Roll centers

You guys should talk...

http://www.lotustalk.com/forums/f3/selecting-setting-springs-shocks-sway-bars-73703/

trompazo said:

Does anyone have the data required to calculate the exact front and rear roll centers?

I'd like to build a spreadsheet that could give the roll center at varying ride heights.

Click to expand...

Really you need the slope of the upper and lower control arm and perpendicular distance from there pivot point to there corresponding ball joint.

Hey try this

It work as long as all units are consistant

For lowering/ raising the vehicle just add/ subtract that amount to the two body points and the sheet should do the rest

This is great.

What exactly are we measuring with the M line 1/M line 2/ and y2?

I don't really understand the position result. The position should be in the centerline and not change (assuming the car is flat and ride height are equal on both sides)

I cleaned up the sheet a little more

To calculate the roll center, don't we need to measure the point where the "virtual" control arms meet, and then draw a line from that point to the center of the tire contact patch?

The roll center height where that line is in the center axis of the car.

I don't see how you can calculate it without knowing the hub measurements, rim width and rim offset.

Maybe I am missing something here.

trompazo said:

This is great.

What exactly are we measuring with the M line 1/M line 2/ and y2?

I don't really understand the position result. The position should be in the centerline and not change (assuming the car is flat and ride height are equal on both sides)

Click to expand...

M1, M2 slopes of the lines
y2 is y intercept of the UCA, I made y1 go thru the origin. You really don’t’ need them there just an interim to get to the final answer.

Position is just the distance from underneath the ball joint to the intersection of the UCA LCA arm vectors. Tells you if its converges away from or towards the center of the car.

THe roll center would be the purple dot

The point in the middle of the car where the line from the center line of the tire converges with the line you calculated.

trompazo said:

actually, I don't think we need to know the wheel measurements and hub measurements, we just need to calculate it based on the track.

The front track is 1457 mm / 57.4 in and the rear track is 1503 mm / 59.2 in. This may change with different wheel setups but would be easy to calculate.

Click to expand...

You just need the ball joint locations and the arm mounting points to the body to get the slope. For the centers height and location from the joint and add in half the tire width and you can get the radius (yellow line). There are some other methods but this will work for instantaneous centers.

FWIW, this is a very simple exercise to do in a 3D modeling program such as Solidworks. No need for expensive suspension modeling software if all you want to find is roll center migration.

Just draw a 2D sketch and make the control arm, track, and hub geometry constant. Then change the ride height or the roll angle. All of the proper constraints are pretty intuitive once you get down to thinking about it.

If you have a kinematics toolbox like CATIA does, generating graphs of RC migration or camber are just one more step away.

Of course knowing the RC migration isn't very helpful without a fairly good guess at the CG location. That is the key to most of the usefulness From there you can even start solving for loads in any suspension member or bushing with almost no more work.

Just fixed it, now it will give you the roll center your looking for

Thanks! That is great.

I will try to get the measurements and positions of the control arms next week when I get the car on a hoist.

One thing though, if one control arm is completely horizontal, the M line is 0 and then it cant calulate anything. I'm not sure why, but it must be possible in some cases to have a completely horizontal control arm.

trompazo said:

Thanks! That is great.

I will try to get the measurements and positions of the control arms next week when I get the car on a hoist.

One thing though, if one control arm is completely horizontal, the M line is 0 and then it cant calulate anything. I'm not sure why, but it must be possible in some cases to have a completely horizontal control arm.

Click to expand...

Yes in that case the height is zero because those lines never intersect. So if the spreadsheet ever comes up with an error than it's zero. I'd have to put in a descent amount of logic for that

Also be careful measuring on a hoist if the wheels move poistion would change some values

No, if one control arm in completely horizontal the lines should still cross. They would only not cross if both control arms are horizontal which would not happen.

IN this formula, if either control arm is horizontal it gives an error, so something seems wrong.

In your diagram, the lower arm is completely horizontal and all the lines cross and the roll center is not zero.