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Discussion Starter · #1 ·
So after years of being a Chevy guy , I bought a new to me 2500 Cummins 18 longhorn. Iv always had lifted trucks but this is my first radius arm and axle truck so looking for so information that the search bar isn’t helping with . I know long term I’d like to do carli or thuren system for ride but I’m the meantime if I do a simple 2.5 spacer and shock extension, how much does it push the axle off? How bad is the ride compared to stock ? Am I going to be more likely to rub one side versus the other?


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how much does it push the axle off?
When you say push the axle off, are you referring to the stock track bar causing the axle to be off center?
 

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Okay. You can figure out how much the trackbar will pull the axle off center when the suspension is raised with simple geometry. An angle finder, garage floor, chalk, and a long straight edge (e.g. 48" level) is all you need.

Measure the length of the stock track bar (center to center), and use the angle finder to determine the degree which the trackbar sits. Next measure the stock spring height. Use the chalk to draw a right angle triangle on the floor. The length and angle of the trackbar should be the hypotenuse, and the stock spring height should be the vertical side of the triangle. The horizontal side of the triangle represents the axle. Add 2.5" to the bottom of the vertical leg, and use that mark to draw a line parallel to the horizontal side of the triangle. This represents the spring spacer. The trackbar functions as a radius and describes an arc. Use the intersection of the hypotenuse and vertical leg of the triangle as a pivot point, and use the length of the trackbar on the straight edge to trace an arc- exactly as you would do with a compass. The horizontal distance where the arc intersects the stock spring height line and the +2.5" spacer line is how much the axle will be moved off center.

That is actually quite difficult to explain in words, and I'm sure is probably very confusing. I've attached a diagram below that hopefully makes it a bit clearer.

926671
 

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Discussion Starter · #5 ·
Okay. You can figure out how much the trackbar will pull the axle off center when the suspension is raised with simple geometry. An angle finder, garage floor, chalk, and a long straight edge (e.g. 48" level) is all you need.

Measure the length of the stock track bar (center to center), and use the angle finder to determine the degree which the trackbar sits. Next measure the stock spring height. Use the chalk to draw a right angle triangle on the floor. The length and angle of the trackbar should be the hypotenuse, and the stock spring height should be the vertical side of the triangle. The horizontal side of the triangle represents the axle. Add 2.5" to the bottom of the vertical leg, and use that mark to draw a line parallel to the horizontal side of the triangle. This represents the spring spacer. The trackbar functions as a radius and describes an arc. Use the intersection of the hypotenuse and vertical leg of the triangle as a pivot point, and use the length of the trackbar on the straight edge to trace an arc- exactly as you would do with a compass. The horizontal distance where the arc intersects the stock spring height line and the +2.5" spacer line is how much the axle will be moved off center.

That is actually quite difficult to explain in words, and I'm sure is probably very confusing. I've attached a diagram below that hopefully makes it a bit clearer.

View attachment 926671
Awesome thank you sir


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You're very welcome!
 

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Our trucks' assembly tolerances tend to be pretty slack. You should check your axle center before you touch a thing. You might find that the axle is so far off center to begin with that a small lift on the order of 2" to 3" actually improves things.

I did.

Stock I found that my axle was almost half an inch off center. If I remember without doing the math, lifting the front end 3" results in a lateral shift of about an eighth or three-sixteenths of an inch. So after I installed my 3" coils and new radius arms, my front axle was STILL off center in the same direction as before, but slightly less so.

To fix that, I installed an adjustable track bar. An alternative would be a track bar drop bracket, which has the advantage of providing better geometry at the expense of having an additional failure point.

But... for such a small lift, what you'll find is that some adjustable track bars will interfere with the differential housing, especially if you have larger bolt heads, add-on hardware like a dual damper center bracket, or a finned cover. I discovered that, too.

Check first. If the off-center is going to get better or reverse slightly to the opposite side, I wouldn't worry about it one tiny bit. If your off-center will increase, then consider an adjustable track bar. The Carli unit's adjustment gear is at one end, not in the center, and therefore less likely to interfere with your center member. The thing is that the Carli unit, while an awesome piece of hardware, is difficult to adjust in-place.

More important than axle off-center is steering and suspension geometry. When you lift 2-1/2", you hit the point at which your radius arms' caster adjustment cams no longer have any significant positive range available. Our 2500 and 3500 front ends have a tendency to death wobble with weak sidewalls, low tire pressure, excess toe-in, loose ball joints, loose steering gear, bad steering damping, or insufficient caster. Bigger and heavier wheels and tires aggravate the issue. You can't eliminate it because the natural frequency of the steering geometry in steering excursion is almost precisely the same as the rotational frequency of 33" to 35" tires at 60 to 65 mph. So any tire and wheel imbalance tends to excite death wobble pretty much exactly at highway speeds. To make matters worse, the natural frequency of the cab floor in at least one primary mode is almost precisely half that -- one perfect harmonic down -- so any tire and wheel imbalance also tends to excite our dreaded seat shake.

Given a solid front suspension, the two most important factors affecting death wobble are tire sidewall stiffness -- tire quality, construction, and pressure -- and caster. Increased caster not only enhances on-center feel and stability, but provides a suppressive damping force that mitigates the tendency to death wobble.

So reducing your truck's ability to dial in more positive caster is not a good thing. You might need it some day.

It's totally possible that you can go with shocks and coils or coil spacers and never see a single issue. If, on the other hand, your truck is one that's just on the edge of wobble and shake now, you might introduce something that means you'll be fighting the wobble and shake nightmare for months or years. My truck apparently is on the hairy edge. If my front tire pressure gets low, or if my steering dampers get weak, or if my cams loosen and my positive caster shakes out, I develop death wobble. And that happened with a fresh alignment and absolutely everything except the axle itself new and tight.

Thuren and Carli both say that you can go with a 2-1/2" lift with stock radius arms. They're experts. I believe them. But at the same time, it might be prudent to dial in as much positive caster as you can once the new coils are in. Or, if you want, you could go pro-actively with radius arm drop brackets or new radius arms, just to be sure. I'm not a fan of radius arm root drop brackets for the same reason I'm not a fan of track bar drop brackets. It introduces more points of failure. Radius arm drop brackets are also precisely in the place that makes them most inconvenient when you're trying to slide under the truck. And if you spend any time off road, they represent new low hanging fruit for the rocks.

Install a steering box stabilizer no matter what you do. The crossmember on which the steering box is mounted acts like a big, stiff torsion spring, and it's shocking to see how much that box can twist around. A lift only aggravates the issue. A steering box stabilizer mitigates that by supporting the Pitman arm shaft at the end, so the torsion load on the box crossmember is pretty much eliminated. For $150 to $170 and half an hour's effort, a steering box stabilizer is the biggest bang for the buck we can put into our trucks. Your on-center feel and steering drift will improve dramatically. If there's ever a chance you'll get crazy and start upgrading antisway hardware, choose a steering box stabilizer wisely. Some won't work with torsion bar antisway kits.

AEV makes a nice leveling kit that moves the axle forward about an inch as well, if I remember correctly, in the interests of vehicle stability and tire clearance. It was pricey when I looked at it, and I wanted a bit more lift, but I think it's a third alternative in there with Carli and Thuren for very high quality 2" to 2-1/2" lifts.

To answer your question directly for the short term: if you retain your stock springs and go only for a 2-1/2" spacer, your ride will degrade ever so slightly because the radius arms will be operating at an increased angle. You will likely see some degradation if you replace your coils. The AEV literature addresses this and says "why should an aftermarket lift rely on nearly one-size fits all coils when Ram supplies dozens of different part numbers dependent on wheelbase and equipment? Why not take advantage of all that engineering?" It's a good point. If you go with very high quality radius arm root drops, spacers, and excellent shocks, and adjust your caster, you shouldn't notice much of a change at all... unless something's loose or you've pushed your axle more off-center.
 
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Discussion Starter · #8 ·
Our trucks' assembly tolerances tend to be pretty slack. You should check your axle center before you touch a thing. You might find that the axle is so far off center to begin with that a small lift on the order of 2" to 3" actually improves things.

I did.

Stock I found that my axle was almost half an inch off center. If I remember without doing the math, lifting the front end 3" results in a lateral shift of about an eighth or three-sixteenths of an inch. So after I installed my 3" coils and new radius arms, my front axle was STILL off center in the same direction as before, but slightly less so.

To fix that, I installed an adjustable track bar. An alternative would be a track bar drop bracket, which has the advantage of providing better geometry at the expense of having an additional failure point.

But... for such a small lift, what you'll find is that some adjustable track bars will interfere with the differential housing, especially if you have larger bolt heads, add-on hardware like a dual damper center bracket, or a finned cover. I discovered that, too.

Check first. If the off-center is going to get better or reverse slightly to the opposite side, I wouldn't worry about it one tiny bit. If your off-center will increase, then consider an adjustable track bar. The Carli unit's adjustment gear is at one end, not in the center, and therefore less likely to interfere with your center member. The thing is that the Carli unit, while an awesome piece of hardware, is difficult to adjust in-place.

More important than axle off-center is steering and suspension geometry. When you lift 2-1/2", you hit the point at which your radius arms' caster adjustment cams no longer have any significant positive range available. Our 2500 and 3500 front ends have a tendency to death wobble with weak sidewalls, low tire pressure, excess toe-in, loose ball joints, loose steering gear, bad steering damping, or insufficient caster. Bigger and heavier wheels and tires aggravate the issue. You can't eliminate it because the natural frequency of the steering geometry in steering excursion is almost precisely the same as the rotational frequency of 33" to 35" tires at 60 to 65 mph. So any tire and wheel imbalance tends to excite death wobble pretty much exactly at highway speeds. To make matters worse, the natural frequency of the cab floor in at least one primary mode is almost precisely half that -- one perfect harmonic down -- so any tire and wheel imbalance also tends to excite our dreaded seat shake.

Given a solid front suspension, the two most important factors affecting death wobble are tire sidewall stiffness -- tire quality, construction, and pressure -- and caster. Increased caster not only enhances on-center feel and stability, but provides a suppressive damping force that mitigates the tendency to death wobble.

So reducing your truck's ability to dial in more positive caster is not a good thing. You might need it some day.

It's totally possible that you can go with shocks and coils or coil spacers and never see a single issue. If, on the other hand, your truck is one that's just on the edge of wobble and shake now, you might introduce something that means you'll be fighting the wobble and shake nightmare for months or years. My truck apparently is on the hairy edge. If my front tire pressure gets low, or if my steering dampers get weak, or if my cams loosen and my positive caster shakes out, I develop death wobble. And that happened with a fresh alignment and absolutely everything except the axle itself new and tight.

Thuren and Carli both say that you can go with a 2-1/2" lift with stock radius arms. They're experts. I believe them. But at the same time, it might be prudent to dial in as much positive caster as you can once the new coils are in. Or, if you want, you could go pro-actively with radius arm drop brackets or new radius arms, just to be sure. I'm not a fan of radius arm root drop brackets for the same reason I'm not a fan of track bar drop brackets. It introduces more points of failure. Radius arm drop brackets are also precisely in the place that makes them most inconvenient when you're trying to slide under the truck. And if you spend any time off road, they represent new low hanging fruit for the rocks.

Install a steering box stabilizer no matter what you do. The crossmember on which the steering box is mounted acts like a big, stiff torsion spring, and it's shocking to see how much that box can twist around. A lift only aggravates the issue. A steering box stabilizer mitigates that by supporting the Pitman arm shaft at the end, so the torsion load on the box crossmember is pretty much eliminated. For $150 to $170 and half an hour's effort, a steering box stabilizer is the biggest bang for the buck we can put into our trucks. Your on-center feel and steering drift will improve dramatically. If there's ever a chance you'll get crazy and start upgrading antisway hardware, choose a steering box stabilizer wisely. Some won't work with torsion bar antisway kits.

AEV makes a nice leveling kit that moves the axle forward about an inch as well, if I remember correctly, in the interests of vehicle stability and tire clearance. It was pricey when I looked at it, and I wanted a bit more lift, but I think it's a third alternative in there with Carli and Thuren for very high quality 2" to 2-1/2" lifts.

To answer your question directly for the short term: if you retain your stock springs and go only for a 2-1/2" spacer, your ride will degrade ever so slightly because the radius arms will be operating at an increased angle. You will likely see some degradation if you replace your coils. The AEV literature addresses this and says "why should an aftermarket lift rely on nearly one-size fits all coils when Ram supplies dozens of different part numbers dependent on wheelbase and equipment? Why not take advantage of all that engineering?" It's a good point. If you go with very high quality radius arm root drops, spacers, and excellent shocks, and adjust your caster, you shouldn't notice much of a change at all... unless something's loose or you've pushed your axle more off-center.
Thanks that is a lot of great information and stuff I hadn’t heard as I’m just diving into this type of suspension other than a little basic knowledge. So let me ask this if I chose to stay with a 2” level to have more room for adjustment on my suspension parts , will I in fact be level with the rear or will I still have slight rake ?


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Thanks that is a lot of great information and stuff I hadn’t heard as I’m just diving into this type of suspension other than a little basic knowledge. So let me ask this if I chose to stay with a 2” level to have more room for adjustment on my suspension parts , will I in fact be level with the rear or will I still have slight rake ?
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No problem. Our front ends are pretty persnickety for such big hunks of steel, so information is always good.

I think that depends on your truck, whether it has air suspension, etc. As a general rule, trucks with factory rear air tend to be at near neutral rake stock, and trucks without factory rear air tend to have an inch or two of rake, stock. Your 2500 with coils all around will be a little different than my 3500 with extra leaves and overload packs in back. After 3" in front and 2" in back -- so a net reduction in rake of 1" -- I still have an inch or two tail high. I'm not the expert on 2500s at all.

You could always put a floor jack under your front axle, take out the tape measure, and see when a billiard ball stops rolling forward in the bed.
 

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Discussion Starter · #10 ·
No problem. Our front ends are pretty persnickety for such big hunks of steel, so information is always good.

I think that depends on your truck, whether it has air suspension, etc. As a general rule, trucks with factory rear air tend to be at near neutral rake stock, and trucks without factory rear air tend to have an inch or two of rake, stock. Your 2500 with coils all around will be a little different than my 3500 with extra leaves and overload packs in back. After 3" in front and 2" in back -- so a net reduction in rake of 1" -- I still have an inch or two tail high. I'm not the expert on 2500s at all.

You could always put a floor jack under your front axle, take out the tape measure, and see when a billiard ball stops rolling forward in the bed.
Yeah Iv done the jack to front end trick before it works but sometimes can be slightly off due to springs settling and all once you do a kit so I’d rather just have it a touch higher in the front then settle down to perfect level rather than be slightly higher in the back over time


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