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Hello, For my rear suspension, I have a pure trailing arm setup. I used the semi-trailing arm template and modified it by deleting the toe link and making the trailing bushings parallel to each other, according to the geometry, the roll center height should be zero should lie on the ground plane but according to the simulation during roll analysis, the roll center height is going up to -45 mm and the lateral displacement is to -12500mm. Best, Premkumar
Hi @premkumar_07,
It looks like you are plotting the results with respect to multiple KnC events. If you enable only the Ride Analysis, do you still see considerable vertical movement of your roll center?
Are you creating these plots manually or are you extracting them from a Standard Report?
Do you have any compliance in your model, such as bushings?
Are you able to attach your model?
It should also be mentioned that the Roll Center is calculated by the following definition:
"The suspension roll center is found by intersecting the line connecting the left tire patch and the left tire patch front view instant center with the line connecting the right tire patch and the right tire patch front view instant center. The roll center height is defined as the difference between the roll center z coordinate and the average tire patch z coordinate."
https://help.altair.com/hwdesktop/hwx/topics/motionview/sdf_roll_rate_tramp_rate_and_roll_center_location_r.htm?zoom_highlight=roll+center
Hope this helps!
Adam Reid
Hi @mauricio_muñoz17,
I have noticed a few things about your model when first reviewing it:
Before:
After:
2. The steering housing mount looks to be located improperly, as the left point is located on the right of the vehicle, and vice versa.
3. The Vehicle parameters in the KnC event do not match your current vehicle configuration, ex. the Vertical CG height shown below:
After those updates, I ran a KnC event with your model with only the Ride Event active, so there is a vertical deflection of each wheel end by +/- 40mm. I plotted the displacement of the wheel end in the YZ plane:
I extracted a few points from this curve and plotted them, along with the arc center and intersection with the Z axis in Inspire:
Based on this method, which is the same method the solver uses according to the Help Documentation, the vertical CG height computes to 267.033 mm, which looks to fall within your calculated values. The SDF report, however, shows a roll center height at time =0.0 of 146.73mm. Odd!
I repeated the same process for a default vehicle assembly from the assembly wizard to see if the solver calculation is incorrectly applied or if it is specific to your model only. I ran the KnC again with +/-40mm of deflection and plotted the displacement in the YZ plane:
Based on the extracted points, the roll center height computes to 55.727 mm. The SDF produces a roll center height of 49.83 mm. So therefore, the calculation looks to be doing the correct job for the example model, but not for yours.
The one obvious difference between your model and the example model is that your model has the bottom of the tire exactly at the same level as the origin, where the example model is 686.5 mm above the origin. So I translated your model up 1000 mm, rebuilt the KnC, and ran the analysis again. Low and behold, the SDF reports a roll center height of 265.32 mm:
Therefore, the issue is resolved and was likely only caused by your tire contact point being perfectly aligned with the global origin. Please find your translated model attached below:
Hello @GTT Adam I tried with only the roll analysis in the simulation modes; however, the results remain the same. But for the ride analysis, the results are as expected. I have attached the plots for both. All my joints are compliant, and I am extracting the plots from the standard report. And is there any way the results can be populated after every iteration? Now I have to reload them every time to get the results. I have attached the model file with the name 'Rear_V10'.
Roll Analysis-
Ride Analysis-
In any HyperMesh Desktop application, you can save the current session as a Flexible Report using File » Session » Save As » Report Template.
Using this report, you can then reuse any plots/animations and specify different files as an input:
Regarding your model, I see that if compliance is removed from the suspension does some strange things. I think it is due to the inclination laterally in the struts:
Are they meant to be purely vertical?
Hello @GTT Adam, Thanks for the prompt reply. I will look at the flex report. Regarding the model, if I make the strut purely vertical, will that solve my issue?
Hello, I simulated without compliance and moved the strut upper joint inwards to make it vertical, and in the second iteration, kept it in its original position. As a result, the roll center height migration has been resolved, but the lateral displacement remains unchanged in both cases.
There should be lateral displacement of the roll center shown based on how the roll center is calculated and how your rear suspension is parallel to the ground.
The instantaneous roll center (IRC) is calculated based on the location of the contact patch as it moves through the roll event. If you trace the location of nodes on the bottom of the tire, you can determine that the lateral location of those nodes do fluctuate (ever so slightly) during the simulation:
The actual travel is very small, which means the radius of that arc will be quite large.
Hello @GTT Adam, I am having a similar issue with the roll center height reported in the SDF file being different from the one calculated on CAD and Adams software. I am using the 2025.0 version (as previously recomended on another post) and the Baja/Fsae libraries; I am optimizing the geometry of my double A-arm front suspension with KnC tests (ride, roll and steer only). As previously stated, the roll center height reported in the SDF file is incorrect, also the anti-dive, side view swing arm angle and lenght results seem off; this in the static condition or at 0s. Could you help me please determine what the issue may be or if I am doing something wrong? NOTE: camber, caster, toe and front view swing arm results are correct and as expected.
I'm sorry that you're having trouble with the calculation of your roll center height.
I believe two important statements should be made, which you may have already considered but I feel should be explicit for others:
#1 The suspension roll center is found by intersecting the line connecting the left tire patch and the left tire patch front view instant center with the line connecting the right tire patch and the right tire patch front view instant center. The roll center height is defined as the difference between the roll center z coordinate and the average tire patch z coordinate.
Therefore, it is entirely defined by the travel of the bottom of the tires and is commonly referred to as the instantaneous roll center. It is not defined using intersecting angles between the top of the shocks and lower control arm. An example can be seen below, where the initial contact location of the bottom of the tire and the current location of the bottom of the tire are used to project a line normal to those lines.
#2 The defined position of the suspension will be different than the position of the suspension at Time = 0.0. The vehicle will settle under gravity and thus comparing the suspension angles before the simulation begins to the suspension angles after the model achieved equilibrium will not be identical. Again, this plays into point #1 in which the method you are using to calculate roll center may not be apples-to-apples.
For more on these metrics, the documentation is linked below
https://community.altair.com/home/leaving?allowTrusted=1&target=https%3A%2F%2Fhelp.altair.com%2Fhwdesktop%2Fhwx%2Ftopics%2Fmotionview%2Fsdf_roll_rate_tramp_rate_and_roll_center_location_r.htm%3Fzoom_highlight%3Droll%2Bcenter
Regardless, how "off" are the SDF values from your calculated/predicted values?
Hello @GTT Adam,
The calculated RC height value was 260 mm using the "traditional" geometric method, Adams reports a value around 270-280 mm, and the SDF file at 0s shows 146 mm. I think the issue may lie with the tire file, I edited the "stock" .tir file that came with the baja library with the values of my tire radius, widht, etc. I cannot think of another thing that may be wrong. Thanks for the fast reply and help!
Did you edit the tire file parameters in the TIR file directly or in the Entity Editor?
Also, did you create the KnC event before editing the parameters or after?
