Steering rack location

[GT6Steve]

Friends,
I was recently reminded of a discussion where a fellow went on at great lengths about all he had to do to put his steering rack behind the front axle centerline rather than in front as delivered. Now as I read and ponder the mysteries of suspension and handling I keep coming back to the WHY of the above.
Other than packaging, is there any intrinsic advantage to a rear steer rack over front steer? Is gyroscopic precession a factor? I don't see any geometrical difference between the two.
Thanx in advance, Steve

 

[ZGuy]

There may be other considerations, but the first that comes to mind is possibly the desire to have Ackerman affect. The is much easier to do with the rach and hence steering arms aft of the front axle.

The need or desire to have partial or full Ackerman is a whole different discusion! ;-)

Tom Higgins

 

[GregLocock]

For production cars there are significant advantages for each location, but really it is driven by packaging as much as anything. I don't think the Ackermann argument is compelling as it is fairly easy to move the rack longitudinally to get Ackermann.

The big argument in favour of a forward rack is that any compliance in the lateral arms in line with the wheel will contribute understeer rather than oversteer.

I agree with the OP, I can't see a gyroscopic advantage




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Aday]

I agree with Greg that the key issue is a rack ahead of wheel center promotes deflection understeer and rack behind wheel center will tend to have deflection oversteer. I would add that a rack behind wheel center does have two advantages 1)often allows a tighter vehicle turn radius by allowing higher maximum steer angles. 2) Allows you to design in the proper Ackermann correction without moving the rack much out of line with the outer tie-rods. If the rack is significantly forward or back of the outer tie-rod joint, steering forces will introduce an undesireable fore-aft force into rack.

aday

 

[SusTestEng]

Personally I like a front steer setup, due to the understeer as Greg described.
But another big reason I like it has nothing to do with dynamics. I like the fredom of routing the exhaust in a more free flowing manor. When the rack is behind the wheel center, you always have to find a way to route the exhaust around it and make sure it doesn't get to close to the inner tierod boots(heat damage). This usually results in a comprimised series of odd angles and massive head losses due to the amount of bends needed to position the exhaust in such a confined space. It can also limit your front legroom by a bit.