Bump Steer Improvement Options

[gearheadE30]

Correct me if I'm wrong, but isn't anti-Ackerman unavoidable when the steering linkages are ahead of the axle line?

What profile are your 275s?
I have 245/65-17 and am thinking of going to 265/65-17. Good idea or bad idea?

Couple of steering issues I have with my C2500 is steering shimmy under braking - but usually only at very low speed - just before coming to a stop.

And a slightly weird feeling at the wheel on fast sweepers where it feels as if the tyre contact patch moves relative to the steering axis at a certain cornering force. Like it tucks under and/or rearwards making it feel slightly 'over centre'. (I have the speed sensitive (EVO?) steering).

I've had a good poke around and kind find anything amiss and it's just been through it's annual (UK) safety inspection where it got a pretty thorough going over. As it sits - by eye, without measuring - camber is slightly positive. Something amiss or are those things just what these trucks do?

It's possible to avoid anti-ackerman, but not with a bellcrank/drag link setup like gmt400s have. It is possible to minimize it a bit more effectively than GM did, though.

Mine are 275/55R17. There shouldn't be anything wrong with going wider, but it does make tramlining and such more of an issue.

The camber spec on these is very neutral so slightly positive can be in spec. There are punchouts in the frame that need to be removed to do a proper alignment. Since these were from the era of trucks being trucks, tire wear and preventing rollovers were more important than ultimate grip and handling prowess.

You should never have an 'over center' feeling, especially not outside of a parking lot where you use a lot of steering angle. EVO can have an effect here, as can worn bushings and ball joints. There is a lot of rubber in the control arm bushings; they can deflect a lot even if they don't appear to be in bad condition. EVO can cause little repeatable steering effort ripples if the radius of that corner happens to be between two of the encoder 'teeth'. I can sit still in my K1500 that still has EVO and move the wheel slightly and make it do this all day long. I don't know if that's normal, just that my truck does it. In the K1500 I ignore it. The Tahoe is intended to be a fun driving experience, so I fixed it.

It may also be play in a joint that just happens to get unloaded and loaded at that radius as well. I have seen a lot of tie rod ends and ball joints that dried up and got notchy because of lack of grease, and they would stick and free up like this in different parts of their travel even though they weren't loose. The only way to check was to remove the spindles and see how they all felt.

Steering feel is never great on these trucks, but deleting EVO helps, as does using a steering box valved for the early trucks if you like heavier steering. My control arms have Delrin bushings; that probably helped steering feel more than anything.

 

[Pinger]

Thanks - good info.
Just to pick up on a few points....

You should never have an 'over center' feeling, especially not outside of a parking lot where you use a lot of steering angle. EVO can have an effect here, as can worn bushings and ball joints. There is a lot of rubber in the control arm bushings; they can deflect a lot even if they don't appear to be in bad condition. EVO can cause little repeatable steering effort ripples if the radius of that corner happens to be between two of the encoder 'teeth'. I can sit still in my K1500 that still has EVO and move the wheel slightly and make it do this all day long. I don't know if that's normal, just that my truck does it. In the K1500 I ignore it. The Tahoe is intended to be a fun driving experience, so I fixed it.

Some play in the rubber is probably the cause.
The ripples - I get them on a constant radius bend off-throttle, slowing, just before I turn off the road into my driveway. I'd put that down to fluctuating rpm (and pump effort) but your explanation will be nearer the truth.

It may also be play in a joint that just happens to get unloaded and loaded at that radius as well. I have seen a lot of tie rod ends and ball joints that dried up and got notchy because of lack of grease, and they would stick and free up like this in different parts of their travel even though they weren't loose. The only way to check was to remove the spindles and see how they all felt.

All greased with grease that's tackier than a tart's boudoir. The motion never feels clicky mechanical - more rubbery.

Steering feel is never great on these trucks, but deleting EVO helps, as does using a steering box valved for the early trucks if you like heavier steering. My control arms have Delrin bushings; that probably helped steering feel more than anything.

I like EVO - though I've never driven one without. There's not much in the way of feel, agreed. But they do steer remarkably precisely with smooth inputs. Much better than I expected.

 

[alignman88]

Without seeing a picture of the suspension sitting at ride height its difficult to know if your lower control arms and tierod ends are parallel to each other. if they are parallel that may rule out regular mechanical bump steer out of the equation. Maybe. The alignment angles you noted when coupled with the wider tire/different offset wheel from stock leads my years of experience down the scrub radius impacted toe values path. Look at the picture, now think of the inner portion of the tire towards outboard as a "lever' with the majority of force on the inner half of the tire causing it to toe out a pre-determined amount. Thats why the majority of vehicles use a positive or toe in static setting. in motion the road, weight and speed cause it to toe outward. Now change tire height, width, wheel offset and the "lever" has changed. Add in the very aggressive -1.0 camber and the disproportionate load across the tire face will exaggerate the way feedback from the road is felt and definitely all that caster isn't helping either. Too much positive caster on steering systems like this aren't favorable to daily IMO. The wider tires, coupled with presumably geometry issues has you feeling "bump steer" but more than likely its tram lining caused by incorrect toe curve change caused by altered scrub radius. This could also create the loose in turn off throttle feel. Unequal individual toe change, and the original toe curve geometry goes out the window with the -1.0 camber. you have a tolerance stacking issue, 2 or 3 little things causing 1 big thing. I would always set front toe values with that in mind. Too much positive toe in will create a darting (road crazy) steering feel. Too little positive toe in coupled with altered scrub radius may cause it to wander (think tires tracking in ruts in road). And again all the inboard contact created by negative camber adds to this as well. Always try to stay within the range set by the factory based off what wheel/tire fitment effects on scrub radius. Toe spec is 0.10 degree=0.125in (1/8). if your tire diameter is 28.9" then you need more toe in. If youre at 1/8th (0.125) tighten it up to .198 or (3/16"). conclusion-less camber more toe in, and use the Baer bump steer tie rod fix if control arms/tie rods not parallel.

I have attached a scrub radius diagram to help paint the picture.

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[kenh]

If you feel an over center in the steering maybe the gearbox is to tight at center? They are normally adjusted at the extreme right and left travel. But, if it is fairly new then maybe it was machined "tight" at center to compensate for anticipated wear????

Ken