Ok you 510 suspension guru's

After reading some information from the great Les Collins, i decided i would go and check my roll ctr front to rear........

History:

Recently i fully rose jointed the rear arms on the race car as the bushes don't put up with the race track for too long, they were all flogged and due to the fully adjustable rear on the car they were wearing badly on opposing sides due to camber angles. I decided to make some inserts and modify the arms to take a full rose joint (i have pics if any one wants to see what i did) Happy with my choice i assembled the car, wheel aligned it and went racing.....(see most recent youtube vid i posted)

Much to my surprise i had gained understeer and more oversteer than i had ever experienced, regardless of engine HP it was worse than what i had before the mod.....

Enter the LCR article i read...

Simply it states that if the rear RC is lower than the front the car will "understeer on turn in and snap oversteer"...... pretty much what i had created!!!!

So tonight i set the car up on proper suspension stands so ride height and spring tension is as per, i string line and found my front and rear RC to find the below

Front - 138mm
Rear - 136mm

My question is, what is an acceptable difference, i understand that the rear must be higher than the front but by how much??? I have many spring combinations for front and rear but im not sure what difference in mm i should be aiming for???

Rear springs - 1600lb
Rear -2 deg
Rear 8mm TI

Front springs - 375lb
Front -3.5deg
Front 3mm TO
Front 3.8 castor



Thanks Crew

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Cheers

Brad 'Cusco' Cuss
Race BRE 510 Replica....on Steroids!

Just realized i made a classic mistake in my numbers......I took my RC from the axle center lines not from the ground!!

Its too cold and late now so ill re measure this tomorrow and report back.....

_________________
Cheers

Brad 'Cusco' Cuss
Race BRE 510 Replica....on Steroids!

Not sure what is ideal front to rear roll center height from the ground. Give Les a call and he will run you through it.

You need to create a adjustable system on the rear so you can raise the inner pivot around 20-30 mm and then compensate and regain camber on the arms. This will allow tuning of the rear roll center whilst retaining the same camber figure.


have a look at this group A BMW photo for ideas of how it could be done(no one has done this on a datsun as far as i know)

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Circuit Race 1600 in the build

Here's my 2C.

The geometry of the rear end is pretty feisty when it comes to changes. By moving the pivot points towards the front or back of the car or up and down, you end up changing the roll centre. I think if you do a search on here, you will find a few diagrams that show how to calculate the roll centre for the semi-trailing arm suspension the Datsun 1600 has.

Now if you move the outer arm pivot point (closest to the tyre) BACKWARDS (towards the rear of the car) you actually move the roll centre towards the ground. Likewise the reverse occurs when you move it inwards, the roll centre moves up. Best bet is to sketch this out and see what the changes are based on how much movement you require.
If you RAISE the outer pivot point (closest to the tyre), you move the roll centre down towards the ground. Likewise if you lower the pivot point, you raise the roll centre.

The question now becomes, what happens to the rear end now that you have made it 'solid'. The squishyness of the bushes are removed and your roll centre is now constant so not lowered/raised depending on what you are doing (under power or under brakes).

Ok, so it's solid now, you gotta deal with that.
The problem is it depends on a variety of variables and the centre of gravity of the car itself (don't get me started with the wing causing downforce changing the moment)
So let's say you have understeer on corner entry and oversteer on exit. One potential cause of this is weight transfer. Let's assume this is the only cause for the purpose of finding out what effect the F/R roll centre can have on this.
If your rear roll centre is higher than the front, this allows a faster response to the weight transfer in the rear. Ideally you want the rear to respond faster to weight transfer than the front. As you turn in to the corner the front tyres load up (come up to and potentially over their slip angle) and the rears haven't started yet. <understeer>. The weight on the front has transferred, and the rear needs to catch up. So the rear tyres slip angle starts as the weight transfers, and then as you go through the corner passes the slip angle for the rears as you power out and the car transfers more weight to the rear <oversteer>.
By raising the roll centre on the rear, you are allowing the rear to 'catch up' with the front and the slip angles are starting to work more even.
Raising the roll centre too much on the rear (compared to the front) will actually get you to oversteer into the corner (DORIFTU) as the weight transfers too quickly. Sometimes this can be a good thing as it is predictable and by using the right foot you can back off slightly.

Raise the rear (or lower the front) roll centre to suit your amount of understeer/oversteer, and correct the roll centre correction with higher roll stiffness (higher swaybar/springs depending on what you need) to balance the car out.

I'm still trying to figure out the roll centre differences on my car (it's bloody hard to 'feel') and how it effects grip...

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
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