Front and rear wheel speed difference?

[RIDER]

That's the most interesting thing ive learned in months. I wonder about how much were talking. Enough that it adds to the difficulty of anticipating/feeling a tire breaking traction in low grip situations? Or can it really never be felt?

[HornetMaX]

That's the most interesting thing ive learned in months. I wonder about how much were talking. Enough that it adds to the difficulty of anticipating/feeling a tire breaking traction in low grip situations? Or can it really never be felt?

I've never been on a track so I can hardly tell for sure. I'm fairly confident it's much easier to feel a small lateral slip (side slip) than a small longitudinal one.

I'm under the impression that at the front you barely feel the longitudinal slip under braking, and when you feel the lateral one is probably already too late

At the rear it's easy to feel large longitudinal slips (both braking and accelerating) and even small lateral slip (rear going side-way).

To give an idea, the murasama rear quali tyre has its longitudianl force (no side slip, no camber, nominal vertical load) that reaches his max at a slip of 0.15 (i.e. 15% difference between "bike" speed and wheel speed). As soon as you have even a modest slip, the force can be fairly big. I guess that's why I think we do't feel the slip (except in extreme cases).

MaX.

[BOBR6 84]

Sometimes you can hear the rear tyre squeak, accelerating hard off the grid  You can always feel the rear sliding and squirming around on the power, its really not a big deal when you are on the bike.. big controlled slides are a different story though lol 

on the brakes you can guarantee the rear will be moving around too!  or chattering because i downshifted like a tw@t 

[RIDER]

Bad question, I was asking if you can feel slip levels that are way too low to feel (obviously not). And sense the slip increases gradually up to a sudden break in traction, it's easier to anticipate rather than more difficult.

You do feel the front sliding laterally easy but usually when trail braking so you aren't on the edge of the tire. 

So whenever a bike is leaned over and moving forward, the tires are slipping laterally due to the small cornering load?

[HornetMaX]

So whenever a bike is leaned over and moving forward, the tires are slipping laterally due to the small cornering load?

Yes, even if in principle you could turn without slipping (laterally) thanks to the camber thrust (a lateral force generated by a tyre with a camber angle, Camber thrust).

Camber thrust is nice because it happens immediately as you camber, while lateral forces generated by slipping have some sort of delay (relaxation length, GPB takes it into account).
But when you take a corner hard enough, camber thrust is not enough so you'll have to slip.

MaX.

[BOBR6 84]

Wow, its amazing the level of detail GPB has within the physics and everything... Really is something special 

[h106frp]

Sort of related..

Had the default 125 on Victoria and noted that at turn 2 at a constant lean, turn rate and speed i lose the front when the bike hits some small bumps toward the exit of the corner even though it was fine earlier in the turn. Obviously taking the corner slower works OK but it does seem that you should be able to go a lot faster. 

The bike appears to have very little compliance for bumps when fully leaned over. In this situation is the tyre lateral stiffness the only form of 'suspension' (is it modelled) or are the frame/swingarm/forks stiffness modeled to allow the frame system to flex a bit and soak up the smaller bumps.

The magnitude of the bumps do not appear as is they should be sufficient to cause the wheel to skip (and cause a fall) but they do with the 125.

[HornetMaX]

The bike appears to have very little compliance for bumps when fully leaned over. In this situation is the tyre lateral stiffness the only form of 'suspension' (is it modelled) or are the frame/swingarm/forks stiffness modeled to allow the frame system to flex a bit and soak up the smaller bumps.

I think tyre stiffness is modeled in GPB. Are the values correct ? Hard to tell but I think they have been changed in beta4 or 5 to be closer to reality.
And yes, chassis/fork/swing-arm flexibility should help a bit too, in principle.

MaX.

[Alone]

Not exactly what you are talking about, but changing some mass points can help a little in the 125cc: http://forum.piboso.com/index.php?topic=2263.msg32177#msg32177 (try the attached geom file)

I fully agree about the chassis/fork/swing arm change the things. Also, from what I try, the rear suspension mass is a point that helps with the prevention of loosing the front in some bumps when leaning constant.
Steer data seems, in principle, an important point on that too, but we doesn´t know what is kdamping and kYaw (unless I´m missing something).
In a bike testing, founded these values to help on bump/leaning issues.
   
   Lock = 30

   Damper = 9
   DamperPower = 0.9

   spg0 = -280
   spg1 = -3
   sdg0 = -70
   sdg1 = 0.3
   sig0 = 0
   sig1 = 0

   KYaw = 20
   
   KDamping0 = 6
   KDamping1 = 0.15


spg, sdg and sig are related to speed and lean angle? Do you have some info about how to calculate them?

[HornetMaX]

spg, sdg and sig are related to speed and lean angle? Do you have some info about how to calculate them?

They are virtual rider parameters: they are the gains of the PID (Proportional, Integral, Derivative) controller that stabilizes the bike dynamics in terms of lean angle.

spg0, spg1 are for the proportional action
sig0, sig1 are for the integral action
sdg0, sdg1 are for the derivative action

The overall proportional gain is something like "spg0 + spg1*V", where V is the bike speed: that makes the gains vary depending on the bike speed.

There's no easy way to compute them: one would need a linearized model of the bike at different speeds, some knowledge about feedback loops, PID controllers etc.

Side note: when I see how fast the steering moves in GPB without any input change (i.e. in the telemtry, how "noisy" the steering signal is), I always think that there may be something wrong in the virtual rider ... bad gains or bad PID implementation ... I don't think on a real bike the steering moves like that.

MaX.

[Alone]

Thanks Max!

[h106frp]

Thanks, I will give the geometry file a try first.