Anti-Roll Bars

[nikpro]

This is the first in a three part series that will hopefully explain some of the very basic fundementals in car handling to the enthusiast. In order to understand why we 'set-up' a car the way we do it is vital that these concepts are read and understood.

The other two articles can be found here and should be read in this order:

(1) This Article
(2) Tyres - Gripping Stuff
(3) Driving the Circle

I will make my apologies now as some of this may be heavy going but hopefully it will explain what the theory is behind ARB's and how they 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.  (Cornering Efficiency = Traction/Vertical Load)

It can also be seen from above that an axle 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 Transfers.

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 affect the wishbone geometry and dynamic camber control (bad) - there's our compromises 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.
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 total grip achievable 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 (Bad)

.........And here is the Compromise!
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. (Good)

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. 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 controlling 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 occurs 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 transfer more weight and hence have less grip overall than the rear causing understeer.

As stated previously, a well designed ARB will increase the overall grip across 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 'ratio' of grip between the front and rear axle by 'tuning' the ARB's)


Another compromise!......

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 (westfield) 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.  

 

 

Information for the above articles taken from the following sources: Milliken & Milliken Race Car vehicle dynamics, Optimum G, Dan Kozakewycz blackartdynamics.com, Smithees Race Car technologies, Herb Adams Chassis Engineering, Alan Staniforth Competition Car Suspension.

[RedditchJay]

OMG...... lost me half way through.... glad I only operate a simple road going Westfield  ;-)

[Andy Banks]

That, with pictures and graphs, must be a candidate for an article in the mag...

[nikpro]

That, with pictures and graphs, must be a candidate for an article in the mag...

Thanks Andy;

I was thinking of submitting it but I think it is probably too 'heavy going' for most?

All I ask is people that are interested read it a couple of times and analyze each section - once you understand it, it explains why stiffening/softnening an ARB, changing springs and altering ride heights changes the grip level of that axle.

The Bullet Points are:

(1) An axle has it's greatest acheivable grip when both wheels are evenly weighted

(2) Load transfer from the inside wheels to the outside wheels occurs during cornering.

(3) The more load you transfer the less grip the axle as a whole will have.

(4) An ARB helps improve the Dynamic Camber Control of a vehicle (Camber control during cornering) and therefore improves overall grip.

(5) An Arb transfers Load quickly from inside wheel to outside (loaded) Wheel.

(6) As more and more weight is transfered the axle as a whole will have less and less grip.

(7) Making an ARB stiffer will give that axle less grip (more load transfer) in relation to the other axle and vice versa.

[B5234FT]

I thought it was an excellent article, highlighting the salient (an commonly misunderstood) points in a clear, concise way without resorting to maths (always the end of the road)

Personally, thats the sort of detail I think is sadly lacking from magazines.

[Rory's Dad]

I thought it was an excellent article, highlighting the salient (an commonly misunderstood) points in a clear, concise way without resorting to maths (always the end of the road)

Personally, thats the sort of detail I think is sadly lacking from magazines.

I agree.  There is always room in the magazine for articles such as this - accessible away from the computer  

Rory's Dad

[nikpro]

I'll 're-hash' it then and submit it; It lends itself to loads of follow on articles in relation to cornerweighting, driving styles etc. that could be explored later.

[Tigger]

Another very interesting article Nick,

I'd submit it to the WW magazine and if you like, I could also publish it in Westfield-world.com too. Obviously with credit to you

Tiggs

[Dave Eastwood (Gadgetman) - Club Chairman]

Be interesting to see, I wonder if as a follow up, there'd be any practical way to demonstrate the effects, (maybe limited to anti roll bar adjustment, maybe a little with tyres as well) on a trackday or similar via data logging. Thinking as much steering angle/lateral g as out and out lap times.

Might help shake out a few old wives tails and take a bit of the dogma out of the topic?

[nikpro]

Another very interesting article Nick,

I'd submit it to the WW magazine and if you like, I could also publish it in Westfield-world.com too. Obviously with credit to you

Tiggs

No probs at all Tiggs - copy what you want from the article or am I best sending it as an email?

[nikpro]

Be interesting to see, I wonder if as a follow up, there'd be any practical way to demonstrate the effects, (maybe limited to anti roll bar adjustment, maybe a little with tyres as well) on a trackday or similar via data logging. Thinking as much steering angle/lateral g as out and out lap times.

Might help shake out a few old wives tails and take a bit of the dogma out of the topic?

Dave,

was thinking of suggesting that I could maybe do a couple of talks at the Cheshire Meet over winter - each would be an hour long and go into the basics of Suspension and handling to help people understand?

Death by flip Chart!

[Dave Eastwood (Gadgetman) - Club Chairman]

I'm more than up for that! If you need any help, just ask  

I also wondered about suggesting a visit to Race Technology for one of their evening meetings, once we're back into the longer evenings of course. I don't know if enough would be interested though.

[Nick Algar - Competition Secretary]

Very clear and well explained Nick, Thanks    

I like the last bit about our cars using tyres used to heavier weights, you are so right with that.

[SteveD]

Very clear and well explained Nick, Thanks    

I like the last bit about our cars using tyres used to heavier weights, you are so right with that.

you speak for yourself slim my tyres are fine with my weight  

[Nick Algar - Competition Secretary]

Very clear and well explained Nick, Thanks    

I like the last bit about our cars using tyres used to heavier weights, you are so right with that.

you speak for yourself slim my tyres are fine with my weight  

The exception that proves the rule as usual wuv