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Old 01-23-2012, 11:31 AM   #1
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Default Roll Centre. Hinge pin Vs upper arm adjustments

Can anyone give reasons why you would use the hinge pin on the wishbone/lower arm to adjust roll centre rather that adjusting the upper arm? I know 0.5mm on the hinge pin needs about 1-1.5mm to equate the same effect on the upper arm but wondered if there was more to it than just that?
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Old 01-23-2012, 11:53 AM   #2
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Mainly, I think it's as you note: pins are macro adjustments, links are micro adjustments. Also, in the heat of battle, thrashing between heats, it can be quicker and easier to adjust the link position rather than re-shim the pins.
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Old 01-23-2012, 01:49 PM   #3
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Just remember that changing the upper link significantly affects camber gain.

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Old 01-23-2012, 03:30 PM   #4
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The advantage of raising or lowering the arm hinge pin is you can get your Roll Centre much lower or higher than you can by just changing the upper link. Also as Magnet Top mentioned when you change the camber link you also affect camber gain. Shimming the lower hinge pin just affects Roll centre.
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Old 01-23-2012, 03:45 PM   #5
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Correction to my post above. Shimming the arm hinge pin will affect camber gain as well. The rest is correct.
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Old 01-23-2012, 06:07 PM   #6
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The camber gain effects seems much more pronounced than roll center effects when you're talking about adjusting the upper arm only. Besides, the lower arm is generally just adjusting the roll center unless you're using kickup or squat. The upper arm is changing both camber gain and roll center, but I always find that thinking of the upper link as adjusting camber gain and the lower arm as roll center generally works and keeps things fairly simple for the non race engineer (me) One thing that is good to keep in mind, though, the that upper link length does have a noticeable effect in how much a car rolls. Longer = more roll, but not like dropping the hinge pins. But as far as raising and lowering RC in general, I stick to the lower arm for that.
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Old 01-24-2012, 01:36 AM   #7
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If I understand roll center adjustments correctly, then there's a few differences. One is how much of a change it makes. From most to least its lower link height, upper link height, upper arm length.

You can raise the roll center by lowering the upper links. Raising the lower links has the same effect, so does shortening the upper arm. If you want to lower the RC then do the opposite. If your lower link had no shims under it then raising your upper links would be the way to go. Or lengthening the upper arms depending on how much you want to change the RC.

Lowering the RC produces more roll (because its further from the CG of the car) and adds more grip to a point. Unless the car has to much roll, then you want to take it away to add grip. This is because the amount of load on a tyre doesn't produce grip in a linear way, so taking load away from the outside tyre only gains grip until that point, where the grip lost on the inside wheel because of the less weight on that wheel is more then the grip gained on the outside wheel from the added load.

Of course, I may have that all wrong because I'm, only just getting my head around it now... lol

On camber gain... If you shimmed the outer link position to remove the camber gain would you still have the same roll center position?
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Old 01-24-2012, 08:34 AM   #8
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As they say a picture is worth a thousand words or in this case a youtube video. I used RCCrewChief to answer the question in a screencast.

http://www.youtube.com/watch?v=wextt1ULI2Q
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Old 01-24-2012, 08:56 AM   #9
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The video works perfectly, but what would all these changes mean on the track ? How would you choose the best setup? Would laptimes be ultimately the key ?
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Old 01-24-2012, 09:19 AM   #10
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The point of this video was just to answer the question posted. The program also has many other features including a handling page that shows how the changes you make to the setup will affect weight transfer and ultimately the handling (push/loose tendency). If you want to see more go to the RCCrewChief webpage and install the demo.

Just like real race cars when you make a setup change, track testing must be used to evaluate the change. Race team Crew Chiefs would use similar tools to make setup changes based on driver feedback. The advantage of this method is you can quickly make a change and know what to expect when you hit the track.
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Old 01-24-2012, 10:32 AM   #11
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Quote:
Originally Posted by bertrandsv87 View Post
The video works perfectly, but what would all these changes mean on the track ? How would you choose the best setup? Would laptimes be ultimately the key ?
Most on-road drivers tend to worry too much about finding the perfect setup. It's a unicorn and doesn't exist; for the modern mega-flexy touring cars it doesn't even matter all that much. The cars have a huge operating window and changing the setup doesn't have a drastic effect like it used to on a stiff chassis.

Just get a decent starting setup, either from a fast driver at your track who runs the same car (if possible), or off the manufacturers website for a similar track type (high/low grip, asphalt/carpet etc).

Then when you're happy that it's the car holding you back and not your driving, try changing one thing at a time and write down the effect it had. At this point you can use something like Bobs software (I use his old Roll Centre calculator which is superb), or a setup guide (the XRay T3 setup guide is available as a free PDF and covers most aspects of TC setup) to give you an idea what to adjust and what direction to go in.
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Old 01-24-2012, 10:44 AM   #12
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Quote:
Originally Posted by BobW View Post
As they say a picture is worth a thousand words or in this case a youtube video. I used RCCrewChief to answer the question in a screencast.

http://www.youtube.com/watch?v=wextt1ULI2Q
Thanks for the video
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Old 01-24-2012, 01:37 PM   #13
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Wow, thanks BobW, what a fantastic way of explaining it. It's also comforting to see my understanding of roll center was correct .

At the end of the video you lowered the inner upper link to replace the camber gain you'd lost by lowering the lower inner link position. Could the same thing have been achieved by raising the outer upper link position? And would that adjustment effect the roll center?

I'm pretty sure I'm going to have to fork out for rc crew chief software now
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Old 01-24-2012, 02:11 PM   #14
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Is there a Rc crew chief app for Android yet ? What a program !!! Thanks again Bobw!!!
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Old 01-24-2012, 03:00 PM   #15
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Thanks everyone. Lots of useful comments.

I hadn't counted on the way top roll centre effects camber gain or reduces it more on the top link than hinge pin. Explained a lot to do with a problem i experienced.

I was running a MI4CX and with alloy blocks which were lower than the standard plastics blocks by 0.5mm. When i changed back to the plastic blocks i had to add 1mm on the top link front roll centre and 1.5mm on the rear to achieve the same feel from the car. Problem was grip roll became ridiculous. The lack of camber gain due to the top arms being parallel to the bottom arms would explain it.
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