Panhard Link Adjustment Methods

[JC3]

People keep saying that laydown chassis mounted servos cause "excessive bumpsteer and weird steering issues." I don't think so. I think that laydown or standup, it's the panhard link adjustment that causes excessive bumpsteer and weird steering issues.

So for either laydown or standup CMS, What steps do you go through to properly adjust your panhard link?

LIST YOUR STEPS ONLY! THIS WON'T BE A STEERING GEOMETRY THEORY DISCUSSION, JUST A LIST OF THE STEPS YOU TAKE TO ALIGN OR ADJUST YOUR PANHARD LINK.​

(Steps below are done after installing the servo in it's final position.)

Let's keep the format like this example. Just the STEPS and NOT YOUR TAKE ON LAYDOWN VS STANDUP:

• Install all links and get things roughly aligned.
  
• Adjust steering and panhard links to be parallel to each other.
  
• Mount shocks and tires.
  
• Check shocks for binding at full droop and extension.
  
• Put the tires on.
  
• Set rig to desired ride height.
  
• Measure the distance from outside of chassis to some point of reference on both sides of the chassis. (I use the inside tire sidewall, a tire lug, or mark on the tire.)
  
• Calculate the difference between the two sides and adjust the panhard link length half of what the calculated difference is in the desired direction.
  
• Repeat until the chassis is centered between the wheels at ride height.
• Verify free movement of suspension and check for bumpsteer.

 

[DrIsotope]

I do all of your stuff, in concert with my old-skool technique of just using a turnbuckle for the panhard. Sometimes it's like 1mm of length that can make a visible difference.

I have a TRX-4 with "normal" servo orientation, and another with a laydown servo, and their suspensions cycle exactly the same. I think excessive bumpsteer finds its roots in the design of the axle-- notably the panhard mounting point itself. Some axles are just easier to tune than others.

 

[HumboldtEF]

Make sure to also turn your wheels to full lock in both directions and cycle the suspension 1 side at a time when checking for bump steer.

A little while back I was prototyping a laydown servo mount for the Sherpa and I honestly couldnt find a good setup and I tried many different positions for the servo and different lengths and mount points for the panhard. I think it came down to the axles (Ar44) and its panhard mount point, I know folks have had better luck with Capra axles and Ar45's.
With the wheels pointed foward it was fine, no bump steer at all. But when I tested with the wheels turned it was a different story. Now It probably could have been drivable but I just wasnt liking it, I was already using a LP servo so the difference in the CG would have been minimal anyway.

 

[ferp420]

when a steering servo lays down the servo horn moves up and down aswell as side to side its a lever its what they do anyway the steering link mounting point will move up and down about a half inch your links might be parallell when the wheels are straight but when you turn full lock there way off causing bump steer
the knuckles can minimize this by using short arms so the servo horn dosent traval as far but then it puts alot more strain on the servo and parts
with a stand up position your secondary motion will be front to back not up and down so your geomitry wont change as.the servo cycles through its range of motion your links will always be parallell and dosent need as strong a servo
not debating whats better personally i dont think its.that big a deal on a crawler the rear axle also plays in to bump steer alittle too so there is no way to eliminate it completely the best you can do is minimize it

now there is a way to make the 2 bump steer motion cancel each other out but i dont have a engineering dagree so i couldent tell you how

 

[JC3]

I do all of your stuff, in concert with my old-skool technique of just using a turnbuckle for the panhard. Sometimes it's like 1mm of length that can make a visible difference.

I have a TRX-4 with "normal" servo orientation, and another with a laydown servo, and their suspensions cycle exactly the same. I think excessive bumpsteer finds its roots in the design of the axle-- notably the panhard mounting point itself. Some axles are just easier to tune than others.

I agree that sometimes it's just a tiny amount that is causing the the problem. I think the turnbuckle is essential for tuning out that troublesome 1mm.