Track bar / drag link geometry

[ksagis]

Figured better to start a new thread versus continuing to hijack @Torkman66 thread.

Here's a pic of my front end showing the drag link and track bar geometry. The drag link is 41.25 inches long and inclined 6.05 degrees above horizontal. The track bar is 34.75 inches long and inclined 9.4 degrees. You can see the COs and the hydraulic ram on back side on tie rod linkage. Things I've checked: ram size is 1.5" (info available to me is bigger than 1.5 inch rams can make things twitchy), and the ram is rigged to be parallel to the tie rod (info available to me is the ram line of action needs to be parallel to tie rod linkage). I've also bounced front suspension per our convo in other thread and don't see any movement in steering wheel (sound like not a great test anyway)

I'm told by PO that the suspension mods were done by a very reputable builder so thinking/hoping it's not a simple as "are the drag link and track bar parallel"? i.e. seems like the relative length plays into how bump steer may or may not be excited with suspension movement - since for a 2" drop of suspension the relative lateral movement is a function of length of the rod since the two bars are going to rotate a different amount. (this gets more complicated for only one tire going up or down if the drag link and track bar don't connect to the same point on the axle, I'm gonna ignore that for now)

My thinking: for equal length arms, you want them parallel, not clear to me how that math changes with different lengths of track bar and drag link, does anybody have expertise in that area? Looking at the picture @lars posted in Torkmans thread, it looks like he very nearly matched lengths and angles which seems to have given super good results.

Several of you like @Yeller @ntsqd @jamesroney had given me some guidance on other thread so tagging them here in case the picture and the additional data gives them more insight of where I should poke next. If anybody know the math versus me CADing out my question above, would certainly appreciate it.

 

[1969]

I would start by removing your drop track bar bracket and drop pitman arm.

What’s your caster at?

 

[toddz69]

Does the truck drive poorly now? I ran out to the garage and measured mine for comparison. I have almost the exact same angles on my trac bar and drag link, although my lengths are about 1.5" different than each other.

Todd Z.

 

[ksagis]

I would start by removing your drop track bar bracket and drop pitman arm.

What’s your caster at?

7.1 left and 7.6 degrees right caster. Alignment sheet attached. Note I adjusted toe myself after they measured and gave me this sheet, I'm running about 0.3 degrees of total toe now which is about 3/16" total toe.

 

[ksagis]

Does the truck drive poorly now? I ran out to the garage and measured mine for comparison. I have almost the exact same angles on my trac bar and drag link, although my lengths are about 1.5" different than each other.

Todd Z.

Honestly, I don't know what I should expect in terms of drive poorly or not. I remember in the 1980s in high school they didn't drive great, but I attributed that to high school mechanics and the relative technology then (versus what I think we have now)

I keep hearing things like "my rig drives great", and "I can drive one handed", etc and beginning to wonder if either they are crazy, or I am crazy! My general summary is straight line tracking is totally fine with me, but hitting a pot hole at freeway speeds seems to handle more poorly than I am hoping for (tho maybe I am mis-calibrated)

Interesting that you have nearly matched lengths and fairly different angles, which maybe blows up my theory a bit. Hmmm.

Appreciate the reply!

 

[1969]

Have you considered adding a sway bar?

 

[DirtDonk]

What’s the distance between the top of the axle tube and the bottom of the frame rail?
Just curious how much your actual lift is.
I wouldn’t get rid of the track bar drop bracket or Pitman Arm. In fact, I’d keep the Pitman Arm and grab (or make) a new, taller drop bracket.
The one you have looks like one of the old Duff’s units. Or a similar copy. Very beefy and nice, but not tall enough for a 3.5” lift. Maybe perfect for a 2 1/2, but it still doesn’t match the Pitman Arm. And a lower angle is almost always better anyway.
Measure the distance between the top and bottom bolt, just out of curiosity.

While I do agree that a shorter track bar probably changes the angle more for a given amount of compression or extension of the springs, I still think keeping them as close to parallel as possible has its advantages over the way it is now.
So, short of changing the tie rod type to get a shorter drag link to better match the track bar length. And short of going through the trouble of extending the track bar to better match, the drag length length, I would simply start by lowering the upper track bar mount slightly to better match the deeper drop of the Pitman Arm.
Seems like a simpler path.

And once you do get them close and dialed in, I would make sure that the track bar bracket is welded as well as bolted.

Have you seen the descriptions of doing the steering test that we have given different names over the years? Where a helper racks the steering wheel back-and-forth while you watch underneath?
It’s a real legit test of everything. Lets you see things like the steering box flexing on the frame, or track bar moving on its bolts, or a mount flexing.
Can be a real game changer sometimes.

 

[Yeller]

I’ll be back with more, not much time today. If it were mine I’d rework the track bar and get it longer and closer to parallel. It’s not terrible now and really close enough for being parallel but the length difference is what is kicking your tail, it does have some bump steer. The overall slope angle looks good.

 

[Johnnyb]

Figured better to start a new thread versus continuing to hijack @Torkman66 thread.

Here's a pic of my front end showing the drag link and track bar geometry. The drag link is 41.25 inches long and inclined 6.05 degrees above horizontal. The track bar is 34.75 inches long and inclined 9.4 degrees. You can see the COs and the hydraulic ram on back side on tie rod linkage. Things I've checked: ram size is 1.5" (info available to me is bigger than 1.5 inch rams can make things twitchy), and the ram is rigged to be parallel to the tie rod (info available to me is the ram line of action needs to be parallel to tie rod linkage). I've also bounced front suspension per our convo in other thread and don't see any movement in steering wheel (sound like not a great test anyway)

I'm told by PO that the suspension mods were done by a very reputable builder so thinking/hoping it's not a simple as "are the drag link and track bar parallel"? i.e. seems like the relative length plays into how bump steer may or may not be excited with suspension movement - since for a 2" drop of suspension the relative lateral movement is a function of length of the rod since the two bars are going to rotate a different amount. (this gets more complicated for only one tire going up or down if the drag link and track bar don't connect to the same point on the axle, I'm gonna ignore that for now)

My thinking: for equal length arms, you want them parallel, not clear to me how that math changes with different lengths of track bar and drag link, does anybody have expertise in that area? Looking at the picture @lars posted in Torkmans thread, it looks like he very nearly matched lengths and angles which seems to have given super good results.

Several of you like @Yeller @ntsqd @jamesroney had given me some guidance on other thread so tagging them here in case the picture and the additional data gives them more insight of where I should poke next. If anybody know the math versus me CADing out my question above, would certainly appreciate it.

Where'd you get those nice frame-mounted bump-stops?

-JB

 

[ksagis]

Have you considered adding a sway bar?

I have, but adding a sway bar doesn’t seem relevant to this particular issue, could you elaborate how it might help?

Also, you mentioned above about removing drop pitman arm and track bar bracket. Seems to me that flatter angles are preferred. Could you elaborate on that comment too please?

Thanks for the reply.

 

[ksagis]

I’ll be back with more, not much time today. If it were mine I’d rework the track bar and get it longer and closer to parallel. It’s not terrible now and really close enough for being parallel but the length difference is what is kicking your tail, it does have some bump steer. The overall slope angle looks good.

Adding track bar length seems hard with the position of the CO, interested in your comment there. I was thinking if length was issue to move the drag bar attach from knuckle to the tie bar sorta like the T style of early models.

Sounds like Todd’s setup is roughly consistent with your guidance to me on the difference in lengths is place to start. (“I have almost the exact same angles on my trac bar and drag link, although my lengths are about 1.5" different).

And Lars pic showed very nearly same length and very similar angles

Totally get the issue of “not much time”, my life has been a blender the last 18 months, appreciate the reply when you have time.

 

[ksagis]

What’s the distance between the top of the axle tube and the bottom of the frame rail?
Just curious how much your actual lift is.

Going off memory, it's 12" in front (think the guidance is 7" is stock so translates to 5" of lift), and 10" in back (think guidance is 6" is stock so translates to 4" of lift in back). Sits quite level so not sure about the difference.

Measure the distance between the top and bottom bolt, just out of curiosity.

Track bar top bolt was 25 3/8" and bottom was 19 5/8"

Have you seen the descriptions of doing the steering test that we have given different names over the years? Where a helper racks the steering wheel back-and-forth while you watch underneath?
It’s a real legit test of everything. Lets you see things like the steering box flexing on the frame, or track bar moving on its bolts, or a mount flexing.
Can be a real game changer sometimes.

I have seen and done those tests. All of front end looks really solid. And even though bracket is bolted in, it's got a lot of material higher on frame and reinforced frame so seems okay. Agree welding couldnt hurt.

 

[Yeller]

In the pic it hard to see the front to back spacial orientation of the components. A lot of builders insist on putting the coilover on the center line of the axle, which is completely unnecessary and complicates track bar and steering terribly

 

[ntsqd]

What is the horizontal distance, and the vertical distance between the frame end of the trac-bar and the pitman arm end of the drag-link? Easy enough to plot the two arcs of travel with those numbers added to the numbers posted at the top.

Looks to me like parallelism could be greatly improved by simply moving the drag-link to the top of the dropped pitman arm. It may not work well in real life there, but it would be closer to parallel. However, that doesn't solve the length difference problem.

 

[Apogee]

I'm surprised it doesn't drive better than you describe...lots of folks running similar looking geometries without the unsettled handling characteristics through the bumps...and are we talking pot holes or POT HOLES? Could it be a spring rate or damping rate issue with the coilovers? Just from what you describe, I would suggest checking/replacing the track bar bushings on a more stock-ish setup, but everything looks new under there. Adding ram-assist typically slows steering down if anything, but the Ø1.5" bore ram should be the least offensive in that way so long as your pump is up to the task. I'm adding a Ø1.75x8 PSC ram from WH4x4 to mine, and other than the added diameter being a little tougher to package (things are tight to say the least), function should be similar.

I haven't driven mine yet with the new axles and steering, but I have a similar ~41" drag link length with GM TRE's, and in order to get my track bar length and angle to match, I pushed the frame side mount down and out a bit, though less down since I'm not running a dropped pitman arm. The attached image shows the suspension at full stuff (or within about 1/2"), noting that my track bar just clears the front sump on my double-sump oil pan. If mine looks a little different, it's because it's a HP D44 from an F150 at the 65" width, so I have a little more axle tube to play with than you do.

Is the coilover in the way, or could you just replace the frame side track bar mount with one that positions the track bar parallel with the drag link and pushes the mount outward to get closer on the lengths? Matching up the drag link and track bar angles and lengths certainly won't hurt anything, but it may not solve the drivability issue described.

 

[DirtDonk]

Going off memory, it's 12" in front (think the guidance is 7" is stock so translates to 5" of lift)…

Well, there’s your problem right there!
OK, not necessarily “a problem“ but it certainly complicates things 5 inches of lift cannot be compensated for simply with a track bar drop and a Pitman Arm. Your angles remain too steep. So yes, in my opinion, that’s part of your issue.

Track bar top bolt was 25 3/8" and bottom was 19 5/8"

So you’re saying that, in that picture of the track bar drop bracket, the spacing between the two visible bolts is 5 3/8 inches? Sure doesn’t look like it!
Or are you referring to the vertical difference between the upper and lower trackbar pivot bolts?

I have seen and done those tests. All of front end looks really solid.

Good. Just wanted to get that out of the way in case something hidden was causing you a little bit of the trouble.

And even though bracket is bolted in, it's got a lot of material higher on frame and reinforced frame so seems okay. Agree welding couldnt hurt.

I think you still have some mocking up to do to find your best set up given the components.
Short of creating a whole new steering system to better match the length of the two bars, you could do other things in the meantime.
Such as going tie-rod-over to lower the draglink angle even more, then adding a trackbar riser to fine tune your trackbar angle to better match the draglink angle.
I still think you need to get your angles down some. Having five inches of suspension lift just makes that high on the list.
The difference in length will still be an issue, but at least your angles won’t be as big a contributor, exaggerating those issues.

 

[toddz69]

I'd love to see side pics of your front suspension too. It appears you have a 4 bar linkage or something similar which means the sway bar effect of the radius arms may no longer be there. That, coupled with your lift and the dropped pitman and trac bar at the frame might be contributors to some undesirable handling. I think your trac bar and drag link angles are fine, personally. I don't have a stock truck handy to measure, but I'm guessing they're shallower than stock rig's pieces.

Todd Z.

 

[ntsqd]

What is the horizontal distance, and the vertical distance between the frame end of the trac-bar and the pitman arm end of the drag-link? Easy enough to plot the two arcs of travel with those numbers added to the numbers posted at the top.

Looks to me like parallelism could be greatly improved by simply moving the drag-link to the top of the dropped pitman arm. It may not work well in real life there, but it would be closer to parallel. However, that doesn't solve the length difference problem.

I guessed at these dims and placed them at 2.0" down and 3.0" to the left. If that is correct, then below is the simple curves plot. The 3.737 dim is the horizontal distance between the axle end of the drag-link and the trac-bar. The 3.661 dim is the same measurement taken at 2" bump; and the 3.966 dim is the same measurement taken at 2" droop. Which means that at 2" bump the RS steering arm is turned .076" to the left, and at 2" droop the RS steering arm is turned .229" to the right. Don't need SW to get these results, but it sure makes it easy to do.

 

[ksagis]

I guessed at these dims and placed them at 2.0" down and 3.0" to the left. If that is correct, then below is the simple curves plot. The 3.737 dim is the horizontal distance between the axle end of the drag-link and the trac-bar. The 3.661 dim is the same measurement taken at 2" bump; and the 3.966 dim is the same measurement taken at 2" droop. Which means that at 2" bump the RS steering arm is turned .076" to the left, and at 2" droop the RS steering arm is turned .229" to the right. Don't need SW to get these results, but it sure makes it easy to do.

Software and CAD for sure way to go, I tried some trig and my head started to hurt. Wouldn't it be more centered on bump and droop cases? You moved the entire axle up and down, versus one side for this case, correct?

I'll get the measurements you mentioned above after work today but probably for the purposes of this exercise, they're close.

0.229" seems like a lot....

P.S. Always love the math you bring along with your strong mechanical expertise.