Measuring Thrust Angle

[Black_Sheep]

What’s the best way to measure thrust angle at home? I know it‘s off because years ago the car was aligned with a Hunter alignment system, the tech said it was off and said not to worry about it…

The print out is long gone

 

[pnora]

If the alignment tech said it was off and not worry about it, why worry now. That is not something that gets easily adjusted as there is no adjustment. Depending on how far off it is shims can be made to shim the front spring mount. As far as measuring it is sort of complicated. 4 wheel lasers dial it to a 100th of a degree. Measuring it yourself you will not even be close. Can be done with strings. Google it.

 

[hunt2elk]

The 4 wheel alignment machines are the only accurate way to measure it that I know of. I messed around with the angle on both my B-bodies after the local shop got a new 4 wheel machine. I would have to look back at my notes, but I'm pretty certain I didn't shim either car more than 1/8".

 

[Black_Sheep]

If the alignment tech said it was off and not worry about it, why worry now. That is not something that gets easily adjusted as there is no adjustment. Depending on how far off it is shims can be made to shim the front spring mount. As far as measuring it is sort of complicated. 4 wheel lasers dial it to a 100th of a degree. Measuring it yourself you will not even be close. Can be done with strings. Google it.

I think the tech was lazy or didn’t have the shims needed to correct it. The car has always pulled to the right when I launch it hard, and feels a little twitchy on long sweeping left handed curves. Yesterday when I did the string alignment to set the toe in, the measurement from the string to the machined surface of the front hubs was over 1/2” different side to side, the right side being greater. That tells me that the diff isn’t square with the chassis. I put on a set of QA1 UCA’s and rebushed the LCA’s this winter so it will be getting an alignment when the weather breaks. I simply want to be the best that it can be. If correcting the thrust angle something I can do while I’m waiting for spring, now would be a good time to do it…

 

[pnora]

I seriously doubt the thrust angle has anything at all with your issues. This is not a great way but it might help. Distance from tire to front wheel well. If it confirms what or where you think it is off you can shim it now. You will pay a shop a hell of a lot more to do it. They will charge time and materials plus the alignment. Here is a simple way thats done with big trucks. Just remember both front wheels and both rear wheels need to be exactly the same. Same offset.

 

[Black_Sheep]

I seriously doubt the thrust angle has anything at all with your issues. This is not a great way but it might help. Distance from tire to front wheel well. If it confirms what or where you think it is off you can shim it now. You will pay a shop a hell of a lot more to do it. They will charge time and materials plus the alignment. Here is a simple way thats done with big trucks. Just remember both front wheels and both rear wheels need to be exactly the same. Same offset.

That‘s the same basic premise that I used to determine that my thrust angle is off. FWIW, I’ve done countless semi tractor and trailer tandem axle alignments using a Bee Line laser system. If I still had access to the equipment this would be a piece of cake. I could probably get close with the string method, but I’m not confident that it can be done with total accuracy.

 

[Cranky]

I think the tech was lazy or didn’t have the shims needed to correct it. The car has always pulled to the right when I launch it hard, and feels a little twitchy on long sweeping left handed curves. Yesterday when I did the string alignment to set the toe in, the measurement from the string to the machined surface of the front hubs was over 1/2” different side to side, the right side being greater. That tells me that the diff isn’t square with the chassis. I put on a set of QA1 UCA’s and rebushed the LCA’s this winter so it will be getting an alignment when the weather breaks. I simply want to be the best that it can be. If correcting the thrust angle something I can do while I’m waiting for spring, now would be a good time to do it…

In most cases cars with power on the 1/4 will try to go to the right because when the car launches, the left rear will try to bury into the pavement while the right rear will try to lift off the pavement. This is the very reason why street cars without a limited slip rear will do the one wheel peel with the right side tire. It's also the reason that 'Mom' added the extra leafs to the right side rear. It's always good to have your thrust angle right but I'm willing to bet you will still have the pull to the right feel on the launch after getting the thrust right. Lots of drag racers are adding the sway bar to the rear.....adjustable anti roll bar is what should be added to any faster drag car.

 

[pnora]

That‘s the same basic premise that I used to determine that my thrust angle is off. FWIW, I’ve done countless semi tractor and trailer tandem axle alignments using a Bee Line laser system. If I still had access to the equipment this would be a piece of cake. I could probably get close with the string method, but I’m not confident that it can be done with total accuracy.

View attachment 1433212

I agree on total accuracy. However I would still check it and look at the tire to front wheel opening. Then if I see something to correct I would shim the mount. At least then when you get the alignment checked it will be closer and maybe right on. One thing I did with the string is on the rear of the rear tire I used a work clamp to clamp the string to the tire. I also used the jack stand like you have.

 

[MoparLeo]

Here is some info for the members that are not familiar with what thrust angle actually means and what it does when out of spec. Mainly a rear axle alignment problem.
Alignment Specs: Thrust Angle and Setback - Tire Review Magazine

 

[pnora]

Here are some shims. I doubt any alignment shop would have these on hand. Looks like there is 6 shims in the kit. Mopar Thrust Angle Shims for Hotchkis Sport Suspension Leaf Spring Kits

 

[Black_Sheep]

The thrust angle can determine the straight-ahead position of the front wheels. So ignoring this angle can undermine even the most accurately aligned front suspension. It can result in a crooked steering wheel as the front wheels steer to align themselves with the desired direction of the vehicle. Also, a misaligned thrust angle can cause the vehicle to handle differently when turning one direction versus the other.

That's why I mentioned how it feels in a sweeping left curve, it has a noticeably different feel than the same type of curve to the right. the wheel was slightly crooked too, even though the front alignment was allegedly in spec.

 

[Black_Sheep]

Here are some shims. I doubt any alignment shop would have these on hand. Looks like there is 6 shims in the kit. Mopar Thrust Angle Shims for Hotchkis Sport Suspension Leaf Spring Kits

I have a set of those shims, ready and waiting...

 

[pnora]

I have a set of those shims, ready and waiting...

Looking at my FSM there is no thrust angle spec. under the front end section. I looked at my print out and there was no thrust specification range. It did show -0.16 as the reading. Not sure what would be considered excessive. As I recall -0.30 to +0.30 was a ball park. I do remember anything over -0.50 or +0.50 was excessive.

 

[dvw]

If the thrust angle was off enough to bother anything the car would be visibly dog tracking. The trouble with string is how to determine if the rear axle square at 90 degrees, has no toe in.ouy, or if it's offset right or left. So you need to establish an acurate vehicle centerline first. If you do that, you can be very acurate. If you're careful. The car pulling right is far more likely to be rear axle weight distribution. If the right rear is light, the car will pull right on hard acceleration. You have to remember we are dealing with 50 year old unibodies that weren't perfect when new. Do you think all the frame rails are going to be perfectly square? Or the rocker pinch flanges? Is the axle housing straight? The answer is probably not. Some are close, some not. The point of origin to start is the lower control arm torsion bar pivots. Drop a plumb bob from each side and draw a line in chaik on the floor between those two points. Measure to the center of those two points. Then plot a line 90 degrees to the first line. You now have a C/L. Compare everything to that and you'll find any issue. I built my 64 this way. Put it on a friends 4 wheel rack. Wheelbase measured the same on both sides less than .1. Axle thrust was 0 zero as well. it probably helps that iv'e probably done 5000+ alignments in my day.
Doug

 

[ckessel]

Depending on what your spring perches look like, there is a little adjustment there too. Loosen the u-bolts and shift the diff around. The perches most commonly used have a hole larger than the spring bolt head. I got some spacers that would alleviate this but when I set the diff on the springs after moving the springs inboard, USCT package, they were just a little too tight on tolerance to drop on. Could have made them go but I had a thought to leave them out which would give me some slack to use when its alignment time as the Thrust Angle is important. With the spring boxes being welded in there is not much I can do there on adjustment. If the room I have on the perches isn't sufficient to make the adjustment, I'll get some blocks to put between diff and spring where the centering pin can be adjusted.