Smike

iTrader: (8)

I'm a little verklempt. Talk amongst yourselves. I'll give you a topic. The rake is neither linear or lateral. Discuss.

In all seriousness. Lets start some protocol for measuring observed rake on different setups. Make this an on-topic comparative comprehensive post.


Post suspension bits, measurement findings, and you belief system. Yes, belief system. There is more than one right answer. And what one driver can do on one setup to another.

My simplified mentality behind rake.

Two schools of thought on rake - level and slight forward bias (rake). The level school of though is that you adjust the rotation by increase front grip via stiffer rear sway bar or high spring rates. The rake school is shift some of the weight transfer forward to act as a pivot point and not adjusting the rear rate as high.

My experiences:
On my Tracks, I ran a level platform. Well, actually, the car still does. My friend is running my exact settings on his car with my old Tracks. But, when I was doing that setup I was reading up on the current Japanese TT cars. They talked about a level rake. So I measured from the welds next to the side skirt about 2-3 inches in from the wheel, or before the wheel for the rears. Then went about leveling the car via that method. I did not swap the spring rates around so the 12k were in the rear. Never worked right for AXing (pushed), but on the track, they were ok. My issue was the daily driving on them - painful.

Megan Tracks Out and In Car:
http://www.youtube.com/watch?v=EnIUYzyaqvc
http://www.youtube.com/watch?v=Q5NVueRu9y0

Currently, I now have the rake school of thought. To be honest, the car swings around when I want it to. And really, honest, its fun! Its like being in RWD again. On the AX I can throttle rotate and on the track I can control the back end.

Here is a outside of car from WW4. Its a scratched run, I went cone hunting, but watch the first turn (that black Evo that goes 2 cars later is actually the car with my old setup - he is new to AXing but learning).

http://www.youtube.com/watch?v=TCgO0HA2p6c

In car same race:
http://www.youtube.com/watch?v=MdHLQr2Y1ys

Here is the first AX of last season (first run on with everything on the car):
http://www.youtube.com/watch?v=EwW8i3JqxjQ

You can see the drift/rotation moments. I was not fighting the car to move.

nightwalker

so you like rake? about how much are you running. My eyes got a little blurry trying to sort out your post.

Smike

iTrader: (8)

I used paragraphs - just a lot of words

I am on Swift Spec R's. The difference is slight 0.6 f/r. I will measure it at the points I mentioned today to see if it is the same as the math (front height - rear height).

http://i85.photobucket.com/albums/k6...o/DSCF0578.jpg

GTLocke13

iTrader: (1)

If you run the numbers, changing the rake of the car has effectively zero effect on weight distribution. I was trying to point this out in the other thread that links to here. Here's how the math works:

The Evo IX has a wheelbase of 103.3 inches. Using this, and assuming 1" of rake (i.e. 1" lower at the front suspension than at the rear) we can calculate how much it rotates the car forward:

arctan (1/103.3) = 0.554 degrees

Now assume that the roof is a 100 lb point mass on the centerline of the car at a height of 57 inches. By rotating the car forward 0.554 degrees, we move this point mass forward by:

57 * tan 0.554 = 0.551 inches

We'll simplify this to 1/2". This move changes the weight distribution by:

103.3/2 = 51.65 <--- distance from center to front wheels
.5/51.65 = .00968 <--- fraction of that distance that is 1/2"

Since if the weight were directly over the front wheels (i.e. 100% distance) the front would see a change of +50 lbs and the rear would see a change of -50 lbs, you can calculate the change in weight by multiplying your % change in distance by your weight:

0.00968 * 100 lbs = 0.96 lbs

So by raking your car 1" you've managed to transfer 1 lb of weight from the rear wheels to the front wheels.

So if it doesn't transfer weight front to rear (or the other way), how does it have an effect on the car's handling?

It works by changing the suspension geometry and weight transfer in a turn. With the car raked, the front tires are deeper into their camber curve (what little we have in a McStrut car) and they will add camber more quickly than the rears under compression. It also changes the roll centers and the CG of the front vs. the rear. The higher CG in the rear should result in more roll force. This will increase the amount of weight transfer on the rear tires when the car starts turning. More weight transfer = less grip.

Smike

iTrader: (8)

Excellent information - exactly why this thread was started.

So I am correct in what I am physically feeling in the car. AX: brake (shift weight forward, enter a turn, pivot on weight, make turn, exit.

GTWORX.com

iTrader: (35)

Just like you can't really affect the front to rear weight distribution by any real amount when corner balancing, rake is not used for shifting weight distribution.

Rake can be used to change the balance of the car by altering the roll axis, the line between the front AND rear roll centers. True "level" rake is not measured by the wheel gaps or anything like that, it's the roll axis. The roll axis is simply the axis the car rolls around, with the roll couple being the a function of the distance from the COG and the roll center. The larger the roll couple, the more roll. The roll couple is a moment arm....for a given cornering force, the farther the COG from the roll center, the larger the moment.

Start with a level roll axis, and lower the front of the car, and you usually end up with a greater roll couple up front --> a shift towards oversteer.

Balance and grip are easier to tune with good spring, swaybar, and damper rates as well as a dialed in alignment. And you need to do this anyway first. If you have coilovers, it's a fun thing to play around with and you can find a nice solution (but remember to check alignment after every ride height change). It's nice to plot the roll centers and use that to determine your rake, but always remember what you're doing to other parts of the car with certain set-ups.

I should also say that ALL of this depends on a variety of other factors, and may not work in all cases. There...the disclaimer.

Hope I didn't miss anything!

- Andrew

Smike

iTrader: (8)

Is there a simple way to measure the roll center on the car? My world of fun, might sound weird, but calculating that would be useful for those who wish to modify it (feeling like James May here).

Obviously, car setup and driver preference play a huge part here.

GTWORX.com

iTrader: (35)

Definitely! As good as all the physics and theory is, making the driver comfortable with the car is HUGE.

Lets see how good my MSPaint skillz are....standby.

- drew

Bster13

iTrader: (2)

http://www.teamassociated.com/racerh..._handling.html

Go to chapter 4.

Brian_08

Ummm im lost. Andrew in that penske manual you sent me i was reading where if the dampers are set almost equal (lets say zeroed out for now) that in a car as front biased as ours are that the cars natural understeer tendencies would as well be almost zero, BUT i may have misread. and wouldn't a person with coilovers that has a lower roll axis in the front naturally harden up the front dampers for this reason or is this something that is normally over looked except by diehards and suspension nerds? lol no offense as i am one well learning. Or are my statements totally off the mark and should i shut up now? lol

GTWORX.com

iTrader: (35)

EDIT: accidentally deleted my MSPaint roll center diagram. crap!

MSPaint to the rescue!

Draw a line perpendicular to the strut from the top.

Draw a line concurrent with the front LCA and extend it to intersect with the first line.

This intersection point is called the instantaneous center.

Draw a line from the instantaneous center to the contact patch. Where the line intersects the centerline is the roll center.

So you can see how lowering the car changes the angle of the front lower control arm and results in a lower front roll center. For the EVO, the rate of change of the roll center height is faster then the change in COG.


- drew

GTLocke13

iTrader: (1)

You're close, but keep in mind that each thing you do to a car's suspension has its own distinct effects. Damping adjustment is completely separate from ride height and spring stiffness. Dampers do absolutely nothing when you are looking at a static snapshot of a car. Static behavior (i.e. skidpad) has to be tuned with a combination of ride height, spring rate, sway bars, and tire pressure. Dampers come into play when you start adding in dynamics. They affect how quickly weight transfers from side to side, and how the car's body moves with respect to the wheels.

Having a low ride height doesn't necessarily mean you have to crank your dampers up. On a well designed suspension, the spring alone should be enough to keep the suspension from bottoming on a smooth surface. Once you add in bumps, you need the shock to help out, but that doesn't mean that it needs to be cranked way up.

Brian_08

thanks, and yes i got all that much. understood, But what i was saying is that with static behavior out of the way and acutally dealing with the dynamic behaviors of our cars wouldn't tuning your dampers (after the car has been lowered spring rates chosen etc) and setting the car up that way have an effect on the roll axis or again is that something that normally goes ignored for the most part? i e. when someone goes to get they're suspension tuned and such when they are getting it done they usually aren't too concerned with getting the roll axis maximized for performance as well. or again am i completely off base. i am trying to learn all i can as i am trying to be able to set up my own car and not have to pay premium to have my suspension set-up until i get into the big leagues if i ever get there that is lol.

GTWORX.com

iTrader: (35)

^ Right. Bringing in too many variables.

Remember on a smooth track in a steady state turn, dampers do not control the balance of the car. They can affect entry and exit and how the weight transfers, as GTLocke said. But you shouldn't turn them up to compensate for other things in my opinion.

- Andrew

GTWORX.com

iTrader: (35)

The dampers should be matched to the springs....that's really it for them. Tweak them a little here and there to get a better feel for entry and exit, but as far as I know they aren't really brought into the equation when you look at rake.

- Andrew