Steering

Hi. I am developing a steering algorithm for my car sim. I experimented with realistic algorithms, but they were too processor intensive AND not as fun to use as a system like Gran Turismo. So, although I might get shot down by saying this, a simple algorithm is what I am looking for. I currently have a system in which the car has 100% grip, (ie) it goes where you point it, and it is simple to put in a 'reduce steering input with car speed' thing like GP3 to control it at high speed, to give a definite understeer affect. I want though, an oversteering affect, with a bit of lift-off oversteer, and a loose back end through the corners, go give a fun, powerslidey feel to the game (it is after all a GAME). so i am thinking the weight distribution would control this (ie) lifting off + braking will lighten the rear end and they would grip less. But anybody wanna tell me how to calculate the lateral forces in the tyres, and what to do when they are bigger than a max force (defined as the coefficient of friction * weight on the particular wheel?). My main attempts have been flawed because the wheels tend to follow 4 different paths and it looks a bit odd when your front inside wheel travels around all by itself (but still reacting to your inputs!). I don't want this slip angle, magic formula bulls**t, to be honest, as all total realism sims are not very good fun (except for the King - long live Grand Prix Legends!!). I don't want to simulate the tyre temperatures, or the resistive force due to flies hitting the windscreen :-) but I just want a fun, fast algorithm. Anybody know where I can find one?? Cheers, Nick.

Can't help you there Nick, but I must say "brave lad" for daring to post! :0) Jim "Nick G" <[email hidden]> wrote in message news:[email hidden]... > Hi. I am developing a steering algorithm for my car sim. I experimentedwith > realistic algorithms, but they were too processor intensive AND not as fun > to use as a system like Gran Turismo. So, although I might get shot downby > saying this, a simple algorithm is what I am looking for. > > I currently have a system in which the car has 100% grip, (ie) it goeswhere > you point it, and it is simple to put in a 'reduce steering input with car > speed' thing like GP3 to control it at high speed, to give a definite > understeer affect. > > I want though, an oversteering affect, with a bit of lift-off oversteer,and > a loose back end through the corners, go give a fun, powerslidey feel tothe > game (it is after all a GAME). so i am thinking the weight distribution > would control this (ie) lifting off + braking will lighten the rear endand > they would grip less. > > But anybody wanna tell me how to calculate the lateral forces in thetyres, > and what to do when they are bigger than a max force (defined as the > coefficient of friction * weight on the particular wheel?). My mainattempts > have been flawed because the wheels tend to follow 4 different paths andit > looks a bit odd when your front inside wheel travels around all by itself > (but still reacting to your inputs!). I don't want this slip angle, magic > formula bulls**t, to be honest, as all total realism sims are not verygood > fun (except for the King - long live Grand Prix Legends!!). I don't wantto > simulate the tyre temperatures, or the resistive force due to flieshitting

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> the windscreen :-) but I just want a fun, fast algorithm. > > Anybody know where I can find one?? > > Cheers, Nick. > >

Nick G <[email hidden]> wrote in message news:[email hidden]... > I currently have a system in which the car has 100% grip, (ie) itgoes where > you point it, and it is simple to put in a 'reduce steering inputwith car > speed' thing like GP3 to control it at high speed, to give adefinite > understeer affect. Does that mean you have no slip angles? > I want though, an oversteering affect, with a bit of lift-offoversteer, and > a loose back end through the corners, go give a fun, powerslideyfeel to the > game (it is after all a GAME). so i am thinking the weightdistribution > would control this (ie) lifting off + braking will lighten the rearend and > they would grip less. > > But anybody wanna tell me how to calculate the lateral forces in thetyres, > and what to do when they are bigger than a max force (defined as the > coefficient of friction * weight on the particular wheel?). My mainattempts I think what you might have to do is some form of four-wheel-steering, if you're not at least providing some slip angle. You're already doing this at the front from what you say, by making the actual steer angle less than the driver input at speed; if you add a similar routine at the back, even if it's just based on front steer angle... <hit by a thought> <lies stunned for a few minutes> This is sounding much like my Final Year Project. What I *think* you need to do, is to put "additional" (i.e. altering the driver command) steering at both ends, in proportion to yaw rate, lateral acceleration, speed or some combination. For oversteer, have the rear wheels steer *outwards* slightly as the speed/latacc/whatever increases; for understeer, steer the fronts outwards. Alternatively, you could model an axle slip angle (basically the same thing) in proportion to the lateral force on that axle, and just cap the lateral force at mu.N (straight line graph to a maximum). That doesn't sound too complicated... and if you kept the max force slip angle constant (maybe fifteen-twenty degrees for a arcader?) and increased the gradient with weight transfer, I *think* you'd get some reasonably playable results... HTH - ask if I haven't made things clear! Jonny

On Thu, 2 Aug 2001 18:50:24 +0100, "Nick G" <[email hidden]> wrote: >Hi. I am developing a steering algorithm for my car sim. I experimented with >realistic algorithms, but they were too processor intensive AND not as fun >to use as a system like Gran Turismo. So, although I might get shot down by >saying this, a simple algorithm is what I am looking for.... >But anybody wanna tell me how to calculate the lateral forces in the tyres, >and what to do when they are bigger than a max force (defined as the >coefficient of friction * weight on the particular wheel?). Then just top them off at the max force. > My main attempts >have been flawed because the wheels tend to follow 4 different paths and it >looks a bit odd when your front inside wheel travels around all by itself >(but still reacting to your inputs!). I don't want this slip angle, magic >formula bulls**t, to be honest, as all total realism sims are not very good >fun (except for the King - long live Grand Prix Legends!!). That's a matter of what you do with gameplay IMHO. Anyway, I wouldn't know how to cut corners with all these physics without endless tweaking and thinking 'hm, it doesn't really behave like the car I drive', so good luck with this route. :) > I don't want to >simulate the tyre temperatures, or the resistive force due to flies hitting >the windscreen :-) but I just want a fun, fast algorithm. Ok, try RARS' algorithm; it's simple, blindingly fast (just paint 1 triangle and you probably see the bottleneck in the graphics already) and a real breeze to implement. It's like: - calculate slip velocity - determine friction coefficient - F=-frictionCoeff*slipVelocity That's it. If you include *some* weight transfer (depending on how much CPU you wish to spend, you only got about a billion per second, lol), that should get you somewhere. Cheers, Ruud van Gaal, GPL Rank +53.25 Pencil art : http://www.marketgraph.nl/gallery/ Free car sim : http://www.marketgraph.nl/gallery/racer/

Take a look at "Race Car Vehicle Dynamics," Chapter 5 that is a good refresher course on stability and control. They give a "Linearized" yaw dynamics system. For example: yaw_moment = N_beta * sideslip + N_r * yaw_rate + N_delta * steer where the N_beta, N_r, N_delta are "Stability Derivatives" (Or "Magic Constants" if you prefer ;) These stability derivatives summarize how the vehicle reacts to its current state (velocity, sideslip, yaw_rate, etc) and control inputs (steer) at any point in the vehicle's "operating envelope". This sounds great. The only problem is that a particular set of derivatives is only correct for a small range of conditions around the conditions they were calculated for. ;( But hey, you want to take shortcuts so having just a single set might even be enough or a fun model. For example, the N_r term provides yaw damping. it physicallyderives from the slightly different "slip angles" at each tire because the vehicle has a yaw rate while turning. (This is why people with more detailed simulations who don't use separate calculations for the contact patch velocity vector of each tire don't get this effect and suffer for it.) The N_r term can be estimated under one set of simplifying assumptions as: N_r = (1/V) * (a*a * Cf + b*b * Cr) where a and b sum to the wheelbase (a is the distance from front axle to cg, b rear axle to cg) and V is the speed. Cf and Cr are the lateral force per radian of the front and rear tires. (Note that they are negative because of usual sign conventions and so this equation basically says that the faster you rotate the harder the vehicle tries to slow down the rotation. As long as N_r is negative! ;) So here we have a simple term for the yaw equation of motion that gives the effect of lots of slip angle calculations ;) without using a single vector equation. The other nifty thing about considering the simplified analytical models for tthe stability derivatives is that it teachs some fundamentals about car stability. For example, the yaw damping is reduced 33% if you increase speed from 60 to 90 mph. Something I wish the idiotic Texas empty bed pickup drivers with illegal overtake speed who sharply lane change past me with only inches to spare had a clue about. You could adjust the derivatives by changing Cf and Cr based on estimates of normal forces. For example, under braking, if you increase Cf and reduce Cr, guess what happens to the static stability derivative N_beta: (The restoring moment that increases with increasing sideslip angle that is POSITIVE for a stable system for usual sign conventions) N_beta = a * Cf - b * Cr Reducing Cr drives it toward a negative (unstable) condition. And now we have instability under hard braking, again without a slip angle or a slip ratio in sight. Enjoy, - Matt

when i said 100% grip, i meant work out the force from the engine on the wheels (this is done accurately), then using F=ma, v=u+at, d=st, you get a forward distance. then calculate the radius of the corner due to the steering input (=wheelbase/tan(steerang)), and then the useful formula: angle/360 = forward distance / circumference (from radius) then just move it that much forward and sideways and rotate it by angle. in this way, slow speed cornering is totally accurate, but when you come to highspeed cornering - well. thats my problem.

Nick G <[email hidden]> wrote in message news:[email hidden]... > when i said 100% grip, i meant work out the force from the engine onthe > wheels (this is done accurately), then using F=ma, v=u+at, d=st, youget a

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> forward distance. > > then calculate the radius of the corner due to the steering input > (=wheelbase/tan(steerang)), and then the useful formula: > > angle/360 = forward distance / circumference (from radius) > > then just move it that much forward and sideways and rotate it by

angle. > > in this way, slow speed cornering is totally accurate, but when youcome to > highspeed cornering - well. thats my problem. I'm definitely thinking you need to use a simple 2DOF (yaw-sideslip) model and add those offsets to the course you're already plotting. Using handling coefficients (I think that was this thread - can't tell since you're not quoting anything) should work quite nicely on that one, though you may need to derive lateral movement from the sideslip value rather than from your curve as you do currently. RCVD, methinks. Jonny

here was my plan: calculate cornering force required from F=(m*v squared)/r calculate max available turning force from F=weight*coeff friction do this for front axle and rear axle (the rear has a smaller radius) then, maybe cap the cornering force at the maximum, or approximate with a sin curve (if you don't understand then mail me). from there, you can wourk out the actual radius of the front and rear wheels. maybe by finding the positions of the centre rear axle and centre front axle, i can draw the car pointing along the line between the two or something.