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[05omegav6]

Mk2 Granada front track 1515mm

Omega front track 1514mm...

[05omegav6]

Re position front or rear, I'm having trouble seeing how the leverage from the road wheels and road imperfections feeding back to the driver through the steering, can be affected by the hub arms leading forward or trailing rearward...? Neither can have an effect on when the wheel hits a bump or road imperfection. The leverage is the same. To my mind.

Perhaps I was thinking out loud tbh...

Worth noting that the vast majority of wwd cars have the rack fitted at the bulkhead/behind the wheel centreline, whereas, rwd cars with racks tend to have it fitted to the subframe, and quite often in front, using an intermediate shaft to connect the rack with the column.

Another detail to consider, is that the Omegas' steering box supports the base of the column, whereas a rack might not

Holden mount the rack behind the subframe, again with an intermediate shaft, but I can find no detail for the column itself

[Kevin Wood]

I think rack in front or rack behind influences the dynamic behaviour of the suspension. Assume, for a minute, that you locate the rack such that the track rods are parallel to the ground at the car's normal ride height.

If you brake, the suspension will dive at the front, the track rods now end up inclined upwards towards the knuckle, reducing their effective length. If you accelerate and the rear of the car settles, weight is transferred to the rear and the front rises. Same effect, the effective rod length reduces. Depending on the interaction between the arc the rod goes through and that of the suspension arms, you'll get a little toe-in or toe-out. Put the rack behind the wheels and you'll get the opposite. If not controlled, this effect will give you bump-steer.

I've no idea how you'd set up all these little subtleties to get a car that drives in a reasonably refined manner, but I'm sure you'd have to. I know this having had to play around with ride height and the exact position of the rack on the Westfield to get it to play nicely.

On a FWD car you've got longitudinal forces from the driven wheels acting on the suspension in acceleration in addition to braking, so I can imagine the rules are all different.

I would imagine that, to get anything drivable by your standards out of a rack setup, you'd need to give 2woody a calculator, quite a bit of paper, and a LOT of beer.

[05omegav6]

So would it therefore be prudent to fit an entirely self contained set up such as the Mk2 Granada one

Spring rates/bushes being tweaked to allow for Omega/V8 weights (given that is obviously the desired outcome:

[Webby the Bear]

Thanks for all the info... sorry didn't mean to digress from the original question and hijack your post, Chris 

[aaronjb]

So would it therefore be prudent to fit an entirely self contained set up such as the Mk2 Granada one

But you'd still have to tweak it such that the pivot points of the track rods are in the right place relative to the lower wishbone pivot points (vertically and horizontally) to avoid excessive bump-induced-toe..

To give an idea of the sensitivity; one particular marque of Cobra replica had (in the past) quite hideous bump steer that someone described as 'frightening' - removing just 10mm of metal from the mounting posts of the rack to move the whole rack upward transformed it and reduced the bump-steer to near zero.

So you've got to hope that the Granny rack has the track rod pivots in the right place, or you end up doing a cut'n'shut on the rack (also used to be quite common in the past on kit cars, before that kind of thing was deemed 'undesirable' and manufacturers started having people like Quaife make bespoke racks to suit their exact suspension geometry).

If doing a cut'n'shut is feasible (technically and mechanically speaking, it is) then you want to find a rack slightly longer than you want that has the input shaft in the right place, and then chop it up, re-weld the insides, machine them and weld the rack back together..

IMHO, etc


[edit] Wait a second, I see what you're saying now - basically ditch the front subframe of the Omega and fit the whole lot from the Granny? Subframe, suspension, mounts, rack, etc etc..

That could work, too. I'll shut up now

[05omegav6]

Got there in the end Aaron

Geometry will be fine as long as the subframe is mounted squarely  only packaging issue is running the exhaust around the intermediate steering shaft, although with nothing behind the subframe, no steering box/tie rod/idler, there should be a shed load of space to play with

[feeutfo]

Interesting gents. As Al says, the space saving aspect of this is purely to keep an ecpensive v8 away from the steering. There's no sense in spending large sums on an engine like that to mess about with off centre position and clearance issues.

So purely with that in mind, initially, I'd be happy to replicate the existing set up but with a rack. Rear position, behind the subframe in the same position as the centre tie rod, and have the ball joint pivot points in the same place as required.

But thinking further, the front polly bush gives way more control it seems, so after talking to 2woody, it seems the front mounted rack would have something much firmer and accurate to pull against, and therefor transmit the steering to the hub more directly.(this is way too simplified a statement but gives an idea on his general description he gave, some of which I even understood) where as the rear rack would push the rear of the hub out, as opposed to the front of the hub in. Wouldn't make the slightest odds if the bushes where not bushes but solid rose joints/or points, but through a rubber or poly bush that gives more comfort, and therefor some play, then the front mounted rack gives a much more direct toe in to the outside wheel that does most of the turning in
of the car.

This is considered worth having, in design terms, as I understand it.

So logic might then be, that if fitting a rack at all, fit the bugger in the best place. Although there's a lot more extra work involved. Swapping hubs to opposite sides so the ball joint fixes to the front. Longer brake hoses as the calipers will then be at the back.... probably fouling the wishbones thinking about it. Aaaaaah

...my thoughts are I'd be happy to achieve the same performance with a rear mounted rack. But depending on the work load, front mounted might be worth having.... If there's room under the sump area.

[aaronjb]

Interesting gents. As Al says, the space saving aspect of this is purely to keep an ecpensive v8 away from the steering. There's no sense in spending large sums on an engine like that to mess about with off centre position and clearance issues.

Before you get too fixated on that one it might pay to work out* the prop shaft UJ angles with your chosen engine & gearbox combination - you might need an offset to ensure that the angles work out to ~1-3º to avoid the UJs binding or wearing the needle roller bearings prematurely..

*work out in this case probably means fitting the engine & gearbox to the car and then mocking up the prop shaft with a scrap shaft, grinder and a welder, then getting some inclinometers out. I'll concede you're unlikely to end up with an angle severe enough to bind the UJs up, but if you end up too close to 0º offset the prop won't last long.

Assuming, of course, the prop has conventional UJs at either end and not something strange and exotic.


So much to consider if you want to do it all 'properly', but the best place to start is sticking and engine in an empty shell.. 

[feeutfo]

That's the thing though, theres a bloody great steering box in the way.


... And an abs pump. Bah, minor issue.

[Kevin Wood]

The problem(s) I see with simply replacing the current setup with a steering rack and maintaining the geometry are as follows:

1) You'll need a steering rack that places the inner track rod pivots at exactly the same place as they currently are, so it will need to be cut and shut. You then need to work with wherever the steering column needs to attach to it, and where its' mountings need to be so more hacking about needed there.

2) The shaping of the centre tie rod in the current setup is done to add clearance around the bellhousing. With a rack, you have the constraint that there must be a straight line between the two inner track rod ends, so this will actually lose you clearance.

I'd want to have offered up an engine and made absolutely certain that it won't ever fit before suffering that pain. If a slightly asymmetrical engine position would achieve it, I'd be delighted. It's really only an aesthetic issue, after all. Look at how many FWD cars have a seriously lop-sided engine bay!

ABS pump is a day's work with a flaring tool to move anywhere in the car. It's even doubtful that you'd be able to use the original one due to its' level of integration with the engine and autobox ECUs.

[05omegav6]

Hence my suggestion

[feeutfo]

There's no doubt in my mind tbh. Box has to go. Project is a non starter otherwise.

[feeutfo]

The rats rack, scroll down.

http://www.rats.no/omega_tg.html

[TheBoy]

Are you going with a Lexus lump, or something else? Thats the first thing to consider.

I'd expect the box/idler to be easier to accomodate than a rack.