Physics Assistance with suspensions.ini and HUB MASS

Mar 23, 2015
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Hello. I am trying to understand how the various parameters within suspensions.ini relate to each other. However, I am rather confused by the HUB MASS value as the comment that goes with it states "Front sprung mass". This seems to be a contradiction, as the hub is normally an unsprung mass... unless I'm very much mistaken, which happens frequently at my age.

Should this value represent the sprung weight at each hub?

(If anyone can point me in the direction of any documentation on these subjects I would be extremely grateful.)

I have also been trying to get my head around exactly how AC processes the relationships between ROD_LENGTH, BUMPSTOP_UP, BUMPSTOP_DN, PACKER_RANGE and wishbone positions. Wheelrate calculation takes into account the lower wishbone length, the lower spring mounting point, the spring's angle in relation to the lower wishbone and how much the spring compresses... or 45% of it is it? I think I understand how AC can 'assume' the spring & damper position from the wheelrate and wishbone mounting points, However, when you try to map out the various positions of wishbones, bumpstops, packers and rod length for some of the cars it seems completely mental.

Am I right in assuming that the bumpstop parameters define the point of maximum spring compression, while the packer range is actually the point of maximum spring extension (from where the packer range begins is another question though)?

Rod Length is a parameter that can be adjusted 'in-game' in order to affect ride height. I assume this works by using up some of the spring's extension travel (packer range). What is the best way to decide on an initial rod length that gives enough room for adjustment and at the same time doesn't restrict travel?

Thanking all those who made it this far in my post for your interest. :thumbsup:
 

Stereo

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Dec 22, 2009
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Yes the comment is wrong, HUB MASS is unsprung weight including wheel and tire (in case of things like control arms, you should only use the portion of their weight that moves, so roughly half)

AC doesn't assume anything about the position of spring/damper, it just calculates them based on vertical travel of the wheel. So it's like if you had an infinitely long spring straight up from the center of the hub. This is what "wheel rate" means.

My understanding of bumpstops/packer/rod length has never been all that clear so I can't guarantee this is correct, but I think bumpstop up/down are hard range limits based on design height, rod length preloads the spring, and packer is the distance the spring compresses from bumpstop_dn before it hits the bumpstop. But rod length decreases the packer range so you have to compensate for that.
 
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mclarenf1papa

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^bumpstop up/dn description right, packer wrong.

To avoid confusion with the below explanation, it’s worth clarifying that bumpstops in AC are the hard stops on the damper, not the rubber “bumpstops.”

Packer range is the distance from the current suspension position (unloaded) to what you’d call a bumpstop IRL. Real-time gap is packer_range - suspension deflection. So it moves with rod length.
 
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Kyuubeey

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As a tip, you can find the "0 point" or design point of the suspension:

Load in 1l of fuel in setup
Tap the gas to get the car to start settling
Open the Telemetry app
Open the Suspension Travel section (Values are in mm)
Match your rod_length to the values per axle in m (ie: 150 -> 0.150)

The car will now load in more or less at "zero" and your bumpstop_up and down values are referenced from that height.

Packer can be found simply via suspension_travel + distance to packer.

Example: 0.150 + 0.020 = 0.170

Remember to find the WHEEL value for these, so if your motion ratio is for example 0.95 like in a strut, and you have 20mm of stroke on the damper until hitting the rubber bumpstop, (0.020 / 0.95) = 0.02105263157

0.150 + 0.02105263157 = 0.17105263157


Here's some things from my RX7 FD3S as an example with hints:


ROD_LENGTH=0.128 ;0.133, 0.128
;-5mm driver mass

;Spring MR = 0.671
;Damper MR = 0.671

BUMPSTOP_UP=0.1043219076
BUMPSTOP_DN=0.05961251862
PACKER_RANGE=0.14290312965

;suspension travel = 0.133 + 0.00990312965
;bump stop travel = 0.010-0.005,
;0.01490312965 - 0.005 = 0.00990312965
;chassis stop travel = 0.070, 0.1043219076
;rebound stop travel = 0.040, 0.05961251862
;bumpstop length = 0.060
;70mm bump from Spirit R height
;40mm rebound from Spirit R height
 

mclarenf1papa

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Packer can be found simply via suspension_travel + distance to packer.
Rephrase to clarify: packer_range value can be found by suspension_travel + real life bumpstop gap at that ride height.

For cars with adjustable ride height, packer gap becomes inaccurate to most applications (where the actual size of the bumpstop is fixed, not the gap to it).
 
Mar 23, 2015
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Ok, so I hope no one minds if I try to summarise my understanding of all this and I apologise in advance for any exasperation I may cause.

In suspensions.ini the actual rod of ROD_LENGTH is described as a push rod, As altering the ROD_LENGTH length in Suspension Editor has no effect upon anything in the visual representation, I assume that this is a component of the damper/spring assembly. In which case I imagine it as a piston rod which acts upon the suspension wishbone at one end while the other end moves inside the damper's cylinder and acts upon the car's bodywork via the spring and the oil/gas inside the damper.

When the suspension is at 'rest' the end of the rod inside the damper's piston should also be resting at a point, let's say about halfway, between the top and bottom of the piston to permit the up and down movement of the wheel.

So, if the bumpstops in AC are actually the ends of the damper's piston rod, then in the scenario above the BUMPSTOP_DOWN value would ve the distance from the piston rod's halfway resting point inside the damper to the point where it hits the lower end of the damper's cylinder and can't move any further down. The BUMPSTOP_UP value would be the opposite measurement from the lower external end of the damper's piston rod when it's at rest to the point where it hits the bottom of the damper's cylinder and can't move any further upwards.

IRL packers are placed between the ends of the springs and the bodywork or wishbones like a kind of protective cushion. (They can also be used to raise ride height). In the above scenario, once the bumpstops have been hit in either direction, the only option left is spring compression or extension, which in turn is limited by the PACKER_RANGE and thus indicates the absolute limit of suspension movement in either direction.

One question regarding the zero design point of the suspension. Is it the centre of the wheel hub, or the hub's Y position at the resting point of an unburdened suspension... or both? :)
 

mclarenf1papa

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Comments added:

In suspensions.ini the actual rod of ROD_LENGTH is described as a push rod, As altering the ROD_LENGTH length in Suspension Editor has no effect upon anything in the visual representation, I assume that this is a component of the damper/spring assembly. In which case I imagine it as a piston rod which acts upon the suspension wishbone at one end while the other end moves inside the damper's cylinder and acts upon the car's bodywork via the spring and the oil/gas inside the damper.

When the suspension is at 'rest' the end of the rod inside the damper's piston should also be resting at a point, let's say about halfway, between the top and bottom of the piston to permit the up and down movement of the wheel.

None of this interpretation/analogy is particularly correct (or anyway particularly understandable). It's best not to try to visualize rod_length because it's not something that exists on a physical car. The simple explanation is that it acts as an offset from the design point ("0 point") of the suspension. Positive values mean positive offset and higher ride heights. It's the distance, in meters, the hub moves down relative to the design height. Alternatively (and more intuitively), it's the distance, in meters, the chassis moves up relative to the design height. This is different from BASEY which shifts the chassis points up and down on the chassis. Rod length actually compresses/decompresses the suspension (arm angle changes).

So, if the bumpstops in AC are actually the ends of the damper's piston rod, then in the scenario above the BUMPSTOP_DOWN value would ve the distance from the piston rod's halfway resting point inside the damper to the point where it hits the lower end of the damper's cylinder and can't move any further down. The BUMPSTOP_UP value would be the opposite measurement from the lower external end of the damper's piston rod when it's at rest to the point where it hits the bottom of the damper's cylinder and can't move any further upwards.
This is correct if I understand what you've written correctly. BUMPSTOP values in AC are the distance in meters from the design point to the hard stops of the suspension.

IRL packers are placed between the ends of the springs and the bodywork or wishbones like a kind of protective cushion. (They can also be used to raise ride height). In the above scenario, once the bumpstops have been hit in either direction, the only option left is spring compression or extension, which in turn is limited by the PACKER_RANGE and thus indicates the absolute limit of suspension movement in either direction.
This one is incorrect. Packers IRL are metal/plastic shims placed along the damper shaft to adjust what, in AC, are the BUMPSTOP_UP/DN values. Some people use bumpstop/packer terms interchangeably, but that's not typically correct. In reality, bumpstops, also known as bump rubbers, are rubber (sometimes plastic) stops added to the suspension that act in compression (hence "bump" in the name) to limit suspension travel by means of a non-linear spring rate in parallel with the main spring. PACKER_RANGE in AC is the distance from the current unloaded point of the suspension (0 + rod_length) to the bump rubber.

One question regarding the zero design point of the suspension. Is it the centre of the wheel hub, or the hub's Y position at the resting point of an unburdened suspension... or both? :)
In the interest of avoiding confusion, best not to mix terminology here. "Zero design point" is an arbitrary value - it's the ride height/orientation you decide your geometry resides at at 0 suspension travel and 0 rod length in AC. The coordinate origin, which I believe is what you're asking about, is the center of the wheel.
 
Mar 23, 2015
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OK, apologies for the nonsense. Due to a back injury I have been able to spend the whole day on the official AC forum. It's all much clearer now.

Thanks for all your time and trouble. :thumbsup: