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The affect of castor angle on self steer
Jayce Lane - 2/4/04 at 12:04 PM

Hi all

I have been informed that the book upper wishbones give you a castor 2 to 3 degrees, which leaves you with no self steer. Self steer is the ability of a motor vehicle to straighten itself up after leaving a corner for example. It has been suggested that I increase in the castor to 6 degrees to improve safety and driving comfort.

your comments on this would be appreciated.

Cheers

Jayce

blueshift - 2/4/04 at 12:20 PM

This has been discussed many times. try using the search feature for things like "book errors" or "castor angle"

JoelP - 2/4/04 at 12:42 PM

i noticed the other day when my sierra hubs were off, that the axis of spin if in front of the line between upper and lower ball joint pivot centers, ie the opposite of trail. this might be why it is so hard to get self centering. i intend to turn the mushroom to put the hole at the front, hence helping it to line up better, and then to also have a lot of castor.

stephen_gusterson - 2/4/04 at 12:49 PM

the book states 5 1/2 degrees, but the drgs are wrong.


atb

steve

Tigers - 2/4/04 at 02:03 PM

Hi!

Can somebody tell what will hapen if I build car with negative steering offset ( I mean - the distance D in the drawing will be negative).


Will there be any self-returning steering or steering wheel will tend to do the opposite?
Of course, I can correct the situation by placing wheel spacers, but is it a real problem?

JoelP - 2/4/04 at 02:33 PM

distance d is a combination of kingpin angle and scrub radius. It depends on which one you make extreme. Too much kingpin will ruin the wheel angle in a corner, scrub want to be low to keep the steering light.

which were you going to change?

britishtrident - 2/4/04 at 07:53 PM

The castor needs to be as much as possible 5.5 is about the max you can use on the Cortina upright trouble is Increase the castor and king pin inclination needs to be increased also -- the only way to do is use negative camber -- quite a lot.

Scrub radius dosen't make the steering heavier to turn when standing still BUT it does when cornering and it greatly increases nasty kick back.

dblissett - 2/4/04 at 07:53 PM

this drawing is from mark alanson i think
any way it should give you 5 1/2 degrees castor
cheers dave Rescued attachment Wisbone Modification.jpg
Rescued attachment Wisbone Modification.jpg

Bob C - 2/4/04 at 08:48 PM

I thought distance D WAS the scrub radius...... I'm putting mx5 uprights on mine & have noticed in the course of measuring up that the scrub radius is 0! And that's also mazda's claim, I've since discovered. I believe some of the (very) old Audi 100s had a negative scrub radius which they made lots of bold claims about in their adverts (in conjunction with their "diagonal dual circuit brakes) - I think I'm talking early 70s here...
cheers
Bob

Mark Allanson - 2/4/04 at 10:00 PM

quote:
Originally posted by britishtrident
The castor needs to be as much as possible 5.5 is about the max you can use on the Cortina upright trouble is Increase the castor and king pin inclination needs to be increased also -- the only way to do is use negative camber -- quite a lot.

Scrub radius dosen't make the steering heavier to turn when standing still BUT it does when cornering and it greatly increases nasty kick back.






Sorry BT, but all of the above has confused me greatly.

Increased scrub will increase the turning effort massively at rest as the wheel not only needs to be rotated, but needs to be moved through the arc of the scrub raduis against the resistance of the tyre on the road surface ie the tyre will not only have to be twisted (zero scrub), but (with my scrub being about 25mm) will have to move the tyre, without rotation, about 40mm about the raduis assuming a steering yaw of about 90 degrees.

Within reasonable limits (lets be sensible), you can run any amount of castor you like without ANY effect on the KPI (the KPI being inbuilt into the upright). The KPI would only be reduced to zero if you had 90 degrees of castor, that is the upright lying on it side!

With a large amount of castor, the steering arm will be raised, so the rack will have to be raised the same amount - as the escort rack being slightly too long, this can only reduce bump steer

As for negative camber, I always set the LH wheel with more camber than the right to compensate for road camber - this gives you less or no steering pull to the left .

If you run zero scrub radius, the steering will be dead, uncommunicative, and the real drag to drive. You would only get any appreciable kick back if one tyre is severely deflated of flat. Audi used negative scrub angles to make the car totally controllable in the event of a blow out, but this would only be drivable with FWD or 4X4

I am not sure which book you have been reading, but perhaps another trip to the library is in order

JoelP - 2/4/04 at 10:44 PM

quote:
Originally posted by Bob C
I thought distance D WAS the scrub radius......

cheers
Bob


quite right bob, my mistake. I meant to say, d is the scrub radius, and it can be changed via KPI of the wheel offset. just missed out the offset bit!

Trident, i think mark is right, big scrub=hard steering at rest. Imagine a setup with 3 feet of scrub radius? you'd be trying to move the tyre thru about 5 feet in 90 degrees. that cant be easy!

Bob C - 3/4/04 at 06:34 PM

Zero scrub radius = dead handling?? thousands of mx5 drivers will give you an argument over that.......
I'll agree it reduces tramlining and kickback, but that sounds like a good thing to me!!!
Cheers
Bob
PS effect of positive scrub radius + kingpin inclination is that the car is actually lifted when not straight ahead - so it will try to self centre even when stationary..... discuss ;^)

Tigers - 5/4/04 at 06:22 PM

Hmm... My probles is that I'm using BMW uprights with shock glass. Normaly KPI is normal and scrub radius is positive, but as I hav to cut off part of shock glass, kpi changes, and scrub radius reduces till some negative value. So I need to find wheels with different offset.