RF shock.

[Bubstr]

Yes moving the upper shock mount in or out changes WHEEL rate (not spring rate) due to the shock angle change. But just moving it up and leaving the angle the same doesn't change the wheel rate at all.
Three things and three things only affect wheel rate on an a-arm suspension:
Spring rate (linearly)
Motion ratio (exponentially)
Shock/spring angle (a function of the cosine of the angle)

At the angles we're dealing with (around 20 degrees) an angle has a pretty small effect on the wheel rate.
For example:
400 pound spring with 14/19 motion ratio and a shock/spring at a 20 degree angle gives you a wheel rate of about 204 pounds/inch.
Changing the shock angle (using the upper mount) to 15 degrees or 25 degrees changes the wheel rate to 210 pounds or 197 pounds respectively.
But move the shock angle to 25 degrees by moving the lower shock mount out 2 inches and you're up to a 257 pound wheel rate.
Moral of the story is that motion ratio is far more critical to wheel rate than shock/spring angle.

The front shocks do not affect a cars ability to roll other than in the wheel rate they provide. The motion of the a-arm suspension is dictated by moment center only. The suspension does not "know" the angle of the shock.

Now rear suspension on a late model is a whole different story. The shocks/spring locations play a part in the roll center, the wheel rate, and the roll rate. But like I said...whole different story and I'm still trying to figure out the math on it.

Good post, So as long as you keep the shocks lower attachment point in the same position, Moving the top attaching point to change the angle, will change the wheel rate, but not in a linear curve, so many degrees for so many pounds. It starts off small and grows as the angle gets greater.

I take it, the angle we are talking about would be measured off the lower control arm? If so would the roll over of the chassis have any effect on this angle and would you do your calculations after the chassis took a set or on level?

I was taught that up to 15 degrees was not worth worrying about and the angle was figured off the lower control arm with a set in the chassis. When you go over that 15, it changes more quickly. But you have the math.

Ever experiment between changing wheel rate with this method and say wheel spacing? I realize, that's what they make adjustable shocks for, but I sometimes am not sold on increasing dampening to increase wheel rate and still getting contact compliance. Sorry mind is wandering on what if's.

[Matt49]

Good post, So as long as you keep the shocks lower attachment point in the same position, Moving the top attaching point to change the angle, will change the wheel rate, but not in a linear curve, so many degrees for so many pounds. It starts off small and grows as the angle gets greater.

I take it, the angle we are talking about would be measured off the lower control arm? If so would the roll over of the chassis have any effect on this angle and would you do your calculations after the chassis took a set or on level?

I was taught that up to 15 degrees was not worth worrying about and the angle was figured off the lower control arm with a set in the chassis. When you go over that 15, it changes more quickly. But you have the math.

Ever experiment between changing wheel rate with this method and say wheel spacing? I realize, that's what they make adjustable shocks for, but I sometimes am not sold on increasing dampening to increase wheel rate and still getting contact compliance. Sorry mind is wandering on what if's.

Correct in your first paragraph. Like you said, the angle change has a non-linear effect and doesn’t make much difference until you get pretty steep because we are dealing with a cosine curve that starts flat and is at maximum slope at 90; but it doesn’t take a straight line to get there. A lot of folks think that since a spring laid all the way over would end up providing no effective rate then a spring laid half way over must provide half the rate. But that’s just not the case. That’s where the trig comes in and in this case cosine of the angle is the correct calculation. Google “cosine table” for a good look. The math is sort of answered for you there in terms of where the change in angle starts to really change the cosine value. Or Google “cosine curve” for more of a graphical view.

The angle you measure is actually the angle of the shock relative to a line perpendicular to the ground where 0 degrees is straight up and down. Obviously if shock angle changes during travel, this would have an effect on the dynamic wheel rate. But wheel rate is relative to the ground which is why we’re measuring against the ground. The wheel doesn’t know that the chassis is moving and rolling. It only knows that it’s feeling some weight.

If you wanted to measure the shock angle relative to the ground (90 degrees being straight up and down) you could do that to but just use the sine of the angle in the wheel rate equation. Example the cosine of 70 degrees is equal to the sine of 20 degrees etc.
Wheel spacing doesn’t change wheel rate on an A-arm suspension. You can space the wheel out three feet and it still feels the rate felt at the lower ball joint. I will add a small but relatively unimportant caveat to this: Camber change produces a bit of a secondary motion ratio but it isn’t enough that it makes much difference at such small scrub radiuses.

I know this part doesn’t make sense at first but imagine the following scenario:

You’ve determined your wheel rate and you decide to put a big 10’ long cheater bar on the snout of the spindle to see what it feels like way out there. Assuming no camber gain, you’d feel the exact same wheel rate at the end of the cheater bar as you would at the lower ball joint. You aren’t generating any leverage against the lower ball joint with the cheater bar because the upper ball joint is pushing back against you and you’re moving in the same up and down line as the wheel would be just further away. There’s nothing to torque, you’re only moving something up and down.

Now introduce a ton of camber gain into this. Let’s say you generate 10 degrees of camber gain with 2 inches of ball joint travel. Your 10’ cheater bar now moved a little over 20 inches but your lower ball join only moved 2 inches! Obviously you’ve got less rate at the end of your cheater bar than at your ball joint. But this is an extreme example of camber gain and REALLY extreme example of offset. Bring these things back to normal racing parameters and they don’t have much of a net effect. But fun stuff to think about for sure :-)

[Matt49]

Sorry for the long-windedness on this stuff. But understanding wheel rates are very important when it comes to understanding why some cars like softer or stiffer springs on some corners. Everybody gets caught up in roll center (which is very important) and effective wheel rates don't seem to get discussed very much.

[Ghopper]

I think Matt49 has explained this subject well.

I only want to add that on an independent frontend you should package the coilovers for your necessary stroke and clearance from other parts. The final Wheel Rate, as mentioned spring rate measured from the wheel, can be achieve by adding whatever spring is necessary to meet your target.


Ghopper