Anti-Roll Bars

[robert]

What Kev wrote:

"I checked the info in a book on suspension design I've been reading, the book is "Designing and Building Special Cars" by Andre Jute, in the section on anti roll bars he says, "A roll bar fitted at the front transfers weight from the inside rear wheel to the outside front wheel and by increasing the slip angle at the front, creates or increases understeer." There is a whole series of chapters on the various suspension permutations with calculations on how to check and set the various parameters.
My point was that on the Cobras I've driven there has already been an amount of understeer so you should be careful of fitting a roll bar if that is the case, as it is likely to make matters worse."

My spin on the topic.

The manner in which the anti-roll bar is mounted to the car has a big effect on its contribution to suspension roll stiffness. For maximum stiffness, the end links (or ends of the arms if end links aren't used) should be attached as close to the wheel as possible. Since the wheels are effectively out on a lever arm from the suspension pivot points, vertical travel of the suspension links near the wheels is proportionally greater than it is near the pivot points. Greater vertical travel at the end links of the anti-roll bar will cause greater twist in the bar, and thus more resistance to roll; also, placing the end links near the wheels means that the suspension will have less of a mechanical advantage with which to twist the bar. Note, however, that a trade-off can exist-mounting the end links outboard to increase the effect of the anti-roll bar may require a longer bar length, which will reduce anti-roll bar stiffness. The net result of this positioning game will depend on the track width and suspension link lengths of the individual car.

It's important that the anti-roll bar should not be pre-loaded when the car is resting at its static ride height with the driver on board, since pre-loading causes a difference in handling between right and left turns. Pre-loading of the anti-roll bar occurs when the bar has to be twisted to connect the end links to the suspension. This problem is a result of a mismatch in the geometry of the anti-roll bar and the mounting points of the end links on the suspension. Pre-loading can be avoided by using adjustable-length end links to dial-in the precise lengt h required to avoid twisting of the anti-roll bar. If the anti-roll bar attaches directly to the suspension without using end links, the bar itself must be rebent to the proper position.

The key to this is understanding the chassis dynamics and the effect of an anti-roll bar on a car. You cannot just blindly add an anti-roll bar for the following reasons.

What Is TLLTD?
TLLTD stands for Tire Lateral Load Transfer Distribution. While this term may sound complex, it simply measures the front-to-rear balance of how lateral load is transferred in a cornering maneuver. It is commonly used to compare the rate of lateral traction loss between the front and rear tires.

Put simply, there is only so much force that a tire can handle. When we ask more of the tire than the tire can deliver, it "saturates," or loses traction. If the front tires saturate before the rear tires, then we call this understeer or push-which means that the car tends to continue moving in the original direction, even though the wheels are turned.

If the rear tires saturate before the front tires, then we call this oversteer or loose-which means that the rear of the car tends to swing around faster than the front, causing a spin. When neither of these conditions prevail consistently, then we describe the chassis as balanced (ALA my GD).

We can measure and compare the steady-state understeer and oversteer characteristics of a vehicle by assigning a lateral load transfer percentage of the front relative to the rear. A TLLTD value equal to 50 percent indicates that the chassis is balanced-or both the front and rear tires tend to lose traction at roughly the same time. A front TLLTD value greater than 50 percent indicates that the front tires lose traction more quickly than the rear tires-resulting in understeer. And a front TLLTD value lower than 50 percent indicates that the rear tires tend to lose traction more quickly than the front-resulting in oversteer.

It is important to note that the discussion of TLLTD only considers steady-state cornering maneuvers, such as a long 270-degree on-ramp or off-ramp. Moderate-to-aggressive throttle or brake application can upset this balance during a transient condition, briefly transitioning a vehicle from understeer to oversteer.


The Effect of Anti-Roll Bars Upon TLLTD
Ideally, you now understand how an anti-roll bar can be used to limit body roll, and you understand that reduced body roll can lead to a reduction in adverse camber changes for better tyre traction. But what may not be obvious is the effect of anti-roll bar changes upon TLLTD (understeer and oversteer.)
In fact, given the above information, one might even assume that a firmer anti-roll bar, which leads to better camber control, would lead to better traction. If we add a firmer anti-roll bar to the front, traction loss diminishes, so understeer is reduced, right?

Wrong. Let's evaluate more closely the meaning of TLLTD-tire lateral load transfer distribution. Stated another way, we might describe TLLTD as the relative demand of side-to-side energy control that is placed upon the tires. Because a firmer anti-roll bar allows less deflection, it will transfer side-to-side energy (lateral loads) at a faster rate.

As the rate of lateral load transfer increases, additional demands are placed upon the tire. So if we install a firmer anti-roll bar in the front, then we increase the distribution of lateral load transfer toward the front tires. This increases the front TLLTD value, which will result in additional understeer, holding all else constant.

The same logic also holds true in the rear. A firmer anti-roll bar in the rear will increase the rate of lateral load transfer, placing more demand upon the rear tires, accelerating lateral traction loss and creating more oversteer, holding all else constant.

This is why blindly adding parts to your car may not produce the desired results.

I Want a 50 Percent TLLTD On My Car, Right?
Since on paper a 50-percent TLLTD indicates a balanced chassis, many enthusiasts are tempted to jump to the conclusion that this is therefore desirable. They may think that all cars should obviously come this way from the factory. Unfortunately, this is not the case-and the considerations are not that simple.

In reality, a car with a 50-percent TLLTD is literally on the constant brink of oversteer. And there are many factors that can quickly and easily take the car from the brink into a full-scale, out-of-control, spinning-in-circles disaster.
For starters, consider the effects of weather conditions that might create a wet or icy road surface. Or imagine that the driver happens to apply too much brake late into a turn-a common mistake among novice drivers. Or consider the effects of varying tire temperatures, tire pressures, or tire wear-all of which will have major impacts upon lateral traction thresholds. And of course, varying weight distribution, as a result of changing fuel tank levels, passengers, or the number of subwoofers in the trunk, will also impact TLLTD.
With all of these things to consider, automotive design engineers are forced to create a more conservative TLLTD. As a result, they intentionally target higher front TLLTD values so that stock vehicles will be prone to understeer-the assumption being that understeer is safer and more predictable for the average driver.

As a general rule, an average street-driving enthusiast is probably willing to accept some compromises-within reason-of a more aggressive TLLTD in exchange for better handling. A suitable target is probably a front TLLTD value of approximately 58 percent, a value that is considered aggressive, but suitable for street driving.

How do I Create the Right Handling Balance?
Since most enthusiasts do not have the knowledge or software needed to calculate chassis characteristics such as TLLTD, the responsibility falls upon knowledgeable tuners.

Obviously, TLLTD and body roll will both be affected by changes to springs and anti-roll bars. While understanding the effects of multiple changes can get confusing, the answer is usually only a phone call away.


Robert
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[russell_ram]

Robert,

Busy again today I see.

Russ

[robert]

Russ

I have 3 tenders to read and mark, however really don't want to do it, so I am hoping they will go away.

Fat chance.

Robert
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[nbracken]

Rob, I have read all, and understand some of the above. I conclude my spring rates are still too soft....

N

[robert]

Nigel

What spring rates are you running,how heavy is the car, what is the weight distribution?

Robert
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[wilf]

Ok - my "feel" for my own car is that the rear rolls more than the front, giving some interesting handling characteristics at/past the limit, although this really only manifests itself on the track. So if I fitted (although I have no idea how) a roll bar at the rear, what would this do? Or is it dependant on the specific car?

remember I am a dunce when it comes to suspension, so don't use any long words worse than "the" or "a".

Wilf

[Mole]

Just a few thoughts if they're of any use...

...Adding (or stiffening) a front anti-roll bar will probably increase understeer so the car will plough on at the limit BUT initially it is likely to reduce it. When the tyre is generating no lateral grip (going straight ahead)it is obviously doing no work sideways (except for maybe a tiny bit "fighting" the other front tyre if you're running lots of toe-in or toe-out). As you start to turn in, weight transfers from the inside to the outside AND (but not really "because") the car rolls a bit. The outside tyre now starts to generate more lateral grip. Tyres tend to generate no grip (lateral or otherwise) if there's no load on them. As the load increases, they grip better and better until they're overloaded or "saturated" as the American book puts it. At this point they start to loose grip.

So I guess what I'm saying is that in practice, a front anti-roll bar is likely to make the car "turn-in" a bit sharper and initially REDUCE understeer but as you approach the limit, they overload the outside tyre and INCREASE understeer.

Obviously, rear anti-roll bars do the same the other way round.

If you're running bars (and suspension) of different stiffnesses at both ends, you need a torsionally stiff chassis otherwise the chasis will just twist a little down its length to try and even the loads out between the outside rear and outside front tyres. Effectively, this means that both ends of the car will roll by different amounts. The original Cobra Chassis doesn't look particuarly stiff but neither is a Morgan (you can actually see them twist as you drive them) and they handle OK (ish). I think possibly they have about the same amount of roll stiffness at both ends so the chassis doesn't need to transmit torsion down its length.

I think talking about "steady-State" handling is a bit of a red-herring because on the road, there's no such thing. Even if you're settled into a long sweeping bend on a constant throttle there's usually small bumps all the way round. As far as transient handling is concerned, the dampers seem to make as much difference as anything because as you initially pitch the car into a bend, it momentarily "props" itself up on its outside dampers if the bump settings are hard enough. This has the same effect as an anti-roll bar (initially increases tyre "bite") but obviously doesn't have the same effect on the limit.

Hope this helps.

[robert]

Another issue is the damper settings and tyre pressures, as these variables can have a major major impact on understeer/oversteer whatever.

It is all down to set-up of a car, if you get it right, it is a dream, get it wrong and it is a nightmare to drive.

Robert
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