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Old 04-29-2006, 05:50 AM   #1
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Default help, where`s my rollcenter ??

Hey ya, people`ss

I`m new here and bla bla blaaa....

Well this is how one finds his rollcenter




But, HELP !! where`ss my rollcenter gone to ???



has it actually moved outside the car ???
or is yust IC on the left where the two lines crosses ??
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Old 04-29-2006, 06:06 AM   #2
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POST A PICTURE OF THE REAR OF YOUR CAR SHOWING BOTH SIDES IN FULL AND I`M SURE SOMEONE CAN HELP; IM SURE YOUR CAR DOES`NT LOOK LIKE A CARTOON!!!
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Old 04-29-2006, 06:45 AM   #3
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Look at your car and at that definition of the roll centre and you'll see that you made two mistakes,

One your drawing is wrong, the upper link should be angled the other way

Second you have drawn the position of the IC, you need to draw the position of the IC for the other side to find where the RC stands.
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Old 04-29-2006, 06:47 AM   #4
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Most of the information around Roll Centers in Radio Controlled publications stays pretty basic. Roll Centers do not always have to stay on the centerline of the car, and do not necessarily stay at one point during suspension travel, however, the drawing you have displayed would produced horrendous positive camber during suspension travel due to the inclined upper link. Let's see a photo of your actual car.

For your theoretical cartoon car, you are correct, the Instantaneous Center (IC) would be at the intersection of the two linkage lines, and the Roll Center very low at the centerline, found by drawing a projection line from IC through the contact patch of the tire.
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Old 04-29-2006, 07:51 AM   #5
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well, here it is ..
first I was running the car without the shims under the toe blocks and I had an even bigger shim under the upper inner balstud / pivot camberlink



sinds then I started experimenting with rollcenters and was figuring where it would have gone to ......
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Old 04-29-2006, 08:09 AM   #6
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From the picture it appears that the upper camber link and lower control arm are parallel to ground level. In that case the roll center is at ground level. If the inner camber link attachment is just below parallel then the roll center is slightly above ground level.
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Old 04-29-2006, 08:47 AM   #7
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But where would rollcenter be if he`s (inner camberlink) just above parralel to the under suspension arm or

where would rollcenter be if in this picture de shims under the toe blocks are removed and the inner side of the suspension arm would face downwards ???
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Old 04-29-2006, 09:37 AM   #8
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Quote:
Originally Posted by duhh
But where would rollcenter be if he`s (inner camberlink) just above parralel to the under suspension arm or

where would rollcenter be if in this picture de shims under the toe blocks are removed and the inner side of the suspension arm would face downwards ???
If inner camber link is just above parallel to lower suspension arm (which is assumed to be parallel to ground level) then the roll center would be just below ground level. In addition this configuration would result in more positive camber (Top of wheel leaning outward) during bump suspension travel.

Lowering the inner hinge pin of the lower control arm will lower the roll center.
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Old 04-30-2006, 12:06 AM   #9
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Been thingkin about it again .....

Quote:
From the picture it appears that the upper camber link and lower control arm are parallel to ground level. In that case the roll center is at ground level.
Is this really true ??? would my rollcenter actually move ouside the chassis ??? so would the whole mass turn around a point that isn`t inside the car itself ???

Quote:
however, the drawing you have displayed would produced horrendous positive camber during suspension travel due to the inclined upper link.
yes !!! but then, wouldn`t it be as simple as saying that is it whise to get the distance beween upper and lower inner turning points as close to each other as possible ?? and for the outher (wheel side) get them as far out each other as possible to get as less change in camber as possible ??? as you wish to keep the tyre as perpendiculair to the racing surface as possible at all times ..... ???

If not, in what situation would it be nice to have this "horrendous positive camber during suspension travel" ??????????
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Old 04-30-2006, 04:13 AM   #10
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Quote:
Originally Posted by duhh
But, HELP !! where`ss my rollcenter gone to ???

i stole it, and u cant have it back
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Old 04-30-2006, 01:44 PM   #11
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duhh-When the camber link and A-arm are parallel and parallel to the ground as is often the case on off-road trucks the instant center is at infinity. When you draw a line from the contact patch to meet the instant center it is parralel to the other two lines. The roll center is indeed at the ground (where the two lines from the tires cross the center line) which is a good place for it to be.

Yes the instant center can be outside the chassis. On our symmetrical touring cars (and the off-road truck) the roll centers will always be on center line of the car. It moves just a little up and out with roll but not much.

My experience with a touring car and Carrol Smiths experience with full size race cars in Tune to Win is that generally an improvement caused by changes to the suspension link positions is usually due to an improvement in the roll center. I find that the touring car is relatively insensitive to camber changes. So when you move a link pay attention to what it does to the roll center. Although crazy roll centers have been used on RC cars as well as full size cars I think that you will have the best luck with rolls centers near the ground or 1/4 inch high or so on a touring car.

When you put the links like you suggest you get a roll center very near the center of gravity. This increases the roll stiffness to an amount so high that traction is reduced. It also increases suspension jacking forces that jack the suspesnion up so high that you cannot use it on high traction surfaces. You can get a snap roll or traction roll. So forget the very high roll center. The optimum is somewhere in between the ground and 1/4- 3/8 inch or so.

Now if you care to know where your roll center is exactly I have a Microsoft Excell spreadsheet that with a few measurements you can calculate the roll center. Usually this is not neccesary, but it has taught me a few things.

Usually one hole up or down on an inner camber link pivot is way too much change. It is better to shim lower A-arms up a little to raise the roll center or shim the outer camber link up a little to raise the roll center. Start low and work your way up after you find the right springs.

[email protected] to request the roll center calculator.

Here is a link to another thread on this web site with a picture of the output of the calculator as well as the measurements to be taken, and also some discussion of roll centers.

Last edited by John Stranahan; 04-30-2006 at 02:11 PM.
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Old 04-30-2006, 05:15 PM   #12
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Your links look near parallel in the photo. I agree with the other racers that the roll center is near the ground.

Last edited by John Stranahan; 05-01-2006 at 09:38 AM.
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Old 04-30-2006, 05:35 PM   #13
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Hope this diagram makes sense?

Darren
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