Re: Drop pitman arms and bump steer

["T.R.E. Jr." <T_R_E_Jr@xxxxxxxxxxx>]

---I don't quite get the bump steer concept myself. I know if you raise the 
distance between the steering knuckle and the pitman arm then you need to 
lengthen the drag link that connects them. I also know that when you lift 
the vehicle and change the caster, you need to correct the caster. Lastly, 
when you lift the vehicle and change the angle of the drag link's rod ends, 
you are sacrificing the angle that the joint can travel and taking a chance 
on binding the joint if you articulate beyond the joint's ability to move. 
So, when the vehicle is lifted, you need to bring the caster back to 
correct, you need to justify the angle of the drag link for the sake of the 
rod ends and you need to bring the pitman arm and steering knuckle to a 
level plane, place the wheels in the correct position for alignment and 
measure the distance between the steering knuckle and the pitman arm. This 
to me seems to be what everyone refers to as bump steer, a combination of 
changes to the steering without the correction of one or more.

---Anyone care to drill a hole in my head and fill me with information?

---Thank you,
-T.R.E.Jr.
-`73 Scout II (StoneThrower)
-`51 Farmall H (Heinz)
-`49 IH fridge (presently unnamed and in need of a compressor)
-`49 Plymouth Special Deluxe 4-door Sedan (Papapalooza)


----- Original Message ----- 
From: "joel brodsky" <travelall1974@domain.elided>
To: <ihc@domain.elided>
Sent: Thursday, July 03, 2003 2:51 AM
Subject: Drop pitman arms and bump steer


> Okay, this is now the 6th Lift I've put in a truck of
> mine, and guess what?  Well, I purposely left the
> front shocks off so that I could more easily
> demonstrate bump steer.  I went over a speed hump in
> my neighborhood, the kind that you can confortably go
> about 15 over without any jarring at all.  As I
> traversed the hump while holding the steering wheel
> straight, the front end of the truck went down/left
> and up/right in the oscillations.  This is clearly
> bump steer.  The reason eludes me though.  It would
> seem that the reason is that the drag link, while
> being held still at the steering box, tries to get
> longer in horizontal distance from the PS box as it
> subscribes the arc traveling upwards.  As this
> happens, it pushes the right front wheel out to make a
> right turn.  then the rebound is the other way, up
> gives a turn to the left.  But, the actual bump steer
> is just the opposite, down and LEFT, instead of the
> explano of down and right.  Anyone?
>
> So, the solution can only be a dropped pitman arm.
> Now, with the drop and stock arms, the difference is
> 2.5".  The stock pitman arm radius is 9.5" and the
> drop is 1.5".  The droped arm is 7" radius and 3.5
> inches of drop.  So will the 2" help with the bump
> steer?  I certainly hope so.  Will the steering be
> affected?  Yes.  Will it matter?  I'm not so sure.
> The stock steering arm at the knuckle is 9.5" making
> the ratio of steering input to wheel reaction
> 9.5"(input)/9.5"(output), thus it is considered to be
> one to one.  The new setup is 7"/9.5" or .736842.
> Thus it runs about 3/4 as sharp as it used to.  So,
> what does this mean to me/you/anyone?  Well, if the
> turning circle was right at 50 feet before, now it's
> 35.7% bigger.  Remember that to get to 100% from
> .736842 it's 1/.736842 so that 1.35714, or 35.7% more.
>  So, that sounds like it sux bad!  Anyone have numbers
> on steering radii for Scout 80, 800, ScoutII,
> Traveler, PU, TA, Travelette, etc.?  I'm wondering if
> my turning will even be close to a Scout 80, which is
> atrocious on the trail anyway.
>
> Better a wider turning circle than bump steer, I
> believe.  I'm going to try the drop arm this weekend
> and report back.  I have been running bump steer for
> about 8 years now and no wonder I've thought these
> trucks handled poorly.  Now that I understand what's
> going on, I can believe that it's this bad!
>
> Bronco II or not, there ought to be a way to get a
> longer radius pitman arm, with about 4" of drop in it,
> to correct a Scout steering to a place where it can
> compare to the stock numbers.
>
> Any ideas anyone?  Now, why again shouldn't I heat and
> bend an stock arm to do the tests?
>
> Thanks for your support,
>
> JoelB
> -time to get some shut-eye
>
> __________________________________
> Do you Yahoo!?
> SBC Yahoo! DSL - Now only $29.95 per month!
> http://sbc.yahoo.com