|
If you want to know what you need, you need to take the measurements and plot some points. Once you know where you stand, you can determine where you want the roll center height to be and figure out how much you need to adjust to achieve, or at least get close to that height.
There are many reasons to correct the geometry. You seem to be concerned mainly with the camber change aspect of it. That being said, any improvement is going to be beneficial here. That being said, you could go ahead and drop the ball joint as much as your setup will allow, for whatever wheel setup you choose.
Now a larger issue here is the roll center height itself. Placing the roll center at an optimum height would be a better way of determining the amount the ball joint needs to drop. There is not some set spot that you need to set it at. It depends on your setup and your own driving preference. In fact, you may never get the geometry perfect, however that doesn't mean you can't get it close.
If you are unsure of what you need to do, you can follow Kojima's setup. I never got a specific measurement from him, but from what I can tell. it only appears he dropped the ball joint pivot one inch from the stock location. I know the pictures make it appear the pivot is way lower, but it really looks to only be about an inch lower. This is the same setup he uses on the Dog Car II and III and his B14 street car (which all sit at different ride heights). His setup seemed to be dictated by the limitations of the hardware (meaning he could only drop the pivot as much as the mono ball pin would allow) and he was trying to achieve as close to zero bumpsteer as possible. Where exactly the roll center height ended up in all this, I'm not sure. But I know Kojima has said two inches above ground is a good spot.
Anyways, anything you can do will probably help. You could drop the pivot a mere 1/4 inch and it would still be better then stock. Like I said, you may never get it perfect, but anything you do achieve will most likely be better then stock.
Put it this way: Lets look at it piece by piece.
In regards to the camber curve, depending on your ride height, you could be either + or - 90°. Regardless, it is safe to say that any improvement (meaning any amount you drop the ball joint pivot) will net some improvement. Suppose in the VERY unlikely situation you drop the pivot so much that you have too much camber gain, you would simply run less static camber. What I'm trying to say is, I wouldn't worry so much about the camber curve as it will only get better.
In regards to Roll Center, one thing is for sure, just as we don't want the camber curve to get any worse, we don't want the roll center to drop even more. More then likely any adjustment in the range you have to work with will net an improvement. At a set ride height, any amount you drop the ball joint pivot, the roll center will get higher. If you were only concerned with the roll center, the only danger you face is lowering the pivot too much and bringing the roll center up too high, which is very unlikely, but something to consider none the less.
Now, something you seem to not mention much is the issue of bumpsteer. I'm not sure what you plans are here, but you should be looking into bumpsteer correction before you do any of the roll center stuff. Even with the stock control arm and ball joint there is a serious need for bumpsteer correction. If you lower the ball joint without changing the steering geometry, you will have unbelievable amounts of bumpsteer. You should REALLY consider doing the bumpsteer correction before anything else. The bumpsteer is really going to dictate a lot. With the stock control arm position, you want to be able to drop the rod end pivot as much as possible while still leaving the rod end above the spindle. Ideally, you would want the rod end pivot even lower but, you can only go so low, and mounting the steering rod end under the spindle is too far. However, when you drop the ball joint pivot and the angle or the control arm changes, you will need to drop the steering rod end pivot lower as well. Here is where bumpsteer correction dictates the roll center adjustments. You need to make sure you are dropping the ball joint pivot far enough so that mounting the steering below the spindle isn't too low (if that makes sense). Say you only drop the ball joint pivot a 1/2 inch, the sweet spot to reduce bumpsteer may be unobtainable due to the the fact that we can only mount the steering arm above or below the spindle, leaving a range of adjustment we can't use.
So yeah, that is all my tired brain can think of at the moment. I hope that helps.
I am working on a similar but different solution, so I will post things as I figure out more.
Last edited by sr20speed; 04-10-2009 at 05:11 AM.
|
