Getting the best out of standard suspension components for touring driving today

This is how I visualise the system

I’ve measured standard mounts in the 250 - 350 N/mm range, adjusting for installation position. (Multiply by 5.7 to get lbf/in). That’s closer to the spring rate than the usual target, raising the possibility of difficult tuning to avoid interactions. Perhaps more significant, the suspension dampers can’t control movement across the subframe mounts because they’re attached to that subframe (instead of to the body as normal practice), so upstream of the mounts.
Hence my thought is to make the subframe mounts significantly stiffer to reduce uncontrolled motion and gain the higher damping inherent in stiffer rubber. Maybe easier installation too.

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Well what a day this was, two new facts to absorb. (The other was that my grandson has a birthday next week. It’s a long and not particularly delightful story).

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Completely agree, but concerned about the fight between the cradle mounts and the trailing links. Should the fore/aft rate of the trailing links be reduced? Funny how the over constrained design keeps coming back to bite us.

Guess that the less than optimal suspension designs worked just fine with bias belted tires of 1961. Especially when compared with the competition of the day.

Radials really were a game changer, coupled with ever-increasing knowledge of chassis and suspension dynamics.

You’re right Tom, I’ve studiously avoided changes to the trailing arm because I don’t have the means to work through that balance with the cradle mounts. Fortunately, the cradle mounts are working in shear in that fore-aft mode, and my bench testing suggests their shear stiffness is little affected by the changes I’ve made - the angled side faces give substantial vertical and lateral response to changes in the rubber isolators, but little effect on the direction that interacts with the trailing arm. (Fore-aft columns in the inevitable chart)


These are all single-direction bench tests, I’m making some assemblies for better understanding of the interactions. These are PU parts for workshop use only, I would only suggest rubber for on-car use for (what I believe are) its higher damping properties.
I started looking at the rear mounts for their lateral (cornering) properties, I now believe they might offer something in ride control. The mount is obviously a critical element in the control and behaviour of the car, I don’t have the ability to make functional prototype parts suitable to use on a car so I’m talking to an established mount manufacturer to make a small quantity of real, functional test parts for evaluation by any interested parties. If they work as I expect, perhaps one of the established parts suppliers would be interesting in picking up the theme.
I’m happy to exchange data and ideas at any point, in exchange for a donation to a cancer-related charity if commercial value is realised.

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For clarity, this is the convention I use for the forces - and their directions - acting on and in the IRS mount:


Open to other suggestions (but not very)

Clive - ya just gotta ask

I took the measurement between inside edges of the holes and got this


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Now - before you release that exasperated sigh: I know you engineer-types use center to center (as clearly denoted on your photo). As the above photo shows, there is a slight raise on the cradle between these holes which made measuring center-to-center challenging.

So – – the diameters of those holes are


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I’ll let you do the math /trigonometry / geometry

AND – damn it – I just realized I’m providing data for a non-US engineer. ALL these figures shoulda been in mm vice inch. Geesh – some people’s kids

Also - sorry for the delay in responding. I had some paint correction and ceramic coating done on Sunday 28APR24 between 1000 to 1700 hrs (<-- I know you’ll appreciate those time hacks)


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Photos don’t do the work justice

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Since its not mounted level but in an inverted V configuration they should all be mixed together, no? And front and rear probably do differently when braking, etc… but if it works for the model

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No problem Craig, I’m ambidextrous having worked in Detroit for several years. In the Buick world, no less, so I really appreciate that piece of glory you have there.

Thanks for the dimensions, I’m going away to do stuff with lines and numbers. I may be a little while.

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:ok_hand: :+1: :smiley:

Since they are mounted at a slope of 45°, both the ride and and fore/aft forces will have vertical and fore & aft components, do you factor that in?

Yes, I modify the “standard” rate defined above to take account of the installed angle when necessary, such as with the assembly (suspension or vehicle). I’m currently looking at the effect of reducing the thickness of the individual rectangle of rubber in a mount to see what range of stiffness is available, for that I’m testing in the plane of the rubber and will adjust as necessary for the whole mount.

Yes, I make an adjustment for the rates I’ve measured in the various directions although I’m not convinced it’s always worthwhile because the fore-aft rate - shearing the rubber - is much lower than the other directions putting it in tension and compression (as in the chart a few days back)
I also have an idea the V-configuration will use the vertical forces to centralise the assembly in the V, reducing fore-aft travel lower than the forces alone would suggest. More speculation :thinking:

Thanks Craig, that took care of a hot afternoon…
I did some calculations of clearances with numbers, strictly Middle School stuff (O-level for the Brits of greater patina), then repeated in CAD. The results were encouragingly similar - before I go further can I just confirm where the assembly process is challenging -

  • if you bolt all mounts tightly to the body then try to raise the cradle into place, at which end of the slot does interference occur with the fixing stud - upper or lower?
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Excuse me, does anyone have a tape or calibrated eyeball at hand?

I’m modelling the cradle attachment of the IRS mount, would appreciate two rough dimensions for the “arch” in the cradle:


It’s purely to make the graphic look familiar, has no effect on results so a value within about 5mm / 1/4in will do fine.

For those undoubtedly few among us who haven’t found this utterly riveting, the purpose is to play with mount size and stiffness, blissfully believing there is a spec that will give better ride and / or handling assurance / authority while being easier to fit.

This is why I think there’s value in adjusting the thickness of the rubber section (blue curve) - it gives a good range of increased stiffness relative to the original, which is far right of the curve.
As discussed above these are sections cut from a mount, tested horizontal (flat). After choosing thickness I’ll assemble a mount and test in “vehicle” orientation.


Curious (to me) that PU doesn’t seem to show the same behaviour. PU sections poured by me in moulds made by me, so many things could be amiss there.

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Clive

There is a minor issue in width measurement as shown in the first photo. There is a notch in the rearmost corner (left side of photo) of “arch” - it is present on both left and right sides of the cradle.


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I wedged a block of wood between the bottom of the arch and the large aluminum bit.

(I tried to keep the gauge closer to vertical than shown in my demonstration-of-technique photo)
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The height came in at 2.4095"

The width

  • Outside to outside edge = 8-7/16" (includes the notch so easier to get good reading because the metal of the cradle touched the wood)
  • Inside to Inside edge = 7-5/8" (had to eyeball the extended terminus of the notch - not so confident of this measurement)

Thanks Craig, that will help me make the model more recognisable

Following should have been posted in the discussion on IRS mount replacement, apologies for my haplessness

The following chart is a summary of bench testing three IRS mounts in three directions, and link to the original post and pontifications. All parts purchased new from regular suppliers immediately before test.
Colours:
blue - Jaguar
green - pattern
red - uprated
As noted above, the pattern part tested significantly softer than the Jaguar, while the “uprated” part was indistinguishable from it.

I’m looking at having a small quantity of IRS mounts made, revised for improved ride & handling and easier fitting. Ideally that would use a small quantity of “core” metal parts - body and cradle brackets - so the supplier is only adding the isolation element.
If anyone here has such brackets available and would be willing to offer them up to support this exercise, please let me know.

I bet @Robert_Laughton has a pile!