Some physics stuff not yet fully understood

[HornetMaX]

Hi all,

I put here a few questions I have no answer for (and nobody I've been in touch with neither). Would be nice if Piboso could provide answers (or others too, if they are really really sure)

• In the .geom file, what is rear_length = 0.06 ? Most seems to think it is the rear shock length (from fully compressed to fully extended), but it seems to have no effect at all. Also it would be not clear to me what this would be used for, given that we don't know where the shock is linked to the swingarm and chassis. I feel stupid.
• In the bike.cfg file, what are Damper and DamperPower in the steer section ? Probably related to some sort of steering damper, but how should we interpret them ?
• In the bike.cfg file, what are spg0, spg1  etc and KYaw, KDamping0, KDamping1 in the steer section ?



• In the bike.cfg file, what is ThrottleMapping0/1/2/3 in the ecu section ? Anybody seems to say "Easy, it's the throttle mapping". But nobody knows how to interpret the rpm0/1/2/3 and throttle params.

Plenty of unclear stuff on tires too, but I'll leave that for later

MaX.

[PiBoSo]

1. Rear spring max travel. It is used to calculate the rear suspension wheel rate.
2. Steer damper. Damping torque = Damper * (steer speed ^ DamperPower)
3. Virtual rider steer torque parameters. Too long and complex to explain.
4. For each engine mapping, it is possible to cap the max throttle for each gear for a specific rpm range.

[HornetMaX]

Thanks for the answers !

1. Rear spring max travel. It is used to calculate the rear suspension wheel rate.

Hmm ... I'm very confused ... How can you compute the wheel rate if you don't know where on the swingarm the suspension is attached (and at which angle, and where the other end of the suspension is attached) ?

As far as I can see,the reduced torsional stiffness is equal (first approximation) to the spring stiffness multiplied by the square of the velocity ratio (sometimes called motion ratio).

But the motion ratio, even in thew simple case of shock attached somewhere on the line joining the rear wheel axis to the swingarm pivot, depends on plenty of things: swingarm length, swingarm angle, distance between the swingarm pivot and the point on the swingarm where the suspension is connected, coordinates of the point where the suspension is connected to the chassis.
Of the needed values, we seem to have only the first two (swingarm length and angle), plus the unclear rear_length.

Or are you using a very simplified model where the suspension is considered always normal to the swingarm (no matter the swingarm angle) and the rear_length indicates the distance between the swingarm pivot and the point on the swingarm where the suspension is connected ?

2. Steer damper. Damping torque = Damper * (steer speed ^ DamperPower)

OK, understood.

3. Virtual rider steer torque parameters. Too long and complex to explain.

I was suspecting that. Way too complex, I agree.

4. For each engine mapping, it is possible to cap the max throttle for each gear for a specific rpm range.

Yeah, but how does it work ? What does the below mean ? Why we have 4 rpm points but only 1 throttle value ?

Code Select Expand

ThrottleMapping0
{
name = qualify
Gear0
{
rpm0 = 8000
rpm1 = 10000
rpm2 = 15000
rpm3 = 17000
throttle = 0.85
}
... ... ...

MaX.

[PiBoSo]

1. Rear spring max travel. It is used to calculate the rear suspension wheel rate.

Hmm ... I'm very confused ... How can you compute the wheel rate if you don't know where on the swingarm the suspension is attached (and at which angle, and where the other end of the suspension is attached) ?

Modern GP motorcycles have an almost "flat" rear suspension curve, so wheel travel and suspension travel are enough for a good approximation in most cases.
For progressive ( or, in some cases, regressive ) wheel rate, it is possible to add points to modify the curve. An example will be featured in MX Bikes.

4. For each engine mapping, it is possible to cap the max throttle for each gear for a specific rpm range.

Yeah, but how does it work ? What does the below mean ? Why we have 4 rpm points but only 1 throttle value ?

Code Select Expand

ThrottleMapping0
{
name = qualify
Gear0
{
rpm0 = 8000
rpm1 = 10000
rpm2 = 15000
rpm3 = 17000
throttle = 0.85
}
... ... ...

rpm0 and below = 1.0
rpm0 to rpm1 = interpolation
rpm1 to rpm2 = throttle
rpm2 to rpm3 = interpolation
rpm3 and above = 1.0

[C21]

Thank you Piboso!

[HornetMaX]

Modern GP motorcycles have an almost "flat" rear suspension curve, so wheel travel and suspension length are enough for a good approximation in most cases.

What do you call a "rear suspension curve" ? Reduced stiffness (at the wheel) vs wheel vertical displacement ?

Do you use the rear_upper and rear_lower in conjunction with the swingarm length and rear_length to determine where the suspension is connected on the swingarm (assuming that the suspensions is fully compressed at rear_upper and fully extended at rear_lower) ?

MaX.

[PiBoSo]

What do you call a "rear suspension curve" ? Reduced stiffness (at the wheel) vs wheel vertical displacement ?

Leverage ratio:
http://www.promecha.com.au/leverage_linkages.htm

[HornetMaX]

What do you call a "rear suspension curve" ? Reduced stiffness (at the wheel) vs wheel vertical displacement ?

Leverage ratio:
http://www.promecha.com.au/leverage_linkages.htm

OK, this is called in a thousand different manners, including "motion ratio" and the more technically accurate "velocity ratio" (ratio between the deformation velocity of the spring and the vertical velocity of the wheel, or the angular velocity of the swingarm for a torsional spring).

But still: how do you compute the value of the leverage (value that does not depend on the swingarm angle in your approximation) from what you have in the .geom and .cfg file of the bike ?
Simply as ratio between wheel travel (function of swingarm length and min/max angle) and rear_length ?

MaX.

[PiBoSo]

What do you call a "rear suspension curve" ? Reduced stiffness (at the wheel) vs wheel vertical displacement ?

Leverage ratio:
http://www.promecha.com.au/leverage_linkages.htm

OK, this is called in a thousand different manners, including "motion ratio" and the more technically accurate "velocity ratio" (ratio between the deformation velocity of the spring and the vertical velocity of the wheel, or the angular velocity of the swingarm for a torsional spring).

But still: how do you compute the value of the leverage (value that does not depend on the swingarm angle in your approximation) from what you have in the .geom and .cfg file of the bike ?
Simply as ratio between wheel travel (function of swingarm length and min/max angle) and rear_length ?

MaX.

How can "velocity ratio" be more accurate than "motion ratio", considering that velocity is just motion / time?

All modern GP motorcycles use a complex linkage at the rear... The end result is usually an almost constant ( less than 10% increase ) leverage ratio.
So wheel travel / suspension travel ( that is the average ratio ) is a good approximation for a GP bike.
Street bikes and MX bikes use strongly progressive links. For them, it is possible to add control points to adjust the leverage ratio. Calculations are complex, though, so it's better to wait for BikeEd.

[HornetMaX]

How can "velocity ratio" be more accurate than "motion ratio", considering that velocity is just motion / time?

It's more with respect to "leverage" that "velocity ratio" seemed more appropriate to me. But I guess it all depends on how picky one is on the definitions. Leverage seems a more intuitive name (people understand immediately why it is named like it is), but velocity (or motion) ratio makes more sense when you derive the equations. No biggie anyway.

All modern GP motorcycles use a complex linkage at the rear... The end result is usually an almost constant ( less than 10% progression ) leverage ratio.
So wheel travel / suspension travel ( that is the average ratio ) is a good approximation for a GP bike.

That's very fine to me, I was just trying to understand the model behind from the exposed parameters. Definitely not criticizing the model used.

Street bikes and MX bikes use strongly progressive links. For them, it is possible to add control points to adjust the leverage ratio. Calculations are complex, though, so it's better to wait for BikeEd.

Not sure what you mean by "add control points": if you mean directly playing with the linkage geometry, I'm not even sure you need this sort of complexity . Maybe just allowing to "play" with the rear suspension curve directly (the part due to the linkage) could be more straightforward and flexible (despite less realistic). Just my opinion of course.

Anyway, big thanks for taking the time to explain all this: all crystal clear now.

MaX.