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Old 05-11-2004, 08:42 PM
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Extending stock MII crossmember

Has anyone ever extended a stock MII crossmember in order to move the wheels out 2" each side? I was thinking of extending the center section 4" and using 2" rack extensions on each side to maintain steering geometry. I also need to do this so that the upper a-arms will clear the inside of the fenders. The center section would have to be cut and a section welded in with suitable bracing added to the inside of the crossmember to maintain strength. A new one piece bottom plate would be used as well. This would extend the hub-to-hub length from about 56" to 60". Any thoughts would be greatly appreciated.

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Old 05-11-2004, 08:59 PM
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I think you will have a problem extending the rack & pinion shafts. The center of rotation of the steering shafts will remain the same, but the center of rotation of your A arms will be changed by how much you move them out. This will cause severe "bump steer".

Vince
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Old 05-11-2004, 09:12 PM
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I believe there are rack extension kits available that move the inner end of the tie rod, and therefore the pivot point, outward, maintaining steering geometry. Is this right?
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Old 05-12-2004, 04:42 AM
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Quote:
Originally posted by Whippetguy
I believe there are rack extension kits available that move the inner end of the tie rod, and therefore the pivot point, outward, maintaining steering geometry. Is this right?
I don't believe that I have seen those. The pivot point of the rack needs to be as close (same relative line) as possible to the pivot point of the A arms.

Vince
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Old 05-12-2004, 04:50 AM
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Quote:
Originally posted by 302/Z28
.... The pivot point of the rack needs to be as close (same relative line) as possible to the pivot point of the A arms.
When using rack extensions this relationship shouldn't change. All they do is move the distance that pivot point outward on the same plane.

Centerline
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Old 05-12-2004, 03:51 PM
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I may be wrong, but I can't see how moving all the pivot points 2" out the same amount would change the geometry.
This brings me back to my original question about extending the crossmember. Am I stepping into new territory here? Has everyone always just settlted for the track width the stock xmember gives you? This would put the outside of the tire about 4" inside the fender, which I don't think is a good look.
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Old 05-12-2004, 04:10 PM
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Extenders will solve the pivot point/bump steer problem but the wider track will also mess up the Ackerman geometry and cause some tire scrub and/or over/under steer. Don't know how critical a problem that would be.
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Old 05-12-2004, 06:35 PM
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Willys36, I know what you are saying about the Ackerman angle, but would this not also occur when installing a crossmember in a car with a different wheelbase? I downloaded Peter Eland's spreadsheet and instructions to chew on for a while.
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Old 05-13-2004, 04:34 PM
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I did some layouts on InteliCad last night. I used a track width of 56" (the hub-to-hub measurement of my stock MII xmember) and the stock wheelbase for a MustangII of 96.2". I then drew 2 lines representing the front axle centrline of the outside wheel turned at 30 degrees and at 45 degrees. I then drew two other lines from the points where these lines intersected the centerline of the rear axle. This gave me the theoretical Ackerman angles for a stock MustangII. I then offset the lines I had drawn 2" to the left fot the left whheel, and 2" to the right for the right wheel. This gave me 2 new intersection points which were now almost 7" behind the rear axle centerline. However, my wheelbase is 4" longer than the Mustang, so now I'm only out a little less than 3". This translates to an error of only .8 degrees on the Ackerman angle. I don't think this will be a huge problem. What do you guys think?
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