I've never seen a Westfield do this ...

[cng1]

The reason that racers fit ARBs is to allow them to fine tune under/oversteer far more quickly and easily than changing springs.

An ARB is effectively a spring.  Now we all know that stiffer springs mean that the tyre is less able to follow the ups and downs of the tarmac, which means less grip.  As a result to maximise grip we all try to run the softest springs we can without screwing up everything else.  Attaching an ARB effectively increases the spring rate thus decreasing grip.

To think about it another way if you have front and rear ARBs and are suffering from understeer you have two ARB adjustments you can make - soften the front ("adding" grip there) or stiffening the rear (removing grip from the rear).  The reason I've put adding in quotes there as I hope you can see the logical conclusion that if softening the front ARB adds grip to the front then ultimate front end grip is with no ARB  == adding/stiffening an ARB loses grip.

[Kevin Jones]

I read the theory pretty much as nicpro lays out here.

In numbers, the weight on the front outer is roughly the same with/without an ARB once the inner has been picked up, but there is a ~60kg difference inner to outer at the rear, so without ARBs the rear will have less grip than with. There is also a front/rear balance issue with this setup, the rear weight is ~40kg more than the front leading to a general oversteer tendancy.

Playing with spring rates can help reduce these effects but not by much. But of course this is just theory culled from a dodgy spreadsheet. I assumed this was a BEC cornering at 1g to get the numbers.

I have tried many ways to balance a westy on paper without ARBs and have never managed it. So while I agree that some cars use ARBs just for easy tuning, a westy really needs them when on a track IMO. That said, these are not massive numbers to work around if you are just out for having a bit of fun.

[stephenh]

cng1: you are oversimplifying the ways in which an anti-rollbar works, and IMHO are drawing the wrong conclusion.

Yes, if you stiffen the front anti-rollbar you MAY increase understeer. But equally you MAY reduce oversteer. So is the anti-rollbar reducing overall grip in the first case? If so then it must be increasing the overall grip in the second case.

Anti-rollbars are a means of managing the transfer of weight between the wheels, without increasing the spring rate when the car is travelling in a straight line.

For example, stiffening the rear anti-rollbar will have the effect of transferring some of the weight (in a corner) from the inside rear wheel to the front of the vehicle. Which may or may not be a good thing, depending on the characteristics/ handling of the chassis. If you want to base your theory on Alan Staniforth's book, then have a look at chapter 9, which sets out the maths involved with a practical example. It certainly doesn't suggest that anti-rollbars are a bad thing!

So to say that an anti-rollbar reduces the overall grip of the car is a bit of an oversimplification, and not necessarily so.

[nikpro]

Sorry - removed post - was written at 5am when knackered and was utter b*****k when I read it later!

I've read the full thread and now understand what cng1 is saying.

It's  hard to give this justification in a single post but will try later.

[stephenh]

What were you doing at 5.00 am, other than writing a post on here? Should be fast asleep at that hour on a Sunday morning!

[SteveD]

What were you doing at 5.00 am, other than writing a post on here? Should be fast asleep at that hour on a Sunday morning!

freakin texting  me   good job i was up and off to work

[Kevin Jones]

One thing that might be being said is that as the inner picks up the effective rate of damping changes because any movement is working against both springs, think that has to be true, but how important that is I don't know.

I don't by the "adding grip"/"removing grip" argument because of the reasons stephenh raises, weight transfer is a zero sum game by definition, there is no add/remove only balance.

[cng1]

I don't by the "adding grip"/"removing grip" argument because of the reasons stephenh raises, weight transfer is a zero sum game by definition, there is no add/remove only balance.

Try separating the two things.  Roll/weight transfer etc is one thing.  Grip is another.  

Softer springs = more grip. Agreed?

An ARB is effectively a torsion spring linking sides.  Softening ARB stiffness (increasing arm length) reduces the effective spring stiffness.  Agreed?

It follows therefore that softening the bar "adds" grip.  The reverse is of course also true.

I hope that explains things.  If not then I am afraid that I think I've run out of ways to explain what is a relatively straightforward mechanical principle.

[stephenh]

Are you sure that softer springs will "ALWAYS" increase grip? I'm not.

[John Loudon - Sponsorship Liaison]

What a load of BS. Softer springs will allow more suspension movement resulting in excessive roll and excessive camber change. Guess what will reduce this? It can be fudged by increasing static camber but then you lose tyre contact patch and guess what that will also do?

[Blatman]

Are you sure that softer springs will "ALWAYS" increase grip? I'm not.

*Basic* car dynamics principal. As CNG said earlier, every race car out there is sprung as softly as practicably possible. Ask any race engineer. I've asked loads of them, the answer is always the same. Softer = more grip. But as I said, this is a basic principal. As soon as you add in other factors like available grip from different tyres, or ARB's or different camber and caster numbers then whilst the basic principle holds true, it is complicated by the additional numbers.

The other side of the coin of lifting the front wheel is that the rears are too soft (and then we're back to diagonal weight shift, corner weights and coffee table analogies that we had a few weeks ago...). But if you want to maintain the rear grip you cannot stiffen it. Given that the car sems fast, altering the rear suspension just to keep the wheel down may well swap the balance of the car such that over a lap, it ends up being slower because of the oversteer that's been dialled in...

[cast iron]

It depends on how you have set your static camber though, if you accept you have body roll you may run more static camber so you gain more cotact in the corners, of course the down side can be less contact under braking.

Harder 1a tyres arent as grippy as 1bs and slicks, the car can benefit from a softer set up rolling more and not to transmiting more load through tyres which cant take it anyway.

[nikpro]

Right, here goes!

I will make my appologies now as some of this may be heavy going but hopefully it will explain what 'I think' cng1 is trying to say and How ARB's effect a cars performance.

Everything with suspension is a compromise, most of the items we add to improve performance also have a detrimental effect somewhere else; it's the balancing act of 'Do the pro's outweigh the Con's?'

The First Item we need to consider are the tyres:

The two most basic concepts when dealing with tyres are understanding tyre load and traction.

(1) The vertical load is the weight on the tyre

(2) Traction is grip from the tyre.

we therefore have an 'input' (Vertical Load) and an 'Output' (Grip)

Tyres can lack grip in two scenarios:

(1) there is not enough Vertical Load Placed on the tyre.

(2) There is too much Vertical Load Placed on the tyre.

These two items; load & grip, are related and can be plotted on a graph - the result is a Curve of the tyres performance in relation to load and grip. (Traction Curve)

The amount of grip available from any given tyre is dependent on how much weight is on it. As weight (load) is increased the grip also increases. The important thing that must be recognized, however, is that the increase in grip becomes less and less as the load is increased. Hence, although there's an increase in total tyre traction, its cornering efficiency decreases.

It can also be seen from above that a vehicle will have most grip when it's tyres are evenly loaded as the unloaded tyre will always lose grip faster than the loaded wheel gains it.

Now lets Move on  

LOAD TRANSFER

With tyre basics covered, we now move onto the next aspect of handling; load transfer. Load transfer is the shift in vertical tyre load from the inside wheels to the outside wheels as a vehicle experiences lateral (sideways) acceleration in a turn.

The vast majority of the weight of a vehicle is a 'sprung mass' and for the purposes of load transfer we are talking about the sprung mass of the vehicle and not the unsprung forces.

Load transfer is a result of a few basic variables in the vehicles design: track (Tr),

height of the centre of gravity (cgh),vehicle weight (W) and lateral acceleration (La). These four variables alone dictate how much load transfer a vehicle experiences and are difficult to change aside from the weight (In a Westfield we can also easily change the Centre of Gravity height by adjusting ride heights). The formula for load transfer is:

Load Transfer = La x (cgh / Tr) x W

As stated earlier, total load transfer is engineered into the vehicle and cannot be changed without adjusting the vehicle weight or ride height, however the Front/Rear distribution of this load transfer can be altered, and is what we change when we alter spring rates and anti-roll bars. As you make one end proportionately stiffer than the other, you increase the amount of the total weight transfer that that axle sees, and reduce the transfer the other sees.

(As an aside we can also see from the above formula that lowering the CoG will reduce the Load Transfer (good) but going to low will effect the wishbone geometry and dynamic camber control (bad) - there's our comprimises again!)

What does an ARB actually do?

An ARB connects the suspension on both sides of the car and is also connected to the chassis of the vehicle.  

As a result, an ARB reduces Body Roll which is extremely good for the following reasons:

When the chassis rolls it has a large impact on the dynamic camber angles of the vehicle - this can be seen in the pic on the first posting.

Basically the body roll 'pushes' the upper wishbone mounts out and 'pulls' the lower wishbone mounts in on the loaded tyre. This puts the loaded tyre into a positive camber attitude whilst cornering which greatly reduces the grip of the loaded tyre due to the decreased contact area of the tyre.

Further to the above - body roll is a relatively slow reacting phenomenon and makes the vehicle un-responsive to direction changes and in the ability to apply power on corner exit as the body returns to its level attitude.

.......Now the bad!

The ARB actually transfers weight from the inside (unloaded tyre) to the outside (loaded tyre). As discussed previously an axle will have it's maximum grip  when it's tyres are evenly loaded as the unloaded tyre will always lose grip faster than the loaded wheel gains it.

In other words - because the ARB transfers weight from inside to outside it actually reduces the grip on that axle; just as cng1 states.

.........And here is the Comprimise!

Although a well designed ARB reduces the total grip available from an axle due to load transfer the axle will gain more overall grip due to the fact it controls body roll making the dynamic camber control better.

This obviously begs the question of why fit ARB's and not control body roll with the coil springs alone?

In order to do this you would have to fit very stiff coil springs. As cng1 states, the purpose of the Coil spring/damper is to keep the tyre in contact with the ground for as much time as possible. Fitting stiff springs increases the wheel rate, the higher the wheel rte the less able the tyre is to follow the track surface - hence the need for ARB's which act in torsion accross the chassis and are better at controling chassis roll.

Using ARB's to tune Understeer/Oversteer

Again going over old ground:

Load transfer is a result of a few basic variables in the vehicles design: track,height of the centre of gravity, vehicle weight and lateral acceleration. These four variables alone dictate how much load transfer a vehicle experiences and are difficult to change.

What we can change is the balance front to Rear at which this Load transfer occurrs by stiffening/softening the springs/ARB's

The stiffer the axle is in roll the more weight it will transfer as a proportion of the total load transfer of the vehicle. Therefore if we make the front axle stiffer in roll in relation to the rear axle, the front axle will have less grip overall than the rear and hence understeer.

As stated previously, a well designed ARB will increase the overall grip accross that axle due to it improving dynamic camber control and hence the overall grip available for the vehicle; all you are doing is altering the 'level' of grip between the front and rear axle by 'tuning' the ARB's)

In a Westfield we can also change the ride heights at the front and rear quite easily. As stated previously the lower the CofG of the axle the less weight is transferred and hence an increase in grip is acheived from that axle.

(Lower front ride height to reduce understeer/Raise rear ride height to increase oversteer.)

Another comprimise!......

It therefore becomes a balancing act of fitting stiff enough ARB's to get the best tyre contact patch area through optimum dynamic camber control during cornering and not so stiff that the axle can not provide enough grip due to excessive load transfer.

There is a caveat though!

A Westfield is a lightweight Sportscar that quite often runs tyres designed for heavier vehicles - they sometimes don't act 'normally' and quite often increasing the roll stiffness can increase overall grip as the vehicle does not put enough vertical load into the outside tyre!............Oh God it's complicated!

I hope the above does help to explain the purpose of an ARB and why it is beneficial to fit them to track cars; they are not primarily an 'understeer/oversteer tuning tool' but a device to improve dynamic camber control through less chassis roll.........hope you stuck with it.  

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edited to add:

The abridged version for 'Peter the Painter'

Yes.......SteveD you were right

(Hope nobody minds but after spending hours writing this post I've also placed it in the FAQ for future reference........if you do mind whinge to the Webmaster and have it removed.)

[SteveD]

can you repeat that please wuv  

you missed the bit where it says 3 wheels are better than four

are you iclke fingers bleeding

[windy]

Soft rear, very stiff front with virtually zero droop travel. He's basically using the chassis as a spring & lucky that the front tyre's grip isn't exceeded, so it appears to work. BUT fit road tyres or run that car as it is in the wet and it wouldn't work = terminal understeer.

I should think the handling of that car is going to be very twitchy / lively if he drives it on a track with zero droop travel on bumpy corners.