Car physics; anti-roll bar units

Hi all, I'm trying to figure out how I should describe an anti-roll bar in my sim. Previously, in SCGT I saw a length value, in mm (millimeters). But when I read about the rollbars, I see it as a spring, where you get something like: d1=suspension_compression_left (relative to the rest position) d2=suspension_compression_right (...) d=d1-d2 k=arb_spring_coefficient (arb=anti-roll bar) F=k*d Where the last force, F, is applied to the left suspension as positive, and negative to the right suspension (or vice versa ;-). Probably 'arb_spring_coefficient' should rather be named 'arb_torsion_stiffness', right? What would the SCGT value describe? I read that the torsion stiffness of the middle part of the ARB goes up proportionally with length^4, so there's a link to SCGT's ARB-length, but there must then be a magic coefficient again where: torsion_stiffness=magic_coeff*arb_middle_length; And ofcourse you get the eternal question 'what is a good value of 'magic_coeff'? I'll have a look at all the books to see if I can get some realistic values for anti-rollbars. I must say in most literature I've come across the anti-rollbar is quickly handled without too much indepth information. I learned the most on a website (http://gallery.uunet.be/heremanss/). Thanks for any ideas on how to define anti-rollbars in an accepted way, Ruud van Gaal, GPL Rank +53.25 Pencil art : http://www.marketgraph.nl/gallery/ Free car sim : http://www.marketgraph.nl/gallery/racer/

Ruud van Gaal wrote in message <[email hidden]>...

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>Hi all, > >I'm trying to figure out how I should describe an anti-roll bar in my >sim. >Previously, in SCGT I saw a length value, in mm (millimeters). But >when I read about the rollbars, I see it as a spring, where you get >something like: >

Was the SCGT setting definately length, and not diameter? In most (road) cars, you would change diameter and keep the basic design the same. In race cars the mounting points of the pushrods to the bar are movable (along the bar) to quickly adjust the rate. Ben

Ruud van Gaal wrote:

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> > Hi all, > > I'm trying to figure out how I should describe an anti-roll bar in my > sim. > Previously, in SCGT I saw a length value, in mm (millimeters). But > when I read about the rollbars, I see it as a spring, where you get > something like: > > d1=suspension_compression_left (relative to the rest position) > d2=suspension_compression_right (...) > > d=d1-d2 > k=arb_spring_coefficient (arb=anti-roll bar) > F=k*d > > Where the last force, F, is applied to the left suspension as > positive, and negative to the right suspension (or vice versa ;-). > Probably 'arb_spring_coefficient' should rather be named > 'arb_torsion_stiffness', right?

Actually calculating the stiffness is complex, because of the complex shapes used. Particularly for systems where the driver can adjust the ARB from the cockpit. They also have the same spring rate to wheel rate geometric issues as the springs. Since your driver will put in a roll bar "stiff enough" for his application, the model you described above will work (as long as the signs are correct :) The goal is to provide additional suspension force when the compression states are different on the two sides. > > What would the SCGT value describe? I read that the torsion stiffness > of the middle part of the ARB goes up proportionally with length^4, so > there's a link to SCGT's ARB-length, but there must then be a magic > coefficient again where: It's more like bar diameter to the 4th power, or outer diameter to the 4th - inner diameter to the 4th power if the bar is hollow (usual). So you would also have to decide on a plan to handle standard bar thicknesses :) The stiffness would be inversely proportional to the length. (the longer it is the weaker.) Staniworth has a good section. Watch for units problems though. Puhn gives equations for angled bars (and several photographs of hardware.) Try setting it up so that the effective spring rate is around half your suspension rate to start. Note that because of the powers of 4 and 2 and the large torsional resisting moment of steel, etc., in the various equations and units conversions, it is easy to make a huge mistake :) So since you are selecting your bar geometry somewhat arbitrarily (unless you go all the way to bar dimensions and installation geometry, etc.) just make sure that the overall effect was as you planned when you test. "How to Make Your Car Handle," Fred Puhn, HPBooks (The Berkley Publishing Group), N.Y. N.Y., 1981. ISBN 0-912656-46-8 "Competition Car Suspension, Design, Construction, Tuning," Allan Staniforth, 2nd Ed., Racecar Engineering / G.T. Foulis & Company / Haynes Publishing, Sparkford, Near Yeovil, Somerset BA227JJ, England, 1994. ISBN 0-85429-956-4 -- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

On Wed, 13 Jun 2001 01:41:16 +1000, "Ben Coleman" <[email hidden]> wrote:

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> >Ruud van Gaal wrote in message <[email hidden]>... >>Hi all, >> >>I'm trying to figure out how I should describe an anti-roll bar in my >>sim. >>Previously, in SCGT I saw a length value, in mm (millimeters). But >>when I read about the rollbars, I see it as a spring, where you get >>something like: >> > >Was the SCGT setting definately length, and not diameter?

Hm, interesting, I should check again how the call it. Probably just 'anti-roll bar: xxx mm'. Thanks for the info, Ruud van Gaal, GPL Rank +53.25 Pencil art : http://www.marketgraph.nl/gallery/ Free car sim : http://www.marketgraph.nl/gallery/racer/

On Tue, 12 Jun 2001 16:51:58 GMT, mjessick-Motorsims <[email hidden]> wrote: ... >Actually calculating the stiffness is complex, because of the >complex shapes used. Particularly for systems where the >driver can adjust the ARB from the cockpit. They also have >the same spring rate to wheel rate geometric issues as the springs. Sounds more complex than what I need, which is like the wheel rate; the effect on the suspension is what I need, and the less I define geometrically, the better, I guess (for now).

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>> What would the SCGT value describe? I read that the torsion stiffness >> of the middle part of the ARB goes up proportionally with length^4, so >> there's a link to SCGT's ARB-length, but there must then be a magic >> coefficient again where: > >It's more like bar diameter to the 4th power, or >outer diameter to the 4th - inner diameter to the 4th power >if the bar is hollow (usual). So you would also have to decide >on a plan to handle standard bar thicknesses :)

Let's not. :) Didn't know the bar was often hollow, let alone know about standard bar thicknesses. ;-) >Staniworth has a good section. Watch for units problems though. >Puhn gives equations for angled bars (and several >photographs of hardware.) Photographs do clarify things often; I'd never seen an anti-roll bar before a month ago on a site, and it gave me a better idea of how this thing actually worked.

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>"How to Make Your Car Handle," Fred Puhn, HPBooks >(The Berkley Publishing Group), N.Y. N.Y., 1981. ISBN 0-912656-46-8 > >"Competition Car Suspension, Design, Construction, Tuning," >Allan Staniforth, 2nd Ed., Racecar Engineering / G.T. Foulis & Company / >Haynes Publishing, Sparkford, Near Yeovil, Somerset BA227JJ, England, >1994. >ISBN 0-85429-956-4

More books! I just obtained 'Tires, Suspension and Handling' from John Dixon, so I guess I'll be busy for a while. :) I can't believe the amount of work that goes into a carsim, with all this ever-increasing detailed theory. But it seems worth it. :) Thanks, Ruud van Gaal, GPL Rank +53.25 Pencil art : http://www.marketgraph.nl/gallery/ Free car sim : http://www.marketgraph.nl/gallery/racer/

Hi Ruud, Matt's post is excellent, I saved it because it's far beyond anything I've tried yet ;-) If you want a simpler approximation to start with, the anti-roll bars (if I remember correctly) are usually measured in units-force/degree-twist. Try taking an orientation vector across the front/rear two wheels, then get the angle between that vector and the chassis roll vector. The difference would be the degrees of twist in the anti-roll bar. Multiply that by the anti-roll bar "torsion rate", which could probably be treated as a torque on the chassis, then divided by the distance from the center of the car to each wheel to get a vertical force at each wheel. I tried this some time ago and it worked pretty well, even though it doesn't anything else into account. The thickness (if that's what it is) is just a way of calculating the spring rate. Perhaps it would be quicker and easier to bypass that. Aside from that, I'm about to rewrite a huge chunk of my engine, seeing as I *finally* got matrices and inertia tensors working properly, so I won't be spending time on anti-roll bars again for a little bit :-) No quats yet, this is tough enough as it is already ;-) Anyway, maybe that's enough to get you going. BTW, when picking anti-rollbar rates with this approach, I tried using a soft spring/shock setting (not the bars, but the regular springs), then raising the anti-roll bar rates to limit roll to around 3 degrees or so. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

I would think that what you use to calculate the interaction between sides of suspension and what the player uses to select it do not need to be the same, as long as they have some constant scale (or exponential scale, anyway). Since the length and geometry is arbitrary, the player only needs to select diameter or length of lever or both and not see the hoo-har used to calculate loads. Ben J. Todd Wasson wrote in message <[email hidden]>... > Hi Ruud, > > Matt's post is excellent, I saved it because it's far beyond anythingI've >tried yet ;-) > > If you want a simpler approximation to start with, the anti-roll bars (ifI >remember correctly) are usually measured in units-force/degree-twist. Try >taking an orientation vector across the front/rear two wheels, then get the >angle between that vector and the chassis roll vector. The differencewould be >the degrees of twist in the anti-roll bar. Multiply that by the anti-rollbar >"torsion rate", which could probably be treated as a torque on the chassis, >then divided by the distance from the center of the car to each wheel toget a >vertical force at each wheel. I tried this some time ago and it workedpretty >well, even though it doesn't anything else into account. > The thickness (if that's what it is) is just a way of calculating thespring >rate. Perhaps it would be quicker and easier to bypass that. Aside fromthat, >I'm about to rewrite a huge chunk of my engine, seeing as I *finally* got >matrices and inertia tensors working properly, so I won't be spending timeon >anti-roll bars again for a little bit :-) No quats yet, this is toughenough >as it is already ;-) > > Anyway, maybe that's enough to get you going. BTW, when pickinganti-rollbar >rates with this approach, I tried using a soft spring/shock setting (notthe >bars, but the regular springs), then raising the anti-roll bar rates tolimit

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>roll to around 3 degrees or so. > > > > >Todd Wasson >--- >Performance Simulations >Drag Racing and Top Speed Prediction >Software >http://PerformanceSimulations.Com

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>I would think that what you use to calculate the interaction between sides >of suspension and what the player uses to select it do not need to be the >same, as long as they have some constant scale (or exponential scale, >anyway). Since the length and geometry is arbitrary, the player only needs >to select diameter or length of lever or both and not see the hoo-har used >to calculate loads. > >Ben >

Absolutely, the player doesn't need to see all the resulting "hoo-har" :-), but just needs to have a couple of inputs to use to set up the anti-rollbars in the first place like you said, such as diameter or length. What I was suggesting to Ruud is to skip that stuff at first, base his force/torque calculations off the angle of twist between the wheels/axle-centerline and chassis, then just set the spring rate directly to get some form of anti-roll bars working in his model. After that, if he'd like to look up equations that produce the spring rate from other types of inputs (length, diameter, mounting point/geometric stuff), it would be an option. Then, he could just plug the new calculated spring rate into the model he'd already written to handle data in Newton-meters/degree anti-rollbar twist. Then, it'd (probably) be an easy little module to change if and when he wanted to add more "complexity" to it, meaning more inputs for the bars, without having to change the underlying code that really just runs off the spring rate anyway. I haven't studied or thought about the actual kinematics of anti-roll bars much at all yet, so at first I would suspect the length and geometry is probably not arbitrary, given what little I've been learning lately about suspension kinematics, but I'd check it out to make sure anyway. However, since Matt Jessick indicated that these are large factors, I'll take his word for it, 'cause he knows a lot more than I do :-). I'm not yet to the point where I can hook up an anti-roll bar system and make accurate calculations based on installation stuff like he is. Am I following what you were saying there? Or am I missing your point ? :-) I do that sometimes.... :-P Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

> > Am I following what you were saying there? Or am I missing your point ?:-)

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> I do that sometimes.... :-P > >Todd Wasson >--- >Performance Simulations >Drag Racing and Top Speed Prediction >Software >http://PerformanceSimulations.Com

I'm not sure I had a point! 8^() I only know what my own car has, which is pretty basic: a pushrod halfway up the lower wishbone acting on the most basic bar. This is 1964 technology at its finest. I haven't seen any system that would contribute any different forces...though I haven't really looked into it either. There's a whole world you guys are delving into that I haven't really looked at beyond posts here and magazines. Maybe some day....or maybe not! Cheers, Ben