Car physics: wheelrates vs springrates

Hi all, I'm getting reports that my Racer sim doesn't deal with low spring rates nicely (at the suspension). I use wheel rates to move around installation ratio's. I'm under the impression that wheel rates are ALWAYS lower than spring rates. But is this really true? I have some ideas on this. With low spring rates, might a regular bump out quickly, or is normally the bump stop on a normal street car never reached? (say on a Ford Focus with 22500 N/m and 17500 N/m springs (not wheelrates)) I have a bug I know of where if the suspension hits the bump stop, I still only take the suspension spring compression (F=k*d) as the upward forces. It should actually then be the tirerate, which is transferred more directly to the body. However, I feel that this situation should normally not arise (hitting the bump stops) during normal acceleration and braking. Any ideas? Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

On Fri, 13 Sep 2002 12:17:21 GMT, [email hidden] (Ruud van Gaal) wrote: >I have a bug I know of where if the suspension hits the bump stop, I >still only take the suspension spring compression (F=k*d) I can't help with your question, but that formula is obcene. =) Jason ----- GPLRank 24.50 N2002Rank -12.995

On Fri, 13 Sep 2002 16:27:11 GMT, [email hidden] (jason moyer) wrote:

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>On Fri, 13 Sep 2002 12:17:21 GMT, [email hidden] (Ruud van Gaal) >wrote: > >>I have a bug I know of where if the suspension hits the bump stop, I >>still only take the suspension spring compression (F=k*d) > >I can't help with your question, but that formula is obcene. =)

LMAO! Hitting the bump stop now sounds obscene too. ;-) Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

[email hidden] (Ruud van Gaal) wrote in message news:<[email hidden]>...

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> Hi all, > > I'm getting reports that my Racer sim doesn't deal with low spring > rates nicely (at the suspension). > > I use wheel rates to move around installation ratio's. I'm under the > impression that wheel rates are ALWAYS lower than spring rates. But is > this really true? > > I have some ideas on this. With low spring rates, might a regular bump > out quickly, or is normally the bump stop on a normal street car never > reached? (say on a Ford Focus with 22500 N/m and 17500 N/m springs > (not wheelrates)) > > I have a bug I know of where if the suspension hits the bump stop, I > still only take the suspension spring compression (F=k*d) as the > upward forces. It should actually then be the tirerate, which is > transferred more directly to the body. > However, I feel that this situation should normally not arise (hitting > the bump stops) during normal acceleration and braking. > > Any ideas? > > Ruud van Gaal > Free car sim: http://www.racer.nl/ > Pencil art : http://www.marketgraph.nl/gallery/

Hi Ruud I don't know if I can help you, but my impression is that wheel rates (don't misstake for tire rate!) in a normal suspension system is always lower than the spring rate. In certain cases the wheel rate is equal the spring rate. One such case is when the spring is considered purely vertical, and is connected directly to the wheel center. Here's a good link showing how to calculate the wheel rate: http://www.eibach.com/ERS_Worksheet.htm

>I use wheel rates to move around installation ratio's. I'm under the >impression that wheel rates are ALWAYS lower than spring rates. But is >this really true? > Wheel rate, yes (unless you've got a really bizarre suspension setup). For it to be equal to or greater, the spring would have to be on the wrong side of the wheel. Or... If you had a straight axle, the spring rate and wheel rate could equal each other in vertical movement. >I have some ideas on this. With low spring rates, might a regular bump >out quickly, or is normally the bump stop on a normal street car never >reached? (say on a Ford Focus with 22500 N/m and 17500 N/m springs >(not wheelrates)) > The only time I've ever hit the bumpstops on a real car is while jumping it :-) During regular driving, or even racing, you probably will never hit them unless you've changed the bumpstop. >I have a bug I know of where if the suspension hits the bump stop, I >still only take the suspension spring compression (F=k*d) as the >upward forces. It should actually then be the tirerate, which is >transferred more directly to the body. >However, I feel that this situation should normally not arise (hitting >the bump stops) during normal acceleration and braking. Well, I guess you're F=k*d ;-) (Thanks, Jason Moyer) :-P If you hit the bump stop you're hitting another, much stiffer, rubber spring in series with the suspension spring. F=k*d then becomes F=k*d^2 ;-) Kidding again of course, but you've got another spring rate to add in at that point. And no, this should never happen during "normal" operation, i.e., tearing around a race track as hard as you can with a stock car. If you wanted, you could probably do an impulse collision at the bumpstop location instead, but I think that would be asking for trouble... That's not what really happens anyway so a stiffer spring would be more realistic. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com My little car sim screenshots: http://performancesimulations.com/scnshot4.htm

On Sat, 14 Sep 2002 00:22:06 +0200, J. Todd Wasson wrote:

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>>I have a bug I know of where if the suspension hits the bump stop, I >>still only take the suspension spring compression (F=k*d) as the upward >>forces. It should actually then be the tirerate, which is transferred >>more directly to the body. However, I feel that this situation should >>normally not arise (hitting the bump stops) during normal acceleration >>and braking. > > Well, I guess you're F=k*d ;-) (Thanks, Jason Moyer) :-P > > If you hit the bump stop you're hitting another, much stiffer, rubber > spring in series with the suspension spring. F=k*d then becomes F=k*d^2 > ;-) Kidding again of course, but you've got another spring rate to add > in at that point.

The bump spring rates are usually very high which put rather annoying demands on the numerics. I don't know how the simulation is performed, but if it is done solving e.g. a state-space using a fixed step ODE-solver like Runge-Kutta 4th-order, the system will probably get unstable... Unless the time-stepping is done adaptively (large CPU-load!).

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> And no, this should never happen during "normal" operation, i.e., > tearing around a race track as hard as you can with a stock car. > > If you wanted, you could probably do an impulse collision at the > bumpstop location instead, but I think that would be asking for > trouble... That's not what really happens anyway so a stiffer spring > would be more realistic.

"Cheating" is sometimes the way to go! /Stefan Larsson

J. Todd Wasson <[email hidden]> wrote in message news:[email hidden]...

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> >I use wheel rates to move around installation ratio's. I'm under the > >impression that wheel rates are ALWAYS lower than spring rates. But is > >this really true? > > > > Wheel rate, yes (unless you've got a really bizarre suspension setup). For it > to be equal to or greater, the spring would have to be on the wrong side of the > wheel. Or... If you had a straight axle, the spring rate and wheel rate could > equal each other in vertical movement.

Single-seaters running push- or pull-rod suspension could have wheel rate higher than spring rate... Jonny

>Single-seaters running push- or pull-rod suspension could have >wheel rate higher than spring rate... > >Jonny Good point, I didn't think of that. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com My little car sim screenshots: http://performancesimulations.com/scnshot4.htm

goodness what's on your mind ? "Ruud van Gaal" <[email hidden]> wrote in message news:[email hidden]... > On Fri, 13 Sep 2002 16:27:11 GMT, [email hidden](jason moyer) > wrote: > > >On Fri, 13 Sep 2002 12:17:21 GMT,[email hidden] (Ruud van Gaal) > >wrote: > > > >>I have a bug I know of where if the suspension hitsthe bump stop, I > >>still only take the suspension spring compression(F=k*d) > > > >I can't help with your question, but that formula isobcene. =)

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> > LMAO! > > Hitting the bump stop now sounds obscene too. ;-) > > > Ruud van Gaal > Free car sim: http://www.racer.nl/ > Pencil art : http://www.marketgraph.nl/gallery/

On Sun, 15 Sep 2002 14:40:05 +0800, "John" <[email hidden]> wrote: >goodness >what's on your mind ? The cinema in my head generally tends to quite interesting. He/she who invents a device to record brain images to DVD will surely prosper. ;-) Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

On Mon, 16 Sep 2002 13:09:22 GMT, Ruud van Gaal wrote:

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> On Sun, 15 Sep 2002 14:40:05 +0800, "John" <[email hidden]> wrote: > >>goodness >>what's on your mind ? > > The cinema in my head generally tends to quite interesting. He/she who > invents a device to record brain images to DVD will surely prosper. > ;-) >

Hey Ruud, just a quick note to let you know your post made it to a.h.b-o-u fame ;-) Regards, uwe -- mail replies to Uwe at schuerkamp dot de ( yahoo address is spambox) Uwe Schuerkamp //////////////////////////// http://www.schuerkamp.de/ Herford, Germany \\\\\\\\\\\\\\\\\\\\\\\\\\ (52.0N/8.5E) GPG Fingerprint: 2E 13 20 22 9A 3F 63 7F 67 6F E9 B1 A8 36 A4 61

On Sat, 14 Sep 2002 10:15:05 +0200, Stefan Larsson <[email hidden]> wrote: >On Sat, 14 Sep 2002 00:22:06 +0200, J. Todd Wasson wrote: >...

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>> If you hit the bump stop you're hitting another, much stiffer, rubber >> spring in series with the suspension spring. F=k*d then becomes F=k*d^2 >> ;-) Kidding again of course, but you've got another spring rate to add >> in at that point. > >The bump spring rates are usually very high which put rather annoying >demands on the numerics. I don't know how the simulation is performed, >but if it is done solving e.g. a state-space using a fixed step >ODE-solver like Runge-Kutta 4th-order, the system will probably get >unstable...

I thought ODE was actually using a modified Euler-type integration, although I'm not sure what the 'modified' entails. :) I actually use implicit integrations for the movement of the wheels, in a fixed timestep (500Hz, can be upped to 1000Hz which is nicer for F1 cars). The thing is that I have wheels that have mass (it seems easier for some to see the wheels as having no mass), and for the road force I use the tire rate (F_road_vertical=tireRate*d where d is the distance the tire is 'in' the track). Tire_rates are probably even worse than bump stop rates (hm, or perhaps just in the same regime). About 10x as high as suspension rates. Given a F1 or Cart car, normal Euler integration was not really handy. With implicit integration this seems to work much more ok (time reversal). So in fact I may be able to just include the bump stop rate and do a combined bumpStopRate and springRate. But as you bump quite unpredictable anyway, just using the tire rate directly and hard-locking the wheel position at its outer limit may be good enough. Once the car really sinks in even then the car chassis bounding box should deal with the collision. Until I want to surpass Stuntman. ;-) (nice game, although it's a shame it seems to have digital controls and some kind of track caching would have been almost vital). I have an idea also that some cars are given too little space for its springs to move in (only 5 cm's up & down for example) which can trigger the problem. Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

On Mon, 16 Sep 2002 16:09:42 +0200, Uwe Schuerkamp <[email hidden]> wrote:

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>On Mon, 16 Sep 2002 13:09:22 GMT, Ruud van Gaal wrote: >> On Sun, 15 Sep 2002 14:40:05 +0800, "John" <[email hidden]> wrote: >> >>>goodness >>>what's on your mind ? >> >> The cinema in my head generally tends to quite interesting. He/she who >> invents a device to record brain images to DVD will surely prosper. >> ;-) >> > >Hey Ruud, > >just a quick note to let you know your post made it to >a.h.b-o-u fame ;-)

My vocabulary unfortunately lacks the full version of that abbreviation. :) Is it possible to clarify? Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

"Ruud van Gaal" <[email hidden]> wrote in message news:[email hidden]... > I have some ideas on this. With low spring rates, might a regular bump > out quickly, or is normally the bump stop on a normal street car never > reached? (say on a Ford Focus with 22500 N/m and 17500 N/m springs > (not wheelrates)) I wouldn't be suprised if some less sporting road cars do hit the bump stops when abused, but bump stops tend to be progressive I think so you might not even notice youve hit them, you wont completely bottom out without a big jump if that is what you mean. > I have a bug I know of where if the suspension hits the bump stop, I > still only take the suspension spring compression (F=k*d) as the > upward forces. It should actually then be the tirerate, which is > transferred more directly to the body. > However, I feel that this situation should normally not arise (hitting > the bump stops) during normal acceleration and braking. It should be some inbetween rate depending on the rate of progressiion of the bump rate, some cars also have rising rate springs. HTH Paul

"Ruud van Gaal" <[email hidden]> wrote in message > I thought ODE was actually using a modified Euler-type integration, > although I'm not sure what the 'modified' entails. :) I think he meant "ordinary differential equation solver" in the general sense, not referring to a particular package, so any integrator can be used. I don't know which modified Euler you're referring to, but one common variant is ye olde a' = a + da*dt, v' = v + a'*dt. -dave

On Mon, 16 Sep 2002, Paul Laidlaw wrote: ...

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> > I have a bug I know of where if the suspension hits the bump stop, I > > still only take the suspension spring compression (F=k*d) as the > > upward forces. It should actually then be the tirerate, which is > > transferred more directly to the body. > > However, I feel that this situation should normally not arise (hitting > > the bump stops) during normal acceleration and braking. > > It should be some inbetween rate depending on the rate of progressiion > of the bump rate, some cars also have rising rate springs.

Many cars will engage their progressive bump stops when a couple of people get in the back seat. These are often cars where the stylists wanted large tires/wheels, and didn't leave enough wheel-arch opening for a decent amount of wheel ride-travel... Take a look at the bump rubbers on some small cars, I'm sure you can find some with very small clearance, even when the car is unloaded. (I go to car dealers on Sunday, when no one is there to complain about my crawling under the cars...)

On Mon, 16 Sep 2002 15:39:38 GMT, Ruud van Gaal wrote: > > My vocabulary unfortunately lacks the full version of that > abbreviation. :) Is it possible to clarify? alt.humor.best-of-usenet ;-) Cheers, uwe -- mail replies to Uwe at schuerkamp dot de ( yahoo address is spambox) Uwe Schuerkamp //////////////////////////// http://www.schuerkamp.de/ Herford, Germany \\\\\\\\\\\\\\\\\\\\\\\\\\ (52.0N/8.5E) GPG Fingerprint: 2E 13 20 22 9A 3F 63 7F 67 6F E9 B1 A8 36 A4 61

Paul Laidlaw <[email hidden]> wrote in message news:am5jcq$2vrtf$[email hidden]...

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> > "Ruud van Gaal" <[email hidden]> wrote in message > news:[email hidden]... > > I have some ideas on this. With low spring rates, might a regular bump > > out quickly, or is normally the bump stop on a normal street car never > > reached? (say on a Ford Focus with 22500 N/m and 17500 N/m springs > > (not wheelrates)) > > I wouldn't be suprised if some less sporting road cars do hit the bump stops > when abused, but bump stops tend to be progressive I think so you might > not even notice youve hit them, you wont completely bottom out without a big > jump if that is what you mean.

I've felt bump-stops on at least a couple of occasions: - Mondeo ST24, two-up with no luggage, over a 'switchback' section of road. Definitely not close to jumping. - Punto 1.2, five-up with five archery boxes in the boot! Neither was particularly progressive... Jonny

> > I wouldn't be suprised if some less sporting road cars do hit the bumpstops > > when abused, but bump stops tend to be progressive I think so you might > > not even notice youve hit them, you wont completely bottom out without abig > > jump if that is what you mean. > > I've felt bump-stops on at least a couple of occasions: > > - Mondeo ST24, two-up with no luggage, over a 'switchback' > section of road. Definitely not close to jumping. I have seen cars produce big sparks from underneath in a big dip I must admit, I guess they must have been well on the bump stops! > - Punto 1.2, five-up with five archery boxes in the boot! Well, 5 up with some boxes in the boot, I can believe it wasn't that progressive :) > Neither was particularly progressive... Paul

On Mon, 16 Sep 2002 16:25:22 +0200, Ruud van Gaal wrote: > I thought ODE was actually using a modified Euler-type integration, > although I'm not sure what the 'modified' entails. :) I'm actually refering to "Ordinary Differential Equations" (ODE). Not "ODE", the Open-Source package. Modified Euler: http://www.cems.uwe.ac.uk/~klhender/NUMAN1/lec10/node4.html Two other nice solvers for mechanics: Runge-Kutta: http://mathworld.wolfram.com/Runge-KuttaMethod.html Adams: http://mathworld.wolfram.com/AdamsMethod.html > I actually use implicit integrations for the movement of the wheels, in > a fixed timestep (500Hz, can be upped to 1000Hz which is nicer for F1 > cars). The thing is that I have wheels that have mass (it seems easier > for some to see the wheels as having no mass), and for the road force I > use the tire rate (F_road_vertical=tireRate*d where d is the distance > the tire is 'in' the track). You will probably end up with a function that only is activated when the tire is 'in' the track and deactivated when it's not. Highly non-linear. :) I guess you won't bother to analyze the stability of that system... *lol* > Tire_rates are probably even worse than bump stop rates (hm, or perhaps > just in the same regime). About 10x as high as suspension rates. Given a > F1 or Cart car, normal Euler integration was not really handy. With > implicit integration this seems to work much more ok (time reversal). I guess it depends... I simulated a single tire modeled as a spring toward the ground and a suspension with a bump stop using MatLab. I set the following numbers: spring damper Tire: 175 kN/m 525 Ns/m Susp: 60 kN/m 460 Ns/m Bump: 5779 kN/m - Wheel mass: 42 kg Chassis: 225 kg Using the standard ODE-solvers in MatLab gave a nice result. Runge-Kutta 4th order solved it nicely too for a fixed time step. But for the same time step the Euler integration failed (unstable) unless the time step was ridiculously low. The bump stop was hard to simulate due to numerical problems I guess. It did work with very small time steps... Perhaps it could be simulated using Newtons first law (the one which handles transfer of momentum when two bodies collide?) with some loss factor? > So in fact I may be able to just include the bump stop rate and do a > combined bumpStopRate and springRate. But as you bump quite > unpredictable anyway, just using the tire rate directly and hard-locking > the wheel position at its outer limit may be good enough. I guess you need to use "ad hoc" methods to obtain a stable enough solution in a realtime simulation. As long as it "feels good enough" it should be ok? I've been reading a book about computational dynamics (Achmed A. Shabana 2001, excellent book!) in which I tried to find some methods to construct general suspensions by reducing the degrees of freedom with constraints such as H-arms, but it does not seem to be functional for a simulation. The joints can "float away" if the time steps are too coarse. And especially when it comes to games, make sure that your methods are robust and numerically stable! > I have an idea also that some cars are given too little space for its > springs to move in (only 5 cm's up & down for example) which can trigger > the problem. I guess you need to stiffen your springs then ;) Hehe, I remembered when I tried a Peugeot rallycar in Racer and the rear axle started to oscillate at a certain speed... But that's probably a flaw in the design of the car. *lol* /Stefan

On Tue, 17 Sep 2002 15:02:35 +0200, Uwe Schuerkamp <[email hidden]> wrote: >On Mon, 16 Sep 2002 15:39:38 GMT, Ruud van Gaal wrote: >> >> My vocabulary unfortunately lacks the full version of that >> abbreviation. :) Is it possible to clarify? > >alt.humor.best-of-usenet ;-) Ah! :) Cool. Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/