Spring Rate/Motion Ratio Calculator
Posted by RALLYRS
Mike Ball Mega Moderator Location: Simi Valley,Ca. Join Date: 07/15/2011 Age: Ancient Posts: 466 Rally Car: Nope...I wish...RWD 2 Door Jeep XJ 4.0 5-spd Dirt-o-cross car(we have no grass!)2.3 ZX3 rallyx car(sold) |
John Vanlandingham Senior Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Mike Ball Mega Moderator Location: Simi Valley,Ca. Join Date: 07/15/2011 Age: Ancient Posts: 466 Rally Car: Nope...I wish...RWD 2 Door Jeep XJ 4.0 5-spd Dirt-o-cross car(we have no grass!)2.3 ZX3 rallyx car(sold) |
Banned Super Moderator Location: La la land Join Date: 01/25/2006 Age: Fossilized Posts: 3,740 Rally Car: Not a Volvo |
John Vanlandingham Senior Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Actually Gene there are lots of whatever each brand calls it "semi-trailing arm" , "swing arm' what evar! Vitrtually all BMW for the longest time had the spring approx 35-50% forward of the spindle and then the shock somewhere else. IRS Mercedes had things obviously delivered from the same sub-supplier as Ford bought their Sierra and Scorpio stuff, Lookie here---a semi-futile attempt to disabuse a couple of the worst sort of "Injur-nears" of their gross blunders about leverage on the Xratty arm 
Even this didn't dissuade them from selling to street guys 750-850 pound/inch springs 
The wheel pushes on the arm, OK? Simple if/then deal IF the spring was directly over the spindle and it was a 300 lb/in spring and you applied 300 lbs it would move 1 " Then if the spring was 50% of the way UP the swing arm, the spindle has an advantage over the spring (the spring becomes like an inverted fulcrum for visualisation).. So IF you wanted the WHEEL RATE to be 300 lbs= 1 inch movement then the spring must be double. In the Xratty case the spring sits roughtly 30% of the way up the arm, the old 510 it was around 50% up (never measured, just recalling) so there's approx 30% or 50% advantage over the spring... these genii said it was a 240% advantage.... Things that pivot are levers, springs are spring and are linear unless they're not most of this is simple arithmetic because mostly there's not truly crazy variable geometry changes, just say the 14-15o angle of the strut and position if front or rear of the spindle..measure, squint make a good gut-feeling guess from known to whatever your big idea is... Why do people always want to obsess on this endlessly and then get things so horribly wrong nearly 95% of the time? John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
Banned Super Moderator Location: La la land Join Date: 01/25/2006 Age: Fossilized Posts: 3,740 Rally Car: Not a Volvo |
starion887 Super Moderator Join Date: 09/06/2006 Posts: 798 |
no-one of consequence Elite Moderator Location: Florin Join Date: 01/06/2013 Posts: 395 |
Evan Horner Godlike Moderator Location: Columbus OH Join Date: 01/03/2012 Age: Settling Down Posts: 109 Rally Car: 1983 Mazda RX7 GSL |
no-one of consequence Elite Moderator Location: Florin Join Date: 01/06/2013 Posts: 395 |
no-one of consequence Elite Moderator Location: Florin Join Date: 01/06/2013 Posts: 395 |
Mike Ball Mega Moderator Location: Simi Valley,Ca. Join Date: 07/15/2011 Age: Ancient Posts: 466 Rally Car: Nope...I wish...RWD 2 Door Jeep XJ 4.0 5-spd Dirt-o-cross car(we have no grass!)2.3 ZX3 rallyx car(sold) |
I'm absorbing all the input you guys are throwing out there,but still think the calculator I posted should work ok with my Focus rear suspension.
Lets say, for classing issues,I need to retain the coil in the arm, as opposed to a "proper " coilover position,but wish to change to a spring with more rate. This is not my car,but a pic off the net and it's yellow coils makes it easy to see the spring position: 
And a pic of a Focus control arm to show where the spring mounts: 
Now I'm not saying this spring is not angled at all,but it's pretty much upright. No trailing arm mounting issues or shock position beyond centerline, etc. Ok,yes it has rubber bushings(which will give some resistance)and the control arm travels in an arc (duh!)but it seems like I should be kinda in the ball park..wouldn't you say? And another question. The calculator says"at the balljoint" concerning rate. Obviously I don't have a ball joint in back. Should I measure to outer arm pivot? Wheel mounting surface? I thought we usually refer to "wheel rate" and measure to wheel mounting surface as opposed to ball joint, as it's several inches difference.. ................................................. ..................... Support your Local North American Rally Forum!! While they are still around-and get the hell off Farsebook!! We still have Specialstage & RallyAnarchy. Post up Here: https://rallyanarchy.com/phorum/posting.php and here: https://www.specialstage.com/forums/forum.php |
John Vanlandingham Senior Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Dammit "classing issues". We--YOU should not be giving a iota of thought towards "classing issues"... 2wd dammit. maybe "2wd n.a." and "2wd not n.a." You should be thinking about things that make your job---learning to drive the car hard and fast---EASIER....even if its a little easy work rather that voluntarily searching for ways to box yourself into some dumb stupid Production realted class where you cannot do anything easy and have to fuck with bullshit like figuring out what spring rate is, ---when the fucking obvious thing is right in the photom, manm: how will you get the spring rate and travel you want and set the car at the right ride height you desire---when you force yourself for no good reason to jam some unknown and unknowable spring between the subframe and the arm? Why make life unnecessarily difficult when there are easy ways to do good? John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
Evan Horner Godlike Moderator Location: Columbus OH Join Date: 01/03/2012 Age: Settling Down Posts: 109 Rally Car: 1983 Mazda RX7 GSL |
No not quite. Here I will use Paint to create a very simplistic 2D representation and explain. So, the tire contact patch is going to always be subject to 3 forces and 3 moments (when the vehicle is in motion and the tire is in contact with the ground). For the sake of simplicity, we are going to ignore all but two of the forces, and focus a little on FY (Lateral force) and mostly on FZ (Normal force). These forces are constrained by the control arms/trailing arms/swingarms/struts/whatever. A component of each force will (nearly) always go into each suspension arm. In looking at my lovely diagram, you can see that in droop, the Lateral and Normal forces are partly going into the spring/damper, but also a component of each force is also going into the lower control arm (LCA). This will effectively increase your ride rate, and stiffen your suspension, because now you are partly using your control arm and chassis rigidity as a spring in vertical wheel motions. In a perfect world, your control arm and chassis have an infinite spring rate. So lets do some math: Ride Rate[FZ, droop] = [component of] Fspring + [component of] Fchassis{infinity} > Fspring This is going to be regardless of what your geometric motion ratio is. Now look at the same suspension in bump. The LCA is at a completely different angle, specifically one that is unable to transfer (compression) loads through itself and into the chassis. This means that all of FZ is now being transfered through the spring, and so math: Ride Rate[FZ, bump] = Fspring = Fspring. This will also have nothing to do with your geometric motion ratio. So in this example, Fchassis{infinity} makes the ride rate in droop stiffer than it is in bump, which creates a regressive motion ratio. 
Please, someone correct me if I am wrong. "The more I learn, the more I realize I don't know and have so much more to learn." - Claude Rouelle, Optimum G lecture June, 2011 |
no-one of consequence Elite Moderator Location: Florin Join Date: 01/06/2013 Posts: 395 |
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