Advertisement Jump to content
  • Advertisement

Edy

Member
  • Content Count

    49
  • Joined

  • Last visited

Community Reputation

371 Neutral

1 Follower

About Edy

  • Rank
    Member

Personal Information

Social

  • Twitter
    @vehiclephysics

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

  1. Edy

    Limited-Slip Differential and friends

    Well, that's exactly how it works in my model. But as said, the final implementation depends on each specific model design.
  2. Edy

    Limited-Slip Differential and friends

    Your point is correct. Torque will always be split 50-50 between wheels. In your example both wheels receive 500Nm. The wheel with traction will push the car with that force and will move the car. But the wheel with no traction will convert that torque into angular acceleration and will gain angular velocity very rapidly. The calculation is easy: angular acceleration = torque / inertia. The large torque and the small inertia causes a huge angular acceleration. So the next moment the transmission will be spinning very fast. In practice, engines and motors have a limitation of RPM they can produce torque at. This means that the next moment the engine won't be able to keep 1000Nm, but maybe 100Nm, or no torque at all. So the next moment you'll still have the torque 50-50 split between wheels, but it will be very small. The wheel with traction will now receive a very small torque insufficient to move the car, while the other wheel is furiously spinning. A theoretically ideal engine that could keep providing 1000Nm no matter the RPMs would be able to move the car in your example. The wheel with traction would receive a constant amount of 500Nm that would push the car, while the other wheel would keep gaining angular velocity indefinitely. However this is not a realistic situation and can't be built in practice. The way of calculating torques depends of each simulation model: design, considered variables, torque flows, inertias, solver, etc. In my specific model the locking torque is a torque amount that is transferred from a wheel to another (same value, as it's taken from a wheel to be applied to the other wheel) and tries to keep both wheels rotating at the same rate. When the clutch pack provides torque enough for keeping both wheels rotating at the same rate then the LSD differential is completely locked. The amount of torque provided by the clutch pack varies on each situation and depends on how much torque is applied at the input, the ramp angle, and the friction coefficient of the clutch pack.
  3. Edy

    Limited-Slip Differential and friends

    It's not about the acceleration. The system could be running at constant speed and the clutch packs being pushed providing locking force. The effect is caused by the reaction torque of the output with less resistance. This also explains why when the less resistance is 0 (wheel in the air) then there's no locking force at all. Looks mostly correct to me. Change "acceleration" with "resistant torque" or "reaction torque" (you name it). I rather not discuss about implementation details, sorry. Each implementation depends on how the specific drivetrain model is designed: considered variables, torque flows, inertias, analytic vs. numeric resolution, etc. You tell me!
  4. Edy

    Limited-Slip Differential and friends

    I don't think the wheel load alone is a good criteria. It's much more important the grip understood as the ability of the wheel to receive torque without slipping. We may simplify the grip as wheel load multiplied by the coefficient of friction of the tire. When a wheel receives torque the grip translates it into tire force that moves the car. If a wheel receives more torque than the grip permits, it start slipping so the torque accelerates the wheel while the resulting tire force decreases. The open differential splits the torque 50-50. If both wheels have enough grip then both wheels will produce the same amount of force. This situation persists in curves as long as both wheels have enough grip (= load x coefficient of friction). When the load is decreased in the inner wheel then it might keep the grip until certain point. Beyond that point, the inner wheel starts slipping decreasing the produced force. Both wheels keep receiving the same torque but as the tire force in the inner wheel decreases, so does the torque required for keeping it slipping (leaving inertial effects aside). As the torque is reduced, both wheels receive less torque so both produce the same reduced tire force (being one slipping, the other adherent). This is the loss of performance issue with an open differential: when a wheel slips, you get an overall acceleration loss. However, going for too much throttle does not cause the outer wheel to slip in an open differential (again, leaving inertial effects aside). All power is routed to the slipping wheel, leaving the adherent wheel producing the same -reduced- force as the slipping wheel. The point of LSD / torque bias ratio is allowing the adherent wheel to take advantage of its unused grip. An open diff is 1:1. This means that the adherent wheel will receive the same torque as the slipping wheel (= reduced torque as for the tire slipping). A bias ratio of 2:1 means that while a wheel is slipping, the adherent wheel will receive twice as torque as the slipping wheel, thus allowing it to produce twice as force. This reduces the performance loss that would be produced in an open differential, as described above. A ratio of 4:1 means up to four times the force of the slipping wheel. I don't think the LSD / torque bias situations are comparable with the locked or viscous differentials. LSD routes the torque based on the wheel producing less tire force. This means that if one wheel is in the air the LSD differential behaves exactly as an open differential: the adherent wheel receives no torque (only the torque caused by the inertial effects, to be exact). However, locked and viscous differentials route the torque in a way so the torque produced by the engine is always transformed, entirely or as a fraction respectively, into tire forces among both wheels.
  5. Edy

    Limited-Slip Differential and friends

    That's it. These differentials typically have a "Lock" function as well to link front and rear axles. It's a way of describing it. It's also a kind of "soft lock": the differential wants both shafts to rotate at the same rate (as in the locked differential), but applies a limited torque to achieve it. I'd describe the effects of a locked diff in more wide terms: Coasting: understeer Accelerating: understeer Too much throttle when accelerating: oversteer, increased risk of loosing control (or initiating drift if done on purpose). Too much engine brake when coasting: oversteer, risk of loosing control. I'm not sure that an open differential would such defined effect on handling. An open diff may reduce the performance due to some wheel getting most of the power. Most of them, except the smallest ones. It's always an open differential. Current LEGO Technic catalog includes these cars with differential: First Responder Rally Car Porsche 911 GT3 RS The Porsche model includes not only differential, also a dual-clutch gearbox with paddle shifters (!!). Check out the video in the page. BTW, I've ordered it today
  6. Edy

    Limited-Slip Differential and friends

    Thank you! I've been playing around with differentials since I was 8. It turns out that most LEGO Technic cars come with differential. Playing with a differential in your hands hugely helps to get the point on how it works and the torques and constraints involved. I recommend you to get one of those sets! Yes, I've modeled a Torsen differential based on the information available, but I hadn't validated the model with actual data yet. Here I'm talking on "standard" differentials with symmetric geometry providing the same torque to both ends. I won't cover asymmetric or epicyclic differentials, which provide different torques to each end. In order to understand differentials we can examine the "spectrum" of the locking effect provided by the differential. At one end we have the open differential, where both shafts rotate freely according to the rotational constraints only. At the other end we have a fully locked differential, which behaves like a rigid rod and both shafts are forced to rotate at the same exact rate. The differential may be partially locked in between. So we have a continuous range of differential looking, from 0% (open) to 100% (locked). This looking percentage is achieved by applying torques to specific parts inside the differential. The car used in my drifting video has a permanent 100% locked differential. This is terrible for maneuvering in the parking area (rear wheels are constantly bouncing when steering in close turns), but it allows to easily break the adherence of both rear wheels for initiating and maintaining the drifting effect. The different types of differentials have different conditions that trigger the differential locking effect and the amount of it. For example: Viscous differential: a constant amount of locking effect, no matter the state of the wheels or the transmission. Clutch pack: the locking effect is triggered by the input torque coming from the transmission. This torque pushes a series of clutches that progressively lock the differential. The amount of locking effect depends on the input torque and the differential setup (clutch friction, cone angles...). If one wheel is in the air this differential behaves like an open differential. Torsen (as I've interpreted it - might not be entirely correct, needs validation). Locking effect is based on the output torques being biased among both shafts proportionally to the output with less resistance. If one wheel is in the air this differential behaves like an open differential (zero resistance in one wheel). The differential has a great influence in the handling. When a differential is fully locked the car tends to go straight. The rear axle becomes a rigid rod that fights against the car steering. However, the locked differential makes easy to break the adherence of both wheels simultaneously, which is useful in some driving styles. An open differential facilitates the steering, but the car might lose traction in some situations due to one wheel slipping when applying throttle or engine brake. The differential typically affects the understeering / oversteering behaviors when entering and exiting the curves. This is why it may be configured with just three parameters: preload: minimum, permanent locking effect in any situation. power: locking effect when applying throttle coast: locking effect when coasting / applying engine brake If one of the wheels (surely the interior wheel) tends to spin when entering or exiting the curves then you'd need some amount of differential locking effect. But if your differential applies too much locking effect, you may find that the car tends to go straight in that situation. The proper balance gives you the best performance.
  7. Edy

    Limited-Slip Differential and friends

    I've implemented a setup like that. This one uses that exact tree of 7 differentials chained: However, in reality these vehicles use a so called H-drive drivetrain. There's a single center differential, then the drive wheels at each side are linked together.
  8. Edy

    Simple ray-cast vehicle

    You have to add some damping to the suspension, otherwise it would hardly stop oscillating. A basic suspension could be implemented like this (pseudocode): contactDepth = maxHitDistance - hitDistance; contactSpeed = (lastContactDepth - contactDepth) / deltaTime; lastContactDepth = contactDepth; springForce = contactDepth * springRate; damperForce = contactSpeed * damperRate; forcePerTire = RaycastDir * (springForce + damperForce); You would then configure the suspension with the springRate and damperRate parameters.
  9. Today's work: implement, test and configure new devices for VPP #unity3d #madewithunity https://t.co/nrthLNaduB
  10. Edy

    raycast vehicle physics problem

    That would be the black arrow in your drawings, right?
  11. Edy

    Car skidding behavior

        Most likely, yes, the car spin will be even worse.    When the wheel is rolling freely (no throttle / brake applied) then it can use all its grip on the lateral force. When throttle is applied causing the wheel to sping faster than the ground then the force direction is deflected forwards, which means that less force is available for compensating the sideways sliding. Thus, the car will likely continue spinning.    I've been doing drifting myself [VIDEO]. The key factor is controlling the throttle carefully. The steering wheel will be mostly rotating itself and all you have to do is apply minor corrections. For example, at around minute 1:20 you can hear how I lift the throttle in order to allow the car to end the spinning. There are many situations in the video where I'm applying throttle gently or no throttle at all while sliding laterally in order to control the car's spin rate.   Later, I applied those lessons to my own simulation model, Vehicle Physics Pro [VIDEO], with great results.
  12. Upgrading your project to Unity 5.5? Ensure to use 5.5.0 patch 3 or newer. Fixes broken RB interpolation. https://t.co/2YvEc9MKSn #unitydev
  13. Hint for Edy's Vehicle Physics: hover the speed labels for showing the unit conversion (km/h, mph) https://t.co/xyvYxIP5d6
  14. I've just added detailed information on tires and tire friction to the VPP documentation: https://t.co/mPJdOxAGJG https://t.co/xAmDgbbzDb
  • Advertisement
×

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

GameDev.net is your game development community. Create an account for your GameDev Portfolio and participate in the largest developer community in the games industry.

Sign me up!