Thread: BMI's DB12R
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Old 09-25-2007, 03:46 PM   #1745
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Originally Posted by SlowerOne View Post
I have had similar problems with AE and IRS lower arms, but the 'cure' was different.

On the AE arms, the bottom face is not flat. As you tighten up the screws, the arm deflects until it 'becomes' flat to the chassis. This gives rise to changes in castor and/or ride height. The other thing it does is distort the chassis, giving rise to uneven ride height under the cells from one side to the other.

By sanding the bottom of the arms until they are flat, these problems are avoided. Sometimes, it seems that if you sand one arm, the ride height problem goes away, but my experience is that it is actually clamping the arm flat, and that's why it works.

On the IRS lowered AE arms, and the new IRS arms, the issue arises from the lack of a countersink. Look at an AE standard lower arm, and there is a big countersink in the bottm - none on the IRS arms of either type. The screw's countersink is deeper than the chassis thickness, so to get the screw flush with the underside of the chassis, the countersink sticks through the top.

When the arms are tightened, it is the countersink that contacts first, and this sends the arm any way it can. Put a countersink in the IRS AE lowered arms, or in the washer closest to the chassis on the IRS arms, and this problem goes away. (I wrote to DiffDude on this, and received a very nice reply).

Before you go sanding/filing the arms, put a countersink into the IRS arms/washers (of either make/type) and then fit the arms. Don't overtighten (that chassis distortion thing again!) and then measure. Every time I do this the dimensions come out spot on, and there is almost zero difference in ride height across the chassis. I've fitted three sets like this (including my DB12R!) and I'd say the IRS arms are simply the best!

Sorry this is a bit wordy, no pictures!! HTH
This is very true. I never did countersink the new IRS arms but i will take a look at it. My ride height is dead on so it may be fine for now. It doesnt hurt to put a countersink on the washer for the hell of it. Thanks for your input
Jason Breiner
BMI Racing
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J Concepts
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