Rear IRS Uprights

[Kenm4187]

I am trying to get some information on the rear IRS uprights. Can anyone tell me if they are a custom Superformance part or if they were made for something else? My car is SPO 2560.

Thanks
Ken

[CWizard]

The uprights for later cars, including yours, are custom made by Superformance. Are you looking for a replacement?

[Kenm4187]

No, I am try to correct the geometry angles. I am looking for something that would be like a drop spindle in the front. I autocross and the tires are shorter and the car lower so the angles and roll center are very bad. I may end up having some made. I was hoping they were from a factory car so there might be some more options.
Ken

[mickmate]

How much do you need to alter the ride height? We looked into this with another hot rod project and found the suspension geometry so good all we needed to do was alter the height. I think you'll find you have plenty of rear height adjustment with the bolt on shock mounts. Have you checked the geometry and camber change with the SPF arms? I think you'll find it is pretty good.

[Kenm4187]

Originally Posted by mickmate

How much do you need to alter the ride height? We looked into this with another hot rod project and found the suspension geometry so good all we needed to do was alter the height. I think you'll find you have plenty of rear height adjustment with the bolt on shock mounts. Have you checked the geometry and camber change with the SPF arms? I think you'll find it is pretty good.

I can get the ride height down where I need it but then the geometry is off, the rear roll center ends up under ground. Then during cornering the roll center gets even worse. The control arms are so short any movement is amplified. I already did this on the front (2' drop spindles and 3" longer control arms) what a nice improvement. If I could get at least a 1-1/2" drop in the rear upright it would be a big help. I am running 335/30/18 and they have a diameter of 25.6". The stock tires have a 28" diameter Big difference.

[vatdevil]

Originally Posted by mickmate

How much do you need to alter the ride height? We looked into this with another hot rod project and found the suspension geometry so good all we needed to do was alter the height. I think you'll find you have plenty of rear height adjustment with the bolt on shock mounts. Have you checked the geometry and camber change with the SPF arms? I think you'll find it is pretty good.

Mickmate,
Can you share data on SPF geometry ?

[vatdevil]

Originally Posted by Kenm4187

I can get the ride height down where I need it but then the geometry is off, the rear roll center ends up under ground. Then during cornering the roll center gets even worse. The control arms are so short any movement is amplified. I already did this on the front (2' drop spindles and 3" longer control arms) what a nice improvement. If I could get at least a 1-1/2" drop in the rear upright it would be a big help. I am running 335/30/18 and they have a diameter of 25.6". The stock tires have a 28" diameter Big difference.

See link on ride height settings. Optimum SPF height setting is roughly based around a 26 inch tire.
Complimentary S.C.O.F. Guest Technical Library Preview

[mickmate]

(28-25.6)/2=1.2" drop at stock control arm positions. That's pretty close to the 1-1/2" you're looking for. 28" is a big tire to start with, most of the 15's are around 26.5" dia. I haven't measured the SPF arms and put the data into CAD but they look good eyeballing them with the lower arms being pretty level at ride height and minimal camber change on an alignment machine. The rear uprights are fabricated parts and could be duplicated (or modified) with different height upper and lower brackets but you would want all your measurements off the car and a known good roll center height to draw and then build them to. That article tells why you shouldn't measure fender heights and then lists all the setting by fender height............??

[Mark IV]

The SPF rear uprights are casting that are custom made for the car. The arms are indeed fabricated pieces that can be modified or duplicated with the desired modifications.

[Kenm4187]

Originally Posted by vatdevil

See link on ride height settings. Optimum SPF height setting is roughly based around a 26 inch tire.
Complimentary S.C.O.F. Guest Technical Library Preview

I have that chart.
For 18" wheels it list the optimum (fender to bottom lip of rim) front at 24" and rear at 23.94". My front is currently 22.75" and my rear is 23". I need to go lower in the back but if I do the geometry and handling gets bad. The shocks have been replaced so they are not bottoming out.

[Kenm4187]

Originally Posted by mickmate

(28-25.6)/2=1.2" drop at stock control arm positions. That's pretty close to the 1-1/2" you're looking for. 28" is a big tire to start with, most of the 15's are around 26.5" dia. I haven't measured the SPF arms and put the data into CAD but they look good eyeballing them with the lower arms being pretty level at ride height and minimal camber change on an alignment machine. The rear uprights are fabricated parts and could be duplicated (or modified) with different height upper and lower brackets but you would want all your measurements off the car and a known good roll center height to draw and then build them to. That article tells why you shouldn't measure fender heights and then lists all the setting by fender height............??

My point with the tire diameter is that with the larger tires the car is higher in relation to the center of the wheel to get the required clearance. If you take a 28" tire off and put on a 25.6" tire the rear geometry does not change (control arm angles), but if you then start lowering the car because you do now have the tire clearance the geometry gets bad. Also measuring from the fender to the lower wheel lip works very well, but measuring from the fender to the ground does not.

The ride height chart is showing an optimum of 23.94" from the fender to the lower lip of an 18" wheel. I am trying to get to at least 22.5". The choice is raising the control arm attachment points on the car or modifying the IRS upright. My hope was that the upright may have been from a Mustang or another vehicle that someone had been making aftermarket parts for. For my fronts I was able to order Wilwood front spindles with a 2" drop.

[Kenm4187]

Originally Posted by Mark IV

The SPF rear uprights are casting that are custom made for the car. The arms are indeed fabricated pieces that can be modified or duplicated with the desired modifications.

Thanks for the picture I have not had a chance to get my car back in the garage and pull them off.