Roll Centre Question

[Strange Rover]

once again its the ratio of wheelbase/COG height for anti-squat under acceleration and wheelbase/COG height/front to rear brake bais for anti-dive, anti-lift under braking....THIS IS VERY SIMPLIFIED.

Serg

Its not simplified it just simple and correct. You guys are looking at suspension dynimics...who cares what the link forces end up being. Leave the link force calculations for when you working out if your links are strong enough...

Your trying to study this by thinking about the individual link forces and all you are doing is making it really complicated. You ask if you can still get anti dive with a downwards angled lower link...so you are looking at forces in this link and looking at how the angle effects things and then what sort of angle you would need in a top link to counteract the vertical component of the lower etc, etc, etc. When in reality who cares what the angle of the lower link is??? Cause all the matters is where the instant centre ends up. So you should be asking yourself is can you get a decent instant centre with a downwards angled lower link?-------And the answer is of course you can....aim the upper link at the lower link chassis mount and you have more anti dive than you probably need.

Bush65 said "But forces can not be transferred through air along an imaginary line angled from the tyre contact patch. They can only be transferred in the links."

Which is true but in terms of vehicly dynamics the net result of all this complicated analysis is a single force from the contact patch straight through the instant centre.

I mean you guys are almost there....

Work out the contact patch force.
Look at the link geometry and correctly determine link forces.
Now again look at the link geometry and determine if the combination of the upper and lower link forces acting at there specifiec angles produces a net lifting or lowering force on the chassis.

Now compare this net lifting or lowering force against the effect of the horizontal inertia force acting at the centre of gravity....

Easy enough stuff.

OR welcome to the graphical solution because the net result of all these calculations and all this ananysis is indeed a single force pushing from the contact patch through the instant centre. Its almost as if the force does travel though this imaginary line...and if you look at the net result it actually does.

Sam

[Bush65]

Thanks for the corrections Sam.

[rockcrawler31]

jesus. My head hurts looking at all of this.

Can anyone provide links to a web resource showing some of this graphically for us suspension noobs. At some point i'd like to make a truggy/comp truck and while i never plan for it to go on road i'd like to apply some of this stuff to make it as best compromise as i can with stability/travel

[Slunnie]

This is an unreal thread on it:

http://www.pirate4x4.com/forum/showthre ... ink+design

[uninformed]

thanks sam.........about time you showed up

getting back to roll centers:(anyone can chime in here)

if your front and rear roll center heights are "x" on your stock vehicle, then you raise it on springs 3 inches. what would you have to do to the roll centers to achive the same driving characteristics when on road. i know theres alot more to do with roll axis's/anti-dive/anti-squat getting changed but lets say they where equal at stock and +3....

cheers Serg

[Slunnie]

thanks sam.........about time you showed up

getting back to roll centers:(anyone can chime in here)

if your front and rear roll center heights are "x" on your stock vehicle, then you raise it on springs 3 inches. what would you have to do to the roll centers to achive the same driving characteristics when on road. i know theres alot more to do with roll axis's/anti-dive/anti-squat getting changed but lets say they where equal at stock and +3....

cheers Serg

Ahhhhh, I'd seen this all through Pirate and here. I thought you must have been planning a buggy or something.

[Strange Rover]

thanks sam.........about time you showed up

getting back to roll centers:(anyone can chime in here)

if your front and rear roll center heights are "x" on your stock vehicle, then you raise it on springs 3 inches. what would you have to do to the roll centers to achive the same driving characteristics when on road. i know theres alot more to do with roll axis's/anti-dive/anti-squat getting changed but lets say they where equal at stock and +3....

cheers Serg

The increase in height of the sprung COG with a 3in spring lift is going to make the vehicle handle worse no matter what.

Now if you were going to run the same rate springs you could raise the roll centres by 3in and you should get the same body roll around corners. The vehicle still wouldnt handle the same cause it will definately roll over more easily but on slow corners it would sort of feel the same cause you will get the same body roll... or course the tyre loads will be different (more weight transfered to the out side tyres) so it still will feel different. Not raising the roll centres and increasing the spring rates will also get the vehicle feeling the same on slow corners (this is what most lifted springs achieve with a higher spring rate). But again the vehicle will still roll over more easily and you still get more outside tyre loads etc.

Either way its going to feel different and not be as safe as stock. Now if you look at the sorts of modified 4wd vehicles that get around it will be fine if you drive it within its limits. 3in of lift is smaller than what alot of people so stiffer springs and stock roll centres will be fine. If you are setting up your roll centres from scratch then lifting them a bit wont hurt but I wouldnt stress about it too much...if it was me I would just set up the roll centres so that it visually looks normal on the axles...if you are going for an A frame put it on top of the pumkin. If you are running a panhard put it somewhere on or just above the axle tube...I would do this whether you are running straight axles or portals. Just make it look normal and tune with springs.

Sam

[uninformed]

funny you should mention A frame vs panhard at rear Sam, i know that the ball joint on rovers is somewhere about 150mm above axle center line, and i was looking at my neighbours stock GU patrol and the panhard intersection to chassis centerline was about axle center line. to me that is a fair amount of difference. the rovers definately have the vehicle roll axis slopping down to the front and the patrol slopping down to the rear.

do you think that 2wd vs full time 4wd plays a roll here? did nissan just get it wrong, or were they looking for a different handling characteristic. i have never driven a patrol but i will ask my neighbour to get a feel vs my rover as rovers are all i have ever diven...

here is another question like the other but in reverse...

what happens if all you change is the roll center height, same spring rates, etc....

say we raise it from stock:

now i know that it makes it more stable on side slopes but has to move the body more when articulation happens, but does it actually make the flex "stiffer" by the body weight restricting articulation?

Serg

[uninformed]

thinking about the roll center heights, A frame vs panhard, (Patrol vs rover)

the ball is the convergence point of the upper links, as in a 4 link. this happens at the axle casing, where as the panhard is behind the axle casing including the gap between the case and panhard, so its convergence is further back. If the rear axle roll axis is doing what i think it is it is parrallel with the trailing arms but starts at the roll center height, which is panhard intersection of chassis width center, so it planes up and would be somewere over the axle casing at the same point as the ball on a rover??????

did that make any sense????

Serg

[uninformed]

having another look at the patrol, its still no where near the height of the rover....

so a fair bit of design difference for 2 vehicles... is 2wd v 4wd the answer???

Steve what have you decided to go with on your zook?

Serg

[Gwagensteve]

Not yet, I am digesting this stuff with interest though - thanks for eveyone's input- one of the better outers threads for ages (except for that one by Yodaboy)

Cheers,

Steve.