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rear axle upper bushing

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Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
Did you consider going with a spherical bearing in place of the rubber bushing on the diff end? I don't like poly on both ends. It tends to bind, but poly on the front with a spherical bearing on the rear works well.
Even poly on one end will bind to some extent; my guess based on beam formulas puts the amount of 'bind' in a poly/spherical UCA at about half that of a UCA with poly at both ends. The short UCA is more severely affected partly because it is short and partly because cornering roll wants to skew the UCA relative to the car's centerline as seen in plan view. The UCA is located several inches above the PHB-defined roll center, so there has to be some relative lateral movement involved.

Poly/spherical LCAs are less affected on both counts, roll motion being essentially 'vertical' at the LCA chassis-side pickups, which joint rotation about the bushing sleeve can accommodate without binding.

Seems there has to be a better way of handling the UCA chassis-side pivot function than either cylindrical poly or rod ends, with at least somewhat better axle location than OE rubber.


Norm
 
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xr7

TMO Addict?
708
823
Exp. Type
Autocross
Exp. Level
10-20 Years
Minnesota
I'm going with the BMR bushing in the diff and using the Metco upper trailing arm. Picked up the Metco part from American Muscle, 1/2 off deal.
 
1,119
1,110
Exp. Type
HPDE
Exp. Level
5-10 Years
Lenoir City TN
Even poly on one end will bind to some extent; my guess based on beam formulas puts the amount of 'bind' in a poly/spherical UCA at about half that of a UCA with poly at both ends. The short UCA is more severely affected partly because it is short and partly because cornering roll wants to skew the UCA relative to the car's centerline as seen in plan view. The UCA is located several inches above the PHB-defined roll center, so there has to be some relative lateral movement involved.

Poly/spherical LCAs are less affected on both counts, roll motion being essentially 'vertical' at the LCA chassis-side pickups, which joint rotation about the bushing sleeve can accommodate without binding.

Seems there has to be a better way of handling the UCA chassis-side pivot function than either cylindrical poly or rod ends, with at least somewhat better axle location than OE rubber.


Norm

By your calculations how does the rubber bushing on both ends compare to the poly/poly or poly/spherical?
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
By your calculations how does the rubber bushing on both ends compare to the poly/poly or poly/spherical?
Somewhat crudely, I'm getting something like 1200 in*lbs/degree of roll with 200E3 psi polyurethane modulus (I think polyurethane can run as high as 400E3 psi). Rubber is maybe 5% as stiff, not counting the presence of intentional OEM voided bushing technology that would soften it still further. 1200 in*lbs/° works out to something like 19 lb/in reflected out to wheel rate. IIRC, that's about as much difference as a one-hole adjustment makes on an adjustable rear sta-bar. 19 lb/in at the wheels vs 1 lb/in, hmmmmm . . .

Years ago, Ehren VanSchmus ran a series of tests for various combinations of LCA (LTL?) pivot details for the Fox/SN95, which ought to provide a reasonable qualitative comparison. As ballpark comparisons, rubber bushings were worth 5 lb/in wheel rate and poly/poly ran that up to 62 lb/in. Of course, this is for a pair of LCAs rather than a single UCA, perhaps balanced a bit by the UCA being shorter. At least this appears to work as an order-of-magnitude sanity check.


Norm
 

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