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Putting the front back and another front in the front
liftarn - 17/4/06 at 05:52 PM

I'm plannign a mid engine using SAAB 900 Turbo as a donor. My plan is to move the entire front suspension and driveline to the rear. The idea is simmilar to the GTM that used Mini parts.

However, some have commented on the idea and mentioned some problems, but I thought I'd get some input from you guys too.

The problems mentions with using the 900 front suspension is
* positive camber when steering
* spindle axis tilting 12 degrees
* no caster
* the A arms are short and heavy and the angles give no caster compensation

Being a beginner I would like to know how serious this is. Should I abandon the idea or go ahead anyway.
I'm planning a build for street use.

kb58 - 17/4/06 at 06:44 PM

You can make your own set of a-arms and fix all the issues. Different in-board mounting points will be needed, though it shouldn't be a problem since you're making a new chassis anyway.

Once you start building your fear of changing things will go away. You may very well end up making a new tube frame structure to connect all bits.

[Edited on 4/17/06 by kb58]

graememk - 17/4/06 at 09:16 PM

gear change might be a prob too

JoelP - 17/4/06 at 09:27 PM

the gear change issues have been solved for many cars in a similar situation, it just takes some good thinking and accurate fabricating

liftarn - 18/4/06 at 08:14 AM

quote:
Originally posted by kb58
You can make your own set of a-arms and fix all the issues.


Yes, I know, but I was hoping to not have to. You don't want them to break and they have to be exact or the steering gets weird.

kb58 - 18/4/06 at 02:43 PM

quote:
Originally posted by liftarn
Yes, I know, but I was hoping to not have to. You don't want them to break and they have to be exact or the steering gets weird.


Well, you don't want the chassis to break either and that's not stopping you. A-arms aren't any more lethal than any other part of the car... just build them conservatively and you'll be fine.

About steering, you still have to deal with it. Since the entire front assembly is being transplanted to the back, what's happening to the steering rack? I assume it'll be removed, so what will replace it? It'll be replaced with toe-control links which are just the right length and mounted to exactly the right inboard point. That point can be found mathematically or by trial and error, and lots of bumpsteer testing.

The point is, you're now making toe-control links and inboard mounting points for them - nearly the same as making a-arms...

[Edited on 4/18/06 by kb58]

NS Dev - 20/4/06 at 11:08 AM

perfectly put KB58

If I were you I would use the engine and box but chuck the suspension and start again. Unless you are throwing the car together it will be as quick to make the arms etc at it will to fit the saab subframe to your chassis.

liftarn - 20/4/06 at 11:22 AM

quote:
Originally posted by kb58
Well, you don't want the chassis to break either and that's not stopping you.


Actually it is. I intend to spot weld and then have a licensed welder do the final work.

quote:
Originally posted by kb58
Since the entire front assembly is being transplanted to the back, what's happening to the steering rack? I assume it'll be removed, so what will replace it? It'll be replaced with toe-control links which are just the right length and mounted to exactly the right inboard point. That point can be found mathematically or by trial and error, and lots of bumpsteer testing.


The design philosophy is "Do as it was done", i.e. carefully meassure how they are mounted in the donor vehicle and replicate that. I'm keeping the same wheelbase too.

kb58 - 20/4/06 at 02:11 PM

quote:
Originally posted by liftarn
The design philosophy is "Do as it was done", i.e. carefully meassure how they are mounted in the donor vehicle and replicate that. I'm keeping the same wheelbase too.


So toe-control links still have to be fabricated, plus the inboard mounting points, no different then a-arm fabrication. And once built it has to be throughly tested, who knows what kind of odd toe-in/toe-out control was designed in by the factory.

The point is, you are designing and building a car whether you do the welding or not. It doesn't get you around the responsibility of putting everything in the right place and more importantly, knowing the consequences of each part. And finally from a practical standpoint, trying to retain all the factory points may very likely take more work, more time, be heavier, and give poorer results then starting over.

These projects are our own personal journey into automotive design and developement and there are many paths to success. Some choose to make their own path while others chose to follow an existing one. Just realize the well-worn path (left from the factory) may not lead to an appropriate solution for a vehicle weighing half what they originally designed for.

[Edited on 4/20/06 by kb58]

Fred W B - 20/4/06 at 07:14 PM

quote:
--------------------------------------------------------------------------------
Originally posted by liftarn
The design philosophy is "Do as it was done", i.e. carefully meassure how they are mounted in the donor vehicle and replicate that. I'm keeping the same wheelbase too.
--------------------------------------------------------------------------------


Agreeing with kb 58, and also not to mention that what was the front suspension will now be your rear suspension. Desirable design criteria for rear suspension can be significantly different to that of front suspension

Cheers

Fred W B

liftarn - 21/4/06 at 06:27 PM

Yes, those are good arguments.

The reason to use the donor's suspension is partially that I donät have to make my own and partially that I then can reuse the coild and damper, thus saving the expense of coil-overs.

Now I have started up another attempt at selling my intended donor vehicle. If it fails this time too I will restart the project. I guess I'll just have to read up a lot on suspension design.

I'll keep the rear spindles and try to source some front spindles från a RWD car with the same bolt pattern.

I'm currenly looking into the Lotus 23 for inspiration. Altough I'm planning on having a 1980s "folded paper" (or rather folded sheet metal) design.

[Edited on 21/4/06 by liftarn]