Castor angle setting

Post Reply
User avatar
RadfordJim
Posts: 88
Joined: Tue Feb 16, 2016 10:19 am
Location: Coventry

Castor angle setting

Post by RadfordJim » Tue Jul 17, 2018 12:01 pm

I've searched the Old and New Forums for info on Castor setting but can't find the answers to my problem.
I had the car checked on a laser tracking rig and the toe was adjusted to zero as recommended by the technician because I've fitted radial ply tyres. Although this gave some improvement, I still had to steer the car back to straight when exiting a corner.

The problem was highlighted when John Box (one of the original SP250 test drivers) recently drove my car on a recreation of the prototype black car's first test drive to Bala Lake 60 years ago (article in DM soon!). His comment was "a little more castor on the steering would be helpful".

The print out from the laser tracking results shows a discrepancy between OS and NS castor. The OS wheel +2deg54' but NS wheel
+0deg03' I suspect the bottom NS wishbone was pushed back to prevent the tyre catching the front of the wheel arch on full lock and I doubt the steering geometry was ever checked after this.
So my questions are:
1. What is the ideal Castor angle?
2. How can I determine what thickness shims are need to correct the discrepancy?
3. Where should the shims be fitted?
4. Should this improve the feel of the steering?
LaserAlignment.jpeg
Thanks in advance!
Jim in Coventry - Home of the Daimler

Sydsmith
Posts: 621
Joined: Sun Feb 14, 2016 11:15 pm
Location: Aberystwyth Wales

Re: Castor angle setting

Post by Sydsmith » Tue Jul 17, 2018 1:00 pm

Hope this is not wrong but are we getting mixed up between caster and camber here.

As I recall caster is the angle of the front suspension top ball joint to lower ball joint relative to vertical and camber is the angle of the wheel relative to the road surface.

Camber on an SP is adjusted by shims under the bottom suspension wishbone, not sure how you would adjust the caster except by moving the wishbone on a front to back axis, don't recall there being any room for adjustment there when I rebuilt my chassis.

But I am sure someone will tell us shortly :D

User avatar
Vortex O'Plinth
Posts: 650
Joined: Fri Feb 12, 2016 8:15 pm
Location: Gloucestershire

Re: Castor angle setting

Post by Vortex O'Plinth » Tue Jul 17, 2018 2:59 pm

There is no direct adjustment for caster in the design of the SP suspension. However, depending on the amount of camber correction required there may be a thickness of shims under the two lower wishbone brackets. Nominally there will be the same thickness of shim under each bracket since it is intended only to correct for manufacturing discrepancies affecting camber.

Theoretically one could remove shims from under one bracket and add them to the stack under the other without affecting the camber, however this would result in a fore and aft translation of the vertical link trunnion causing the link to rotate about the upper ball joint and thus alter the caster angle. Some very approximate trigonometry indicates that removing 0.065" of shim from under one bracket and adding it to the shims under the other would alter the caster by one degree. I have not checked this but perhaps someone could confirm - or otherwise - this calculation.
Nick

Pas d'elle yeux Rhône que nous.

User avatar
RadfordJim
Posts: 88
Joined: Tue Feb 16, 2016 10:19 am
Location: Coventry

Re: Castor angle setting

Post by RadfordJim » Tue Jul 17, 2018 3:08 pm

Thanks Syd, yes it's the Castor angle that I'm trying to correct (3rd row on print out) to help steering return to straight. The Camber (2nd row) is pretty much exactly the same on both sides as shown in the print out.
Jim in Coventry - Home of the Daimler

Ian Slade
Posts: 377
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: Castor angle setting

Post by Ian Slade » Tue Jul 17, 2018 3:41 pm

To set up the steering geometry one would first need to set the caster angle, then the camber and finally the tracking. The caster is set as Nick says, but by removing shims from the front to rear or vice versa you will alter the camber, you then would need to add or subtract shims in equal amounts front and rear until the correct or required camber is achieved, having completed the caster and camber you have in effect pushed or pulled the trunnion forward or rearwards, doing so means the trunnion has rotated about the centre point, thus by moving the caster rearwards would result in toe out from the original setting, opposite for moving the trunnion forward. I suspect the caster angle is the secret to lighter steering with the original steering worm and peg steering, though which way I have no idea. :?
Owner since the 70's, Genghis is slightly to my left.

bakergh
Posts: 50
Joined: Wed Feb 24, 2016 1:57 pm
Location: New Forest, Hampshire

Re: Castor angle setting

Post by bakergh » Tue Jul 17, 2018 3:47 pm

Front suspension information including all steering angles is at

http://jag-lovers.org/saloons/daimlan/s ... uspens.pdf

Graham

daimlersteve
Posts: 276
Joined: Wed Mar 09, 2016 11:20 am

Re: Castor angle setting

Post by daimlersteve » Tue Jul 17, 2018 10:56 pm

Hi all,
yes ,the way to change castor is by shimming the lower arms.
Take care here .
Depending on the state of the chassis ( variable) , mounting holes and tower position /top arm hole alignment , the settings can be all over the place
Too many shims will mis-align the vtl. link at the top as the trunnion ange is sq.off the vtl link. . The parts can be pushed together but this will cause premature wear on the trunnion . ( a ball joint at the bottom would have no problems with this ,but we're stuck with trunnions.
Best done on axle stands in your garage. Remove the spring on the incorrect side.
Find a strip of ally 150x25x1.5 , drill a hole 100 from one end that will take the grease nipple in the top b/j. (this clears the suspension parts. Mount the strip on the top b/j , drop a plumb bob from the extended end down past the trunnion g/nipple , measure that distance --string to nipple on the 2+ degree side of the car. The difference /the ht. x tan ( Tan op/adj ) is your castor. --- just make the other side the same and you're there.
Correct the other side to the same with shims or chassis adjustments
Have done it this way numerous times.
steve

User avatar
RadfordJim
Posts: 88
Joined: Tue Feb 16, 2016 10:19 am
Location: Coventry

Re: Castor angle setting

Post by RadfordJim » Fri Aug 17, 2018 5:32 pm

I've just had the castor angle adjusted by my favourite "old school" workshop. Before starting, they checked the castor using a digital inclinometer to compare with the print-out above. Results were similar so we decided to adjust the nearside only as that was almost zero deg.

They used ratchet straps to compress the road spring and on removing the lower wishbone arm bolts, found 8 x 15 thou shims under the front arm and none under the rear, so as a starting point 6 shims were swapped from the front to the rear. On re-assembly the castor was rechecked and showed 2.4 deg nearside and 2.6 deg offside so near enough! Camber remained the same then the tracking was rechecked and the change in castor angle had caused a slight toe-in which was expected so this was re-adjusted.

I took the car out on the road and found the steering lighter and more positive. Self-centring is better and the oversteer has gone, so a good result and well worth the effort.

Many thanks to all contributors for the technical advice which the workshop guys found very helpful.
Jim in Coventry - Home of the Daimler

Post Reply

Who is online

Users browsing this forum: No registered users and 2 guests