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#1
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GPL Salisbury diff preload
Hi all,
Not sure if RAS is stilled filled with car sim creators as back in the day. ;-) I'm currently revising my differential code (in Racer) and trying to get Salisbury diffs to work as a variation on LSD's (limited slip diffs). For an LSD, I define: - power_ratio; torque bias ratio for power-on - coast_ratio; same for coasting (letting go off the throttle) - preload; base spring-loaded torque GPL defines diffs as 60/30/2 for example (power/coast/clutches). From back in the day Niels Heusinkveld explained that: - ratio=cos(angle)*(1+clutches). So for example, cos(60)*(1+2) = 0.5*(1+2) = 1.5. So for the above diff you get 1.5 torque bias ratio. Anything inbetween that keeps the diff locked. But for preload; how does GPL do that? Without some preload, the diff would become a bit unstable, esp when one wheel is on the grass. Surely all the clutches result in some preload torque. But how much friction torque exactly? Thanks for any ideas, Ruud |
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#2
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GPL Salisbury diff preload
A Salisbury is just an LSD that can have separate coast and power
locking, if I'm not mistaken. No difference in the math there. You're probably already doing a Salisbury without knowing it. As for the torque bias ratio with power/coast/clutches: I'd think there'd be a friction coefficient in there too, but maybe in GPL Dave just put it at 1. I don't know if there was any preload in the GPL model. From what I can remember from driving it (it's been years and years since the last time, mind you) there didn't seem to be any, or at least very little. I'm not entirely sure quite frankly whether changing the number of clutches changes the preload. From page 742 of the great Milliken's "Race Car Vehicle Dynamics", the triangular regions are controlled by number of clutches (and their friction coefficient I'd think) and ramp angle, while the preload area is a pretension torque that has to be overcome separately from this and is controlled by the springs or a washer or whatever. Increasing the number of clutches might increase the preload, but I'm not so sure about that. Here's a nice way out of that conundrum for you that I'd use: What the engineer really wants in the end is to specify the preload *torque* at the wheels rather than the preload spring force. That's the important part for handling (anyone know what the spring rate of the belleville washer or whatever is providing the preload is?). You adjust the preload spring force to whatever gets you the desired preload torque. So if you have preload torque as the input instead of preload spring force, you bypass this problem entirely by keeping them seperate. If you add a clutch and want 50NM preload torque before and after the change, then you can just keep them separate. You might assume somebody goes in and adjusts the preload spring a bit to maintain that torque if indeed it changes. If I change the diff's torque bias ratio, I want to make sure I'm not changing the preload too without intending to. |
#3
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GPL Salisbury diff preload
Let me expand on this just a bit. To start out with, let's define
"preload torque" as the amount of torque required across an axle (inputs from the wheels) to allow differential action to occur, which is to allow the axle to go from locked to slipping. I.e., the old adage about jacking up the car, grabbing hold of one wheel so it can't rotate, then twisting the other wheel with a torque wrench to the point where finally starts rotating. The torque that this occurs at is the "preload torque" as used here. "Preload spring force" or "preload force" or "spring force" here refers to the force at the preload spring itself. So references to "torque" refer to the torque across the axle, while "force" references force at the actual "preload spring" which is just a spring like any other. Ultimately in reality the preload spring's force controls the torque. Ok, moving right along: What folks have been doing (myself included) that have figured out non- viscous diff modelling at all is to specify preload torque in the setup screens directly, and understandably so. It makes sense from the engineer's perspective because this is something that you want to adjust separately from the torque bias ratio. Same goes in reality. So if you go with this approach, don't model any change in preload torque unless you change the preload torque directly. Not the number of clutches, not ramp angle, nothing. You set preload torque directly like described earlier. However, you could try something else: Another way to go (you'd almost certainly be the first to do it in a sim which might be appealing to you and win you even more points with your Racer fans that go back almost as far as you and I do :-P ) would be to allow for the possibility that changing the number of clutches does actually change the preload torque. However, if you do this, then you need to specify preload differently. You aren't setting the preload torque directly anymore. Think suspension preload here for a minute as being analogous. On the "far" side of the spring you have a fixed spot in space that comes from how long the space between the spring stops at either end are. On the other ("near") end you have the same thing, but it's adjustable with a screw or something equivalent to a shock collar or what have you. You have a spring rate that doesn't change. Whatever spring compression you get out of that gives the preload spring force which would change the preload spring torque. Here's where the clutch pack number might (emphasis on "might," I'm guessing here) have an effect on "preload" as you'd specify in this new user input replacing the old "preload torque" could come in: That distance between the 'shock collar' of the diff's preload setting and the "far" side of the spring might get eaten up a little bit by that extra clutch pack. This is what I suspect from looking at a cutaway diagram of a Trac Loc LSD at least. I could be wrong, but if it indeed does, then this would compress the preload spring a bit and indeed change the preload force (and therefore preload torque). So, with your whizbang new diff preload model that nobody's ever done in a sim before, you could specify "preload" by letting people select different length or rate springs, different washers of varying (but not infinitely adjustable) "lengths" to eat up some of that space between your "near" and "far" end points, while at the same time displaying "preload torque" traditionally. Only in Racer now the preload torque is not adjustable directly like it is everywhere else. Now it's like showing the real downforce in Newtons or something that's there for display so you know what you've got, but is really controlled and adjusted in the setup screens by changing the wing angle or ride height or whatever. This might be more realistic and interesting for the die-hard gearheads that love to tinker with every screw and bolt on the car you give them. If you change the number of clutches you'll see the "preload torque" change in the display, but are free to tune that back to how you had it before you changed the number of clutches, which is sort of assumed to just happen by magic in other sims if the number of clutches actually does any of this stuff :-P So for the not so mechanically interested player who is aware of "preload torque" effects on handling, this doesn't complicate things too much because they could still see the "preload torque" and adjust things to get it close to how they want pretty easily just fiddling with the adjustments, but adds some depth to the diff model which now could let them choose from a slew of springs with various combinations of rates and spring lengths, number and thickness of washers in some diffs, and "preload length" in others. Perhaps the "preload length" here might be adjusted on some diffs by turning a screw/collar. You could specify the number of turns of the collar and multiply by the pitch of the screw thread and use that. I don't know if "put five more turns in the diff" is something you'd ever hear at a track, but if it is, you could model it this way. Anyway, just throwing an idea out there to help get you thinking if you want to try something new in this area. If not, just set "preload torque" directly and be done with it. Don't let changing the number of clutches alter the preload torque at all. |
#4
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GPL Salisbury diff preload
Thanks for the elaborate reply (as always, most enlightening ).
.... > of clutches, not ramp angle, nothing. You set preload torque directly > like described earlier. However, you could try something else: That's how Racer (v0.8.6) does it now indeed for LSD's. > Another way to go (you'd almost certainly be the first to do it in a > sim which might be appealing to you and win you even more points with > your Racer fans that go back almost as far as you and I do :-P ) > would be to allow for the possibility that changing the number of > clutches does actually change the preload torque. However, if you do > this, then you need to specify preload differently. You aren't > setting the preload torque directly anymore. In Racer you can specify the type of diff; for an LSD, you directly specify power_ratio, coast_ratio and preload (a torque in Nm). For the Salisbury, internally I just use an LSD. I derive the ratios from the ramp angles. I was just wondering how preload could be calculated in a GPL kind of way. But atm, Racer can handle: ; Salisbury diff { ratio=4.2 type=2 power_angle=60 coast_angle=30 clutches=2 clutch_factor=1.0 } or ; LSD diff { ratio=4.2 type=3 preload=500 power_ratio=1.5 coast_ratio=1.5 } So the Salisbury diff is just turned into an LSD internally, but tweaking it is just like in GPL. I'll stick with adding an explicit preload in Racer v0.8.7 for the Salisbury diff. That just seems a bit more flexible and still leaves room for preload=0 (perhaps) GPL diffs. Cheers, Ruud |
#5
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GPL Salisbury diff preload
Sounds good. :-)
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