MORE ON 2005 MUSTANG REAR SUSPENSION 2

[BillyShope]

Though few tears will be shed, the 2005 Mustang will probably be the last RWD beam axle car with significant production numbers. But, since many of the owners will be taking them to the dragstrip, I find the possibility of offsetting the upper link fascinating. Retired and with nothing better to do, I've written a little BASIC program to iterate the upper link position and angle, for a given offset, to provide equal rear tire loading and no squat or rise during acceleration. But, I need some dimensional information if it's to be of any value. Anyone out there, who pulls a paycheck from Ford, who can help me out?

 

[BillyShope]

This hasn't worked out as well as I had hoped. I orignally wrote the program so that I input the lower link angle and it solved for the upper link angle. I assumed that I could iterate for an upper link angle and position for a given offset, but I got fooled! It only works if you iterate on the offset, which means that the necessary offset might be larger than the space available. I'd still like to try it, though, if someone can provide the dimensions.

 

[BillyShope]

Wish we were able to edit our posts here! I usually have to say things 3 or 4 times before I get it right. I meant to say that the original program iterated on the lower angle with a fixed offset. This often (for "reasonable" numbers) means the lower links are angled down from the axle, which is highly unlikely for the Mustang. If you input the lower links as horizontal or angled up, the necessary offset can quickly become quite large.

 

[Joest]

Here is some data I measured personally the other day when an '05 was on the lift.

Rear:

Track: 62"

LCA: 18.5"

3rd Link length: 8.25" and 7-8" above axle CL centered on axle

LCA bracket on axle: 4.75" below axle CL

Spring base: 37.5"

Since the car was up on the rack, the suspension was in full drop position so I could not measure the angle of any links.

 

[BillyShope]

Thanks, Joest. That's a good start. I still need wheelbase, effective tire radius, the angle of the lower links, and an estimate of the CG height. When it comes to geometry, the spacing of the lower links cancels out, but it is necessary to calculate link loads.

In case anybody's wondering why I mentioned using an iterative procedure, rather than solving directly, it's simply because of my limited patience, coupled with a high propensity for error. I did solve directly for the case when the unsprung mass is not considered and my work is available in the student workbook which accompanies the Millikens' "Race Car Vehicle Dynamics." But, when a finite unsprung mass is considered and the no squat/no rise line, consequently, does not pass through the rear tire patch, the algebraic manipulation becomes overwhelming! I did put together an EXCEL program which tackles it in steps and this BASIC program is kind of a check. I'd certainly like to see a direct solution, but, due to its very limited applicability, I doubt if anyone will go to the trouble.

 

[Joest]

I believe the wheelbase is 107.1" and the rolling radius of the tires is close to 12.4". Angles are still unknown. Good luck.

 

[GregLocock]

A bit late now Joest, but if you had measured the eyebrow height, ie wheel centre to fender, fully drooped, and then when the car was back on the ground, you'd know the answer. Oh well, maybe next time. I'd take a stab that the lower link is horizontal at design load (2 passengers)

CG height can be got from the NHTSA website I think (the rollover page), either directly from a table, or indirectly from the SSF (Track/(2*CGZ)), of 1.53. CG height typically changes very little between kerb loading and design (2 PAS)






Cheers

Greg Locock

 

[Joest]

Yes, I know. It was on the lift when I went to the shop I consult for, but I never saw it down. If they still have it next time I visit I will take some measurements with a digital level.

 

[BillyShope]

Thanks, guys. I've got some numbers:

Assuming an all up weight of 3250 (yeah, that's light) with 250 in the axle assembly, the lower link horizontal, the CG height 24, and a 4.11 axle, the offset comes out to 14.6. This is quite reasonable and much less than I feared. With a 3.08 ratio, it's still only 19.5 inches. The IC is a little closer than I would have liked, being only 38.75 ahead of the axle with any ring and pinion.

Still, everything's within reason and it looks like I could help some kid with his drag Mustang...if I'm ever asked.

 

[Sundance7]

Bill, I'm trying to design a three link suspension for my 65 Mustang. So far all those I have talked to say it would require severely invasive modifications to the interior to allow enough room for the upper link. However, the 05 Mustangs geometry will fit within the confines of the drive train "hump" without major modification to the interior.

When I mentioned the use of the 05 geometry or similar, it was not received well by the group in which I presented the idea (another forum).

Any help you or others could provide would be greatly appreciated!

 

[Sundance7]

No edit sorry...

My thought if it was not clear, was to fabricate and test your design.

 

[BillyShope]

If you can get your hands on the student workbook which accompanies the Millikens' Race Car Vehicle Dynamics, it contains my equations for a 3link which provides equal rear tire loading and no squat or rise. You can get it through Amazon, of course. (No, I don't get a percentage.)

(The thrust loss due to acceleration of the the rear axle assembly is ignored, but this introduces a relatively small error.)

 

[Sundance7]

Bill, The workbook has been discontinued. I haven't exhausted all my resources yet, hopefully I can track one down used.

 

[BillyShope]

Okay, I'll try to type the equation into this post. Actually, this is an improved version of the equation I supplied for the workbook, as it takes the mass of the rear axle assembly into consideration.

the tangent of the link angle(s), as measured at the rear mounting point(s), positive up from the horizontal:

h/l-KR(1+m/M)/yG

where (h/l) is the ratio of CG height to wheelbase, "m" is the mass of the rear axle assembly, "M" is the remaining mass of the car, "y" is the offset of the single link, "R" is the tire effective radius, and "G" is the axle ratio.

"K" has a value of unity for the symmetrical links and a value of:

(mR + Md_o)/(mR+Md_s)

for the "odd" link. "d_o" is the vertical height, at axle centerline, of the single link, and "d_s" is the same for the symmetrical links.

(Note that the odd link can be either above or below the symmetrical links. For a tubbed dragstrip car, the lower position is more easily implemented.)

Hope that's clear and I haven't made any typos. Wish you could have found the book. I'm surprised it was discontinued.

 

[Sundance7]

Bill, sorry it's been a while. Thank you for the equations.