Playing around with bikeED and interested in the effect that moving the 'center of mass' has on the observed bike handling.
Curious if there is a source of data for bikes like the Honda to compare to the GPB bikes models. I always though it would be around the crank somewhere but this assumption does not seem to be correct from what i can see.
Thanks
That sort of data I found almost impossible to find or acquire. Though I've only really searched for the old classic bike data.
I've also tried emailing bike companies and some classic racing guys for real bike data and Dyno Chart copies, but all I got back from those, that bothered to replied, was that they were not allowed to release that sort of technical information and my best bet was to actually go to classic race meets and talk to the riders and mechanics about it.
If your lucky you can find data that some have posted online, but centre of mass data I really cannot see anyone posting info like that, but there must be a mathematical formula for calculating that if you can obtain the other figures about the weight of the chassis, swingarm, etc, etc needed? Other than that you could just use trial and error testing until it feels right. ;)
Maybe Max would know the CM formula?
Hawk
Trial and error is what got me scratching my head ;)
I was just playing with the new Honda and found that moving it close to the crank centre makes the bike much more 'flickable' at high speed and generally nicer handling all round, but most of the bikes have it positioned higher and further back so trial and error might be misleading.
Keep reading about 'mass centralisation' but cant find any useful info.
I don't believe you can find the centre of mass with just a formula. I mean the vertical point, cause the horizontal you can find it easy. I'm very curious though to learn how they find the centre of mass cause i was doing a research myself lately... let's wait and read opinions.
Quote from: tseklias on December 06, 2015, 06:39:22 AM
I don't believe you can find the centre of mass with just a formula. I mean the vertical point, cause the horizontal you can find it easy. I'm very curious though to learn how they find the centre of mass cause i was doing a research myself lately... let's wait and read opinions.
It seems quite critical with the way the physics are calculated in game - it makes a major impact to way the bike 'feels' when turning/accelerating/stopping etc. If you want bikes that feel definitely different and unique to each other rather than Mura clones this seems to be effective.
Pictures of the newer Ducati without its fairing would suggest it is very low and forward, virtually no bike in the area with the 'classic' mass point.
This is an older bike, but with the lightweight clutter removed where would you now place the centre? I would have guessed somewhere near the crank ???
(http://www.sportrider.com/sites/sportrider.com/files/styles/large_1x_/public/import/header_images/146-1201-01-z%2Bducati%2Bchassis-structure_0.jpg?itok=LCyAK2ol)
The CoG of a body can be computed with a formula, knowing the exact geometry of the body and the density of the material at each body point. You can look it up on wikipedia (https://en.wikipedia.org/wiki/Center_of_mass (https://en.wikipedia.org/wiki/Center_of_mass)).
You basically can't do it by hand, except for trivial bodies (e.g. sphere, disc, rod etc). So most of the time this is done numerically.
Given a body, there are ways to localize its CoG experimentally (i.e. you can "measure" it).
Given he CoG of two bodies, computing the CoG of the resulting assembly of the two is trivial. So if you have the CoG (and mass) of each body, it's easy to compute the COG of the bike.
Keep in mind that GPB bike is an assembly of multiple bodies: front and rear wheel, rear swingarm, steering head, chassis and rider (plus the CoG of the fuel in the tank, which of course varies with time).
Notice however that the CoG varies in time too, not only due to the fuel, but also (and mostly) due to the fact the bodies move relatively one to the other (swingarm, steering head and, of course, the rider).
Good info Max. ;)
Hawk.
Quote from: h106frp on December 06, 2015, 12:17:33 AM
Keep reading about 'mass centralisation' but cant find any useful info.
This is related to inertia: a bike that is slimmer (less wide, e.g. a v4 vs a 4 in-line) has a smaller inertia along the longitudinal axis so it will be easier to flick.
Forgot to add: moving the CoG fwd/backward and up/down alters the load transfer (and other things). This dictates how hard you can accelerate before the front lifts up.
@h106frp: V.Cossalter, "Motorcycle Dynamics". That's all you need man :)
Thanks,
I think that's useful ;) Anyone lend me a GP bike so i can measure it ;)
@MaX actually something you could explain that has puzzled me about the inertia boxes;
steer
{
Mass = 11
InertiaBox = 0.4, 0.8, 0.2
The second value appears to fix the top yoke in the longitudinal axis of the bike, reducing it allows it to visibly move. Fair enough that must be how fixed points are tethered in the model
but;
rear_suspension
{
Mass = 8
InertiaBox = 0.4, 0.2, 0.6
seems to contradict this, surely the rear pivot should be rigid in the longitudinal axis as well.
Just puzzled ???
Now i can pan and zoom in biked i can see easily that chassis center mass position varies greatly between mod bikes, back to trial and error i guess :)
Centre is quite a bit higher on the motogp15 Honda MOD than on the PB Mura which has it in the area (crank) i ended up with on the Honda by trial and error - lucky coincidence?. Noticed on the CBR250R it is very high ???
QuoteKeep in mind that GPB bike is an assembly of multiple bodies: front and rear wheel, rear swingarm, steering head, chassis and rider (plus the CoG of the fuel in the tank, which of course varies with time).
And each of it has an influence of the bike handling....including fuel mass, chassis mass, aero pressure point :o
That´s the "problem" modding new bikes AND let them behave different.
It´s easy (more or less) to make a new Muru Clone....but to make the clone an individuum it´s harder and sometimes the art of try and error. ;)
You can make the bike oversteer, understeer, wheelie , bottomed, changing the values......in the right or wrong direction.
Then we have the steering damper and PID controller which can mess up good bike physics when improperly set.
QuoteV.Cossalter, "Motorcycle Dynamics". That's all you need man
That´s the bible.....but not for everyones mind (too much formulas) ;)
Quote from: C21 on December 07, 2015, 08:26:52 AM
QuoteV.Cossalter, "Motorcycle Dynamics". That's all you need man
That´s the bible.....but not for everyones mind (too much formulas) ;)
True, but there's only one way to understand it for good.
If one needs something a bit easier on the maths, there's this: Motorcycle Handling And Chassis Design by T.Foale (http://books.google.co.uk/books?id=84hF-qoR5I8C)
QuoteMotorcycle Handling And Chassis Design by T.Foale
i find that one more practicable then the Cossalter one.
Cossalter is very theoretical but it´s the mathematical view of Foale´s book.
I'd say more the other way around, the Foale book is the "practical" view of the Cossalter :)
The problem is that in some occasions, you do need a formula to do the job.
Might be some guidance, its for the road bike version but i guess they would be similar mass distribution
(http://www.asphaltandrubber.com/wp-content/gallery/2016-honda-rc213v-s/2016-Honda-RC213V-S-street-bike-92.jpg)
QuoteMight be some guidance, its for the road bike version but i guess they would be similar mass distribution
I doubt ;)
For $185,000 you would hope it had some resemblance. :o ;D
Quote from: h106frp on December 06, 2015, 08:14:29 PM
Noticed on the CBR250R it is very high ???
its tuning to banking speed and balance, and some moment
BTW,
chassis
{
Mass = 47
Inertia = 5, 3.4, 2.5
and
steer
{
Mass = 8
InertiaBox = 0.4, 0.8, 0.2
I'm still not understand "Inertia" and "inertiaBox" parameta well,,,, anyone can explain it? :'(
Quote from: Yohji on December 09, 2015, 08:23:20 AM
Quote from: h106frp on December 06, 2015, 08:14:29 PM
Noticed on the CBR250R it is very high ???
its tuning to banking speed and balance, and some moment
BTW,
chassis
{
Mass = 47
Inertia = 5, 3.4, 2.5
and
steer
{
Mass = 8
InertiaBox = 0.4, 0.8, 0.2
I'm still not understand "Inertia" and "inertiaBox" parameta well,,,, anyone can explain it? :'(
Hello Yohji,
Not a comment on your bike - it seems to handle quite nicely, just an observation r.e. 'real life values' vs 'GPB physics optomised (trial end error method)'.
As i understand it so far, the bikeED 'chassis center of mass' should be for the chassis component only so you might expect it to be quite low and somewhere around the crank/gearbox, the total bike center is then dynamically calculated in game from sum of the various components.
I am also looking for an explanation of 'inertia box' it seem to be some sort of matrix of ratios
Quote from: Yohji on December 09, 2015, 08:23:20 AM
I'm still not understand "Inertia" and "inertiaBox" parameta well,,,, anyone can explain it? :'(
Inertia is ... weel, the inertia. What you don't understand ?
InertiaBox to me is just the inertia of the steering, don't know why it's called InertiaBox instead of simply inertia.
For the chassis we have values, Inertia = 5, 3.4, 2.5, OK
for steering we seem to have ratios of 0 to 1, InertiaBox = 0.4, 0.8, 0.2
If you reduce the second value (say 0.2) you will observe the top yoke physically moving in the longitudinal axis, A value of 1 fixes the yoke in this axis - they seem more like 'rigidity' values than calculated values for inertia. So i guess it has been scaled from 0=no restraint to 1=totally restrained for each axis.
Just guessing from observation
Quote from: h106frp on December 09, 2015, 01:34:40 PM
For the chassis we have values, Inertia = 5, 3.4, 2.5, OK
for steering we seem to have ratios of 0 to 1, InertiaBox = 0.4, 0.8, 0.2
If you reduce the second value (say 0.2) you will observe the top yoke physically moving in the longitudinal axis, A value of 1 fixes the yoke in this axis - they seem more like 'rigidity' values than calculated values for inertia. So i guess it has been scaled from 0=no restraint to 1=totally restrained for each axis.
Just guessing from observation
0.4, 0.8, 0.2 are the dimensions (in meters) of the bounding box: a 3d box that is assumed to have an uniform density and the specified mass (8 for the steering head).
The inertia is then calculated as (https://en.wikipedia.org/wiki/List_of_moments_of_inertia (https://en.wikipedia.org/wiki/List_of_moments_of_inertia)):
Ix = 1/12.0*mass*(dy*dy+dz*dz);
Iy = 1/12.0*mass*(dx*dx+dz*dz);
Iz = 1/12.0*mass*(dy*dy+dx*dx);Where dx, dy, and dz are the values of InertiaBox. So bottom line: it's a "proxy" for the inertia of the element.
I really doubt it has anything to do with "rigidity" or "restraint".
But don't ask me why we have real inertia for the chassis and a box for other elements: I don't know.
Thanks for the explanation, this is the sort of info that needs documenting for meaningful physics mods.
Quote from: h106frp on December 09, 2015, 02:51:10 PM
Thanks for the explanation, this is the sort of info that needs documenting for meaningful physics mods.
Everything should be documented. Better if officially :)
Quote from: HornetMaX on December 09, 2015, 12:47:11 PM
Quote from: Yohji on December 09, 2015, 08:23:20 AM
I'm still not understand "Inertia" and "inertiaBox" parameta well,,,, anyone can explain it? :'(
Inertia is ... weel, the inertia. What you don't understand ?
InertiaBox to me is just the inertia of the steering, don't know why it's called InertiaBox instead of simply inertia.
ex) Inertia = 5, 3.4, 2.5
I know inertia's word mean, but what is first,second, and third parameta?
it's Diffusivity?
x,y,z axis values or y,x,z or z,y,x i can never remember ;)
Yep, inertia along X, Y and Z axis.
Careful about GPB convention about axes: best is to double check it in BikeEd, it seems to be left-handed ...
I see, I'll tune it. thx.