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Author Topic: Mk2 golf VW Custom Suspension Uprights hubs Motorsport design  (Read 4611 times)

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Offline Smudge

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Mk2 golf VW Custom Suspension Uprights hubs Motorsport design
« on: November 09, 2013, 02:15:04 pm »
Hello all

I'm working on a project for the Mk2 and thought I would share my thoughts and progress. I appreciate others inputs and thoughts on this and there are plenty of you who have experience and knowledge in this area.

A long time ago I sketched the mk2 front suspension geometry in an attempt to better understand it. When you start modding the suspension, by lowering, wide tracking, adjustable top mounts etc things change in the geometry that you may not have realised and may not want because of the undesired effects they can have.

For example, lowering the car results in wishbones that point up and this seriously alters the role centres detrimentally. Wide tracking the car alters the king pin inclination (KPI) angle and altering the wheel ET effects the scrub...

After seeing these effects (and feeling them) I have wanted to design an upright that will allow a lower ride height but have none of the negative effects this creates. It should also be affordable, strong and ideally be easy to change the bearings in situ.

I have looked into, and have now designed an upright that can deliver what I class as desirable geomoetry. Below is a sketch of the mk2 geometry, the KPI and the scrub are the focus, the car is set to 2 degrees camber and has been lowered, wide tracked, wheels are ET33. Altering the wheel ET has a very big impact on the scrub and for me is something I think should be avoided if possible, scrub should be kept as close as possible to 0.



The scrub is 0.8mm
The KPI is 12.59 degrees
Caster is 3.5 degrees (it should be close to the KPI from what I have read)
The wishbone angle is +5 degrees

The same car now with 2.5 degrees of camber.




The scrub is 2.49mm
The KPI is 12.98 degrees
Caster is 3.5 degrees (it should be close to the KPI from what I have read)
The wishbone angle is +5 degrees

The scrub barely changes and is within an acceptable limit IMO, this is how I run my car now. The problem with this is the KPI is too different to the caster and more importantly the wishbone angle which effects the roll centres.

Add a 50mm ball joint spacer to the same car with 2.5 degrees camber and we have...



The scrub is 15.38mm
The KPI is 12.06 degrees
Caster is 3.5 degrees (it should be close to the KPI from what I have read)
The wishbone angle is -2.66 degrees

The wishbone angle is much better, but the scrub is now increased, ball joint spacers IMO are weaker than the standard setup, I have never been a fan of them because of the extra force they exert onto the ball joint when braking and cornering. So, I want to keep the wishbone angle, but reduce the scrub and try to get the KPI and caster figures closer. something that is not easily (or even possible?) with the std upright.

If you could choose your own geometry settings for the mk2 what would you pick? What range of adjustment would you want? (adjustment because different circuits, drivers, type of event require different settings).

Camber?
Caster?
KPI? (should be kept close to caster, or do you not think this?)
Wishbone angle?
Scrub?
« Last Edit: July 22, 2014, 07:38:52 am by Smudge »
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Offline Tristan

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Re: Custom Suspension Uprights
« Reply #1 on: November 09, 2013, 02:26:50 pm »
I like the idea Tom but I can't see pics?

Offline Smudge

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Re: Custom Suspension Uprights
« Reply #2 on: November 09, 2013, 02:35:13 pm »
I like the idea Tom but I can't see pics?

You were to quick for me, I was still uploading them from the PC! Now up for you to see.
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Offline RobT

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Re: Custom Suspension Uprights
« Reply #3 on: November 09, 2013, 02:53:47 pm »
Seem to recall Brian mentioning he had made a front suspension geometry model???? Would be worth comparing the dimensions with what you have Tom
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Offline sparrow

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Re: Custom Suspension Uprights
« Reply #4 on: November 09, 2013, 05:11:00 pm »
This is really good, I'm going to be following this closely.

I maxed out the castor on mine using tubular wishbones. You can see in the picture below how far forward the wheel sits in the arch. That's about 5 degrees of caster.


I'm going to get adjustable top mounts, so should be able to up the caster some more. Future plans though.

Camber: 1 to 4 degrees
Caster: As much as you can I think. Not sure if it's possible to get too much in our geometry.
KPI: I don't know the answer to this one.
Wishbone angle: Level when the car is at rest
Scrub: Must be as close to zero as possible

On KPI, I think the KPI is higher than the caster to increase camber as the steering is turned. This allows the wheel to be relatively vertical when traveling in straight line, but have increased camber when cornering. I don't know if this is a compromise for road use or not.
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Offline Smudge

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Re: Custom Suspension Uprights
« Reply #5 on: November 09, 2013, 06:01:51 pm »
I have adjustable top mounts maxed for caster, with stock wishbones I can only get around 5 degrees. My design should see plenty more caster than that.

I'm interested about your take on the wishbone position, the further they point down the more the roll centre is moved up which is a positive. Or do you know something different?  ???

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Offline RobT

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Re: Custom Suspension Uprights
« Reply #6 on: November 09, 2013, 06:11:34 pm »
Wishbones want to be level at average suspension position, to minimise geometry change about this mean position. And as long as possible for same reason.

This inevitably means they need to point down at rest, but maybe not by much if the spring rate is high.

Mine are just pointing downhill a couple of degrees
« Last Edit: November 09, 2013, 06:13:28 pm by RobT »
2008 Audi A6 2.0TDI S-Line
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1996 Seat Ibiza 16V F1.5 Race car

Offline Smudge

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Re: Custom Suspension Uprights
« Reply #7 on: November 09, 2013, 06:36:00 pm »
Agree with you Rob pointing down is good at rest, its also very interesting to see the geometry changes as the spring compresses! Scrub pretty much stays the same but camber can change to almost negative readings! Well that's what I have found if the camber is much less than 2.5 degrees.

Here are three pictures of the roll centres.

The lowered car with std upright, 2.5 degrees camber, lowered, wide tracked. The roll centre is the red circle, it is below ground level! Tut tut.



Next same car setup as above but with level wishbones at rest. Roll centre is now 39.03mm above the ground.



And finally, same car and setup but with my upright design. Roll centre is 80.93mm above the ground level.



Edit, new drawings added with more detail and accuracy.

« Last Edit: November 13, 2013, 09:38:35 pm by Smudge »
See my MK2 Golf 16v in the members section.


Offline RobT

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Re: Custom Suspension Uprights
« Reply #8 on: November 09, 2013, 06:52:34 pm »
Need to factor in car roll, thats why camber goes positive
2008 Audi A6 2.0TDI S-Line
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Offline sparrow

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Re: Custom Suspension Uprights
« Reply #9 on: November 09, 2013, 07:20:40 pm »
I've assumed that the suspension is in average position when the car is at rest, so that may be incorrect.
Rob's definition is more accurate.

When I looked in to this in more detail (nearly 20 years ago, so could be getting some wrong), I seem to remember the optimal suspension was double wishbone with the upper arm pointing down slightly when the lower arm was parallel to the ground. The lower arm needed to be as long as possible, and the length of the upper then controlled camber change during suspension compression.

In F1 cars of the time, this resulted in the roll centre being below ground level, which I remember as good.
Based on what you've said, this is wrong. Either I'm remembering it correctly, and that theory has now been disproved, or I'm remembering it wrong.

Also, isn't roll centre in relation to CoG key? In that the greater the distance between the CoG and the roll centre, the greater the leverage, meaning the more the car will roll. I seem to remember trying to work out what happened when the roll centre was above the CoG, but I can't remember the outcome. Or was that moving the inner wishbone point past the chassis centre point? I can't remember now.
I suspect I'm confusing some of the above with this.

I'm really going to have to read up on this again. I'll have to try and remember what the book was that I read back then.

If Brian.G does have a working model, that would be really interesting to see. There was another guy on CGTi who started making a string representation (can't remember the scale). He has a 20vt Mk2 with Berg cup arches. Last I saw the car was in pieces getting raised suspension turrets, and then the thread wasn't updated again.
Edited to add: I think it's this one. Just out, haven't got time to check:
http://www.clubgti.com/showthread.php?204978-Mk2-Golf-IHI-track-car-Re-lighting-the-flame!&highlight=raised+front+turret
« Last Edit: November 09, 2013, 07:31:23 pm by sparrow »
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Offline RobT

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Re: Custom Suspension Uprights
« Reply #10 on: November 09, 2013, 07:32:57 pm »
Allan Staniforth was one of suspension gurus and came up with the 'string computer' to show how suspension changes dynamically. Its a good method.

Lean is due to the rotation of the cofg around the roll center. They want to be in same place. I had my car properly evaluated by a btcc suspension engineer and have the roll center figs somewhere.....will dig them out. Cofg is a couple inches above crank center line.
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Offline Smudge

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Re: Custom Suspension Uprights
« Reply #11 on: November 09, 2013, 07:46:20 pm »
Rob, knowing where the CofG is in our cars is a real help! It's very hard to guess but just above centre of crank seems to be fair, was this a measured CofG point? I have read that the roll centre should be 15-30% of the CofG. But there are so many contrasting views against all setups it makes it difficult to know what is best for our cars.

I think with F1 the roll centre can end up below the ground, but there maybe reasons for this, it might be better there so they can control the ride height better from aero effects. Something I don't have to consider.

The closer the CofG is to the roll centre means there is less force around to make the car roll when cornering, this means you can use softer springs to control body roll, which is a benefit because it means the setup will be better at riding bumps than a car with a lower roll centre and harder springs to compensate.

If the roll centre gets above the CofG the car wont roll at all! Not good when cornering.
See my MK2 Golf 16v in the members section.


Offline RobT

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Re: Custom Suspension Uprights
« Reply #12 on: November 09, 2013, 08:27:21 pm »
Roll center with my drop-pin modified uprights is 16mm.
2008 Audi A6 2.0TDI S-Line
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Offline Smudge

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Re: Custom Suspension Uprights
« Reply #13 on: November 10, 2013, 12:36:47 pm »
Is that 16mm from the ground up Rob? Because that seems like a small amount (level wishbones create more than that as seen in my drawings).

I have be looking at the roll centre in general and have been reading around the subject.

Firstly the stock MK2 the roll centre was around 102mm off the ground according to a drawing i made (accuracy of this is not great because i don't have a stock ride height anymore). Now what relationship this has to the CofG is anyone's guess until an accurate CofG can be established, Rob suggest its a couple of inches above crank centre line (just above inner CV centres), others suggest it could be up as high as the camshaft! It could be calculated but to do that I need to use car scales which i dont have access to, it would be good to get an accurate figure for my car.

The VW engineers may have set the front role centre at what they thought was a 'good' height and what's most important is the relationship with the rear roll centre, which it is I believe on a Golf mk2 would be higher than the front. However, I am stuck when trying to calculate rear roll centres on our cars. Any ideas guys?

My design puts the front roll centre at around 80mm off the floor, well within this 15-30% bracket of the CofG (if CodG is assumed to be just higher than crank centre line, but it can be considerably higher and still fall into this 15-30% bracket). I'm pretty happy with the 80mm front roll centre, its not as high as the original VW engineers intended but I don't think with a lowered car it could be that high anyway!

I think next is to work out the rear roll centre and an accurate CofG for my car, this will give real data to work off as apposed to assumptions.

Any ideas how to measure rear roll centre on our cars?
See my MK2 Golf 16v in the members section.


Offline RobT

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Re: Custom Suspension Uprights
« Reply #14 on: November 10, 2013, 01:07:08 pm »
Yep, 16mm above ground he said. Btcc engineer and lecturer in automotive engineering. Rear is 40mm.
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