Power Steering Rack Ratio???

[Duk]

Does anybody know the actual gear ratio of the power steering rack?

I've had an idea about making a quicker steering rack, but need to know what the actual ratio is for the Alfa rack.

[Duk]

Found it (maybe): http://www.alfabb.com/bb/forums/milano-75-1985-1993/42086-gregs-anti-power-steering-conversion-momo-wheel-2.html#post366561 "rack is about 18:1 steering ratio..........."

Another question, does anybody have a good photo of the actual rack? Or does anybody know for sure that the rack is a constant ratio?

My thinking is that I may be able to make fit the rack and pinion from a VN onward Commodore, but they are a variable ratio rack and pinion: "17.2:1 on centre to 11.8:1 on lock." http://www.uniquecarsandparts.com.au/holden_commodore_VN_technical_specifications.htm Not sure I'd want that, but I believe the Commondor racks are of ZF origin, so am thinking that making the guts fit into the Alfa rack housing could be reasonably straight forward............ 

[scott.venables]

I've often wondered what the ratio meant so I just read this on Wikipedia:

The steering ratio, is the amount of degrees you have to turn the steering wheel, for the wheels to turn an amount of degrees.  In most passenger cars, the ratio is between 12:1 and 20:1. Example: If one complete turn of the steering wheel, 360 degrees, causes the wheels to turn 24 degrees, the ratio is then 360:24 = 15:1 (360/24=15).

So won't you need to compare the length of the steering arms as well as the ratio? 

I'm pretty certain the 75 rack is constant ratio but I hadn't considered it might be when I had mine apart. Here's a picture of the rack assembly and another of the pinion.  I have some more photos of the pinion assembly/spool valve if you want.

[colcol]

One of the more interesting jobs to do on a car, power steering is interesting, it actually used to frighten me, so i decided to fix one on the 33, never knew what the ratio actually meant, thanks for posting it,......is that a Mini in the background, Duk?, Colin.

[Duk]

I've often wondered what the ratio meant so I just read this on Wikipedia:

The steering ratio, is the amount of degrees you have to turn the steering wheel, for the wheels to turn an amount of degrees.  In most passenger cars, the ratio is between 12:1 and 20:1. Example: If one complete turn of the steering wheel, 360 degrees, causes the wheels to turn 24 degrees, the ratio is then 360:24 = 15:1 (360/24=15).

So won't you need to compare the length of the steering arms as well as the ratio? 

I'm pretty certain the 75 rack is constant ratio but I hadn't considered it might be when I had mine apart. Here's a picture of the rack assembly and another of the pinion.  I have some more photos of the pinion assembly/spool valve if you want.

Yeah, the length of the steering arms will be a factor, that's for sure. But if the amount of rack movement for a given rotation increases, then the steering rate will be faster.
The ratio is depends (amongst other things) on the angle of the rack teeth and you can see this in 1 of the pictures on http://www.jagtas.org.au/torque/techtorque/mk-2-steering-notes/ showing a VL Commodore variable around centre, rack.

[scott.venables]

That variable ratio rack is really something.  I've looked at the pictures a lot and I can't figure out how the mesh and lash kept within reasonable limits along the whole traverse.  I'd love to have a good look at one.

If the 75 housing can accomodate a bigger pinion, a batch of fast ratio rack/pinion sets might not be ridiculously expensive.

colcol, that's my Dad's Clubman GT which he's owned since 1978. On the other side is his 1930 Oakland V8

Cheers, Scott