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r3dn3ck · 04-03-2006, 10:20 AM

I've had 2 heavily modded IRS's. Usually what you're hearing is the loading and unloading of the diff causing it to pitch on its crappy mounts.

In the back the diff cover is also the rear support bracket for the diff and that bolts laterally to a bushing dealie that's a totaly piece of criznap. you also don't have a vibration damper at the front of the diff like the later models came with. That rubber bushing and the soft rubber bushings on the front of the diff are all the time bouncing back and forth. They're great for keeping vibration down but jeebus. Anyway, the basic principle of IRS is that the differential is fixed to the car firmly and the wheels and halfshafts do all the moving around. Ford's implementation didn't work quite so well in that respect since it would have made too much NVH for it to pass and hit the market with the 99's. Part of making it quiet seems to have materialized in looser gear setups as well. I don't know for sure but it was a concern of my diff guy when he set up my gears.

Fixing the diff case to the subframe in as firm a way as possible takes care of most of the bounce that leads to clunk and allows you to run a tighter setup on your gears for less clunk there. There'll always be some clunk I'm afraid. There's just too many bushings and replacing them all with delrin is expensive.

The ball joint issue would most likely cause the clunk on every accel/decel cycle or every time that side of the suspension articulated a little (I'd think)

Here's how to fix this long term at the expense of some minor increase in the sound of the gears (provided that the issue you're having is "cobra clunk"):

MM Aluminum Differential Bushing Kit
MM IRS Subframe bushings (Poly Urethane)
03 cobra front IRS crossmember.
high speed Balanced aluminum drive shaft.

That'll fix your diff in place and help keep the clunk from getting a place to start. It'll also reduce the rotating mass/energy and reduce wheel hop a good bit.

I did those things and was solid as a rock till I found another way to break my IRS. After a while I said screw it and went back to a live axle. they're just lighter and more durable.

04-03-2006, 10:20 AM	#7 (permalink)
r3dn3ck Wowbagger hates me too! Join Date: Nov 2005 Location: Magrathea/California Posts: 7,311	I've had 2 heavily modded IRS's. Usually what you're hearing is the loading and unloading of the diff causing it to pitch on its crappy mounts. In the back the diff cover is also the rear support bracket for the diff and that bolts laterally to a bushing dealie that's a totaly piece of criznap. you also don't have a vibration damper at the front of the diff like the later models came with. That rubber bushing and the soft rubber bushings on the front of the diff are all the time bouncing back and forth. They're great for keeping vibration down but jeebus. Anyway, the basic principle of IRS is that the differential is fixed to the car firmly and the wheels and halfshafts do all the moving around. Ford's implementation didn't work quite so well in that respect since it would have made too much NVH for it to pass and hit the market with the 99's. Part of making it quiet seems to have materialized in looser gear setups as well. I don't know for sure but it was a concern of my diff guy when he set up my gears. Fixing the diff case to the subframe in as firm a way as possible takes care of most of the bounce that leads to clunk and allows you to run a tighter setup on your gears for less clunk there. There'll always be some clunk I'm afraid. There's just too many bushings and replacing them all with delrin is expensive. The ball joint issue would most likely cause the clunk on every accel/decel cycle or every time that side of the suspension articulated a little (I'd think) Here's how to fix this long term at the expense of some minor increase in the sound of the gears (provided that the issue you're having is "cobra clunk"): MM Aluminum Differential Bushing Kit MM IRS Subframe bushings (Poly Urethane) 03 cobra front IRS crossmember. high speed Balanced aluminum drive shaft. That'll fix your diff in place and help keep the clunk from getting a place to start. It'll also reduce the rotating mass/energy and reduce wheel hop a good bit. I did those things and was solid as a rock till I found another way to break my IRS. After a while I said screw it and went back to a live axle. they're just lighter and more durable.