aerosam
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| posted on 1/3/09 at 07:00 PM |
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rear suspension geometry
Hi Guys,
I've now fitted my donors rear axle assembly into my chassis, and the suspension mounting points on the BMW wishbones are looking a little too
far back. I could quite easily use them with 18" coilovers but they would be at a really nasty angle, over 45 degrees.
Alternatively I can weld a mount to the top of the wishbone, and using a 16" coilover, it would sit perpendicular. However, this would mean there
would be only 7.5" from the pivot point on the axle carrier - is this too little?
See Pic below. Red ring is around the original mounting point, green line is the proposed path of the shock if mounted on the top of the
wishbone.
 
Rescued attachment rear suspension idea.jpg
Had enough of this dictatorship known as LCB. Gone elsewhere, not coming back. Kiss my ass ChrisW.
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mr henderson
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| posted on 2/3/09 at 09:01 AM |
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Well, I'll have a go if no-one else will.
First of all, those are not wishbones but trailing or semi trailing arms.
I think that neither of the possible solutions mentioned are satisfactory. As you say, using the original mounting point is going to put the coilover
at an awkward angle, and welding a mount to the top of tha arm is going to put the coilover in an unsatisfactory position too.
I think the solution is more radical, you need to alter the chassis to give a more vertical position to a coilover picking up on the original
mounting. This could be done by moving the tailing arm pivots forward, or by altering the chassis to move the upper coilover mounting point
rearwards.
I suspect you might be think along the same lines?
John
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MikeR
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| posted on 2/3/09 at 09:20 AM |
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What angle is the front shock at?
Having it angled is not ideal as it means as the car goes over a bump it gets more and more angled. This actually means the spring gets less effective
so the spring gets softer the larger the bump. To counter this you end up with a harder spring so small bumps are more jarring.
then again, as i said when i started - what angle is the front shock at?
As for welding it vertical - are you sure the metal at that point is man enough. your going to introduce a bending force in the middle of the trailing
arm. If it is man enough (and it might well be considering its designed for a BMW - but its your call) then you could weld it there. The 7.5"
thing is nothing to worry about as the trailing arm is actually from the car mount to the wheels - so plenty long enough - in fact its almost a
positive as it will give you more wheel movement (1" shock movement would be 2" wheel movement).
The best solution is to go de-dion instead of using the bmw system but you don't want to hear that 
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aerosam
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| posted on 2/3/09 at 09:22 AM |
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afraid not John, I've been looking around and the Robin Hood seems to use much the same setup as I'm proposing. See Pic below.
Unfortunately I can't move the axle carrier forward as it already slightly impedes the passenger cabin, and the upper mounts can't move back
as the coilovers would then be in contact with some other chassis tubes.
I'm considering making some special brackets to mount the bottom of the coilover towards the rear, welded on to the top of the trailing arm
(correct terminology now - thanks) but increasing the dimension given to 9-10". I'm having the afternoon in the garage today so I'll
take some measurements and see what I can do.
 
Rescued attachment RHchassis.jpg
Had enough of this dictatorship known as LCB. Gone elsewhere, not coming back. Kiss my ass ChrisW.
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mr henderson
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| posted on 2/3/09 at 09:57 AM |
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Basically you are using an unsuitable system, and you need to decide where to make the compromises necessary to proceed. There simply isn't a
satisfactory answer to your original question.
When I was planning to market my own kit I looked into the question of donor components, and how much fabrication and modification would be needed to
use any particular system.
The trailing arm system is used on a good many cars, even FWD and 4WD derivations of those cars, such as the Audi A3 Quattro. In the end I decided
that trailing arms were just NFG on this type of car, and that a fabricated upright would be needed. That's a problem as on most TA cars the
bearing is pressed into the arm itself, unlike the Sierra bolt on system.
What would I do if I was in your postion? I think I would ditch the TA's and get a set of MK Indy or Haynes roadster type uprights, and get
special driveshafts made to mate with the BMW diff.
Another way might be to see if you could get a pair of rear uprights from Marlin, as their roadster uses BMW bits.
Not what you want to hear, I'm sure.
John
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ettore bugatti
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| posted on 3/3/09 at 12:29 AM |
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Mounting the shock on orginal mounting point of the upright and weld a bracket on the upper section of the chassis?
See yellow line.
Like the deDion suggestion
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britishtrident
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| posted on 5/3/09 at 09:07 AM |
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Difficult to see from the picture but I assume it is a BMW "Z axle" or the older semi-trailing arms.?
If Semi-trailing arms wet weather handling will be err... interesting.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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aerosam
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| posted on 5/3/09 at 08:02 PM |
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quote:
Originally posted by britishtrident
Difficult to see from the picture but I assume it is a BMW "Z axle" or the older semi-trailing arms.?
If Semi-trailing arms wet weather handling will be err... interesting.
It's from an E34 (5 series), no need to worry about wet weather handling as this car is being built for track use only.
Had enough of this dictatorship known as LCB. Gone elsewhere, not coming back. Kiss my ass ChrisW.
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britishtrident
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| posted on 6/3/09 at 07:41 PM |
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quote:
Originally posted by aerosam
quote:
Originally posted by britishtrident
Difficult to see from the picture but I assume it is a BMW "Z axle" or the older semi-trailing arms.?
If Semi-trailing arms wet weather handling will be err... interesting.
It's from an E34 (5 series), no need to worry about wet weather handling as this car is being built for track use only.
My most enjoyable races were in the rain. :-).
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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wozsher
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| posted on 7/3/09 at 05:07 PM |
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What about inboard coilovers via pushrods works on my roadster but I am
on wishbone rear end,just a thougt!
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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mr henderson
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| posted on 7/3/09 at 05:25 PM |
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quote:
Originally posted by wozsher
What about inboard coilovers via pushrods works on my roadster but I am
on wishbone rear end,just a thougt!
What needs to be considered is how the weight of the car is going to be presented to the suspension.
If a rocker system were to be fitted, the pivot points would still have to be in pretty much the same place as the top of a conventional coilover. And
that's the problem here, there is no suitable place on the chassis that is in a satisfactory relationship to the suspension.
As was said earleir, the suspension system being fitted isn't really suitable, and big compromises are going to be have to made if the project
is to proceed as is.
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