative camber throughout suspension travel if you moved the lower strut mount inwards.

Irentat

iTrader: (1)

Can you please be clearer.

So if I gain negative camber, you make it sound like a good thing because currently, we gain POSITIVE camber through suspension travel. The best camber I have for my car right now is around -2.5 degrees for road racing. If I were to not have positive camber gain, I would be very happy.

90FC

I'm not sure about camber gain, but I know you will change the scrub radius quite a bit.



"Scrub radius is the distance between where the SAI intersects the ground and the center of the tire. This distance must be exactly the same from side to side or the vehicle will pull strongly at all speeds. While included angle problems will affect the scrub radius, it is not the only thing that will affect it. Different wheels or tires from side to side will cause differences in scrub radius as well as a tire that is low on air. Positive scrub radius is when the tire contact patch is outside of the SAI pivot, while negative scrub radius is when the contact patch is inboard of the SAI pivot (front wheel drive vehicles usually have negative scrub radius).

If the brake on one front wheel is not working, with positive scrub radius, stepping on the brake will cause the steering wheel to try to rip out of your hand. Negative scrub radius will minimize that effect.

Scrub radius is designed at the factory and is not adjustable. If you have a vehicle that is pulling even though the alignment is correct, look for something that will affect scrub radius."

From here: http://www.familycar.com/Alignment.htm

90FC

Also, I think It would be more practical to just run lower offset wheels and wider fenders.

Irentat

iTrader: (1)

OK,

How about this:

Increase the angle of the strut the opposite way. Then by Turbo II's description it would be the opposite effect.

I am just thinking but here is maybe how it can be done: On the lower strut mount: slot the upper bolt hole (remember there are 2). Now with camber plates, increase the angle of the whole strut by pivoting on the lower lower bolt.

Just a thought.

jgrewe

iTrader: (1)

It can be done. It won't effect the scrub radius by itself because that is set by the location of the ball joint in relation to the top of the strut and the wheel offset.

Scrub radius isn't the huge issue people think it is. The biggest problem with having a bunch is when you turn the wheel you are essentially 'lifting' the outside wheel in relation to the chassis. This gets worse if you are running more castor too trying to get some camber gain with the strut suspension.

I made some strut housings that give me about 3/4" more clearance to the wheel. It will effect your wheel rate though. Two things are happening, you are moving the spring to a more upright position(increases effectiveness) BUT, you are moving it in at the same time which decreases it effectiveness.

jdmsuper7

iTrader: (1)

You would also be increasing the side loading on the strut, which is never a positive.

Irentat

iTrader: (1)

What is "wheel rate"? Is it the spring you are saying is losing and gaining effectiveness?

Thanks much for someone who has done this.

In the end, what did it do performance-wise for you (not counting the clearance increase).

Ben

jgrewe

iTrader: (1)

Wheel rate is the final spring effectiveness the wheel sees. Think of the lower control arm as a lever working against the spring. If the attaching point is near the end you don't get any mechanical advantage on the spring. (It might be easier to think of the whole strut and arm assembly upside down, and you are pushing down against the spring from where the spindle is.) If you move the spot the spring is attached to the control arm(or the imaginary point it would intersect in the case of a strut) almost to the inside mounting point of the arm you will have a lot of mechanical advantage on it from the spindle end.

That mechanical relationship(motion ratio) is very close to 1:1 on a strut for our purposes here. So a 400lb/in spring will get you very close to a 400lb/in wheel rate. Now when you move the spring attaching point in you get a huge change in wheel rate with the same spring because of the formula involved. Play with this little program to see what happens to the wheel rate numbers when little changes in motion ratio are made (motion ratio is how much the spring moves compared to how much the wheel moves) If you move the wheel 1 inch and the spring moves .75 inch you have a .75 motion ratio.

http://www.bmw2002faq.com/content/view/43/32/

It takes a lot of very precise measuements to get all the info you need to choose the right springs when you start changing stuff from stock but its worth it. All by itself the mod is hard to identify as any great change for anything but wheel room. I don't have a catalog part on my car anywhere so I really can't isolate the strut housings, they are just a link in the suspension chain.

Irentat

iTrader: (1)

Thank you as I now understand wheel rate and the mechanical advantage changing the spring location will have.

I am fully intending to change springs at a later date so that is a non issue...other than calculating correctly.

My main focus in starting the discussion was to find a way of decreasing the amount of positive camber that comes from our front supsension design in cornering. Knowing this, can you comment on how your change impacted this specific item for you?

Thank you,
Ben

jgrewe

iTrader: (1)

Camber is a whole different can of worms. There is only a few 10ths of a degree change from the suspension itself. It comes from the lower arm swinging and becoming effectively longer or shorter as it travels through the place where a line drawn through the inner pivot point and the ball joint is parallel to the ground. So, in the range it travels you aren't getting much but every 10th of a degree matters on the track.

You get the positive camber gain from the body itself leaning and if your lower arms are already passed the point of being parallel if your car is lowered(not the arms themselves though, the line from the inner pivot to the ball joint).

You can get some negative camber to show up but it comes from higher castor numbers. Picture the strut from the top with it leaning back a few degrees and the wheel straight. Now as you turn the strut the wheel will actually lean IN as the spindle travels around the axis the strut is turning on. So, you thnk, great lets crank a bunch of castor in this thing and get a bunch of negative camber when I turn a little. The problem with that is something called kingpin inclination and it can't be changed very much on a strut car. We can get a little change if we have camber plates because we can change the camber at the top and at the spindle. Do the opposite adjustments at each end and you will effect the kingpin inclination. Still there is only so much you can get and the amount of castor in relation to kingpin has a small range that they work nicely together.

I mentioned the other issue of lifting the outside wheel in post #7 and it gets worse with more castor.

So, the short easy answer about positve camber gain is to limit how much the car rolls and be sure its not too low. A lowered strut car has roll center issues as well, the roll center drops faster than the center of gravity so you end up with a car that wants to roll more than a stock one and they usually end up on the bump stops which will kill your grip.