Achieving a Handling Balance Formula Ford 1600

Since the hasty rebuild of our Mondiale M89s back in April the car has never really been easy to drive.  A few people have remarked how entertaining it was to watch but that’s not fast and it’s not why we go racing.  And you know, going sideways gets a little boring after a while.   At least it’s not understeer.

Too much of this!

So, we’ve worked on ways of improving the handling balance over the last few rounds and it’s got better but it’s still on the oversteery side. In fact during testing for the last round at Oulton Park the car was almost perfect with a knackered 4 year old front left tyre and three newish tyres. New tyres all round just ruined it.  Anyway we’ve got it sussed now but before I explain what we’ve you may wish to read the following post to understand how suspension affects the balance of a chassis…

To summarise, we found the higher the weight pushing down on a tyre, proportionally it’s grip level is reduced.  That is to say it will grip more when pressed down harder because it’s being pushed down harder but with a lighter load the ratio of grip to weight pressing it down will be higher.

ACB10 Tyre Data
ACB10 Rear Coefficient of Friction for 84kg and 306kg Loads

We also concluded the end which can roll more will have more grip.  The more we stiffen one end the less it rolls when cornering.  The stiff end of the car will transfer weight to the outside tyre more than the soft end when cornering.  This would mean the outside tyre on the stiff axle would have more weight on it than the outside on the soft end.  This soft end therefore will have more grip on it’s outside tyre as it is more lightly loaded and the whole axle will have more grip.

To summarise the summary; the end with the most roll is the end with the most grip!

Blade Style Roll Bar
Anti-Roll Bar Blade in Softest Setting

We have too much oversteer so we need to allow the rear end to roll more and reduce the amount the front end can roll.  First thing we do is look at the rear anti-roll bar and obviously we need less of it!

The roll bar effectively tries to twist the chassis back to a non-rolled position when one wheel is displaced upwards opposing roll.  On the rear of the Mondiale we have two blade type roll bars.  We set them both to the most bendy softest position – 90 degrees to the link pushing them upwards.  This is unusual in that one of them is normally fixed in the stiffest position but we still had too much oversteer so we tried both (the guys in Ireland run both these full stiff).

Blade roll bars
Both Blades are in ‘Soft’ Position

This helped improve the car after qualifying at Donnington but it was not enough so next we tried to stiffen the front anti-roll bar.

Anti-roll bar
Front Anti-Roll Bar in Stiff Position

The higher the link is up the front anti-roll bar the more stiff it appears to the car.  This is because, the closer links are to the axis the bar rotates around, the less leverage the suspension has on it so more force is required for the same amount of displacement of the roll bar.  We had this as far up as the link would go!  This is already a thick, stiff bar.  At least in the second race at Donnington the car became a little more driveable like this but it was way too oversteery!

We looked at using a different hole in the upright for the rear suspension which would give a lower roll centre but this limited the rear droop which was already a bit skinny and causing oversteer problems of it’s own.  So we really had to look at the spring rates next.

Originally this car came with 400lbs springs front and rear.  We had a slight issue of the car bottoming out at the rear so we fitted the 425lbs springs to the rear to avoid this (you don’t want a FF1600 hitting the floor it damages the engine).

Suspension Springs
Assorted Springs

A stiffer spring feels good to the driver as the car becomes more responsive to the brakes and steering.  However if you keep going stiffer, it eventually starts crashing over bumps and kerbs and effectively just reduces the grip on the road due to the time the wheels are airborne!  As someone who regularly rides over 200 miles a week on a bicycle I know all about this!  You tune a bicycle’s handling with just tyre pressures (particularly in the wet).  You always want more front grip than rear grip so I run my bike tyres at about 5-6bar front and 8-9bar rear.  If you’re riding over some bumpy road it will be the rear which crashes around and slides first!

So to reduce the oversteer we could have gone softer on the rear (not really an option due to the bottoming over bumps) or stiffer on the front.  I had some 450lbs springs.  This gave a very high frequency wheel rate but it felt about right given the balance we had.  I first tested the car at Oulton Park after fitting these and sure enough it felt a lot better and equally pleasing was the fact the front still ran over the kerbs fairly well.  Also, given the chicanes they have there is was useful to have a stiff car in roll as less time is wasted changing direction.

Droop Stops Screws
Zero Droop Front Suspension – the screws stop the dampers extending beyond ride height.

We also tried running zero droop with a slight preload – a slightly heavier person sat in the car when we set the droop stops.  Running zero droop front suspension means when the car corners the inside wheel cannot move downwards and take the weight of the inside of the car putting more load on the inside rear wheel and outside front wheel.  This sounds like just what we need especially when accelerating when the front wheels would experience droop.  It did help traction but still mid-corner it was all about oversteer.

The problem now was we still had too much oversteer and no more scope for adjustment anywhere – this would not have been ideal in the wet.  The final tactic was to reduce the rear ride height to the legal limit so the car ran level instead of with a little rake (this lowers the roll centre height).  Again this helped and for the first time the car felt good in the next test session at Oulton running in a respectable low 1m51s times on old tyres.  But still there was too much oversteer around corners like Druids where the car was sometimes going through the entire corner on opposite lock.  With new tyres it reverted back to being slow.

Suspension Wishbone
Spacer allowing two positions for the rear lower wishbone

Last year the car had a bit of understeer sometimes without having to resort to these extremes.  That could have come from the semi-seized front rockers we had but I don’t think so.  The difference was subtle.  I reassembled the lower wishbones in such a way as to increase camber gain by fitting the spacers the other way around.  This meant the lower wishbone would flatten out in roll making it longer and pushing the rear wheel outwards at the bottom.  Unfortunately I’d forgotten all about this and it happens to moves the rear roll centre upwards considerbly!

roll centre picture
The point the chassis rotates about is called the roll centre

The roll centre is the point around which the chassis rotates.  It’s obvious in the picture above where that is but with wishbone suspension you have to draw it.  Firstly though why is it important?

Before we begin the amount the chassis rolls is related to the roll centre but the weight transferred from inside wheels to outside wheels in total is not affected by the RC – this is purely down to the position of the CoG and the cornering force.

Low roll centre
Pretend car with ultra low roll centre

As the distance h is so large the leverage the cornering force at the centre of gravity has on the chassis about the roll centre is very large compared to the leverage the spring has on the chassis through the distance ‘a’.

Axle height roll centre
The centripetal cornering force has less leverage on a higher roll centre

Now compare that to the leverage the cornering force has on a much higher roll centre as above.  With the same springs and weight, the axle with the lower roll centre would roll considerably more given all of the leverage the cornering force has on the chassis through the roll centre.  If we had the low roll centre at one end and the higher roll centre at the other what would happen?  Well the chassis can only rotate through one angle as it is rigid (well should be compared to the stiffness of a spring!).  That means the low roll centre end would still be rotating further at the angle the high roll centre end side would have reached equilibrium.  So the high roll centre end starts taking some of the load from the low roll centre end increasing the load on the high roll centre end’s outside tyre and reducing it’s grip.  Obviously this reduces the load on the high roll centre end’s inside tyre as we know the total load on the axle is fixed by the position of the CoG.

Again we could have simply reached this conclusion from the basic principle ‘the end which rolls more has more grip’.  Obviously the end with the low roll centre will roll more with all that leverage.

Suspension Wishbone
Spacer allowing two positions for the rear lower wishbone
Wishbone Roll Centre
Moving the lower wishbone joint down just a little really lowers the roll centre

In the above picture we can see with the spacer in the lower position the angle of the lower wishbone has it pointing upward and this gives a much higher RC than if we put it above the wishbone joint (the yellow lines).  It could be about 2″ lower in this position.  Remember this will cause more rear roll and hence will give more rear grip.  For reference the front RC is on the floor or lower depending on ride height.

The small change we made to the placement of the spacer really made a difference to the roll centre height – far more that a ride height change could make.  We could get this even lower by using the low hole in the upright but as mentioned this will limit rear droop which is already insufficient.

The handling as it was on Oulton was just on the outside of the tuning window.  So, with the rear roll centre much lower we expect the handling to be much more tunable with a understeer/oversteer balance being able to be achieved within the range of the rear bar.

If this isn’t the case there must be a crack in the chassis allowing it to flex.  Anyway before we next run it it’ll need a new engine and will have to be put back together!

Mondiale M89s
The 89 is back in a pile of bits


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