Moving the IRS rearward

[lal Naja]

How many of you have moved the Jag IRS rearward in your Butler Cobra?

It seems simple enough to do and it would provide a little extra tire clearance.

Would love to hear the pro's and con's.

Thanks,

Arthur

[soulsurpher@hot]

I think I purchased mine with the setback allready done. I'd be willing to share photos if you like. right now I've got truck tires on it so I dont know how it helps with clearance. BTW I'm looking for some wheels if somebody has changes them out or have them laying around so name your price thanks.

[lal Naja]

Originally Posted by soulsurpher@hot

I think I purchased mine with the setback allready done. I'd be willing to share photos if you like. right now I've got truck tires on it so I dont know how it helps with clearance. BTW I'm looking for some wheels if somebody has changes them out or have them laying around so name your price thanks.

Thanks for the reply. If you have photos showing how the gap is filled between the differential mounting plate and chassis, that would be great. This is the gap that would be created when the diff is moved back.

What type of wheels are you looking for?

Arthur

[Monster]

If you are looking for 15" Trigo wheels with new tires I have a set on my car (Arntz/Butler) that I've been thinking of replacing with 17" 's. I put a couple of pictures in my gallery.

Let me know if your interested.

Mike

[lal Naja]

Bit the bullet today and removed the IRS. Started modifications to the frame to move the IRS back 3/4". Should be a piece of cake and hope to have it reinstalled by Sunday night.

[Butlers24rth]

I did mine last year. I moved it 1 1/4 inches back.
Not too bad
I can outline
Remove IRS, have drive shaft xtnded 1 1/4
Shock mounts: Welded and additional 2 inch pc of steel to the back of the mount. drilled new holes 1 1/4 inches back. I also added gussets to the top to strengthen it.
Front tray mount: made a 1 1/4 X 2" solid pc of alum as a spacer bar, bought longer grade 8 bolts
Cut loose the rear Tracking/ adjustment bars from the frame. I simply seperated the angle from the frame keeping the Hime joint mounts all together then when I re installed the IRS I welded it back to the frame 1 1/4 back
The most work was the upper mount to the top of the diff. I used a pc of 2 x 4 x 1/4 tube on end and 4 in axis front to back, had to cut a contured pc to fit the frame crossover bar on the other end of the tube I cleaned up and re used the plate that attaches to the top of the diff. I gusseted this as well. You have to make/ buy new bolts to thread into the top of the diff as the old ones are too long to get between the frame and the mounting plate. Check before you weld.
I did all of the welding in the car using pcs of sheet metal as heat barrier.I am very happy with it and I have not broken it, at least not yet!!
Good luck, Scott

[Ralphy]

If you don't mind me asking. What sort of spring rates are you guys running with four coil overs? Also are you running the same front/rear springs?

[lal Naja]

Originally Posted by Butlers24rth

I did mine last year. I moved it 1 1/4 inches back.
Not too bad
I can outline
Remove IRS, have drive shaft xtnded 1 1/4
Shock mounts: Welded and additional 2 inch pc of steel to the back of the mount. drilled new holes 1 1/4 inches back. I also added gussets to the top to strengthen it.
Front tray mount: made a 1 1/4 X 2" solid pc of alum as a spacer bar, bought longer grade 8 bolts
Cut loose the rear Tracking/ adjustment bars from the frame. I simply seperated the angle from the frame keeping the Hime joint mounts all together then when I re installed the IRS I welded it back to the frame 1 1/4 back
The most work was the upper mount to the top of the diff. I used a pc of 2 x 4 x 1/4 tube on end and 4 in axis front to back, had to cut a contured pc to fit the frame crossover bar on the other end of the tube I cleaned up and re used the plate that attaches to the top of the diff. I gusseted this as well. You have to make/ buy new bolts to thread into the top of the diff as the old ones are too long to get between the frame and the mounting plate. Check before you weld.
I did all of the welding in the car using pcs of sheet metal as heat barrier.I am very happy with it and I have not broken it, at least not yet!!
Good luck, Scott

Thanks Scott.

I've just finished moving mine back 3/4". I guess I could have gone more but felt that 3/4 was all I needed. If I had your post to read before I started, perhaps I would have gone the extra 1/2" too. Ah well!

With regard to your drive-shaft. How far into the transmission tail does yours go?

As there is no sliding of the spline in the transmission because of the rigidly fixed IRS, I'm making my drive shaft longer with only 1/4" of clearance in length. That's long enough to slide it all the way into into the transmission and bolt it to the diff flange. Previously, my drive-shaft was 1 1/8" shorter and I think that was far too short.

Arthur

[lal Naja]

Originally Posted by Ralphy

If you don't mind me asking. What sort of spring rates are you guys running with four coil overs? Also are you running the same front/rear springs?

I will have to look up my invoice from the rear shock supplier and see if the rate is listed as I don't remember.

My front suspension is not the typical MGB. It's from Fast Cars in Michigan. and I don't remember what rate Ted from Fast Car supplied. I believe the springs may be color coded so I will take a look. The suspension is 80lbs lighter than the MGB so the rate would be different I'm sure to MGB.

[Butlers24rth]

Arthur,
I don't remember how far in the spline shaft I had mine made. There is a certain minimum and I beleive that I relied on the advice of the Driveline service people to guide me on that.
The only reason I went with 1 1/4 inches back was that it centered the tires nicely in the well. I suspect that all of these kits were built slightly different so what works for me might not be best for you.
Luck
Scott

[Ralphy]

lal Naja,
Thanks for the reply. I hope you can find the info.

[razerwire]

Arntz made a deal with the owner of csx3xxx , who had bent his car, that he would fix his car if he would let him make a mold off his repaired body. it was agreed and a mold was made. Alum hard to work with and in repairing the body came out 1" longer. Arntz made his wheel base 91" to fit fiberglass body. Butler and Bennett did not realize and made frame 90" wheelbase, which is what original cars were. Paul Bennett and I changed mine to 91" to fit body and looks better. We did not move rear end but changed upper and lower arms. If anyone has Bennett pushrod,full space frame car and needs pics can send. Push rod car has sheet metal ford 9" I.R.S. with cv joints so Butler cars would be different fix like you have done.

[n6opv]

Arntz never made it to 91" until the very late Type 4 cars. The most common Arntz, the Type 3, still had a 90" wheelbase although many have been modified, as you guys are discussing. Ron Butler made 60 of the Type 3 chassis for Arntz before he went under and subsequently, the first 30 or so Butler chassis remained unchanged until Ron decided to lengthen the wheelbase by 3/4" in the mid 80's.

Jason

[lal Naja]

Originally Posted by n6opv

Arntz never made it to 91" until the very late Type 4 cars. The most common Arntz, the Type 3, still had a 90" wheelbase although many have been modified, as you guys are discussing. Ron Butler made 60 of the Type 3 chassis for Arntz before he went under and subsequently, the first 30 or so Butler chassis remained unchanged until Ron decided to lengthen the wheelbase by 3/4" in the mid 80's.

Jason

Jason, My car was produced July 86 #42 Do you know if mine is of the final generation chassis?

I moved mine back 3/4" because that was "relatively" simple to do. This however, will still not center the wheel in the fender arch, it will just provide a little bit more clearance for slightly larger diameter tires.

Arthur

My Cobra build: http://apps.facebook.com/wildfire/fb...oid=1023096501

[n6opv]

Originally Posted by lal Naja

Jason, My car was produced July 86 #42 Do you know if mine is of the final generation chassis?

I moved mine back 3/4" because that was "relatively" simple to do. This however, will still not center the wheel in the fender arch, it will just provide a little bit more clearance for slightly larger diameter tires.

Arthur,

Sorry I didn't see your post until now. All the Butlers had the same chassis with the exception of the upper diff mount, which was moved back 3/4" in the later cars to extend the wheelbase for the reasons you're all discussing. There is no variation in the construction of any of the chassis; the very substantial jig would not allow for it.

Ron originally thought there were about 30 cars built before the wheelbase was lengthened. Since yours is beyond that, obviously there were more. It was done sometime in the 80's though That I guarantee! The 3/4" does not completely center the wheel, as you mentioned, but it does provide plenty of clearance for a properly sized tire and presents a look very consistent with original Cobras (their rear wheel was forward in the well also).

Two related parts changed as a result of this. The upper shock mount was modified to have the tube offset to the rear and the front piece of the differential stiffening plate was redesigned to take up the gap that was now there. Both of the old parts could have simple spacers inserted instead, since Ron doesn't have any more sitting around.

Jason

[lal Naja]

Thanks Jason for the update!

Arthur