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.

Why?

Also, don't get me wrong. I have .25 rake on mine and my car works, like really really works.

Sorry, the pinch weld is where the panels are joined on the chassis behind the front quarter?

Dann

On a normal road car, they are all set up with the front roll center slightly lower than the rear roll center. Reason it is this way has to do with how the wheels are loaded under driving conditions. (maybe MR cars are different?.?.? IDK, but probably not) Its a little difficult to explain. In general, the higher your center of gravity is the higher your roll center needs to be. On good suspension designs, as you compress your suspension your roll center raises. The closer the roll center gets to your center of gravity the less the car will roll. (btw, as you drive the roll center shifts all over the place and is a force center and not a fixed point...)

With the rear roll center too low the car will just over steer over the place, with it too high it will be mad tail happy... With the rear slightly above the front you get a happy neutral.h

Yep. Its the ridge that you attach your factory jack to.

In that sentence is "roll center axis" referring to the same thing as "roll axis"? Roll axis as I understand is just straight line drawn between the roll centers, so how could it be out of sync with respect to the roll centers? It's obvious that having the roll axis slope down to the front some is helpful for a miata, but has anyone sat and figured out why, other than testing?

Level roll centers means the rear wants to track the front end perfectly. This is a car that pushes, though, since the car has momentum in a straight line. In order to actually make it turn and rotate, you need to have the rear roll center higher than the front.

I was quoting someone else. I assume he was referring to the original roll axis compared with the new after height adjustment, but I don't know.

You are correct that the roll center axis is just an imaginary line connecting the front and rear roll centers. To me though, the its not very useful information other than visualization the relationship between the front and rear. Even though front and rear suspension movement effects each other, I prefer the think of them as functioning separate unless I am "doing the math" to keep from going crazy (too much to think about all at once. I also prefer to think of each corner as separate islands of suspension, and I like to forget that the chassis likes to flex..:vash:.ect lol) I like to break it down Barney style as much as possible when I am conceptualizing suspension dynamics in my head...lol

Yes, mostly correct (%100 percent correct if the rear suspension was exactly like the front -camber curve, mounting points, arm angles,..) Even on front wheel drives the rear roll center is slightly higher than the front to keep handling balanced, but I suspect you are right [RWD more than FWD.]

Can you even drive? Or are you trying to build the perfect track car with ride heights set to the "t" before actually going out and tracking it?

This thread is so silly.

Hater! No actually I'm only 13. I like the Miata because it looks cool and its small like my penis. Whoops, ball just dropped....whoops, there goes the other one.

<\pissing contest> You win :fawk:

Jerk.

It's a ROLL center, not a "role" center.

FFS...

rofl

To the OP:

To answer the original question, I would setup the car under the conditions it will be driven. That means setting the ride heights (and setting 'rake') with your weight in the car. That also means having an average amount of fuel in the tank (ie: 3/8 to 1/2 tank).

Go out and get the book tune to win by Carroll smith and read. /thread.

Have a great day,
Jared

On page 54.

"The front roll center will always be lower than the rear. If it is too much lower, we will have a car that does not enter corners well and which exits corner on three wheels. The big trick here is to keep the front and rear roll center movements approximately equal to each other--and in the same direction--as the car does its various things while negotiating a corner."

Thank god this thread came around. I've been wondering why my car seems to want to track straight on turn in - I can work with it, and have altered my technique accordingly, but even watching videos online I could tell other's miatas had much more rotation capability...

All my tools and tent, etc. are still in my trunk from Chuckwalla so I'm going to unload the car tomorrow and then measure... it's probably all wrong.

Whoops :facepalm: I don't know why I keep doing that. Gotta quit with the Drunk typing. lol Edited some of the mistakes out... (I will be playing the role of "that guy" :slap: who haz them grammar issues)

Makes me happy. That book has been on my want list for a little while.

So yeahhhhh... my front is 1/8" higher than the rear. My pinch welds are pretty well molested, so it's hard to be super accurate, but adding 1/4" has got to help

Welp, it guaranteed that this thread helped someone. Methinks there will be an "OMG, my car handles GREAT" post coming soon.

^ LoL



I read something last night on AutoSpeed.com that was interesting:

MX-5/Miata Magnificence - Part Two
Part Two of our look at one of the most successful sportscars in modern motoring...
By Michael Knowling

"[2nd Generation Miata] Suspension - while similar in layout to the original - was heavily revised. Several pick-up points were relocated - caster was increased by two degrees and front roll-centre lowered - and the springs and dampers were revised to suit (including having more travel). Interestingly, concerns of steering kickback caused by increased castor were addressed by re-bushing the steering rack - it was made relatively soft in vertical compliance to reduce transmitted shock."

I didn't realize that the front roll-center had been lowered for the second gen. Very interesting. That means that you could possible get away with less rake on a NB. Wonder how much the front roll center was lowered. Time to start Googling...

This, god dammit.
I'm going to measure my ride height today, my car feels shittier than it used to.

I know its been said but its worth mentioning again, DON'T measure from the fender lip like I did. :facepalm: