caster on AWD cars

[Serge]

a long time ago, it was taught to me that on a FWD car, since the front wheels are doing the pulling, less castor is best, up to even zero

makes steering a bit easier too

for rwd cars, since the car is being pushed from behind, more castor is better (shopping cart argument)


caster is generally good in cars with poor dynamic camber curves, but if you can fix that problem through other means (LCA length, balljoint extenders, and angle of strut from spindle), then you wont have to rely on caster to give you some camber in a corner


so what is the desired setup of castor in an AWD vehicle

reading various forums it seems everyone wants more caster.. but whats the rational?

also, would this be depended on what kind of a center diff you go? From what i understand my 2001 RS doesnt really transfer a heck of alot of power to the rear ... esp once the center diff heats up on the track :)

comments?

[JoeT]

Stock caster on the 2.5 RS is around 3 degrees, rule of thumb is similar to RWD cars. The Mac Struts setup has a pretty poor camber curve and it needs all the help it can get.

Most time attack guys in the usa actually almost try to set their static camber pretty steep, aka around 3.5 degrees and heavy duty spring rates to prevent loss of camber during compression.

Back to Caster, the more the better to help the car.

BTW: The center diff on your RS is a viscous coupling type, which actually gets more viscous when hot (to a point). It does not transfer power to the front or rear, rather it controls slip similar to a Viscous LSD. Picture the center diff as an open differential with spider gears, the difference is, the Center diff applies friction to the slipping side to try to transfer some torque to the non slipping side. It can be overpowered, but if you're not slipping, unless you're in a WRX with a 1.1:1 center diff ratio, it'll be 50/50 all the time. You'll notice it on the trak.

Subaru's understeer like fat pigs, it's easy to drive fast, but not easy to drive at the limit.

[Robin]

yup... i gained a bit of caster when i used camber plates with my hotbits coilovers.... on 225-50-15 ra1/r888 15x7 wheel (45 offset), i get about -3.5 front camber, and about -2.7 rear camber..... 0 toe all around.

I think i gained a bit of caster around 3.5 but I'd have to look for the last time i did an alignment on the machine...

I do my own alignments at home (old string / digital caliper)...

this is all on a 1999 impreza rs.