I just tried that roll center calculator and it shows a different height (3.35)
then the one from Wishbone (2.044)
I did a test case in CAD compared to the www.arengineers.com/RollCenter
calculator and they
matched. I don't have wishbone installed... keep putting it off because its DOSiness. You
probably have an input error somewhere if it's off that far.
By the way, there's a picture (Chetcpo post Oct 25, 2009) of Wishbone above where it appears RC construction line goes to the inside edge of tire... or maybe to the kingpin/ground intersection... not to tire center.
My CAD model, and apparently the arengineers calculator, draw the construction line from the
instant center back to the center of the tire. Where it crosses car centerline is of course the
roll center height. Performance Trends' Suspension Analyzer v2.0 agrees also.
I have an original (survived our garage fire!) Maurice Olley 1962 Suspension Notes II, the bible of vehicle dynamics. In it, he clearly constructs the RC line to tire center, which is clearly not where the kingpin intersects the ground.
Len Terry's 1973 Racing Car Design and Development agrees... clearly drawn to tire center.
David E. Cole's 1973 Elementry Vehicle Dynamics also agrees, with extensive explanations and examples. (A great textbook if you can find it...)
However, Collin Cambell's 1973 Design of Racing Sports Cars clearly shows and
states it's drawn to the kingpin/ground intersection.
Allan Staniforth's 1988 Competition Car Suspension is remarkably vague on roll center and the only clear SLA suspension drawing on the subject implies the kingpin/ground solution like
Cambell, but a photo of the "string computer" appears to use the tire center. Maybe newer
editions have improved?
Anywho... Chet's picture reminded me of the anomoly, and I wondered if Wishbone used the Collin Cambell approach. It's mostly an academic question... even my test case which has a really big scrub radius to more clearly shown the difference (but many rear suspensions are like that) only changes the roll center height from 4.87 to 4.70. If someone wants to throw my test case into Wishbone for kicks, here it is...
rollcentertestcaseCAD.jpg [ 53.74 KiB | Viewed 1184 times ]
rollcentertestcasePTsusAnalyzer.jpg [ 122.75 KiB | Viewed 1185 times ]
rollcentertestcaseWebRollCenter.jpg [ 107.81 KiB | Viewed 1188 times ]