Ball joint extender bumpsteer.

Its been issue'd before but can't find the link to it.
I have bumpsteer issues on my KVA B chassis which I am trying to sort. I allready raised the steering rack by 12mm ( about 7/16 ).

I have seen ball joint extenders to extrent the tie rod from the steering arm ( not to extend it from the track rod).
I prefer the ball joint extender over a rose joint conversion but any leads to a rose joint conversion will do. track rod side is M14x2.

Cortina mk3 rack & tie rods.
 

Ian Clark

Supporter
Hi JP,

Just wondering if you have large diameter front tires? Big tires with a racey kind of low ride height will throw the steering arm/steering rack geometry off. Are you using all the available caster? More caster angle will raise the steering arms. Just some things that come to mind, if you're not already there...

Cheers
Ian
 
Ian, guess you'r right about caster.
I think I have to go to a specialist for a four wheel alignment to set this car up properly which hasn't been done yet, as i am just finished building it.
It seems I chasing ghosts now, going to fast in attacking the last issues............ cause I wan't to drive it more then just around the block.

Tires are 225 60 15 and 295 50 15
 

Mike Pass

Supporter
I think in the first instance you need to check that the suspension actually has bump steer. As the front axle is Cortina Mk3 it seems unlikely that Ford built it with bump steer.
To check for bump steer you can use a bump steer gauge ( not cheap £200+) or the laser pointer method (cheap £1). Bump steer is caused by the steering arm moving in an different arc than the suspension arms causing the end of the steering arm to move in or out relative to the hub thus effectively steering as the suspension moves up and down.
Remove the front clip and jack the front of the car off the ground. Remove the wheel and the spring and damper. Buy a cheap laser pointer from Ebay and glue it to a flat magnet. Stick the magnet on the hub and aim the dot at a wall or flat board at least 2 metres away ( the longer the better - 3/4 m if possible). Set the suspension on a trolley jack at about the normal ride height point. Now raise and lower the suspension 1cm at a time and at each stop mark the position of the laser dot on the wall or board. The line of dots on the wall board should be pretty much a straight vertical line. The hub will actually move in a slight curve because the wishbones move in an arc but any bump steer will have a much bigger effect and due to the length of the "optical lever" effect of the long laser beam, it will show up as the line of dots will be a curve. Whist it is impossible to remove all bump steer because wishbones ends move in an arc as long as the line of dots is close to a vertical line especially for a couple of cm above and below the normal ride height it will be OK.
Bump steer can be brought under control by moving the rack position until the best position is found.
I don't understand why you have raised the rack from the original Ford position by 12mm. This may be the thing that has caused the bump steer. Try returning it to the original position in the first instance and test. The raising of the rack is a well known way of reducing bump steer on the GTD but that method is specific to the GTD. I have not heard that the Cortina MK3 front needs the rack position changing.
If it turns out that you have bump steer which cannot be eliminated by moving the rack there are companies that will do custom Ford racks of different lengths which will effectively change the point that the steering arm arcs from.
Hope this helps
Cheers
Mike
 

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Ian Clark

Supporter
Hi JP,

Your front tires are approx 25.63 inch diameter. If you go for a ride front height under 5.00 inches you will aggravate the bump steer to the extent that other methods of working around the steering geometry inherent to the uprights is your only choice.

I agree with Mikes suggestion about moving the rack back down to see if things improve and max out your caster too. Extended tie rod pins or tie rod eliminators (rod ends on dedicated tall bolts) will give you all the adjustability needed.

Start looking for a race car shop to dial in the suspension and corner weights. The regular tire/alignment shops are way out of their league with a GT40.

Cheers
Ian
 

Neil

Supporter
Set your front caster at 7 degrees and move the rack up & down to find your minimum bump steer point.
 
Mike, steering rack is Cortina mk3. Suspension & upr
I don't understand why you have raised the rack from the original Ford position by 12mm. This may be the thing that has caused the bump steer. Try returning it to the original position in the first instance and test. The raising of the rack is a well known way of reducing bump steer on the GTD but that method is specific to the GTD. I have not heard that the Cortina MK3 front needs the rack position changing.

Mike

Mike, KVA "B" chassis, Jaguar XJ6 SII suspension & uprights (as per KVA "B" type), Cortina mk3 steering rack. You know the chassis as you identifiet it at Adrian Beale.
Very obvious bumpsteer. Loads of toe out at spring decompression and toe in at compression. Dont need gauges to see it happen.
So I moved the rack up with two 6mm spacer plates (12mm total now) and its way less now, but not there yet.
Not much room left to raise the rack unless I modify the steering collum to sit in a different angle which means dash out and that I would like to avoid.

Only three "B" types seems to be build so it must be prototype as they very quick moved on to the "C" type chassis.
I have to develop it myself to get it working.

Next step, let a specialist sort it. four wheel aligning first as the GT hasn't been aligned properly at all.

Old pic from before the car was finished,
1651590388690.jpeg


Pic with a 6mm spacer plate to raise the rack. (not sure why the pic is tilted)
1651591481380.jpeg


We experianced the drivebillity issues after the finished build.
1651591151768.jpeg
 
I am i no way an expert and its been some time since i last "dived" into this field;

If you draw a line between your inner pivot points, the inner balljoint (on the steeringrack) should be as close as possible to that axis.

Second your trackrod should be parallell to your lower controlarm.
Is it possible that you have swapped sides on the bolted steeringarm on your uprights?

With the two above incorporated you should get a minimum og bumpsteer.
 

Mike Pass

Supporter
Ah, I remember. I forget as I see so many cars and the ancient grey cells are on the wane. Looking at the pic showing the front suspension from the front I can see why you are having problems. As Ole says the simple way to get the bumpsteer somewhere near is to ensure that the track rod inner pivot point at the rack end sits on the same plane as the 4 wishbone inner pivots. On your suspension the steering arm inner pivot is well inside the plane of the 4 wishbone pivots. Therefore the arc followed by the outer track rod end is quite different to the arc followed by the upright. So the amount moved in or out of the track rod end is different to the amount moved in or out by the upright and hence bumpsteer. As it will be difficult to change the wishbone mounts the best route forward is to change the rack to one which is longer between the steering arm inner pivot points. The steering column does not need to be changed but the connecting links from the steering column to the rack may need to be changed depending on the rack chosen.
It may be possible to get a custom Ford rack made up with the correct length for your needs.
A bit of careful measuring will be needed to get the correct length between the ends of the rack to get the outer pivots in the plane of the inner wishbone ends. I will look up the place that does custom Ford racks so you can ask them if they can make a rack of the correct length for your setup.
Looking at your suspension pic the angle of the wishbones looks a little odd, Usually the upper wishbone is angled upwards at about 10 degrees and the lower wishbone angled down at about 5 degrees. This is done so that in bump/cornering the outer end of the upper wishbone moves inwards and the outer end of the lower wishbone moves outward. The effect of this is to increase the amount of negative camber in bump to counteract the roll of the car. The opposite happens at the other wheel in cornering. To me it looks like the inner mounting of the top wishbone is too high and needs to be lowered to achieve an upward angle. Note that by the wishbone I mean the line between the actual pivots - not the metal bars as the distance between the pivots is the actual effective wishbone.
Once you know the correct length of rack bar try this place for a custom rack. - http://www.kellybraysteering.co.uk/pages/manual_steering_racks_12938.cfm
Cheers
Mike
 
If you draw a line between your inner pivot points, the inner balljoint (on the steeringrack) should be as close as possible to that axis.

Second your trackrod should be parallell to your lower controlarm.
As close as (steering rack in neutral position)
1651666161058.png


Lower wishbone angle
1651666273495.png


Trackrod angle, just about 1½ degree out
1651666376776.png


Upper wishbone angle
1651666427173.png


Bumpsteer measurement.
Using a laser from center hub to a piece of paper at 2.2 meter distance from the hub.
Black line is without tie rod fitted, 3cm suspension lift
Red line is with trackrod fitted, again 3cm suspension lift.
1651666745731.png


Haven't worked out camber. As per Jaguar it uses shimm plates between to upper wishbone halfes.
 

Terry Oxandale

Skinny Man
Seems to be a lot going on here in the geometry (rack being forward, camber inducing arm geometry, etc). Assuming no change in the inner pivot location of the upper arms, and no change in rack length, It would appear the next change to get one closer to some reductions of bump steer is to raise the rack further upward due to the "abnormal" geometry of the arms. If the lower arm in the photos is perfectly horizontal (0º), and the upper arm is at a downward slope (16º) toward the wheel, then the track rod rod should be also include a downward slope that is in a ratio of it's location vertically between the upper and lower arms (an example would be if the track rod is exactly located vertically between the upper and lower arms, then the angle is a split between the upper and lower arm's angles, or 8º of downward angle of the upper arm being the lower are is horizontal). If it is closer to the lower arm, that downward slope would be less by the ratio of between the upper and lower arm's vertical dimensions. Given the imperfect geometry here, at least that should help. Another way to look at the above explanation, is to draw a line to where the upper and lower arm axis planes intersect (in this case it is outside the wheels instead of the normal inside), and then make the track rod centerline intersect at the same point as the arms do. You will still have other issues, but at least the bump steer will be about as good as you can get given the other issues with this end's geometry.
 
Terry, your comment is very welcome. It made a lot clear.
I allready thought about reversing the upper wishbone to an upwards slope.

If I understand it correct, what youre telling, If I reverse the upper wishbone so its pointing 16º upwards, the track rod should be at an 8º angle upwards so lowering it back to its original position.

And with the upper wishbone pointing upwards, both wishbones geometric should be as it should.

This chassis was build by a coachbuilder in Birmingham in order of KVA in the late eighties. They build three of those chassis.
I only found out during the build what I had.
Anything can be made to work so mods to the upper wishbone are considerable.
 

Mike Pass

Supporter
As the suspension geometry as is will gain positive camber in bump I would suggest that you lower the inner pivot of the top arm to a point which gives an upward slope of about 10 degrees ( currently about 16 degrees down). This will help the camber geometry and will make the angle of the track rod more in line with the wishbone angles. The coilover mount can stay where it is. From your other pics it looks like there is a suitable frame member inside the wheel arch panel to mount the inner wishbone mount firmly in the new lower location. If you make the bracket adjustable or so you can use shims this will allow you to adjust the static camber to about 0.5 degree negative as a starting point.
Once this is done (1) follow Terry's suggestion of making the track rod angle a proportion of the wishbone angles. And (2) change the rack length so that the track rod swivel is on the same plane as the inner wishbone pivots.
Cheers
Mike
 

Chris Kouba

Supporter
Upper wishbone angle
1651666427173-png.122515

Is the suspension loaded in this photo? Looking outward from the chassis pivot, the top arm should point upward (at least slightly) at nominal ride height. Lowering the inner pivots would be a good place to start. You could probably benefit significantly from a bit of reworking.

The other consideration is that you can't just lay an angle finder on the wishbone unless the ball joints and pivot points are all co-planar with (or parallel to the plane of) the axis of the suspension arms. The true angle is found by using the pivot points themselves, which can be displaced significantly from where the actual A-arm hardware is located. Your actual measurements may be close, but different.
 
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Mike & Perry, thanks. Very helpfull and understandable.
Chris, close is good enough for now to highlite the issue. With the math from Perry I can work it out.

I will reposition the upper wishbone as Mike mentioned, there's a suitable frame member to do so, and I will make it "Camber" adjustable so camber can be set as jaguar XJ SII with shims.
My aim is the 10º upwards slope Mike suggested.
Will get on it after the Spa Classic event.
 
The one thing you all seem to be ignoring is the KPI , the most important and the first thing to design in relation to the pivot positions and the wheel angle / position in relation to the wheel /road support point. The KPI angle on the standard Ford uprights is way insufficient, and this will affect everything else, including the above. Think about it !
 
The one thing you all seem to be ignoring is the KPI , the most important and the first thing to design in relation to the pivot positions and the wheel angle / position in relation to the wheel /road support point. The KPI angle on the standard Ford uprights is way insufficient, and this will affect everything else, including the above. Think about it !

And what exactly is the background for kpi being the most important?
Everything needs to correspond to eachother and its a matter of making the correct compromises.

To threadstarter; if you measure all pickuppoints i can run it through a analysisprogram to get graphs for everything through the suspensiontravel.
 

Terry Oxandale

Skinny Man
For me, the most important part of KPI is how it impacts camber in a turn. You really can't have one (caster) without the other (kingpin) and they need to be "coordinated" for lack of a better term. Add to this the fact that KPI also impacts where the scrub radius point is, and now it's more complicated. IMHO, it's way more information than can be shared in a post, but read up on it, and I think you'll appreciate it's importance. There is a lot going on in a front suspension that is not easily identified visually, but so important in how a car performs and feels to the driver. Everything on a car is a compromise, and finding that sweet-spot, for what you want out of the car, is the challenge.
 
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