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PostPosted: February 20, 2016, 1:40 am 
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I am trying to convince myself that this layout could work? It may do some strange things under braking though??


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PostPosted: February 20, 2016, 1:53 am 
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Sorry about my ineptitude. Repeated images - D'oh!!! I ran a motion simulation and this (the second image) is the setup under droop - mind you the axle centre is about 200mm below ride height. Hopefully the first shows the setup under about 75mm of, "ahh, What do you call it", jounce! Anyway, pushed up!!


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PostPosted: February 20, 2016, 12:13 pm 
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Location: Cotswolds - UK
Your design is basically a hybrid of the rear suspension arrangements used on the Caterham and live axle variants of the Sylva Striker/Phoenix cars, but with the added disadvantage of a high roll centre (which will enforce a steeply angled roll axis, if you're using double wishbone front suspension designed in accordance with current practice).

Both the Caterham and the Sylva are known for their excellent handling characteristics, but both are also noted for the limitations inherent in their rear suspension designs, which you may find it instructive to study:

* Caterham has found it necessary to cobble on a very crudely-conceived upper longitudinal Watts link arrangement on each outer end of the de Dion tube to restrain it against torque reaction from braking and acceleration, on its higher-performance variants (and has reverted to a more conventional 5-link, Panhard rod arrangement on its budget live axle model).

*The conflict in the vertical links of the longitudinal Watts link arrangements on the Sylva cause it to try to twist the axle like an ARB in roll, which on the Sylva is accommodated by rubber bushes in the leading/trailing arms. In your arrangement (with upper restraint in the centre of the axle), roll-steer would be the result, I think.


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PostPosted: February 20, 2016, 4:15 pm 
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Thanks for the instructive reply.

Yes the motion simulation does show roll steer. I was considering the advantage of no binding through the range of motion as something that was worth considering when comparing against trailing 4 link with watts linkage. The motion simulations were done over a very large range, left side +-200 mm with right side held at approx ride height.

To tell you the truth I did not even consider roll centre - purely conceptual at this stage!


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PostPosted: February 20, 2016, 5:35 pm 
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It will work fine. The whiner didn't mention that most everything has some roll steer; probably because he didn't know.

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Last edited by Miatav8,MstrASE,A&P,F on February 21, 2016, 7:42 am, edited 1 time in total.

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PostPosted: February 20, 2016, 6:00 pm 
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Location: Cotswolds - UK
Roll steer isn't the biggest issue; the other two factors I mentioned (steep roll axis inclination and deflection of the de Dion beam between the outer and central pickup) will be your bigger headaches.

Nothing's insurmountable - just factors to be aware of - but ask yourself why no other car places its rear roll centre so high, these days, and why Caterham saw it as necessary to come up with their Watts link bracing arrangement (whilst admitting that it was only because they didn't have the money to develop a proper IRS at the time) as a bolt-on 'fix'.

Or you could just believe Miatav8,MstrASE,A&P,F when he tells you it'll be fine. :roll:



As an aside, the Strathcarron SC5a used a de Dion with a transverse mid-rear engine installation. It didn't work terribly well and when the car was resurrected as the Javan R1, they changed it to double wishbones, but that's NOT to say that the concept was necessarily flawed - just that that particular installation didn't work very well.


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PostPosted: February 20, 2016, 8:50 pm 
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Cheers!


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PostPosted: February 20, 2016, 9:12 pm 
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The reason for putting third link on top was to allow the lower links to take the braking/acceleration forces forward to a "strong" part of the frame. I believe that the lower links will have to deal with the largest forces (have I got this wrong?).

Thanks for your replies - "nuff said"


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PostPosted: February 21, 2016, 1:52 am 
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Yeah, that, of course, is another disadvantage of any arrangement that uses leading links (or even potentially a Panhard Rod): you need structure behind the engine to support the links.

By way of comparison, take a look at something like the (GRP monocoque) British GTM Libra or the very similar (but welded aluminium 'spaceframe') Renault Spider, both of which effectively use double wishbones turned through 90 degrees to become double trailing wishbones, so that to all intents and purposes, the structure finishes at the bulkhead behind the driver:

Renault Sport Spider:

Image

Image

Image

GTM Libra:

Image

Image

Image


Last edited by Sam_68 on February 22, 2016, 8:33 pm, edited 1 time in total.

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PostPosted: February 21, 2016, 2:23 am 
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Being a Northerner (Australian Northerner!!), I have had a few beers at this time. Isn't this a VW bug with ability to correct/help increase bump steer??

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PostPosted: February 21, 2016, 7:45 am 
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pdwalsh59 wrote:
"nuff said"


Ok. Right back at you.

You can lead a mule to water....

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PostPosted: February 21, 2016, 10:42 am 
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Sam_68 wrote:
By way of comparison [snip] double wishbones turned through 90 degrees to become double trailing wishbones
Wow, that is brilliant! Thanks for that, Sam_68, I would never have thunk it up myself. And by mounting the upper arm at an angle (inside pivot lower than outside pivot) one can control camber...with vertical bolts in the rod ends and shims to raise and lower the pivot points, once could make it adjustable...all around clever, that.

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PostPosted: February 21, 2016, 3:15 pm 
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Very clever and from what I have read also a very good handling car, often called a poor mans lotus elise/excige. The problem I have read most in researching the GTM is with a fairly long control arm length (from bulkhead mount to hub) and narrow width of the arm they wear the inner joints fast and the smallest amount of movement at the bulkhead translates to a lot of movement at the hub.


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PostPosted: February 21, 2016, 9:16 pm 
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I not only don't see anything wrong with a DeDion with a midi, I'm doing that myself. I'm using a fairly standard four trailing arm setup with a Watts link, though.

My original plan was to use a plain Panhard rod, but the Watts will give me room to run the exhaust under the axle, which will greatly simplify exhaust routing.


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PostPosted: February 21, 2016, 9:35 pm 
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If I recall correctly, the "La Bala" uses/used that setup. Check their wbsite to see if there's any pics, though the site seems like it's no longer actively maintained.

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