MX Bikes Official Forum

Yop. Need some input - will start technical but got a TLDR at the end :p .

Found some new data about mass distribution on dirt bikes. Basically, there is ~52% rear bias (52% of the total mass is on the rear tyre, 48% on the front one). Measured with one bathroom scale under the frontwheel one under the rearwheel, Now I'm trying to port this in properly.

If we assume the other masses locations are correct or very easy to place(steer, suspensions, wheels and brakes have to be on point), and neglect the height différences between front and rear in BikeEd, the only unknown is Chassis weight position. Steer and Rear Susp weight could be moved around but they are ~15% of chassis weight each and leave not a lot of room for positioning, so we'll neglect that for now and keep their positioning free handed.

This means chassis weight forward position (between front/rear wheel) can be calculated.

I made an excel thing for that, and it comes out that our chassis weight in BikeEd should be much more toward the back. Trying out the results in game, the bike is much, more more stable from the rear and doesn't lose much front grip. This is especially noticable when breaking hard, the rear doesn't wiggle around. SX behaviour is much more natural too (front suspension doesn't work as hard - less bottoming out - more even behaviour).

Height-wise, the engine is around 66% of the weight of the whole chassis thing. So I'm placing it 1/3 of the way between the lowest chassis point ("Ground Clearance" cursor in BikeEd) and the seat height. This is still a WIP - the important bit is how forward the weight is. The tests I ran also had some "classic height" positioning and gave very similar results for most purposes.

TLDR: it appears chassis weight should be located a bit lower and significantly more towards the back of our geometry models and it seems to give better in game behaviour.

New:


Old:


Thoughts?

seems ok, but wouldnt that contribute to the fact that the virtual rider gets the front end to swap?
not to mention the problems with the pitch of the bike, always seems to compress too much in the front and launch you in the air with the front wheel up, dont know tho...
would be interesting to try!

All seems logical Asdreal.

Quote from: Mace-x on February 10, 2017, 02:07:22 PM
not to mention the problems with the pitch of the bike, always seems to compress too much in the front and launch you in the air with the front wheel up, dont know tho...

I wondered this but if there's less weight on the front I'm thinking it'll compress the suspension less in that situation reducing the front high effect?

yeah, that´s what i tought aswell...  ???

I love what asdrael has done, it´s awesome to have such a complex setup in a bike in a accurate way but i feel the sim, more specifically the virtual rider is not ready for it  :'(

Current methodology:

- Define center of gravity (CoG) coordinate from weight distribution
- Calculate horizontal distance from each weight location to CoG
- Mass x Distance everything (except chassis obviously)
- Balance it out with Chassis weight finding it's proper location so total sum of Mass x Distance = 0

Did I mess something up? Follows levers law, the "only" approximation is that both Wheel axle are horizontal (not far from the truth but not exact in BikeEd).


Edit: so far, easier to turn, more controlable rear wheel - also when it s sliding when breaking / gasing it - better SX balance, more forgiving (slightly, but I'm running 450F in SX atm). Stock settings - almost not bottoming the fork out in SX. And taht's when hitting stuff hard, 100mm oïl only. (for reference, on the YZ450F OEM, I can now hit triple triple triple double in the back rythm without any body movement with perfect balance using stocks setting with this new weight distribution). Only downside is that it requires a harder spring rear and it messes with the sag.

Very interested in feedback on this:

Ran the YZ250F (standard, not OEM) geom in my "weight distribution" excel sheet with 52% rear bias. At that stage, no idea if I'm seeing things, doing too much wishful thinking, whatever. This is the geom file (please rename it to yzf250.geom when using it, and BACKUP YOUR NORMAL GEOM):

https://mega.nz/#!9IoTiToI!PP27Kqvhch8mitSHmir3wp1ro6UlKSogR6ZrjW07K5U



Especially looking for feedback on:
- handling and stability
- rear and front wheel behaviour in different situation (braking / wide open / lean changes)
- balance when jumping (SX comes to mind)
- suspension setup changes


Why is the weight so low and back?
Don't forget the fuel tank is a separate entity ;) Chassis is everything from engine, frame to exhaust. Fwiw - the engine is between half and a third of the weight of the entire "chassis" thing.

Thanks for the feedback. I've read the article as well, but it refers to road bikes (who have a shorter wheelbase and overall higher center of gravity).

Digging a bit in it for MXB, it appears the chassis weight position helps with stability and balance, the rider weight should be right above the bike's own center of gravity so it feels more natural moving around. The problem being, that the neutral position is then too forward for the sag measurement to be "correct". I can live with that, just need to imagine adding 5mm to the rear rider sag roughly.

For whatever reason, with weight placed the way I do, I cannot get a good setup sag wise at the same time as a good on track behaviour. Will dig deeper, but I do'nt think I'll update my bikes this week with better weight.

DINGDINGDING



(extract from the "Race Tech's Motorcycle Suspension Bible").

Let's try to see how that fits now. If it does, expect updates :p

Doesn't work: the correct rider weight position + correct bike weight position makes it easy to set sag but impossible to ride because you lack forward movement amplitude (starts are wheelie hell for example and it's not fun). Will settle for correct bike weight position, neutral rider position (right above the bike's own center of mass). Will make sag values a bit off but much more enjoyable to ride.

BikeEd explanation and pictures here: http://forum.mx-bikes.com/index.php?topic=1551.msg22904#msg22904

Found today, thanks Vortex_Damien for the link:



Hey I'm pretty much spot on :p It varies with brank and engine cubic size but that's a pretty good ballpark, give or take a few cm.