Everything You Ever Wanted to Know About Rake

[Brian_08]

ok so yea i was WAY off base there. well back to reading. he he i love this man, read something and then come here and get a better evaluation of what i read and a better in-depth analysis of what i am learning. WOO HOO, i feel smart. well kind-of. Thanks guys, lets keep this goin i want to learn more lol : puts nerd glasses on:

[theshadow]

Hmm so I'm curious...if rake/ride height affect transient behaviour through roll centres and damping affecys transient behaviour through weight transfer then how do the two interact?

Basically I guess I'm wondering whether my assumption that lower cars should roll less, and exhibit smoother transitional behaviour, at a given spring and damping rate vs. a higher car, is true or not.

If a lower roll centre or a lower roll axis would mean that the car would exert a lower moment of force around the pivot point, would it be possible to get away with softer damping settings to acheive the same rate of roll or weight tranfer than a higher car? Or with rake i.e. different roll centres on each end of the car, would you have to adjust the dampers differently on each end than you would with a zero rake setup if you wanted to acheive the same transitional behaviour?

I'm also a little comfused about rie height and damper stiffness...on a higher car, if the higher car's centre of mass is higher vs. its roll centre vs. a lower car, then when the car is pitching,during a change of direction, if you look at the difference between the roll centre and the car's centre of gravity as being a fulcrum them wouldn't the tipping force be higher (and rate of tipping be slower) than with a lower car?

I've observed since lowering the rear of my car that the breakaway characteristics of the rear are much more progressive, so just wondering whether the above theory might explain that. Also curious how I might be able to adjust my dampers softer in terms of both bump and rebound (which are quite stiff in the rear) to further decrease understeer.

[Bster13]

If a lower roll centre or a lower roll axis would mean that the car would exert a lower moment of force around the pivot point, would it be possible to get away with softer damping settings to acheive the same rate of roll or weight tranfer than a higher car? Or with rake i.e. different roll centres on each end of the car, would you have to adjust the dampers differently on each end than you would with a zero rake setup if you wanted to acheive the same transitional behaviour?

Ok I am an extreme novice, here, so take my advice w/ a grain of salt, but from reading this:

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

...your roll axis ~is~ your pivot point when talking about lateral forces. Whether I raise or lower your roll center, it all is in reference to the car's center of gravity. The bigger the difference between the Cg and Roll Axis, the more force is being transferred side to side as your corner. If you minimize the distance between the two, you could then run lower spring rates and dampening because there isn't as big a lever arm (the distance between the Cg and Roll Axis) acting side to side. If you rake the car (or anti-rake it ), then yes your Cg is going to be closer or farther to one of the roll centers, and thus a short or longer lever arm to throw the car's Cg at. So u would need bigger springs and more dampening the farther away the car's Cg is compared to the front or rear Roll Centre.

I'm also a little comfused about rie height and damper stiffness...on a higher car, if the higher car's centre of mass is higher vs. its roll centre vs. a lower car, then when the car is pitching,during a change of direction, if you look at the difference between the roll centre and the car's centre of gravity as being a fulcrum them wouldn't the tipping force be higher (and rate of tipping be slower) than with a lower car?

If you had a higher car with a center of gravity farther from the car's roll axis vs. an identical car with a center of gravity lower (and closer) to the car's roll axis, then the 'higher' car would pitch/roll more than the lower car. You multiple the Mass (center of gravity) times the Distance (between the center of gravity), the bigger the # the bigger the lever arm cranking to pitch the car when cornering about the Roll Axis (entire car) or Roll Centre (front or rear of the car). The "tipping force" as you call it would be higher, yes, but given the same springs ans dampers in each car, the "rate of tipping" would be ~faster~, not slower.

I don't know how to explain your situation with lowered rear....I'm just learning and I'm sure someone more knowledgeable will come along and correct both of us shortly.

[theshadow]

Yep I think we're thinking along the same line of thought...and i actually do understand what youre saying

I'm busy trying to calculate where my rear roll centre might be at the moment (as well as the front) so I can try some informed damper adjustments now that rake is out of the way.

With the car lowered in the back as it is (LESS RAKE setup), the rear lower suspension arms are lower in the middle than the outside so they point UP from the centre...which would seem to indicate that by lowering my car I've made the roll centre higher, and not lower ???

Instead of condusing people, let me just put it this way - car is lowered and set up "anti raked" so that my inner chassis weld clearance is about 6.5 inches in front and 7 inches in the rear...which means that the rear is really really low (almost zero rake).

The car handles VERY neutral, responds very well in sharp transitions and is very progressive on the limit - extremely agile and easy to drive fast right now. Just trying to figure out how to fine tune in a little more front grip. This is probably my best all around track/autoX setup so far in terms of handling (note that it's a combined setup). I've tried TONS of stuf)...but always looking for that little extra.

I think it's very near where it should be, just need to fine tune my dampers now...a little more front grip would really put it over the top!

Current setup:

Low rake setup (6.5 inches weld clearance in front, 7 in rear)

-3 camber front, -1.3 camber rear, 0 toe all around.

KW V3s with 9k springs all around - front bump set at 1 turn open, rear bump 0.75 turns open, front bound 1.25 turns open, rear bound 1 turn open.

The Team Associated link was excellent BTW (I used to race R/C cars) - only thing is they don't mention dampers!

[RT]

ot

[killthelights14]

awesome thread. thanks for posting guys!

[theshadow]

Regarding rake, I can sum up my observations thus:

High Rake Setup (nose down):

- More "binary" handling - wants to understeer OR oversteer, especially when pushing hard
- More unstable mid corner, especially in high speed corners
- More power oversteer on corner exit
- More abrupt breakaway characteristics

Overall, this setup seems to favour a point, lift, rotate and shoot driving style but you need to be on your game, ready to catch the car, as this setting seems to be more sensitive....car seems to want to oversteer or understeer, not settle down and balance in between. Doesn't seem to work as well with high speed, steady state cornering (either wants to oversteer or understeer, more twitchy, requires more corrections which scrub speed. Possibly good for really tight, high grip tracks though if you can harness the increased tendency to wards power oversteer coming out of corners to good effect.

Low Rake Setup (more level):

- Generally more settled in higher speed corners
- More neutral/controllable on high power corner exits
- Better mid corner balance
- More progressive breakaway characteristics

Overall, this setup seems to favour a more fluid "textbook" circuit-racing driving style i.e. proper line, progressive throttle, slow in fast out. I find I can press harder with this setup since the car doesn't want to bite; settles more easily into a neutral cornering attitude with good adjustability. A looser, point and shoot driving style can work as well though if the situation merits it since you can provoke the tail to come out more aggressively and catch it easier when it does...just chuck the car into the corner, modulate the throttle slightly - "set it and forget it". When pressing hard the car is more forgiving, warns the driver of imminent breakaway sooner which allows smaller corrections and more confidence, more throttle to be applied, more cornering speed to be carried and less scubbing off of speed due to the fact that you don't have to do as many little mid corner corrections.

Of course my camber and damper settings etc. play a part but my setup is pretty conventional for competition/circuit guys: -3 camber up front, -1.3 rear, 0 toe all around.

[SmikeEvo]

Thanks RT!

Umm, info, drool...

[Bster13]

Enough of the guesswork...I gots me some readin' ta do!

[killthelights14]

mmmmmmm those books look good, i might have to go pick those up.

[GTLocke13]

Just to add something since we're talking about roll center. There are many misapprehensions about what a roll center is and what it does. It reality it is an imaginary point that allows us to simplify the math for forces in the suspension arms at a given chassis location. This means that as the chassis starts to move vs the suspension (i.e. pitch/roll) the roll center moves. This means that using it to predict any kind of dynamic behavior is relatively useless. One thing that you can look at is roll center migration - calculate roll center at various chassis displacements and see how far it moves: this will give you a vague idea of whether your suspension geometry is conducive to smooth handling. Of course unless you have some better software than I've run across, figuring out what your suspension is doing under various combinations of roll and pitch and then optimizing it is beyond anything I would want to tackle.

Again, this doesn't take into account how your springs and shocks interact with the suspension, it only looks at chassis movement and how smooth that will be given smooth force inputs (i.e. how linear is it?). It also doesn't take into account camber curves, steering axis inclination, and all the other geometry that is important in keeping your tires in their happy place. (See here for a description of a tire's "happy place".)

What springs and shocks do is manage weight transfer. They change the speed at which load is moved from tire to tire. We generally simplify things to side-to-side analysis in turns and front-to-back analysis for acceleration/braking and don't include combinations of the two. Again, I'm sure F1 teams with budgets of $100M or so have access to that software; I don't.

Now, when you start to turn in, the car will begin to roll and also begin to transfer weight from the inside tires to the outside tires. The shock and spring combination (which, with the mass of the body is a complex version of a simple spring/mass/damper system that everyone studies in physics) should allow the body to roll smoothly to its steady-state position. Too much spring and shock and you'll "shock" the tire, causing a spike in the force on it, which means the tire will slide. Too little spring and shock and the body will move too quickly, overshoot it's steady state position, and start wobbling. This also generally takes a tire out of its happy place.

[RT]

ot

[GTWORX.com]

Originally Posted by Bster13

Enough of the guesswork...I gots me some readin' ta do!

Have fun!

How to Make Your Car Handle is a bit old but a great starting book....not too heavy.

- Andrew

[RT]

ot

[Bster13]

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