Whiteline Jim
11-07-2009, 10:20 AM
Taken from a Whiteline Discussion Paper titled
"Hollow versus Solid Swaybars - Comparison of the 2 separate design strategies and how they differ in practical use and manufacture."
by Jakub Zawada BS Eng (2003)
The following are extracts from the above paper prepared for a discussion of this topic. The full version is attached to the post as a PDF.
Comparison of Hollow and Solid Swaybar Design
In this paper the differences between solid and tube swaybars will be discussed. Namely in the stress distribution under load, weight differences and solid tube equivalents.
The reason why tube is chosen over solid is for weight. Although for a one-piece design it is harder to implement. Through searching for examples of hollow swaybars I have found the most sound design to be that of a series 5 RX7 (93 – 01), where a tube is used as the bar and blades are attached by splines. These are the only ones that I found I could get ID and OD dimensions, most manufacturers just give OD and state they are hollow. Generally they are of imperial sizes, and most have common imperial wall thickness.
Assuming that bar rate is only effected by the torsional member or the bar (blades are stiff) then the rate can be calculated and an equivalent solid found. The following page shows typical OD and wall of tubes, solid equivalent, % solid OD to tube OD, and % weight increase by going to solid bar.
For example a 31.75mm (1¼ “) by 1.24mm (0.049”) tube bar is the equivalent of a 23.0mm solid bar, this solid bar has an OD which is 72.6% (23/31.75 x 100) of the tube OD, and an increase in weight of 71.5% over the tube bar. Another example is a 25.4mm by 6.35mm tube will act like a 25.0 mm solid bar as in torsion and bending (swaybar).
The sizes have been gained from a RACEtech stock list from British International Trading which deal in high strength materials such as “chromoly” and “aircraft grade aluminium” with all sizes coming in common imperial measurement.
Conclusion
Tube torsion members can be used successfully as swaybars with some benefit in weight reduction.
As an example, 2 individual, rate equivalent (approximately), common sized generic bars 1100mm long with 300mm blades, one solid 24mm the other tube of 25.4 x 3.96mm (1” x 0.156”) gives a weight reduction of about 2.5kg with about 200g of that as unsprung mass per wheel.
However the downside to these is the increase in stress levels for equal OD or rate, and therefore reduced strength. Also the complications seen in manufacture swaybars from tubular medium, and the importance of geometric control under bending make these more difficult and expensive to make. The attachment points at the blades can also be a problem with these swaybar designs.
The additional complications and downsides of hollow bars seem to outweigh their advantage in overall weight and unsprung mass, which can be seen to only be marginal in street car application. Even OE manufacturers rarely use this type of design in their swaybars. However with the right conditions and setups, they can bring some gain in racing situations, which can require weight reductions to the gram.
"Hollow versus Solid Swaybars - Comparison of the 2 separate design strategies and how they differ in practical use and manufacture."
by Jakub Zawada BS Eng (2003)
The following are extracts from the above paper prepared for a discussion of this topic. The full version is attached to the post as a PDF.
Comparison of Hollow and Solid Swaybar Design
In this paper the differences between solid and tube swaybars will be discussed. Namely in the stress distribution under load, weight differences and solid tube equivalents.
The reason why tube is chosen over solid is for weight. Although for a one-piece design it is harder to implement. Through searching for examples of hollow swaybars I have found the most sound design to be that of a series 5 RX7 (93 – 01), where a tube is used as the bar and blades are attached by splines. These are the only ones that I found I could get ID and OD dimensions, most manufacturers just give OD and state they are hollow. Generally they are of imperial sizes, and most have common imperial wall thickness.
Assuming that bar rate is only effected by the torsional member or the bar (blades are stiff) then the rate can be calculated and an equivalent solid found. The following page shows typical OD and wall of tubes, solid equivalent, % solid OD to tube OD, and % weight increase by going to solid bar.
For example a 31.75mm (1¼ “) by 1.24mm (0.049”) tube bar is the equivalent of a 23.0mm solid bar, this solid bar has an OD which is 72.6% (23/31.75 x 100) of the tube OD, and an increase in weight of 71.5% over the tube bar. Another example is a 25.4mm by 6.35mm tube will act like a 25.0 mm solid bar as in torsion and bending (swaybar).
The sizes have been gained from a RACEtech stock list from British International Trading which deal in high strength materials such as “chromoly” and “aircraft grade aluminium” with all sizes coming in common imperial measurement.
Conclusion
Tube torsion members can be used successfully as swaybars with some benefit in weight reduction.
As an example, 2 individual, rate equivalent (approximately), common sized generic bars 1100mm long with 300mm blades, one solid 24mm the other tube of 25.4 x 3.96mm (1” x 0.156”) gives a weight reduction of about 2.5kg with about 200g of that as unsprung mass per wheel.
However the downside to these is the increase in stress levels for equal OD or rate, and therefore reduced strength. Also the complications seen in manufacture swaybars from tubular medium, and the importance of geometric control under bending make these more difficult and expensive to make. The attachment points at the blades can also be a problem with these swaybar designs.
The additional complications and downsides of hollow bars seem to outweigh their advantage in overall weight and unsprung mass, which can be seen to only be marginal in street car application. Even OE manufacturers rarely use this type of design in their swaybars. However with the right conditions and setups, they can bring some gain in racing situations, which can require weight reductions to the gram.