CHASSIS - Alignment Specs: Thrust Angle and Setback

 CHASSIS  - 

Alignment Specs: Thrust Angle and Setback

By Andrew Markel on Jun 8, 2012

A member of Talk Morgan from Dallas asked the question 'I have a 2005 Roadster. I was wondering what the "normal from the factory" rear thrust angle measurement is for a Roadster. Is it possible to change the setting?'

Having confused many members, who like me had not even heard of the expression before, another member introduced a link to this topic. 

http://www.brakeandfrontend.com/alignment-specs-thrust-angle-and-setback/

I thought that it was worth adding for those of you who would have an interest in such matters. 

Frank

The thrust angle is an imaginary line drawn perpendicular to the rear axle’s centerline.

 It compares the direction that the rear axle is aimed with the centerline of the vehicle.

 It also confirms if the rear axle is parallel to its front axle and that the wheelbase on both sides of the vehicle is the same. 

It is one of the most important diagnostic angles during an alignment.

To measure the thrust angle on a vehicle, you have to perform a four-wheel alignment. Even if the rear axle is non-adjustable, you need to take rear axle readings to properly align the front suspension.

A thrust condition exists when the rear individual toe is not equal. The thrust angle of a vehicle can be generated by two conditions or angles.  This makes it difficult for some technicians to properly diagnosis the problem. 

First, the thrust angle could be generated by the angle of the axle or a misaligned rear suspension cradle that can change the toe angles. Also, a thrust angle can be generated by rear toe settings that are independent of the axle angle or implied axle angle.

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.

RWD VEHICLES WITH LEAF SPRINGS

Most rear-wheel drive cars and trucks with rear leaf spring suspensions don’t have adjustments built into the suspension. But, the thrust line is a very important angle that can help you diagnose other problems.

If the rear live axle vehicle has a greater than normal angle, thrust angle is an indication that the axle has shifted or the mounting points on the frame have shifted.

To get a better picture of the damage, look at the setback of the front wheels. Setback is a diagnostic angle that measures the difference in distances between the centers of the front wheels. Differences in the setback angle can indicate damage in the frame or within components like control arms and bushings. Take a closer look at caster angles from side to side to see if there is a larger problem.

A setback and thrust angle misalignment could be an indication of frame damage. If the vehicle has suffered a recent collision that was offset, the frame may be suffering from a condition known as a “diamond frame.” This occurs when one side rail shifts in relationship to the other side rail. On a vehicle with an independent front suspension and a rear live-axle, the shifted rails will cause the front suspension to have an increased setback and thrust angle. This is caused by the mounting points of the suspension moving.

Another piece of diagnostic information to look at is the ride height. On rear suspensions with leaf springs, the leaves of the springs can become damaged and can change the ride height and the position of the axle.

One remedy for this problem is a plate that can go between the axle and springs, and allows some fore and aft repositioning of the axle to equalize rear toe readings on both sides.

Install the plate on the side of the vehicle which will help to equalize ride height. Installation of this kit may change ride height 1/2 inch. If ride height is negligible, then installation should be done on the right side for leaf springs above the axle (left side for leaf springs below the axle) to account for road crown.

Axle housings can become bent from impacts. If you see an axle with a difference in toe greater than .50º look at the axle for possible damage.

Rocking the Cradle

More and more automakers are offering all-wheel-drive on an increasing number of vehicles from small SUVs to compact sedans. On these vehicles they are mounting the differential and suspension components of a cradle that may only connect to the uni-body in four to six locations. While this may make for easy assembly, it makes the alignment technician’s job more difficult.

When aligning these types of vehicles, pay attention to rear wheel setback and the thrust angle. These diagnostic angles can help you determine if the cradle or suspension components are damaged. Most thrust angle problems on these suspensions can be resolved with toe adjustments. But, if the cradle has shifted, you may quickly run out of adjustment on the toe links.

The Morgan Roadster Limited Slip Differential

 The Morgan Roadster Limited Slip Differential.

It was this comment from Terry Seymour that prompted me to compile this topic.

"I started to move back and heard a clonking sound from behind me, checking my mirrors again, saw nothing, started maneuvering again only to hear the awful noise. 

While I was on the shoot, MMC engineers had Frog Mog up on a ramp and ascertained the noise was coming from the rear axle Limited Slip Differential. 

Having explained it was quite normal and not unusual ”they all do that sir”, and not dangerous to drive."

Noisy Limited Slip Differentials During Cornering.

Limited Slip Differentials lock the two rear wheels together when traveling in a straight line but allow one wheel to slip when turning.

 They provide better traction on slippery surfaces. During a turn the wheel on the outside ~ 10% faster than the inside wheel. 

By overcoming Clutch Tension the two wheels can rotate a different speeds during turns. 

The Roadster 100 Differential contains friction plates when other types may contain Cones.

A multiple disc clutch differential consists of two sets of multiple disc clutch packs located on each of the two side gears. 

These clutch packs have friction plates splined to the side gear, interlaced with steel plates that have tabs locking them to the differential case.

The Spider and Side Gears are bevelled. The turning pinion gears force the side gears outwards towards the case. 

The mechanical force of the bevelled gears and spring tension apply the clutch. For a set of friction plates to slip they must overcome the preset spring tension.

Service the differential fluid at recommended intervals. Special Limited-Differential Fluid compliments the engagement and release of the limited slip components. 

Adding standard differential fluid to a limited slip differential can result in noise and vibration while turning.

A vehicle with a Limited Slip Differential 'Chatters' while only taking turns. Using the wrong differential fluid causes chatter. 
.........................................................

A Morgan Plus 4 Exploded Diagram Showing both Normal & Limited Slip Differential Options.

  
A Morgan Dana Spicer Rear Axle Diagram with a Cone Type Limited Slip Differential. 

A Newly Fitted Roadster Axle as seen in The Morgan Factory.

The Dana MRA 0061 - 001403 Rear Axle with Limited Slip Differential. 

The Large Hexagon Differential Filler Plug can be seen just above the lower right yellow blob.

These axles were supplied by the Spicer Axle Australia PTY Ltd. The 0061 LSD model is used on all Roadster 100's has a gearing Ratio of 3.73 : 1 

Interesting that the Series 1 Roadster used a 0060 model with a Ratio of 3.08 : 1.

The Axle Label on my car Series No 015

The Offside End of the Axle Casing and Air Vent.

The small protrusion to the right of the label is an Air Vent presumably to equalise any pressure build up within the axle.

No 015's Differential Casing seen from above. 

The Oil Drain Plug is at the lowest pint of the rear cover adjacent to the Fuel Tank. 

The Filler Plug is on the same cover mid way down and towards the offside featuring a Large Hexagonal Head..

(The polished area seen here is where the factory fitted Tool Rack 'Jack Recess' bounced and rubbed on the Diff. Housing during rear suspension bottoming. )

The Nearside end of the Axle Casing.

The Complete Axle Unit.

Changing The Limited Differential Oil.

Changing the differential fluid is not that easy on a Roadster due to the location of both Drain Plug and Filler Caps to the Fuel Tank.

Terry's Rear Axle and the Proximity to the Fuel Tank.

The Drain Plug at the Lowest Point.

The Hexagonal Filler Cap with Specification Tag.

Dave Wellings - Draining the Fluid.

Dave - Refilling using a Funnel & Rubber Tube.

The Recommended Oil is: 80W/140 Full or Semi Synthetic. such as Morris LODEXOL XFS Semi Synthetic or Penrite Pro Gear 80W-140 GL5 GL6 Fully Synthetic.

A Friction Modifier is also recommended such as: Red Line Limited Slip Friction Modifier from Demon-Tweeks

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Comments found on Talk Morgan and Piston Heads Forum.

Before we all get euphoric about LSDs there are few points to consider:

1. The Morgan Motor Company  "standard" in the 'Dana' Axle from Spicer Axle Australia PTY Ltd. which is the Friction Clutch Type, and is quite susceptible to begin set up wrong from the start. Some versions need a special additive in the oil.

The Component Diagram for a Dana Clutch Plate Limited Slip Differential.

2. The Quaife torque biasing diff recommended by Tim (which I fully endorse, by the way) is a much better option for most applications and Morgans in particular.

The Quaife ATB BMW Limited Slip Differential

3. The LSD will promote understeer at most times when used with a gentle throttle, this might detract from your enthusiastic driving but it is the "safe" option

4. When you reach the point at which you do break traction , now both rear wheels will be spinning, and you had better be used to this , and good at catching it. If you are, and can, then there is no doubt that an LSD is a great addition to your car and will enhance your Morgan driving experience.

I summary, think about what you want from your Morgan and how you drive it. If you regularly go off road then an LSD provides much needed traction, if you compete with the car then it gives far superior traction out of corners, but if you just want to tour around (at legal speeds?) and are a fair weather driver, you might not get as much from it.

Andy G at Talk Morgan.

Noticed a similar knocking with my Chimaera, I thought it might be the CV's so I asked Dave Batty to take a look when it went to him to be serviced last week. 

He couldn't find anything wrong and he reckoned it was the Limited Slip Diff causing the knocking, nothing unusual and nothing to worry about. - 

Furry Dice at PistonHeads

Noticed it on mine too, full lock in reverse or pulling out of side turning, glad it's a case of 'they all do that sir'

Raceboy at PistonHeads

I think we're talking about how the rear wheels judder and slip slightly as you turn on full lock at slow speeds. With a Limited slip diff a certain amount of drive is always supplied to both wheels, unlike a normal diff where you can hold one of the drive wheels still and all the drive will go to the other wheel.

I think the cause of the juddering on a LSD is that one of the wheels doesn't 'freewheel' and ends up been dragged along at the speed of the (slower moving) inner wheel.
Dannyboyo at PistonHeads

You are right to say a certain amount of drive is always supplied to both wheels, that is why a LSD exists. The juddering, if it is coming from the rear is more likely to be due to the LSD "working" than anything to do with tyres scrubbing. If that is how a LSD worked tyres would be scrubbing all the time - I think not.
Ivan at PistonHeads

Visiting The Morgan Factory with Combat Stress & MOG Magazine.

Combat Stress Building a Morgan

Good afternoon all,

I just thought I’d give you a quick update on how the photo shoot for our fund raising calendar went at Morgan Motor Company on Wednesday.

The Mission Statement and Advertisement.

Well it was an early start as I headed out from Lymington. 

Our planned meet time with the team at the Morgan visitors center was 09.00.

A Very Early Arrival 

The run up was pretty uneventful until it came time to park.  Upon arrival I swung into the car park ready to reverse into my chosen spot.

Hard lock complete I started to move back and heard a clonking sound from behind me, checking my mirrors again, saw nothing, started maneuvering again only to hear the awful noise, popped into neutral, applied hand brake got out and checked around/under the car, still nothing!! How bizarre.

Undeterred but confused I returned to the drivers seat to finish parking only to hear the noise again, straightened up and silence returned, so I parked up and went to meet the team inside.(more on the noise later).

The Factory Visitor Center Car Sales Area.

The Visitors Car Park

The New Visitor Center Refectory 

The Photographers Team Gathering.

The First Veteran knows how to wear his Combat Stress Cap.

Anticipation of a Busy Day Building Morgans.

'Masking Up' before heading to the Factory Workshops

Everyone arrived on time, We split the team in 2 and moved off with our respective photographers(3 per team), Veterans(3 per team), MOG magazine and MMC representatives,

My Team taking the Visitor's Route to the Factory.

Passing a Chassis & Finished Car Storage Area.

Receiving some Technical Information along the way.

Familiarising themselves with their surroundings. 

Heading for their Work Stations to begin building.

Mmmm! What is going on over there?

Our task was to complete 6 separate shots per team depicting our intrepid volunteers from Combat Stress Building a Morgan for the calendar.

I headed up team A. 

We were tasked with the Chassis, Engine, Panel Shaping, Louvre Manufacture, Ash Wheel Arch Manufacturing and finally Quality Control/Detailing and Inspection.

We were in the Chassis Shop with its line of CX Chassis in build, but it was soon time to cut some Louvres into a Bonnet 

As we worked our way around the factory I was really taken back by how quickly the student photographers picked up the baton from their briefing and our CS Volunteers became fully fledged Morgan Motor Engineers, it was a joy to watch and be a part of.

Checking the Bar Codes.

Multi Tasking to get it right.

Tightening the Compression Bolts on the Rear Wheel Arch Laminating Jigs

Nigel from MOG was in my team, recording step by step what was happening with a view to make a story from the proceedings as they unfolded before us.

Nigel, undertaking some Quality Control

Job Done - Tough Work but Very Satisfying.

And no Glue spilled on the floor below.

Although our Combat Stress volunteers had challenges in their lives honestly you wouldn’t have known as they were determined to enjoy the spirit of the day getting into every shot no matter how tricky access seemed, they really were characters, filling the day with one liners and anecdotal stuff that made everyone on the team smile and feel at ease.

Readying themselves to tackle that Engine.

Up on the Gantry, ready for the car.

Now that we've Built it, Its time to try it for size.

We are actually undertaking the Quality Control and Final Detailing 

Suffice to say Team A&B completed the shots required and every one enjoyed a quick look around the museum and shop before a hearty and well earned lunch.

Inspecting the Range of Models in Dispatch.

The Teams making their way to Lunch

Some interesting items viewed along the way.

A Little Offcut of  Veneer 

A Display of Polished Dashboards 

A Place for Everything

During the return to the Visitor Center much interest was shown in the very limited edition of only 8 New Plus Four CX-T (for Trials) 

Our final shots required every student and CS volunteer to be photographed in a finished Morgan. Alas it was Blue but there were smiles abound as they each had the opportunity to sit in an British built Iconic beauty. 

Then Nigel offered a quick spin around the block to the team in his Plus 8, one by one they came back looking windswept and disheveled but none the worse for the quick blast around the block.

I’ll leave the story there as it will be covered with decent photos and editorial in the December edition of MOG Magazine.

If you haven’t subscribed to receive MOG yet please do so. They support the R100 team in all we do above and beyond the call of duty promoting everything we do.

So please support our worthy cause and order your calendars, photos contained here are clearly mine, taken at random times as we went round, they are not intended for use in the calendar,  don’t let their quality put you off, every one was taken with pride and a smile.

All the best

Terry 

Now back to Frog Mogs rear end noise!

While I was on the shoot MMC engineers had Frog Mog up on a ramp and ascertained the noise was coming from the rear axle Limited Slip Differential. 

Having explained it was quite normal and not unusual ”they all do that sir”, and not dangerous to drive, I undertook to get it looked at in a couple of weeks time at Williams when she’s in for her annual service and MOT.

I didn’t hear another peep from behind until I put it into reverse on full lock to get into the garage at home and there it was again! 

Oh well I’m sure it will still be there tomorrow!!