Lesser steering effort at Higher vehicle speeds 1

[senjim]

Hi all..
I've come across the performance rating of a minibus (7.5Ton GVW). This is having integral power steering which is reported to have higher steering effort (about 5Kg with a 450mm dia stg wheel) at lower speeds (say 20-30kmph) and a very low stg effort(Controlling of the vehicle becomes difficult-feeling like the vehicle changes the lane on it own) at higher speeds (above70kmph).

Vehicle having a castor of 1.6deg in static. I suspect that the castor is getting reduced at higher speeds due to suspension spring (leaf) winding causing vehicle instability.

Can anyone thro some light to understand and define the problem.
Thanx in advance for your comments

Cheers
Sen

 

[GregLocock]

More likely to be due to pneumatic trail migration due to speed, and aerodynamic yaw instability, which also gets worse with speed, as the centre of pressure moves forward.

Cheers

Greg Locock

 

[GregLocock]

Oh, and of course the steering wheel angle for a given lateral g drops as speed increases, so the sensitivty of the vehicle increases. This can be corrected by using speed sensitive power steering, or careful tuning of the torsion bar and boost curve.

Cheers

Greg Locock

 

[senjim]

Thanx..
But is this phenomina applicable only to power assisted steering??? Can i quantify the effect of aerodynamic instability by running the bus at higher speeds with steering power assistance disconnected mode??

Cheers
Sen

 

[GregLocock]

Yes, of course.



Cheers

Greg Locock

 

[senjim]

Thanx Mr.Locock. I'll let u know after few trials

Cheers
sen