Help, setting rake...
 

After lots of reading on the topic over the past few days, I have stumbled on something that I am having trouble wrapping my head around. :vash: adjust rake with driver ballast or without...


I want to set the front ride height at 4.5" front (Pinch weld) and the rear with +.25" rake to the front (pinch weld.) My plan is to disconnect the sway bars, ballads drivers seat, bounce each corner, role the car back and forth, measure the ride height....


Then comes the dilemma: When the car is driver ballast I should get the car level, then un-ballast and add .25" forward rake from the rear? Or, just dial in an extra .25" to the rear while it is ballasted?


I would have just done the former to begin with, but 949 says

"The Miata seems to work best with about .25" positive rake (rear higher) measured at the pinch welds without driver in car and about 1/4 tank. It seems the the roll center axis doesn't like to be too far out of sync with the roll centers."

I'm confused. I was expecting more of this:
http://www.gardeningoncloud9.com/wp-...en-rake-01.jpg

*

I corner weighted my car with me in it a while ago, and there's a .25" DR and ~.12" DF difference between me in it and not. So what the info on the 949 site is trying to approximate is the weight of the driver.

So, why don't we set the rake with the driver in the car? :ugh:

Either I didn't explain that properly, or you didn't comprehend it.

What I'm saying is that a ~200 lb weight in the driver's seat causes the DR to sag .25" and the DF to sag ~.12". So, when setting your ride height you can account for this by adding those amounts to the ride height when setting it yourself without ballast in the driver's seat. It's an approximation of corner weighting and should get you in the ballpark.

IOW:

If you were going for, for round numbers' sake, 12" all around, you'd set the corners to:

PR - 12"
PF - 12"
DR - 12.25"
DF - 12.12"

And when you sat in it, it'd be more like:

PR - 12"
PF - 12"
DR - 12"
DF - 12"

Is that better?

The question is still -why approximate the drivers weight (if that is the reason why 949 wants to measure .25" rake without the driver in the car) when you can just set the rake with the driver in the car? See what I am asking?

Why specify that the driver is in the car for setting overall ride height and the specify driver out of the car when talking about setting rake?

Say with the driver in the car, your set the ride height (using your number examples):

PR - 12"
PF - 12"
DR - 12"
DF - 12"

When you get out its:

PR - 12"
PF - 12"
DR - 12.25"
DF - 12.12"

Then you add .25" to the rear (per 949):

PR - 12.25"
PF - 12"
DR - 12.5"
DF - 12.12"

And when you sat back in it, you would think it would be: (even thought this is not exactly the case due to weight transfer)

PR - 12.25"
PF - 12"
DR - 12.25"
DF - 12"

^ So, if this were the case (which I suspect it wouldn't work exactly like that), why wouldn't you just adjust the rake with the driver in the car and save your self a heap of trouble?

What I don't understand is why 949 says to add the rake with the driver out of the car.

If you have a buddy to help, go for it. I thought I made it clear that it was an approximation/not ideal/YMMV...

You're talking as if "setting rake" is some quantum leap away from setting the ride height. It's just setting a slightly different ride height front and rear, that's all.

EDIT - 949 probably says to add rake with the driver out of the car because once you've already balanced the car to account for the driver's weight, you don't need to do it again if all you want to do is add height to the rear. You pretty well explained it to yourself in your own examples.

I just set my height 4.25/4.5 without me in the car and call it a day.

Car drives fine, is fast on the track and my tires wear evenly. that's all that matters.

I think we set our cars higher in the back, and they came like that, because our cars like to bottom out in the back. Have fun bro.

best post in the thread.

Thats true, but it also has a lot to do with how the car steers and grips in a turn.

Oh, and if you're trying to be so accurate at the fender lips... news to you, chances are they are all different anyways. So after all that time trying to dial in 1/8ths and 1/4s of an inch, they are still most likely .25-.5 out of what you think they are. The hub/fender measurement is the least accurate. Tis' why I only measure at the pinch welds.

Hope your floor is level, BTW - a 1/4" difference isn't much, but it will wreak havoc with the ride height measurements.

Measuring the fenders is a joke - measure at the pinch panels.

Meh. Ever since my car was corner-weighted, I just measure on the coilover sleeve itself. Was just using the fender measurement since that's what a lot of people use. Trying to give OP a "general idea".

What is the reason that you are wanting to have .25in rake? Are you just trying to follow what 949 says? Is your car not rotating enough?

Miatas don't really work unless you have .25" of rake at the pinch panels.

Why is that?

Last sentence of the first post.

Plus, I've tried flat, and the car won't turn in for shit. I've had customers echo the same comments. Lift the rear a quarter inch, and it goes from undriveable pig to perfection.

I am actually measuring at the pinch welds... was only referring to the fended measurement because it was the example used in the discussion. The pinch weld is more accurate I guess, but even then it is not perfect. Close enough for me. It would probably more more accurate to measure from a suspension pivot point on the sub frame.

Yep, level floor...I have it on boards that have been shimmed to my floor. (and somewhat secured in place)

For a car suspension to work properly, the rear role center needs to be higher than the front. Rather than calculating where the front role center is, then calculating where the rear role center is, and then doing the math to figure out what would be optimal (which I will probably end up doing in the future when I get really board someday) I am just going to go with 949's suggestions and call it a day.