Mid Engine SVX concept.

[bazza]

Originally Posted by Dessertrunner

- In one of the artical I read about that car they said the first ones (early models) of the car, the back subframe was not strong enough and it effect the cars handling, must have flexed.
- Bazza mentioned that there is such a thing as having the engine weight to low, it will effect the handling, I think he said its like a boat. Can't get my head around that.

Yeah flex is a nasty thing - suspension geometry changes due to flex (created during G loading) and is why roll cages and tubular subframes help reduce the flex and can increase grip.

With the engine weight to low theory - imagine putting 235 kg on top of the roof and driving - the car would want to try and tip over and would assist the roll. This is due to having raised the COG (center of gravity) and moved it further away from the RC (roll center). This creates a bigger lever arm (like a big torque wrench) trying to roll the car over.

In reverse if you put 235 kg very low in the car you might effectively move the COG beneath the RC. This would probably make the car behave like a yacht with a keel as the lever arm would be in reverse. Most RC's in the Subaru WRX and STI's are usually at ground level or 1-2 inches above it so you most likely won't have such a nasty issue but it's just something to be aware of as you're changing a lot of things.

[Dessertrunner]

Bazza I think the Renault had issues due to the ratio of wheel base to trank as well. I didn't run the numbers but figure they had issues because they tied to lengthen the wheel base.

Has anyone found out why that Mid engine Hyundai Genesis dispeared off the radar. I saw no more of it and would love to know why.
Tony

[Crazy_pilot]

Given how strangely the geometry seems to line up between the engine, suspension and subframe, would you consider doing away with the stock subframes entirely and building up a custom tube frame? It seems that trying to build from the stock parts is just going to result in a hodge-podge of welded together chunks that won't really suit the application. A custom subframe could be made modular to allow separate engine and transmission sections, or an upper cage that would wrap over the transmission to tie into the strut towers and stiffen the rear up.

[Dessertrunner]

Originally Posted by Crazy_pilot

Given how strangely the geometry seems to line up between the engine, suspension and subframe, would you consider doing away with the stock subframes entirely and building up a custom tube frame? It seems that trying to build from the stock parts is just going to result in a hodge-podge of welded together chunks that won't really suit the application. A custom subframe could be made modular to allow separate engine and transmission sections, or an upper cage that would wrap over the transmission to tie into the strut towers and stiffen the rear up.

I think the answer is yes with exception of the strut top mounts they should stay with the current postion in the body.

[Dessertrunner]

Here is a draft design of the new subframe design. I have had to change the trailing link but kept the rest of the suspenion the sames.

[bazza]

Originally Posted by Dessertrunner

Here is a draft design of the new subframe design. I have had to change the trailing link but kept the rest of the suspenion the sames.

Very nice.

One suggestion - where the lateral arms bolt to the frame - create a flange with 3 x holes rather than one. Use a circle to get the holes in the right spot as the centre point will be where those arms join the hub. Basically this will allow you to raise and lower the ride height without destroying the roll centre. A major issue with all Subaru's is when you lower them - you fk the roll centre and the handling - hence why Whiteline / Perrin make "roll center adjustment" parts. It's also why my front and rear subframe also have adjustable roll centers and why if you look closely at the STI from say 98 to 06, the point at which the front and rear lateral arms mount hole moves to address changes in COG and ride height. Also the V8's used to run live roll centre height adjustment as the COG changes with fuel loading and reduces performance etc etc.

I'd probably also use 5mm steel there - my 3-4mm stuff tore under huge load at Mallala and needed some re-enforcement.

[Dessertrunner]

Got any photo of what you mean?

[Dessertrunner]

Had a look at the White Line units and as I see it some of there option for the front only move the bottom ball joint in or out to enable a correction. In realiaty they don't change the angle of the bottom trailing arm as you suggest Bazza. From my rough experiance your suggestion is the correct one not the White Line one.
As I understand it what counts is the angle & distance relationship for the 3 pivot points with in the gemotory of the suspension. Lowering the car has a number of effects as I see it. I am talking as regards the rear suspension.
- The angle of the suspension arms in relation to the ground has now changed and the gemonitory is as if the the wheel is in the up position.
- Distance between the inside pivot point of the arm and the pivot point at the top of the strut has now reduced.

Wild guess the Camber of the wheel has increase and when the wheel has to ride up for any reason the chanber will go past its design limits. In addiation I would expect that the toe in of the wheel would also change.


If my heads right then the only way to handle this issues is as Bazza suggests and put addiational pivot points on the centre support/pivot point.

Thanks for the suggestion I will take it on board.
Tony

[Tireiron]

Just for reference with the front whiteline kit (I've installed them) they use a taller balljoint and tie-rod end joint. This is so that when the wheel is in the "up" position because of lowering the car the hub ends of the control arm and tie-rod will be moved back down to keep the same angle in reference to the ground. I actually had to remove them from my friends car when he went to a different, taller, spring because they put too much angle on the arms at that point.

The factory rear suspension design doesn't have any adjustment for arm angle if you lower the car. It doesn't even have any camber adjustment in the rear, just mild toe adjustment to keep the wheels pointing straight.

Jason

[bazza]

Originally Posted by Dessertrunner

Had a look at the White Line units and as I see it some of there option for the front only move the bottom ball joint in or out to enable a correction. In realiaty they don't change the angle of the bottom trailing arm as you suggest Bazza.

Check again mate - Whiteline move the mount point lower at the point of the ball join for the front. You'll need to compare a stock WRX item vs the whiteline item to see as it's hard to understand without comparing them side by side.

From my rough experiance your suggestion is the correct one not the White Line one.

My way is also the way Subaru WRC did their setup. However they also used adjustable ball joint mounts as well - doing so changes bump steer and scrub so both used in combination work rather well up front.

As I understand it what counts is the angle & distance relationship for the 3 pivot points with in the gemotory of the suspension. Lowering the car has a number of effects as I see it. I am talking as regards the rear suspension.
- The angle of the suspension arms in relation to the ground has now changed and the gemonitory is as if the the wheel is in the up position.
- Distance between the inside pivot point of the arm and the pivot point at the top of the strut has now reduced.

Wild guess the Camber of the wheel has increase and when the wheel has to ride up for any reason the chanber will go past its design limits. In addiation I would expect that the toe in of the wheel would also change.

If my heads right then the only way to handle this issues is as Bazza suggests and put addiational pivot points on the centre support/pivot point.

Thanks for the suggestion I will take it on board.
Tony

You'd use adjustable strut tops and camber bolts to sort out the camber. I'd even ditch the one piece SVX lateral rear arm and use some two piece whiteline WRX units (if they fit or custom make something) - this then gives you the ability to adjust toe and camber easily.

Moving the mount points on the subframe is purely for the roll center only. Do you have a rough idea on spring rate btw?

[icingdeath88]

Originally Posted by bazza

You'd use adjustable strut tops and camber bolts to sort out the camber. I'd even ditch the one piece SVX lateral rear arm and use some two piece whiteline WRX units (if they fit or custom make something) - this then gives you the ability to adjust toe and camber easily.

Yes, the impreza lateral links fit. The stock WRX links have a bracket for the swaybar link to mount to - a lot of aftermarket ones make this a separate piece, which is what you want if you are putting them in an SVX. These should be good: http://www.performanceracesolutions....?productid=354

I have the Megan ones, they work, but I think the eibach ones will give more useful adjustment, and are the cheapest I've seen so far, without the swaybar part.

[Dessertrunner]

Originally Posted by bazza

You'd use adjustable strut tops and camber bolts to sort out the camber. I'd even ditch the one piece SVX lateral rear arm and use some two piece whiteline WRX units (if they fit or custom make something) - this then gives you the ability to adjust toe and camber easily.

Don't agree as the SVX has the third arm coming from the front to the wheel hub the same as your car. using flexiable trailing arms will require that to stay. Given that the arm has a very short length it would be effected by any rid heigth change the most of all.
A rigid arm can still offer toe in adjustment because one bush can be adjusted on a ecentic.

It maybe possiable to move the inner trailing arms to origanl position and leave all that fixed as Matt suggested using standard arms just with a couple of pivot positions in height for adjustment.

Tony

[bazza]

Originally Posted by Dessertrunner

Don't agree as the SVX has the third arm coming from the front to the wheel hub the same as your car. using flexiable trailing arms will require that to stay. Given that the arm has a very short length it would be effected by any rid heigth change the most of all.
A rigid arm can still offer toe in adjustment because one bush can be adjusted on a ecentic.

It maybe possiable to move the inner trailing arms to origanl position and leave all that fixed as Matt suggested using standard arms just with a couple of pivot positions in height for adjustment.

Tony

That third arm is a trailing arm and that controls rear dive and squat and cannot be removed unless you change suspension type (ie double wishbone setup instead). Anyway it's your car - just letting you know what works and why it works.

[Dessertrunner]

Originally Posted by bazza

That third arm is a trailing arm and that controls rear dive and squat and cannot be removed unless you change suspension type (ie double wishbone setup instead). Anyway it's your car - just letting you know what works and why it works.

I think we are saying the same thing,
if the bottom arms are rigided arm then the third arm is not needed. In the same way the the front of a SVX does not have the third arm.

Do you know what White Line suggest people do with the third arm when people lower the car?

[bazza]

Originally Posted by Dessertrunner

I think we are saying the same thing,
if the bottom arms are rigided arm then the third arm is not needed. In the same way the the front of a SVX does not have the third arm.

Do you know what White Line suggest people do with the third arm when people lower the car?

If I were aiming to make the SVX into a handling machine, I would actually see if I could remove the front subframe completely and use the subframe as found in all the other Subaru's or at least use that design instead. The reason being is Subaru had the ability to change the front suspension geometry and use the SVX design yet they've used the same design from the 1989 Legacy to today's STI with a basic change to how the "a-arm" attaches near the gearbox and they did collect a few WRC titles and many wins with this setup so it's certainly quite good and strong. However they did completely change the rear suspension in 2008 in the Impreza to double wishbone (Legacy's did this in 2000).

Looking at the SVX front subframe, straight away without that longitudinal arm mounted back I can see you lose rigidity that the longitudinal mount point gives and you lose the ability to add castor via the longitudinal arm and also play with dive and squat properties - whiteline call this "anti lift kit". When you lower a car the dive and squat characteristics don't change that much to effect performance in a bad way (and as i said you can get the Whiteline ALK) - it's the roll center that kills performance.

So I think copying what I would see as a less than ideal front design without the strong longitudinal support in the rear is a bad idea. I can straight away see it would be very hard to control flex and 0.5-1 mm of flex is far to much for example. The wear on the bushes be it metal bearing or rubber would be massive and I just cannot see any benefit in doing this - especially if this mid engine concept is going to be a handling machine. I'd actually see if you can stick some the modern new age double wishbone WRX rear suspension in your beast.