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MKI / MKII S type 240 340 & Daimler 1955 - 1967

Mk2 Steering Geometry Bump Steer and hopefully more

 
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Old 03-02-2019, 03:58 PM
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Default Mk2 Steering Geometry Bump Steer and hopefully more

Following on from some discussion on Doug great thread , I am hopefully going to try and assist with some aspects of the steering geometry on the conversion to power rack and pinion steering on the Mk2.

Below is a pictorial/CAD view of the front suspension on a Mk2, this has a ZF Servotronic rack and pinion power steering.

Pictorial of the Mk2 front suspension

You will see that the suspension is in full droop (lowest point of travel) the bump stop has approx 100 of travel to hit the stop on full compression of the suspension.

The green segment shows the lower wishbone, and the arc of travel for the ball joint from full droop to full compression.

The yellow segment shows the travel of the tie rod for the same amount of travel.

You will see that the two arcs of travel for both the wishbone ball joint and the track rod end follow a pretty close parallel path, so there will be very little movement in the tie rod end during the travel of the suspension (near zero bump steer)

If the rack were mounted slightly lower, then the tie rod and track rod end would follow the pink line, and you will see that the track rod end would move inward relative to the bottom ball joint as the suspension compresses, therefore as the tie rod is behind the axle as the steering compresses the wheels would turn out and they would turn in on droop (assuming 0deg of Toe in/out at the static suspension position)

There are other factors at work here which require 3D modelling to show, but this is the basics of bump steer on the Mk2

There are other positions/lengths of rack that can work, but the principle is the same, the track rod end and lower ball joint should follow a parallel path to avoid bump steer. (this applies to the Mk2 because of the proximity of the track rod end and lower ball joint on the almost same plane)

If the track rod end of a vehicle is above the lower ball joint significantly the movement in the upper wishbone bears more into the equation unless the upper and lower wishbones are the same length and parallel, I will explain that more if anyone wants me to draw that up.

I will attempt to CAD another drawing showing how Ackerman angles work on the Mk2 shortly, unfortunately I am not very quick on CAD and 3D modelling is above my skill level.

Any comments, criticism, corrections, or questions please fire away, hopefully I or someone else can answer any questions and any errors I may have made can be pointed out and corrected.

 
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Old 03-02-2019, 05:12 PM
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Great post TJ. Look forward to your Ackerman steering/angles post. Some R&P conversions on our Mk2's & S Types cause the most incredible levels of tyre slip & scuff.
 
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Old 03-02-2019, 05:24 PM
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I still do not understand what bump steer is, but never mind, I'm not doing rack & pinion in my S type. The car was not designed for it. I have my XJ with rack & pinion if that counts? .
 
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Old 03-03-2019, 01:04 AM
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Originally Posted by Jose View Post
I still do not understand what bump steer is, but never mind, I'm not doing rack & pinion in my S type. The car was not designed for it. I have my XJ with rack & pinion if that counts? .
Bump Steer is pretty simple, it is when the car steers itself under suspension compression or droop without any movement of the steering wheel, if the effective length of the tie rods change under suspension movement you have bump steer.

Most cars have some degree of bump steer, it is almost impossible to have none at all, it's simple physics and geometry.

 
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Old 03-09-2019, 09:38 AM
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TilleyJon….Thanks for your CAD drawing. I would be interested in seeing one with the lower wishbone arm about 1 to 1.5 inches sloped below horizontal. If you draw out an Ackerman steering layout it will be obvious immediately why these rack and pinion converted cars wear front tyres quickly. If the steering idlers are displaced, as in a rack conversion, it becomes impossible to attain “toe out” on turns. You could put all the corrections possible into ensuring that you have zero bump steer and it will make no difference. Equally it would not matter which rack you used.On converted cars, in each case you will find both front wheels are turning identical angles whilst in practice you need say 22.5 degrees on the front inner wheel, when the outer wheel is 20 degrees. If the steering arms were to be altered to better the system it would achieve very little and then probably introduce more bump steer effect.Most of these cars are sporting lower wishbone arms at totally incorrect angles, which normally cause camber changes on unequal length wishbone systems, another problem causing excess tyre wear. On this type of front independent suspension the normal vertical movement of the lower arm outer joint is between 2 and 3 inches (50mm to 75mm). Only in extreme circumstances are these measurements exceeded and the lower figure is quite usual. In practice the lower arm horizontal position needs to be around the middle position of normal suspension movement. If this position is achieved by correctly setting front spring length and poundage, any camber, castor or track variation changes will be very little in normal service. This however will not cure a rack and pinion’s inability to obtain toe out on turns.The best that can be achieved from a tyre wear point of view, is to get the bottom arms about 1 to 1.5 inches (25mm to 40mm) below horizontal, when the car is in kerb weight condition and the tracking just erring to the toe out position.I have an S type with a power rack conversion and I am toying with a few ideas to sort the toe out on turns.
 
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Old 03-10-2019, 09:18 AM
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Welcome to the forum NWG, you are completely correct in saying there is more to this than simply bump steer, the above CAD was more by way of describing bump steer as it relates to the Mk2, below is a CAD of how the camber changes with steering compression etc.

The Camber angle becomes more negative on compression and positive on droop. I am working on the Ackerman angle drawing, but it is not a quick process as I am only OK with CAD and not good, so it takes me time, I also need to measure and set out the CAD drawing from existing components as there are not enough dimensioned drawing available if any at all.

You have said that the front wheels will follow identical angles on turns, I am not convinced that is the case, but when I draw it out I will soon see if I am right or not, you seem to know your onions so I am not saying you are wrong.



Camber changes with suspension movement




I will hopefully get the Ackerman angles drawing done soon as it will be interesting to compare R&P to the existing arrangement.
 

Last edited by TilleyJon; 03-10-2019 at 09:24 AM.
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Old 03-12-2019, 06:06 AM
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TilleyJonI have tried to reply once but the text seems to have disappeared in the ether! So I will try again, I also complete web discussions on Word and use copy and paste, not very successful first time around……must do better!

I have measured my own car (1965 Jaguar 3.4 S MOD) and a few more using Weaver Turntables, in each case the rack converted cars show identical steering lock angles with no angular increase of the inner front wheels. This was not unexpected, but might show better results with a shorter rack (longer track rods) most probably from a different car than Jaguar. It could be that some gain might result from fitting a rack with the inner track rod joints that are adjacent to each other at the centre of the rack. Of course any replacement rack would need to pass a number of tests including the measurement from the rack centre to the steering column centre line.

The problem on rack converted MK 2’cars can best be described as follows….

Assume the rack assembly is represented as a straight rigid track rod. Attached through a joint to each end is a steering arm. In the straight ahead driving position each end will mirror the opposite end. For illustration purposes in plan view, the left joint will be at approximately 4 o’clock and the right joint at 8 o’clock.

In actual practice if we put this arrangement through a standard steering test the following will happen. From a straight ahead position, LEFT lock will be applied until the RIGHT wheel has turned through 20 degrees and the RIGHT joint moved from 8 o’clock anti-clock wise to say 6 0’clock. The LEFT joint needs to move anti-clockwise from roughly 4 o’clock to 1 o’clock to give us about 22.5 degrees, but it will only move to the 2 o’clock position to provide a 20 degree lock angle. This description mimics the rack converted Mk 2 situation when both front wheels are providing matching steering angles, albeit I have used clock points to illustrate the problem..

Your finding of the camber angle turning negative on suspension compression and positive on suspension droop confirms my comment regarding the lower wishbone arm position, and its importance. If you view various web threads, including those specialising in even more high end cars than Jaguar you will find owners obsessed with camber, caster and toe in angles and dimensions, what is not understood or discussed, is the starting point which is the correct angle of the lower wishbone arms.

You have just proved the case by showing the movement from positive to negative camber changes as the lower arm swings through its operating arc. This always happens with unequal length wishbone arrangements. As you progress, your CAD illustrations will show how important that lower arm static position is to the steering and general handling of the vehicle.

In the static non driving position the Jaguar design allows positive camber (just) with a move towards negative camber if sporting driving is undertaken. The actual front suspension sectioned drawing in the workshop manual is fairly accurate and it shows the bottom arm at about 10 / 12 degrees below horizontal statically. The manual also contains measurements of the major steering components, which may help you in your endeavour. From my view point, thank you for your considerable effort, it highlights the suspension issue very well.
 
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Old 03-15-2019, 04:22 PM
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Originally Posted by NWG View Post
TilleyJon

The problem on rack converted MK 2’cars can best be described as follows….

Assume the rack assembly is represented as a straight rigid track rod. Attached through a joint to each end is a steering arm. In the straight ahead driving position each end will mirror the opposite end. For illustration purposes in plan view, the left joint will be at approximately 4 o’clock and the right joint at 8 o’clock.

In actual practice if we put this arrangement through a standard steering test the following will happen. From a straight ahead position, LEFT lock will be applied until the RIGHT wheel has turned through 20 degrees and the RIGHT joint moved from 8 o’clock anti-clock wise to say 6 0’clock. The LEFT joint needs to move anti-clockwise from roughly 4 o’clock to 1 o’clock to give us about 22.5 degrees, but it will only move to the 2 o’clock position to provide a 20 degree lock angle. This description mimics the rack converted Mk 2 situation when both front wheels are providing matching steering angles, albeit I have used clock points to illustrate the problem..

Your finding of the camber angle turning negative on suspension compression and positive on suspension droop confirms my comment regarding the lower wishbone arm position, and its importance. If you view various web threads, including those specialising in even more high end cars than Jaguar you will find owners obsessed with camber, caster and toe in angles and dimensions, what is not understood or discussed, is the starting point which is the correct angle of the lower wishbone arms.

You have just proved the case by showing the movement from positive to negative camber changes as the lower arm swings through its operating arc. This always happens with unequal length wishbone arrangements. As you progress, your CAD illustrations will show how important that lower arm static position is to the steering and general handling of the vehicle.

In the static non driving position the Jaguar design allows positive camber (just) with a move towards negative camber if sporting driving is undertaken. The actual front suspension sectioned drawing in the workshop manual is fairly accurate and it shows the bottom arm at about 10 / 12 degrees below horizontal statically. The manual also contains measurements of the major steering components, which may help you in your endeavor. From my view point, thank you for your considerable effort, it highlights the suspension issue very well.


Although I have not completed the CAD work as yet as I need to take some more accurate measurements, I do not agree with the fact that the steering on a MK2 will have the same angle on both wheels throughout a turn, my initial CAD whilst not completely accurate proves that there is some difference, (3 deg toe out), this can also be backed up by geometry, whilst I will not bore you all with the full maths behind all this at the moment, (I can do so if you wish of course), the image shows the basic geometry behind this, which goes on to show that there will always be some difference in the angles of each wheel as the wheels turn due to the linkage involved. Once I have made the actual measurements and applied these I will post further details of both the mathematical geometry and the CAD, hopefully both will be the same.

I can then apply this to the MK2 and show how far away from Ackerman the setup is, and also what changes would need to be made to correct it, if thse changes are actually physically possible.

The change in Camber angle is not affected by the rack and pinion conversion, it is a fact of the unequal wishbones, so was not part of this particular point as such, I am only looking at the kinematics of the steering in relation to Ackerman angle at this point. I have imported the suspension drawing into CAD and scaled it as per the dimension for the lower wishbone, I have found that the drawing is actually not a completely accurate drawing, as when the lower wishbone is shown at full size, the upper wishbone is 5mm short on the drawing, so I cannot take any accurate measurements from the manual drawing and the manual does not give the dimensions for the relevant parts, so I will need to physically measure them on the ground so to speak.



Geometry of rack and pinion steering behind front axle.

From the above diagram, y˛=[(B−(p+2r))/2−xsinβ]˛+ [d-xcosβ]˛



Steering turned right


from above y˛=[B/2−(p/2+r−q)−xsin(δi +β)]˛ + [d−xcos(δi +β]˛

the same can be applied to the outer angle Y˛=[B/2−(p/2+r+q)− xsin(δo -β)]˛ + [d−xcos(δo -β]˛

Here is my inaccurate CAD which shows the differences in the inner and outer wheels, the second pink line on the right wheel when turned left is the same angle as the left wheel .



CAD showing rack movement and changes in wheel alignment during turning.
 

Last edited by TilleyJon; 03-15-2019 at 04:59 PM.
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Old 03-16-2019, 05:14 AM
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There are some apparently learned, if complex, white papers on the internet on maintaining Ackerman steering with rack & pinion systems.

I have also wondered but not given serious thought to making up a hybrid system of centre steer power rack with some of the existing system although it would defeat some of the objective.
 
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Old 03-16-2019, 09:57 AM
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Thanks Glyn, this was supposed to be a fairly simple explanation on steering geometry specifically on the Mk2 / S-type with real life scale diagrams, I had not intended to go into the mathematics behind it, but it has come to pass that I have ended up giving further detail .

I will hopefully get the CAD stuff done shortly as it will be interesting so see, so far it appears that the rack on my car should be moved rearward about 60mm, to get close to Ackerman, but without accurate measurements this may be a bit off, or way off !

I want to also CAD up the existing system, it would be interesting to see how that works with relation to Ackerman.
 

Last edited by TilleyJon; 03-16-2019 at 11:12 AM.
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