RALLYRS Mike Ball Infallible 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) |
Saw this posted elsewhere and thought it was kinda nice and simple.
If anyone has their actual ratio/rate figures for their car handy-please let me know if it's actually accurate-math ain't my strong suit! http://www.proshocks.com/calcs/imotion.htm ................................................. ..................... 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 John Vanlandingham Mega Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Too simplistic, does not account for shock angle or shock postions beyond "ball joint" centerline.. Fail. 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. |
RALLYRS Mike Ball Infallible 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) |
Gotcha-too good to be true-oh well...... ................................................. ..................... 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 |
heymagic Banned Mega Moderator Location: La la land Join Date: 01/25/2006 Age: Fossilized Posts: 3,740 Rally Car: Not a Volvo |
JV is more than correct. The old 510s are one of the best examples. Rear springs were mounted inboard on the trailing arm. Spring rates had to be something around double or triple of a conventional strut spring. I'm sure the chart will work for some springs on some cars tho.
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john vanlandingham John Vanlandingham Mega 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. |
heymagic Banned Mega Moderator Location: La la land Join Date: 01/25/2006 Age: Fossilized Posts: 3,740 Rally Car: Not a Volvo |
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The formula cited is right; the squaring factor of the arm ratio in the formula is correct and is something that simplistic views typically fail to account for. But like John and Gene say, it does not account for things like:
- angle of the spring versus the control arm which can change a lot in the full range of motion ('specially in a rally car) and which would require a stronger spring as angle between spring centerline and arm increases - the effective spring rate of fixed rubber bushings in the control arm eyes (which adds spring-like force in the compression direction but fights the spring in extension) So as a general formula, to account for some basic changes, it is OK but does not get you the whole story. |
JVL is completely correct. For example, over the entire range of travel, my front motion ratio varies from .94 to .97 back down to .95. I can't imagine anyone who is serious just using .96 (which is correct at static ride height) for anything except arguing on the internet. Same goes for the rear, where the Great Unwashed often uses .84 for everything when we know it can be as low as .83 or as high as .85. [/snark] |
MeCalledEvan Evan Horner Godlike Moderator Location: Columbus OH Join Date: 01/03/2012 Age: Settling Down Posts: 109 Rally Car: 1983 Mazda RX7 GSL |
I also agree with JV/others. Since suspension arms travel in arcs, the motion ratio will change continuously thoughout the full length of wheel travel.
Additionally, a motion ratio is dependent on both the geometric position of the suspension links (resulting in wheel displacment vs. spring/shock displacement), but also dependent on the force distribution from the spindle into spring, shock, suspension arms/bushings, etc. I saw a whole lot of people misunderstanding this concept when doing Formula SAE in school. Students would design their car to have (geometrically) linear motion ratios, but not run the statics or trig and then end up with highly progressive or regressive ride rates. Resulted in a lot of confusion on why their open-wheeled race car was handling like shit. The only vehicle that I have come across that has a 'linear', well, 1D motion ratio is a go kart. But then if you consider chassis and tire flex, not even they do... "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 |
Are you saying that some forces can cause suspension components to bend and that this can alter the motion ratio? If so, then I agree.
Otherwise, please explain why you believe that motion ratio depends on force (of any sort). The only place where I can see force coming into play is if you relax the typical simplifying assumption that the "tires" are infinitely narrow and infinitely stiff. |
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RALLYRS Mike Ball Infallible 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 John Vanlandingham Mega 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. |
MeCalledEvan 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 |
Wow. Lots of crossing posts. This is to Mike.
Given that the knuckle moves with the eye of the outer pivot on that lower LCA: you measure the distance between the two pivot-points (at the ends of the lower LCA) and then measure from either end to the center of the spring's seat. From this you calculate the location of the spring's seat as a proportion of the distance from the inboard to outboard pivots. This is your motion ratio. To convert a spring-rate to a wheel-rate, you multiple the spring-rate by the square of the motion ratio. To convert a desired wheel-rate to a needed spring-rate, you divide the wheel-rate by the square of the motio ratio. In general, your wheel-rate is much lower than the spring-rate due to the mechanical advantage of the spring not being out at the end and, therefore, not moving as much as the wheel. If you want an analogy: think nut-cracker. Note: some people will jump all over this simplistic approach on the grounds that the motion ratio changes as the lower LCA moves. To such people I say: "yes, you are correct, but have you actually done the trig needed to see by how much it changes?" When they admit that they haven't (as has always been the case so far), I smile. You see, I've done the trig. And the changes for most suspension designs are ignorably small. If you're buying HyperCo or Swift springs, for example, which are only guaranteed to be within 5% of stated value, then that's a bigger deal than the changes in motion ratio as a function of bump/droop. Edited 1 time(s). Last edit at 10/21/2013 02:21PM by Iowa999. |