832019 Rod Ends in Bending
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Pats Column - Rod Ends in Bending
Category Patacutes Corner
By Pat Clarke
ldquoRod ends in bendingrdquo How many times have we heard that phrase
Teams are told regularly that it is a design feature that must be avoided and there is an unwrittenrule at all Formula Student or Formula SAE events that a car with REIB in the suspension or
steering system will not advance to the Design Finals
At the Australian event a few years ago as the Tech Advisor I was determined that the Australian
teams would not employ rod ends incorrectly and advised every team on their correct use I thought
it was a design demon that had been exorcised yet at the very next competition they were back
Some teams argue passionately that if the rod ends are correctly sized then the adjustability of
threaded rod ends justifies their use They point out that at one time most junior formula race cars
used rod ends in this way as a matter of course One of the most ardent FSAE judges against REIB
was the late Carroll Smith yet ironically here is the cover picture from his Nuts Bolts Fasteners
and Plumbing book showing him posed beside a REIB To be fair the heavily loaded lower wishbone has a proper encapsulated spherical bearing The picture was taken by David Gooley
Trying to justify REIB by saying they were used on some old FFord or F3 racecar is a flawed
argument because these cars had neither the tyre grip nor the brake performance of modern cars
and so had lighter loads to react through the suspension links
Certainly every year cars with REIB finish the event and some even finish high in the points but
832019 Rod Ends in Bending
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this is not an excuse for poor design So lets talk about why teams like REIB and why Design
Judges hate them
Teams like threaded rod ends because of the ease of adjustability When a car uses suspension
components incorporating them the adjustment of camber and caster becomes a simple matter of
twisting the rod end(s) in and out Unfortunately this usually means they are being loaded
incorrectly Threaded rod ends are designed to be loaded in compression or extension That are not
designed to be used when loaded laterally as this tends to bend the threaded shank and usuallyapplies shear loads across the thread roots Bending loads tend to distort the eye causing the
spherical element to seize This quickly leads to failure as shown below
A rod end failure as shown here can be very dangerous as it can result in a sudden and complete loss
of control At the very least it will result in an instant DNF ruining an entire yearrsquos work for the
team
Judges can never condone a design feature that is inherently dangerous
Of course if the rod ends are big enough they will not break but the Design Judges argue that if thesystem is correctly designed then smaller and lighter components can be used and this has a flow
on effect to the sizing of other components
So the second objection that Judges have is related to Newtons Second Law Cars that are heavier
than necessary are not good designs
The shear loads are not only in the vertical plane Front wheel brake torque is the biggest single
force in most cars and this is reacted through the suspension links If the car has outboard rear
brakes then there is a smaller brake torque force which must be reacted at the rear
832019 Rod Ends in Bending
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These forces are reacted in a horizontal plane and if threaded rod ends are used then a shear load is
introduced Failure of a rod end under hard braking can have catastrophic results and as a result the
judges hate this aspect too
Finally the Judges do not like the offset load paths when rod ends are used in the wishbone
construction The cranked load path from the tyres into the chassis is not desirable Sure this aspect
of design is not as critical as the potentially dangerous situations mentioned earlier but it is still a
factor in Design Judging
Wishbones can be made with the rod ends mounted along the line of the legs These are easy to
make but pose an installation problem if adjustments need to be made on the chassis end of the
wishbone This adjustment will change the length of the base and will require the chassis mounts to
have some form of adjustment or heaven forbid the wishbone legs to be bent or stressed Needless
to say the design Judges donrsquot like this solution either
832019 Rod Ends in Bending
httpslidepdfcomreaderfullrod-ends-in-bending 44
The problem with wishbones using encapsulated spherical bearings is that they do not allow for easy adjustment The usual method being the use of shims behind the chassis or upright mounted
clevis and here is another dilemma The Design Judges hate thick stacks of shims so the range of
adjustment is limited
At the design stage teams will not know the optimum suspension settings and during the chassis
build construction errors may creep in so suspension adjustability very desirable when testing and
setting up the car
My suggestion would be to make a set of adjustable wishbones using threaded rod ends These can
be used to set up the car prior to the event Once the optimal geometry has been arrived at then a
set of fixed geometry wishbones using encapsulated spherical bearings bushes or even flex plates
can be constructed for the event The adjustable wishbones can be kept for use in an emergencyI am looking forward to inspecting all the cars at Hockenheim in August and smiling because I see
no ldquoREIBrdquos Well even Design Judges can dream
832019 Rod Ends in Bending
httpslidepdfcomreaderfullrod-ends-in-bending 24
this is not an excuse for poor design So lets talk about why teams like REIB and why Design
Judges hate them
Teams like threaded rod ends because of the ease of adjustability When a car uses suspension
components incorporating them the adjustment of camber and caster becomes a simple matter of
twisting the rod end(s) in and out Unfortunately this usually means they are being loaded
incorrectly Threaded rod ends are designed to be loaded in compression or extension That are not
designed to be used when loaded laterally as this tends to bend the threaded shank and usuallyapplies shear loads across the thread roots Bending loads tend to distort the eye causing the
spherical element to seize This quickly leads to failure as shown below
A rod end failure as shown here can be very dangerous as it can result in a sudden and complete loss
of control At the very least it will result in an instant DNF ruining an entire yearrsquos work for the
team
Judges can never condone a design feature that is inherently dangerous
Of course if the rod ends are big enough they will not break but the Design Judges argue that if thesystem is correctly designed then smaller and lighter components can be used and this has a flow
on effect to the sizing of other components
So the second objection that Judges have is related to Newtons Second Law Cars that are heavier
than necessary are not good designs
The shear loads are not only in the vertical plane Front wheel brake torque is the biggest single
force in most cars and this is reacted through the suspension links If the car has outboard rear
brakes then there is a smaller brake torque force which must be reacted at the rear
832019 Rod Ends in Bending
httpslidepdfcomreaderfullrod-ends-in-bending 34
These forces are reacted in a horizontal plane and if threaded rod ends are used then a shear load is
introduced Failure of a rod end under hard braking can have catastrophic results and as a result the
judges hate this aspect too
Finally the Judges do not like the offset load paths when rod ends are used in the wishbone
construction The cranked load path from the tyres into the chassis is not desirable Sure this aspect
of design is not as critical as the potentially dangerous situations mentioned earlier but it is still a
factor in Design Judging
Wishbones can be made with the rod ends mounted along the line of the legs These are easy to
make but pose an installation problem if adjustments need to be made on the chassis end of the
wishbone This adjustment will change the length of the base and will require the chassis mounts to
have some form of adjustment or heaven forbid the wishbone legs to be bent or stressed Needless
to say the design Judges donrsquot like this solution either
832019 Rod Ends in Bending
httpslidepdfcomreaderfullrod-ends-in-bending 44
The problem with wishbones using encapsulated spherical bearings is that they do not allow for easy adjustment The usual method being the use of shims behind the chassis or upright mounted
clevis and here is another dilemma The Design Judges hate thick stacks of shims so the range of
adjustment is limited
At the design stage teams will not know the optimum suspension settings and during the chassis
build construction errors may creep in so suspension adjustability very desirable when testing and
setting up the car
My suggestion would be to make a set of adjustable wishbones using threaded rod ends These can
be used to set up the car prior to the event Once the optimal geometry has been arrived at then a
set of fixed geometry wishbones using encapsulated spherical bearings bushes or even flex plates
can be constructed for the event The adjustable wishbones can be kept for use in an emergencyI am looking forward to inspecting all the cars at Hockenheim in August and smiling because I see
no ldquoREIBrdquos Well even Design Judges can dream
832019 Rod Ends in Bending
httpslidepdfcomreaderfullrod-ends-in-bending 34
These forces are reacted in a horizontal plane and if threaded rod ends are used then a shear load is
introduced Failure of a rod end under hard braking can have catastrophic results and as a result the
judges hate this aspect too
Finally the Judges do not like the offset load paths when rod ends are used in the wishbone
construction The cranked load path from the tyres into the chassis is not desirable Sure this aspect
of design is not as critical as the potentially dangerous situations mentioned earlier but it is still a
factor in Design Judging
Wishbones can be made with the rod ends mounted along the line of the legs These are easy to
make but pose an installation problem if adjustments need to be made on the chassis end of the
wishbone This adjustment will change the length of the base and will require the chassis mounts to
have some form of adjustment or heaven forbid the wishbone legs to be bent or stressed Needless
to say the design Judges donrsquot like this solution either
832019 Rod Ends in Bending
httpslidepdfcomreaderfullrod-ends-in-bending 44
The problem with wishbones using encapsulated spherical bearings is that they do not allow for easy adjustment The usual method being the use of shims behind the chassis or upright mounted
clevis and here is another dilemma The Design Judges hate thick stacks of shims so the range of
adjustment is limited
At the design stage teams will not know the optimum suspension settings and during the chassis
build construction errors may creep in so suspension adjustability very desirable when testing and
setting up the car
My suggestion would be to make a set of adjustable wishbones using threaded rod ends These can
be used to set up the car prior to the event Once the optimal geometry has been arrived at then a
set of fixed geometry wishbones using encapsulated spherical bearings bushes or even flex plates
can be constructed for the event The adjustable wishbones can be kept for use in an emergencyI am looking forward to inspecting all the cars at Hockenheim in August and smiling because I see
no ldquoREIBrdquos Well even Design Judges can dream
832019 Rod Ends in Bending
httpslidepdfcomreaderfullrod-ends-in-bending 44
The problem with wishbones using encapsulated spherical bearings is that they do not allow for easy adjustment The usual method being the use of shims behind the chassis or upright mounted
clevis and here is another dilemma The Design Judges hate thick stacks of shims so the range of
adjustment is limited
At the design stage teams will not know the optimum suspension settings and during the chassis
build construction errors may creep in so suspension adjustability very desirable when testing and
setting up the car
My suggestion would be to make a set of adjustable wishbones using threaded rod ends These can
be used to set up the car prior to the event Once the optimal geometry has been arrived at then a
set of fixed geometry wishbones using encapsulated spherical bearings bushes or even flex plates
can be constructed for the event The adjustable wishbones can be kept for use in an emergencyI am looking forward to inspecting all the cars at Hockenheim in August and smiling because I see
no ldquoREIBrdquos Well even Design Judges can dream