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Gpl Suspension Article


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#1 Lee200

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Posted Jul 07 2014 - 05:59 PM

Introduction:

This will be a four part article that covers what we know about GPL's suspension system.  The first part will cover the springs, the second will cover the bump rubbers, the third will cover ride height, and the fourth will cover what this knowledge might imply for setups.

Much of this information was orginally discovered by Gene Fryman.  I have independently confirmed Gene's findings and added to them.

Suspension Deflection:

GPL normally uses a suspension travel of 19 centimeters (7.5 inches) from full droop to full bump.  However, it may be possible for the suspension to be compressed more than 19 centimeters and the code extrapolates the suspension force beyond the 19 centimeter compression point.

Springs:

A big misconception has been that spring rate and wheel rate (as specified in the setup menu) are different.  This is normally true in real life, but in GPL, they are exactly the same as the code does not apply any mechanical advantage from the suspension arms.  So when discussing the rates in GPL, wheel rate and spring rate can be used interchangeably.  Figure 1 is a picture of a real world shopping cart spring that is a very good depiction of how GPL's wheel and spring work together.  This clearly shows a real world suspension where wheel rate is the same as spring rate.

GPL simulates non progressive springs that are vertically mounted .  The spring uses a constant rate measured in pounds per inch of spring compression.  As the spring is compressed, the force it exerts increases at a linear rate.  Althought the code slightly tilts the springs for rather obsure reasons, for our purposes we can assume that the springs are always vertical.  Again, Figure 1 is good depiction of this.

Figure 2a is a graph that shows the force exerted by a 100 pounds per inch spring as the suspension is compressed.  The vertical axis is the spring force in pounds and the bottom axis is the suspension deflection in centimeters.  The graph shows the linear increase in spring force that continues in a straight line up to and beyond the 19 centimeter suspension deflection point.

Figure 3a is graph that shows the force exerted by a weaker 80 pounds per inch spring.  You will note the more gradual slope and lower force than with the 100 pounds per inch spring.

Attached Files


Edited by Lee200, Jul 11 2014 - 08:55 AM.


#2 Lee200

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Posted Jul 07 2014 - 06:00 PM

This is the second part of the GPL suspension article.

Bump Rubbers:

The bump rubbers are auxillary springs whose main purpose in real life is to prevent the suspension components from hitting and damaging each other when the suspension fully deflects in bump.  They are typically made from rubber and only come into play when the suspension is almost fully compressed in bump; hence the name of bump rubber.

In GPL, the bump rubber is treated as a progressive spring and its force is simply added to the normal spring force.

The bump rubber force is always 800 newtons/179 pounds when fully compressed at the 19 cm suspension deflection point.  As players, we cannot adjust the bump rubber's maximum force, but we can adjust the bump rubber length in the setup menu.  The bump rubber length is the distance before the full suspension deflection of 19 centimeters at which the bump rubber begins to compress.  So if you set a bump rubber length of 2.5 inches/6.35 centimeters, the bump rubber will start to apply force at 19.0-6.35=12.65 centimeters of suspension deflection.

The bump rubber force is progressive so there is very little force added when it first starts to compress.  Most of the force comes in the last bit of compression.

Like the springs, the code will extrapolate the bump rubber force so that it continues to increase, but at a linear rate, if the suspension deflection is more than 19 cms.

Figure 4a is a graph that shows the combined force produced by a 100 pounds per inch spring and 2.5 inch bump rubber.  You can see that the total suspension force increases at a linear rate solely due to the spring until the 12.65 centimeter point is reached.  Then the bump rubber gradually starts adding its force so that at 19 centimeters of suspension deflection, it adds 800 newtons/179 pounds of force.  Above 19 cms, the spring and bump rubber forces together increase at a linear rate.

Figure 5a is a graph that shows the combined force produced by the same 100 pounds per inch spring and a .5 inch bump rubber.  You can see that the bump rubber doesn't come into play until almost the very end of the suspension deflection and its force is added very abruptly.

So a long bump rubber adds force more gradually than a short bump rubber.

Attached Files


Edited by Lee200, Jul 11 2014 - 08:57 AM.


#3 Lee200

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Posted Jul 07 2014 - 06:00 PM

This is the third part of the suspension article.

Static Suspension Deflection:

By definition, the chassis' sprung weight sits on the springs.  So the suspension deflects and the spring compresses to oppose the weight of the car at rest.  The amount of spring compression and the force it produces in return is equal to each wheel's portion of the overall chassis sprung weight.

The amount of sprung weight on each wheel is found by adding up the chassis weight (which includes the oil and water), the fuel weight, and the driver weight and apportioning that total weight by the weight distribution percentage (longitudinal Center of Gravity position as a percentage of the wheelbase).

In GPL, the driver's weight is always 77 KGs.  As an example, the '67 Lotus chassis weighs 500 KGs and if we add 30 KGs of fuel, the total sprung weight is (500+30+77=607 KGs/1,335 pounds).

All GPL cars use a weight distribution of about 60% give or take a few percent.  So the front wheels will bear 40% of the total sprung weight which is evenly divided between the two front wheels.  In our example, the left front wheel will bear (1,335*.40/2=267 pounds).

Now if we were to use a wheel/spring rate of 100 pounds per inch for the left front tire in the setup menu, the static suspension deflection at rest would be (267/100=2.67 inches/6.8 centimeters).

Figure 6a is a graph which shows the total suspension force for a 100 pounds per inch spring and a 2.5 inch bump rubber.  The left most red circle is the static suspension deflection point of 6.8 centimeters where the spring is producing exactly 267 pounds of opposing force to the chassis sprung weight.

Ride Height:

GPL has magic springs.  Unlike real world cars where ride height changes with spring strength, GPL first computes the static suspension deflection value and then adds the player setup menu ride height to determine the suspension deflection where the chassis tub bottoms out on the track surface.  In other words, it adjusts the spring's attachment point to the chassis so that the desired ride height is achieved when the car is at rest.

Continuing with our example, if we were to use a 4 inch ride height, then 4 inches/10.2 centimeters would be added to the static suspension deflection of 6.8 centimeters.  (10.2+6.8=17.0 centimeters).  So the suspension deflection would be 17.0 centimeters if the chassis tub bottoms out on landing from a big jump.

With vertical springs and no mechanical advantage, each inch of chassis drop corresponds to one additional inch of suspension deflection/spring compression.  So if you use a 4 inch ride height, the suspension deflection/spring compression will increase by 4 inches from static when the chassis tub bottoms.

In Figure 6a, the right red circle is the suspension deflection point at which the chassis hub will bottom.  As you can see, it occurs when the bump rubber force is active.

Figure 7a is a similar graph that shows the suspension deflection and chassis hub bottom points with the same spring and bump rubbers, but with a 2.5 inch ride height.  The static suspension deflection point is the same as before, but the chassis hub bottom point occurs at only 13 centimeters of suspension deflection.  This point is about where the bump rubber begins to add its force.  So in effect, the bump rubber is never used.

Attached Files


Edited by Lee200, Jul 11 2014 - 08:58 AM.


#4 Lee200

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Posted Jul 07 2014 - 06:00 PM

This is the fourth and last part of the suspension article.

Implications For Setups:

You now know how GPL's models the suspension as far as the springs, bump rubbers, and ride height are concerned.  All three interact to determine the static suspension deflection at rest, the amount of force the suspension produces as you drive around the track and the suspension moves up and down, and the suspension deflection at which the chassis hub will bottom on landing from a jump.

Setups are very personal and there are no right or wrong settings.  What works for you may not work for others, but perhaps there are some general rules based on our knowledge of how GPL models the suspension.

Here are some thoughts:

If you look at the total suspension force graph in Figure 4a from Part 2 (reproduced below), you can see that there is an initial portion where the spring contributes all the force, then a middle curved portion where the bump rubber force is added, and lastly, the portion where the suspension force continues to increase, but at a linear rate.

I submit that you never want to operate in the far right portion where the suspension force rapidly increases as that may remove any feel for how the car is reacting to suspension movement.  So very low wheel/spring rates probably aren't good.  Remember that GPL does not model bumps in the track other than artificially so the real world need to use lower wheel/spring rates for improved grip on bumpy tracks does not apply to us.

So that leaves the first two portions of the suspension force curve as our operating area.

You could use a higher wheel/spring rate and .5 inch bump rubber length so that the bump rubbers never come into play before the chassis hub bottoms out.  Suspension forces will increase in a linear fashion until the chassis hub bottoms.

Or you could use a lower wheel/spring rate and a 2.5 inch bump rubber so that the chassis hub would bottom just as the maximum suspension deflection of 19 centimeters is reached.  This may provide a more gradual increase in suspension forces throughout the suspension's total range and give better feedback to the driver than the first option.

Only tests and your personal preference will tell which option is better for you.

I've included an Excel spreadsheet that will calculate these values for you.  Based on the chassis and fuel weights, wheel/spring rate, bump rubber length, and ride height, the spreadsheet will compute the static suspension deflection point and the chassis hub bottoming point.  You can adjust the inputs to place your suspension into the operating range you prefer.

Attached Files


Edited by Lee200, Jul 11 2014 - 09:02 AM.


#5 Lee200

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Posted Jul 07 2014 - 06:00 PM

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#6 Frenchy

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Posted Jul 07 2014 - 11:23 PM

View PostLee200, on Jul 07 2014 - 06:00 PM, said:

This is the second part of the GPL suspension article.

Bump Rubbers:

The bump rubbers are auxillary springs whose main purpose in real life is to prevent the suspension components from hitting and damaging each other when the suspension fully deflects in bump.  They are typically made from rubber and only come into play when the suspension is almost fully compressed in bump; hence the name of bump rubber.

In GPL, the bump rubber is treated as a non progressive spring and its force is simply added to the normal spring force.

The maximum force that the bump rubber produces is always 800 newtons/179 pounds when fully compressed.  As players, we cannot adjust the bump rubber's maximum force, but we can adjust the bump rubber length in the setup menu.  The bump rubber length is the distance before the full suspension deflection of 19 centimeters at which the bump rubber begins to compress.  So if you set a bump rubber length of 2.5 inches/6.35 centimeters, the bump rubber will start to apply force at 19.0-6.35=12.65 centimeters of suspension deflection.

The bump rubber force is progressive so there is very little force added when it first starts to compress.  Most of the force comes in the last bit of compression.

Figure 4 is a graph that shows the combined force produced by a 100 pounds per inch spring and 2.5 inch bump rubber.  You can see that the total suspension force increases at a linear rate solely due to the spring until the 12.65 centimeter point is reached.  Then the bump rubber gradually starts adding its force so that at 19 centimeters of suspension deflection, its force is maximum.

Figure 5 is a graph that shows the combined force produced by the same 100 pounds per inch spring and a .5 inch bump rubber.  You can see that the bump rubber doesn't come into play until almost the very end of the suspension deflection and its force is added very abruptly.

So a long bump rubber adds force more gradually than a short bump rubber even though their maximum force is the same.


Hi Lee.

Thanks very much for your excellent work. very insightful and I'm sure will be most helpful.

Just one small error I noticed in Post #2 about bump rubbers where you contradict yourself as highlighted above. Also, figure 1 shows the bump rubber force being applied as a curve whereas in Figure 2 it appears linear. ("Two people say they're Jesus, one of them must be wrong")

Thanks again
Cheers
David

#7 John Woods

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Posted Jul 07 2014 - 11:46 PM

Excellent.
Read thru once, will again and probably a few more times.

#8 Frenchy

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Posted Jul 08 2014 - 04:49 AM

I also can't quite understand the calculations for ride height. eg if all factors remain the same but you have a smaller fuel load, the amount of suspension travel to bottom out would be less when really it should be more.

#9 Fat Rich

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Posted Jul 08 2014 - 05:08 AM

Great stuff Lee, really helpful. :bravo:

I think I was assuming ride height was calculated differently so your info will probably make me look at setups in a new way.

I'm also assuming there's no tyre flex calculated in GPL, maybe the sidewalls were stiffer in the 60s than modern tyres?

View PostFrenchy, on Jul 07 2014 - 11:23 PM, said:

"Two people say they're Jesus, one of them must be wrong"




;)

Edited by Fat Rich, Jul 08 2014 - 05:25 AM.


#10 JonnyA

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Posted Jul 08 2014 - 05:23 AM

Interesting post, Lee. Thanks.

#11 Lee200

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Posted Jul 08 2014 - 07:08 AM

View PostFrenchy, on Jul 07 2014 - 11:23 PM, said:

Just one small error I noticed in Post #2 about bump rubbers where you contradict yourself as highlighted above. Also, figure 1 shows the bump rubber force being applied as a curve whereas in Figure 2 it appears linear

Yes, that was a typo.  The bump rubbers are progressive springs.

The figures were created with an Excel spreadsheet and the points were plotted every 1 cm.  With a .5 inch bump rubber, the curve is virtually a straight line between 18 and 19 cm suspension deflection points which is why Figure 2 shows it that way.  If I had plotted the curve using 1 mm points, there would be slight curve to the plot.

Edited by Lee200, Jul 08 2014 - 07:08 AM.


#12 Lee200

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Posted Jul 08 2014 - 07:48 AM

View PostFrenchy, on Jul 08 2014 - 04:49 AM, said:

I also can't quite understand the calculations for ride height. eg if all factors remain the same but you have a smaller fuel load, the amount of suspension travel to bottom out would be less when really it should be more.

No, it would be exactly the same.

With a lighter fuel load, the static suspension deflection would be less.  But if the ride height remains the same, it will take the same additional suspension deflection to reach the bottoming point.

With vertical springs and no mechanical advantage, each inch of chassis drop corresponds to one inch of suspension deflection/spring compression.  So if you use a 4 inch ride height, the suspension deflection/spring compression will also be 4 inches when the chassis tub bottoms.

Edited by Lee200, Jul 08 2014 - 07:50 AM.


#13 Frenchy

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Posted Jul 08 2014 - 08:50 AM

View PostLee200, on Jul 08 2014 - 07:08 AM, said:

View PostFrenchy, on Jul 07 2014 - 11:23 PM, said:

Just one small error I noticed in Post #2 about bump rubbers where you contradict yourself as highlighted above. Also, figure 1 shows the bump rubber force being applied as a curve whereas in Figure 2 it appears linear

Yes, that was a typo.  The bump rubbers are progressive springs.

The figures were created with an Excel spreadsheet and the points were plotted every 1 cm.  With a .5 inch bump rubber, the curve is virtually a straight line between 18 and 19 cm suspension deflection points which is why Figure 2 shows it that way.  If I had plotted the curve using 1 mm points, there would be slight curve to the plot.

Ok cool

#14 Frenchy

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Posted Jul 08 2014 - 09:03 AM

View PostLee200, on Jul 08 2014 - 07:48 AM, said:

View PostFrenchy, on Jul 08 2014 - 04:49 AM, said:

I also can't quite understand the calculations for ride height. eg if all factors remain the same but you have a smaller fuel load, the amount of suspension travel to bottom out would be less when really it should be more.

No, it would be exactly the same.

With a lighter fuel load, the static suspension deflection would be less.  But if the ride height remains the same, it will take the same additional suspension deflection to reach the bottoming point.

With vertical springs and no mechanical advantage, each inch of chassis drop corresponds to one inch of suspension deflection/spring compression.  So if you use a 4 inch ride height, the suspension deflection/spring compression will also be 4 inches when the chassis tub bottoms.

Ok I get it now. Sorry for being thick. So that means if I qualify on light fuel and then fill up for the race, the mechanics automatically pack up the suspension to maintain my selected ride height. or to put it another way, adding fuel doesn't mean you'll start bottoming out if you weren't previously, so no need to increase ride height.

Can you also confirm that GPL takes in to account the slope of the track and the length of the car when calculating when the chassis tub bottoms out? eg I've always felt that braking too late in to the rise at Moss at Mosport is what made the nose bottom out with catastrophic consequences. Changing ride height or even stiffer springs there doesn't seem to make much difference (without going to extremes) but braking a bit earlier and gentler seems the only solution.

#15 Lee200

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Posted Jul 08 2014 - 09:41 AM

View PostFrenchy, on Jul 08 2014 - 09:03 AM, said:

So that means if I qualify on light fuel and then fill up for the race, the mechanics automatically pack up the suspension to maintain my selected ride height. or to put it another way, adding fuel doesn't mean you'll start bottoming out if you weren't previously, so no need to increase ride height.

Can you also confirm that GPL takes in to account the slope of the track and the length of the car when calculating when the chassis tub bottoms out? eg I've always felt that braking too late in to the rise at Moss at Mosport is what made the nose bottom out with catastrophic consequences. Changing ride height or even stiffer springs there doesn't seem to make much difference (without going to extremes) but braking a bit earlier and gentler seems the only solution.

That is correct.  The mechanics magically adjust the spring attachment point so that the suspension starts with your selected ride height regardless of fuel load and sprung weight.  :)

Yes, I believe it true that the track's slope is taken into account.  So if the track begins dropping away from you when entering a dip, then the front part of the chassis is momentarily farther away from the track's surface than the rear and it's instantaneous ride height is more.  The opposite occurs when you exit the dip and track is rising to meet you.  Hope that makes sense.

Edited by Lee200, Jul 08 2014 - 12:34 PM.





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