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Old 03-10-2006, 09:14 AM   #1
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Default Roll Center

Quick question about roll center. I have the XXX-Main book and just had a question.

How hard is it to get your CG equal to your RC and do you really want them to be equal?
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Old 03-10-2006, 08:04 PM   #2
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Can anybody help me on this?
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Old 03-10-2006, 08:56 PM   #3
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Default roll center

To get the rc above or equal to the cg would not be possible with any touring car on the market today to my knowledge with their current adjustments.

Having said that, one way to raise the rc high enough to be equal to the cg is to have the lower arm inboard pivot point on the hinge pin be raised to just below (eg. just a few mm) the pivot point of the upper link (assuming the upper link is about parrallel to the ground).

If the rc and cg are identical in theory the car will not roll, but as the car transfers weight to the front or rear of the car the suspension would compress, changing the rc to not be equal to the cg, then the car would roll. It certainly would be an interesting theory to test out . If the rc is above the cg the car should actually lean into the corner like a motorcycle.

Since car balance has a lot to do with weight transfer and how you control that weight transfer, you will want the car to roll at least a little bit so that you can make the car balance in the corners they way you want it to. So to answer you question, you really would not want the rc to be the same as the cg, because that would limit your ability to some degree to control the weight transfer to achieve the balance you want.
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Old 03-10-2006, 10:53 PM   #4
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http://home.tiscali.be/be067749/58/ check out this site it is a good rc car tuning guide it explains cg and rc in detail it is a cool site i found it doing a google search.
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Old 03-11-2006, 10:45 AM   #5
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What type of car do you have?

the reason why i ask is ive been, trying different roll center's on my car to see how is would effect my car, and its been a valuable tuning aid, for me at least!
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Old 03-11-2006, 02:08 PM   #6
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Try this;

Hudy Setup Guide

First half is about adjusting Xrays, then there's the theory on what all the adjustments do.

I belive it says that if the roll centr and CoG are the same the car will not roll at all, which means that roll bars and to a lesser extent springs, would be rendered virtually useless.

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Old 03-12-2006, 01:23 AM   #7
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Quote:
Originally Posted by Martin Crisp
To get the rc above or equal to the cg would not be possible with any touring car on the market today to my knowledge with their current adjustments.

Having said that, one way to raise the rc high enough to be equal to the cg is to have the lower arm inboard pivot point on the hinge pin be raised to just below (eg. just a few mm) the pivot point of the upper link (assuming the upper link is about parrallel to the ground).

If the rc and cg are identical in theory the car will not roll, but as the car transfers weight to the front or rear of the car the suspension would compress, changing the rc to not be equal to the cg, then the car would roll. It certainly would be an interesting theory to test out . If the rc is above the cg the car should actually lean into the corner like a motorcycle.

Since car balance has a lot to do with weight transfer and how you control that weight transfer, you will want the car to roll at least a little bit so that you can make the car balance in the corners they way you want it to. So to answer you question, you really would not want the rc to be the same as the cg, because that would limit your ability to some degree to control the weight transfer to achieve the balance you want.
HOLY COW!!!! The Martin Crisp!!!!! Man I cannot believe this man I love your book thanks for helping me out with this!!!
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Old 03-12-2006, 01:26 AM   #8
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Thanks guys for the help I really appreicate it
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Old 03-12-2006, 05:36 AM   #9
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Martin.... You da man! See you in Vegas
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Old 03-12-2006, 06:36 AM   #10
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On my JRXS you would raise the inner A-arm pivot approximately .3 inches to have the roll center height the same as the center of gravity which is approximately .9 inches above the ground. I have a spreadsheet where you can insert measurement from your car and calcute your roll center to answer this type of question.

You would not want to raise your roll center this high and this is why. Lateral weight transfer on an axle reduces cornering traction. Roll actually delays this lateral weight transfer at the beginning of a corner (I have seen this exactly backwards in many RC print medias). Roll, also, does not increase lateral weight transfer (this is often printed wrong as well). When you raise the roll center that high, you reduce the roll and decrease your cornering traction by more rapidly transfering weight to the outside tires.

Another bad effect of raising the roll center is that you get suspension jacking. When you corner with a high roll center, part of the cornering force is directed upwards. This raises your center of gravity and reduces cornering potential on asphalt. On carpet this causes traction roll overs from the high center of gravity that is produced.
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Old 03-12-2006, 06:52 AM   #11
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you guys are all 100% correct, i am currently running a really high roll center on my RDX and have been having extremely good luck with it.

as for the weight jacking, it is completely and 100% possible if you run the high roll center with the same shock set ups, by running the high roll center, it allows you to soften the springs and the oil more than you would think, this is where you get your traction back.

i have read about the motorcycle theory in my XXX set up book, there is a nifty little diagram in the roll center section that shows it very nicely.

when you guys hit a dot on the track, what does you car do, kinda of get pushed away from the corner? when i hit a dot yesterday with mine, it leaned into the corner and didnt effect me much at all.
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Old 03-12-2006, 10:38 AM   #12
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i tried expierimenting with changing my roll centers front and rear, due to my track being green most of the time, and i found that raising the front did give more steering and grip but maybe a bit too much, so i reduced the pivot and pivot block spacers to .035 instead of .050, and made the car much easier to drive with a calmer front end versus with the .050 up front(Jrxs), also increased the read Roll center as well, and with that i went with .050 and that worked excellent, not my setup is on rails, im quite happy with both changes to the front and rear roll center, wish i would have done this earlier....
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Old 03-12-2006, 12:54 PM   #13
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Attached is a sketch of the JRXS with a very high roll center. Here is a description of the lines. The view is from the rear. The left rear tire is on the left of the chart.

The top red line is the camber link. The diamonds show the location of the pivots.

The middle green line is the A-arm line. The squares show the location of the pivots.

the lower blue line connects the bottom of the rear tire with the instant center which is where all the lines meet on the right.

The roll center is where the blue line crosses the line up through the center of gravity (The vertical black line). The center of gravity is the top of the black line.

notice on this car we can also achieve a very high roll center by lowering the inner camber link pivot one hole and then making a more modest change to the inner A-arm pivot.

All that said, this roll center is too high. The car willl not be supple. The tires will tend to slide more and be less compliant with the road as the roll stiffness is almost infinite. You can retune it with very soft springs to compensate, but I don't think this will give you the best lap times. The Losi XXXS originally came with very high roll centers. Almost immediately low roll center blocks were produced.
Attached Thumbnails
Roll Center-very-high-roll-center-b.jpg  

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Old 03-13-2006, 06:21 AM   #14
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WisRacr1 - Glad you like the book. It is so kool to hearing from people like yourself that get something out of the book. Thanks again.

Boomer - Thanks buddy. I can't wait for vegas....just trying to decide if I should run both mod classes or run the 19t and mod rubber like last year, because rubber is more my thing.
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Old 03-14-2006, 07:19 AM   #15
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Hey Martin,

I am confused about 1 thing with roll center, possibly you could explain it please.

If i was to raise my inner (upper and lower) mounting locations, say higher by exactly the same amount, what would it do in terms of handeling? Would it do nothing?

Thanks
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