Ford tolerances suck (IRS perfection in progress)

[Makoto]

I wanted to share this for anyone who was looking to do an IRS swap.
Its possible this is an issue in the 99+ cobras but the flared fenders mask this subtle issue that in no way affects drivability.

alright so as many of you know I slammed this 99 cobra IRS into a 97 cobra. Allegedly this is nearly a bolt on affair. That much is true if you are happy with the way ford does things. Short and sweet? the front holes for the IRS are such that the IRS, when bolted in, is a little to the driver's side. Not enough to get out of alignment spec but enough to necessitate a spacer. I thought, perhaps, the whole subframe was just a little to the left but today I noticed the passenger tire is about 1/4" more rearward than the driver's side.

so, from the factory there is a bit of play in the IRS and one fix for that is to drill out the holes for 9/16" bolts. Solving the movement in the IRS cradle may have introduced a new issue of precisely lining up the thing... wrongly. solution? wallow out the front passenger bolt some measurement so that the whole cradle can rotate a hair.

This could solve a couple things.

1. the need for the spacer on the passenger side (5mm)
2. the need for the passenger wheel to move forward some
3. the need for the driver' side to move rearward some



has anybody noticed this besides me before?

 

[Makoto]

Might as well give it a gooo

the caliper is set to the driver's side (62mm) and this is the passenger side (79mm)... bit of a gap





i cut about 2mm forward this hole on each side because math (based on totally guessing so its only a ballpark)






pretty significant improvement. the difference is only 1/4" now.



so I'm going to try taking another 2-4mm out today, see where it sits, and then make some washers that have tabs to lock the bolt forward when its all buttoned up.

The weird thing was it brought the other side forward a smidge and pushed the other side wheel back a bit so there's no real way to do this without just going slow.

 

[RAU03MACH]

Only true way to find out if your aligned from front to rear is laser alignment
I'm not saying you can't figure it out
Just saying tires can get expensive especially the rears

 

[Makoto]

Only true way to find out if your aligned from front to rear is laser alignment
I'm not saying you can't figure it out
Just saying tires can get expensive especially the rears

The car is aligned and drives perfectly, you're barking up the wrong tree.

this thread is about the alignment of the IRS assembly in relation to the chassis, not the alignment of the suspension to the road.

In the interest of clarity I'm speaking purely to the alignment of the IRS assembly to the chassis in order to position the wheels correctly in the wheel wells.
This is NOT the suspension alignment in relation to itself as there is no way to adjust fore and aft in these suspensions.

The rear needs to rotate counter clockwise a smidge which means I'll need to correct the toe a smidge.

 

[RAU03MACH]

I gotcha
Your talking about forward or back thrust

 

[RAU03MACH]

I am thinking about trying on the 98 once my engine is in
Good info to know

 

[RAU03MACH]

Your trying center the irs to the car

 

[badass98svt]

I'd be willing to bet that measurement varies from car to car. These Mustangs weren't exactly precision built. Good for you for getting it done though. My OCD would want that fixed too

I've also seen cases where the IRS or SRA are off-center, meaning the right side wheel sticks out more than the left or vice versa. It's just Ford's "precision" haha

 

[lwarrior1016]

I just went and checked my car. The passenger wheel of forward maybe 3/16” when compared to the driver side.

 

[lwarrior1016]

I think every one of these irs cradles is built a little off when compared to the next one

 

[Makoto]

Here's how i confirmed the distance discrepancy. the metal caliper and the fingers are the gap from the other side.






the distance of the other side minus this side equals roughly 13mm. i then measured other stuff and kept seeing a familiar sight...



the purple mark on the yellow string is taken from the other side of the car. the string extends to the front rim (spanning jackstands)


Basically i kept seeing this measurement EVERYWHERE so that gave me the confidence to proceed.

so here's what i ended up doing:


I made some washers with tabs to fill in that gap to lock the bolt forward. it shouldn't move but I don't wanna risk it.





this is how far forward this thing shifted. remember that 13mm? here it is. you can see where the bolt was originally. the other side was all the way forward and I figured that's why ford put the slot there. well, maybe, but turns out these rear bolts should be all the way forward (at least on my car).

 

[ttocs]

I think I would put a tack weld or two on that washer to ensure it didn't move.

 

[Makoto]

the washers on each side have tabs that fill in the void to prevent the bolt from walking back. similar to the one below but this is before i added more material and made the shoulders more pronounced. it wedges in the hole pretty good, essentially filling it completely.



I also put a tab of aluminum to fill in the void on the rear bracket to make sure the whole right side can't move. This is basically what I did with the front bolt, the tab is just welded to each washer on either side of the mount.




final result? not perfection but really darn close. this gap was half an inch on the passenger side, now its virtually imperceptible without tools.

 

[Makoto]

So in addition to being too far back there is also the common issue of it not being centered. This is how i fixed that.

1. 2.5" ARP studs on the side that needed spacers (had to drill out the holes to 39/64 per ARP's instructions)
2. 2 5mm spacers
3. ground about 1/4" off the tips of the studs so i wouldn't have to use open faced nuts
4. reinstall wheel and bask in perfection

an added benefit of having one axle "longer" than the other is the resonance of symetrical axles will often create wheel hop if you hook hard enough. unequal length rotational assemblies can keep that resonance from happening (in theory, this comes from corvette world where they intentionally did it in later generations).





left compared to right is now perfectly symetrical. this has bugged me for a long long time.