Suspension misalignment

[John Quilter]

@ Glyn Ruck. You are correct, I do not believe even after much investigation, that the Ackermann angle can be correctly accommodated with a rack & pinion conversion which my car has. The downside of not not having a built in toe out when making a turn is some tire scrubbing and in slow maneuvering a reduced turning circle. The tire scrubbing contributes to early front tire wear (outside edge as I recall). Having said this I still like the lack of steering gearbox leaks and the very positive steering that the rack provides. Of all the 185 X 15 tires out there which is the most hard wearing? I should fit those next time but my annual mileage is not great.

[John Quilter]

Firstly it is necessary to establish the current ride height, with full fuel tanks and correct tyres and pressures. I find the acceptable way to clone the factory build position is to attach masking tape, lined up along the gap formed by the front and rear doors and cill. The extension of this tape at the rear end forms a tangent with the top of the circular section of the spinner. At the front, an extension of the tape passes approximately through the centre of the front spinner. This position matches the Jaguar dimensions for the body shell and images in brochures, it also produces correct front and rear lower suspension arm angles.

And another comment. I run an Austin 1300 (Austin America) and with these cars with the Hydrolastic suspension the workshop manual give measurements for ride height. It was a figure measuring from the center of the hub cap to the underside of the wheel arch directly above. (Interestingly it even varied slightly between marques!) I wonder if there is such a figure for our cars.

[Glyn Ruck]

@ Glyn Ruck. You are correct, I do not believe even after much investigation, that the Ackermann angle can be correctly accommodated with a rack & pinion conversion which my car has. The downside of not not having a built in toe out when making a turn is some tire scrubbing and in slow maneuvering a reduced turning circle. The tire scrubbing contributes to early front tire wear (outside edge as I recall). Having said this I still like the lack of steering gearbox leaks and the very positive steering that the rack provides. Of all the 185 X 15 tires out there which is the most hard wearing? I should fit those next time but my annual mileage is not great.

It is possible but difficult with potential items like the sump getting in the way. I believe a centre steer rack like an Astra uses & intermediate levers (idler arms) likely the best way. A friend is messing around at present trying to get an end steer rack to work (accurately) but rack selection, length, mounting & a number of custom manufactured parts seem necessary ~ I'm not sure he is going to get it right. With some conversions I've driven the tyre scrub would drive me nuts. All are not equal. End steer rack direct to the standard Jaguar hub arm close to impossible.

Back of the envelope calcs ~ one needs to achieve approx 9 degrees of differential toe out inner wheel to outer wheel at 30 degrees of steering angle. (a number of assumptions being made & centre of kingpin unknown). Toe out needs to increase exponentially with steering angle.

Fortunately with our high sidewall tyres they are a little more accommodating than low profile tyres.

The hardest wearing tyres in 185 X 15 are IMHO

The Michelin XVS & Michelin XVS-P

Page 2 here:

http://www.jagstyperegister.com/forum_n ... +xvs#p6002

VERY basic concept & of course wheelbase dependent.

1280px-Ackermann_turning.svg.png (138.46 KiB) Viewed 2739 times

Ideal Ackermann.JPG (49.23 KiB) Viewed 2738 times

[JCS]

Glyn

“…………one needs to achieve approx 9 degrees of differential toe out inner wheel to outer wheel at 30 degrees of steering angle. (a number of assumptions being made & centre of kingpin unknown). Toe out needs to increase exponentially with steering angle…….”

If I understand your comment correctly, you are suggesting that when the inner wheel on a turn is at 30 degrees steering angle, then the outer wheel needs to be approx. 21 degrees.

Have I understood your comment correctly?

Norman

[Glyn Ruck]

Outer wheel, inner wheel angles
10 deg, 10+1 deg
20 deg, 20+4 deg
30 deg, 30+9 deg
The number after the plus is added to achieve exact Ackermann. The angles depend on the ratio of front track to wheelbase. I've used a ratio that I've guessed as close to S type. A 30 deg turn of the outside wheel should be about the S Types full lock and give a turning circle of just under 36ft. The outer wheel defines the turning circle.

So reference from the outer wheel. i.e. outer wheel at 30 deg steering angle = inner wheel at 39 deg. Ackermann angles are approx the square of the angle of the outer wheel. i.e. exponential with increasing steering angle.

Checked a colleagues calcs with an old HP41C that I still own. My trigonometry is rusty to say the least so let calculator do it for me & we concur.

I live on a steep slope overlooking the sea. I have to do 4 full lock turns to reverse out of my garages & up the driveway just to get to the road above.

I've just checked the Service Manual for S type & turning circle is in fact quoted at 33 feet 6 inches. My colleague is working on a Mk2 which is quoted at the same. Both wheelbase at 107 inches. Above numbers remain approx correct just not quite full lock. As I said back of envelope.

[Orlando St.R]

As an addendum to this topic, referring back to discussion in the opening posts about ride height, the FIA Homologation Form https://historicdb.fia.com/sites/defaul ... roup_1.pdf does contain a diagram with ride height specified. What unfortunately isn't specified is whether the vehicle has fuel in the tanks or not, though there is a note alongside the weight stating that the weight of the car is to be specified 'with normal equipment, water, oil and spare wheel, but without fuel and repair tools'. So maybe that's a reasonable assumption.

fia-ride-height.jpg (185.78 KiB) Viewed 1933 times

[Orlando St.R]

Just measured my ride height and, according to that FIA document, it's an inch too low all round!

[jaguar&mg]

… and mine is correct at the front and two inches lower at the back. Hmmm.

[John Quilter]

I think we might be obsessing about this stuff. Just measured my car (all original springs and 169,000) miles and from using the handwritten drawing above, my car is at front 6.5 inches to floor and rear 8.0 to floor. I'm leaving well enough alone but I do wonder if anyone has tried, or knows about, those old shade tree mechanic's use of those screw in wedges that you could fit to coil springs to raise ride height on old tired springs.

[John Quilter]

At the risk of obsessing about this matter, I have recently corrected the 1/2 inch low on the LH (driver side) front of my 3.8S which after 170K miles has all its original springs. I found on the internet these hard rubber spacers and being quite inexpensive, ordered a set. Using these eliminated the time consuming and complex process of dismantling the front A frame and fitting (if they could even be found) the spacer rings under the coil spring. These rubber spacers blocks were cut down a bit and two were inserted into the coil spring on the LH side. The result is that side has been raised 1/2 inch and now matches the RH side. The measurement from center of hub cap to underside of the wheel well lip is now 15.5 inches on both sides. Jack point measurements to the ground is: RH side, rear 8.0, front, 7.0 and LH side rear 8.25 and front 6.75. I deem these measurements close enough. A quick and dirty method of small ride height adjustments.