What's bump steer ... and is this it?

[dremu]

Just buttoned up my truck, after doing 52's up front, 56's in back, crossover, highsteer, Ford towers up front and inboarding shocks in the back. I didn't MEAN for it to snowball like that, but one thing leads to another... So I test drove her briefly around the yard, didn't take her out on the street yet... draglink is off slightly, and I'm debating toeing her in a little more. No big deal, all easy stuff.

The thing I don't get, though, is this ... when I turn the wheel, with the truck stationary, as I'm setting the wheels straight, is that the nose waggles side to side something fierce. I donno if it's a steering thing, or a shock thing ... more like a lack of shock thing? I mean, she rolls enough there's no way I could take her on the road like that!

I can be more specific about all the stuff I've done, but I figured I'd throw this out and see if anybody says "Oh, you did this specific dumb thing"

Thanks!


-- A

 

[sled_dog]

waggles? Like it leans whe you steer? I know when I first installed crossover on my 86 sitting in the garage if I turned the wheel the front end would lift and lower depending how i steered. It was a combination of steering axis inclination and the force of the drag link pushing up on the frame rail as it pushed out on the steering arm. Nothing to be worried about there.

But is your truck physically moving side to side in relation to the spring mounts on the axle? If so it sounds like the springs are weak, bushings are shot, or something is loose somewhere.

Don't tell me about thinks snowballing, I went from a Daily driver K5 / trail rig to a frame and 4 link buggy project. Doh...

 

[sled_dog]

oops. also thats not bumpsteer.

Bumpsteer is "steering angle change during suspension travel". Aka if you hit a bump and the wheels turn because there is something improperly engineered in your steering system, that is bumpsteer.

 

[85mudblazin]

Thats death wabble.
Adjust your toe. I had to adjust mine in about 1/8" incriments until it stopped.

 

[dremu]

oops. also thats not bumpsteer.

Bumpsteer is "steering angle change during suspension travel". Aka if you hit a bump and the wheels turn because there is something improperly engineered in your steering system, that is bumpsteer.

Yeah, I was stinking it was unrelated. As for the problem ... it's almost like it's a dang low-rider on hydraulics, like the corners go up and down. Maybe it's what you described ... I'll get the alignment closer and take her out for a veeery gentle spin =)) Thanks! -- A

 

[sled_dog]

have you upgraded to bigger tires? Bigger tires enchance the visual appearance of the SAI effect. I was freaked the first time I saw how much crossover effected my suspension too.

 

[sled_dog]

Thats death wabble.
Adjust your toe. I had to adjust mine in about 1/8" incriments until it stopped.

how would death wobble appear sitting still in the driveway though? I've experienced it once(when my tie rod bent after hitting a car) and it didn't relaly happen at low speed and most certainly not sitting still. But if there is a way or something I'm not getting, please let me know, I'd like to.

 

[dremu]

Thats death wabble.
Adjust your toe. I had to adjust mine in about 1/8" incriments until it stopped.

They wobble even when not rolling? Wow. Well, toe-in tweaking is on the list, so I'm good. -- A

 

[BigOrange90Jimmy]

What it sounds like is the drag link is using the steering knuckle to actually raise and lower the driver's side of the truck, since you said you were stationary.

To simulate the effect, take a broom handle and hold it in your hands. Now, raise one end to where it's slightly higher than the other. With your left hand, try to push the right hand to your right, while also trying to maintain that angle you have established. It will try to "raise" your left hand. Now, try pulling against your right. It will "lower" your left.

The 52" springs only exaggerate this effect since they flex better. To eliminate this problem, you will need to fabricate a panhard bar like is used on link-style suspensions.

 

[dremu]

have you upgraded to bigger tires? Bigger tires enchance the visual appearance of the SAI effect. I was freaked the first time I saw how much crossover effected my suspension too.

Nope, same tires as before ... but with the 52"'s comes a good chance I've botched one of the steering angles ... umm ... caster, I think? If the axle is all tilted funny, would that do it? -- A

 

[85mudblazin]

Ow didnt read that he was sitting still

My truck does the same thing. What is going on is that you have more steering in the box then you do at the axle, as in your steering stops.
it is most likely caused by the draglink being out of adjustment. I havent gotten around to messing with mine.

 

[bernman]

it seems as though you will need a trac-bar, i did a similar conversion on my truck, you will definitely need this if you want to get the full potential from the crossover steering. you will notice most trucks, that come stock with crossover type steering have a trac bar.

 

[85mudblazin]

NEVERMIND what I said.
I just went out and messed with it. it is the soft springs thats doing it. But that is sitting still. Those tires have alot of grip so when driving you wont have that swaying motion.

 

[bernman]

when your driving it is less, but when you get to a turn you defentitly can feel the front end walking a little, especially at moderate speeds, all this went away after i installed the trac bar

 

[Greg72]

You're not going to want to hear this....but it sounds like a crack in the frame near the steering box....or loose steering box bolts???There is a LOT of force generated when you crank the wheels from side to side and the truck isn't moving. It should be easy (with a helper) to slowly crank the wheel and be on the lookout for whatever part is flexing or reacting unnaturally to the forces.

 

[ntsqd]

Yeah, I was stinking it was unrelated. As for the problem ... it's almost like it's a dang low-rider on hydraulics, like the corners go up and down. Maybe it's what you described ... I'll get the alignment closer and take her out for a veeery gentle spin =)) Thanks! -- A

Ever notice the comp guys steering back and forth when they're hung up? What they're doing is using that corner "jacking" to load each front tire in search of traction. As Sled noted, it's an effect of the SAI (Steering Axis Inclination angle) and the Castor angle when turning. That same effect is what makes the truck want to go straight down the road at speed b/c each front tire is trying to get to it's lowest (unloaded) position. The lowest position for the left front is at full right turn and vice versa. At some point (hopefully going straight) they reach a balance.

 

[dremu]

You're not going to want to hear this....but it sounds like a crack in the frame near the steering box....or loose steering box bolts???There is a LOT of force generated when you crank the wheels from side to side and the truck isn't moving. It should be easy (with a helper) to slowly crank the wheel and be on the lookout for whatever part is flexing or reacting unnaturally to the forces.

@&*$^@&*$^@. I'll duly check ... there's the bolt-in brace and I loc-tited the hell out of those bolts, as I recall ... but I'll check. Thanks! -- A

 

[BadDog]

It's just a "feature" of cross over. As mentioned, the cross over drag link is causing left and right forces to deflect the soft springs. Arched springs also have side leverage on bushings that allow more deflection. The drag link is basically pushing side to side changing the horizontal relationship of axle and frame.

The angle of the drag link also causes some vertical forces that cause the front to jack on one side or the other depending on direction. Softer springs make this more noticeable.

I don't think it's a broken frame or loose box, at least not doing what you describe. Not to say it won't be cracked or loose, just that I don't think that's doing what you describe. It usually manifests as "loose" steering along with potential popping noises.

"Bump steer" is when you're moving and hit a bump or ditch causing the truck to veer or wheel to want to turn in your hand. Basically the same forces but in the opposite direction to what you describe. The axle is moving up or down relative to the chassis which causes the horizontal relationship between the 2 ends of the drag link to change as the drag link changes angle due to the vertical relation ship changing. Pythagorean theorem and all that…

Death wobble is unrelated and has to do with harmonic oscillations in the tire rod and knuckles that act a lot like what happened to the Tacoma Narrows bridge (search Google). Basically, you hit a bump or some other "input" and the front end starts doing a medium-high frequency jerking back and forth that often requires you to slow down to get it to stop, though sometimes a quick stab of break or steering input can make it stop. This often happens with Dana 60s that have a little wear on them and you add larger tires. Whether you have it and how bad will be determined by the size and weight/mass of the tires, tie rod material, spring rate, shock rate, lift height, alignment, and steering component wear. Opinions differ as to how much, if any, each of those contribute. Most axles other than D60s are not known for have DW issues.

 

[azblazer]

Russ...stuck in wyoming once again...day number 10 this time....and wouldn't ya know it i'm on the rig directly in front of the last one i was on when i tried to have a conversation with you. Same deal....no cell service. End hijack....Anyways in reviewing Dr. Emu's pictures i noted that he had not added caster shims to the rear springs he moved up front...my deal is to allways get as close to the stock specs as possable and so far i have never gone wrong by doing this. The 79 has camber plates behind the spindles and she handles great on or off road. So that being said if he was to align tankie after adding the correct caster shims to the front spring pack with new longer center pins and then aligned tankie....and added camber shims if needed it would make a big difference? Out of the two options ava to correct camber on these trucks the camber plate that goes behind the spindle is the best method out there. The first method does have a drawback as it will tend to wear the brake pads at a slight angle...the second option allways was hoky to me and just never worked very long. Upper balljoint preload is set with a fishscale in the older manuals but the newer joints just give you a torque spec of 100Ft. Lbs. or so...so when trying to dial in the camber angle on the machine its kinda hard to achieve proper preload when doing so....then they give you a hoky beveled washer that will breakin two even with just light duty wheelin thus throwing off what ever angle you had built into her....So russ...long story short. In school i remember there is a posative and negative caster/camber angle...one points to the truck and the other away from the truck....since i never ended up running a alignment rack in my career i never comitted to memory which is what. Without having caster wedges under the front springs how is he affecting things. To much caster makes it way to easy to steer and not enough makes it a bitch to steer....Doug Wright

 

[BadDog]

Good point. Proper alignment (caster, camber AND toe) will help eliminate much of the most troublesome aspects of the bad characteristics I listed above. It will even help with bump steer indirectly by making it FAR less "twitchy" and "wandering" while experiencing "bump steer". That same tendency to "follow the road" is amplified when you get bump steer inputs constantly "adjusting" your angle of attack. Most (including me) also feel it will have considerable impact on both frequency and severity of death wobble, particularly if you have a "tight" (aka rebuilt or at least not badly worn) front end (balls/kings/box/all of it) and STILL get DW (that's death wobble, not Doug Write, and which is worse is a matter of debate ).

BTW, one problem that often comes up is the lack of ability to sufficiently adjust camber using any of the more traditional means when dealing with various lifts and crossed up or custom spring packs. Another thought for the fabricators out there is that you can change the spring mounting points to help deal with caster, or at least bring it into a range that can be fine tuned with the normal adjuster/adapters, which can also set camber as long as the housing is still straight.

Man, I feel for you Doug. Good luck in getting back soon to warmer lands. Give me a call when you can, got some updates on the chip and tuning.