Castor Angles 350i

[John042]

I'm in the process of replacing some of the front suspension bushes. Aren't poly bushes fun? Anyway the front tie rod bushes were non existent. The replacement poly bushes are a good deal thicker and therefore the forward mounted tie rod nuts needed adjustment to reconnect the the drop links to the roll bar. At the moment I 've set them equally each side by measuring the threaded bar from the lock nut to the chassis. My manual tells me me that the correct castor angle is 3.5 degrees positive. Can this be DIY measured? If so, is positive the measurement from the vertical forward? Thanks for any ideas. J C.

[mrzigazaga]

Hi John

this info is in the FB group in the file section ...along with loads of other stuff I put in there...

Here are the generic settings for all Wedges:
15 inch rims 3.2mm toe gives 0.24 degrees wheel to vehicle centre-line or 0.48 degrees wheel to wheel.
The tracking should be set at 3.2mm toe-in +/- 1.5mm
Caster: 3.5 - 4.0 deg. pos.
Front
Camber 1.0 deg. neg.
Toe (in) 0.50 deg.
Rear
Camber 1.0 - 1.5 deg. neg.
Toe (in) 2MM - don't know why MM not deg!
2mm on 15inch rims = 0.30 degrees = 18 minutes
2mm on 17inch rims = 0.27 degrees = 16 minutes
Either way it's just a smidge toe in.
14 inch rims 3.2mm toe gives 0.26 degrees wheel to vehicle centre-line or 0.52 degrees wheel to wheel.
For the front
Toe is 3.2 +/- 1.5mm
Camber is 0 +/- 1.2 degrees (not adjustable anyway is it?)
Caster is 31.2 +/- 0.5 degrees
KPI 6 degrees
For the rear
Toe is 4.5mm
Camber is 0.5 +/- 0.5 degrees (this seems small as the wheels have a visible camber)

[John042]

Hi Mark, loads of info' thanks. Simplicity though is the Castor angle the amount of perpendicular angle of of the wheel centre to the vertical? In which case "Leaning" forward is positive? Negative, rearwards? Therefore could it be set using a simple vertical data at the wheel centre and adjust the offset accordindly? J C. Seems to easy?:laugh

[RCK974X]

For tie bars, this adjusts TOE-IN , not castor. I don't think castor or camber is adjustable in std form.

You can do a DIY job for toe-in. Not perfect, but I've done this several times, and tyres lasted for a long time, so couldn't be all that far out...

I use two long pieces of angle iron pushed up against the tyres (make sure they are straight !!)
then measure the distance between the contact patches (angle iron to tyre that is)
for a 13 inch tyre this will be somewhere about 15 inches (because bulge of tyre is about the middle of the side wall)

First you need a bit of geometry.....

0.5 degrees positive. Take the tangent of this (which is 0.00872687).
so, for every inch forward, 0.5 degrees will cause a sideways movement of 0.00873 inches.

right..... so across the tyres at 0.5 degrees positive, the FRONT contact patches to angle iron are going to be 0.00873*15 = 0.13 inches less than the back...
and two tyres gives 0.26, or a quarter of an inch.

so, move to outside the nose, and measure the across angle iron pieces. get the measurement parallel to the car as much as poss. this is measurement 'A'

then another 15 inches forwards, measure again, and it should be 0.26 inches less than 'A'.

Or do it in mm.....

All you are doing is using the bars to enable you to 'move' the measurement to outside the car, so you can use an ordinary tape measure.

and it works for the rear too....

substitute your angle and contact patch measurement, and it works for any toe-in adjustment for any tyre size.

Edited by RCK974X on Saturday 1st December 01:04

[RCK974X]

positive camber means wheels lean inwards vertically
positive toe-in means wheels point inwards (forwards, fronts nearer than back)

castor is err..... I think the effective angle of the 'pivot' as the wheel turns.
It's a bit hard to see with a twin wishbone , but think of top ball joint being in front of (or behind) bottom ball joint, to give an effective pivot angle.

I think that's right....

Typically toe-in is positive so that as you drive, the flex in the suspension/bushes will draw the wheels parallel.
Front wheel drive cars typically have toe-out for the same reason, as they 'pull' the car along.

Camber tends to be zero or positive at the front, and zero or negative at the rear for best handling,

Castor angles, to do with handling and steering and the self-centre effect required.

(both posts edited for typos)

Edited by RCK974X on Saturday 1st December 01:07

Edited by RCK974X on Saturday 1st December 01:17

[John042]

Thanks Andy. Perhaps too many beers, Friday! Surely castor angle is affected by the tie rod from the bottom wish bone to the chassis? Screwing in would pull the bottom wish bone further forward therefore the wheel vertical would be affected? No doubt the toe in/out would also alter. The original problem was the fitting of thicker poly bushes and connection of the roll bar drop links.

[RCK974X]

Yes it probably does, they all interconnect by a small amount, but officially tie-bars only adjust toe-in....(as far as I know) especially if they are horizontal (more or less)


If you read about suspensions (and it can get VERY complex), the angles all change by design as things move.

Examples -

Twin wishbones - the camber (lean in) changes according to the relative length of the wishbones and their 'at rest' position as the wheel goes up and down.
Castor is fixed (I think) as the upright effectively locks the ball joints and wishbones at the same angle (??)

toe-in can change as the steering wheel turns .... and the height/position of the steering connectors can also affect how the wheels move

and as the body rolls, the effective angles of the wheels (to the road) can also be tuned for grip and handling.

[adam quantrill]

You could make a small adjustment by shifting the top and bottom ball joints if you have ones that are bolted down, but it would be a rough adjustment.

Also, if you lower the front of the car, or raise the back (bigger wheels?) you'll get a small reduction in the angle, maybe a couple of degrees.