This subject has been dealt with many times, but I had to re-adjust mine and checked the basic information given by Jaguar.
The way to check the height under chassis changed in January 1954 as mentioned in Jaguar SB 139.
I also found the same height of 11 ¼ " mentioned by Vandiest (“Jaguar Cars” reprint 1961). The height relates to “the distance from the ground to the underside of the front cross member”.
I made wooden blocks based on the “old method” positioned "below the lower face of the most forward parallel section of the chassis". But also according the later method.
Has anybody ever tried the later method? Well I did and I found that with the correct height set according the old method, the wooden block of the later method as at least 1" too long. It is absolutely impossible to get the wooden block under the front cross member.
Very strange! I took some pics to show the difference for my OTS. First pic shows the wooden block under the “parallel” section of the chassis and the second picture the block at the front cross member of the chassis.
I do have new springs at the rear, but would that impact the height at the front that much? The overall stand is reasonably “horizontal”. (pic 3)
I had a look and the rear of the car is too high, also due to an almost empty fuel tank! I guess that a full tank might add about 40 to 50 kilogram (or 100 lbs) of weight over the rear axle. I will simulate that and see what happens with the front.
If no improvements are observed, I will have to take a look at the new springs.
Yes, that’s the best way to get the required weight simulating 2 passengers and a full tank.
I tried to simulate a full tank and the rear end dropped by about 3/4 of an inch. But the front of the car didn"t raise about the inch I was looking for, to get the correct height.
I still have my doubts whether the early approach and the later recommended version are comparable. May be Jaguar went in 1953 for a higher position of the chassis at the front, in any case more than the 180 mm of 1949.
200 pounds of occupants would be between the front and rear axles, but closer to the rear. However, the deflection of the front torsion bars and rear leaf springs is not identical. Each could be calculated, and undoubtedly were calculated by Heynes et al.
A full tank of fuel 15 imperial gallons, 18 US gallons, 68.2 liters is 108 pounds or 49 kgs.
I keep coming back to the thought that your new rear leaves may not be arched properly at 5-1/2".
However, a missing item of information is whether there was any difference in the free camber of the early C.3661 spring vs. the later C.5721 spring from Nov/Dec '52 onward.
I went for a different approach this time. The upper part of the chassis looks on drawing C.6762 like a straight line up to the point were the rear spring section starts.
The box section at the point "below the lower face of the most forward parallel section of the chassis " is about 6 15/16" (or 176 mm) high. If we go further to the front the box section at “the underside of the front cross member" measures about 4” (or 102 mm) in height.
This means a difference in height of 2 15/16" (or 74 mm). Even if there are some mistakes in my measurements, this difference is by far less than the 4 1/8 " or 104 mm) difference in height that resulted from the new definition of SB 139.
Wouldn’t it be possible that Jaguar opted for a new definition of the required chassis height for setting camber and castor for the later XK 120 models and the XK 140?
I understand that most people always use the earlier method with the 7 1/8" high wooden blocks under the two side sections of the chassis. The newly defined height according SB 139 would lead to a much higher “stand”.
Wouldn’t this at the same time also affect the behavior of the car (positive or negative)? We know that Jaguar used less camber and castor on the XK 140.
Or should the XK 120 after the arrival of the heavier torsion bars follow the defined camber & castor values of the XK 140? Again: more questions than answers…
Notice the bottom of the chassis is parallel to the ground from the master cylinder mounting bracket to the front attachment of the rear springs, but the top surface and forward of the bracket it is angling upward. XK120_chassis_drawing.pdf (2.1 MB)
So I think that the 7-1/8" measurement and the 11-1/4" measurement are consistent with each other.
They are on my car anyway.
