Steering rack Retrofit

[sethmal]

Hi all,
I'm an interloper here, more of an electrical engineer, but had a chassis/steering question. I'm planning to retrofit a rack and pinion system into a chassis that had a steering box originally. The rack is needed because of space constraints due to an engine change. I expect to lose some travel, but are there other considerations I'm missing? Assuming I'll have the rack made to have the nearly the same inner pivot points(not sure I'm using the correct terminology here) and position it in the same plane as the track rod. After speaking with the custom manufacturer, because of limitations, the width of the inner pivot points on the rack may be 2.5cm wider on the rack than my existing track rod/control arms. Will this difference cause excessive bump-steer?

To increase travel, could I reposition the tie-rod location on the steering knuckle without moving the rack?

Thanks for your help!

 

[GregLocock]

"Nearly the same" is not good enough. The tolerance on inner tie rod vertical location is about 1mm on a modern car.

Changing the tie rod length by 12.5 mm is probably acceptable.

"To increase travel, could I reposition the tie-rod location on the steering knuckle without moving the rack?"

Yes, but that'll change the Ackerman, and alter all your other curves as well.

What you are proposing is not especially difficult, but it would be worth looking at the locost message boards to see how much of this steering geometry you can do. Your aim, if you have no other preferences, would be to ensure that the ackerman and bump steer curves and max lock remain the same as with your current setup.

This may give you a new steering arm length in which case you might want to change the gear ratio of the rack.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[dynatune]

Sethmal,

if you can try to make the rack as long as it needs to be. 2,5 cm is quite a significant change and most probably this will get your bumpsteer into trouble. But it depends on how your original kinematics were designed. If the car did have relatively long track rods and did have for instance a lot of bumpsteer there might be room to play. Still you need a kinematics program to be sure.

Cheers

Dynatune,

http://www.dynatune-xl.com

 

[cibachrome]

There will be quite a few issues with your change to R&P steering.

With regard to the ride/roll steer (I dislike the 'bump steer' terminology, its a 'bum steer'...

The front view elevation angles of the tie-rods define the amount of ride and roll steer your suspension has. If the outer tie-rod end is higher than the inner, you have understeer, etc. The length is also important, That defines the sensitivity (derivative) of your ride/roll steer functions. They are usually pretty nonlinear, so resetting the ride static height will need to be considered. A wheel aligner and a pencil and paper can be used to plot this curve. Getting it wrong can wear out a set of tires pretty quickly.

A R&P gear is also usually considered to be a 'light-weight' mechanism. They are not generally strong enough for ordinary misuse (curb hits, pot holes, jumping driveway ramps, zero speed parking maneuvers.

Moving the outer ball along the steer arm will change the overall steering ratio of the vehicle. You can change a 16:1 set (R&P + steer arm length) to a 10:1 pretty easily. THAT vehicle will be horribly unmanageable at high speed but fun to park if you have enough hydraulics. Having a log splitter in the trunk may be necessary.

Speaking of which, the hydraulic valve profile and pump flow rate will pretty much define your effort vs. lateral g cornering relationship. Get that wrong and you will again be either squirrely at high speed or hard to park.

Recirculation ball gears (as you call a box gear) usually have quite a bit of compliance when subject to tie-rod loads. This is taken into consideration when the overall handling of the OEM production vehicle is defined and designed. A R&P gear is usually quite a bit stiffer mechanically (less compliance). This will dramatically reduce the understeer of the vehicle with a corresponding increase in the steering gain (g/100 deg-SWA).

The gear mounting system you choose will also be important. Solid mounting will get you additional compliance understeer reduction but will risk rack damage and fracture. Jumping a tooth is common for these misuse conditions. Rubber mounting gets your understeer and durability back with rubbery steering feel (from the rubber !!).

Yes, there are long rack, hollow rack, strong pinion, equivalent gear types, but the valve profile, the pump flow rate and the plumbing MUST be factored into your design unless it just for show.

A Corvette, Camaro, or Trailblazer gear and pump set comes to mind. The Trailblazer/Envoy gear is one of the longest for rack travel I've ever seen. Its beefy with wide mounts. Trailblazer is a TRUCK. Some of the newer lightweight load pickups now have good R&P gears.

 

[sethmal]

Thanks all for the great advise. I've shelved the idea of a rack retrofit for the time being. I'll just work around the space constraints of the steering box. When looking at the space available, a rack may introduce just as many problems, just in a different area. For reference, the car is a 1971 BMW Bavaria (E3), it uses a mac strut front suspension with fairly long control arms. I'm installing a V8 from a much newer BMW and there are tight clearances for the exhaust around the steering box area.

Thanks again for the help