How to determine and adjust centerline thrust?

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TrailBeast

TrailBeast

AKA Mopars4us on Youtube
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Hey all, I have a question about centerline thrust.

I know what centerline thrust is, and why it is important but how does one determine what the centerline thrust is, and when it is necessary to adjust it?

I have a feeling mine is out of line a little due to a slight sideways hop of the rear end over certain types of bumps.
For example, a line or crack in the pavement that both rear tires go over at the same time.

Thanks
 
I've never heard the term. I googled it, and only got hits re: aircraft.

If you are talking about general frame alignment, there are measurements somewhere in the shop manuals in the body section. Just like modern cars with IRS, you can check front/ rear alignment.
 
I'm wondering if it is as simple as your rearend being a little out of line. When you hit a bump one tire grabs a split second sooner than the other giving you the feeling of sideways motion.

I know what you are talking about as I have felt it before on numerous vehicles, never really gave it much thought though.
 
67Dart273 said:
I've never heard the term. I googled it, and only got hits re: aircraft.

If you are talking about general frame alignment, there are measurements somewhere in the shop manuals in the body section. Just like modern cars with IRS, you can check front/ rear alignment.
Click to expand...

Hey Del, thanks for answering.
Centerline thrust in cars is how the rear axle of the car is lined up with the rest of the car as it goes down the road.
They (auto accessory) places and alignment places sell and use plates between the front spring hanger and body to adjust it.

What I was wondering is what measurements and measuring points they use to determin it.

I googled it also and found this article about it that says,


"The thrust line denotes the actual direction of the rear wheels (a right-pointing thrust line or a left-pointing thrust line). This is the real-world direction in which the rear wheels are aimed, irrelevant of the geometric centerline. You can also view the thrust line as a line that divides the left and right rear wheel toe.
Thrust angle is the angle formed by comparing the chassis geometric centerline to the rear wheel thrust line. This angle is measured in degrees. A right-aimed thrust angle that deviates from the centerline is referred to as positive, while a left-aimed thrust angle is negative. "

Here's the article, but it doesn't say how to determin it.
http://www.motor.com/article.asp?article_ID=1094
 
Very common problem for those who measure their cars carefully before aligning. I developed a shim that fits between the front spring hanger and the body to adjust rear toe. Chances are its short on the pass side. PM me if you are interested in these and more information.
 
Bad Sport said:
I'm wondering if it is as simple as your rearend being a little out of line. When you hit a bump one tire grabs a split second sooner than the other giving you the feeling of sideways motion.

I know what you are talking about as I have felt it before on numerous vehicles, never really gave it much thought though.
Click to expand...

This is exactly what I am thinking, and it is also what centerline thrust adjustment is for.


GMachineDartGT said:
Very common problem for those who measure their cars carefully before aligning. I developed a shim that fits between the front spring hanger and the body to adjust rear toe. Chances are its short on the pass side. PM me if you are interested in these and more information.
Click to expand...

Appreciate the offer, but I am capable of making them if needed.
You said "chances are it's short on the passenger side"
What are you basing this on? (are they commonly short on that side?)

And "short" measured from where?
This is what I need to know.
 
The best way IMO is to get a printout with your alignment specs including the thrust angle. Shimming the rear will move the thrust angle, but will not adjust rear toe.
 
6pk2goDemon said:
The best way IMO is to get a printout with your alignment specs including the thrust angle. Shimming the rear will move the thrust angle, but will not adjust rear toe.
Click to expand...

Ah Ha!
So it is measured off the rims then.
That makes sence.

Thank you

For some reason I was thinking it was measured from the rear axle center to some point on the frame or body.
Duh :D
 
6pk2goDemon said:
The best way IMO is to get a printout with your alignment specs including the thrust angle. Shimming the rear will move the thrust angle, but will not adjust rear toe.
Click to expand...

+1

It really isn't something easily measured and it is mush easier to correct on an alignment rack. Loosen your front spring hanger and install shim between hanger and vehicle frame. When dealing with shims a little goes a long way. We shim rear ends on fwd solid rear axle toyota cars all the time here.

You will need to find a friendly alignment shop or a friendly alignment tech that doesn't mind a time consuming after/off hours alignment. Most shops don't like to tie up a rack during business hours with old cars and alignments that are more than toe N go.
 
mcnoople said:
+1

It really isn't something easily measured and it is mush easier to correct on an alignment rack. Loosen your front spring hanger and install shim between hanger and vehicle frame. When dealing with shims a little goes a long way. We shim rear ends on fwd solid rear axle toyota cars all the time here.

You will need to find a friendly alignment shop or a friendly alignment tech that doesn't mind a time consuming after/off hours alignment. Most shops don't like to tie up a rack during business hours with old cars and alignments that are more than toe N go.
Click to expand...

The only shop my car ever see's, or will see is a tire shop.
I can handle it at home, just needed some info.

Thanks all
 
Chances are, the wheelbase is short on the pass side needing shims on the pass side. Shimming WILL change the toe. Cross measure from the centerif the leaf spring bolts that peek through the shock plates to similar measuring holes in the frame. Make sure both sides are equal. If the toe us off, it's all in one direction.
 
It can be done easily in your garage using laser level(s). The spec you are shooting for is zero, or, equal measurements on each side. If the rear housing is bent your total toe will not be zero and many factory rears are off slightly and not causing any problems. Adding shims will not change total toe but will correct an incorrect thrust angle. Example: if you have toe in on the drivers side and toe out on the passenger side, you add shims to the drivers side until both sides are the same. I start by setting up a string line down the center of the car measuring at several points off the frame. I extend that string out the front as far as possible. Set up a laser level mounted on the face of the rear wheels, sometimes you need a standoff. I use a piece of 2" x 2" thinwall square tubing. Aim the beam toward the front of the car and level to the floor. Now make your measurements just in front of the rear tire to the centerline string and again as far forward as you can from the laser dot to the center string. Both measurements on the pass side should be the same and both measurements on the drivers side should be the same but both side dont have to match because very few Mopars have the rear perfectly centered. I hope this gets you started. I probably left out a few things but I hope you get the principle.
 
Also don't forget that MANY vehicles do not have the same track WIDTH front to rear.
 
...............Del isright...........mopars have set back designed into there geometry, as in the passenger front wheel is farther ahead than the drivers for better handling.........kim.......
 
Ok, thanks guys.
I use lasers to do most of this kind of stuff already, and I used them to get the frame flat before the subframe connectors went in.
 
Just FYI from one of my class lessons from when I was teaching.

Thrust Angle

Thrust angle is the direction that the rear wheels are pointing in relation to the centerline of the vehicle.
The vehicle will "dog track" if the thrust angle is not zero and the steering wheel will not be centered.
The best solution is to first adjust the rear toe to the centerline and then adjust the front toe. This is done during a all wheel alignment if the rear toe is adjustable. If the rear is not adjustable, then the front toe must be set to compensate for the thrust angle, allowing the steering to be centered.
If the thrust angle is not correct on a vehicle with a solid rear axle, it often requires a frame straightening shop to correctly reposition the rear axle.
So in addition to the handling problems that are the result of incorrect toe settings, thrust angles can also cause the vehicle to handle differently when turning left vs. right.
.

Axle offset: Axle offset is when the rear axle cocks sideways in the vehicle causing both rear tires to point in the wrong direction. Remember just like a forklift if the rear tires point to the left the vehicle will steer to the right. This condition is typically caused by a broken leaf spring center bolt or problems with the shackle. On vehicles with trailing arm suspension worn or damaged bushings or a bent arm can cause a pull in the steering wheel.
 
run it through water and look at the track...I see old beater vans that the rear track is an entire tire width off the front track, funny as heck drving behind one. The laser measurement assumes the rear is centered, which is not the case in mine as I got a tad more clearance on one side. Another way would be to take a laser 90 degrees off the rim to a point a measured ways away and then off the centerline or somewhere that is parallel with the body line up front. Measure between the dots, than take the same off the other side. The distance should be equal if the rear track is square, itll be off if its not, the more distance you take the reading off, the more exagerated the angle will be. Buddy has a cool laser that projects a +, set that up in front down the center and then you got your centerline and reference points out in space.
 
No need for any fancy lasers, just use a string and plumb-bob. Drop the bob off various points (torsion bar mounts, K frame bolts, rear end housing, spring mounts, etc..) on the car and mark those spots on the floor, then crisscross the measurements to see if anything is out of whack.
 
Kim is correct. I back-haled a Duster, went to 4 link. You have to put it on jack stands,(rear end has to be jacked up to ride height, same with lca's), level the car, drop plumb bobs from all 4 wheels. Each one from the same point; like, lower ball joint fitting, rear axle face, Takes a level to do that.

Put the marks on the floor. Mark though the points, across, and front to rear.

Measure; a mopar is off 1/2" front to rear. No matter how you move it; shims, 4 link, etc; once front to back measurements are good. Now pull a diagonal measurement from back to font, opposite sides of the car. It will be off, as the suspension isn't square. adjust, get it squared.

New ride, launch straight, no squeirely trap action.
 
oldkimmer said:
...............Del isright...........mopars have set back designed into there geometry, as in the passenger front wheel is farther ahead than the drivers for better handling.........kim.......
Click to expand...

I have a rebuilt Kframe waiting for me to install it. I checked the measurements from the lower control arm spindle hole to the mounting holes and they are slightly forward on the passenger side. I was going to shim the strut rod to bring it back. Now I am thinking it might not be a good idea. It's about 1/8th+".
 
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