rod ends in bending

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



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



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



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



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






















setting up the car



























setting up the car






rod ends in bending

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