Castor angle limitation

[senjim]

Hi all
I'm having a minibus with a front axle load of 3tons. (power assister steering). The castor angle was 3deg and i had lot of problems like the steering was very light (floating like feeling) at high speeds & poor self centering. Now i've increased it to 6deg & now the steering grip is better at high speeds & the self centering is also very good.
Is there any adverse effect in having a 6 deg castor? (The vehicle has a rigid axle front axle beam with reverse elliot type stub axles)

Cheers
Sen

 

[NormPeterson]

Changing static caster will affect the static toe setting slightly, so did you reset the toe as part of this change?

You've also altered the bumpsteer. But whether this consequence is adverse, benign, or even beneficial cannot be determined without more information.

Norm

 

[senjim]

Thanx.....

our vehicle has 0mm toe setting. hence i think tht there won't be any toe variation by altering the castor..
But is this 6deg per say is ok?? what will be normal castor in LCVs with rigid front axle beams

sen

 

[NormPeterson]

When you change caster, the whole steering knuckle assembly rotates (as seen in side view). This means that the ends of the steering arms must move vertically. Since the rack or centerlink is presumably supported by the chassis which has not moved, the tierods must adopt slightly different inclinations (as seen in front view). If you don't change their lengths, dimensional consistency requires that the lateral positions of the outer tie rod ends move to accommodate the change in relative vertical positions.

The above toe effect is small, perhaps under 0.1* of total toe, so you may well still be within tolerance.

Bumpsteer is the dynamic version of the above geometric description, where the relative vertical separation of the inner and outer tie rod ends varies as a consequence of suspension movement. You'd most easily feel this from the driver's seat as changes required in the amount of steering input to maintain a precise position and heading within your lane as you encounter a slight pavement 'heave' within a turn taken at speed.

Norm

 

[manic59]

I would think that if you had the caster adjusted on an alignment machine that the person making that adjustment would also correct the toe accordingly as part of the procedure. You are on the right track. If you had no power steering then the lower caster would be correct,but with power steering you need a more stabilizing force. You get that when you increase the caster angle. If you like,you may increase it further to more stabilize the system if you have the adjustment available. Do not increase it to a steep caster angle if you are doing alot of intown driving or you may have a premature tire wear problem due to the excess lean in turns caused by the high caster abgle. Good luck!!