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PostPosted: Thu Sep 11, 2014 3:17 pm 
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Crazy! I had to look twice at the trailing arm with the bolt missing!! I love the scalloped tube to minimise stress risers on the locating arm in the foreground.

Some good photos man! Not sure I will be going that far with my setup, although it may be tempting...

I think you may be right with the homoligation side of things, but I would be very curious to know the story behind going back to the semi-trailing arm design. Maybe the removal of the bolt in the inner joint didn't make the scrutineers happy :D

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PostPosted: Thu Sep 11, 2014 3:23 pm 
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Thanks for these Bluebird pictures, and this last one in particular, as I have been pondering for years how they were able to run what is essentially an over constrained suspension design in those cars. I did think that they may have done something like this, ie. leave out a pivot bolt on the factory arm that (presumably) needed to remain unmodified by the Group A rules of the day, but could never prove it...this does and it all makes sense. Was going to build something based on this for a 1600 of mine many years ago....might just have to dust off the old proto parts again....

Great topic of discussion, guys, thank you!


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PostPosted: Thu Sep 11, 2014 3:55 pm 
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Hey Pista_510,
As Lampy was commenting previously, what kind of knuckle/hub where you going to run to allow fitment of the parallel arms mounted off the diff.
The sandwich plate on the diff is quite clever and fairly easy to have made up.
One way would be to simply weld a few 'features' onto the axle carrier to allow fitment. I can feel myself making a few scale models and making some measurements...

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PostPosted: Thu Sep 11, 2014 6:00 pm 
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The more i look at it i think it would be fairly easy.
I think i would use the standard 1600 hub and weld mounts to it, similar to what they have done on the bluebird. As long as it is appropriatly braced it should be strong enough.
Coliovers would be necessary
Attachment:
[ attachment ]
Bluebird038.jpg [ 502.25 KiB | Viewed 7807 times ]

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PostPosted: Fri Sep 12, 2014 10:20 am 
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Yes, my initial idea was to use the 1600 rear trailing arm, cut off the inner pivot "leg" and then weld on bracketry to hub carrier for the tranverse links. If I can recall, using the stock 1600 shock location would encroach on the tranverse linkages so an idea was to then use the spring pocket in the trailing arm as the basis for mounting coilover spring / damper - this would then need to create a new upper spring seat that becomes a shock mount instead (which would obviously then intrude on the rear seat area - OK for race I guess).

I think that if I was to do it now, I would want to approach it to ensure the load paths and reactions for the spring and shocks, and the dynamics of the new geometry, would probably be best to create a new knuckle arrangement and linkages to replace the 1600 trailing arm? I think it should be doable within the envelope of the 1600 rear structure - will get out to the shed on the weekend to have another look.


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PostPosted: Mon Sep 15, 2014 1:24 pm 
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It seems Porsche 930's (and other Porsche's of this vintage) have semi-trailing arm suspension. For aftermarket, they have this thing called an adjustable spring plate (ASP), which is pretty cool. You can adjust camber, toe and ride height by just undoing a few bolts, and moving things around.
Check out this more complicated version:

[ img ]
[ img ]

The bloody great big hole on the LHS of the photo is where the axle goes, and the Aluminium swing arm goes into the page. The system pivots (as shown by the gif) on a mount on the side of the chassis. Pretty interesting!

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PostPosted: Tue Sep 23, 2014 8:28 pm 
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Interesting that everyone's raising the rear pivot, got me a little worried about my thinking now. I've actually gone the other way and lowered the pivot in the hope of controlling the squat in the rear end, by searching for more antilift, of course there are limitations within the standard crossmember, so my change isn't huge. I hadn't thought about modifying the crossmember mounting to gain more.
I've mocked up the brackets and swung the arms from anticipated ride height thru to full bump, with in the order of 1º camber gain, so a base -0.5º camber increases to max 1.5º
Toe if set at 1-2mm in at ride height, increases toe as the suspension compresses, so it only gets more stable in compression.
I'm using 240K arms into a 180B. Previous 180B's I've played with have always suffered in the motorsport I enjoy (motorkhanas and standing start hillclimbs etc.) with the camber increasing as the weight transfers, meaning you end up sitting there spinning your wheels as those nice wide race tyres have decreased in width to 1/3 due to the camber increase. Also a problem when you're braking.
For the most part I'd ignored the rollcentre changes as they seemed to be tied more to the angle of the crossmember than anything else and I wasn't prepared to go that far with my modifications.

I remember reading on one of the rally forums that a member started modifying their semi-trailing arms closer to full trailing arms an inch at a time. Every modification made the tyres wear a little better and made the stage times a little quicker, though you reach that point where the arm hasn't got enough lateral support. Ways around I guess if you wanted to get involved in a lot of custom engineering.

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PostPosted: Wed Sep 24, 2014 8:59 pm 
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A rally car is probably ok with the roll center as is.
It the lowered tarmac cars that have problems with the roll center being too low.

Anti squat would be nice for a rally car but i would have thought it would sacrifice traction. Transfer of weight onto the rear wheels when launching seems to be good for grip in a 1600/180b providing the wheels dont gain too much camber.

I had a good look at adding lateral links to the diff today.
Looks like there will be a clearance problem with the rods to the mustache bar and the exhaust. Both seem fixable with a bit of work. Not sure im keen to take the plunge just yet.

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PostPosted: Thu Sep 25, 2014 4:03 pm 
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Boyracer, I thought lowering the pivot point actually increase your squat under load, which technically would be a good thing for drag racing as you are putting more load on the rear wheels which gives you greater traction. Unfortunately, the camber curve goes in the wrong direction and you get less of a tyre contact patch. A balance between squat (or the distance the wheel travels under hard acceleration) and the camber change as it goes through that distance needs to be found. If you are running soft, sticky, low pressure tyres, they would be more forgiving of the camber change.
Also for drag racing, the roll centre doesn't really have that much effect as you aren't doing any cornering.

(which is pretty much what Lampy said...)

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PostPosted: Tue Sep 30, 2014 8:24 pm 
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Does anyone have any links on how to calculate/measure the anti-squat/anti-dive for a semi-trailing arm suspension or even a trailing arm suspension?
I'm trying to find some details on brake forces and acceleration forces.
I'm more interested in braking forces and how the system is unsettled by braking and how the suspension lifts the car under brakes.

My current train of thought is to:
1. Reduce the 'sweep' angle from the current (approximately) 25deg to 20 or even 17 degrees to be similar to the BMW. This would involve a few 'mods' that wouldn't be that easy.
2. Have some roll centre adjustment and possibly raise the pivot points due to reducing the sweep angle (as reducing the angle lowers the RC, so raising the points points the RC back up. The closer you get to full trailing arms the closer the RC gets to the ground)
11ty. Do nothing and sit on my bum and just enjoy that unsettling feeling at Phillip Island as I exit turn 1 into turn 2.

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PostPosted: Wed Dec 10, 2014 8:17 pm 
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Here's another idea from the guys at BMW.
It's called a pitman arm or dogbone.

You have to use rubber bushes so they deflect a known amount.
[ img ]

Here's a website showing how to service the dogbone pitman arm which shows a few more photos.
http://www.exx.se/maintenance/dogbones/index.shtml

Looks like it provides toe correction as the suspension travels in bump and rebound. Simple and effective.
Typically used on the E32 1992 BMW 750iL and other models of this vintage using the semi-trailing arm suspension

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PostPosted: Mon Mar 30, 2015 5:16 pm 
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Lampy, have you taken the plunge yet?
I have, but I'm still working on it.
Might post a few pictures up soon when I have something to show.

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PostPosted: Tue Mar 31, 2015 11:55 am 
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Unfortunately I haven't done any work on the rear end. Once I get my new shed done I'll be back into it. I would be interested to see which way you have gone. I am thinking a chapman strut system these days.


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PostPosted: Wed Apr 01, 2015 7:51 pm 
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Les & I are also working on something atm that will negate the trailing-arms & include a Chapman strut style arrangement.

It's very early days atm, but Les is hoping to sell it all as a bolt-in kit :wink:

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PostPosted: Thu Apr 02, 2015 1:30 pm 
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Sounds interesting.
Estimated time to completion?
Will he do a DIY version?

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