How to lower roll centre? (1 Viewer)

[OpTimuS]

hey guys after reading the thread on Francesco_from_italy and his air-shock'd 40, I did some searching on this site and could find nothing to help me out.

If I want to lower my roll-centre (thus making my truck morestable on side slopes) what can I do?

I was guess the following would help:
widen wheel track
lower CoG

but I understand they would only be a band-aid.

So to the guys out there with 4-link experience what can be done with the links in a 4-link setup (or any other means) to lower the roll-centre of a link'd air shock'd truck? cheers,
Davo.

 

['74 UA FJ]

Check out the other thread and wait for the results. He is going with the anti rock bars.

 

[Mace]

Track width will do nothing for your roll center..

Read up on Roll center, I think you have a bunch of misconceptions..



Besides, what are you worried about??

"Thanks friend but I'm not calculating anything. Im just going on the "if I can get it to fit" calculator"

 

[bull]

Good read..

http://www.neohio-scca.org/comp_clinic/hand_out_reprints/Vehicle Dynamics2007.pdf

like everything else online take it with a grain of salt

 

[OpTimuS]

Besides, what are you worried about??

"Thanks friend but I'm not calculating anything. Im just going on the "if I can get it to fit" calculator"

hey there mace not calculating anything just finding more knowledge to read and devour over 2-many beers

 

[OpTimuS]

Good read..

http://www.neohio-scca.org/comp_clinic/hand_out_reprints/Vehicle Dynamics2007.pdf

like everything else online take it with a grain of salt

printed off and added to scrapbook cheers dude.

 

[OpTimuS]

Check out the other thread and wait for the results. He is going with the anti rock bars.

Anti-rock bars are really not an option fr my off-road only truck.

 

[mookie]

don't use air shocks on a heavy, full bodied cruiser would be a good starting point. anti-rock bars ARE for offroad only use vehicles, many comp rigs have them. and i think a HIGH roll axis is what you want to shoot for to get stability for off camber situations and to reduce body roll, not that i'm an expert by any means, i'm still trying to figure it all out myself....

check this out...http://www.pirate4x4.com/forum/showthread.php?t=168577

if you can sift through and ignore all the b.s. and read this, you'll either be a genius/link suspension expert or brain dead...

 

[Mace]

and i think a HIGH roll axis is what you want to shoot for to get stability for off camber situations and to reduce body roll, ...

You are correct, but there are other things that matter too..

 

[OpTimuS]

You are correct, but there are other things that matter too..

like horizontal triangualation on links, separation of links at diff, separation of links at chassis, and wheelbase?

 

[peesalot]

track width has no effect on roll center ?

lets put some 55" wms axles under your rig and see what happens ?

Not arguing just lookin for some education.

I fully understand the triangulation used in figuring roll center and how weight distribution can affect it, but an extra 3 " of trac width sure changed the lines for me. It was not night and day but subtle differences.

 

[OpTimuS]

I just found exactly what I was looking for and hi-jacked it from pirate, now bear in mind that this is one (well-ejumicated) man's opinion.

The roll centre is the point that the body wants to rotate about on the axle or the point that the axle supports the body laterally (or supports the body in a sideways direction). The simplest example is a triangulated 3 link with the upper links triangulated to a single point on top of the axle. This point is the roll centre. If you are talking about roll centre what most people are concerned about is the height of the roll centre. If the roll centre is high it will reduce the amount of body roll that you get when on side slopes. This is a good thing because it keeps everything stable in the off camber stuff and is why a lot of people try to get their roll centre up high - to reduce body roll. The roll centre can be thought of as the point where the body is supported laterally (or sideways). So that if the body is supported laterally up high it wont have as much tendency to induce body roll than if it was to be supported down low. If a rig uses a panhard rod then the roll centre is the mid point of the panhard rod.

When you start to look at triangulated 4 links things start to get a bit more complex and you should be really talking about the roll axis as apposed to a single point. But in simple terms if only the upper links are triangulated then the roll centre is high (means less body roll) and if only the lowers are triangulated then you get a low roll centre (means more body roll). If both the uppers and lowers are triangulated then the height of the roll centre will still be high and the angle of the roll axis will be more horizontal. Now without going into it too much a horizontal roll axis produces little roll induced rear steer (which may or may not be usefull on a rock crawler). Also if you can get the roll axis to be sloping down a bit (going from the rear to the front) then it will behave better on the road (again this may not worry you on you trail rig)

Again this is hard to explain without going into things too deeply or writing 1000 words.

To further complicate things a high roll centre (or axis) isnt the be all and end all of rock crawler suspensions because what a high centre also does is if a wheel moves upwards (over a bump) then the body must move sidways as well (really should be drawing pictures here to help explain this). Now as you go faster for the wheel to move upwards quickly it must push the bodysidwards quickly as well - which is hard to do. What this means is that when you are throttling up a climb quickly (still talking rock racing speeds here ie say 40:1 and 4000rpm) when you hit a bump on one side your springs wont want to compress when you have a heigh roll centre because to compress a spring it must move the body sidways as well so you may as well think of it as having no springs at all and the rig will just bounce off the tyre.

Running a low roll centre will let a wheel compress much more easily without pushing the body sidways as much.

Lots of successful comp buggies run high roll centres and lots of then run low roll centres. One isnt definately better than the other.

So in summary:

If you want a high roll centre trianglate the uppers.

If you want a low roll centre triangulate the lowers.

If you want the rig to behave well on road at speed then you should look at triangulating both the uppers and lowers and try to get your roll axis sloping downwards a bit towards the front.

If you want to minimise roll induced rear steer then you should also do the double triangulated thing to get a flat roll axis (although having the axis a bit up hill or down hill wont matter just as long as its flattish).

If you want your suspension to work well on the heigher speed throttling climbs and soak up bumps then you should go for a low roll centre.

If you want your rig to work well on the slower crawling speeds and reduce body roll and remian stable in the off camber stuff then you should go for a high roll centre.

Finally the hieght of the roll centre or the angle of the roll axis wont really make a huge difference to the capability of a rig - we are only talking degrees here. Lots of very good comp buggies run every variation of roll centre and axis.

I was going to do a bit on the IC and anti squat as well but that just took me an hour and it probably didnt come out very well anyway. So I will do it later of nobody else does or I dont agree with them.

 

[Mace]

track width has nothing to do with Roll center. the axles can be 100" wide or 30" wide, if the links define a certain roll axis then that is what you have..

The width of the axles will prevent you from rolling, but they won't stop the car body from swaying back and forth..

 

[Isotel]

4 link Calculator - JUST USE IT PEOPLE!!

all these threads about band aids on how to fix bad link design, Plug in your numbers move some stuff around till you get the numbers right. its the only way to lower roll center, and make your rig work the way 4 links are intended.. if you don't want to do all that work and research, put on some leaf springs and call it good.. leafs will work better than ill planned 4 link!

 

[peesalot]

got it, roll axis confused with rolling.

 

[ntsqd]

I'll put this out there, see if it helps anyone still in the fog.

Consider one axle's suspension. It has a single roll center that is defined by the geometry of the locating linkage. Depending on what the linkage layout is, there may be a axle specific Roll Axis (not to be confused with the vehicle's Roll Axis; more on that later). This axle specific Roll Axis is the pivot line that produces Rear-steer (& controls how much of it there is) in a rear suspension. The axle specific Roll Axis has that suspension's Roll Center located along it somewhere. It is located by what ever feature does the lateral location of the axle. This can get crazy and it is too easy to get lost in the details. Just know that there is a Whole bunch of stuff contained in breaking down & detailing those last couple of sentences, and the sublties can make the difference between a vehicle that works well and one that does not.

So now we've got the Roll Center of one axle. The other axle also has this point in space. The imaginary line thru space that connects the front suspension's Roll Center to the rear suspension's Roll Center is known as the Vehicle's Roll Axis. When the body rolls over on the suspension, say when leaning in a corner on Hwy 50 on the way up to the Rubicon, this imaginary line is the pivot line that it rolls or hinges around.

OK, now is the fun part. Where is the Center of Gravity (CG)? A really rough approximation in a push-rod V8 powered vehicle has it at the same height as the cam, and on the friction surface of the flywheel. So now where is the Roll Axis relative to the CG?

IF the CG is above the Roll Axis the vehicle will want to flop over. The higher the CG is above the Roll Axis, the more easily it will flop. It's like trying to balance a spinning top that isn't spinning.

IF the CG is on the Roll Axis, then nothing happens since the weight distribution above and the weight distribution below the Roll Axis are the same.

IF the CG is below the Roll Axis then it is like a pendulum. A sidehill will actually cause the body to lean into the hill.

Lowering the Roll Axis is not necessarily a good thing. Lowering the CG almost always is.

 

[OpTimuS]

IF the CG is above the Roll Axis the vehicle will want to flop over. The higher the CG is above the Roll Axis, the more easily it will flop. It's like trying to balance a spinning top that isn't spinning.

IF the CG is on the Roll Axis, then nothing happens since the weight distribution above and the weight distribution below the Roll Axis are the same.

IF the CG is below the Roll Axis then it is like a pendulum. A sidehill will actually cause the body to lean into the hill.

Lowering the Roll Axis is not necessarily a good thing. Lowering the CG almost always is.

That is a very interesting spiel. Thank-you for more information.
I have always been of the understanding that the lowering my CoG is a good thing.

 

[Mace]

That is a very interesting spiel. Thank-you for more information.
I have always been of the understanding that the lowering my CoG is a good thing.

lowering the COG is a good thing..
the roll center is defined by your links.
The COG is defined by the rig/springs..

You wanna make your rig more stable. Lower the COG.

My 40 is 22" to the frame under the doors. And that is with 39" Irocks..

 

[bull]

lowering the COG is a good thing..
the roll center is defined by your links.
The COG is defined by the rig/springs..

You wanna make your rig more stable. Lower the COG.

My 40 is 22" to the frame under the doors. And that is with 39" Irocks..

Mace this thread could use some pics of your 40 set up.. for reference

 

[OpTimuS]

lowering the COG is a good thing..
the roll center is defined by your links.
The COG is defined by the rig/springs..

You wanna make your rig more stable. Lower the COG.

My 40 is 22" to the frame under the doors. And that is with 39" Irocks..

Wow I have done some measuring and guessing and I was hoping for 25" from the ground at the same place as you measured.

Some pics would be sweet.