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Old 04-06-2007, 08:19 PM
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T-bird IRS geometry??

I think Iíve brought this project up about three times now, but Iíve finally torn into it and I have a few questions.

Probably most important is the angle of the control arms. From the measurements Iíve taken from a functional T-bird, the front mounting points of the lower control arms are angled up by about 1 3/4" in relation to the rear mounting points. Should this angle be maintained as opposed to parallel to the ground? Looking for optimum handling with this setup.

Another concern is pinion angle. This would be changed while adjusting the control arm angle (I would have to rotate the entire subframe to accomplish this). What could I get away with as far as pinion angle is concerned?

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Old 04-07-2007, 05:44 AM
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This matter of keeping the lower control arm horizontal pertains to BEAM AXLES ONLY. When, with a beam axle, the lower arm is horizontal, roll steer effects are minimized. But, with an IRS, forward motion of the right rear, for instance, does not necessarily coincide with rearward motion of the left rear.

This does not mean, of course, that you can't have roll steer effects with an IRS. You can be confident in assuming, however, that the engineers at Ford took these matters into consideration. In other words, keep what you've got.

A few more words on roll over/understeer: Since roll steer does not affect wheel loadings, its only effect is on steering wheel angles. (If a racecar is inclined to go into the wall headfirst, you can add all the roll oversteer you like and it will still go into the wall headfirst. The driver will simply be holding the steering wheel differently at the time.) In other words, all those nasty dynamic consequences of oversteer, for instance, are not present in ROLL oversteer. For this reason, engineers sometimes use roll oversteer to hide...from the driver...a car's severe understeer. If a driver doesn't have to apply a lot of steer angle when going through a turn, he might assume that the car doesn't push badly. But, a certain amount of push can be concealed with roll oversteer.
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Old 04-07-2007, 06:43 PM
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Thanks for the info. Iím going to start building the jig to relocate the mounting points for the subframe soon. I will try to retain that angle if at all possible.

The only other thing that may do this is pinion angle. Iíve read that this angle should be parallel with the angle of the trans. output. As I've mentioned above, to adjust pinion angle I must rotate the subframe. So, if the pinion angle doesn't measure up with the control arm mounting points set at their factory position, which angle should I adjust?
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Old 04-07-2007, 09:16 PM
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Ah! I see what you mean. If you adjust for the pinion angle, I doubt if you'll be far enough off on the link geometry to make a noticeable difference. But, without being there and seeing what you've got, I certainly wouldn't be dogmatic.
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Old 04-08-2007, 09:59 AM
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I think youíre right. Itís not a very hard pitch up and it may locate the pinion perfectly the way it is. Unfortunately, I just pulled the differential apart to install new gears and clean the housing up, so it will be a little while before I can measure for certain. Thanks for your help on that one.

I hope you donít mind, but Iíve got another couple of questions about the lower control arms; this time about the best angle to have, at ride height, from the mounting points on the subframe, outboard to the knuckle. I believe Iíve read somewhere that this should also be as parallel as possible to the ground for best handling. Is this also incorrect?
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Old 04-08-2007, 11:35 AM
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It's not a matter of such comments being "incorrect." It's just that they don't directly apply to an IRS. With The Tbird IRS, the 4 pivot points of the lower arm are not in the same plane. This is how the engineers controlled roll steer and toe changes. Nothing you can do to change that (short of fabbing a new arm). The angle of a line through the inner pivots, when viewed from the side, determines the amount of squat on launch. Like the stock Tbird, you're going to have a lot of squat. This is pretty much a characteristic of an IRS, since the 100% anti-squat line passes through the axle centerline, rather than the tire patch (as with a beam axle).
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Old 04-08-2007, 11:41 AM
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On those lower control arms that is a rule of thumb that allows full travel of the suspension..On what you are doing we now have some software that allows one to do design simulation so one can get closer to right in the first place..it is not free however..As far as the pinion angle I would work to get the suspension to work properly and then if I had to rework the third member mountings to get the correct pinion angle so be it..

Just my take..

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Old 04-08-2007, 03:46 PM
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Again, BillyShope, thanks for all the information. This is a complicated topic and Iím glad youíve been so forthcoming.

Sam, you donít happen to know the name of that software package? Iíll see if I can track it down.

Would it be a better idea to set the suspension up to be adjustable? I was considering using coil-overs. The problem then would be: where to mount the shock on the control arm. I could use the original shock location, but Iím not sure if that would be strong enough. Iíve included a shot of the mounting point. The control arm is cast iron, but it seems like asking a lot from a single bolt. What do you guys think?

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Old 04-08-2007, 10:02 PM
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http://www.truechoice.com/prodinfo.a...M%20GEO3%20PRE this is the location of the William Mitchel software..Kinda spendy tho..

Also try Steve Smith autosports http://www.ssapubl.com/productList.aspx?media=Software

Lot less money but maybe not all the capability..

On the coil over mount I think I would fab a reinforcement so both sides of the bolt are supported..

Sam
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Old 04-09-2007, 07:25 AM
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Thanks, Sam. You werenít kiddiní about the ďspendyĒ on that first one. Thatís almost the entire budget for this project.

The other option that Iíve been considering for the coil-over mount is in the spring bucket (you canít really see it very well in the picture). Iíd end up cutting the rise out of the center and mounting a plate in the bottom of the bucket. There would be enough room to put several mounting positions for the coil-over, in a row. That way I could adjust the angle of the shock and only have to have a single mounting point on the frame.

Would this be a better setup?
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Old 04-10-2007, 03:43 PM
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Mounting the coil over in the original spring bucket would be a better choice to me..I would not mess with multiple mounting points my self..Just put the coil over vertical if possible..Makes life a bunch easier..



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