What causes bump steer... in the road?

[99wishes]

I have read several posts about what causes bump steer on the jeep, but why do some bumps do it while others don’t? I can drive over 10 seemingly identical bumps and my tail end wiggles on 1 of them. And what is crazy is I can drive over those same bumps again and that same odd bump is the only one that causes my tail end to wiggle again.

What is it about the bumps that triggers this reaction from my Jeep?

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[TrewJL]

is your jeep stock or lifted? what's your tire pressure?

 

[99wishes]

is your jeep stock or lifted? what's your tire pressure?

All stock, but I am trying to understand what in the road creates this. I would think a bump is a bump and the vehicle would react the same, but it doesn’t. Some bumps are different. I don’t understand why two bumps on the same road that are the same size are handled differently each time by the same vehicle. Heck, sometimes the smaller bump is worse.

 

[HealthRebel]

I have read several posts about what causes bump steer on the jeep, but why do some bumps do it while others don’t? I can drive over 10 seemingly identical bumps and my tail end wiggles on 1 of them. And what is crazy is I can drive over those same bumps again and that same odd bump is the only one that causes my tail end to wiggle again.

What is it about the bumps that triggers this reaction from my Jeep?

What is your current tire pressure?

 

[entropy]

I have read several posts about what causes bump steer on the jeep, but why do some bumps do it while others don’t? I can drive over 10 seemingly identical bumps and my tail end wiggles on 1 of them. And what is crazy is I can drive over those same bumps again and that same odd bump is the only one that causes my tail end to wiggle again.

What is it about the bumps that triggers this reaction from my Jeep?

It is not just about hitting a bump. It is called harmonic frequency, every object has a harmonic frequency. What do I mean by that? well if an object vibrates at its harmonic frequency, the amplitude of the vibration increases exponentially making it vibrate more and more violently. It is like when the opera singer breaks the wine glass example, it is not how loud the opera singer is that makes the glass break, it is actually the frequency, she sings a note at the harmonic frequency of the glass which then makes the glass vibrate, increasing its amplitude exponentially would eventually break the glass. Of course the louder she sings, the higher the initial amplitude that transfer to the glass.

So what's happening to your jeep is that when you hit the bump an impact wave travels through your jeep's components, if the frequency of this wave is just right, it will make it shake violently as it has hit the harmonic frequency of your steering components. A lot of times people find out the culprit of their "death wobble" is due to lose components, and this makes sense as having lose components decreases damping and allows the structure to violently vibrate. Get your Jeep checked.

 

[99wishes]

Interesting. I have no basis for my thinking but the only thing I could think of is the bumps having slight angles which pushed your vehicle one direction or the other. Or did one bump have a different approach angle that caused the vehicle to remain in the air long enough for your wheels to be slightly off. I tried different speeds and different angles of attack, neither of which impacted the bump steer.

What lead to this post was when I tried it with a completely different jeep! I had a dealer let me take a new Rubicon for a test drive and it had the same, but slightly less, reaction to the same set of bumps. Kind of fascinating regardless.

What you are saying makes sense considering both essentially have the same suspension. Anyways, I am having the dealership look at but thats a different story for a different thread.

 

[99wishes]

What is your current tire pressure?

Right now it depends on the temperature but between 33 and 36psi. Did the same at 40psi though.

 

[fdFifty]

I find that shock valving and wheelbase tend to be the biggest factor it what bumps you feel in the seat and the steering wheel.

The profile/shape of a particular bump can really effect how the valving reacts at a given speed. Not always 1:1 with the height of that frost heave or the depth of that pothole. When the shock valving is overcome and the spring rapidly compresses, you’ll feel the reaction in the steering wheel (bump steer). It’s just the geometry of your steering linkages reacting to the quick change in height.

The relationship between your wheelbase and the particular distribution of bumps across the road are what generate the above mentioned oscillations. This is often the cause of that feeling when the rear steps out a bit as it becomes unweighted.

So if the other Jeep you drove had a different wheelbase, shocks, or both, you would see that different behavior.

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