Antiroll bars
cj440 - 23/8/15 at 08:45 PM
I see images of Westfields, some have antiroll bars some dont, is there pros & cons?
CJ.
Sam_68 - 23/8/15 at 09:20 PM
Yes, very much so.
Anti-roll bars are often a necessary evil, required to limit body roll and (far more importantly) to balance the ratio of front:rear roll
stiffness, hence controlling oversteer/understeer balance.
But they degrade grip and worsen ride, because they act to link one wheel to the other - so that a bump at one wheel is partially transmitted to the
wheel at the other side of the car.
As a general rule, you should fit the lightest anti-roll bars you can, commensurate with achieving the right chassis balance, and get as much of the
roll resistance as you can via the main road springs.
phelpsa - 23/8/15 at 10:20 PM
quote:
Originally posted by Sam_68
Yes, very much so.
Anti-roll bars are often a necessary evil, required to limit body roll and (far more importantly) to balance the ratio of front:rear roll
stiffness, hence controlling oversteer/understeer balance.
But they degrade grip and worsen ride, because they act to link one wheel to the other - so that a bump at one wheel is partially transmitted to the
wheel at the other side of the car.
As a general rule, you should fit the lightest anti-roll bars you can, commensurate with achieving the right chassis balance, and get as much of the
roll resistance as you can via the main road springs.
There's lots of half truths in this info.
Anti roll bars are often necessary, but not at all evil.
They link one wheel to another, but so does a corner spring (via the chassis).
What you're looking to do essentially is control roll and pitch to a certain set of limits, which is usually defined by tyre performance under
camber change for this type of car along with obvious physical constraints (you don't want to be running on the bumpstops).
Seven type cars tend to have a much longer wheelbase than track, hence they tend to require much more roll stiffness than pitch stiffness. Some of
this additional stiffness is given by the fact that the geometric roll centre tends to be higher than the geometric pitch centre, however this is
rarely enough.
So... you apply an anti-roll bar to give the additional roll stiffness required. An anti-roll bar will effect the single wheel bump stiffness much
less (in the region of half) than adding equivalent stiffness to the corner spring. Plus the damper rates required to control the lower wheel
frequency will be lower.
phelpsa - 23/8/15 at 10:23 PM
quote:
Originally posted by cj440
I see images of Westfields, some have antiroll bars some dont, is there pros & cons?
CJ.
So in answer to your question... who knows! As Sam said, being able to adjust the roll stiffness balance front to rear easily is a definite pro, but
if your springs are so stiff already that their overall stiffness contribution is negligible then they going to make sod all difference.
Give a bit of background on your car and current setup to one of the reputable Westfield specialists and they can probably make some pretty educated
suggestions.
Sam_68 - 23/8/15 at 11:12 PM
quote:
Originally posted by phelpsa
Anti roll bars are often necessary, but not at all evil.
They link one wheel to another, but so does a corner spring (via the chassis).
Not at all in the same way. The ARB effectively bypasses the chassis, and thereby the inertia of the sprung mass.
I stick by my statement that they are a necessary evil: there is no way that any rational suspension designer would link across the wheels in the way
that an ARB does, if the weight transfer management and roll-limiting functions of the ARB could be achieved in another way.
The direct linkage across the wheels is a bad thing in an absolute way; in no scenario can it be considered beneficial. No matter what other
benefits an ARB offers, this factor alone means that it will degrade mechanical grip.
With regard to damper stiffness and single wheel bump rates: if the damping is calibrated correctly (and I appreciate that's a BIG
'if', in terms of the typical Seven/Locost type car), then the high velocity damping will be matched to the single wheel bump rate,
regardless of whether that rate is achieved solely by the road springs, or by road springs in combination with ARB. And the damping stiffness in roll
would be matched to the low velocity damping, again regardless of whether the roll stiffness is achieved by road springs alone, or in combination with
ARB's.
More realistically, with the less sophisticated dampers available within the average Locoster's budget, the bigger the discrepancy between roll
rate and ride rate caused by the use of an anti-roll bar, the more of a compromise the damper setting will be.
But I think we can safely assume that we're drifting into discussion of complexities that will completely befuddle anyone who needs to ask the
basic question of whether ARB's have pros and cons... hence my quite simplistic initial response to the OP.
ETA: All the above applies to conventional, torsion bar type ARB's, of course.
There are arguably more sophisticated solutions, though they have yet to catch on in a big way.
[Edited on 24/8/15 by Sam_68]
britishtrident - 24/8/15 at 07:33 AM
quote:
Originally posted by Sam_68
Yes, very much so.
Anti-roll bars are often a necessary evil, required to limit body roll and (far more importantly) to balance the ratio of front:rear roll
stiffness, hence controlling oversteer/understeer balance.
But they degrade grip and worsen ride, because they act to link one wheel to the other - so that a bump at one wheel is partially transmitted to the
wheel at the other side of the car.
As a general rule, you should fit the lightest anti-roll bars you can, commensurate with achieving the right chassis balance, and get as much of the
roll resistance as you can via the main road springs.
Hmm -- not really
Anti-roll bars have a lot of advantages, all major manufacturers use them because they improve ride quality when compared to controlling roll by
increasing wheel rate by fitting stiffer springs. In addition front anti-roll bar allows a reduction in front spring rate which lowers the front
suspension frequency which reduces pitching when the car crosses a bump. It is desireable to to have the front suspension frequency lower than the
rear suspension frequency with the aim the car remains more level as the front and then rear wheels traverse the bump this avoids that "bucking
bronco" feeling due to changes in pitch.
The big adavantage of an anti-roll bar for a home builder or racer is they allow quick adjustment of the front-rear oversteer/understeer balance
without changing springs or roll centre heights.
On any front engined rwd car a front anti-roll bar is a very good idea, but fitting a rear anti-roll bar needs a more cautions approach. The rear bar
on a front engined rear drive car should always have significantly lower stiffness rate than the front bar.
On a very light car like a Locost or Westie a front anti-roll bar needs to be fairy low stiffness to allow the springs to work. The negative effects
of what happens if too stiff a bar is used can be seen on some Robin Hood models where the the Sierra anti-roll bar was used which was much too stiff
and removed any independent movement in the front suspension.
Because anti-roll bars restrict axle articulation off road vehicles are another area where anti-roll bars are sometimes not the best solution.
[Edited on 24/8/15 by britishtrident]
phelpsa - 24/8/15 at 09:43 AM
quote:
Originally posted by Sam_68
quote:
Originally posted by phelpsa
Anti roll bars are often necessary, but not at all evil.
They link one wheel to another, but so does a corner spring (via the chassis).
Not at all in the same way. The ARB effectively bypasses the chassis, and thereby the inertia of the sprung mass.
I stick by my statement that they are a necessary evil: there is no way that any rational suspension designer would link across the wheels in the way
that an ARB does, if the weight transfer management and roll-limiting functions of the ARB could be achieved in another way.
The direct linkage across the wheels is a bad thing in an absolute way; in no scenario can it be considered beneficial. No matter what other
benefits an ARB offers, this factor alone means that it will degrade mechanical grip.
With regard to damper stiffness and single wheel bump rates: if the damping is calibrated correctly (and I appreciate that's a BIG
'if', in terms of the typical Seven/Locost type car), then the high velocity damping will be matched to the single wheel bump rate,
regardless of whether that rate is achieved solely by the road springs, or by road springs in combination with ARB. And the damping stiffness in roll
would be matched to the low velocity damping, again regardless of whether the roll stiffness is achieved by road springs alone, or in combination with
ARB's.
More realistically, with the less sophisticated dampers available within the average Locoster's budget, the bigger the discrepancy between roll
rate and ride rate caused by the use of an anti-roll bar, the more of a compromise the damper setting will be.
But I think we can safely assume that we're drifting into discussion of complexities that will completely befuddle anyone who needs to ask the
basic question of whether ARB's have pros and cons... hence my quite simplistic initial response to the OP.
ETA: All the above applies to conventional, torsion bar type ARB's, of course.
There are arguably more sophisticated solutions, though they have yet to catch on in a big way.
[Edited on 24/8/15 by Sam_68]
Sam, not necessarily disagreeing with you on the physics. In the ideal ride scenario of large sprung mass, small unsprung mass and low corner
stiffness you are spot on. However the smaller the ratio, the more of a blur it becomes. In reality the body displacement under single wheel bump is
far from negligible and affects the load distribution across all 4 tyres quite considerably, especially when (as you rightly pointed out) dampers may
not be that we'll suited to the application.
It may not be that relevant to the OP, but still an interesting topic to discuss!
coyoteboy - 24/8/15 at 10:43 AM
We have the ideal person to ask already on the forum....
Of Race Car Vehicle Dynamics fame...
Sam_68 - 24/8/15 at 06:36 PM
quote:
Originally posted by phelpsa
Sam, not necessarily disagreeing with you on the physics. In the ideal ride scenario of large sprung mass, small unsprung mass and low corner
stiffness you are spot on. However the smaller the ratio, the more of a blur it becomes. In reality the body displacement under single wheel bump is
far from negligible and affects the load distribution across all 4 tyres quite considerably, especially when (as you rightly pointed out) dampers may
not be that we'll suited to the application.
Likewise I'm not disagreeing with your comments... I think we perhaps just differ a little in terms of the degree of emphasis we place on the
various factors. And when I say that I think ARB's are a necessary evil, you need to read the word 'necessary' as well as the word
'evil'. 
And I agree absolutely that body displacement under bump is a very significant factor with lightweight cars - far too many people seem to regard the
sprung mass as an immovable object with infinite inertia, which it most certainly is not!
But conventional ARB's don't help counter that problem, either: if anything they add to it. It's interesting to muse on the
potential of shuttle-type ARB set-ups, which deflect the opposite wheel down instead of trying to pick it up, when they hit a single wheel
bump.
I think you have to sit back and consider why you need the advantages that ARB's confer in the first place, though:
* You only need a small amount of adjustability in terms of diagonal weight transfer, if you get the basic understeer: oversteer balance right
to start off with.
* Body roll is quite limited on a car with such a low CoG as a Seven, and you need to balance the other advantages of limiting roll (more favourable
camber control and reduced jacking) against the degradation in grip caused by ARB's and the increased roll stiffness required to counter it. The
latter can quite easily cancel out the former, if you're not careful, particularly once you get away from the nice, smooth surfaces of race
tracks.
I wouldn't deny for a moment that the balance is in favour of ARB's for competition, but on the public road, with a car as light and low as
a 'Seven', my personal feeling is that the bias tips the other way. I prefer a more compliant set-up, with which ARB's are not
necessarily a requirement.
FERRARIST - 28/8/15 at 08:43 PM
My humble experience with roll bars:
I make pictured below mid-engined single seater with Alfa 166's 3.0v6 engine. No even single drawing on paper or design software - it was in my
mind only. Well, i spend many nights with suspension calculators.
690kg, very light front end, each front wheel supports 105kg, each rear - about 240kg.
Previously same engine was on my trackday Alfa 156.
I was really impressed to feel 300kg less weight when i drive it on track for first time, but was shocked how much understeer it had.
4 new GAZ shocks, 22mm ARB in front, no rear ARB.......260lbs front springs, 460 rear's........
Had 1-2 trackdays in order to get use to it, handling, balance and so on.....it was all new for me......cure few problems but same amount of
understeer remains.......and becomes even worse when apply throttle on corner exit.
Then i changed front springs to 160lbs, rear's a little bit softer - 380lbs. And most important - front ARB from 22 to 26 - huge difference, not
even slight understeer until corner exit(trial braking used as prefered style), however there still amount of understeer when throttle applied on
corner exit.....but not as before.....
Fitted again old 22mm ARB - worse.
There is one so called "modern" type of suspension setup - SS - BB - soft spring - big bar - it works for me for now.
I'm also support the idea that "competition car can pitch a lot & roll a little … OR … pitch a little & roll a lot"
Forgot to mention - less spring in front give me bit more pitch and braking improves as well....
But as i said - it's just my experience, and it may not work on any car.
Description
[Edited on 28/8/15 by FERRARIST]
MadMaxx - 11/10/15 at 10:00 AM
Which dimensions for an ARB for a BEC Fury of 424 kg with front 180 lbs and rear 130 lbs springs?
mark chandler - 11/10/15 at 10:14 AM
Impossible to answer unless you know the effective ratio of the shocks, how much shock movement compared to wheel movement.
My BEC around 450kg has no ARB, 400lb front springs with an effective ratio of maybe 1.25:1 with rear 350lb springs 1:5:1
Live axles tend to have 1:1 ratio as the shock is vertical, IRS you have wishbones so the shock becomes a lever.
MadMaxx - 11/10/15 at 02:30 PM
quote:
Originally posted by mark chandler
Impossible to answer unless you know the effective ratio of the shocks, how much shock movement compared to wheel movement.
My BEC around 450kg has no ARB, 400lb front springs with an effective ratio of maybe 1.25:1 with rear 350lb springs 1:5:1
Live axles tend to have 1:1 ratio as the shock is vertical, IRS you have wishbones so the shock becomes a lever.
The Fury is an IRS and actually stripped down to fully re build the chassis and I'm evaluating if to add an ARB having some opinion from other
owners on a tested solution or have some indication about how to dimension it related to the rigidity of the shocks. Should the ARB be more or less
rigid of the shocks? How much?
The effettive rigidity of the shocks could be calcoulated once I will put together both front and rear suspensions to misure them.
AdamR20 - 12/10/15 at 08:55 AM
A general figure I have seen mentioned (and one I have employed on my car with good results) is that the rear ARB should contribute around 15% of the
overall roll stiffness for that axle, and the front approx 50%.
[Edited on 12/10/15 by AdamR20]
