Wheel alignment shim thickness

#1
Recently had the wheel alignment checked on my 68 SC after noticing the drivers side steering arm boot rubbing on the inner guard. Could potentially also see the negative camber when sitting on flat ground. Check confirmed 1.4 degrees of negative camber ve 0.2 on the passenger's side. I have shimmed the inboard top mount 3mm (1/8") and it appears to have moved the steering arm away from the body (yet to take it back for the check)

Just wondering how much (thickness) shim people have seen used in the past? Is there a max amount before you start changing any other geometry?

Finally, this all occurred after doing a few thousand miles after fully rebuilding the front end. Anything else I should potentially check. Ride height is OK and even so I don't think its a spring.
 

jp928

Active Member
#2
If you measure the distance from the outer top mount to the inner mount (that you shimmed) - X, and then from the outer mount to the top ball joint on the upright - Y, you will get the amount that the ball joint moved outwards = 1/8" x Y / X - assuming Y> X. Then use that value (Z) against the distance (D) from the top ball joint to the bottom ball joint, as thats where its pivotting in camber - artan (Z/D) should give you the degree change. IIRC the spec for camber is 0 +/- 1 degree. Did you get a figure for caster as well as camber? I have experienced high steering loads due to increased caster caused by the bulkhead moving backwards under top mount loads.
 
#3
DId not check the caster value this time around as it was within spec the first time I had the check done. Probably is sometihng I should check now I've started playing around
 

jp928

Active Member
#4
Now that you have adjusted the top pivot it has probably changed, so suggest you ask for caster as well. If you measure camber for some left and right lock you can calculate caster - measure camber at 20d left turn (from centre) and 20d right turn - then take camber change (degrees) divide by turn change - 40degrees, multiply result by 57.3 (180d / pi). do this for EACH wheel to get its caster.
 

SydneyRoverP6B

Well-Known Member
Staff member
#5
Recently had the wheel alignment checked on my 68 SC after noticing the drivers side steering arm boot rubbing on the inner guard. Could potentially also see the negative camber when sitting on flat ground. Check confirmed 1.4 degrees of negative camber ve 0.2 on the passenger's side. I have shimmed the inboard top mount 3mm (1/8") and it appears to have moved the steering arm away from the body (yet to take it back for the check)

Just wondering how much (thickness) shim people have seen used in the past? Is there a max amount before you start changing any other geometry?

Finally, this all occurred after doing a few thousand miles after fully rebuilding the front end. Anything else I should potentially check. Ride height is OK and even so I don't think its a spring.
The shims are fitted by the business undertaking the steering alignment, so the right thickness is obtained to correct the problem, that has been my experience.

DId not check the caster value this time around as it was within spec the first time I had the check done. Probably is sometihng I should check now I've started playing around
The castor is not adjustable on our Rovers. Placing shims behind the top link changes the camber but they have no effect on castor. Unless the relative position of the swivel pillar ball joints to each other, and their relationship relative to a vertical datum changes (which it doesn't when changing the camber), no change in castor occurs. To change castor you would need to redesign the swivel pillar in such a way that the reference point that currently exists between the top and bottom ball joints is subject to displacement.

Now that you have adjusted the top pivot it has probably changed, so suggest you ask for caster as well. If you measure camber for some left and right lock you can calculate caster - measure camber at 20d left turn (from centre) and 20d right turn - then take camber change (degrees) divide by turn change - 40degrees, multiply result by 57.3 (180d / pi). do this for EACH wheel to get its caster.
Castor does not change JP regardless of how many shims a person may fit behind their top link. Shimming the top link is an angular displacement within a horizontal plane that does not change the relationship between the top and bottom ball joints, whereas changing castor is a change in displacement within the vertical plane of the swivel pillar relative to a chosen vertical reference line which is usually referred to as the steering axis.

Ron.
 

jp928

Active Member
#6
Ron says "Castor does not change JP regardless of how many shims a person may fit behind their top link. Shimming the top link is an angular displacement within a horizontal plane that does not change the relationship between the top and bottom ball joints, whereas changing castor is a change in displacement within the vertical plane of the swivel pillar relative to a chosen vertical reference line which is usually referred to as the steering axis. "
My direct personal experience is that I changed a very heavy steering TC back to normal by shimming BOTH the top mounting points . At the time I was advised by mechanics who had done this in the past, citing an apparent depression of the firewall mounting points. Looking at the system in a side view, where else but forward (reducing caster) can the top ball joint go when both the mounts are shimmed? I agree that shimming one mount will mostly change the camber, but since shimming one mount will move the ball joint in an arc (around the other mount), there will also be a small change in the caster. If the top ball joint moves , as you say, in a horizontal plane, while the bottom ball joint does not move, any movement fwd or backwards (in a side view) must change caster, it being a measure of the angle between a VERTICAL line at the wheel centre, and a line through the two ball joints that the wheel angle changes around.
Check this thread, where others record the same effect I saw with shims...
Alignment specs

Does anybody have any printed results from alignment checks that show deviation from specified caster?
 
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SydneyRoverP6B

Well-Known Member
Staff member
#7
Ron says "Castor does not change JP regardless of how many shims a person may fit behind their top link. Shimming the top link is an angular displacement within a horizontal plane that does not change the relationship between the top and bottom ball joints, whereas changing castor is a change in displacement within the vertical plane of the swivel pillar relative to a chosen vertical reference line which is usually referred to as the steering axis. "
My direct personal experience is that I changed a very heavy steering TC back to normal by shimming BOTH the top mounting points . At the time I was advised by mechanics who had done this in the past, citing an apparent depression of the firewall mounting points. Looking at the system in a side view, where else but forward (reducing caster) can the top ball joint go when both the mounts are shimmed? I agree that shimming one mount will mostly change the camber, but since shimming one mount will move the ball joint in an arc (around the other mount), there will also be a small change in the caster. If the top ball joint moves , as you say, in a horizontal plane, while the bottom ball joint does not move, any movement fwd or backwards (in a side view) must change caster, it being a measure of the angle between a VERTICAL line at the wheel centre, and a line through the two ball joints that the wheel angle changes around.
Check this thread, where others record the same effect I saw with shims...
Alignment specs

Does anybody have any printed results from alignment checks that show deviation from specified caster?
When you say both the top mounting points JP, do you mean both the inner or outer, or do you mean both sides, i.e., all 4 mounting points?
The photo below shows the shims fitted to my Rover's top links (both RH and LH side of the car have the same application). These shims changed the camber allowing a greater clearance between the inside surface of the steering arm and the base unit. The inner mounting points of both top links have no shims fitted.

Ron.
 

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jp928

Active Member
#8
Ron,
I meant both mounts, inner and outer, on one side. I did say "BOTH" mounts, several times, obviously not clearly enough.
Here is another way to think of it. What movement occurs at the top ball joint if the spring is shortened? I think it moves backwards towards the bulkhead, in an arc around the lower ball joint, which will INCREASE the caster, with ZERO change in the camber. A Lengthened spring would move the ball joint forward, REDUCING caster, without changing camber. If you agree with this proposition, how is the effect different from that obtained by shimming BOTH mounts (on one side)? Without wanting to verbal him, in the thread I linked to above, Harvey seems to agree with the caster effect in his remark that it would not be considered a normal service item.
Looking at your shims on the outer mounts, they should make camber more negative, moving the top ball joint inwards, which to me would move the steering arm CLOSER to the inner guard...?
 

cobraboy

Well-Known Member
#9
I would bet a good steak dinner that any shimming for camber, that is under either the inner mount ( to promote positive camber ) or under the outer mount ( to promote negative camber ) would as a side effect lessen positive caster, maybe not much, but I bet it would.
 

SydneyRoverP6B

Well-Known Member
Staff member
#10
Ron,
I meant both mounts, inner and outer, on one side. I did say "BOTH" mounts, several times, obviously not clearly enough.
Here is another way to think of it. What movement occurs at the top ball joint if the spring is shortened? I think it moves backwards towards the bulkhead, in an arc around the lower ball joint, which will INCREASE the caster, with ZERO change in the camber. A Lengthened spring would move the ball joint forward, REDUCING caster, without changing camber. If you agree with this proposition, how is the effect different from that obtained by shimming BOTH mounts (on one side)? Without wanting to verbal him, in the thread I linked to above, Harvey seems to agree with the caster effect in his remark that it would not be considered a normal service item.
Looking at your shims on the outer mounts, they should make camber more negative, moving the top ball joint inwards, which to me would move the steering arm CLOSER to the inner guard...?
This is why what appears to be intuitive with the Rover's front suspension and how it operates JP is not what often happens. My Rover has been running shims just on the outside attachement of the top link (as in the photo) since circa 1988. The reason they were fitted was to increase the clearance between the steering rod and the inside surface of the base unit. They were making contact on the driver's side, and after fitting, this was no longer the case. The distance had increased, which is the opposite to what you have surmised. I remember the service manager at the Rover dealer where I took the car on occasions said I had rubbing and needed to have it corrected. I asked what was involved and he said they will 'shim the top link' and a good steering place will do it. I took it to a business that specialised in truck steering. The fellow had worked on Rovers before as he knew about the glove boxes, which I had removed in advance. He surplied the shims which he placed to correct the rubbing, and correct it it did. I have seen other local Rovers with shimmed top links, in some cases I can place my fingers between the steering rod where it joins the swivel pillar and the base unit.

Ron.
 

SydneyRoverP6B

Well-Known Member
Staff member
#11
Viewing a side elevation of the front suspension, my thoughts are these. If the top link at its point of attachment with the bulkhead is allowed to move a distance x forward, provided its orientation remains static, then indeed the top ball joint will move forward relative to the bottom ball joint. This will, if nothing else changed, result in a change of castor. However, the system is not so abliging and does not remain in this current configuration. The ball end is no longer in contact with the inside surface of the spring cup, if we allow the spring to remain in its current static length. The length of the spring will not change as the mass of the vehicle has not changed. Newton's third law tells us that the spring will exert the same force on the ball end via the spring cup as the spring cup exerts on the spring. This therefore means that the top link must rotate through the angle phi as shown on the diagram in order for the spring to remain at its normal static length, as the force f = kx has not changed, and the system must remain in equlibrium. Consequently, there is a rotation of the top link given the point of attachment has moved forward. This rotation removes any change in castor returning the steering to the same degree of castor that it possessed prior to translating the top link.

Ron.
 

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jp928

Active Member
#12
Ron,
I think I follow your logic, but... The effect of shimming is/was very pronounced in my case. Symptom was heavy steering - noticeable as our other 2000 was much lighter. Advice from those experienced in the model was to shim both attachment points on each side equally. At first, not knowing how much shim was correct, I obviously over did the thickness - result was little or no self centering, and the test drive was VERY short and scary. Steering was much lighter. Reducing the shim thickness produced the desired result- good self sentering, and lighter steering. No other changes were made - same tyres and pressures and loads. Tyre wear did not suffer in later usage. What else could have produced the observed changes than a change of caster?
Why would the rotation of the top ball joint EXACTLY cancel out the caster change caused by the shimming? I can see that the ball joint will move fwd and upward a fraction, pivotting around the stationery ball end in the spring cup.
HOWEVER - what angle of rotation of the top ball joint would be needed to produce a REARWARD movement of the joint? As I see it the angle would be enormous, impossible . An arc pivotting around the spring ball end (do you agree thats the pivot point when the mount is shimmed?) will not go rearwards until the ball joint has proceeded HIGHER than the spring ball end, from its starting position BELOW the ball end at the spring, which would have it well out through the inner guard and bonnet. And until the top ball joint has arrived level with the spring ball end, its moving FORWARD, reducing caster.
Note that there are other threads here referring to this shimming effect.
Serve returned, Your ball.
 

SydneyRoverP6B

Well-Known Member
Staff member
#13
Ron,
I think I follow your logic, but... The effect of shimming is/was very pronounced in my case. Symptom was heavy steering - noticeable as our other 2000 was much lighter. Advice from those experienced in the model was to shim both attachment points on each side equally. At first, not knowing how much shim was correct, I obviously over did the thickness - result was little or no self centering, and the test drive was VERY short and scary. Steering was much lighter. Reducing the shim thickness produced the desired result- good self sentering, and lighter steering. No other changes were made - same tyres and pressures and loads. Tyre wear did not suffer in later usage. What else could have produced the observed changes than a change of caster?
Why would the rotation of the top ball joint EXACTLY cancel out the caster change caused by the shimming? I can see that the ball joint will move fwd and upward a fraction, pivotting around the stationery ball end in the spring cup.
HOWEVER - what angle of rotation of the top ball joint would be needed to produce a REARWARD movement of the joint? As I see it the angle would be enormous, impossible . An arc pivotting around the spring ball end (do you agree thats the pivot point when the mount is shimmed?) will not go rearwards until the ball joint has proceeded HIGHER than the spring ball end, from its starting position BELOW the ball end at the spring, which would have it well out through the inner guard and bonnet. And until the top ball joint has arrived level with the spring ball end, its moving FORWARD, reducing caster.
Note that there are other threads here referring to this shimming effect.
Serve returned, Your ball.
Sorry JP but I don't agree that the pivot point when the top link is shimmed would be the ball end. The line of rotation is through the axis of the top link bushes. When the top link is moved forward from the bulkhead, it translates without rotation, meaning the ball end also moves forward by the same amount. If returning to the initial poistion is prevented due to the placement of shims, then the only way to return the ball end its initial position is to rotate the top link through an arc with the centre of rotation being the axial line that passes through the top link bushes. Given the spring static length has not changed, the rotation through the axial line must be sufficient to allow for that to happen. If I had a scale drawing I could calculate the angles and the displacement, or do a drawing in AutoCAD and let it tell me the angles and offsets. I can't say with certainty that translation through rotation would return the top ball joint to precisely the same position that it maintained prior to shim placement. In normal operation, there is very little movement in the top link and in the spring in terms of compression and expansion, else the inner top link bush would be destroyed in a very small time interval.

Ron.
 

jp928

Active Member
#14
Okay, so the top arm rotates around the bush centre as usual, and arcs upwards. My contention is that the horizontal component rearwards is miniscule compared to the shim under the mounting points.
eg. assume the horizontal distance from the pivot point to the top ball joint is 20". We have shimmed the mountings 0.125". The angle the top arm rotates through is , say, 2deg. The rearward movement of the ball joint is 20 - 20Cos(2) = .01218 - an order of magnitude less than the shim. So the forward movement of the ball joint is 0.1128 in this scenario of guesswork as to angle.To get a rearward movement equal to the 0.125" shim the arm would have to rotate through 29deg, which IMHO is pretty improbable. An angle change of 5 deg would only produce a rearward movement of 0.076", still less than the shim.

Anyway, can you address why my case of shimming produced results consistent with reduced castor, and seems to have done so for others here?
 

cobraboy

Well-Known Member
#15
So by shimming the pivot castings this raises the arm as the spring takes up the 'slack', this raises the swivel pillar, which lowers the car ! and does not lessen the caster ?
I am not convinced.
If all this were to be true then by having an adjustable spring seat, which I have, then one could then raise the car back up again and have lessened caster.
I am interested in this topic as I would like to be able to adjust caster. Real world differences matter to me not paper calculations. If I were not so busy I would get a hold of a caster gauge and stick a 1/4" shim behind the pivots and do a before and after check.
Which is what someone ought to do, otherwise this is going nowhere.
 

mrtask

Well-Known Member
#16
I'm following all this with interest, as the front ball joint gaitor of my left hand steering side rod sometimes scrubs the inner arch. Once I completely understand what does what (!?!?!?), I'd like to know what size shim(s) to put behind which side of which top link to cure the problem, without creating new ones... o_O:hmm:
 

cobraboy

Well-Known Member
#17
By shimming the top link to lean the swivel pillar out in order to stop rubbing you are altering the camber toward positive. To alter the geometry to cure an interference issue is bonkers, you should remove the interference and run correct geometry. What if the arm only rubs on one side ? Are you going to lean out one side only ? Christ, think what the thing is going to drive like afterward.
Dressing the affected area of the inner wing would be the way forward.
I fitted larger wheels which would not clear the rear of the wheel arch on lock. I did not move the wheel forward :rolleyes:, I dressed the wheel arch to get clearance.
Correct alignment is paramount, bodywork is merely the clothing and is a minor consideration,
Edit
It may pay to get an alignment done, it may be that the side that rubs has too much negative camber anyway, and bringing it to the correct spec cures the rubbing.
 
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jp928

Active Member
#18
Without some sort of jig to verify the location fore/aft of the spring base and its effect on caster the question is unresolvable. In my mind the fact that there were 2 cars with very different steering loads, and the bad one was fixed by shimming says that caster changes with shimming - of both mounts on the one side. Some cars have obviously suffered some pretty severe overloads - I have seen an inner guard that has had the top ball joint nut come through it. I suspect that that event would have involved a coil bound spring which could have depressed the spring mount, producing the increased caster that will lift steering loads. The steering loads increase because the movement of the tyre contact centre means changing lock is trying to lift the front of the car, as it was explained to me. I am going to measure my caster as soon as I can, because I am concerned that the PAS is masking high steering loads, which can be bad for the relevant ball joints.
If somebody can provide the distance between the top and bottom ball joints on the upright (as close to ball centres as can be estimated) one could work out the caster change produced by moving the top ball joint fwd/back. eg, if the distance is 18", then 1degree moves the top ball 0.314". Measurement of caster is imperative before looking at any adjustments.
Ron's diagram above shows the ball end into the spring cup is threaded into the bellcrank - making this adjustable would enable caster adjustment, but would also change ride height. Locating the lower arm mount bolts on eccentrics would enable caster and camber adjustment....
 
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#19
I'm following all this with interest, as the front ball joint gaitor of my left hand steering side rod sometimes scrubs the inner arch. Once I completely understand what does what (!?!?!?), I'd like to know what size shim(s) to put behind which side of which top link to cure the problem, without creating new ones... o_O:hmm:
Mine was scrubbing in the same manner on thr LHS only. By putting 1/8" shim behind the inboard top mount it looks like I've moved the pilar outboard approximately 5 mm. I did start with -1.4 degrees camber so (as per above) I can afford to make an outwards ajustment. Going back for an alignment check next Saturday morning and will proably leave it with them once I can show them how to make the adjustment to get it correct.
 

jp928

Active Member
#20
"I'm following all this with interest, as the front ball joint gaitor of my left hand steering side rod sometimes scrubs the inner arch. Once I completely understand what does what (!?!?!?), I'd like to know what size shim(s) to put behind which side of which top link to cure the problem, without creating new ones. "

Strongly suggest you get an alignment done that tells you camber and caster and toe before moving anything. I would have suggested shim the inner mount only, until Ron said his rubbing was fixed by shimming the outer mount, which doesnt make sense to me - shouldnt work. Also check that the top ball joint is in good condition - no slop.
 
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