Everything You Ever Wanted to Know About Rake
Jan 2, 2009, 10:48 AM
#16
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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:
Last edited by Brian_08; Jan 2, 2009 at 10:53 AM.
Jan 2, 2009, 06:09 PM
#17
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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.
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.
Last edited by theshadow; Jan 2, 2009 at 06:19 PM.
Jan 2, 2009, 09:12 PM
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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?
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?
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.
Jan 3, 2009, 06:17 AM
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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!
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!
Jan 3, 2009, 06:25 AM
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Mike, thanks for starting this thread, I think this is an area of set-up thats been overlooked and misunderstood (on the Evo) for far too long.
First off, agreed on the measurement location. The jack points are about 60" apart so those that wish can do the math to calc the degree of rake. And agreed, this has nothing to do with static weight distribution, its all about the dynamics of what takes place when everything starts moving.
It also has nothing to do with your shocks, your springs, your sway bars, nothing, nada, zip! This is actually something that needs to be address during the initial set-up of the chassis. In the case of the Evo, Mitsu already decided what to do based on the requirements of a street car, and for good reasons.
Mike, I've cut and paste my comments from the "How low to go" thread that member theShadow started -
"Lets start with chassis rake. I come from a long family history of open wheel dirt car racing, unlike todays racers that buy everything with a credit card we built our cars from the ground up. The only suspension pieces not made by hand were the shocks and the rod ends. The advantage was that we already knew the technical details of our chassis so it made ride height and those type of things much, much easier to deal with. The Evo was an unknown for us so like everyone else we just had to start somewhere and work with it.
Most the posts I see involving rake are attempts to improve cornering and while it will have an impact on cornering, chassis rake is really there to help you go straight! Since the Evo was set-up for the average Joe on the street it has a bunch of chassis rake. Like the inherant understeer, this is good for an average person, but not a race car driver.
Now of course we only had to go left, but its not as easy as it looks! For a winged midget on a short track, 1/8 mile or less, no rake. For a quater mile oval increase the rake to 1/4", for a one mile track, increase to 1/2". The reason we did this was because of our tire stagger, but even at that, doesn't take much to make a differance.
No long straights in autox, your almost always in some form of transition, I would start with 0 rake and if anything, lower the rear of the car, maybe up to a 1/2" or more. For a road course I would start at 1/2". If I was at a track with long straights that would be O.K. but if I was at a track that was mostly curves and short straights I would again lower the rear at least another 1/4".
For ride height I measure the bottom of the pinch seam weld just ahead and just behind the jack points. Right now I'm 6" in front, 7" in the rear, this gives me about 1 deg of rake which works well on the street. For tracking the car next year I'll take another 1/2" out of rear and start with 1/2 a deg of rake.
Only ballpark figures, but a place to start."
Nils mentioned roll center kits, so I made a change, shadow did too -
"Ha, that’s funny, I did the same thing last week, only I went the other way. I raised the car ½” and went back to a previous alignment set-up, the car actually feels better. Height at the bottom of the weld seam is now 6 ½” front, 7 ½” at the rear. I still want to lower the rear ½“, but I wanted to go back to a known set-up first and hopefully minimize the impact of lacking the roll center modification."
Both cars felt better after the changes -
" Shadow, looks good man, I like the stance more now than your first picture. In the first photo it looks more like a classic drag car, I’ll bet the rake was close to 1 ½” before your change. And change is good, especially on suspension. I think we’re in a good ride height zone for a street driven, track, autox type car.
So what did we impact with these changes? Basically we’ve helped the roll axis of the car by flattening it out. By doing more at the rear we’ve also changed the way weight is transferring through the chassis, we‘ve slowed it down just a tad. This tends to improve cornering characteristics in general, but for the Evo it’ll help reduce the tendency to lift the inside rear and reduce the diving to the outside front at the same time."
Rick
First off, agreed on the measurement location. The jack points are about 60" apart so those that wish can do the math to calc the degree of rake. And agreed, this has nothing to do with static weight distribution, its all about the dynamics of what takes place when everything starts moving.
It also has nothing to do with your shocks, your springs, your sway bars, nothing, nada, zip! This is actually something that needs to be address during the initial set-up of the chassis. In the case of the Evo, Mitsu already decided what to do based on the requirements of a street car, and for good reasons.
Mike, I've cut and paste my comments from the "How low to go" thread that member theShadow started -
"Lets start with chassis rake. I come from a long family history of open wheel dirt car racing, unlike todays racers that buy everything with a credit card we built our cars from the ground up. The only suspension pieces not made by hand were the shocks and the rod ends. The advantage was that we already knew the technical details of our chassis so it made ride height and those type of things much, much easier to deal with. The Evo was an unknown for us so like everyone else we just had to start somewhere and work with it.
Most the posts I see involving rake are attempts to improve cornering and while it will have an impact on cornering, chassis rake is really there to help you go straight! Since the Evo was set-up for the average Joe on the street it has a bunch of chassis rake. Like the inherant understeer, this is good for an average person, but not a race car driver.
Now of course we only had to go left, but its not as easy as it looks! For a winged midget on a short track, 1/8 mile or less, no rake. For a quater mile oval increase the rake to 1/4", for a one mile track, increase to 1/2". The reason we did this was because of our tire stagger, but even at that, doesn't take much to make a differance.
No long straights in autox, your almost always in some form of transition, I would start with 0 rake and if anything, lower the rear of the car, maybe up to a 1/2" or more. For a road course I would start at 1/2". If I was at a track with long straights that would be O.K. but if I was at a track that was mostly curves and short straights I would again lower the rear at least another 1/4".
For ride height I measure the bottom of the pinch seam weld just ahead and just behind the jack points. Right now I'm 6" in front, 7" in the rear, this gives me about 1 deg of rake which works well on the street. For tracking the car next year I'll take another 1/2" out of rear and start with 1/2 a deg of rake.
Only ballpark figures, but a place to start."
Nils mentioned roll center kits, so I made a change, shadow did too -
"Ha, that’s funny, I did the same thing last week, only I went the other way. I raised the car ½” and went back to a previous alignment set-up, the car actually feels better. Height at the bottom of the weld seam is now 6 ½” front, 7 ½” at the rear. I still want to lower the rear ½“, but I wanted to go back to a known set-up first and hopefully minimize the impact of lacking the roll center modification."
Both cars felt better after the changes -
" Shadow, looks good man, I like the stance more now than your first picture. In the first photo it looks more like a classic drag car, I’ll bet the rake was close to 1 ½” before your change. And change is good, especially on suspension. I think we’re in a good ride height zone for a street driven, track, autox type car.
So what did we impact with these changes? Basically we’ve helped the roll axis of the car by flattening it out. By doing more at the rear we’ve also changed the way weight is transferring through the chassis, we‘ve slowed it down just a tad. This tends to improve cornering characteristics in general, but for the Evo it’ll help reduce the tendency to lift the inside rear and reduce the diving to the outside front at the same time."
Rick
Last edited by Smike; Feb 6, 2010 at 04:02 AM.
Jan 3, 2009, 07:14 AM
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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.
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.
Last edited by theshadow; Jan 3, 2009 at 05:31 PM.
Jan 4, 2009, 08:51 AM
#26
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.
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.
