November 20

Corner weights

Corner Weight – Vehicle Balance
As you study wheel alignment and vehicle handling, you pick up real quick, the value of proper corner weight. But how do you set that? I’ve found four different ways, that vary from expensive to DIY at home.

What the big guys use
If you’re a well funded NASCAR team or a serious track racer, you probably have a LongAcre computer scale set in your set-up trailer. We all can wish. LongAcreRacing.com

Bathroom scales
An enterprising Tuner figured out a way to use bathroom scales in conjunction with a platform. Using levers, he improvised a less expensive way to determine the corner weight. But still out of my range. The link has since disappeared so try a Google search on Ruggles Scales. You’ll get the idea

Hydraulic readout
This racer used a modified slave cylinder and pressure gauge to read out the weight when a vehicle was lifted, one corner at a time. Pure-ists will argue the weights are influenced by not measuring all four at the same time, they’re probably correct. But what difference does 1/2oz make in 600 lbs? Article

Local scales?
If you have a friend that runs the local grain elevator, CAT scales or similar operation, maybe he’ll let you pull one corner at a time onto the scale. If you have to pay for the time/service, there may be another way.

DIY
A recent article in the 2008 Cobra Guide shows a way to measure the tire footprint and air pressure and turn that into a value. Add all four and do the math and you have front end / rear end weight, corner weight, percentages. Maybe the accuracy is compromised slightly but if it’s within 5% that’s only 30 lbs per corner. Close enough for my purposes.  Weight calculation

October 8

Driveshaft and IRS updates

Driveshaft

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.

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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


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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.

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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
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 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

September 21

Bumpsteer

Spent the last month fighting alignment and bumpsteer problems. I won! On Aug 4th, the total bump steer totaled almost 1″. Now it is less than .040″

Just about the time I was doing my rough alignment, the article in Kit Builder magazine by Dean Lampe regarding bump steer appeared. I should have adhered to the old premise, “ignorance is bliss”. There are many Hurricane kits being successfully driven and no complaints on the owners site. Many happy campers, huh.

I studied how bumpsteer is measured and the affects it has on a vehicle. I then measured my kit, as assembled, and found it to be drastically incorrect. At normal settings, I found the bump steer to be approximately 3/8″-1/2″ at 2″ of bump or jounce. And even worse, those movements were crossing neutral. As the car rolled, the inner wheel would toe out 3/8″ and the outer wheel would toe in 1/2″. This would have been one squirrelly ride.

I consulted with Dean, and a couple others. All felt it was something that should be addressed, especially if I ever wanted to take the car onto a track. The paradox is there are about 50 Hurricane kits presently on the road and no one had ever complained of it. Dean built one of the first cars and he acknowledged he never checked bump steer and felt the car handled OK. But with that same sentence, he added, “I don’t think most manufacturers even check it. I thought my new GT40 handled OK but after correcting for bumpsteer, I am amazed how much better it handles.”

As I studied the car and design and finally determined the steering rack was mounted too high, by about 1″. Moving it would be a major task since the left mounting boss sits on the lower frame extension. Moving the standard Mustang II rack down meant cutting and welding this part, something I didn’t want to do. I decided to modify the rack  by cutting the boss off and fabricating a clamp mount for it.

In the process, I learned the rack was also mounted off center about 1/2″ Moving the rack till the boss on the right side touches the frame rail will remove about 3/8″ of the offset. Close enough.

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Jury-rig to test if I was doing the correct thing. Cut the boss on the left side of the rack. Test fit
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Rack after clean up. I left a small part in case I ever need to bolt to the new clamp Welded gusset  to frame and drilled new holes Completed Pieces
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New position, about
1 1/4″ lower
Clamp Passenger side. New hard bushing
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Outer view Modified Steeda bump steer adjuster Final


As part of the initial analysis, I had purchased a Steeda bumpsteer kit. It would move the outer tie rod pivot up and down. But problems: I need to move the point up, right to the middle of the steering arm, and the Steeda kit is really designed for the Fox body and SN95 cars. The threads (9/16-18) are correct but the adjuster is about 1 1/2″ too long. I liked the idea of having some adjustment so I cut both the inner and outer ends to shorten it.

The mounting clamp on the drivers side is fabricated from  1 x 2 x 4 aluminum. Two pieces were clamped together and drilled for the mounting bolts. It was then bored on the centerline for the rack. The assembly is then bolted thru the frame extension. On the passengers side, I welded a 2 x 2 x 3/16″ gusset below the front crossmember, in line with it. I then drilled a new mounting holes. I used a die grinder to relieve the edges of the rack to miss some welds.

Once the mods were completed, the true test came – did I fix it or have I just made the problem worse. Measuring my bump steer at the finish (and adjusting it to optimum settings), I ended up with .034″ max at 2″ of bump and .012 max at 2″ of droop. I was one happy camper. Measuring the right side, I wasn’t as fortunate, I was getting .022 max at 2″ of bump but at droop, the toe was going to .048 at 1 1/2″ then reversing and going to .020″ at the 2″ point. The tie rod requires some tweaking. Part of the analysis revealed the inner tie rod pivots are about 1/2″ too far out. The Mustang II rack is 24.50″ between pivots. On my build, it would steer better if the pivots were 23.50″. For now, I’ll just live with what I have.

On the passenger side, I fabricated a nylon bushing to hard mount the rack. I chose to move the rack back about 1/4″ to imrove ackerman settings.

The modification to do this was not complicated but it was a little more than bolt on parts. I’ll be happy to share details with anyone who decides to tackle it.

Roll Steer
My car also has an Independent Rear Suspension, and it can add to the steering issues. I was pretty fortunate, the roll steer is less than .030″ in four inches of movement. A trick when setting the rear, it is opposite of the front – set the toe first, then set camber by adjusting in the heim joints an equal amount on each.

These links proved very useful in my research of Bump Steer and how to correct it.

Morris Clements comments: http://www.gasholes.com/smf/index.php?topic=3232.0 
this is a very lengthy thread, the bumpsteer section is around page 20 and 30. It is worth the time to read the whole thing. Morris provides the .021″ per inch tolerance basis.

More comments by Morris on the ClubCobra site: http://clubcobra.com/forums/showthread.php?t=60445&highlight=roll+steer&page=2

Longacre Racing: http://www.longacreracing.com/articles
This info provided by Longacre tells how to measure and what needs to change to correct what.

Modified Mustangs: http://www.mustangandfords.com/techarticles/mufp_0611_bumpsteer_explained/index.html
A good article, very helpful

str-427-cobra.blogspot  :Click here
About half way down the blog, he discusses his findings. The use of a magnet laser light on the the rotor for rough evaluation really sped up the process of tuning.