Thread: BMI's DB12R
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Old 09-25-2007, 02:20 PM   #1742
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Join Date: Jun 2005
Posts: 900

Originally Posted by Scottrik View Post
I ran into the same problem last fall setting up three new cars all with IRS lowered Associated arms. They were all off by about .015. No obvious burrs, etc. I was very surprised by this as I'd been fat dumb and happy (like always) figuring they'd be square since they're milled...I'm sure in a fixture of some sort.

Another thing that's come up...I started getting about a .5mm difference in height side-to-side. Measured springs, etc, re-set spacers, etc. Finally figured out that my pivot balls (always Niftech polished bronze) are floating up and down in the lower arm. No doubt an impact (impacts) have enlarged the socket.

I'm really wanting to try the new IRS lower arms. Also really wanting to try a Laje SpeedEvil front end but they're out of a couple key parts. He'd also not thought of drilling the new IRS arms for his front end when I asked if this was available (and he's a IRS dealer/importer).

Looks like the DB12R will have great success Jason. I was interested to see someone describe it as, basically, the missing link between T-bar and link cars. That was what it looked like to me too. I may have to try one...
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
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