Metal cloak 3.5 and death wobble

[Jeep&dogs]

Hi Old Jeeper,

The caster was set to 6.3 earlier this year. It was 5.4 prior to adjusting. I didn't experience death wobble again until last week. We only put about 1,000 miles during that time. The weather is about the only change that stands out.

Metal Cloak recommends 6 to 6.5 degrees of caster. Other people I have spoken with recommend ~7 degrees.

I haven't checked the rear. Based on what I feel when the DW happens the problem is with the front..

Thanks for everyone's input.

Jeff

I have seen and heard dozens of recommendations on caster settings on the Jl. 5-7+. One thing that would be interesting to see is a graph of caster vs tire size and what setting works for what diameter tire. Caster actually changes where the contact patch of the tire sits vs the centerline of the spindle. 6.3 on a 33” vs 35”’ or 37” tire is different. The contact patch if the 37” tire is almost a full inch further in front of the spindle vs a 33”.

The further the contact patch is in front of the spindle centerline the more weight the wheel needs to lift when the tire is turned. The weight it applies is actually what forces the wheel to return after a turn.

No doubt that measurement (contact patch distance from spindle center line) has something to do with the DW. Unfortunately that’s a hard measurement to get and caster is easy so that’s what everyone goes with.But it’s only 1/2 of the measurements that are really needed.

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[roaniecowpony]

I believe the "trail" or "caster trail" is what returns the steering to straight ahead. That's the distance the contact patch is behind the point on the ground projected by the balljoint axis.

 

[Jeep&dogs]

I believe the "trail" or "caster trail" is what returns the steering to straight ahead. That's the distance the contact patch is behind the point on the ground projected by the balljoint axis.

Negative caster puts the contact patch behind the centerline of the spindle and causes the shopping car effect. Positive caster puts the contact point in front of the spindle centerline and increase stability.

 

[roaniecowpony]

Jim, I think it's the other way around.
KPI – Static Castor and Castor Trail – Suspension Designer

 

[Old Jeeper]

I have seen and heard dozens of recommendations on caster settings on the Jl. 5-7+. One thing that would be interesting to see is a graph of caster vs tire size and what setting works for what diameter tire. Caster actually changes where the contact patch of the tire sits vs the centerline of the spindle. 6.3 on a 33” vs 35”’ or 37” tire is different. The contact patch if the 37” tire is almost a full inch further in front of the spindle vs a 33”.

The further the contact patch is in front of the spindle centerline the more weight the wheel needs to lift when the tire is turned. The weight it applies is actually what forces the wheel to return after a turn.

No doubt that measurement (contact patch distance from spindle center line) has something to do with the DW. Unfortunately that’s a hard measurement to get and caster is easy so that’s what everyone goes with.But it’s only 1/2 of the measurements that are really needed.

Somewhere here but I cannot find it there must be an APPLAUSE button. Because i certainly applaud you.

I have argued time and time again on what Big as in Tall tires do, they change the length of the wheelbase and thus impact the caster setting.

You can get a measurement by drawing a vertical line thru the spindle down or a straight edge and chalk a vertical on the sidewall of the tire. Now measure the distance forward to daylight under the tire. As an addition to that process. Let the air out of your tire until the forward patch no longer grows. So as you let air out the tire increases the size of the contact patch, but at some point in psi it no longer does in length. This would be the optimal length of the contact patch and optimal psi. I found on my BFG KM2s that between 3-5 psi gave me the optimal contact patch. NOTE this mostly is applicable to the guys that play in the rocks. My GY MTRs did not have as good a contact patch as my KM2s.

So this would beg the question: Do you ever get DW as a result of airing down??? I have not, but I have seen folks get it and no doubt they were on the edge of getting it to begin with.

 

[Jeep&dogs]

Jim, I think it's the other way around.
KPI – Static Castor and Castor Trail – Suspension Designer

Caster tail refers to both directions front and rear of centerline. Positive caster projects it in front of the spindle and negative projects it behind the centerline.
In front increases stability behind decreases it.

 

[Jeep&dogs]

Somewhere here but I cannot find it there must be an APPLAUSE button. Because i certainly applaud you.

I have argued time and time again on what Big as in Tall tires do, they change the length of the wheelbase and thus impact the caster setting.

You can get a measurement by drawing a vertical line thru the spindle down or a straight edge and chalk a vertical on the sidewall of the tire. Now measure the distance forward to daylight under the tire. As an addition to that process. Let the air out of your tire until the forward patch no longer grows. So as you let air out the tire increases the size of the contact patch, but at some point in psi it no longer does in length. This would be the optimal length of the contact patch and optimal psi. I found on my BFG KM2s that between 3-5 psi gave me the optimal contact patch. NOTE this mostly is applicable to the guys that play in the rocks. My GY MTRs did not have as good a contact patch as my KM2s.

So this would beg the question: Do you ever get DW as a result of airing down??? I have not, but I have seen folks get it and no doubt they were on the edge of getting it to begin with.

100% agree with every word of this. Unfortunately most will argue this till they are blue in the face that you don’t know what you are talking about. You are spot on.

 

[Jeep&dogs]

Caster tail refers to both directions front and rear of centerline. Positive caster projects it in front of the spindle and negative projects it behind the centerline.
In front increases stability behind decreases it.

This is also why letting air out of the tires “fixed” the wandering on some Jeeps. It actually alters the caster

 

[Old Jeeper]

Jim, I think it's the other way around.
KPI – Static Castor and Castor Trail – Suspension Designer

Not sure what you are saying on this.

The pics are correct insofar as Positive, neutral and negative...IMO as one Retired Engineer to another...

 

[Old Jeeper]

This is also why letting air out of the tires “fixed” the wandering on some Jeeps. It actually alters the caster

My experience having been on Jeep forums since the early 80's there are a LOT of experts and then there are a FEW who know what they are talking about.

Several years when TJs were the Jeep there was a guy who had a website on TJs and it was the #1 Jeep site on google search. I also had a website I know it one of the top hits, he put a LOT more stuff than I did, I just put stuff I worked on. One day I get a e-mail from him. Turns out he lives in the same city as I do. He asks if I can do some testing for him and a write up. Sure. I do and posts it, not under my name, his name. Does not bother at all. I get more stuff from him and I discover. He had never owned a Jeep, what he did was search out folks to install stuff. Well he was getting stuff free and he would call me up and say can do a write-up and pics on this or that. Everyone thought he was a Guru and he a section on his website where you could ask questions. LOL he send me some and say can you answer this? He had people everywhere and he would send them a part and have them do the install, write up and pics. In trade they keep the part and answer some questions and they get to keep the part. He was truly a legend in his own mind...

 

[roaniecowpony]

Not sure what you are saying on this.

The pics are correct insofar as Positive, neutral and negative...IMO as one Retired Engineer to another...

Don, with positive caster, the axis of the knuckle pivot (the line between the two ball joints or kingpin) is slanted forward at the bottom/back at the top. This axis line, projected to the ground, is forward of the tire contact on the ground.

Negative caster has the tire contact with the ground ahead of the knuckle pivot projected axis.

...Until you air down, as you mentioned.

 

[Jeep&dogs]

Don, with positive caster, the axis of the knuckle pivot (the line between the two ball joints or kingpin) is slanted forward at the bottom/back at the top. This axis line, projected to the ground, is forward of the tire contact on the ground.

Negative caster has the tire contact with the ground ahead of the knuckle pivot projected axis.

...Until you air down, as you mentioned.

You Are right. I had to think about it for a second but yes. Negative does project the contact patch in front of the center line. And positive is rearward

 

[roaniecowpony]

You Are right. I had to think about it for a second but yes. Negative does project the contact patch in front of the center line. And positive is rearward

And as you mentioned, when the tire is softer and diameter is larger, the contact patch lengthens and often puts some of the tire contact patch ahead of the axis of the knuckles. This is destabilizing, but it is mostly offset by the greater contact to the rear as well.

 

[Jeep&dogs]

And as you mentioned, when the tire is softer and diameter is larger, the contact patch lengthens and often puts some of the tire contact patch ahead of the axis of the knuckles. This is destabilizing, but it is mostly offset by the greater contact to the rear as well.

With this all being said I feel the answer to some, not all, of the DW cause on the JL is actually the distance from the centerline to the center of the projected contact patch of the tire. Also why some have it and others don’t. I think the “window” for that measurement is actually very small. And at some point camber comes into play also because that will alter that to distance to some degree depending on toe setting.

Even wheel offset will change scrub radius and comes into play.

 

[Old Jeeper]

OK guys lets get this straight, is the picture posted and its annotations correct if so we are all saying the same thing. LOL I think we are...

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