Why use front and rear ARBs

[colinmseries]

This is something I've not been able to resolve from reading. My understanding is that an ARB adds roll resistance to that already provided by the wheel springs/bars and so allows a better compromise of wheel spring/bar rate. I realise that it adds to the overall rate at a particular corner in bump but does not do so in pitch or dive.
Assuming most practical vehicles require more roll resistance at one end than the other, why do I see production cars using an ARB at both ends? Why not optimise the wheel spring/bar rates and add a single ARB to the end needing more help in roll?
I have Milliken and a couple of other books I have seen you recommend to others. Thankyou, Colin.

 

[GregLocock]

A good question, and I have some sympathy for your position, but in practice have not been able to get rid of many sta bars.

Fundamentally, the ride guys set the spring rates on production cars, and the handling guys set the roll rates. You can't use RCH in place of sta bar, it has the wrong time constant. A combination of RCH and shock tuning might work, but that impacts on ride again.

Also in some markets there is a customer/journalist perception that a good suspension has a sta bar.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

From a racers standpoint, bars just give us one more thing to tinker with. A necessary evil. (Like as if I don't already have enough thrashing about in my head)!

BUT---

Also in some markets there is a customer/journalist perception that a good suspension has a sta bar.

By golly, I think you've got it!

Rod

 

[BrianPetersen]

Theoretically, you can achieve any desired roll stiffness using spring rates only at all four corners (the front and rear springs need not be the same rate and usually aren't) ... but most likely the spring rates would be so high that the ride quality will be abysmal.

Antiroll bars have no effect in a two-wheel-bump situation and allow compliance based on the (softer) spring rate alone. In a one-wheel bump, since only one end of the ARB is moving, it still acts less stiff than a spring of a rate that you would have to use to achieve the roll stiffness using springs alone.

Having said that, there are plenty of vehicles that lack an antiroll bar at either the front, the rear, or at all.

 

[Bribyk]

Most design judges in FSAE weren't even interested in your suspension if you didn't have ARBs.

 

[colinmseries]

Just a thought you might comment on: After examining the underside of a friend's late model TVR which had a fixed front ARB and an adjustable rear one, was that perhaps they had built in a very high front roll stiffness and left the owner to play with rear bar to redistribute the effects?
Certainly the car was not bought for it's ride qualities but it occured to me that perhaps that was what they were up to, although I am still wondering in that case why not just make the front bar adjustable and dispense with the rear?

 

[hemipanter]

Q
1.
This is something I've not been able to resolve from reading.
2.
My understanding is that an ARB adds roll resistance to that already provided by the wheel springs/bars.
3.
I realize that it adds to the overall rate at a particular corner in bump but does not do so in pitch or dive.
4.
Assuming most practical vehicles require more roll resistance at one end than the other,
5.
Why do I see production cars using an ARB at both ends?
6.
Why not optimize the wheel spring/bar rates and add a single ARB to the end needing more help in roll?

A

1.
Many books describes how to calculate things but leave "what and why" to calculate things out.
2.
Right.
3.
You are on the right track to answer your Q here.
4.
The car has a longer wheelbase than track width. This makes the car having a greater deflection in sideway roll than in pitch roll, for the same G-load. Therefore we should add ARB to a rate of the relationship Tw/Wb.
5.
When the car is subject to braking forces, we like it to do so in a straight line. I other words, we need a uniform side to side spring setting.
6.
As for side roll, we are using the ARB even as an over under steering balance tool, which is a big difference.
The Tw/Wb gives us an ARB rate number that is to be added.
This number has then to be distributed between the front and rear axle in the right proportion.


Regards
Goran Malmberg

 

[JohnLear]

I suspect the main reason why a front ARB might usually be used on FWD cars is to create understeer (i.e. even more understeer...).

I've completely removed the front ARB on a couple of my FWD cars (road cars), and the handling improved markedly in both cases (significantly less understeer), with surprisingly little increase in body roll motion, despite the front ARB being nominally the stiffer of the two ARBs (though rear ARB stiffness was also increased to some degree).

 

[hemipanter]

A manufacturer will many times compromise what I state as a starting point to reach their goals. I had a job sorting out the suspension of a Viper sportscar. These cars have a fairly good ride compared to their cornering capability.
What they did is to add a great deal antisquat and antidive to handle braking and acceleration forces and large swaybars together with a balanced antiroll for the corners. That way the car becomes relative soft in heave for a smooth ride while still being able to handle hard driving.
If looking at the spring wheelrate, together with roll wheelrate, of such car it will appear a bit confusing if not taking the anti’s in to account. Not to forget the complex spring motion ratio involved for the car in question. The Corvette C6 is using a similar technique.

It may look like this is a way to have the cake while eating it. It is not, even if it is a great setup for a street sportscar. Both acceleration and braking show backsides using other type tires or in special track situations. There is also possible to bottom the suspension out in compression sections.

This was just to show ONE special situation for selecting special swaybars.

Regards
Goran

 

[colinmseries]

Thank you all for your responces, I think I have a better appreciation, Colin.

 

[Yawing]

The reason for using both front and rear antiroll bars is because they are probably after a certain total roll stiffness of the vehicle (set by comfort/performance targets), yet need to tweak the load transfer distribution to set the understeer gradient (balance). quite often these 2 events cant be satisfied with only using 1 ARB on either end of the car

 

[bvanhiel]

My unerstanding, and it's one that I'd love to have corrected if I'm too far off, is this:

Springs are mostly used to tune ride by targeting a specific harmonic frequency based on the rate and unsprung mass. That frequency is low in street cars (~1Hz) and higher in race cars (~2Hz+). ARB's can then be used to tune the amount of body roll and the weight shift from the front to rear outside tires in a turn.

So let me test my thinking... if I wanted to convert an everyday sedan into a race car I would first quadrouple the spring rates to get the natural frequency up to the 2Hz range. Then I'd put on F/R ARB's to keep the body roll managable at my anticipated turning g's. And finally I'd tweak the ratio of front and rear bar rates to get the understeer/oversteer balance of the car. Sound right?

 

[bvanhiel]

That should be "sprung mass" not "unsprung mass". And here's the "d" that got away...

 

[GregLocock]

Well that's certainly a reasonable approach.

On the other hand, who cares how much the car rolls on a racecar (non aero), so long as the suspension geometry works in roll.

The fundamental requirement of a suspension is that it should move vertically, adding stiff springs and what-not PREVENTS it from doing its job.

An approach I'd think about is that to maximise the grip from each tire (on a non aero car) we should fit the very softest springs we can get away with, and then add the weakest anti roll bars available to give us a supension that in the course of one lap sees every wheel just move through its entire desirable (weasel word alert) travel. These low rates (and zero antidive and so on) maximises the grip of the tire because we aren't upsetting it by thumping it around.

cat...pigeons

Of course if you have lousy suspension geometry, or aero, then you have other priorities.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Greg

Off road racers certainly use that appoach, though quite probably for different reaons.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[GregLocock]

The way I look at it is rather than thinking about the friction circle for the tire, what is the friction circle for the CAR? It is the GGV plot, and most of the shape of that plot at a given speed is just the sum of the friction circles for all the tires.

The reason I think it works is to come back to Goran's zero car, and the other truism that very little really matters except the horizontal forces at the contact patch.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hemipanter]

As Greg states, softest possible springs should be used for best grip. But it takes a higher ride height not to bottoming out
For a race car we are looking for a low cgh to minimize weight transfer. We must therefore lower the car as much as possible. This calls for a compromise.
The track is not flat and it has straights and turns. This will dictate how much spring stiffness-travel the car will need in heave, pitch and roll. And the limit is ground contact in the different directions.

For a road car there is quite other criteria’s like comfort that set spring stiffness, not to mention the shocks that is almost the reverse in extension-compression dampening. And the fact that a road car is not changed in setting for each and every ride to use spring travel in the most efficient manner all the time.
Goran

 

[evelrod]

On the other hand, who cares how much the car rolls on a racecar (non aero), so long as the suspension geometry works in roll.

I don't often disagree with Greg, but---

I've raced several cars over the years from several different owners, each of which had his own idea of how to set up the suspension. I am of the "EBJ" persuasion (eye ball jiggler). The stiffer and lower the better. Soft suspension is easy to handle for the novice and you can get to 8/10ths easily...the last 2/10ths is a tough call, even for an expert. There are times that call for a softer suspension setting, rain, off road (although I have never raced off road in a four wheeler) and, perhaps, a very rough track. I don't like soft suspension with the attendant roll...scares me!

An example, I helped set up, and later drove, a semi tube-framed Lotus Cortina that was a mid western national champ two years with a 1250 lb. front and 525 lb. rear "wheel rate" and all up race wt. including driver of 2030 lbs. with a 205hp David Vegher/BRM twincam. The car was a dream to drive...you could put it anywhere you wanted just by "thinking it there" ! This car was definitely NOT a car for a novice, but in the right hands---

Additionally, my Mini is set up "stiff"...can't say the rate, the 'springs' are very old rubber cones...Rear bar but NO front bar...However, the front is setup to "go infinite" in a corner...Not all that unusual in a racing Mini, I am told. It works well for me.

Rod

 

[GregLocock]

Aye, but if you were designing a race car from scratch you wouldn't end up with a Mini.

I agree that in practice race cars end up with stiffer springs than road cars, but that is not to get more grip, that is to cope with other problems. In your case you have increased the roll stiffness at the rear to get rotation at the apex (which is a nice feeling, but a rotating car may not be the fastest round the track) and to help keep the front tires square to the road without increasing the front roll stiffness excessively, so maximising traction and braking.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hemipanter]

1250 lb. front and 525 lb. rear "wheel rate".

Each wheel ratio for respective axle?
Goran