Metric Rear Suspension

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turtle
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Metric Rear Suspension

Post by turtle » Wed May 07, 2014 8:22 am

I run a metric chassis on dirt ovals. I am finding limited information, and contradicting information on rear moment center location for the stock mounting location points.
We are allowed to run a nine inch rear-end, with the top mounting points no higher than 7 3/4" from the "centerline" of the axle. The lower trailing arms can be no lower that 2 3/4" from the "bottom" of the axle tube. The front locating points must be in the stock locations.
One of the things we have done, is to raise the rear ride height. This is done to add more angle to the lower trailing arms, which adds roll steer to the rear, as the body rolls. It also increases the left rear trailing arm angle to help with drive off of the corner.
Now to my question.....
On the upper mounting angles. By raising the ride heights (raising the chassis mounting point), what effect does that have on roll center location????
Also.... I know that all the mounting points have intersecting lines creating instant centers.
What happens to the roll center if the mounting points on top of the rear-end, are at different heights? Does have an effect on the left to right placement of the moment center??
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Re: Metric Rear Suspension

Post by ZIGGY » Wed May 07, 2014 10:55 pm

Right or wrong, I have used the 'symetric angled 4 link' rear option in Performance Trends sw later versions to help me see where I 'was at'.
I well understand why you want more lower angle but I expect you've already figured out some bad things happen as the rear ride height increases.
One way to skin that cat in the last few years has been using a LR shock with a ton of extension. I also expect you can figure
out the details if you decide to try it, especially If you're the same Turtle I think I've seen on dirt forums for a long time.

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Re: Metric Rear Suspension

Post by turtle » Fri May 09, 2014 8:20 am

Speaking of the Left rear extension... I was going to try some real soft spring packages to get increased suspension travel. I have already made extra clearances in the suspension for the extra travel and extra shock mounting positions to allow for more travel.
I well understand why you want more lower angle but I expect you've already figured out some bad things happen as the rear ride height increases.
I know about the side effect of roll steer... Is there something other than moment center movement that needs to be considered?? That also goes into my question about the moment center movement left and right.... The extra body roll will change the chassis mounting points/bar angles as it goes through it's travel.
I also expect you can figure
out the details if you decide to try it, especially If you're the same Turtle I think I've seen on dirt forums for a long time.
I'm not sure if I am the same turtle or not. I have been reading this forum for a few years, then I joined and posted a few times over the past couple of years. :D
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Re: Metric Rear Suspension

Post by ZIGGY » Sun May 11, 2014 11:59 am

As my pathetic brain understands it, raising your rear ride height raises the roll or moment or whatever the hell you want to call it center and CG. Too high gets the car jacking & flopping. Authorities say that in general :) wherever it is, it's best that its movement during suspension travel be relatively limited. I don't know what handling effect your changes are going to have on your car. You seem smart enough to know the whole damn thing (front, rear, tires, track, engine) has to work together in one big compromise. I expect you've considered front & rake in your rear changes, yes? And then there's the driver and what he likes.

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Re: Metric Rear Suspension

Post by turtle » Sun May 11, 2014 11:37 pm

I have taken into consideration the total combination... front end geometry, anti dive, CG height, weight percentages, roll steer, blah blah blah.... lol :lol:

I was working on the rear suspension, checking to see the limitations of travel during body roll,(shock travels, suspension binding) when I noticed how much the upper trailing rods were moving in opposite directions. That is when I started to question what it was doing to the "roll center"...... Nobody EVER talks about this in the racing circles that I am associated with.

I know that when the front control arms start changing angles, the "roll center" starts moving all over the place. That, among other things, is something that needs to be addressed when changing mounting locations.

You had mentioned the Performance Trends software. I think that I might try to look into something like that. Then I can better understand the effects of the suspension movements, with this style rear suspension.
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Re: Metric Rear Suspension

Post by ZIGGY » Mon May 12, 2014 12:27 pm

Do use Performance Trends, not something else. Just checked and see the price has increased but it's still a good way to spend $250.
You mentioned trying to go real soft with springs. Be careful - typically it'll bite good (especially on slick) but go too far with them & a high RC
and the car will wallow like a 1960's Buick when turning. Good luck.

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Re: Metric Rear Suspension

Post by SLPRACINGENGINES » Thu Jun 05, 2014 4:48 pm

ziggy we use to use stings and figure all this mess out ..either buy a performance trend or afco also has a good program..both are about the same cost..it will make your life alot easier..both are seperate programs..one for the front and one for the rear...buy both and start at the front get it right and then work on the rear..i build tons of metric street stock cars here in alabama and in missiissippi..there is alot that goes into figuring it out.ill give you a hint ..you want somewhere around 4 inch of roll center in the front and 6 to 61/2 in the rear ..but it has alot to do with who is driving the track .

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Re: Metric Rear Suspension

Post by turtle » Sun Jun 15, 2014 7:02 pm

@SLP
can this 6" - 6 1/2" be obtained using the stock mounting locations??
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