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Do Bump Rubbers Work The Same In '69 As Other Mods?


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#21 ginetto

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Posted Aug 02 2010 - 04:40 AM

Thanks for the info :thumbup:
Another question; how exactly the bump and rebound ("clicks" of the suspensions right?) get involved into this?
Should these 2 values be set according a formula with the variation of the wheel-rate or just following the weight of the car?

#22 Lee200

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Posted Aug 02 2010 - 06:18 AM

 Ginetto, on Aug 02 2010 - 04:40 AM, said:

Thanks for the info :thumbup:
Another question; how exactly the bump and rebound ("clicks" of the suspensions right?) get involved into this?
Should these 2 values be set according a formula with the variation of the wheel-rate or just following the weight of the car?

Hi Ginetto,

My understanding is that the damper force is simply added to the wheel/spring rate force, bump rubber force, and ARB force.

Instead of adding force per inch of suspension compression, dampers add force per speed of the suspension compression.  So the faster the suspension moves, the more force is added.  As speed is distance over time, GPL just takes the difference in suspension compression between each cycle and adds force based on the difference.

When hitting a dip, heavier cars should produce more downward force resulting in faster suspension compression in bump; however, they MAY in fact resist upward force at a crest resulting in slower suspension decompression in droop.  So you'd think the bump force should be more than the rebound force.  When in roll; however, the two opposite suspensions are moving at the same speed although in opposite directions so the bump and rebound force should be about the same.

In GPL, the bump resistance is less than the rebound resistance.  Here's a chart of GPL's damper force courtesy of Gene.  The bottom axis isn't labeled, but it the units are in distance per time.

To answer your question, seems to me that the heavier the car, the higher the bump and rebound settings should be.  But since the wheel/spring force gets added in, a high wheel/spring setting may allow lower bump and rebound settings.   :idunno:

Lee

Attached Files


Edited by Lee200, Aug 02 2010 - 06:44 AM.


#23 John Woods

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Posted Aug 02 2010 - 09:24 AM

This kind of stuff makes me wish I was smart. All this time, and it never occurred to me bump/rebd was asymetric.
First, it never occurred to me lower shock rates could complement stiffer springs...I thought exactly the opposite; that stiffer shocks were needed to control stiffer springs. (If I read things correctly).
From the graph, it appears a setting of 4 for bump and 1 for rebd would equal a 50%-50% bump/rebd setting? But 50/50 may not be a good idea because?
An ALMS, (or maybe Rolex Series), Chief Mechanic once said in an interview, "Well, we worked on the car until the last minute and got no practice time, so we sat the shocks up 50/50 and we'll have to make changes as we see how things go in the race." So I've been starting out with settings like 3/3 and tweaking...thinking I knew what I was doing.

Edited by John Woods, Aug 02 2010 - 09:35 AM.


#24 Lee200

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Posted Aug 02 2010 - 11:53 AM

 John Woods, on Aug 02 2010 - 09:24 AM, said:

This kind of stuff makes me wish I was smart. All this time, and it never occurred to me bump/rebd was asymetric.
First, it never occurred to me lower shock rates could complement stiffer springs...I thought exactly the opposite; that stiffer shocks were needed to control stiffer springs. (If I read things correctly).
From the graph, it appears a setting of 4 for bump and 1 for rebd would equal a 50%-50% bump/rebd setting? But 50/50 may not be a good idea because?
An ALMS, (or maybe Rolex Series), Chief Mechanic once said in an interview, "Well, we worked on the car until the last minute and got no practice time, so we sat the shocks up 50/50 and we'll have to make changes as we see how things go in the race." So I've been starting out with settings like 3/3 and tweaking...thinking I knew what I was doing.

Me too...wish I were smart.   :blink:

Don't get me wrong; I think in the real world and probably in GPL, you should use higher damper settings and wheel/spring rates for heavier cars.  But it all comes back to how you want to make the car feel.

Yes, you are reading the graph correctly.  4 to 1 results in the same force in bump and rebound.

Well no one took my bait so I'll throw in what we know about roll bars anyway.   :D

The sprung weight transfers from the inside tires to the outside tires in a turn and as I said before, we have no control over this as for any given lateral G, the load transfer will always be the same.  Load transfer is resisted by the springs and bump rubbers and roll bars (ARBs).  The sum of the spring, bump rubber, and ARB forces is the total roll resistance force.

In the real word, what is totally magic is that the proportion of total load that is transferred to the front or rear outside tires is directly proportional to the roll resistance at each end.  For example, if the front has 75% of the total roll resistance and the rear has the remaining 25%, 75% of the total load transfer will go to the front outside tire and only 25% to the rear.  This is great stuff for fine tuning the handling of a car as we can use the ARBs to proportion how much load we want to go to the front and rear tires thus making the car understeer or oversteer.  The more you load a tire, the less grip it has and vice versa.

So much for the real world...how about GPL?

In GPL, the ARBs resist roll by adding roll resistance to the outside wheel just like the real world.  It does this by adding force to the outside tire based on the difference in suspension compressions on opposite wheels and the ARB setting, but also removes the same amount of force from the inside tire.  This is very strange and I'm not at all sure that real world ARBs do this.  Looking at a ARB which is attached to opposite side suspensions, you might expect that as the outside suspension is loaded with force under roll, the inside suspension would unload by the same amount of force, but is that really true?

I've read several books and articles on this and have found no definitive answer.  Milliken's "Race Car Vehicle Dynamics" has only one line in 800 pages that says, "Twisting of the bar adds load to one wheel and removes it equally from the other.", and Staniforth's "Competition Car Suspension" says the roll bar transfers load from the inside to the outside tire.  In both books when a numerical example of load transfer is given, nothing is shown about load being added back to the inside tire.

Regardless of whether GPL's is correct or not, that's how it works.  What is important though is that GPL's method does not provide load transfer from one axle to the other...you can't apportion more load to the front or rear based on the relative proportions of roll resistance on each axle.  All you can do with the ARBs is to even out the load between the two wheels on the same axle which SHOULD even out their grip and temperatures.

Remember that total roll resistance includes the springs and bump rubbers (and dampers too when the suspension is moving) so they have an effect on load transfer just like the ARBs.  What is different about the ARBs is that they only work when the chassis rolls; they have no effect under straight line dive or squat.

EDIT:

As the ARBs only work in roll, clever suspension geometry can also have a similar effect during pitch.  Called antidive and antisquat, these effects come into play whenever the chassis pitches forward under deceleration or backward under acceleration.  GPL properly models these effects by adding force in the opposite direction so that the pitch change is lessened during longitudinal load transfer.

Lee

Edited by Lee200, Aug 03 2010 - 06:41 AM.


#25 John Woods

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Posted Aug 02 2010 - 01:20 PM

Amazing. Information nowhere else available? Incredible!
IMHO

What more can there be?

#26 FloP

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Posted Aug 02 2010 - 02:37 PM

Thank you very much for this wealth of information, Lee! I'm reading every post of yours with great interest! :hat-tip:

#27 John Woods

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Posted Aug 02 2010 - 04:18 PM

Two questions I'd like to know answers to:

How to optimize correlation of shocks and diff to achieve maximum launch, sustained acceleration, and top speed.

How to correlate that to original 11 track specific grip coefficients.

Just wondering. Figured I'd skip the bait and go the whole way.

But I still want more bait.Posted Image

#28 Lee200

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Posted Aug 02 2010 - 04:41 PM

Well since we have a few crazies like me who are interested in how GPL works, here's a bit on the how the computer simulation actually operates.  It's not essential to know and probably has no practical application to making your car run any faster, but does give you some more insight on this wonderful program.

Inside the guts of GPL are constructs call "rigid bodies" which are used to simulate parts of the car.  I think of a rigid body as being a homogeneous sphere that has no size and cannot be compressed or twisted, but it can move.  In a "Six Degree of Freedom" model, a rigid body can move in six dimensions:

Linear Motion:

1)  Forward or backward along the longitudinal axis
2)  Left or right along the lateral axis
3)  Up or down along the vertical axis

Rotational Motion:

4)  Pitch about the lateral axis
5)  Roll about the longitudinal axis
6)  Yaw about the vertical axis

GPL uses twelve...yes, twelve different rigid bodies.  They are:

1)  Chassis
2)  Fuel
3)  Front Left Wheel Hub
4)  Front Right Wheel Hub
5)  Front Left Wheel
6)  Front Right Wheel
7)  Rear Left Wheel
8)  Rear Right Wheel
9)  Engine
10)  Clutch
11)  Outshaft--differential
12)  Head--player's head

Not all of these rigid bodies are allowed six degrees of movement.  The chassis and head do, but the wheels can only rotate about one axis and move up and down in another while the fuel has no freedom of movement at all.

The front wheels have hubs while the rear do not.  This allows the front wheels to be steered.

Note also that the head, which simulates the driver's head, is given its own rigid body, but apparently Papy didn't have time to finish the code that would make it move independently  to simulate bumps and loads on the driver.   :(

The most important rigid body is the chassis as all the other rigid bodies are connected to it.  The chassis has one other important characteristic...it has mass.

A few hundred years ago, a rather clever Englishman named Newton came up with his three laws of motion while watching apples fall on his head from a tree.  His second law states that Force = Mass * Acceleration which can be rewritten as Acceleration = Force / Mass.  So if we apply a given force to an object of given mass, the object will accelerate according to his formula.

GPL measures all the different forces acting on the chassis and combines them into a single force for each of the three linear directions of motion.  When applied to the chassis mass, an acceleration results which between two successive time points can be used to determine  the change in speed and also distance traveled for each of the three linear axes.  The mathematics of how GPL does this is not fully understood as it involves solutions of differential equations and is stuff that is way, way beyond my comprehension, but this is the gist of what happens.

So GPL keeps track of the rigid body's acceleration, speed, and position and repeatedly updates these 388 times per second!  No wonder that 1990s computers sputtered.  GPL also keeps track of the rigid body's linear momentum (which is speed times mass) for use during collisions with other objects such as the hay bales I so often visit.

GPL does exactly the same thing for rotation.  It keeps track of the rigid body's rotational acceleration, speed, and direction.  And like the linear components, it also keeps track of rotational momentum which explains why your GPL car can do all those 360s coming off a tight turn.

The rotational analogs of force and mass are torque and moment of inertia.  As we all know, torque is a force applied over a distance and is measured in pounds feet or some such.  Moment of inertia is a harder concept to grasp, but there are formulae for figuring out the moment of inertia for all sorts of differently shaped objects.  As I've mentioned before, GPL figures an overall car moment of inertia at startup and uses that throughout in its rotational computations.   The formula for rotation is Acceleration = Torque / Moment of Inertia.

To wind this up, here's a list of forces that act on the chassis.  As the forces act over a predefined distance, they also generate a torque on the chassis.  For example, the tire acts upward through the wheel to the spring which is located outboard of the chassis based on the track size.

1) Gravity
2) ARBs
3) Springs
4) Engine
5) Clutch
6) Head!

Lee

Edited by Lee200, Aug 02 2010 - 05:17 PM.


#29 Lee200

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Posted Aug 02 2010 - 05:08 PM

 John Woods, on Aug 02 2010 - 04:18 PM, said:

Two questions I'd like to know answers to:

How to optimize correlation of shocks and diff to achieve maximum launch, sustained acceleration, and top speed.

How to correlate that to original 11 track specific grip coefficients.

Just wondering. Figured I'd skip the bait and go the whole way.

But I still want more bait.Posted Image

Ah Grasshopper, that is in Lesson #2.  But first you must learn to walk on the rice paper.   :)

Seriously John, drivetrain physics is an entirely separate subject and so far, we've only touched on suspension physics.  But if you're interested, I'll get into that later.

I don't know what you mean by 11 track specific grip coefficients though.  Are you referring to the 11 original tracks?  If so, each track does NOT have different grips as they basically all use the same track surface.  In GPL, the surface grip is set according to its composition with the best being asphalt.  So as long as you are on the track and not off in the weeds, you're getting the best grip possible.  I can tell you though that the hay bales don't offer much grip.   :D

Lee

#30 John Woods

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Posted Aug 02 2010 - 10:38 PM

Okay, I can be patient.

A vague memory involves reading about track builders setting grip or traction variables. Isn't there something about that in the GPLEA Guides? (Stefan just posted them, guess I could check). Or somewhere. And it is difficult for me to believe Mosport, Kyalami, and Monza all have the same grip. I always thought the factorials of the variables offered a wide range of choice for track surface grip, and matching a setup to a particular unknown grip recipe was part of the art of being fast.

But I'll just sit quietly and try to not get too far ahead. Except, I just noted the exclaimation sign after "head." As in, head motion figures into force acting on the chassis?!!! Or is that the unfinished part? And clutch?

Well?Posted Image I'm waiting. Quietly...

Edited by John Woods, Aug 02 2010 - 10:53 PM.


#31 MECH

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Posted Aug 03 2010 - 04:48 AM

View PostJohn Woods, on Aug 02 2010 - 10:38 PM, said:

But I'll just sit quietly and try to not get too far ahead. Except, I just noted the exclaimation sign after "head." As in, head motion figures into force acting on the chassis?!!! Or is that the unfinished part?

Well that got my attention  :huh:
So they have the data for the head movement but failed to implement that?

Pity  :duh:

Mental note: must figure out a bypass to get the wheel turning applied to the head movement :think:

#32 Phil

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Posted Aug 03 2010 - 06:06 AM

View PostJohn Woods, on Aug 02 2010 - 10:38 PM, said:

And it is difficult for me to believe Mosport, Kyalami, and Monza all have the same grip. I always thought the factorials of the variables offered a wide range of choice for track surface grip
All Papy tracks use Asphalt surface type (the most grippy) except the following corners, which use slippery Concrete:

Rouen: Nouveau Monde hairpin, Scierie corner and the straight towards Paradis
Nurburgring: Karussell and Mini Karussell.

The variation you feel is probably due to the amount of camber in the turns.

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Posted Aug 03 2010 - 06:13 AM

View PostJohn Woods, on Aug 02 2010 - 10:38 PM, said:

Okay, I can be patient.

A vague memory involves reading about track builders setting grip or traction variables. Isn't there something about that in the GPLEA Guides? (Stefan just posted them, guess I could check). Or somewhere. And it is difficult for me to believe Mosport, Kyalami, and Monza all have the same grip. I always thought the factorials of the variables offered a wide range of choice for track surface grip, and matching a setup to a particular unknown grip recipe was part of the art of being fast.

But I'll just sit quietly and try to not get too far ahead. Except, I just noted the exclaimation sign after "head." As in, head motion figures into force acting on the chassis?!!! Or is that the unfinished part? And clutch?

Well?Posted Image I'm waiting. Quietly...

Track makers can choose from various surfaces which have different grip coefficients, e.g. asphalt, concrete, curbs etc. which does affect traction. As an example look at Rouen where the grip varies between the penultimate and last corners to be concrete not asphalt, changing the grip to a factor of 0.96 as opposed to 1.00.

However, the bigger grip variation comes about due to camber on the road whilst using the same surface. An uncambered track with asphalt grip actually feels very slippery and is surprisingly hard to drive on, but adding even a modest amount of camber makes the road much more appealing and fun to drive on. It is actually then all about choosing the correct line and entry speed if the road is cambered or not which makes the difference as in effect the camber is changing the effective radius of the turn. It seems that Papy tried to fudge the laptimes they got by using constant grip for all main track surfaces and then adjusting the road width and/or camber to give approximately the right lap time result, or if not, then to give something that was a pleasing/fun result to drive on.

Rob

#34 Lee200

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Posted Aug 03 2010 - 06:26 AM

View PostJohn Woods, on Aug 02 2010 - 10:38 PM, said:

Except, I just noted the exclaimation sign after "head." As in, head motion figures into force acting on the chassis?!!! Or is that the unfinished part? And clutch?

Yes.  The head's mass and any movement affect the chassis.

It is unfinished and the driver's head doesn't actually move.  But if you want to know how much Jackie Stewart's head weighed, it was about 1 thousandth of a pound.  Apparently, that was before he was knighted.   :lol:

The clutch code does work, but its rigid body is only allowed freedom to rotate about the longitudinal axis and put a torque on the chassis.  It causes the chassis to rock back and forth as you rev the engine.

Lee

#35 Lee200

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Posted Aug 03 2010 - 06:31 AM

View PostMECH, on Aug 03 2010 - 04:48 AM, said:

Well that got my attention  :huh:
So they have the data for the head movement but failed to implement that?

Pity  :duh:

Mental note: must figure out a bypass to get the wheel turning applied to the head movement :think:

Hi Martin,

Have no idea how to do that.  Nigel did, but he's long gone now.

In fact, just before he left he was working on adding another two rigid bodies; one hub for each of the rear wheels in order to correct a bug where they weren't tilted properly.

Lee

Edited by Lee200, Aug 03 2010 - 07:12 AM.


#36 Lee200

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Posted Aug 03 2010 - 07:04 AM

Back to the rigid body stuff, I was a little remiss in not mentioning how important linear and angular momentum are to GPL.  Momentum is mass times velocity so at any given moment (no pun intended), GPL knows the chassis momentum in each of the six degrees of freedom.

When a force is applied over time, that value is called impulse and when GPL applies impulse to the existing momentum, a new momentum is reached which is stored for the next calculation cycle.

Momentum is used not only to determine how much the chassis rigid body is accelerated, but also for collision reaction with objects such as other cars and hay bales.   :wave:

Lee

#37 John Woods

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Posted Aug 03 2010 - 07:42 AM

Hay bales again?

#38 Lee200

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Posted Aug 03 2010 - 09:48 AM

View PostJohn Woods, on Aug 03 2010 - 07:42 AM, said:

Hay bales again?

Yes, I'm an expert on hay bales.  Grass hay is much more forgiving than alfalfa so I usually look for those to hit.  It's also much better to run into the rectangular or cubed hay bales versus the large round ones as the round ones contain much more moisture and can rust the brake rotors.

Also after a particularly bad shunt, the grass hay is particularly tasty to chew on while waiting for the ambulance.   :hat-tip:

Lee

#39 John Woods

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Posted Aug 03 2010 - 12:56 PM

Now that's some information I can use.

Just going all crossthread and off-topic, wondering about GPLTime vs. real time and the validated see-saw laptime effect, is there any chance factors like momentum and inertia have some relation to these effects with unknown causes? For instance, might they have limits at which values are re-set to zero, or re-set every two laps?

Edited by John Woods, Aug 03 2010 - 01:02 PM.


#40 brr

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Posted Aug 03 2010 - 01:07 PM

View PostJohn Woods, on Aug 03 2010 - 12:56 PM, said:

Now that's some information I can use.

Just going all crossthread and off-topic, wondering about GPLTime vs. real time and the validated see-saw laptime effect, is there any chance factors like momentum and inertia have some relation to these effects with unknown causes? For instance, might they have limits at which values are re-set to zero, or re-set every two laps?

The laptime effect is supposed to be based on full rotations of the car. Spinning the car 360 degrees has the same effect as driving one full lap.




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