[ODE] Bike Balancing!

Wed Nov 7 08:28:58 MST 2007

Hmmm....that's a good article, and I guess as you say; for low speeds, 
counter-steering would work ok to balance the bike.  But as this is a 
motor bike and typical reaches speeds of 210-250kpm, I think a different 
approach may be needed. Counter-steering at those speeds could lead to 
massive under-steer and loss of traction which is not so great :)

I was thinking of using contact joints at one point too, and still am.  
But my current line of thought is that it might limit the flexibility of 
the engine/tire model and for now, just applying forces on my own works 
without any issues. 

Eike Decker wrote:
> >From the sources I've read back then, it is the way how we stabilize the bike
> when driving it - little steers to the left and right. One said that you can
> see that if you drive slowly with wet wheels or on sand - one path is smooth,
> which is the back wheel, and one path is oscillating around that smooth path,
> which is your own stabilizing.
>
> There's an article here which covers the balancing 
> http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics
> or 
> http://www.diablocyclists.com/bicyclescience.htm
>
> I've tried many things out, and adjusting the steering is enough to balance it.
> And for a nearly upstanding bike, the sphere wheel geometry is not different
> from real wheels. Without that stabilizing movements, the bike tips over very
> quickly, thus, the spheres do not seem to stabilize the bike.
>
> Thanks for explaining your tire physic model. I thought lately if I could use
> some ray tests and using dcontact joints for simulating the tire physics. I
> wouldn't want to make the slip/contactforce calculations myself...
>
> Btw, putting the center of mass below the bike does help, but is also causing
> troubles in certain conditions.
>
> Eike
>
>   
>> Eike I suspect uch of the balancing behaviour that your experiencing is 
>> from the sphere themselves?  Not sure if just altering the steering 
>> angle will do what I want as I'm casting rays against the world for 
>> wheel collisions.
>> ----
>>
>>
>> This is OT :- but maybe people here might benefit from how I approached 
>> my tire model. Maybe I can be persuaded to do a ODE example when I have 
>> some free time :)
>>
>> The tire model is very simplistic and comes in two parts :-
>>
>> 1) the engine/transmission simulation which deals with the torque/revs 
>> of the engine and gearing
>> 2) the forces produced by the tires
>>
>> The first part is just numbers, and not really that hard to do. It just 
>> calculates the engine and rive torque bases on the throttle input and 
>> current drive gear.
>>
>>  The second part really does nothing more than cast a ray for each wheel 
>> against the scene.  When a wheel is colliding with something it produces 
>> the wheels forces in this way :-
>>
>> 1) Caclulate the world matrix of the wheel (this will include any 
>> steering angle on the wheel)
>>
>> 2) Project the wheel matrix on to the contact normal The At and Right 
>> vectors of the matrix will now give you the longitudinal and lateral 
>> force vectors
>>
>> 3) If the wheel is a drive wheel, calculate the desired force from the 
>> drive torque and multiply this by the longitudinal patch vector from step 2
>>
>> 4) If the wheel is braking, then produce calculate the brake force 
>> produced by the wheel
>>
>> 5) Get the velocity at the wheel contact point, and normalize it (call 
>> this vector patchVel)
>>
>> 6) Perform a dot product between the patch vel and the patch right 
>> vector and multiply this by the wheel load (I get this from the 
>> suspension force) and also the tires friction coeffiicient.
>>
>> 7) Calculate the maximum load the tire can take.  I calculate this by 
>> taking the wheel load (again from the spring force) and multiplying it 
>> by the tire friction coefficient and the surface friction coefficient. 
>>
>> At this point you wil have three things :-
>>  i. The desired longitudinal force of the tire ( a scalar value called 
>> forceLong)
>> ii. The desired lateral force of the tire ( a scalar value called forceLat)
>> iii. The maximum force the tire can produce ( a scalar value forceMax)
>>
>> 8) Check to see if the tire is braking the force limits. If it does, 
>> workout the total amout of force the tire produces and flag the wheel as 
>> braking traction.  An exmaple of the code is :-
>>
>> forceMaxSqr = forceMax * forceMax;
>> totalForceSqr = (forceLong*forceLong) + (forceLat*forceLat)
>>
>> if (forceMaxSqr < totalForceSqr)
>> {
>>     float k = totalForceSqr / forceMaxSqr
>>     forceLong *= k;
>>     forceLat *= k;
>>  
>>    // You may want to set flags in your wheel structure to indicate the 
>> wheel has lost traction
>>
>> }
>>
>> 9) Combine the forces into one vector with something like
>>
>> forceVec = (patchLat * forceLat) + (patchLong + forceLong)
>>
>> 10) Apply the force at the contact point for the wheel
>>
>> 11) Update your wheel rotation velocities.  It's a bit complicated to 
>> explain everything I do here, but in a nut-shell; the wheel rotation 
>> velocities are updated depending on thier drive/brake/traction state.  
>> i.e. if a drive wheel is slipping, it's rotational velocity is that of 
>> the rpm of the axle, if not, then it's the same as the longtudinal patch 
>> velocity.  If the brakes are pressed and the wheel has lost traction, 
>> then the angular velocity of the wheel is just assumed to be zero (locked)
>>
>> 11) There is also a feedback loop for the engine revs to be constrained 
>> to the wheel rotations, but just taking the average of all of the drive 
>> wheels rotational velocities in contact with the ground and not slipping.
>>
>> The tire model isn't complete though.  I haven't added 
>> lateral/longitudinal tire deformation and the maximum force calculation 
>> should really use something like Pacejka curves, but as a basic model it 
>> works very well.  I prototyped the tire model using a car with similar 
>> parameters to a Opel/Vauxhall/Holden VXR8 and it was pretty fun to drive.
>>
>> I added some additional things to the tire model for more arcade 
>> handing. Things like :-
>>
>> i) simple/complex load calculation flag
>> ii) simple/complex patch matrix calculation flag
>> iii) fake global downforce multiplie which acted on the max force 
>> calculation
>>
>> The car is pretty fun to drive and exibits very realistic behavours like 
>> brake lock up, wheel spin, traction loss as the tires reach thier 
>> limits.  The lack of tire deformation make loss of traction happen very 
>> suddenly though, sort of like driving on very bad tires at high speed.
>>
>> It worked on the bike first time too using engine and gearing parameters 
>> from a Yamaha, apart from the falling over bit :)
>>
>> I hope this was informative!
>>
>>
>>     
>
>
>
>
>   

-- 

James Steele   

3D / Physics Programmer

james.steele at greentube.com

Greentube Internet Entertainment Solutions AG

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A-1060 Wien

FN 197003k, HG Wien

Tel: +43 1 494 50 56

Fax: +43 1 494 50 56 - 14

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