camber and offset

[ramong24]

I recently got some BBS RSs and the rear offset is 43, and at first I rubbed pretty bad, so I added 5mm spacers but I still rub a bit, but not as bad. I have a decent amount of neg camber (~6 deg).

Hypothetically, if I correct the camber to almost 0 (or as close as i can get with the camber kit) will I be able to run the higher offset and not rub on the suspension?

My logic is with the camber kit, it will push the upper control arm or whatever away from the car, thus pushing the top of the wheel away, allowing more space for it to travel when hitting a bump.

Please help!! I need to know if I should to invest in a larger spacer or do the camber kit and I would rather not spend $400 on 2 new barrels.

[Oh Damn, it's Sam]

You can do some easy trigonometry to achieve a decent approximation of what you're trying to figure out. It won't be exact, since to get an exact answer we would need to know a few precise measurements off your suspension system, like where exactly your suspension pivots in terms of camber, and so forth. But, for most suspension systems, the following will get you close:

First, we need to know whether it's the wheel or the tire we're interested in here, and that just depends on what's rubbing. For the sake of argument, let's say it's the tire. To do our calculations, we need the radius of the tire. For convenience's sake, lets say you're running 205-40-18s, just to pull a tire size off the top of my head. Just substitute your numbers for what I have in bold:

205 * 0.40 + 18 * 12.7 = 310.6mm

Now that we know the radius, we can do just a bit of trig to see how much moving from 0º of camber to -6º pushes the top of the tire inward. Imagine a right triangle, where one side is the axle centerline, one extends vertically downward from the innermost extremity of the tire, and one connects that same innermost extremity of the tire to the point where the plane of the tire sidewall meets the axle centerline. It would look something like this:



Now we know a few things about this triangle. It's a right triangle, of course, so we know that the angle between the axle centerline side and the vertical side is exactly 90º. We also know the other two angles: one is 6º, which means the other must be 84º. We also know the length of the angled side: 310.6mm. Let's lay those numbers on the diagram to see how they fit.



Notice the X on the chart? That's the number we're interested in. If we solve for the length of that side, we'll know how much the top of the tire is pulled inward from the 6º of camber. Turns out, we have exactly enough information to figure that out. First make bloody sure your calculator is in degree mode. If it's in radians, you're going to have bad day. Let's calculate:

Sin (6º) * 310.6mm = 32.5mm

So we now know that correcting from -6º of camber to 0º will give you an additional 32.5mm of space.

Again, this is an approximation based on a bunch of assumptions about the exact layout of your suspension (since I don't know what kind of car you have) and so forth. We could get closer to exact with a handful of measurements of the suspension. These calculations are exact should your suspension's pivot in terms of camber be exactly coplanar with the inside sidewall of the tire. If it pivots inboard (relative to the car) of the sidewall, the number in reality will be somewhat larger than we calculated. If it pivots outboard of the inner sidewall (that is, under the middle of the tire), the real number will be somewhat less than we calculated. If you can provide the position of the camber pivot point relative to the inner sidewall plane, I can get you a highly precise number, but absent that information, this should get you close.

EDIT: To put it into a nice, single calculation, try:

(tire width * tire aspect ratio * .01 + wheel diameter * 12.7) * sin (camber)

[ramong24]

Wow. That's helpful! I did the calculation based on my tire size (215/45r17) and it was about the same. Buuuut actually my wheel is hitting. The tire is stretched on a 9" wheel. It is 17x9 et 40. So a 7.5" barrel. The car is a Mitsu Galant on coilovers. Does any of that help to do any other calculations?

[Oh Damn, it's Sam]

If it's the wheel rubbing, substitute the radius of your wheel for the radius of the tire in the calculation above. A 17" wheel has about a 225-240mm radius, but measure to be sure.

If you have a known radius you're working with (i.e. you're not calculating from a tire size), this is the equation:

radius * sin (camber)

We can make some finer adjustments to the calculations and make it more accurate if we know where the suspension's camber pivot is in relation to the wheel. We would need to know it's position along both the vehicle's width and height axes relative to any specific point of the wheel - preferably the center point of the backpad. If you can provide that, I can refine the equation.

[ramong24]

I think I'm confused about where the "suspension camber pivot". Is it basically at the bottom of the coilover, where the bottom mount would move resulting in taking away the camber? And from there, it depends if that point is within the barrel of the wheel, or outside the barrel?

[Oh Damn, it's Sam]

The pivot point we're interested in is the point on your suspension where the rest of the components would rotate around in order to gain or lose camber. Depending on the configuration of your suspension, this could be a lower bolt on a strut, a bushing, or a balljoint.

For example, on my front suspension, I have a semi-MacPherson strut setup with independent spring. The pivot would be the ball joint at the control arm / hub interface. If I were to adjust the camber by sliding the strut in my (non-existent) camber plates, the hub assembly (and thus the wheel) would rotate around this point.

In the rear, I have a 5-link. Here, were I to adjust my camber using an adjustable camber arm, the hub and wheel would rotate around a particular bushing on the spring arm. That would be the camber pivot point for the rear suspension.

Depending on how that point is located relative to the wheel, you may see more or less space generated by adjusting camber in the calculations above. To see why this matters, imagine holding a 2x4 vertically in your hand. Stretch your arm out, and then move your arm upward until the 2x4 leans back toward you at a 5º angle. The end of the 2x4 moved quite a bit, both toward you and upwards, thanks to the long lever (your outstretched arm) you've placed it on. This is the equivalent of having the suspension pivot point quite far from the wheel. Now imagine you pull your arm inward so it's only your wrist that is tilting the 2x4. Perform the same adjustment to angle, and you'll notice the end of the 2x4 doesn't move nearly as much, barely upward and inward by comparison. This is the equivalent of having a suspension pivot point that's quite close to the wheel.

Whatever your exact suspension design, what we would be interested in knowing is how many mm inboard the suspension pivot is relative to your wheel's backpad, and how many mm down from the axle centerline. That, combined with the wheel specs you've provided, would give us enough information to be very precise.