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Suspension geometry questions


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#1 Shtstr

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Posted 12 March 2018 - 05:20 PM

Ok for the people that are smarter than myself with suspension geometry I have a few questions I need answers to.
In the picture below is a diagram of how I am moving the suspension on the cross 8 front cross member to suit the Torana.
As you can see the diagram the pivot point on the cross member stays the same and all I am doing is moving the strut top and lower ball joint 57 mm each side.
By doing this I also have to shorten the steering rack and front cv's by 57 mm as well. The cv is simple as all I do is Shorten the intermediate part of the CV
Do I need to shorten the main body of the steering rack or can I just shorten the rack ends by 57 mm.
The lower control arms will still be in a downward angle as per factory in The Cross 8.
Can anyone see any problems I will be running into.
I myself am thinking it is pretty straightforward just by moving the strut tops in the same amount I am shortening the lower control arms and steering rack ends.
Point a is the original mounting point and point b will be the new mounting Point.

Attached Files


Edited by Shtstr, 12 March 2018 - 05:24 PM.


#2 Shtstr

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Posted 12 March 2018 - 05:54 PM

So basically the crossmember all mounting points and steering rack and front diff all stay in the same location.
All I want to do is move outside mounting points 4 steering lower control arm and Macpherson strut Inwood 57mm. From point A to point b.
Is this possible or will it cause problems

#3 StephenSLR

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Posted 12 March 2018 - 05:56 PM

Here's what properly designed suspensions should look like.

bsdrawing.jpg

mustang-bump-steer-correction_2617.jpg

 

s



#4 Shtstr

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Posted 12 March 2018 - 06:29 PM

As I said before if all inner points stay the same and all outer points are moved exactly the same amount 57 mm what issues am I going to have.
1 lower ball joint moved in 57 mm
2 strut top moved in 57 mm
3 steering rack end Shorten 57 mm
4 main body of rack left the same
all mounting points on cross member left the same.

#5 Bigfella237

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Posted 12 March 2018 - 06:58 PM

I'm not sure how any of those diagrams relate to MacPherson Strut type suspension though? Struts are much easier to figure out all the angles on because there are less moving parts, but the problem is there's much less you can do to fix any problems.

 

fig-14.gif

 

The problem with shortening the LCAs and moving the strut assembly inboard is that you will raise the roll centre significantly. In that sense you would be better to keep the LCAs at the original length and move the lower pivots inboard by the same distance, but then you'd need to shorten the actual steering rack itself rather than just the tie rods, and that opens up an all new can 'o worms.

 

If someone has one of those fancy suspension analyzer programs you could enter all the dimensions and work it out, but without that I'd say just remove the springs, mock the whole suspension up, set the ride height and take a heap of measurements, but you really need to have data from both a standard Adventra and a standard Torana to work from?

 

I'd imagine the roll centre of a standard Adventra (being 4WDish) would probably be higher than a Torana, so if you're going to raise that even further you may end up with an unstable handling car?



#6 Shtstr

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Posted 12 March 2018 - 07:08 PM

I'm also shortening the length of the strut by 60-70 mm overall and changing the angle of the lower arm by lowering the ride height by 20mm. I believe that should help with the roll center.

Edited by Shtstr, 12 March 2018 - 07:10 PM.


#7 Shtstr

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Posted 12 March 2018 - 07:13 PM

Also the tyre height will be less. The adventra has a 225 55 17 at 679mm high
I will be running a 255 35 18 at 635mm high.

#8 StephenSLR

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Posted 12 March 2018 - 07:16 PM

I'm not sure how any of those diagrams relate to MacPherson Strut type suspension though

 

Oops, I missed that part, lol

 

s



#9 Bigfella237

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Posted 12 March 2018 - 07:34 PM

I'm also shortening the length of the strut by 60-70 mm overall and changing the angle of the lower arm by lowering the ride height by 20mm.

 

Also the tyre height will be less.

 

Yeah WOW... with such wholesale changes I'd be investing in some decent suspension software and figuring all this out before you start cutting.

 

I'd GUESS that you may have to move the LCA pivot points anyway if you want to keep the Adventra steering rack, if moving the strut tops in and down by that much the rack ends will likely not line up with the LCA pivots and you'll have bump steer issues?



#10 Shtstr

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Posted 12 March 2018 - 07:47 PM

I will try and draw out a full-scale diagram tomorrow on the concrete to see if the roll centre decreases or increases

#11 Shtstr

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Posted 12 March 2018 - 08:12 PM

If all parts are shortening by same amount and top and bottom points are moved in proportion to inwards im assuming that it should work.

Edited by Shtstr, 12 March 2018 - 08:13 PM.


#12 Toranamat69

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Posted 12 March 2018 - 08:20 PM

I would just draw it up as you are planning. a scale model on paper will work just as well.

 

Since you are making multiple changes, it is tough to predict exactly what will move where. 

 

I'd draw it in 3 configs too, at ride height, full droop and full compression, will give you enough to know if its a goer or not.

 

Shorter LCA means you will have a sharper camber curve throughout the suspension travel, might not be that significant, but it might be too.  It might even help.



#13 Bigfella237

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Posted 12 March 2018 - 08:48 PM

There's just so many other things to consider beside all the instance centres and roll centres (both static and dynamic), like where is your rear roll centre now compared to the Adventra, or the new centre of gravity, and how do the front suspension mounting points relate to the rear suspension and overall wheelbase (which I assume is significantly shorter now)?

 

Will changing the length of the struts alter the total suspension travel, will that mean a change in spring rates, how about anti dive characteristics?

 

You also have to work out the static camber and caster, plus gain (or loss) of either as the suspension travels, you're also altering the scrub radius, bump steer and probably even Ackerman angles, in fact just about every single thing you can change is being changed except the included angle built into the strut / spindle assembly... actually come to think of it, cutting and shortening the strut may well change that too!

 

I don't mean to be negative but teams of engineers spend years figuring all this stuff out when designing a car, you can't just go making wholesale changes and expect everything to be sweet as.



#14 dattoman

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Posted 12 March 2018 - 09:16 PM

I'm in awe of you smart blokes



#15 Shiney005

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Posted 12 March 2018 - 10:21 PM

Have you got a build thread on your hatch? Watching with a lot of interest.



#16 Shtstr

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Posted 12 March 2018 - 10:34 PM

Have you got a build thread on your hatch? Watching with a lot of interest.


Nope

#17 bat 53

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Posted 13 March 2018 - 09:53 AM

g'day all 

i fit a standard vp p/s rack onto hr cross members with no bump steer

i moved the rack to suit the road height travel up or down 

fully engineered with 4mm bump steer either side of centre the rack is stock

 

cheers 

bat



#18 axistr

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Posted 13 March 2018 - 10:07 AM

Going through the steering geometry individually, the Ackerman angle would only be affected very slightly and in fact could end up being slightly better. The Ackerman angle change has more to do with the difference in wheel base between the Adventurer and Torana. The steering arms are fixed from memory to the strut. Unless you bend them it shouldn't change much.  Either way the toe out on turns would be much better than the original Torana set up which was at best pathetic.

 

The scrub radius and king pin inclination will not be affected unless for some reason you decide to bend the stub axles, its a fixed design angle between the strut and the stub axle assembly. However a wheel offset change from the original Adventurer offset will effect the handling. Its not the width its the off set that will cause the car to handle different. But the wider the wheels the more they tend to tram track on some roads.

 

Caster and camber is directly related to the position you place the top strut tower mounts in. When setting the top mount positions I would try and get hold of a decent angle indicator to ensure they are both laying back the correct amounts for caster and the struts are equal both sides, and the strut angles are leaning in the desired amount for the cambers you want to achieve, Try to leave a bit of movement in the top mounts for minor wheel aligning changes when its all finished. Correct me if I'm wrong but from memory the radius rods have shim spacers at the front for minor caster adjustments.

 

Because your only moving the outboard components equally in by 57mm I don't see to many dramas effecting the geometry causing any major bump steer issues so long as you use the original control arm and ball joint.

 

Any change in steering rack inner ball centres and lower inner control arm mount positions is where the real issues start and can cause bump steer. For good handling characteristics suspension ride height is important and I think you should be mindful in this area. Lowering the suspension ride height can cause quite a bit of camber change. As previously stated its the roll centres that will be effected the most due to the modification.

 

I cant see any change to anti dive under braking unless you dramatically change caster. One matter to consider is the spring and shock rates. Having slightly less leverage due to the shorter arms will reduce the leverage. Probably not  enough to have any major effect but should be considered.            

 

 



#19 Shtstr

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Posted 13 March 2018 - 02:34 PM

Finally someone who knows and understands what is being done.
Only outer points are moved inward and yes same balljoint and hub assembly used and all set exactly how it was in the adventra only ride height is lowed by 20 mm with the strut and 20 mm with tyre height bringing the bottom of the x member from 190mm adventra to 150mm torana height off ground and due to the body height differences from adventra to torana the strut top is lowed aswell and i use a shorter strut/coil over to do this.

#20 Bigfella237

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Posted 13 March 2018 - 03:38 PM

Finally someone who knows and understands what is being done.

 

Sorry for trying to help, I won't waste my time next time...



#21 Shtstr

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Posted 13 March 2018 - 05:19 PM

Its ok bigfella. I even went to a suspension specialist today and he agreed that if all geometry lines are kept as per the adventra there should be no issues at all.

#22 rogered

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Posted 13 March 2018 - 08:22 PM

Essentially what your are doing, (looking from the front  as you are) , is altering the camber as the suspension moves up and down.  What you can do is make a Stiff cardboard "mockup" of the set up, put drawing pins in the pivot points and move the " suspension" up and down to see you results, Lesser of an issue when travelling  in a strait line, but if you determine the vehicle "roll center", then model the car in " roll"  you can then aim keep the wheel  " camber " vertical with the road surface. I personally like to start with the bottom arm straight, (parallel with the road)






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