A while ago I added a significant amount of caster to the car and I've had positive and negative outcomes.
Positive: The car feels better. The RF tyre is wearing magically - its done 4 race meetings and hardly has worn, tyre temps are within a couple degrees across the treads. Life is good!
Negative: With teh changes in geometry Ive created a side scrubbing effect on the LF tyre, the outside portion of the tread is quite fine, but hte inside has worn significantly and has notable graining across the treads. It looks as though it is over-turning.
Have I messed with the ackerman setup on the car by increasing caster? Its gone from about 3 degrees to 9 degrees. Camber is even side-to-side.
Bear in mind that Teretonga is a anticlockwise track with only one significant right hand corner.
Any suggestions as to what I should be investigating in order to fix this problem. If I can fix this then a set of tyres looks like it would last two seasons!
skin1235,
Feb 28, 8:34am
I love your problems MM, always a little outside the square changing caster shouldn't effect it like that, but you either have an unbalanced system or an artifact of the track you do most of the driving on first - whats the toe second - get under there and check that the distance between the outer tie rod end and say the rotor are the same both sides, allowing for toe in they should be the same assume R&P so no issues of idler arms whats the toe like at 20 degrees perhaps for that track you need to adjust the toe for a tighter right outer bias, ie bring the left in a little more by increasing caster you have increased the footprint of the right tyre under cornering pressure, the added weight of momentum shoves that side down but this will mean the left will be tending to lift ( momentum / inertia ) due to body roll and suspension travel limits, there may even times when it is lifted off the surface altogether, and in times like this the inner edge is doing all the work that left wheel has to do artifact of the track could be that you need to swing that wheel in a little more to accomodate for scuff from the right becoming wear on the minimal contacting left
skin1235,
Feb 28, 8:37am
a lot of oval racers have no steering control rods or mechanism on the inside wheel - it can flap about at will, when its on the ground caster does the job, while its off it doesn't matter anyway
morrisman1,
Feb 28, 9:00am
interesting skin!
Toe is zero, it had some toe out but I got rid of that. Eyeing it up, it looks like the inside wheel turns significantly more than the outside, more so that what I would consider normal.
skin1235,
Feb 28, 9:18am
, its supposed to, thats what the ackerman angle measures, there can be 3 or 4 degrees more turn inside to outside, its following the arc of a different diameter circle, the inner wheel on any corner turns sharper than the outer the ackerman is calculated to lay the tyre in the correct path, track width, track length dictates the inherent static angles so when turned the axis lines of the front wheels dissect the axis line from the rear axle at the same point
skin1235,
Feb 28, 9:22am
I'd be tempted to pull a bit of toe in, its FWD so toe should be toe out about 1.2mm ( but thats a standard vehicle with standard geometry and for street road driving ) have you increased the tyre size?, wider, rim sizes, not sure if your class allows such or has limits to such
I guess somewhere in the racing world there are boffins playing with this searching for the ultimate angles for given directional turning meanwhile given that you have stubs with an inbuilt ackermann ratio you have to make the best you can it would not be easy to adjust the angle and keep it drivable lol, perhaps some do, maybe have an adjustable length on the hub arm, or adjustable angle on the hub arm too technical for me though
skin1235,
Feb 28, 9:41am
plus in a race car you'll have completely out of boundary scrub patterns, tyre slip etc
morrisman1,
Feb 28, 9:54am
just clubmans so no restrictions. The tyre size is 205/50r15 and thats very close to stock. Ride height is lower but not sacked to the ground.
When I mentioned the inside wheel turning more, I know its meant to turn more but it looks like its doing it too much, and to the point that the inside tie-rod and steering arm are in a straight line while at full lock
skin1235,
Feb 28, 10:00am
did you remove the steering lock stops at some time, it shouldn't be a straight line, a bump at the wrong time could turn it inside out ( Model T Fords used to do this when they got a bit worn, but on the steering drop arm, next minute you had to turn left to get the wheels to go right - and you never knew if it had dropped back the right way while you where steering straight ahead, you gave a little flick before the corner to know which way you had to steer - great fun, even greater if you were not expecting it lol)
skin1235,
Feb 28, 10:02am
how often do you use full lock?, if its sliding that much ackermanns theory isn't going to do much at all, all the angles are different for each corner depending on how hard you've got it hanging out
will catch up tomorrow
snoopy221,
Feb 28, 10:05am
inside tie-rod and steering arm
As in you are not talking a rack and pinon but actual steering arms and steering box and steering idler. Ya cannae stuff up ole ackerman on a rack. However way back in the nineties had a fulla working for us that actually fitted the steering idler incorrectly on a falcon-upside down believe me that will seriously confuscate ole ackerman-end of the day check your wheel angles with turntables-or park on two straight chalk lines on concrete and turn wheels and use chalk and a protractor. Personally 9 sounds a bit high on castor 6 to 7 is usual for racing and with the change in footprint one usually doubles up the sway bar to help the inside wheel to gain traction.
morrisman1,
Feb 28, 10:40am
eureka, I think Ive just worked it out. Moving the steering rack forward and aft apparently changes ackerman, cos really acherman is the angle between the steering arm (the arm on the cast hub) and the tie rod (arm which the tie rod end threads onto). I defined them just incase I had the names wrong.
Heres a pic of what I think is going on, excuse the crappy drawing & read my story to go with it!
please let me explain. If I am right, the ackerman angle is not set by the angle of the steering arms to the hub face, but more accurately by the angle between the steering arm and the tie rod when the wheels are straight ahead. With no ackerman this will be 90 degrees.
Adding ackerman increases this above 90 degrees. If the steering rack was to move forward then the angle would decrease back towards 90 = less ackerman angle. If the steering rack was to move rearward then the angle would increase = more ackerman.
What I have done when adding caster was to move the strut tops rearwards but also the lower ball joint forwards in the region of 20mm. This in effect is moving the rack rearwards, increasing the angle between the steering arm and the tie rod and thus increasing ackerman to the point that the inside wheel is over-turning and the steering arm approaching in-line with the tie rod. Moving the rack forwards would correct this. Alternatively I could decrease the caster a bit and put stock lower arms back in which should reinstate the correct geometry.
Am I nuts (dont answer that!) or have I possibly cracked it?
Tomorrow I can take a comparison pic between a stock n14 pulsar, and the modified one to see just how much there is in it.
budgel,
Mar 1, 11:54am
By adding castor the relative angle of the steering arms to the horizontal plane has changed. This will affect the ackerman angle somewhat.
The position of the rack doesnt determine ackerman angle, even if the tie rods are now at a different angle from before. It may well increase or reduce toe in which would need to be adjusted , but the ackerman angle is dictated by the steering arm angle in relation to the hub axle. Has your increase in castor resulted in more self centering of the steering?
The tyre wear sounds more like it is caused by camber issues.
mechnificent,
Mar 1, 7:42pm
You are wrong about the ackerman angle being set by the angle betweent the tie-rods and the cast steering arms MM. The ackerman angle is set by the angle between the centre line of the hub bearings, and the cast arms. When the wheels are aiming straight ahead, the cast arms ain at the centre of the rear axle.
However, that said. you do have a point.
If you moved the steering rack to the rear, and then steered to the left, the left steering rod would be pulling in an increasingly straight line to the cast arm as the wheel came around to lock, which would cause less relative angular movement compared to the right wheel which would be having it's tie rod pushed at increasingly nearer to right angles as the wheel got near full lock. The effect would be the opposite to ackermans principle. Conversely, if you move the rack forwards, and turn left, the left rod will pull the cast arm at nearer to a right angle as the wheel gets near full lock, and the right arm would be pushing at an increaingly acute angle as that wheel got near full lock. you would get excessive ackerman principle.
All of that said, I suspect that Skin is right about you needing to set it up favouring the predominant turn, left or right for the track you are on. In theory, whatever you do with the rack should effect both wheels equally if the steering is normal road use and getting even use left and right. The only thing that causes uneven ackerman angle on a rack and pinion normally is the cast arms being bent on the hub.
skin1235,
Mar 1, 7:43pm
beg to differ there budgel, the ackermann is the angles on the arc that the arm travels through, shifting the rack from its original position, or effectively shifting by altering the mount points of the wishbones will alter the horizontal arc the arm travels through, altering caster also alters it by introducing yet another different to original arc, this one being a more vertical axis arc than normal
MM you maybe on the right track but to remedy? how much does it oversteer that left, ie how much can you toe it in to bring it closer to understeer or neutral, and what effect does that have in the straight ahead position what have you got in the way of sway bars ( as snoopy suggested ) keep a little more downward on that left being that the track is predominantly left turns
mechnificent,
Mar 1, 7:44pm
Assuming that all the steering angles are symmetrical, unless your cast arms are bent, the tyre wear being uneven will be the result of harder/more cornering one way. Or a weight imbalance ?
Not the ackerman anyway.
skin1235,
Mar 1, 7:54pm
while doing the comparison pics, check the ackerman angles if yuo ahve time and the privilege - a large builders square and large builders protractor if you don't have swivel plates to put under it, a straight line along each side and measure the angle from that at say 20 degrees and I suspect the more common 16 to 18 degrees you use on the track interesting topic, don't stop tossing these in MM, its good to exercise the mind on occasion
now another thought, how much does rear scuff under momentum effect the ackermann angles, rear tyre side slip will alter the effective point that the ackermann should be aimed at, I suspect it would move that point back behind the rear axles, ackermann is only effective if the rears are following straight behind the fronts, twist that 150mm to one side by controlled slide, normal side slip under cornering etc and what have you got
skin1235,
Mar 1, 7:56pm
actually it may shift it forward of the axle cos the axle is travelling away and twisting its physical axis back, its theoretical axis would actually be forward of the physical
esky-tastic,
Mar 1, 8:26pm
Aaargh! One of those details that wold have been handy to know right at the start!
But well done figuring it out!
esky-tastic,
Mar 1, 8:31pm
Yep, at straight-ahead position you should have a line (imaginary) from the equivalent of your king-pin (ball joints) that runs through the tie-rod eye in your steering arms that then meets in the centre of your rear axle.
The above is ideal but seldom actually 100% achieved.
bill-robinson,
Mar 1, 8:40pm
just put lower rate springs on the rear of the car and let it pick up the inner wheel. that will stop tyre scrub. you are over thinking the problem
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