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Author: Subject: Unusual Suspension Design
renrut

posted on 27/6/11 at 02:16 PM Reply With Quote
Unusual Suspension Design

Wasn't sure where to put this as its entirely suspension and not really Middy but its definitely not typical locost either and thought I'd get a better response here. Mods feel free to move as you see fit.

Anyway the question is for suspension gurus.

I've got a serious lack of space in the back of my little middy and having already built one suspension setup (mac strut) but I don't have sufficient space and clearance for the track control rods. So I'm considering doing away with the lower ball joint and making new uprights and lower wishbones so that the lower connection between upright and wishbone can only rotate in one plane for bump and droop. I would keep the upper strut in place and not be fitting an upper wishbone but that seems to work fine for normal macpherson strut systems.

This all seems to make sense to me and should be lighter than the current setup albeit with less adjustability. But I'm starting to get worried that I can't find an example of where this system has been used anywhere else. Is there some fundamental problem with it that I can't see?





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Dick Axtell

posted on 27/6/11 at 02:31 PM Reply With Quote
quote:
Originally posted by renrut
So I'm considering doing away with the lower ball joint and making new uprights and lower wishbones so that the lower connection between upright and wishbone can only rotate in one plane for bump and droop. I would keep the upper strut in place and not be fitting an upper wishbone but that seems to work fine for normal macpherson strut systems.


Sounds, to me, very like a Chapman strut arrangement. Will scuffle thru old paperwork for a pic.

Pic I had in mind shows upper wishbone, therefor not fitting your description. However, thinking a little more about your proposal, you seem to imply that there will have to be fixed length drive shafts, which will then fill the function of the absent upper wishbone.

Is that correct? Or have I pictured wrongly?

[Edited on 27/6/11 by Dick Axtell]





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ashg

posted on 27/6/11 at 02:32 PM Reply With Quote
this is a case of.... a picture speaks a thousand words.





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renrut

posted on 27/6/11 at 02:33 PM Reply With Quote
That's what I initially thought but on looking most chapman struts appear to still have a track control rod. Or are they just misnamed macpherson struts?

ETA found some pics of an old lotus (elan possibly) chassis and that appears to not have the track control arms. If this is the case then I may be worrying about nothing.

[Edited on 27/6/11 by renrut]





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Dick Axtell

posted on 27/6/11 at 02:46 PM Reply With Quote
Looking at pics of some replacement suspension parts for Lotus Elan/Europa, I note that the lower wishbone has a markedly wider spread of the attachment points. This could allow you to avoid track control arms. Maybe.




[Edited on 27/6/11 by Dick Axtell]





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renrut

posted on 27/6/11 at 02:52 PM Reply With Quote
quote:
Originally posted by ashg
this is a case of.... a picture speaks a thousand words.


A picture of the elan mk1 rear suspension which appears to be exactly what I was on about without realising it.

[img]http://lotuselan.files.wordpress.com/2010/03/lotusrearsuspension017.jpg?w=1024&h=722[/img]



quote:
Originally posted by Dick Axtell
Looking at pics of some replacement suspension parts for Lotus Elan/Europa, I note that the lower wishbone has a markedly wider spread of the attachment points. This could allow you to avoid track control arms. Maybe.



It does look to have quite wide spacing between the mounting points on the hub side and that would make it less susceptible to twisting which I can't see the strut helping with much at all.

I should phone my bro, he has an 1970 Elan+2 in bits in his garage and could have a look and take some dimensions for me.





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MikeRJ

posted on 27/6/11 at 02:56 PM Reply With Quote
I think what you are proposing is to replace the lower balljoint with a simple bushed pivot? If so this this method is widely used on Locost IRS designs. By using a rectangular "wishbone" (suitably triangulated!) you can attach the bottom of the upright via two rod ends to give you some toe adjustment.

This scheme doesn't allow you to add/remove toe-in on bump as a separate track control arm would, but some may regard this as an advantage, since getting this correct can be tricky and getting it wrong will make a very ill handling car.

[Edited on 27/6/11 by MikeRJ]

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renrut

posted on 27/6/11 at 03:25 PM Reply With Quote
I had a look over a lot of locost IRS designs but they all still seem to have a top wishbone which this wouldn't but yes thats pretty much it.

It's amazing how many ways other people can say the same thing simpler than myself!





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MikeRJ

posted on 27/6/11 at 04:24 PM Reply With Quote
I don't foresee any major issues translating this to a simple McPherson strut design. I'm certain this scheme has been used on some production cars though I don't know which ones off hand.

I guess this shows the basic design (though probably not with drums and universal jointed driveshafts):



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ashg

posted on 27/6/11 at 05:48 PM Reply With Quote






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Volvorsport

posted on 27/6/11 at 05:57 PM Reply With Quote
is the gearbox in the way of a de dion setup ?

lotuses have non plunging driveshafts as part of wheel control .





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renrut

posted on 27/6/11 at 06:51 PM Reply With Quote
quote:
Originally posted by Volvorsport
is the gearbox in the way of a de dion setup ?

lotuses have non plunging driveshafts as part of wheel control .


Not the gearbox but the chain differential. Going up means I hit the exhaust, going down I hit the ground and going backwards puts me outside the car.

I do have plunging driveshafts. Is that likely to make a big difference as surely it will need some sort of wheel to diff movement with the axle swing?

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MikeRJ

posted on 27/6/11 at 07:20 PM Reply With Quote
quote:
Originally posted by Volvorsport
is the gearbox in the way of a de dion setup ?

lotuses have non plunging driveshafts as part of wheel control .


So do Jaguars, at least the older ones.

quote:
Originally posted by renrut
I do have plunging driveshafts. Is that likely to make a big difference as surely it will need some sort of wheel to diff movement with the axle swing?


You will need plunging driveshafts with the scheme you have outlined, it will bind up if you don't.

[Edited on 27/6/11 by MikeRJ]

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Doug68

posted on 28/6/11 at 06:27 AM Reply With Quote
I don't know of any 1/2 shafts that don't plunge in some way unless they're being used as part of the swing axle for example.
Most I believe achieve it by plunge in the CV joint itself.

Back to your original question, it sounds to me like you've designed yourself into a corner. Either:

a. You compromise and do what you've describe above.

b. Layout the suspension you really want and rework the rest of the design until it fits.

It may seem like a lot of work to do now, but in a few years when you are driving the thing and its too late to change will it bother you or not?





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renrut

posted on 29/6/11 at 12:16 PM Reply With Quote
Well mine plunge as part of the inner CV joint so thats that.

I'm well aware I've designed myself into a corner, mostly through making too many assumptions at the starting point (I'd have used a shorter engine for a start!) and trying to retain too much of the original car to save effort which has backfired. Laying out the suspension I should really use (double wishbone) is basically rip it all up and start again type of problem as the whole rear chassis will need remaking.

Having internet investigated the 60s Elan and Elan+2 which is the only example of this suspension style being used on a production car it works well but it doesn't handle high suspension loads very well (e.g. powerful engines and sticky track tyres) and tends to lose its rigidity as a result. Lotus stopped using it after the Elan and went to double wishbone and there are a lot of kits out there to convert Elans to double wishbone which suggests it was an experiment that didn't quite work.

Having said that I don't intend to put sticky track rubber on (its a weekend toy and occasional track car rather than dedicated) and the Elan suspension was some of the best in the day anyway so its hardly like I'm moving over to cart springs and wooden wheels.

I think this is the way to go. If it doesn't work I'll still need to rebuild the whole back of the car anyway. If it works then I've got a nice simple lightweight suspension setup.

If I make the lower joint between upright and wishbone out of rose joints rather than using a bush setup this would hopefully improve its location as well. I'll still have the bushes at the wishbone to chassis joint so should still maintain some compliance.





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renrut

posted on 4/7/11 at 11:20 AM Reply With Quote
Now I'm going down this route I'm looking for some advice and experience from the knowledgeable chaps on here.

It looks like I'm going to have to fabricate my own uprights unless I can weld to the existing ones. I believe the existing ones are steel so should be doable but is there a big IVA/MOT no no about doing so or is it ok as long as the welds are good? This would obviously save me a lot of hassle as the only bit I really want to change is the lower wishbone link.

Secondly what sort of size rose joints should I be looking for to attach the lower wishbone to the upright? It'll be 2 so I can nicely adjust the static toe to how I want it. I figure I need something at least as strong as the existing lower ball which is attached to the upright by a typical taper into the upright which ends with a M10 thread so probably average about 12mm diameter.

Would 2x M12 rose joints (one either side of the upright) be sufficient or should I be looking for something beefier like M14? I'm struggling to find proper strength information on rose joints.





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Neville Jones

posted on 4/7/11 at 07:22 PM Reply With Quote
What uprights are you contemplating welding to?

If they are Ford, they may well be cast SG iron. Non weldable.

Check them with a file first.

Cheers,
Nev.

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renrut

posted on 4/7/11 at 08:46 PM Reply With Quote
quote:
Originally posted by Neville Jones
What uprights are you contemplating welding to?

If they are Ford, they may well be cast SG iron. Non weldable.

Check them with a file first.

Cheers,
Nev.


They're fiat items. How does a file check them? What should I be looking for?

Also I've heard rumours that welding to suspension parts can be an MOT failure, which we all know is a bit silly but is there any truth to this rumour as obviously its a lot easier to tell if someone has done aftermarket welding to an OEM cast upright than say a fabricated locost one.

[Edited on 4/7/11 by renrut]





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renrut

posted on 5/7/11 at 08:52 PM Reply With Quote
Done some test welds on a spare upright. Looks to have taken, repeated bashing with a large hammer doesn't seem to have done anything other than round the edges a little.











Yes I know they're not pretty, its to test if it works or not. Any wise thoughts?

Next I guess would be making up the brackets to go on the bottom.





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