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Author Topic: Calculated KP Inclination Angle mismatch to CAD Data
arbartz
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Post Calculated KP Inclination Angle mismatch to CAD Data
on: January 5, 2021, 15:19
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Alright, so this is my first time using this tool, so maybe I'm doing something wrong, but it appears that the calculated KP Inclination Angle in the RA tool does not match the measured KP Inclination Angle in CAD from which I'm pulling my points from.

See the attached photos for measurements. The important things to note is that all distance measurements match up to what I measure in CAD. (KP/Spindle Offset, KP/Hub Offset, & KP Length) However, RA seems to think my KP Angle is 13.6deg, while CAD says 7deg. Is this a measurement error on my part or is there something funky with how RA is doing the math?

(Side note, it'd be REALLY nice if we could make our own uprights/spindles that are "fixed", sorta like the 3 canned spindles that are available.)

Link to model:

FieroRev1

Right now I'm just trying to make sure my upright matches the one I'll be using (C5/C6 Corvette Upright), then I'll go and start nailing down the control arms, hard points, etc.

Thanks!
Image
Image
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racingaspi-
rations.co-
m
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Posts: 107
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Post Re: Calculated KP Inclination Angle mismatch to CAD Data
on: January 5, 2021, 16:06
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It looks like the measurements don't quite match.

What is the coordinate system displayed in the CAD drawings? You can use one of the vector coordinate input methods within the calculator to compare:

ISO 8855
SAE J670e

Click the pencil button then click the form button then select one of the above.

Can you upload the CAD drawing without the dimensions? Can you also send over the following measurements:

king pin inclination,
stub axle / kpi intersection distance from top pivot,
stub axle length from kpi to wheel mounting face
distance between top upright and lower upright pivots

I can then see about creating a pre-defined part.

arbartz
Newbie
Posts: 5
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Post Re: Calculated KP Inclination Angle mismatch to CAD Data
on: January 5, 2021, 18:58
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Which dimensions don't quite match? (See the two additional images I attached which show direct point-to-point measurements that should match those in the RA tool.)

As for the coordinate system displayed in the CAD image, it's just with reference to the part itself, not the ISO or SAE system that references from the center of the vehicle.

I could email you the SLDPRT file for it that I have, or export as a STEP file. I don't have a 2D print for it, but I could work on generating one.

Here's the requested measurements:
king pin inclination: 7.04deg
stub axle / kpi intersection distance from top pivot: 188.78mm
stub axle length from kpi to wheel mounting face: 108.74mm
distance between top upright and lower upright pivots: 320.58mm

I just want to note that this CAD model was created by someone else who took a ton of hand measurements on a real part. So it's not GM original CAD and I can't speak for it's accuracy, but it seems to jive with what I've seen elsewhere for measurements online.

Thanks!
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arbartz
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Posts: 5
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Post Re: Calculated KP Inclination Angle mismatch to CAD Data
on: January 6, 2021, 05:48
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Alright, this was really bothering me today that there were so many differing measurements popping up. So I dug around and found another CAD model of a C5/C6 upright, along with datapoints of a C5 from another suspension design tool that were given as examples.

From everything I've looked at, here's the list of possible KPIs for these spindles.
8.3-8.6 - Forum
8.5 - Forum
8.8 - Forum
8.1 - Performance Trends Suspension Analyzer 2.4
7.0 - CAD Model #1 (the one above)
8.5 - CAD Model #2 (the new one I found)

I also found two possible UBJ to LBJ lengths:
320.4mm - Forum
327.1mm - Performance Trends Suspension Analyzer 2.4
(Since I do not have CAD of the ball joint itself, and how low it's center sits from the mounting point on the spindle, I need to set the length of the KP axis, it's angle, and the location of the UBJ)

In any case, I'm thinking I'm going to use a KPI of 8.5deg and UBJ to LBJ length of 320.4mm. From there I got a new KP/Hub Offset of 110mm and a KP/Spindle Offset of 182mm. Which aren't too far off on my original entries I suppose, but the RA calculation of KPI is still wrong I think. Actual KPI is certainly not 13.5deg.

arbartz
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Posts: 5
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Post Re: Calculated KP Inclination Angle mismatch to CAD Data
on: January 6, 2021, 05:59
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Annnnd I got it... Looks to be a order of data entry sort of thing. After playing around with the LBJ connection point to make the KPI = 8.5deg, THEN adjusting the KP offsets, then going back to adjusting the LBJ connection to make the KP length correct, everything looks as it should.

Apologies for the flurry of responses.

racingaspi-
rations.co-
m
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Posts: 107
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Post Re: Calculated KP Inclination Angle mismatch to CAD Data
on: January 6, 2021, 14:29
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Well done. I'll take a look at your steps and see what I can improve.

arbartz
Newbie
Posts: 5
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Post Re: Calculated KP Inclination Angle mismatch to CAD Data
on: January 6, 2021, 17:43
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Awesome, thanks for looking at improving it! It's a great tool, and I'm happy to hear you are actively working on improving it!

As long as you're looking at improvements, might I suggest a way to "lock" certain inputs? Something like a checkbox in the data entry screen, or when I click on a draggable component have a checkbox to "fix" it in place.

Right now the issue I'm running into is as I play with my static camber setting, my KP offsets change. So I need to change static camber, then go back and enter my KP offsets again. Not super critical, but a minor annoyance.

racingaspi-
rations.co-
m
Moderator
Posts: 107
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Post Re: Calculated KP Inclination Angle mismatch to CAD Data
on: January 6, 2021, 18:45
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That is a request which has been asked a few times over the last year. It is one of the next items on my list.

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