Roll center?

[gearhead]

Has anyone ever bothered to calculate the roll center of a 620 at stock height and after it was lowered?I'm looking to lower mine for better handling,not so much to have it low.I know there is a point where lowering something actually decreases handling because the geometry gets all fubared. The rear roll center is easy,its basically in the center of the pumpkin,I'm just not sure where the front roll center is located.Center of gravity is about at crank centerline give or take a few inches.So Has anyone calculated the roll center for a stock 620?

thanks in advance.

 

[jesusno2]

I haven't calculated factory ride height yet. I do know that lowering a 521 620 king pin truck (via torsion bar method) the roll center drops under ground which creates body roll. And the alighnment is completley fucked. Under just 1 degree of body roll lots of postitve camber on the side its rolling on. Instant center is way way WAY out their in no mans land. If you watch my 521 build thread were going to be changing all this soon .BTW IC on leaf springs can be higher or lower then the center of the third depending on how the mounting points are configured on the frame. Judging from my software the stock roll center is about 2 inches maybe a bit more.

[Ooph!]

Search/Google "finding front roll center" I learned by working on Dirt track stock cars.

Basically you measure your front suspension as it sits then put it on graph paper. By measuring the height of each suspension point you can get the angles onto a graph.

Upper A arm angle and Lower A arm angle = instant center left and right

Contact patches to instant centers = roll center

For the rear draw a line between the two leaf spring attachment points, the roll center is on that line directly above the axle center line.

So if your springs have a lot of arc to them or longer shackles the roll center will be higher a flatter spring it will be lower.

 Depending on the year of truck the king pin front will be limited on what can be done to raise or lower roll height.

With the ball joint suspension a taller or shorter ball joint can be used to help move it to where you want by changing angles on the top or bottom instead of both.

[gearhead]

so lowering a 620 moves the roll center underground huh?I'll go measure up everything and figure out where it is stock, and lowered.Then I will see about changing the upper or lower arm to get a good roll center.I'll draw everything up in autocad to get a good idea of static angles,then play around in solidworks to get an idea of dynamic angles...educational software packages are great. AH YEA! :)

[jesusno2]

were going to be moving the chassis points and rebuilding the whole shock tower/ upper mount. messing around with anti-dive numbers right now. The worst is the alignment issues that plauge this chassis.

[Ooph!]

were going to be moving the chassis points and rebuilding the whole shock tower/ upper mount. messing around with anti-dive numbers right now. The worst is the alignment issues that plauge this chassis.

Alignment issues, care to describe a little more?

[jesusno2]

Camber changes drastically with body roll with stock arms. So we are fine tuning the control arms to help get rid of the positive camber issues and figuring out what kinda castor number it takes to help with all that also. Just doing one step at a time I'm waiting for my parts to get cut so we can see proof of concept, make sure were on the right track or if I need to go back to my chassis program and re design

[gearhead]

so is that what causes the understeer? tires go positve camber?

[jesusno2]

positive camber and toe and combined with skinny tall tires all contribute to understeer. 

[K_trip]

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[jesusno2]

Enlighten me about this J bar thing? I'm planing on adding a panhard on my 720, but I haven't pulled any dimensions off the chassis yet to know where it will work and be adjustable.

[jesusno2]

Nevermind I know what your talking about now. For some reason I was drawing a blank. Its past my bed time. Just remember low roll centers create body prolly which is what will jack the font outta whck. Finding. The balance between frt and rear roll centers can be very touch n go and will very a lot depending on driver preference, and sway bar sizes and adjustability.

[K_trip]

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