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Why use front and rear ARBs

Why use front and rear ARBs

Why use front and rear ARBs

(OP)
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.

RE: Why use front and rear ARBs

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Why use front and rear ARBs

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---

Quote:

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

RE: Why use front and rear ARBs

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.

RE: Why use front and rear ARBs

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

RE: Why use front and rear ARBs

(OP)
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?
 

RE: Why use front and rear ARBs

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
 

RE: Why use front and rear ARBs

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).  

RE: Why use front and rear ARBs

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
 

RE: Why use front and rear ARBs

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

RE: Why use front and rear ARBs

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
 

RE: Why use front and rear ARBs

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?

RE: Why use front and rear ARBs

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

RE: Why use front and rear ARBs

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Why use front and rear ARBs

Greg

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

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers for professional engineers
 

RE: Why use front and rear ARBs

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Why use front and rear ARBs

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
 

RE: Why use front and rear ARBs

Quote:

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


 

RE: Why use front and rear ARBs

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Why use front and rear ARBs

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

Each wheel ratio for respective axle?
Goran

RE: Why use front and rear ARBs

I have to say hard cars low cars feel better on a smooth surface.

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers for professional engineers
 

RE: Why use front and rear ARBs

yes the maximum grip for a non aero car without any geometry issues with lots of wheel travel usually means soft springs bars...BUT race tracks/roads quite often have chicanes or turns after each other in quick succession, where a quick change of direction is desired. hence why it is best to aim for a reasonable roll per G rate as it correlates to weight transfer rate (ignoring damping and dependant on inertia) so we are back at stiffer ARBs/springs

RE: Why use front and rear ARBs

McPherson struts, Goran.  Front spring rate was 650# and the front bar was 600#/inch non adjustable. Rear was live axle, coil over @ 450# springs and 75#/inch nominal, but adjustable to higher rate. 1778 lbs. semi dry wt. (had 12 qts oil and some ballast. I don't recall how much as it varied from driver to driver. SCCA driver wt. is "180". At the time (early 80's) Dennis weighed 200+ and I weighed about 165.

Now, on our Lotus Cortina, I have 300 front/Koni/250 bar and ~150 rear leaf springs/ Panhard bar/ Koni/75+ bar. Car weighs ~1900 wet with 175hp Evelrod/Omnitech twincam. Decent race/show car with my 120 lb. son in it. Not a beater.

Greg, it's a lot easier to design it right in the first place, I guess.  Making road cars into race cars requires many compromises not leading to the optimum setup. Certainly the Mini Cooper was, and still is, a challenge.  Actually it is quite good and is probably the safest racing saloon I have ever driven.  It has saved my ass several times where in a Cortina, Alfa or, BMW I would have been toast! It's not the fastest Mini but it's a loooong way from being a "back marker". Ck the stats on mylaps.com. Several forum members have stopped by at the track and I would be happy to chat with any member, time permitting.

The way I have the suspension (oxymoron?) set on the Mini, the front upper control arm only has one inch between the bump pad to the mounting pad for the bump stop which is removed.  In a hard corner at speed things hit and the rate goes off the chart.  I need the rear bar to rotate the rear properly.  Without it the thing pushes like a plow.  It sounds terrible, but it seems to work. Mostly, no one even notices. Vintage rules don't allow much and I already bend the rules to the breaking point.

Rod

RE: Why use front and rear ARBs

"New Postpatprimmer (Automotive)      
5 Jan 09 17:46
I have to say hard cars low cars feel better on a smooth surface. "

We're dealing here with response as well as total grip. A softer car will have more grip, but be harder to drive, because of the movement.

Ben

RE: Why use front and rear ARBs

Just to add some more counter intuitive swaybar stories, my blasted, yet beloved BMWs..

I've found that a big front sway bar on an E30 or E36 will actually combat understeer by limiting roll and compression and thus keeping out of the ugliest places of the camber curve.

Actually, same with VWs (particularly MK4s), you need to raise them above factory ride height to get into a reasonable part of the camber curve! wretched things...

RE: Why use front and rear ARBs

Yup, as Chapman meant to say, even a bad suspension can be made to work if you don't let it move.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Why use front and rear ARBs

Works for me, Greg!

Rod

RE: Why use front and rear ARBs

Just to add a surprise I found by accident.
I had taken the front sway bar off of my 66 Mustang and mounted it on the rear.
Although it was my second mustang build time got away from me and I was unable to put the bigger bar on the front.
I drove it a couple of weekends and was surprised how well the rear only kept it from rolling up on the outside front tire.
I forgot to mention that it had 600# springs in the front and 250# center eye in the rear.
I didn't leave it that way, but it was an interesting find.
Later I put a small ARB on the back of a Ford van and it really made a difference...  

Cheers

I don't know anything but the people that do.

RE: Why use front and rear ARBs

Yup, as Chapman meant to say, even a bad suspension can be made to work if you don't let it move.  CheersGreg Locock

This above means stiff springs, not stiff ARBs.

In real life no suspension setup is perfect, there are always compromises. ie camber will roll off on the ourside, or you will have roll steer on the rear axle,etc.

also if your car is as soft as physically possible to just about bottoming out you might have maximised contact patch load variation, but that doesnt help much when you cant get your tyres up to temp to generate any grip :)

 

RE: Why use front and rear ARBs

Some comments from an old timer:

Fifty years ago, the goal was to simply not have any sway bars. The first test cars of a new design went to the proving ground "naked," where the trunks were overloaded and they were put through a series of handling tests. If "problems" were encountered, a front sway bar was added. And, of course, the bean counters would then complain about the extra cost.

I remember a '59 model from one of the big three was scheduled to begin the proving ground tests when a VP showed up, asked about the test, and declared that he would conduct the test himself. He drove it one lap around the high speed oval and declared that it did not need the sway bar. Bootlegged tests later indicated that it definitely should have had one.

One other thing: I know it will never happen, but it surely would be nice if we could all speak the same language. An anti-roll bar (ARB)...to a drag racer...means something entirely different from that which I was trained to call a "sway bar" or "anti-sway bar." To the drag racer, an ARB is a REAR sway bar with a ridiculously high rate and adjustable linkage. The adjustable linkage is used for static preloading and the high rate brings more of the driveshaft reaction torque back to the rear wheels as an aid in equalizing rear tire loads. (This is, of course, for a RWD car with beam rear axle, which is still so common at the drag strips.)
http://home.earthlink.net/~whshope
over 130,000 visitors

  

RE: Why use front and rear ARBs

Thanks for that link.  So often the young folks ask questions that I cannot give an appropriate 'non complicated' answer.  The link should satisfy all general questions...especially 'drag racing' which is the most common form of automotive racing in my neck of the woods.  I did a bit of drag racing in the mid 1950's and it was great fun (for me, not my parents)! ;o)

Rod

RE: Why use front and rear ARBs

Lotus elise and exige as well as the Mclaren F1 have only a front sway bar.  I know for a fact that the F1 has relativly soft springs and both cars use slightly wide tires in the front based solely on weight distribution, and especially considering the very high power to weight ratio of the F1.  Maybe this is to take some of the extra load from only having the front bar without getting unbalanced...  So I guess its possible to even things out with a Front sway bar only if you are willing to have wider front tires to balance understeer.

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