Three-wheeling it, good or bad?
09-01-2013, 07:47 PM
#52
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Re: Three-wheeling it, good or bad?
Hmmmmmm...your picture smells like a deliberate provocation. It looks like you're trailing the brake on entry to a tight little downhill corner. "It doesn't Really lift like it looks like it does". Now if you want to "fix that", keep going up on the front spring rate, down on the rear, increase rear droop travel and don't turn in till you're all the way off the brakes. That'll also help with the slow entry part of the popular philosophy.
Scott, who thinks that's the most aggressive rear bumper trimming he's evar seen on a DC2...I almost kinda like it except for the spare tire well is ugly...
Scott, who thinks that's the most aggressive rear bumper trimming he's evar seen on a DC2...I almost kinda like it except for the spare tire well is ugly...
09-01-2013, 08:01 PM
#54
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Re: Three-wheeling it, good or bad?
I think you might have missed the humor intended. Not much to suggest - looks like the outside front might be able to use a little more negative camber. I look at that pic and mostly what I see is FUN!
Scott, who cut his rear bumper to about where Michael Jackson was after his first surgery...yeah, I keep thinking about cutting it a little more...
Scott, who cut his rear bumper to about where Michael Jackson was after his first surgery...yeah, I keep thinking about cutting it a little more...
09-02-2013, 12:43 AM
#55
Re: Three-wheeling it, good or bad?
GSpeedR, who I hold in considerable regard and who is my superior in many ways, repeats something Horrendous when he says that as soon as a wheel lifts the roll stiffness of the car changes. That's not as right as it sounds. The car with biased front/rear roll stiffness has two roll "rates" (in weight per unit time): front and rear. There is no disruptive change in the rate of front weight transfer after the inside rear lifts. AND it's likewise not completely right that rear roll resistance goes to zero as soon as the inside rear lifts. Don't believe me? If I'm not right, then at that moment you would theoretically be able to flip the rear of the car with your pinkie finger. Forgetting about gravity? Also, when that inside rear is up, the other rear tire should just be starting to slide.
To prove this theory, here's a test on a K&C rig.
http://www.morsemeasurements.com/wheel-lift/
There is a limit to this game of stiffening the rear axle however. Under constant lateral acceleration each axle can only transfer as much weight as it has available statically. In other words, the total axle load remains constant as the vehicle corners. Obviously the rear axle has less load to transfer than the front, around 40% of the vehicles weight. Once the rear axle has transferred all the weight it has available the inside rear wheel lifts off the ground and any additional weight must be transferred at the front axle. Since the car was set up with a much lower front roll stiffness, it is left with the challenge of having to resist the roll moment of the same vehicle mass but with a fraction of the roll stiffness available. Remember that due to the much higher roll stiffness of the rear axle the remaining front roll stiffness is not half of the initial stiffness. Instead, it is the percentage of the front axle stiffness relative to the total stiffness, which may be as little as a third or less. With the full mass of the car only being supported by a third of the roll stiffness, the roll angle per G of lateral load is much higher and any small increase in lateral acceleration results in a huge amount of body roll.
Basically, AIUI, setting up your car so that the IRW lifts at a lower LT% essentially just reduces the window you have to work in where you can tune the balance with the springs and bars. If you're operating in the 30-40% region, but you gave up that tuning parameter at 25%, then good luck finding the extra time with suspension tuning. Moving the lift point to within your operating range gives you that extra tuning parameter. Sure, it'll understeer up until you're in the sweet spot, but in racing you should be on the ragged edge, right?
Again, only AIUI.
Last edited by Kozy.; 09-02-2013 at 01:19 AM.
09-02-2013, 06:33 AM
#56
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Re: Three-wheeling it, good or bad?
I think you might have missed the humor intended. Not much to suggest - looks like the outside front might be able to use a little more negative camber. I look at that pic and mostly what I see is FUN!
Scott, who cut his rear bumper to about where Michael Jackson was after his first surgery...yeah, I keep thinking about cutting it a little more...
Scott, who cut his rear bumper to about where Michael Jackson was after his first surgery...yeah, I keep thinking about cutting it a little more...
I keep thinking about cutting my rear bumper too...
How much and where to cut is the question...
09-02-2013, 08:18 AM
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Re: Three-wheeling it, good or bad?
And I thought the 3 legged dog was extinct! If I could just find some magic way to move weight to the back of the car, all would be good.
09-02-2013, 08:29 AM
#58
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Re: Three-wheeling it, good or bad?
holy **** lolz.
09-02-2013, 09:37 AM
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Re: Three-wheeling it, good or bad?
Does anybody think that a super expensive/high tech system that disconnects and adjusts sway bar stiffness actively (like the new Porsche Ceyennes) would solve this? If so would lap times improve?
What would this graph look like? A front axel graph would have to be over laid onto a rear axle graph correct? What would it look like and could it help in setting up the same car without the system?
Who's going to Frankenstein one onto their car first?
What would this graph look like? A front axel graph would have to be over laid onto a rear axle graph correct? What would it look like and could it help in setting up the same car without the system?
Who's going to Frankenstein one onto their car first?
Last edited by miamirice; 09-02-2013 at 04:48 PM.
09-03-2013, 02:12 AM
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Re: Three-wheeling it, good or bad?
A bit more from here: http://www.morsemeasurements.com/wheel-lift/
"Fundamentally we need to keep a four wheel vehicle on four wheels, even if the inside rear is only touching the ground as a token gesture. The first thing that will make all subsequent tuning easier is to eliminate any preload or droop limiting of the rear wheels, so that when the tire achieves zero load the spring is fully unseated and has run through its full range of linear travel. Moving as much weight to the rear as is practical will also help, giving the rear more weight to transfer in a corner. Once that is done, if the inside rear is still lifting it is actually time to make the front stiffer. While it may seem counter-intuitive based on driver feedback of the car plowing across the track, it will serve to reduce the roll angle of the car and correct the camber without changing the load distribution. The goal is to sync up the inside rear wheel running out of load to transfer with the entire car running out of lateral grip, so that no more load transfer will occur and the car is left on all four wheels. In a FWD car the driver can buy a bit of rear load by applying the throttle. This causes forward acceleration, transferring load to the rear tires and helping the inside rear stay touching the ground. With enough power it might be desirable to compromise corner entry by lifting the inside rear in order to have the car in the optimum spot on exit when the driver gets back on the throttle. By tuning the car to get the inside rear just touching under corner exit acceleration you have maximized the inside front load under acceleration, which can aid traction."
In the picture below, once the inside rear lifts at point B (about 2 degrees of roll), the rear roll stiffness no longer increases with more roll angle, seen by the flattening out of the red curves. The car is now pivoting about the rear outside tire. The front roll stiffness in green keeps on increasing past 2 degrees since both front tires are still on the ground and the load transfer keeps on increasing at that axle with more roll, while the rear load transfer has reached its limit, since all the rear axle weight is on the outside rear. The total roll stiffness in blue is the sum of the high rear roll stiffness in red and the lower front roll stiffness in the green curve, and at 2 degrees the rate of increase or slope of the roll stiffness per degree of roll takes a step down, with a lower slope starting at the point the inside rear tire lifts.
Of course this is measured on a kinematic rig. Whether or not you can get more roll past 2 degrees on track depends on getting more grip from the front axle in steady state cornering on a flat track, which is doubtful since one would increase load transfer which would reduce grip for that axle. Probably angular momentum about the cg line keeps the chassis rolling more once the rear lifts. And the camber change on the outside front past 2 degrees won't be helping things either as reduced roll stiffness would quickly change the camber on the outside front. Thus the car is likely to understeer more and more once the rear lifts.
One should do a skidpad experiment with different amounts of rear roll stiffness to check maximum g's achievable once the inside rear lifts. and then trail braking and curb hopping don't enter into it.
"Fundamentally we need to keep a four wheel vehicle on four wheels, even if the inside rear is only touching the ground as a token gesture. The first thing that will make all subsequent tuning easier is to eliminate any preload or droop limiting of the rear wheels, so that when the tire achieves zero load the spring is fully unseated and has run through its full range of linear travel. Moving as much weight to the rear as is practical will also help, giving the rear more weight to transfer in a corner. Once that is done, if the inside rear is still lifting it is actually time to make the front stiffer. While it may seem counter-intuitive based on driver feedback of the car plowing across the track, it will serve to reduce the roll angle of the car and correct the camber without changing the load distribution. The goal is to sync up the inside rear wheel running out of load to transfer with the entire car running out of lateral grip, so that no more load transfer will occur and the car is left on all four wheels. In a FWD car the driver can buy a bit of rear load by applying the throttle. This causes forward acceleration, transferring load to the rear tires and helping the inside rear stay touching the ground. With enough power it might be desirable to compromise corner entry by lifting the inside rear in order to have the car in the optimum spot on exit when the driver gets back on the throttle. By tuning the car to get the inside rear just touching under corner exit acceleration you have maximized the inside front load under acceleration, which can aid traction."
In the picture below, once the inside rear lifts at point B (about 2 degrees of roll), the rear roll stiffness no longer increases with more roll angle, seen by the flattening out of the red curves. The car is now pivoting about the rear outside tire. The front roll stiffness in green keeps on increasing past 2 degrees since both front tires are still on the ground and the load transfer keeps on increasing at that axle with more roll, while the rear load transfer has reached its limit, since all the rear axle weight is on the outside rear. The total roll stiffness in blue is the sum of the high rear roll stiffness in red and the lower front roll stiffness in the green curve, and at 2 degrees the rate of increase or slope of the roll stiffness per degree of roll takes a step down, with a lower slope starting at the point the inside rear tire lifts.
Of course this is measured on a kinematic rig. Whether or not you can get more roll past 2 degrees on track depends on getting more grip from the front axle in steady state cornering on a flat track, which is doubtful since one would increase load transfer which would reduce grip for that axle. Probably angular momentum about the cg line keeps the chassis rolling more once the rear lifts. And the camber change on the outside front past 2 degrees won't be helping things either as reduced roll stiffness would quickly change the camber on the outside front. Thus the car is likely to understeer more and more once the rear lifts.
One should do a skidpad experiment with different amounts of rear roll stiffness to check maximum g's achievable once the inside rear lifts. and then trail braking and curb hopping don't enter into it.
Last edited by descartesfool; 09-03-2013 at 02:42 AM.
09-03-2013, 05:08 AM
#62
Re: Three-wheeling it, good or bad?
I once did a little math model of load transfer rates and plotted front load transfer percentage against lateral G.
In a front heavy car with a rear roll stiffness bias, it started off understeer biased and became increasingly oversteer biased as the G's rose, until the point the inside rear lifted. Beyond this, it started upwards on an understeer gradient.
The ONLY things which altered that post lift gradient was weight distribution.
I'll have to add it onto my load transfer calculator at some point, it depicted the effect quite nicely.
In a front heavy car with a rear roll stiffness bias, it started off understeer biased and became increasingly oversteer biased as the G's rose, until the point the inside rear lifted. Beyond this, it started upwards on an understeer gradient.
The ONLY things which altered that post lift gradient was weight distribution.
I'll have to add it onto my load transfer calculator at some point, it depicted the effect quite nicely.
09-03-2013, 05:58 AM
#63
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Re: Three-wheeling it, good or bad?
Ahh yes I remember that article.. and it does a very good job at explaining this. Good stuff once again...
Now that it's fresh in my brain again I have to adapt that to RWD... fak
Now that it's fresh in my brain again I have to adapt that to RWD... fak
09-03-2013, 06:07 AM
#64
09-03-2013, 06:45 AM
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Re: Three-wheeling it, good or bad?
Christian,
I'm gonna take that winkie at the end of that whole thing as indicating that it's all a joke, else I'm gonna be up late writing some more.
In an effort to keep it short, if you don't do any trail braking, and immediately go to maintainence throttle on turn in, then you're overbraking and not taking full advantage of the available physics in that sector and you could be going faster and still coming out faster.
We NEED the rear end to want to come around so we can take steering lock off and put More power down. That rear tire "height" is nothing more than a tell tale for a less or more unloaded inside rear and hence more or less loaded inside front. We simply want to maximize the load on the inside front, short of diminishing returns of course, in virtually ALL sectors of the turn.
Scott, who has posted the following graphic before - those areas between those curves are weight on or off the inside front or rear respectively...when you choose a negative value for lift you are choosing to carry less load on the inside front thru a significant portion of the whole corner. When you take into consideration how little grip capacity is left for tractive work on the inside front, the limiting factor with the most commonly used diffs, you cannot generally want less weight on the inside front...
* Note to Gringotegra: We have been having these threads for Years...they're always the same and they always recur...it's alot like arguing about torque and horsepower...
I'm gonna take that winkie at the end of that whole thing as indicating that it's all a joke, else I'm gonna be up late writing some more.
In an effort to keep it short, if you don't do any trail braking, and immediately go to maintainence throttle on turn in, then you're overbraking and not taking full advantage of the available physics in that sector and you could be going faster and still coming out faster.
We NEED the rear end to want to come around so we can take steering lock off and put More power down. That rear tire "height" is nothing more than a tell tale for a less or more unloaded inside rear and hence more or less loaded inside front. We simply want to maximize the load on the inside front, short of diminishing returns of course, in virtually ALL sectors of the turn.
Scott, who has posted the following graphic before - those areas between those curves are weight on or off the inside front or rear respectively...when you choose a negative value for lift you are choosing to carry less load on the inside front thru a significant portion of the whole corner. When you take into consideration how little grip capacity is left for tractive work on the inside front, the limiting factor with the most commonly used diffs, you cannot generally want less weight on the inside front...
* Note to Gringotegra: We have been having these threads for Years...they're always the same and they always recur...it's alot like arguing about torque and horsepower...
I'm glad to see the posts from Claude and Kozy as the mechanics of lifting a wheel are described in the same way that they work in my head. Always nice to see the reality inside my head mirrored in the outside world.
09-03-2013, 10:25 AM
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Re: Three-wheeling it, good or bad?
Damn you guys are distracting me from my reading of Tsai-i-chan...that's Portent and Omen Divination for you barbarians.
Here's what I think: when you think about it, and think it has to be, you're likely to think it has to be. And sometimes that leads us to construct a defensible case, and sometimes it doesn't.
You must think about this seriously.
The Morse Technical Article is marketing document with both reasonable and erroneous information.
"Doctor Morse" is describing a set of "symptoms" to educate you about something you don't understand...let's call it "a disease", that can get out of hand if not "treated".
The narrative is laced with selected scenarios that take advantage of your lack of understanding.
You must examine this well.
This is Very Different from how we actually drive a car.
You must bear this in mind.
MY GOD! The driver was lucky to escape all this with just a complaint.
This person in the VW Rabbit is Obviously a Showroom Stock racer from the late 1970's who didn't get his tire pressures right.
Why is YOUR car grinding away it's outside front tire's outside shoulder?
REALLY? What about with Front spring and bar? I know, I know - but I also know that you're just quibbling.
NO DUH! Yep, doing more of what's ceased working WON'T WORK. Doctor, I think you're really onto something.
ANOTHER ONE OF THESE? It's like the Doctor is just having fun with you now.
Doctor, my tires aren't getting along. Three of them are complaining that the fourth is lazy. Can you talk to them? In something other than gibberish? ("no mechanism for the tire to pull the car downward" is gibberish if you didn't catch it).
The Doctor has an answer: It's easy - all four tires should always be in contact with the ground.
You has got to be ****** kidding me. Well I guess in a world where tires are friends, you might deal in token gestures as well.
Why are you running so big a battery that this would make Any difference?
Granted, if you have So Little power to put down that you wouldn't miss any weight that could have been on the front...
Credit where credit is due - the only problem they really seem to be having with this fwd car is UNDERSTEER. How about that.
That's funny - kinda sounds like there's just too much Front grip. Counter-intuitive? Yeah, Counter-intuitive alright.
Yeah, I'm on the gas mainly to try to keep that inside rear down.
If I could put a little more power down I think I could. But since I moved the battery to the back of the car I'm getting too much front wheelspin.
I'm gonna hafta call the Doctor again. Maybe get a referral to a Diff Specialist.
AND HERE WE ARE:
NO ****! So I guess it IS ok to lift the inside rear during trailing brake turn in after all. THANKS DOCTOR! Hey - what compromise?
You know part of why you're trailing brake is to overload the outside rear and encourage it to come around?
Oh lookey! Charts!!! I looooove charts.
That doesn't sound right. And the deviation would be in the other direction.
I'd sooner believe that both segments of trace from Zero to A and A to B, are softer and stiffer respectively because of compliance and binding effects.
Now, there's no getting around that if you put that car on their machine you get that data as output.
So we should think about how that machine works.
It grabs the chassis and moves it around (very slowly remember) while logging the wheel pads.
Have you ever tried to tip over a car?
Once you've got it to where both the front and rear wheels on the side you're lifting are off the ground, is your work done? Or is there still a lot of lifting work to be done?
Do the wheels on the other side of car know anything at all about that work?
No.
Neither does the K&C machine - vertically anyway.
Surely they normalize this out somehow...surely...I wonder...
Based on the chart data, if both inside wheels lifted the traces would both be flat in the first chart, and their inverses would be vertical in the second.
And that wouldn't be right.
Hmmm...that's interesting.
The green trace (front roll moment) just barely changes it's slope (softens) about the same roll angle at which the inside rear lifts.
That can't be what people like Komodo are referring to when they take issue with my saying that "There is no disruptive change in the rate of front weight transfer after the inside rear lifts".
And it sure doesn't look like what we'd expect if we were going from 100% of roll stiffness to something like 30%. That's the blue trace.
Hmmmm, so what is causing that change in the green trace?
Here's how we actually Drive a car: We pitch or throw it into the turn. It doesn't matter if we're a clod or we're smooth. If we're a decently fast driver the turn in to the average corner is violent. And completely unlike the K&C machine, there is Velocity associated with the displacements at the four corners of the car, and with them associated damper forces. Oh what would the K&C pads make of That? I guess they're on the short list of things that a 7-post rig does better.
What's happening as the car is reaching it's maximum roll angle in real life? It's rate of roll is DECREASING. By the time that inside rear has lifted most of the lateral weight transfer has occured - there's maybe a couple hundred pounds left to transfer across the front axle and the car is rolling Slowly.
You really want to talk about the Whole Mass of the Car going up against that poor little old front roll stiffness after the inside rear ditched band practice? The fact of the matter is that the inside rear went "over the top" giving it's all, while the inside front was still safe in the trenches behind it's parapet of load writing a love letter to the drivers right foot.
And of course, somebody had to facilitate the obligatory appearance by "from the time the inside rear lifts understeer increases" in any thread on this subject.
It's ludicrous. It doesn't fit into the sequence of a fwd racecar as actually operated. When that inside rear comes up the outside rear should be letting go.
*It's probably the case that a well setup (fast, balanced, the competent driver not complaining about it) typical fwd club racer driving around a large radius skid pad with some throttle on won't have the inside rear off the ground.
*It's probably the case that that same car will lift the inside rear under trailing brake turn in.
*It may be the case that once the brakes are off, near peak lateral load, no throttle yet, the inside rear will be down. I don't spend alot of time doing this, and so don't think I care that much either way.
You must think about all of this properly.
The 15th year of having a Type-R, the 3rd day of the 9th month.
Scott, who needs to get back to his study of the divinations of the portents coming thru the steering wheel force and what's happening at the hardest working tire of them all...
Here's what I think: when you think about it, and think it has to be, you're likely to think it has to be. And sometimes that leads us to construct a defensible case, and sometimes it doesn't.
You must think about this seriously.
The Morse Technical Article is marketing document with both reasonable and erroneous information.
"Doctor Morse" is describing a set of "symptoms" to educate you about something you don't understand...let's call it "a disease", that can get out of hand if not "treated".
The narrative is laced with selected scenarios that take advantage of your lack of understanding.
You must examine this well.
K&C testing is not really 'dynamic' testing at all...K&C tests are quasi-static - all loads and displacements are applied quite slowly.
You must bear this in mind.
Anyone who has seen a VW Rabbit teetering through a turn ...the car is rocking along the outside rear and inside front wheels and the chassis is bouncing upward and pitching forward as it does so. This increase in bounce travel raises the CG, further increasing the weight transfer and exacerbating the problem. Luckily this feedback loop is interrupted by the outer edge of the outside front tire grinding away from the positive camber gain, and the front of the car begins to slide as the driver complains of understeer.
This person in the VW Rabbit is Obviously a Showroom Stock racer from the late 1970's who didn't get his tire pressures right.
Why is YOUR car grinding away it's outside front tire's outside shoulder?
A car on three wheels is statically determinate. This is an important point to note because it means we have lost our ability to tune the balance of the car with roll stiffness.
They may increase the stiffness of the rear axle in roll, which only serves to make the problem worse as it runs out of weight to transfer even sooner.
They may try some form of droop limiting or preload, hoping that it will “hold the inside rear wheel down”, but again physics isn’t on their side.
Preload and droop limiting just reduce the amount of travel available before the inside wheel rear runs out of vertical load, and with no mechanism for the tire to pull the car downward it is perfectly happy hanging out in the air watching its other three friends do all the work.
The Doctor has an answer: It's easy - all four tires should always be in contact with the ground.
...even if the inside rear is only touching the ground as a token gesture.
Moving as much weight to the rear as is practical will also help, giving the rear more weight to transfer in a corner.
Granted, if you have So Little power to put down that you wouldn't miss any weight that could have been on the front...
Once that is done, if the inside rear is still lifting it is actually time to make the front stiffer. While it may seem counter-intuitive based on driver feedback of the car plowing across the track, it will serve to reduce the roll angle of the car and correct the camber without changing the load distribution.
The goal is to sync up the inside rear wheel running out of load to transfer with the entire car running out of lateral grip, so that no more load transfer will occur and the car is left on all four wheels.
In a FWD car the driver can buy a bit of rear load by applying the throttle.
If I could put a little more power down I think I could. But since I moved the battery to the back of the car I'm getting too much front wheelspin.
I'm gonna hafta call the Doctor again. Maybe get a referral to a Diff Specialist.
AND HERE WE ARE:
With enough power it might be desirable to compromise corner entry by lifting the inside rear in order to have the car in the optimum spot on exit when the driver gets back on the throttle. By tuning the car to get the inside rear just touching under corner exit acceleration you have maximized the inside front load under acceleration, which can aid traction.
You know part of why you're trailing brake is to overload the outside rear and encourage it to come around?
Oh lookey! Charts!!! I looooove charts.
At point A the rear roll moment starts increasing at a higher rate, which indicates the droop limiting has started picking up the left rear wheel...But at point B the left rear wheel lifts off the ground, causing the rear roll stiffness to completely flatten out.
I'd sooner believe that both segments of trace from Zero to A and A to B, are softer and stiffer respectively because of compliance and binding effects.
Now, there's no getting around that if you put that car on their machine you get that data as output.
So we should think about how that machine works.
It grabs the chassis and moves it around (very slowly remember) while logging the wheel pads.
Have you ever tried to tip over a car?
Once you've got it to where both the front and rear wheels on the side you're lifting are off the ground, is your work done? Or is there still a lot of lifting work to be done?
Do the wheels on the other side of car know anything at all about that work?
No.
Neither does the K&C machine - vertically anyway.
Surely they normalize this out somehow...surely...I wonder...
Based on the chart data, if both inside wheels lifted the traces would both be flat in the first chart, and their inverses would be vertical in the second.
And that wouldn't be right.
Hmmm...that's interesting.
The green trace (front roll moment) just barely changes it's slope (softens) about the same roll angle at which the inside rear lifts.
That can't be what people like Komodo are referring to when they take issue with my saying that "There is no disruptive change in the rate of front weight transfer after the inside rear lifts".
And it sure doesn't look like what we'd expect if we were going from 100% of roll stiffness to something like 30%. That's the blue trace.
Hmmmm, so what is causing that change in the green trace?
Here's how we actually Drive a car: We pitch or throw it into the turn. It doesn't matter if we're a clod or we're smooth. If we're a decently fast driver the turn in to the average corner is violent. And completely unlike the K&C machine, there is Velocity associated with the displacements at the four corners of the car, and with them associated damper forces. Oh what would the K&C pads make of That? I guess they're on the short list of things that a 7-post rig does better.
What's happening as the car is reaching it's maximum roll angle in real life? It's rate of roll is DECREASING. By the time that inside rear has lifted most of the lateral weight transfer has occured - there's maybe a couple hundred pounds left to transfer across the front axle and the car is rolling Slowly.
You really want to talk about the Whole Mass of the Car going up against that poor little old front roll stiffness after the inside rear ditched band practice? The fact of the matter is that the inside rear went "over the top" giving it's all, while the inside front was still safe in the trenches behind it's parapet of load writing a love letter to the drivers right foot.
And of course, somebody had to facilitate the obligatory appearance by "from the time the inside rear lifts understeer increases" in any thread on this subject.
It's ludicrous. It doesn't fit into the sequence of a fwd racecar as actually operated. When that inside rear comes up the outside rear should be letting go.
*It's probably the case that a well setup (fast, balanced, the competent driver not complaining about it) typical fwd club racer driving around a large radius skid pad with some throttle on won't have the inside rear off the ground.
*It's probably the case that that same car will lift the inside rear under trailing brake turn in.
*It may be the case that once the brakes are off, near peak lateral load, no throttle yet, the inside rear will be down. I don't spend alot of time doing this, and so don't think I care that much either way.
You must think about all of this properly.
The 15th year of having a Type-R, the 3rd day of the 9th month.
Scott, who needs to get back to his study of the divinations of the portents coming thru the steering wheel force and what's happening at the hardest working tire of them all...
09-03-2013, 05:04 PM
#68
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Re: Three-wheeling it, good or bad?
A simple example: Car weighs 2600 lbs, unsprung weights assumed to be zero for simplicity, with front axle load of 1600 lbs and rear axle load of 1000 lbs.
Static corner weights are 800_LF, 800_RF, 500_LR, 500_RR. Cornering stiffness is 200 lbs/degree of roll for the front axle, and 250 lbs/degree for the rear axle, giving a total roll stiffness of 450 lbs/degree.
At 2 degrees of roll, we transfer 400 lbs across the front axle and 500 lbs across the rear axle. New corner weights are 1200_LF, 400_RF, 1000_LR and 0_RR. The right rear is fully unloaded. Now the rear roll stiffness is irrelevant as all the weight across the rear axle than can be transferred has been transferred, and no more load can be added or subtracted from any of the rear tires.
If we can add 1 more degree of roll, the car will pivot about the LR tire while the front suspension deflects at the same angle as the rear axle , one tire of which must lift more, and only the front axle will contribute to the roll stiffness, which will now be 200 lbs per degree total. So roll stiffness has dropped by over half, and corner weights are now 1400_LF, 200_RF, 1000_LR and 0_RR. So the car rolls much more easily now, since roll stiffness drops from 450 lbs/degree to 200 lbs/degree once the RR lifts. Just like in the chart for steady state cornering, which happens slowly through a carousel for example.
So does the car understeer more and more as you try to push it past the point of rear lift due to an increase in the front wheel load differences, or can we get more grip out of the 3 tires still on the ground? How much more roll can we induce past the initial 2 degrees that just causes the inside rear to fully unload?
Static corner weights are 800_LF, 800_RF, 500_LR, 500_RR. Cornering stiffness is 200 lbs/degree of roll for the front axle, and 250 lbs/degree for the rear axle, giving a total roll stiffness of 450 lbs/degree.
At 2 degrees of roll, we transfer 400 lbs across the front axle and 500 lbs across the rear axle. New corner weights are 1200_LF, 400_RF, 1000_LR and 0_RR. The right rear is fully unloaded. Now the rear roll stiffness is irrelevant as all the weight across the rear axle than can be transferred has been transferred, and no more load can be added or subtracted from any of the rear tires.
If we can add 1 more degree of roll, the car will pivot about the LR tire while the front suspension deflects at the same angle as the rear axle , one tire of which must lift more, and only the front axle will contribute to the roll stiffness, which will now be 200 lbs per degree total. So roll stiffness has dropped by over half, and corner weights are now 1400_LF, 200_RF, 1000_LR and 0_RR. So the car rolls much more easily now, since roll stiffness drops from 450 lbs/degree to 200 lbs/degree once the RR lifts. Just like in the chart for steady state cornering, which happens slowly through a carousel for example.
So does the car understeer more and more as you try to push it past the point of rear lift due to an increase in the front wheel load differences, or can we get more grip out of the 3 tires still on the ground? How much more roll can we induce past the initial 2 degrees that just causes the inside rear to fully unload?
09-03-2013, 05:24 PM
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Re: Three-wheeling it, good or bad?
I'd say that the amount of "grip" available afte the rear wheel takes off will be primarily dependent upon: 1-static camber of the outside front, 2- camber curve of the outside front. Assuming the outside front isn't rolled over and grinding off its outside edge, the car stands to pick up grip as the chassis rolls it more parallel to the track. Add in the car being into the steep part of the camber curve and it seems to me like there's more grip to be had... Up until there isn't. Or until our hero presses the gas pedal a little more and transfers weight back... hopefully getting that pesky inside rear back on the ground. 
09-03-2013, 05:42 PM
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Re: Three-wheeling it, good or bad?
Claude - within the context of your example please tell me how it is possible for the car to continue rolling after the point at which there's zero cornerweight on the inside rear? As I understand the case you're making the car Cannot go any faster around your carousel at that point, for if it did it would only transfer more weight across the front axle, Reducing the grip, and so the car would understeer offline having Not lifted and Not rolled any more.
Here's how it is:
There's how much peak lateral force the back of the car can produce while working.
There's how much peak lateral & tractive force the front of the car can produce while working.
An inflection point in the relationship between the two is NOT indicated by the inside rear lifting.
The inflection point in the relationship between the two is indicated by the outside rear "letting go" and there being enough tractive capacity in the front to outrun it.
THAT is the sweet spot. THAT is The Edge.
Each axle has it's greatest capacity for lateral load at Zero lateral load doesn't it. As we DEVELOP lateral load we have two traces converging: increasing Developed Load and decreasing Load Capacity. Where they converge - That's a Real point with a correlate in the real world. There's one at each end of the car. The vertical position of the inside rear wheel is a minor coincident datum useful in that it is easily observable.
Scott, who drives cars, not test rigs, not equations, CARS...
Here's how it is:
There's how much peak lateral force the back of the car can produce while working.
There's how much peak lateral & tractive force the front of the car can produce while working.
An inflection point in the relationship between the two is NOT indicated by the inside rear lifting.
The inflection point in the relationship between the two is indicated by the outside rear "letting go" and there being enough tractive capacity in the front to outrun it.
THAT is the sweet spot. THAT is The Edge.
Each axle has it's greatest capacity for lateral load at Zero lateral load doesn't it. As we DEVELOP lateral load we have two traces converging: increasing Developed Load and decreasing Load Capacity. Where they converge - That's a Real point with a correlate in the real world. There's one at each end of the car. The vertical position of the inside rear wheel is a minor coincident datum useful in that it is easily observable.
Scott, who drives cars, not test rigs, not equations, CARS...
09-03-2013, 06:13 PM
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Re: Three-wheeling it, good or bad?
There's how much peak lateral force the back of the car can produce while working.
There's how much peak lateral & tractive force the front of the car can produce while working.
An inflection point in the relationship between the two is NOT indicated by the inside rear lifting.
The inflection point in the relationship between the two is indicated by the outside rear "letting go" and there being enough tractive capacity in the front to outrun it.
THAT is the sweet spot. THAT is The Edge.
Scott, who drives cars, not test rigs, not equations, CARS...
Last edited by rice_classic; 09-03-2013 at 06:18 PM. Reason: Added
09-03-2013, 11:58 PM
#72
Re: Three-wheeling it, good or bad?
Yet another thread where I am totally perplexed as to what side of the fence Scott is sitting on.
Or if he can even see the fence.
Or if he can even see the fence.
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