Thanks Steve, they look like real specialists, I’ll get in touch with them.
Cheers
Sorry Luis, I don’t know enough about XKs or their steering type to do any better than general speculation.
On an instinctive level I could imagine rubber bushes absorbing more effort to deflect them than solid bearings, but I thought typical PU bushes were a similar stiffness to rubber ones. Then it might be a matter of the inherent damping in the materials - subjectively I feel PU has less damping than rubber so might absorb less energy when deflected, but have no data to support that.
The condition of the joints and pivots in the suspension would obviously be a factor in a comparison, as would the castor angle. I think the XK, like the E-type, has very low castor so a variation would probably be more apparent than in a car with more typical (higher) castor.
Stirring those two factors together with a bit of guesswork, it’s possible that rubber bushes allow more castor change as the wheels are turned, so some effort increase. I’ll take a look at that in the E-type model some time.
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Hi Luis, I have a bit more hazy speculation supported by a small amount of analysis.
I took my standard E-type model and looked at the effect of making the steering rack mounts significantly softer, then stiffer, in the lateral and vertical directions (one by one). That’s an estimate based on a guess because I’ve never measured the stiffness of the original mounts, so I was looking for trends not exact results.
RACE software doesn’t model the entire steering system up to the driver’s hands so won’t give steering torque directly, it must be implied from other suspension parameters - I used castor change and toe-link loads.
From that, I could see no likely effect on steering loads from varying the rack mount stiffness in the axial (cross-car) direction.
Making the mount stiffer in the radial (vertical or fore-aft) direction slightly reduces toe-link load, but it is slight. Perhaps more significant is that a soft mount shows some hysteresis during a steer manoeuvre - the effort to turn the wheels into the corner is greater than the effort to straighten after the turn. I could tentatively see that “lost energy” giving a feeling of greater effort, or inertia, to turn with those softer mounts. Conversely, a stiffer mount does not show that hysteresis so might feel lighter as the output force matches the input.
All this is totally speculative, in real life engineers would have actual steering torque data to guide the discussion. Overall I think the difference is too slight to be a “make or break” factor. If you’re seeing anything above “slight” there’s probably another vehicle factor at play, or I’ve overlooked something. Both likely, I’m very willing to be proved wrong since my data is as scant as my knowledge here.
Perhaps more significant is that the radially softer mount adds a slight understeer tendency, but going stiffer than my nominal does not have the opposite effect. That’s the trend on the chart, I can’t say more than that without actual stiffness values for real parts.
I had to take some time away from this, I do have a couple of dampers at Cornering Force for test.
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Thanks so much for the time. I am sure that many “feelings” when it comes to things such as steering or ride are entirely subjective, or dependent on so many variables, such as wear, tire pressures, compounds… that it is very difficult to compare.
… which is why really good test drivers are very, very difficult to come by, and worth their weight in gold.
Had I’ve been able to monetize the abilities I did have, at one time, 35 to 40 years ago, I might actually have created a different career path for myself.
I also had to take time away from testing various bushings and mounts. I plan to resume that in a week or so.
Get paid for driving cars and dreaming up tech-sounding comments? How could that be?
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Finally found the time and energy
to disassemble the adjustable valve on the GAZ shock absorber.
Starting at the bottom.
- Knob with a set screw that holds it on to shaft #4.
- Tiny detent ball bearing.
- Retainer that threads into the base of the shock.
- Shaft with threads.
- Plunger with internal threads.
- Spring
- Plastic piston.
The knob turns the threaded shaft, adjusting how much the plunger is preloading the spring .
Funny to me how small these parts and passages are compared with the McPherson struts I used to work on.
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I see it as a mix of art and science. Traditional wisdom is to change only one variable at a time, which makes for a long and tiring process. Around the time I retired we were being encouraged by program managers to develop a process which allows changing multiple parameters at a time - based on the Taguchi process, I vaguely recall. That is attractive for saving time and resources but it needs variables which can be measured precisely and plotted numerically - far away from a slight feeling that the last damper change increased road noise “a tad” while improving steering connection “a bit more”. My belief was that a good development engineer subconsciously does an informal version of the Taguchi analysis, focussing on the most significant effects at a given time.
Current tools do allow us to take the work a level deeper than the doughty heroes who did the original car(s) could attempt. Even twenty or thirty years ago we could simulate and compare charts of changes and events that would have previously required hours pounding up the A5 with gritted teeth and flies in the nostrils.
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I got swift results from the very helpful gentlemen at Cornering Force. (Thanks for the suggestion, Steve)
To recap, I bought a Monroe and Woodhead rear damper from David Manners (DMG) and sent them to Cornering Force for industry standard force-displacement and force-velocity tests that would allow me to compare them with Gaz and Girling dampers previously tested. The Monroe and Woodhead were listed for E-type S1 & S2, I don’t know whether they are exact original specs or a later re-creation / substitute, as the Girling recently tested appears to be.
The force-displacement tests look reassuringly like traditional twin-tube dampers in shape.
The Woodhead has a classical jounce:rebound force ratio, while the Monroe has a higher proportion of compression forces. That direction can work well on a car with sporty aims as - within reason - it can give a more lively and “alert” feel. Obviously I’ve no idea if either is a faithful copy of an original, or why they are so different. They would feel distinctly different on the road.
The Monroe also has a strange asymmetry in the compression (upper) curves. Maybe a factor of the valve design, I hope we’ll learn more when we take them apart.
I’ll add the previously tested Gaz and Girling curves to this chart for comparison. My first reaction is that I’d prefer Monroe or Woodhead, simply from the curve shapes. That only addresses one part of the question, of course - the actual forces are equally important, a pretty curve is not useful if the overall control force is inadequate. That might be where the others come into the reckoning.
More to follow.
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Here is the second part of the damper tests, peak force / velocity curves. These show the (peak) velocity at the 6 o’clock and 12 o’clock positions on the prior force-displacement curves, one point for each of those curves building up the curves here.
Without having driven any of these cars or dampers, I’m guessing I might prefer the shape of the Woodhead. That’s mainly for the higher rebound forces at the low end, which would tend to give stronger roll control (the red dotted line has a “fuller” shape below 0.1m/s).
The differing jounce:rebound force ratios are also clear here.
I’ll add the data for Gaz and Girling, it could take a little while…
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Hi Clive…compareing the characteristice is one thing but dont forget to compare the spring perch positions top/bottom with relation to the fixing bolt position…as this will affect ride height if tney do not all have the same dimensions…Steve
Very strange. Compression damping builds up very quickly, but then bleeds off as the shock is slowing down? I forget who, but somebody tried a displacement based bleed in the form of a groove on the id of the inner tube.
Getting the right original damping curve has always been a problem. Once a car goes out of production, aftermarket and even the factory service part is often just close enough.
Yes, that will be checked before the dampers are dismantled. Were Monroe and Woodhead original production fitments on the E-type? I’m curious whether these are actual Jaguar specs or something that’s dimensionally close enough to fit.
Yes, seems curious but the curves look regular enough to be deliberate. Maybe it would give a feeling of enhanced roll control and / or agility, with a gentle ride feel?
I suppose that effect would be position sensitive then, depending on the position of the piston relative to the groove? These tests were run with +/- 12.5mm travel, centred approximately (but not precisely) at mid-travel.
We’ll learn more when we get the units apart in the near future.
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I think Woodheads or Gitling.
Girling but posssibly Woodhead (with a Girling label) as well …Steve
Girling vs Woodhead front shocks: Dazed and confused - The 'E' Type Forum
Steve, is there a set of agreed dimensions for the original rear dampers that I can use as reference for the parts we have?
Ideally:
- fixing hole centres
- spring perch spacing
- spring perch relative to fixing hole
- travel
I think parts of this data have been previously supplied here - apologies for any duplication
Hi Clive…I dont have the dimensions and not sure they were exactly the same through the models…its cleàr from parts catalogues that spacers were used as well at certain times…im sure this has a lot to do with members haveing widely differing ride heights…replaceing shocks without bothering to compare the dimensions…Steve