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MGB Sway Bars (anti-roll bars)
Their purpose and use on the MGB
by Gary Pegg

Their purpose
Lloyd Faust quotes Lotus creator and engineer, Colin Chapman, when discussing the purpose of sway bars: "The springs are there to prevent suspension bottoming under severe braking, if they do this, they are stiff enough.......control body roll with bars".

So why the discussion of sway bars on MGBs?
This discussion, of course, is relevant to all MGBs, but it is especially relevant to MGBs from the ’74.5 model year through the ‘76 model year for reasons we will discuss below.

In order to understand the discussion of sway bars on the MGB, we need to understand both their purpose and a design change brought about by US Federal regulations that necessitated a change in the MGB’s body design beginning with the 1974.5 model year.

The purpose of sway bars:
There are several things that affect an automobile’s handling characteristics. The most important are weight and ride height. An automobile’s weight must be offset by the correct springs and shocks in order to control body movement. An automobile’s ride height and weight affects both it’s center of gravity and the tendency for the body to “roll out” during cornering. It is easy to understand if you imagine a car whose body rises ten feet above the wheels of the vehicle. If that body weighs no more than a feather, the “roll out” caused by its weight will be negligible. If, however, that body weighs a ton, it is easy to imagine that the centrifugal force on that automobile would cause it to roll over in even the slightest of turns. Sway bars are used to limit the effect of centrifugal force on the car’s body; reducing body lift during cornering.

Why is this so important for the 1974.5 through 1976 model years?
When the MGB was first introduced, it included a front sway bar of 9/16th and its handling characteristics were very good. Due to financial considerations, however, sway bars were not included in the 1975 through 1976 MGBs. In addition, beginning in 1974, US Federal regulations were introduced that required that the bumpers on all automobiles sold in the US be of a standard height (set by Federal regulations) and be able to withstand a 5mph crash without damage to the vehicle. This necessitated an increase in the ride height of the MGB and the addition of the heavier rubber bumpers beginning the 1974.5 model year. This increase in both height and weight changed the car’s center of gravity and the effect of centrifugal force on the car. The adverse effect of the change was not compensated for until the re-introduction of sway bars beginning with the 1977 model year when, according to John Twist, all RB cars received 5/8th sway bars front and rear. The lack of a sway bar on the 1975-76 model year and the poor handling characteristics of the 1975-76 MGBs is what has principally driven most of the interest in three conversions to all rubber bumper cars: add-on sway bars, lowering of the ride height of the later cars and the conversion back to the original chrome bumpers (although for some the change to chrome bumpers is driven more by a consideration of the look of the vehicle rather than in consideration of its handling characteristics).

Their use on the MGB

As we noted earlier, the 1975-76 Rubber Bumper (RB) model's handling is significantly improved by the addition of sway bars with or without making the Chrome Bumper (CB) conversion and is a desirable modification for all RB models not equipped with sway bars. This does not mean that other models of MGBs cannot profit from a change in which sway bars are used and the thickness of the sway bars used in order to fine-tune a particular car’s handling characteristics.

It is important to point out, as both Lloyd Faust and Kelvin Dodd have pointed out in posts on, that an incorrect size sway bar on either the front or back can actually be detrimental to the car’s handling characteristics. And you should never increase the size of an existing rear sway bar without balancing that out with an increase in the size of the front sway bar. Although sway bars and kits are available for both the front and rear, the addition of only the rear sway bar (as noted by Bill Boorse) "will cause oversteer: the condition whereby the rear end breaks free and tends to travel toward the outside of the turn." The best handling is achieved when both the front and rear are considered when conversion is done. In cases where conversions have been done to both the front and rear in order to stiffen the cars handling to its maximum, a sudden loss of traction in the rear without the usual ability to feel any change ("no gradual feedback") occurs when the vehicle is pushed to the limit and on slippery roads. Some, who have done both conversions to their RB cars, have later removed the rear sway bar because of this characteristic. In some cases, this may be caused by the addition of the rear sway bar without adding a stiff enough front sway bar thereby unbalancing the car’s suspension.

Front sway bars
Although most 1974.5-76 RB cars did not have the mountings or holes for the front sway bars, the A-arms can be swapped out for earlier ones which did, or the existing A-arms can be modified by the addition of a gusset and hole. Kits are also available as aftermarket add-ons. Kelvin Dodd points out that “Since the MGB was originally fitted with a front sway bar that used solid end links, most non-original street sway bars follow the original pattern and have an arm length similar to the original and use the original links.”

Rear sway bars
Most 1974.5-76 RB cars also did not have mountings for rear sway bars. There are aftermarket kits available online for this conversion as well; some of which are adjustable, making fine-tuning possible. There will, of course, be discussions regarding adding the rear sway bar from later models to the earlier model cars. Although the sway bar assembly from the later RB cars can, in theory, be added to the earlier cars with certain modifications and some welding, as noted by Llyod Faust " order to mount the rear bar, the factory moved the fuel pump out of the way (into the trunk). The 77-80 bar will also not clear the battery boxes in the two battery cars. To mount a factory bar in a pre 77 not worth the effort.”


It is not only the size of the sway bar, but also the length of the arms that determines how stiff it is. Anyone who has ever used a “cheater bar” to break loose a stubborn nut, knows that the longer your lever, the less force you need to twist something in a particular direction. Shorter arms will not be able to twist the sway bar as easily as longer arms. This means that a sway bar with shorter arms will resist lift more than a sway bar with longer arms even if the bars are the same thickness! As Lloyd Faust said in one of his posts “one has to factor in the length of the lever arm causing the sway bar to ‘twist’…the longer the arm, the easier to twist the bar…you need to know both of the dimensions (bar diameter and lever arm length)…esp when dealing with aftermarket bars.” All of the add-on sway bars that I am aware of have shorter arms giving them a higher rate! What that means is that a 9/16th add-on might have the same rate as a stock 7/8th sway bar might have had if the factory had produced one; thereby creating a dangerous oversteer condition. As Kelvin Dodd noted “None of the aftermarket bars use the same mountings as the factory 5/8th rear bar.” (77-80) “The three that I am familiar with…mount to the corners of the boot floor and have pretty short arms making them a much higher rate than the factory bar.” “These aftermarket rear bars MUST be fitted with a higher rate front bar or the car will oversteer dangerously.” “Bolting one of these bars on while retaining the original front bar will cause disastrous handling.”

Please note that when adding a rear sway bar or any aftermarket kit, great care should be taken to insure that safe practices are observed when working. It is also important to note that incorrectly mounting these kits can create a dangerous condition if any part of the system should fail.

Kelvin Dodd has noted that “if a 1974.5-76 MGB is lowered using higher rate, shorter springs, there may not be a need to install (even) a front roll bar. The higher rate spring and lowered height may be all the owner needs for confident driving.” This is due to the fact that the lower center of gravity may be a sufficient deterrent to body roll-out to obviate the need for a sway bar for some drivers. However, to return the MGBs of these model years which have been lowered, to their once ideal handling characteristics, a front sway bar, at least, should be added. As Kelvin also notes: “Adding a front bar to a 1975-76 is an improvement.” and “If you do a lot of high speed cornering in a B, upgrade to a 3/4" front bar. If you really want to carve turns at high speed on the street, upgrade to a matched front and rear bar and increase damping appropriately.”

Bill Spohn provides the formulas for determining stiffness and comparing sway bars:
1. “If you want to know how one bar compares to another of the same design (when only the diameter of the bar changes) –

% increase in stiffness = (new diam./old diam.)^n

In the case of solid bars, n=4, so if you add a small amount in diameter you get a large increment in stiffness. Going from an 18 mm. bar to a 19 mm. bar, for instance, gives you a % increase of 23% !!”

2. “The other very important factor in how stiff your new bar will be is the length of the moment arm. Intuitively, it makes sense that a bar mounted with a long arm (i.e. in an MGA, with the bar mounted far forward on the frame horns) has more ‘give’ in the arms than a short bar mounted close in. The actual relationship for a solid bar is arithmetic, so if you have a ¾” bar that has 10” arms, it will only be half as stiff as one with 5” arms.

That is why it is meaningless to talk about diameter without mentioning arm length. A short armed 5/8” bar can be stiffer than a long armed ¾” bar, and when one car guy tells you “I needed to go to a 7/8” bar to get it stiff enough” he isn’t imparting much in the way of information unless he also gives you the length of the arms on his bar (properly measured from the centre of his bar to the centre of the attachment point of his link).”

Barney Gaylord of adds “Stiffness of a sway bar increases as 4th power of the diameter, and changes inversely proportional to the length of the arms. What is commonly overlooked is stress level in the bar. You can always make the bar thicker to be stiffer with less stress, but if you install a small diameter bar with short arms, you could break the bar from stress.”

I have included a chart showing the sway bars used for all years of MGBs. John Twist notes that “Nearly all the MGBs brought into the USA had sway bars from 1962 through 1974. The sway bar was dropped somewhere in the 1974/2 range, but was reintroduced on all MGBs front AND rear, 77-80. The sway bars supplied were 9/16th for the roadsters and 5/8th for the GTs on the earlier models.”

On most American market MGBs (except those sold in 1974.5-1976), the stock configuration is adequate for normal driving.

Note: I would like to thank Lloyd Faust, Kelvin Dodd, John Twist of University Motors and Barney Gaylord of for their assistance with this article in varying degrees as well as the many comments and insights from members here at Where I have quoted individuals such as Bill Boorst and Bill Spohn, I have noted the source in the article. (Gary Pegg – thepeggasus)


(Your MGB may not be equipped with sway bars even though a sway bar is indicated in the chart, as sway bars were optional in some markets.  Almost all American MGB’s were equipped with sway bars.  The factory size sway bars for each model year, when equipped, have been indicated in the chart below.)
Model Year Front as Originally Equipped Rear as Originally Equipped Recommendation
Cars 75-76
Recommendation All RB Cars with Modified Height Recommendation All RB Cars with CB Conversion Recommendation All RB Cars with both conversions
  Front Rear Front Rear Front Rear Front Rear Front Rear

9/16th None

63 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
64 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
65 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
66 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
66GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
67 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
67GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
68 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
68GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
69 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
69GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
70 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
70GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
71 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
71GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
72 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
72GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
73 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
73GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
74 9/16th None N/A N/A N/A N/A N/A N/A N/A N/A
74GT 5/8th None N/A N/A N/A N/A N/A N/A N/A N/A
75 None None 9/16th None N/A N/A N/A N/A N/A N/A
76 None None 5/8th 5/8th 9/16th 9/16th 5/8th 5/8th 9/16th 9/16th
77 5/8th 5/8th N/A N/A N/C N/C N/C N/C N/C N/C
78 5/8th 5/8th N/A N/A N/C N/C N/C N/C N/C N/C
79 5/8th 5/8th N/A N/A N/C N/C N/C N/C N/C N/C
80 5/8th 5/8th N/A N/A N/C N/C N/C N/C N/C N/C

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