Bump Steer

Guys,

I seem to recall a thread a week or so ago from a guy wondering why his Jeep pulls to the left when he stomps on it and dives to the right when he lets off. Can't find the thread but maybe the poster will find this if he didn't get his answer already. I think there was speculation about lockers, loose motor mounts etc. Not sure if this was resolved or not. Reaching for my soapbox I say "excuse me while I whip this out".

I don't make a habit of romping on the 73 with 304 but I did so today. Haven't had it on the road too long so we're still learning each other. Immediately I knew what the poster was talking about. The issue is simply a design function of the kind of steering linkages these old Jeep have. When you jump on it the center of mass shifts rearward and the nose pitches up. The axles aren't coming along for the ride so the steering linkage articulates the flex between chassis and front axle. Unlike newer vehicles the Jeep has a drag link connected to the right knuckle, and the tie rod connects left and right knuckles. So when the nose pitches up the angle of the drag link increases, pulling the front of the right knuckle inboard and steering the right tire to the left. The tie rod shoves the left wheel into a left turn also. Now when you let off the gas the nose pitches down, squatting the front suspension and decreasing the drag link angle and steering the wheels to the right.

Bump steer is aggrevated by spring lifts. The steeper your drag link angle the more the impact will be when the nose pitches up or down. A 4" lift without a dropped pitman arm will be a nightmare. My 2-1/2" lift with stock pitman has some bump steer. Not sure if it's been done but a later model Y-type steering linkage refit to a CJ would reduce bump steer because instead of both wheels steering left or right they will affect toe-in. Still a squirrely affect but not as noticeable as our old-timey rigs.

Chilly

 

Your describing torque steer not bump steer... like when you stomp on the gas and the front left corner raises up, the hemi charger I drove last week did the same thing but it wasnt bump steer

 

No, I'm talking bump steer. Torque steer is from twisting the frame due to drivetrain torque. Bump steer happens when the front suspension loads and unloads, affecting the angle of the drag link to the right knuckle. I don't need any torque at all to induce bump steer. While I have a 304, it's hardly a frame-twisting torque monster.

 

torque from the engine, any engine.
in a narrow, short wheel base vehicle.
isn't the drag link only moving up and down in your scenario ?
don't see how it can move rearward, in the scenario you've presented.
even my F4 would take me to the left on accel
the 73 steering is totally different than say a stock 65 CJ
anyway, I think it's related to engine.

 

Drag link doesn't move front to back, it swings in an arc pivoting about the pitman arm. If the drag link was horizontal you'd barely notice it. But if the drag link is at a downward angle from the pitman arm to the right knuckle then compressing the front suspension lessens the angle. The geometry is a right triangle. Throw some Pythagorian's theorum on the problem and you'll see what I'm talking about. Easy to draw, difficult to describe in text. Bump steer happens any time you have significant height difference between pitman arm and steering knuckle attachment point. The reason is that the drag link becomes effectively longer horizontally when the angle is reduced. If the pitman arm doesn't move then the knuckle gets shoved outboard. If you deny the geometric facts then why bother with dropped pitman arms? My unlifted TJ had nary a hint of pump steer but the pitman arm was at the same height as knuckle so I wouldn't have expected any.

 

Your evidence speaks against your own argument. If it's a torque related phenomenon then having the same issue with an engine having virtually no torque does not support the hypothesis. While your F4 doesn't have enough torque to twist a frame, it does have enough to shift center of gravity on accelleration, pitching the nose up and squatting the rear a bit. Decel pitches center of gravity forward compressing the front suspension.

Chilly

 

Here's a decent description I found. Not exactly like a CJ because we don't have track bars or other coil spring contraptions but the physics is similar. Forget about the track bar arc stuff, pay attention to drag link arc.

http://www.quadratec.com/jeep_knowledgebase/article-68.htm

 

without reading thru all the gibberish
then I suppose if we were to reattach the drag link to the LH knuckle, the Jeep would then go to the right on acceleration, correct ?
or, a RH drive Jeep will pull to the right ?
check back on that and then report your findings.

 

What about an early CJ with bellcrank steering, why will they do it? The pitman arm is in the middle...

 

It's about the height of the pitman arm relative to the knuckle attachment. And if the RHD steering is in fact a mirror image of the LHD it will behave opposite.

Don't know anything about the bellcrank geometries. Never examined one in a significant way. But once one understands what causes bump steer you can just about look at a front end and discern whether it would be affected.

If it's a torque-related phenomena then why does it happen when one hits the brakes? Why does it happen to vehicles with hardly any torque? That's like saying smoking causes pregnancy because my mom and grandma and great grandma had babies and they DON'T smoke. What kind of argument is that?

My goal isn't to convince anyone. But I may enlighten some people with a basic understanding of geometry. I'm not inventing a new philosophy on steering behaviors. It's already a well known effect. You may continue to tell yourself that a flathead 4 has sufficient torque to twist up your chassis if you like.

 

I dont know if your understanding..How about this video, this is me in my flatfender, watch it and at one point when I stomp on it the front left tire comes off the ground, well it has full hydro steering and no steering linkages from the frame to the axle...http://www.youtube.com/watch?v=2luz29X2QkQ

Some of what your saying is correct, but your also describing alot of whats going on with torque steer. Bump steer is just that, its normally when one side of the axle compresses thus changing the steering geometry.

 

Is that a flathead 4-cylinder in that flatty? It looks pretty badass. I'll guess that if you have installed full hydro it's no longer poking around on a 4-cyl. In any case, nice buggy! I acknowledge that an engine with significant torque, or a gutless engine at high revs suddenly acquiring traction, will flex a frame. Not sure how much it contributes to unintended steering input. Certainly having unequal loading of the front suspension components can cause some sqirrellyness. Your video shows some rather uncommon behavior that I'm not sure corresponds to road behavior under accell/decell unless someone is pulling a hole shot (strangely, my Jeep tracks like an arrow when I pull a hole shot, and with 32" Swampers it'll make a 100' strip even with a 304).

Please explain the physics or geometry of how twisting a frame under high torque input will cause unintended steering input and maybe I'll get it. The fact that compressing a front suspension with an angled drag link will, and must, result in uninvited starboard rudder. If the driver holds the wheel and the front suspension compresses that extra "X" length due the reduction in "Y" (for a fixed hypotenuse) will shove the front of the right knuckle to the right. Simply has to happen. We can wish it weren't so, we can call it different things, but if your drag link isn't horizontal the physics and geometry will have their way with you. Stomping the pedal with four tires spinning may lift a corner but I don't think one can correllate that to squirrelly road manners under driving condition.

 

Bump steer is when you hit a bump and it turns without steering input.

Torque steer is when the torque of the engine causes unintended steering input, whether is be tilting the jeep back, rolling the jeep left, or twisting the frame.

 

Jeep had a problem with the V6 when they first came out of the frames twisting and cracking due to the torque of the V6.
The field fix was a pair reinforcement plates to bolt/weld to the frame to stiffen it up.
This is a fact, not a myth.

Why don't full size Jeeps exhibit the same tendency ?
Same type of steering components/geometry, correct ?

they do, but you don't notice it as much because of the frame width.

carry on......

 

The length of the chassis will have an effect as well. The longer the chassis the more flex between the rear axle and the front so there normally will be less effect of torque steer. That's one of the reasons short CJ's suffer from torque steer much more than say a pickup or wagon.

 

Yep,,,X2

Same problem I had before swapping the power steering.
Bump-steer is gone, now I have torque-steer with the 5.0 V-8 (when I slam my foot into it)