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PostPosted: Thu Apr 01, 2010 1:31 pm 
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I am wondering if anyone has any thoughts on getting the roll center and the C/G close to the same axis?

Staniforth recommends the front roll center to be at ground level up to a couple of inches above ground and the rear R/C to be a couple of inches above the front R/C. Anyone have any thoughts on that and how it has worked for them?

As a side note: I am getting close to starting the build. I am soon going to have to start a build log. One of the key components showed up last night: The HeadLamps. After all, the entire build revolves around the headlamps. :lol:


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PostPosted: Thu Apr 01, 2010 4:26 pm 
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That's why my build is going so slowly. I haven't decided on and bought my headlights! :shock: :lol:

If I understand things correctly with the roll center the same height as the center of gravity the body would not roll when cornering but you would get a lot of jacking forces and the outside tires will roll under like with a swing axle.
Someone please correct me :|

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PostPosted: Fri Apr 02, 2010 12:55 am 
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MYTF wrote:
I just tried that roll center calculator and it shows a different height (3.35) then the one from Wishbone (2.044) :?: :?:


I didn't try it but I might compare it later today if I have a moment.

Can you give the measurements you put it or a screen capture and I'll run it through a 3rd program for you.


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PostPosted: Fri Apr 02, 2010 1:15 am 
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I would have to agree......if I could only source a good set of headlights, the whole thing would just come together perfectly. :wink:


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PostPosted: Fri Apr 02, 2010 1:45 am 
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I read in a Mark Ortiz article that if a wishbone suspension has a roll center higher than about three inches, it will jack itself up as MYTF describes. Think pole vaulting.

The amount of load that gets transferred from side to side during cornering is determined by CG and the vehicle's track width, so a low CG is always desirable. If the roll center height is 25% of the CG height, then 25% of the weight transfer will be reacted through the wishbones or axle and will not produce body roll. Less body roll is a good thing; however, the load transferred via the roll center results in more lateral force at the tire patch (possibly skid-producing), rather than more downward force as would be be the case if the load were transferred via the springs.

There's no lack of compromises in car suspension design!

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PostPosted: Fri Apr 02, 2010 5:42 am 
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MYTF wrote:
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...

Attachment:
rollcentertestcaseCAD.jpg
rollcentertestcaseCAD.jpg [ 53.74 KiB | Viewed 1483 times ]


Attachment:
rollcentertestcasePTsusAnalyzer.jpg
rollcentertestcasePTsusAnalyzer.jpg [ 122.75 KiB | Viewed 1484 times ]


Attachment:
rollcentertestcaseWebRollCenter.jpg
rollcentertestcaseWebRollCenter.jpg [ 107.81 KiB | Viewed 1487 times ]

.

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PostPosted: Fri Apr 02, 2010 8:35 am 
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Ask and ye shall receive.

LOL at the program's 'DOSiness'. I'm an old DOS hand but still found it a bit of a chore today. It works, though! I used it to check my CAD modeling once, and it came out on the button, to three decimal places.

Edit: Oops, I added a higher resolution picture of the screen print and now have two images.


Attachments:
scd.jpg
scd.jpg [ 156.41 KiB | Viewed 1481 times ]
scd.jpg
scd.jpg [ 78.13 KiB | Viewed 1479 times ]

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PostPosted: Fri Apr 02, 2010 10:28 am 
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:oops: False alarm. I reentered my numbers and found my error. Both programs give the same answer. Thanks All

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PostPosted: Fri Apr 02, 2010 12:04 pm 
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Phew:

I was holding my breath on that. I was thinking that 2 weeks of pounding in numbers in Wishbone and another program was going to disagree.

Yikes.


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PostPosted: Fri Apr 02, 2010 2:24 pm 
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Concerning the Roll Center vs C/G:

I dug up and spent a good part of my morning (Mark Ortiz referenced) reading Carroll Smith' Tune to Win.

"We can use High roll centers which result in low roll moments (the distance from R/C to C/G). We do not want to follow this approuch because we will then have poor camber curves and high jacking forces"


Thanks MYTF.


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PostPosted: Fri Apr 02, 2010 4:13 pm 
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... not to mention an inability to tune the handling via anti-roll bars and shocks. If the car doesn't lean, changing these two items will have no effect.

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PostPosted: Sat Apr 03, 2010 2:46 pm 
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So, after doing a ton of reading over the last couple of days and sitting at the computer last night until F1 qualifing came on, I ended up with this.
Attachment:
Front-CA-vince1c.jpg
Front-CA-vince1c.jpg [ 59.44 KiB | Viewed 1497 times ]


This was basing my design on a 59" front track and the upper frame width set up for Jack's TTL nose (22" wide)

The result is a upper frame width of 22" (cough, cough, duh!) and the lower frame width of 16".


Attachments:
Front-CA-vince1.jpg
Front-CA-vince1.jpg [ 90.68 KiB | Viewed 1486 times ]
Front-CA-vince1b.jpg
Front-CA-vince1b.jpg [ 52.66 KiB | Viewed 1490 times ]
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PostPosted: Mon Apr 05, 2010 1:02 am 
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KB58 wrote:
... not to mention an inability to tune the handling via anti-roll bars and shocks. If the car doesn't lean, changing these two items will have no effect.


That won't affect tuning. The chassis physically moving doesn't really have much to do with how stuff is tuned - its all about how stiff one end of the car is when compared to the other that changes the roll stiffness distribution.

The roll moment only controls how much roll resistance you need to control chassis roll - not how much weight is transferred.

If the roll moment at one end of the car is significantly different than the roll moment at the other end, allegedly it can mess with dynamic weight transfer but not static weight transfer or roll stiffness distribution.

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PostPosted: Mon Apr 05, 2010 9:20 am 
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I was adding my thoughts to, "We can use High roll centers which result in low roll moments (the distance from R/C to C/G)...." If the roll-center is placed as high as the CG, suspension tuning is impossible.

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PostPosted: Tue Nov 16, 2010 10:05 pm 
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Probably a silly question, but is Wishbone applicable for IRS applications? I'm probably overthinking it but I really don't want my driveshafts to act as a stop because I designed it wrong.

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