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Old 03-26-2008, 04:05 PM   #1966
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Ammo-Here is a link to building that front end. I only do as much as will make it smooth to preserve as much plastic as possible for crashes. Crash worthiness is one of the strengths of this associated suspension.

http://www.swiftracingproducts.com/Page.cfm?InfoID=5192


Fat head thanks for the support. Interesting diagram isn't it.

JevUK-that rocker strut is interesting. I would need to see the animation. the rocker makes it looks like it fights itself on first glance. Or is the rocker just doubling the movement of the spring with bump?

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Old 03-26-2008, 04:11 PM   #1967
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Quote:
Originally Posted by John Stranahan View Post

Fat head thanks for the support. Interesting diagram isn't it.
Yes it is, I was very surprised how much it moved the R.C., I've only modeled double wishbone suspension to get the R.C. before and they generally don't move it that much with such a small change, it is something I will look at on my fullsize car now (that has macpherson struts) when I look at lowering it some more!
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Old 03-26-2008, 04:21 PM   #1968
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Old 03-26-2008, 06:08 PM   #1969
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John, I think part of the issue is that the angle of the upper A arm will also change. Doesn't look like that's accounted for in your diagram, although I could be wrong.

As the suspension compresses, the upper A will increase in angle, and that mean the IC wouldn't be as far apart as indicated... at least thats as I understand it

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Old 03-27-2008, 12:00 PM   #1970
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Try Hard-The angle of the upper A-arm does change but only a very small amount. Maybe this is the .001 Jason is talking about. It mostly changes in length when you use the tie rod to adjust camber.

I will repeat this here
"The instant center on this inverted strut (inverted McPherson Strut) is determined by extending a line at right angles to the kingpin at the lower pivot. A second line is extended on the upper A-arm axis. Where the two meet is the instant center. A line from the tires contact patch to the instant center meets the roll center at the vertical center line of the car."

You should be able to draw your own roll center sketch using the above procedure.

I drew one more sketch. This is a roll center diagram for the Oval car which has different camber on left and right. The left side is + 2 degree, The right side is -3 degrees. We are looking at a front view so the sides are reversed.

When you get the roll center on a car with front suspension assymetry you have to do an additional step. You have to get the instant center of the second side now. Where the two lines from the tires cross is the roll center.

Note the roll center is off to the left of the car (right on the sketch). It is also at chassis height when I would prefer it to be at road height. The camber on the left side tire has moved the roll center upward about 5 mm

Note also that I have labeled symmetrical roll centers on the diagram(What if both sides have the same camber). There is over an inch difference between the two one at -3 camber and one at +2 camber. I have called this a little defect and will do so again. You can adjust around it by tilting the A-arms once your cambers are set. There are other problems with this spring on kingpin suspension, mainly huge stiction, that could be eliminated with a more modern race car suspension.

I have written a spreadsheet that will calculate roll centers for twin A-arm cars. Link below. I also have discussions of roll center and nicer diagrams on twin A-arm roll centers on the TC5 tips and tricks thread.

The program below calculates roll center based on a few easy measurements of the car. For dual A-arms only.

http://hometown.aol.com/johnstranahan/RollCenterCalculator.xls

A clever enthusiast could modify it to do McPherson Strut.

There is a better copy of this sketch on my next post.


John
Attached Thumbnails
Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-inverted-strut-roll-center-oval-car006.jpg  

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Old 03-27-2008, 01:07 PM   #1971
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Old 03-27-2008, 07:58 PM   #1972
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Roll Center
So to the best of our knowlege, the previous two sketches on roll center are correct. So what the racer wants to know is which adjustment does what to the roll center. We should be able to make a little chart for the oval car using the previous sketch. Here it is again. I have darkened the suspension parts and added a few more labels.

Now from this sketch it appears that the roll center for my oval car is up around chassis height. There is a dot on the diagram labeled "roll center" off to the right in the diagram. I would rather lower it a bit to the ground. What this will do is add steering traction and perhaps reduce scrub. It seems the softer front tires helped in this regard. That was worth a huge .2 seconds along with other small changes.

Lower the outer (right hand) upper A arm pivot. This will move the instant center outward and lower the roll center some. I can do this by cutting plastic off the top of the steering arm with the stock suspension. I have done this before to good effect. (The inner A-arm end can also be raised with aftermarket parts.)

I can move the inner left hand A arm pivot up. This will move the instant center out on this side and the roll center will come down. This will require aftermarket parts.

Now I am wondering how my Full size Ford Focus Race Car (with Mcpherson front struts) gets away with about a 5 degree tilt on the kinkpin (strut) axis with only a modest -1 degree camber (all that I can adjust in at the moment). Well it has to do with the construction of the spindle and knuckle. The spindle in the Fords case is not perpendicular to the strut; it is at a 90+4 degree angle to the strut. So the two adjustments can be made independent with the addition of different steering blocks. It might be nice to construct a +5 degree block for the pan car. I could incline my left side kingpin -3 degrees like the right side and then add the +5 degree block to get my positive camber. It would be easy to fill and superglue a trailing arm steering block solid and then redrill it. I'll have to give it a test sometime.

I have read quite a few pages on tuning for the oval. Absent was much discussion on roll center as it is difficult to determine. What was discussed is camber gain. The author liked to use A-arm parallelism only as a means to get negative camber gain or not.

Arms (top and bottom) more parallel the less camber gain. Good for the left side.

Arms more angled, more negative camber gain. Good for the right side.

Now if you have a little table on roll centers like the above for an oval car (not a road car) let us hear about it.
John
Attached Thumbnails
Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-inverted-strut-roll-center-oval-car007.jpg   Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-protoform-dodge-charger-top-rear-view-001.jpg  

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Old 03-28-2008, 12:16 AM   #1973
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Ask and it shall be given to you. An anonymous reader sent me a roll center calculator that was built for the Associated front suspension (or CRC etc.) You have to have Excel to use it as is the same for my calculator.

Yes the diagram and method agree with my own just above. There is a sketch included with the sample data much like my own. Yes you have to input camber angle for it to calculate properly.

What you are doing, when you find an instant center, is trying to find the center of the circle around which the wheel spindle moves going up into bump. It takes two lines to do this. You cannot do it with one line! Finding roll center involves another set of lines.

This is not my software but is labeled sample software so here it is.

http://hometown.aol.com/johnstranahan/StrutRollCenterCalculator.xls

John

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Old 03-28-2008, 04:09 AM   #1974
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Old 03-28-2008, 10:49 AM   #1975
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"lower arm angle" is incorrect for this inverted strut suspension. Instead you draw a line at 90 degrees to the kingpin for this lower line. That is why camber affects the roll center.

Steering block (arm) experiment
Here is a pic of the parts I plan to modify. The small rod at left will be glued into the steering block. Then I can redrill it probably in 3 steps. .089, .120, then ream to .125 inch. I will set the angle in a small angle vise to 5 degrees. When it is installed it will give me my 2 degree positive caster on the left front and allow a -3 degree kinkpin angle. I may get my 5.5 mm of lost roll center back and move it closer to the ground. This should improve front grip (act like lowering the spring tension). I may or may not be able to notice this without a clock running.
John
Attached Thumbnails
Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-steering-block-002.jpg  

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Old 03-28-2008, 12:40 PM   #1976
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Old 03-28-2008, 12:51 PM   #1977
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Old 03-28-2008, 01:33 PM   #1978
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On a Mcpherson strut the upper part of the strut is fixed to a pivot. The strut can pivot; it cannot move up and down on an arm. On this suspension you would draw a right angle to the strut at the top of the strut. The reason is that the strut is on a tangent to the circle that the wheel moves on. The radius at this point is at right angles to the tangent, right angles to the strut.

The Associated suspesion does not have an active lower arm; It does not move up and down. It only allows pivoting of the kingpin. It merely supports a pivot for the bottom of the kingpin. This pivot could just as easlily be attached to a pillar coming straight up from an extended frame.



John

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Old 03-28-2008, 01:41 PM   #1979
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Old 03-28-2008, 01:56 PM   #1980
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Gurney Strips Revisited

Here is a pic of an HPI wing that I purchased for my oval car. You will see on the sketch that it has a trailing lip. It is hard to see but the lip is at 90 degree angles to the back of the wing. Actually points forward.

This is called a Gurney Strip. The purpose of this strip is to increase the effectiveness of the wing or body element without causing a large increase in drag.

The HPI set comes with two wings whose only difference is the angle and size of the strip. The second wing has a less angled strip that is slightly wider.

We had a pioneer test this on his pan car earlier in the thread. I tested it on the back of my high speed body, but did not do a very good job of forming the lip.

Anyway this is worth a try if you use this style of wing or can form a lip yourself.
John
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Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-hpi-200-mm-wing.jpg  
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