12" lift questions

[Engineer]

Greybeard-

I'm really busy right now. I want to answer your question and to do so will take some time to properly explain it. Check back in a couple of days.

Also, when my wife and I were dating and first married we used to go to Lesterville. The last time we were there a guy I went to high school with got his '79 Ford F-150 4x4 smoked by a dude on an ATC-250R. The guy on the 250R was helicoptered to StLouis. The guy I went to high school with had to have his Ford towed home do to the extensive front end damage. Never found out what happened to the guy on the 250R. Darryl's Ford had to have the front end sheet metal replaced and the crash ripped the steering box apart.

And check out my build thread. I have a post in there where I describe visiting a guy's shop who built Bigfoot's last two race trucks.

[Greybeard]

NP.

For thirty or more years I have lived by the rule that Chevy steering has to have the pitman end at the draglink and the steering arm end at the drag link at as close to parallel with the truck and ground as is possible and practicable. Plus or minus an inch does not hurt too bad. Four inches causes steering problems, all a bent drag link does is connect the pitman to the steering arm so the truck can be steered, it does not fix any misalignment between the two...it's band-aid.

enaberif

Have you played with air pressures? There is a sweet spot for every tire and that spot is not necessarily the pressure on the door jamb or the sidewall. My Mickey T Tall Bajas (39-15/15) took 30 psi from the sidewall. I ran about 13psi all the time on road and went as low as 3psi offroad and never had any problems. Of course they were bias belted tires. My current Dodge 2500 I have ten ply rated tires on it and run about twenty pounds less than the sidewalls ask for and have for the last three years. Just sayin....sidewall pressure is for max loads on that one tire. I have never come close to loading my truck to that max weight. A couple weeks ago I did air them up to max pressure because I had one ton of catfood in the shortbox and three tons in a trailer. I'm certain I still never came close to loading those tires to near their maximum capacity though.     

[enaberif]

Its not tire pressure its straight up rough suspension. I live in a City where the roads will bottom out a lot of cars so on a solid axle truck it just jars the crap out of you everywhere. Now it could simply be a choice of springs and shocks which I could improve down the road.

[Irish_Alley]

have to disagree its a combination of both. i know one set of tires i have are max at 85lbs. theyre now on my ½ton with only 40psi even the tire shop asked if i would be hauling anything, i told them no. they said they would put 40 lbs in them so it doesnt ride so rough. its not a bad ride and the wife doesnt complain.

[enaberif]

I'm running with 32-35psi in my tires so I know its not that. I don't want to derail the thread it was just a comment I made about how I think a 12" lift would be rough.

[Greybeard]

I don't know, I think we've beat the 12" horse to death already. 

But just to make certain....I believe the OP has determined that everyone here cannot be totally wrong and has determined that good judgement is to sell the 12" lift and use the proceeds to buy something more practical. The way I see the 12" lift riding is similar to writing the letter U and putting a truck on top of it. Both ends of the U point nearly straight up. How on earth can that flex? And without 12" long shackles there is not enough movement in the spring end anyway. Of course my numbers are not accurate but the idea is still valid.

As for tire pressures in over sized tires...I read on offroad magazine years ago (one of the magazines anyway) the way they figured it was most practical to find the desired tire pressure for a given tire on a given truck. I used the technique and found it worked pretty good. It may not work as well with radials but it still works OK.

It's simple, use some kids sidewalk chalk, make a good wide, about 1/2" wide will do, mark across all the treads from sidewall to sidewall in a couple of different places on the tires. No need for uniformity. Do this in a parking lot or someplace with fairly clean, really dry, and smooth asphalt. Drive the truck in a figure eight two times at least at a reasonable speed but not to fast. Try not to make the figure at steering lock tightness either. Look at the chalk stripes. If they are not worn off drive another lap or two and check them again. The chalk should wear faster off the high spots of the tire tread IE, the sides if under-inflated, the center if over-inflated. Some experimentation on the speed and tightness of the turns used might be necessary. It works though. It, like a lot of other things, just takes time and patience (and no cops wondering what the heck you are doing). 

[roundhouse]

Ive done the same thing with chalk.   But drive in a straight line and see how it wears of


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[Irish_Alley]

i know with racing, if the tire temp isnt even you dont have maximum tire contact. which if you want top speed less is better but if you want acceleration more is better. i keep that in mind when airing my tires. max psi will cause a high center, wont be good for traction but better for mpgs. low center will be good for traction but more drag so less mpgs. too much of either or will have more problems.

[Greybeard]

Yup, but the issue here is ride quality, there is no all around perfect air pressure for all. Like all this automotive, the stock tires had the pressures set for a compromise. That's the best one can hope for I believe.

And maybe the test with the chalk was in a straight line, I read that article about 20 years ago. Details elude me at times.     

[Engineer]

I'd like more of your thoughts on the stock steering setup. You mentioned that you had an 8" lift on your truck and used a block under the steering arm and a dropped pitman arm. With a D60 this is all that is necessary with a 8" lift to get the steering as close as it can be. A D44 will still come up rather short of the desired alignment IIRC. However, beyond 8" I shudder to think of those that use the bent drag links. They do work (if reduced lock-to-lock steering is acceptable) but like every piece of steel that has a bend in it the overall strength of the rod has been severely compromised under compression not to mention that the angle between pivots, like you said, should remain as level as possible no matter the style of steering used. Even though the axle housing does move in a rearward plane (arc?) during spring compression, the actual movement does not cause as much feedback through the steering as does the misaligned pivot points. Am I correct?

Greybeard think of it like this:

(Before you start reading don't think I assume you to know nothing. I realize you already know most of what I am posting here but for the benefit of those who don't it may read a little elementary. I just hope to explain what I have to say so that everyone can understand it.)

Position yourself outside of our trucks (73-87 Chevy/GMC) looking at the driver's side front tire. Assuming a stock suspension the spring is flat. With this spring you get very little extension/contraction of the length of the spring as it cycles. Additionally the spring is only slightly canted downhill toward the rear of the vehicle by about 3° (you have about 4° built into your axle housing for roughly 7°total caster) to facilitate suspension cycling as the truck is moving forward. Therefore on an unmodified truck it is most optimal to have the steering pivots in a level plane with the horizontal axis of the truck.

Now as lift is added to the truck typically this is done by adding arch to the front spring. With the arch, as the suspension cycles the length of the spring will grow, or elongate. Since the front of the spring is anchored to the frame the axle will deflect rearward with the elongation of the spring. The more arch you have the more the spring will elongate for a given deflection.

One thing that is constant regardless of how much arch a spring has is the fact that the axle will deflect exactly 1/2 the distance of the total deflection of the spring. Now this is dependent on having a spring anchored on one end and a pivot on the other end. And the axle must be located in the center of said spring. (As is the case on our trucks)

Now back to our truck. Assume we have an 8" lift. If we are to look at our left front tire from the side and we can envision the axle moving upward from a bump in the road, let's say the axle deflects rearward 1" as it is cycling. In order to prevent bump steer with our stock steering we would need the rear steering pivot to motion rearward the same 1". Now we know the front steering pivot will stay stationary to the horizontal plane of the truck because the steering box is mounted solidly to the frame in the same manner as the front of the spring. We also know the steering pivot on the axle will deflect up and down exactly as the axle does. With our factory steering, look at the front steering pivot as if it is the center (or axis) of an imaginary circle. Now picture the rear steering pivot as the out side of the same imaginary circle. The typical drag link on our (73-87 Chevy/GMC) is about 15" long. If we were to drop the rear steering pivot, use 3" as a baseline, below level this would cause the pivot to move rearward a measured amount as it rotates around that imaginary circle as the axle cycles upward, and of course rearward as we compress our 8" rearched spring.

The way to limit or stop bump steer altogether is to build correction in our steering that finds the "sweet spot" in the imaginary circle that will allow the correct forward and rearward travel of the rear steering pivot in a way it matches the forward/rearward travel of the axle.

[Engineer]

Graybeard to answer one of your questions that I missed in my last post, I should've added that you want your crossover link as flat as possible because if everything is correct in your suspension the axle shouldn't cycle sideways.

But this flat link theory does not apply when using modified factory steering. With modified factory steering you definitely can't run the steering pivots level with the ground and have the proper geometry as I have previously explained.

[enaberif]

The spring cycle being talked about is one of the reasons why longer shackles exist for the front. Its quite common and possible that with lifted springs the spring itself can and will hit the frame.

[Greybeard]

I see what you are saying, however, in order to lesson the total bump-steer the drag-link needs to be in as neutral a position as possible to facilitate both up and down motions. That rearward movement can never be eliminated as you say. Therefore, by starting in a position that does not initially push against the pitman arm any bump steer is reduced to the minimum during the upward and rearward movements of the housing. If the drag link starts at an upwards angle towards the pitman arm, that imaginary arc (as the truck frame/body moves down in relation) and the very real rearward movement of the housing moves it (the drag-link) directly into the pitman and subsequently through to the steering wheel.
A drag-link that starts level between the steering arm and pivot arm ends reduces the total amount of deflection the pitman will realize as the suspension starts to move in either direction.
The way I visualize it is that as soon as the drag-link moves out of horizontal between the end of the pitman and the steering arms the effective length of the drag-link starts being different (meaning the distance between point A and point B changes). As you described, the rear pivot point moves down (or up) in an arc while the front pivot point basically remains moving in a relatively flat linear motion moving from static to forward during droop to rearward as it moves into compression. Nevertheless, the wheels in a moving truck are seldom ever pointing in one direction for very long which helps to negate the feel of the bump-steer even more.

Regardless of whether it can be eliminated completely or not, it can be reduced to negligible amounts by keeping the drag-link as level as possible no matter the amount of lift. However, as the lift increases the returns diminish because the components involved lose strength very quickly assuming the loads that are placed on them by the resulting very over-sized tires that are commonly the reason for the ultra-tall lifts in the first place.

And I completely agree with the crossover idea, anyone familiar with solid coil sprung front ends understand the sideways movement that setup brings into the equation. Leaf springs would break or bushings would quickly be destroyed with that kind of constant flexing.

Not arguing here, just trying to see if we or I am on the same page. The science may say I'm wrong, physical experience says I didn't have any perceptible bump-steer with a six inch lift, 4" steering arm, and 2" dropped pitman.   

[Greybeard]

The spring cycle being talked about is one of the reasons why longer shackles exist for the front. Its quite common and possible that with lifted springs the spring itself can and will hit the frame.

With a 6" lift or taller this is a real possibility. However, the reality is that even when I rock crawled with my truck in Colorado I could not compress or droop my suspension enough to cause that. At least with a SWB the weight of the rear of the truck allows the frame to flex first, then the rear wheels lift off the ground because they are short and light (even with a 14 bolt). I can't say what a LWB does in this regard because I have never wheeled in a LWB truck. I know I had my truck so twisted up the front side of the box had about four inches of it hanging outside the cab line (with no body lift). Maybe the shocks were the limiting factor, I don't know. Going slow is different from jumping the truck however. Mine got jumped once by a buddy. He broke the front driveshaft is all. I didn't see it happen but the group told me he got about five feet of air under the front tires. The rear not so much because he had a thousand pounds of monster truck tire in the bed. That's what I get for being the nice guy and lending my truck out so the monster truck with the flat tire could get back to camp.