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Hydraulic Power Steering Racks differences?

Hydraulic Power Steering Racks differences?

Hydraulic Power Steering Racks differences?

(OP)
I'd like to add a GM power steering rack on '88 Lotus Esprit turbo (2900 lb gross weight).
I'm looking for a suitable part which has similar((+/-) attachment spread/points to the chassis.
I found several potential donors, but have difficult time to find a right attachment spread.
One of the candidates is a Silverado pick-up truck (6000 lb GW), which seems to fit well.
I'd like to confirm,
that perhaps a truck stock rack may be "too strong" (different valving)for a passenger car.
Should I rather look for a Regal of Bonneville part, instead?
Thank you

RE: Hydraulic Power Steering Racks differences?

This is not a double post of the magnasteer one.

The amount of boost in an HPAS is governed by the system pressure, the piston diameter, the C factor of the pinion to rack, the valving of the T bar, and the stiffness of the T bar. A truck rack is likely to have a big diameter, low C factor, and different valving and T bar stiffness to a car's rack.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Hydraulic Power Steering Racks differences?

GM pickup trucks that I've driven - granted, older ones - have ultra-light super-over-assisted, steer-with-a-pinkie-finger power steering with zero, and I mean ZERO, perceptible road feel. It feels like the steering wheel is mounted on a ball bearing and connected to nothing else; that the truck magically changes direction when the steering wheel is turned is some sort of remarkable coincidence in the universe. Video-game steering would perhaps be another way to describe it. Is that what you want in a sports car? If the rear swings out while cornering, you're going to be guessing how much to counter-steer to catch it, because you won't be able to feel what the right amount is.

My van has pretty lousy steering feel, too, but I don't drive it hard. And, being front wheel drive with lots of built-in understeer, there's not much need for having to catch a slide.

RE: Hydraulic Power Steering Racks differences?

What you need to know to add HPS to any vehicle is equivalent tierod loads per Ayg, C-Factor (rack displacement per 360 degrees), pump flow rate and max parking loads. Then you need to decide what effort/work RATES you want in order to get the best 'feel' for the amount of steering gain your sled has. Adding caster makes things worse in general because this is a 'false' gear loading signal (it arrives late because of friction and strut mount compliances & preloading). Then you need to pick an intermediate shaft with the proper impedance to achieve the best transmissibility. A good set of tires with fast relaxation AND Mz relaxation characteristics that are coincidental to Fy relaxation will make your project worthwhile. (BMW's Continentals from a 5 Series [the Star construction] have the best properties out of thousands I have ever measured.

Finally you need a pump that's built for the job, too. If it were up to me, I'd pull parts from a BMW 5 Series and stuff them in. [The lines with tuned dampers, too].

RE: Hydraulic Power Steering Racks differences?

(OP)
Thank you all very much for your valuable opinions.
After investigating hundreds of them, I selected a potential donor: 94 Buick Regal rack.
It has the most acceptable attaching mounts (slight rework required), and is comparable dimensionally. Also it has the same pitch and range and uses the same rod ends as my old manual part. All specs can be seen on Rockaut0 website when you click on the blue "Info" button next to the part. Terrific research tool.

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