Drag racing mobile app
Heya guys. I haven't posted here in a while so I hope you'll all be kind enough to let me engage in a small amount of shameless self promotion. 
I made a mobile version of my drag racing simulation tool to see what mobile app development was like. I figured there may be a few drag racing tech nerds like me around here that might like to know about it. A handful of the folks at the iRacing forums thought it was pretty cool anyway.
This one's Android only. I designed the GUI for phones, but I hear it works ok on tablets too.
https://play.google.com/store/ ... anceSimulations.SASMobile
Feel free to shoot me an email through the "contact developer" link at the store if you want.
I made a mobile version of my drag racing simulation tool to see what mobile app development was like. I figured there may be a few drag racing tech nerds like me around here that might like to know about it. A handful of the folks at the iRacing forums thought it was pretty cool anyway.
This one's Android only. I designed the GUI for phones, but I hear it works ok on tablets too.
https://play.google.com/store/ ... anceSimulations.SASMobile
Feel free to shoot me an email through the "contact developer" link at the store if you want.
Yeah, that's multiplayer stuff that you'll only see with the opponent cars. I don't notice it when I'm racing.
Thanks for pointing it out though. That gives me an idea on how to fix it. Won't be any time soon though I'm afraid.
Thanks for pointing it out though. That gives me an idea on how to fix it. Won't be any time soon though I'm afraid.
We just released offroad for VRC Pro, our R/C car simulator, now with 1:10 electric short course trucks.
http://www.vrcpro.com/
We went live about 30-45 minutes ago, so right now download times are really long (1h34m for me), so here are a couple vids to fill up 1% of that time at least.
http://www.youtube.com/watch?v=n9vR9B0C8Io
http://www.youtube.com/watch?v=KILszcaQ-pI
http://www.vrcpro.com/
We went live about 30-45 minutes ago, so right now download times are really long (1h34m for me), so here are a couple vids to fill up 1% of that time at least.
http://www.youtube.com/watch?v=n9vR9B0C8Io
http://www.youtube.com/watch?v=KILszcaQ-pI
As I understood it, it's the member's real name. A 2 gets appended to the end when it's the second person to use the name. The name comes from the credit card so there's no way to change it unless you ask iRacing support to do it for you.
I've heard of the 7 year itch, but the 7 year bump? 
No kidding. Why on earth would anyone go to a racing simulator forum to get diagnosed for a medical problem? Seriously, go see a doctor if you haven't already.
Not really. Tire models aren't quite as cut and dry as that. Everything (especially tires) is subject to garbage-in/garbage-out to an extent. The model could be perfect, but the stiffnesses or load sensitivity or anything else could still be a little too far off just because of the input data you used in an attempt at recreating a particular tire. A set of tires on one car is bound to be closer to the real set than it is on another. There's always room for a little bit of improvement on any given car or tire, so I take the updates to tires on various cars as a good thing. It means they're paying attention.
The split between front and rear tires on a particular car is especially critical. Changing the grip or lateral stiffness on one end or the other by only 1% can have a noticeable effect.
Make sure to get the latest version. The first couple I posted had errors.
There's no way it's anywhere near 10 degrees on the F1 car during normal cornering of course. I'm not getting anywhere close to that even on the Solstice.
Did you change the functions to have the correct wheelbase and so on? It's very easy to make a mistake there. All four functions have to be changed in order to switch cars.
There's no way it's anywhere near 10 degrees on the F1 car during normal cornering of course. I'm not getting anywhere close to that even on the Solstice.
Did you change the functions to have the correct wheelbase and so on? It's very easy to make a mistake there. All four functions have to be changed in order to switch cars.
Hi, guys. I just wrote up some functions to get the vehicle and tire slip angles out of the iRacing telemetry. Check forum here if this sounds like your cup of tea:
http://members.iracing.com/jfo ... list/1477369.page#4240355
http://members.iracing.com/jfo ... list/1477369.page#4240355
We released 1:12 modified with 4.5T motors yesterday. This and the 1:12 spec are probably the best handling cars in VRC Pro right now (I'll have to do something about that, now won't I?)
It's still a 30 day trial so give it a go if you're into RC cars. The spec 1:12 car (10.5T motor, not nearly as fast) is included as part of the free trial.
It's still a 30 day trial so give it a go if you're into RC cars. The spec 1:12 car (10.5T motor, not nearly as fast) is included as part of the free trial.
People can complain about free speech only after game forums become owned by the US goverment
Agree. That was the only time I really enjoyed the Skippy.
Haha, thanks..
When Doug Milliken and I were working together on the tire testing for VRC Pro, he told me a lot about Hard Drivin' and the guys that worked on it with him. Doug is listed in the credits only as a "test driver." The way he explained it to me is that since they were the first people to make a video game using a real vehicle and tire model, they didn't want to give any hints as to how it was done. So Doug's name was slipped in at the bottom of the credits as a "test driver" so competitors wouldn't catch on. They apparently never did, either. No other games came out by competitors that were remotely similar.
By the way, have you ever played San Francisco Rush that came out years later? That one to me felt very much like Hard Drivin'/Race Drivin' did and I often wondered if it was done by the same people. Years and years later after meeting Doug, he confirmed that it was done by the same core group of guys.
I think I was 15 when Hard Drivin' came out. I too was absolutely floored by it after a friend dragged me to an arcade insisting I'd love it. That and Race Drivin' are what got me interested in vehicle modelling and got me thinking about physics modelling (aside from the first MS Flight Simulator which got me interested in 3D graphics). Everything I've done since was inspired by this. To "meet" Doug Milliken so many years later in a newsgroup (rec.autos.simulators) and have fascinating talks with him over so many years and find out he was the reason Hard Drivin' and Race Drivin' drove like it did, was wonderful. To later work with him personally and call him a friend has been a privilege.
I learned to drive a stick shift on Hard Drivin' and is really where I learned to drive a car. I learned to slide a car and drive fast on that. It translated directly to real driving too which made it all the more mesmerizing to me. I'd say it launched me into this whole vehicle and physics modelling stuff to a large degree...
And of course, the corkscrew was fun. So was the tunnel which I always had to drive around the roof. Sooo much fun
The arcade my friends went to started with Hard Drivin'. The owner of the place thought it was the coolest thing in the world too and was usually there watching us play and sink endless quarters into it. Eventually Race Drivin' came out. The big, expensive cabinet version. The guy bought two of them and had them linked together. Yep, you could do multiplayer races on it. I remember at 15 or 16 years old (before I could drive a real car) beating a guy at the autocross course that did SCCA racing. My driving style was hugely drifting. It was really the fastest way to go around the last turn. I'd drop it down a gear to get the back end out and just sail around it. It's fun now to actually understand why what I was doing worked. I didn't know back then how I was "supposed to drive." I just did whatever was fast (and fun). Fun times
This is actually when I first started paying attention to yaw velocity and yaw acceleration, although back then of course I didn't know what those terms meant, but the concepts were solidified in my head way back then. At some point I noticed how the horizon moved as the car rotated and started paying a lot of attention to that. Eventually I learned to drift the car and not overcorrect it. Maybe that's partly why now I can jump in a sim and see things immediately that most others seem to never notice. Been paying attention to that kind of thing for 22 years now I guess. Oh dear, has it been that long already?
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A couple ways to check are to watch videos or study vehicle response plots of real vehicles via telemetry, cross checked against simulation. In both you can see what the driver is doing and how the vehicle responds, and why it does so. When a simulation does the exact opposite of what the real car does in a given situation, there's a problem, and nobody needs to feel any g-forces to see that.
I'm talking about things like you get in iRacing's oval cars, where if you slide the car, the easiest and quickest way to recover is to simply steer into the spin. Most of the iRacing cars will straighten up immediately. Real cars do not do this. If they did, you'd see it in Nascar and other series all the time. Seeing things like this does not require any g-forces to be felt. My example about having somebody sit down and play with tires in a simulation for an hour or so in order to see what drop-off in tire force curves after the peak does was meant to illustrate another example where the g-force argument falls on its face. Some things are obvious without feeling anything. If you turned the wheel and the car did a back flip, you would know immediately something was wrong. Sure, with some g-force feeling you could have anticipated the backflip coming sooner and been more likely to prevent it, but that doesn't mean the backflip is not just fundamentally a wrong reaction. I'm talking about the backflip while other people are talking about how they could anticipate backflips earlier if only they had g-forces like the real drivers have.
For me it's more subtle than this. I see steering into a spin immediately straightening up a car right away, and other similar things, within minutes (sometimes seconds) of trying a new sim. I don't need g-forces to see things like this, and I doubt your racing buddies who were talking about feel would either. Just tell them to slide the car and steer the opposite of what they would normally do and watch their reaction. It'll be priceless.
My point is that people use the "missing g-forces" argument everywhere for everything, even in cases where it really doesn't apply such as in the example of steering into a spin straightening up a car immediately, or countersteering a little bit causing the car to spin out twice as quickly as it would have if you had done nothing with the steering at all, or simply steered a few more degrees into the turn. This type of thing is blantantly obvious visually even without any g-forces or force feedback. I don't think your friends in your example were talking about the same thing I am.
That's fine.
I don't buy that for a second. If anything, I'd argue the exact opposite. If the car's "too easy to drive and drift" the quickest way to "fix that problem" is to add a bunch of drop off after the peak. If you really want the car to whip around quickly, add more drop off at the rear than the front, like the FGT and HPD had when the NTM first went on them. Then dial it back a bit when people complain it's too hard and the cars are doing really funny things. This is what Hyperactive was arguing and I agree 100% with that.
I've seen the tire force curves used in rFactor, and in iRacing you can get a pretty good idea what the force curves are doing after the peak quantitatively through the telemetry. Like I said in another post, they come in somewhere between LFS and rFactor. LFS is closest of the three in this area which is why you don't have these problems in LFS, and it's a drifters dream in comparison to any other sim. An unfortunate result of that is that a great deal of simmers think that's wrong and it's too easy to be right. The trouble is, like you seemed to be suggesting, they're comparing it to other sims rather than to real cars.
Most sims these days are quite good up to the limit, so guys that are really fantastic drivers and ride the limit all the time without going very far over it are happy as can be. They're likely to have no idea what I'm talking about. Dancing around right on the slip angle limit is indeed helped by having g-forces and physical feedback. This is what your friends were talking about I think and I agree with that. But passed that where you're sliding a lot, most of the sims don't do a very good job, and in this area you don't need physical feel to see something wrong.
LFS handles this area better than any other sim I'm aware of, besides my own and two others that friends have made that aren't public. The original Hard Drivin' back in the 80's didn't have this problem either, because Doug Milliken knew what tire force curves looked like out in the slidey area when he made it. This was way before GPL.
If you were here at my house I could show you exactly what I mean by playing with force curves and letting you try all these things yourself as I've done probably hundreds of times over the last eleven or twelve years or so. There are vehicle behaviors that you get when the force curves drop off significantly that you do not get when they don't. It's really that simple. And in many cases these are the very same vehicle behaviors people end up having a realism debate over. These types of things don't require g-forces, or even force feedback for that matter, to see. To me they're as obvious as the proverbial back flipping car.
A g-force argument is fine, but it has its place. More often then not it finds its way into arguments that do not require g-forces to settle the issues, which can be seen visually quite easily if one knows what he's looking for.
I can see where some people might have a tough time swallowing all this because they haven't written their own car simulators to play around with these things in and see what does what. Even a lot of people who have written car simulators fail to see connections between many of their inputs and the resulting outputs, as evidenced by just about every sim ever made.
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Do we know if Dave does the cars himself? I don't think he does, actually. My impression is that Eric Hudec and maybe one or two other guys do most, if not all, of this.
Dave's one of my heroes so I don't want to bash him too much, but come on.. Even if Dave does this himself, look at GPL. GPL was awesome, but the slip angles in steady cornering were around DOUBLE what they were supposed to be. Some people still hail that as the most realistic sim ever, even with 50% accuracy in probably the most important area of tire behavior. They don't know what they don't know, but thanks to a healthy dose of Male Answer Syndrome, will go on and on about it as though they do, gleefully throwing out the opinions of real drivers in the same cars because they can't feel the g-forces or something like that.
Knowing how the real car feels and then getting the sim version to do that is harder than it seems. I even have troubles doing this in VRC. I know what I want the response of the cars to be, but yet can't seem to get them exactly as I want them to be most of the time. I doubt Dave is much better at that than I am. I wouldn't assume the iRacing version of any particular car handles exactly as Dave would want it to.
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An hour of so of tweaking tires oneself in a sim would probably convince most people that the role of a lack of physical forces in all this is highly overrated. When you get a bit sideways and do something, say release the throttle, what does the car do? Does it spin, do nothing, or straighten up on its own? That's very easy to see without feeling g-forces. People routinely tweak their setups in sims and feel the differences right away, even very subtle ones. We see the same thing in RC car racing, both real and simulated. Yet there are accelerations involved in neither.
I'd argue that drifting is probably made harder by g-forces rather than the other way around. One of the reasons drifting on slicks might be a little harder than it is on street tires is the accelerations are higher and response times are lower. In a sim the accelerations are missing completely, making this easier to do rather than harder. You wouldn't know that from driving most sims outside of LFS though, where real drivers routinely complain about how difficult the sim version of the car is to catch a slide in. Operating exactly on the slip angle peaks all the time is probably helped a lot by feel, where you can more immediately sense small variations in rotational acceleration for example, but many of the behaviors we're talking about here are well outside this region and are plenty easy to spot without any g-forces whatsoever.
The trouble is a lot of times people will see something like this and comment on it, and even though they're probably dead right, somebody else will dismiss the observation in one fell swoop by simply saying "you can't feel the g-forces," and wins by default. After spending way too much time in my hobby sim over years and years playing with this, and seeing how easy it is to make a car that spins uncontrollably, or self corrects in a slide and won't spin unless you try really hard to do so, or anywhere inbetween, these "lack of real feel" and "it's something in the FFB" arguments irritate me to no end. These people haven't experienced and played with these things themselves at all and really don't have a clue what they're talking about.
Sometimes the g-force argument is just plain silly. If you stomped on the brakes in a straight line and the car took off 1000 feet into the air, would you say "well maybe the real car does that. After all, I can't feel the g-forces?" I see this being just about as ridiculous as that would be to anybody here.
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The guy iRacing contacted to arrange the tests at CALSPAN is a friend of mine. iRacing expressed interest in testing beyond the peak lateral force slip angle. I don't know specifically what was done though because this was a few years ago, before iRacing was released.
CALSPAN can go up to 30 degrees slip angle. What seems likely to me is they might have tested one tire out to that angle at the highest. It's expensive to do this and 30 degrees eats up a tire in a hurry... Anything higher than that would still be a guess on their end.
It's not hard to find out roughly how much drop off iRacing's various tires have in their force curves with some quick skidpad tests. In Atlas watch the vector sum of lateral/longitudinal acceleration at a steady state cornering peak (all lateral), then kick the car out as far sideways as you can, release throttle/brake steering, and see what the percentage difference is.
If the car pushes hard at the limit in steady state cornering, you should be seeing an INCREASE in lateral acceleration when you slide. Instead, on the cars I tested, there's a rather large decrease. I want to say 15-25+%, but don't quote me on that. I remember that on at least two of the cars lateral acceleration during steady state cornering would drop 7% during the first 20 seconds of running due to the rapid rise in tire surface temperature. So to test the drop off you have to get the tires to that point first or else it'll look like there's a lot more drop off then there really is. Circle around for 30 seconds or so before doing the test to get passed most of the temperature effect.
Skip Barber: I can't stand this car now. When the NTM came out on several cars besides the FGT and HPD, the Skip was the best one. They tweaked it the next patch and now I can't bear to drive that thing anymore.
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@Shotglass and Hyperactive
Both ways of thinking about it have their benefits. I tend to go more with Shotglass' way because it includes situations that the steady state doesn't. I.e., Hyperactive's example of steady state slip angle definitions works only for steady state. If the rotational velocity is anything different from this it doesn't quite get the whole picture. I.e., if the car was spinning into the turn more or less quickly there's another component added to it that changes the slip angle away from the steady state picture.
In my hobby sim (and every other sim these days probably) I use the velocity and orientation vectors directly as Shotglass described, so I tend to think of slip angles as working that way since that's how they're defined. But at the same time, in a steady state corner those are equal to Hyperactive's description and excellent illustrations, so sometimes I think of it that way.
To be a truly powerful vehicle dynamics sorcerer, it's best to be able to think about it both ways so you can adapt to the situation, I suppose
Both ways of thinking about it have their benefits. I tend to go more with Shotglass' way because it includes situations that the steady state doesn't. I.e., Hyperactive's example of steady state slip angle definitions works only for steady state. If the rotational velocity is anything different from this it doesn't quite get the whole picture. I.e., if the car was spinning into the turn more or less quickly there's another component added to it that changes the slip angle away from the steady state picture.
In my hobby sim (and every other sim these days probably) I use the velocity and orientation vectors directly as Shotglass described, so I tend to think of slip angles as working that way since that's how they're defined. But at the same time, in a steady state corner those are equal to Hyperactive's description and excellent illustrations, so sometimes I think of it that way.
To be a truly powerful vehicle dynamics sorcerer, it's best to be able to think about it both ways so you can adapt to the situation, I suppose
Wow, Tommy. Thanks.
I told Pieter (the owner) what you said on IM and he wrote "that's the kind of customer we need!"
Shotglass: I've wondered the same thing for a long time. What the top drivers usually say is they're a little quicker with one-ways than they are with diffs, at least on some of the cars. They're harder to drive that way though because as you said, it's rear wheel braking only then.
There are probably a few areas that could be pointed to here as possible benefits. In the iRacing thread where we were talking about braking and cornering, there was the state 2 and state 3 example where the braking reduces lateral force on the front. With one-ways this doesn't happen. On 1/8 scale nitro cars at least, the rear tires are enormous compared to the fronts and have a great deal more grip and downforce, so they can get away with this a bit more. They get better turn in under braking.
Another thing is that braking in straight lines is done hardly at all. At the end of a long straight with a hairpin at the end of it you might stab the brakes for a fraction of a second before you've slowed the car enough to start turning in, but I think most of the braking is done while cornering. It's sort of like this: Stab the brakes, hold it and flick the car into the turn, wait while it slows down and goes to the apex, then jump on the power. So most of the time when you're braking you're also cornering. I imagine with these cars you're probably getting down to the limit of what a human can do in such a short amount of time. A lot of the guys in some classes use anti-lock braking on their controllers that pulses the brakes when they use full brake input. This is at the world champ level and even they do this to get an advantage. Humans can only do so much in a 0.2 second long straight line braking situation I suppose.
In VRC Pro this is how I'm usually driving. I set the brake strength to give balance while braking/cornering even though I'm giving up a little bit on the straight.
With electric 1:10 cars (in VRC) that have the same tires front and rear, this doesn't work well for me at all so I use a solid axle on the front. In reality this is what most people seem to do too, even at the world championship level. Occasionally you'll find somebody using a front one-way, but I think they're a minority. I tried two different real electric cars at the LRP Worlds in 2007 (I wasn't part of the competition of course, a couple guys handed me their cars to try out between heats). One had a one-way and the other had a solid axle. This was the only time I ever drove a real 1:10 electric car, and I couldn't drive the one-way version to save my life. If I touched the brakes I couldn't control the car at all. The solid axle version was a lot easier precisely because of what you pointed out: The difference in braking/cornering.
I told Pieter (the owner) what you said on IM and he wrote "that's the kind of customer we need!"
Shotglass: I've wondered the same thing for a long time. What the top drivers usually say is they're a little quicker with one-ways than they are with diffs, at least on some of the cars. They're harder to drive that way though because as you said, it's rear wheel braking only then.
There are probably a few areas that could be pointed to here as possible benefits. In the iRacing thread where we were talking about braking and cornering, there was the state 2 and state 3 example where the braking reduces lateral force on the front. With one-ways this doesn't happen. On 1/8 scale nitro cars at least, the rear tires are enormous compared to the fronts and have a great deal more grip and downforce, so they can get away with this a bit more. They get better turn in under braking.
Another thing is that braking in straight lines is done hardly at all. At the end of a long straight with a hairpin at the end of it you might stab the brakes for a fraction of a second before you've slowed the car enough to start turning in, but I think most of the braking is done while cornering. It's sort of like this: Stab the brakes, hold it and flick the car into the turn, wait while it slows down and goes to the apex, then jump on the power. So most of the time when you're braking you're also cornering. I imagine with these cars you're probably getting down to the limit of what a human can do in such a short amount of time. A lot of the guys in some classes use anti-lock braking on their controllers that pulses the brakes when they use full brake input. This is at the world champ level and even they do this to get an advantage. Humans can only do so much in a 0.2 second long straight line braking situation I suppose.
In VRC Pro this is how I'm usually driving. I set the brake strength to give balance while braking/cornering even though I'm giving up a little bit on the straight.
With electric 1:10 cars (in VRC) that have the same tires front and rear, this doesn't work well for me at all so I use a solid axle on the front. In reality this is what most people seem to do too, even at the world championship level. Occasionally you'll find somebody using a front one-way, but I think they're a minority. I tried two different real electric cars at the LRP Worlds in 2007 (I wasn't part of the competition of course, a couple guys handed me their cars to try out between heats). One had a one-way and the other had a solid axle. This was the only time I ever drove a real 1:10 electric car, and I couldn't drive the one-way version to save my life. If I touched the brakes I couldn't control the car at all. The solid axle version was a lot easier precisely because of what you pointed out: The difference in braking/cornering.
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@hyperactive
I see your changes and the new plots. That looks more like it. Your point about not needing constant slopes is still correct though. The important part you showed was that the rear slip angles are bigger than the fronts. How much so then isn't too terribly important.
My feel for how much it probably would be comes from doing similar maneuvers in my hobby sim while displaying the slip angles. I want to say that in "slow" (whatever that means) slides that 5 degrees difference is a lot. That may be a memory failure though. Your data would be more reliable than that
I see your changes and the new plots. That looks more like it. Your point about not needing constant slopes is still correct though. The important part you showed was that the rear slip angles are bigger than the fronts. How much so then isn't too terribly important.
My feel for how much it probably would be comes from doing similar maneuvers in my hobby sim while displaying the slip angles. I want to say that in "slow" (whatever that means) slides that 5 degrees difference is a lot. That may be a memory failure though. Your data would be more reliable than that
I haven't driven iRacing since shortly after the latest patch. I lose interest and take a break for a couple months occasionally.
I enjoy some of the cars and not so much others. My favorite as of the last patch was the HPD. That felt soooo much better than before. I like a car you can be aggressive with and don't have to constantly battle just to keep pointing somewhat forward. The FGT was a lot more enjoyable too. Enough to make me annotate my video with 'this has been fixed' anyway.
Good to see, but it must be said that this doesn't invalidate your point in the slightest
The low speed spin problem was precisely that. It seemed to get worse the slower you were going, at very small turn radii. Even smaller than 50 or 100 radius.
Also, something that would actually increase the slip angle difference further is the spinning motion of the car itself. Your plots are steady state which is perfect for finding out at least what the minimum slip angle difference would be. As we add additional yaw velocity to spin the car (so we have some rate of change to the slip angles), the difference grows beyond that. So your mistake reduces the slip angle differences a bit, but the spinning motion that's been left out increases them again somewhat, so it may not be as far off as you might be thinking.
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I imagine that's right, but don't know for sure.
Bingo. Plus the weight transfer that's already there. This is going from state 3 to state 2 (or whatever it was).
@hyperactive
My quick estimation of a 1 degree larger rear than front slip angle was probably way too low for the case in my video. I agree with that of course. What I was trying to point out was that the front tires would also be dropping off, not just the rears, which takes away a great deal of the effect I thought you were referring to. Usually when I run across this thinking, people seem to be only considering the rear drop off and forget about the front entirely. My mistake if you weren't.
On your point about not needing different slopes: Strictly speaking you're right about that of course. The slopes don't need to be different as can be shown with a ridiculous example tire like I came up with where all the force disappears in 10 degrees slip angle. I meant it more as a matter of practicality with real tires. With the exception of what appeared to be some kind of Nascar tire at -8 degrees camber, I haven't seen a tire force curve that drops off more than probably 5-8% at quite high slip angles (in the dry, the wet is another matter), and even that is somewhat questionable because of the noise involved in the raw sampling and how much the forces can change due to temperature during the sweep test. I.e., the curve can appear to drop off due to temperature effects as the tire is being steered up to the high slip angle. Usually on the return trip to the lower slip angles you end up with a much flatter force curve, which is more like what a real car is probably running on most of the time. When this is taken into consideration, most real tires are dead flat after the force peaks right out to 90 degrees, believe it or not, and some continue a slow rise after the nominal peak. We need to keep in mind that these tire plots we see are not always processed the same way, and drum tests usually show more drop off than flat belt tests do. So when somebody pulls out a graph that shows a lot of drop off, I'm immediately skeptical. My thinking and view on this was largely shaped by conversations I've had with Doug Milliken on this while we worked together on tire testing for VRC Pro.
If we take a rear tire that peaks at a 10% higher lateral force than the front does and let the slopes after the peaks be identical, at least in scale, and something along the lines of a slope you might find with a real tire like the FGT's, my gut says the slip angle difference needed would be beyond what you would get at cornering radii on the scale of what was shown in my video. So if both tires dropped as much as 9% after the peaks, even 90 degrees slip angle difference front to rear wouldn't be enough to reverse the yaw moment. So if I picked 1 degree or 10 or 20 slip angle difference it might not have mattered much. This was coming partly from practical experience with my own sim. This then led me to the conclusion that in practice the slopes would really need to be quite different. In my sim it took quite a bit more of a difference in slopes than I originally thought it would in order to get behavior roughly similar to the FGT and HPD.
The way to proceed from here, as you seem to already be going toward, is to grab some real tire data and see just how large a slip angle difference you'd need in order to get the yaw moment reversal happening. We'd need a rear tire that produces a great deal more force than the front so we have something like the FGT: A stronger pusher at the limit.
My quick estimation of a 1 degree larger rear than front slip angle was probably way too low for the case in my video. I agree with that of course. What I was trying to point out was that the front tires would also be dropping off, not just the rears, which takes away a great deal of the effect I thought you were referring to. Usually when I run across this thinking, people seem to be only considering the rear drop off and forget about the front entirely. My mistake if you weren't.
On your point about not needing different slopes: Strictly speaking you're right about that of course. The slopes don't need to be different as can be shown with a ridiculous example tire like I came up with where all the force disappears in 10 degrees slip angle. I meant it more as a matter of practicality with real tires. With the exception of what appeared to be some kind of Nascar tire at -8 degrees camber, I haven't seen a tire force curve that drops off more than probably 5-8% at quite high slip angles (in the dry, the wet is another matter), and even that is somewhat questionable because of the noise involved in the raw sampling and how much the forces can change due to temperature during the sweep test. I.e., the curve can appear to drop off due to temperature effects as the tire is being steered up to the high slip angle. Usually on the return trip to the lower slip angles you end up with a much flatter force curve, which is more like what a real car is probably running on most of the time. When this is taken into consideration, most real tires are dead flat after the force peaks right out to 90 degrees, believe it or not, and some continue a slow rise after the nominal peak. We need to keep in mind that these tire plots we see are not always processed the same way, and drum tests usually show more drop off than flat belt tests do. So when somebody pulls out a graph that shows a lot of drop off, I'm immediately skeptical. My thinking and view on this was largely shaped by conversations I've had with Doug Milliken on this while we worked together on tire testing for VRC Pro.
If we take a rear tire that peaks at a 10% higher lateral force than the front does and let the slopes after the peaks be identical, at least in scale, and something along the lines of a slope you might find with a real tire like the FGT's, my gut says the slip angle difference needed would be beyond what you would get at cornering radii on the scale of what was shown in my video. So if both tires dropped as much as 9% after the peaks, even 90 degrees slip angle difference front to rear wouldn't be enough to reverse the yaw moment. So if I picked 1 degree or 10 or 20 slip angle difference it might not have mattered much. This was coming partly from practical experience with my own sim. This then led me to the conclusion that in practice the slopes would really need to be quite different. In my sim it took quite a bit more of a difference in slopes than I originally thought it would in order to get behavior roughly similar to the FGT and HPD.
The way to proceed from here, as you seem to already be going toward, is to grab some real tire data and see just how large a slip angle difference you'd need in order to get the yaw moment reversal happening. We'd need a rear tire that produces a great deal more force than the front so we have something like the FGT: A stronger pusher at the limit.
Last edited by jtw62074, .
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