Driveshaft and IRS updates


Man, it sure is short ! (how many times have you heard that?) One of the benefits of using an independent rear suspension is you don’t get any third member movement. I can’t imagine the arc one of these driveshaft’s move thru when a live axle moves thru its normal range.

I am fortunate to live only a few miles from a very good driveshaft firm. A quick jaunt with my axle flange and transmission yoke and I was in business.


One of the beauties of the HMS design is the “X” frame that mounts under the transmission. Easy to drop and remove components as necessary but also a great platform to mount the emergency brake and the driveshaft loop. The loop came from Summit Racing.

Just slightly evident in the picture is the offset in the driveline. This is necessary to get the roller bearings in the U-joints to roll and lubricate themselves. If the total angle is less than 3º, they end up just brinnelling their races and will quickly fail.

The third member shows the rubber bushings supplied with the kit. I am told it is wise to change them to hard mounts.

Independent Rear Suspension
I chose to include an IRS with my build. The live axle rides exceedingly well on the HMS design, but to be period correct, and to optimize resale value, and IRS is almost mandatory. The HMS design uses the proven Thunderbird SuperCoupe axle with a special frame and suspension arms. Very similar to the Factory Five design however the upper arm is triangulated to improve handling


I chose to take delivery with the unit, un-assembled. That created some problems in itself when I learned the assembly process also served as a quality control check.Many of the bolt holes required additional tweaking. Once assembled and bolted in, I learned there was some localized yielding in the support structure. I am familiar with the problems this can create, having done failure analysis on a truck suspensions earlier in my career. A number of conversations with HMS and reinforcing plates were added to may car and to future builds.

IMG_3234 IMG_3236 IMG_3244
Never force anything, just get a bigger hammer. Since my unit had never been assembled to the frame, a few holes required some tweaking. PortaPower to the rescue You can see the localized yielding/deflection I encountered as the fasteners were tightened. Look at the reflection around the bolt and washer. Beginning assembly.Make sure to think ahead. A little anti-seize will keep these adjustable
IMG_3247 IMG_3256 IMG_3387
 Start fitting all the pieces together The brake calipers are reversed to gain cable and hydraulic line clearance Closeup of the late model SuperCoupe parking brake cable. The eye does require slight opening to accept the Lokar cable end
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 Front view
I was told this driveshaft may be a little small
 Finished assembly

I chose to include the IRS option with my car. Three items entered into the decision
1) I was trying to be period correct –  the original car also carried an independent rear suspension
2) I did not plan to race the car, but was interested in the best ride and handling possible
3) Re-sale value with an IRS is higher

As stated earlier, I also chose to have the car provided in an “Un-assembled” condition. This adds a few complexities to the IRS.
1) Handling – it isn’t the lightest piece in the build. I found my floor jack would serve as a make-shift tranny jack, by adding a sheet of plywood to the saddle.
2) Fitting the frame: When the IRS is welded, there is a certain amount of weld draw. When factory assembled, they have to deal with the issues this creates. When it is delivered un-assembled, one of the items included is “mis-matched” holes. A little ingenuity works these out.

I strongly suggest you take documenting photographs before you dis-assemble anything. They may come in handy. With my pictures taken, I broke the suspension down and tackled the finishing process.
A) I cleaned up the center section and painted it.
B) the axle shafts were not painted – to accomplish this, I had to dis-assemble the half-shaft boots, paint, then reassemble
C) The upper and lower arms, and the rear housing bracket fit my blast cabinet. They were blasted and painted.
D) The frame was blasted and cleaned outdoors, it was too large for the cabinet.

On re-assembly, I noted these items:
1) The rear housing bracket did not fit the differential tightly. The fit was improved by adding 1/32″ thick washers inside the bracket. When torqued down, the bracket was then tightly clamped to the differential.
2) There are articles on the internet about the use of a differential brace. Under hard acceleration and cornering, some aluminum differential covers have failed. I have the opinion this is caused by a combination of the failure of the forward mounting bolts/bushings and the design of the Mustang rear differential hanger. The rear bracket on the Mustang is rubber mounted and can induce some twist into the components. I don’t think this is an issue on the Hurricane mounting. Hope I’m right.
3) At the time of my build, instructions for assembling and mounting the IRS had not been published. I have since been given this advance copy of the IRS Assembly

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Posted 10/08/2008 by Paul Proefrock in category "Drivetrain", "Suspension