02 November 2012 12:34

Pat’s Seven Deadly Sins of FS Design

Category: Pat´s Corner

In my last blog, I mentioned that some ‘beginner errors’ are creeping back into the designs we are seeing from some very experienced teams. I will consider the following list as ‘deadly’ sins because they are likely to lead to a structural failure and hence a DNF.

One thing I find a little difficult to come to grips with is that the teams change but I don’t! By that I mean, when advice is given I forget that it wasn’t given to the team members who are designing this year’s car, so mistakes creep back in. This was brought to my notice by a post on FSAE.com where a poster asked why teams use encapsulated spherical bearings in their suspension rather than using threaded rod ends, which allow easy adjustment. So number one of the seven deadly sins has to be...

1.    Rod Ends in Bending (REIB)

So much has been written about this subject over the years it is hard to think what to say. Simply, you must not put bending loads through the threaded shank of a rod end (sometimes called Heim joints or Rose joints). Unless the rod end is enormous, the load will cause the rod end to break at the stress riser.

In the illustration above, this is particularly evident in the lower/outer rod end. As the judges say “That rod end is ‘GTB’*”

The team must analyse the loads in the suspension arms. In particular, factor in the brake torque!

There are differing levels of loads in the A Arms. Only linear loads should be fed through rod ends and as the load path deviates, so the bending moment increases until at 90°, which we see in rod ends at the wheel end of the A Arm, where failure is almost guaranteed.

Teams justify their use of rod ends by citing ease of adjustment. That is a ‘cop out’! There are smarter, stiffer and just plain better ways to adjust such parameters as Camber and Caster. In the example illustrated above for instance, shimming between the lower clevis and the upright would permit Camber adjustment without disturbing the Toe setting!

In an earlier edition of this blog, I have shown how to properly design suspension arms.

2.    Rear toe compliance

This is another topic which has been covered in a previous blog. Lack of proper control of rear wheel toe will make a car impossible to control accurately.

Usually, this is caused by reacting the rear wheel toe loads over too narrow a base at the upright (in this case), by reacting the load into a compliant part of the chassis or by feeding the load through an oblique angle which causes both compliance and bending issues.

Compliant toe control can cause either unwanted Toe-out or Toe-in, depending on the layout. If the wheel toes out under cornering loads, it causes the car to tuck in.

The driver reacts by relaxing the steering input which reduces the lateral load, reducing the toe out and starting the whole steering sequence again.

If the wheel toes in under lateral load it causes the rear of the car to fight the steering inputs by the driver. This is often described as ‘terminal understeer’. Once the driver has completed the corner and starts to straighten the car, the rear to in decreases causing the driver to put in a further steering correction.

Both scenarios can be seen on track as the driver struggles to keep the car on track between the cones. The result is always scattered cones, slow lap times and a tired and frustrated driver who makes even more mistakes.

A related issue is rear bump steer or roll steer. This happens when the geometry of the toe control arm is not in harmony with the suspension wheel control. Again, this can be either toe out or toe in, depending on the geometry. Sure, some car makers will deliberately engineer in ‘passive rear steering’, but they do that to tame FWD understeer and so the feature has no place on an FSG car!

3.    Unsupported rocker pivots

Forces like to travel in straight lines. When they are sent around a corner it creates a vector force that must be reacted into the chassis.

Mounting a bell-crank pivot in the middle of an unsupported tube can, depending on the geometry, impose a large bending force into that tube. This will lead to compliance and, over time, probably a failure of the tube. The problem is increased if the load vector is offset from the tube creating an overturning moment as seen in the illustration shown here.

There are other issues with the mechanism illustrated above, not the least being the rocker and damper chassis mounts being in single shear. This design is definitely GTB*!

4.    Tapered wheel nuts on alloy wheels and wheels not spigoted

This subject almost deserves an entire column on its own! Ever since the Paderborn wheel hit me at FSG 2009, I have taken a closer interest in errant wheels.

Firstly, wheels must be centred on a spigot; it is not the job of the wheel-studs to centre the wheel! Any run out in the wheel, radial or axial, will excite the tyre at the contact patch and reduce grip, especially in the wet (something to keep in mind for the FSG skidpan event). By the way, so will wheel imbalance, even if you cannot feel it in the car. Remember all that when you fit your brand new expensive racing tyres in search of extra grip. Wheels must run true!

So, wheels must be spigoted and aluminium wheels should not be attached using tapered wheel nuts or this can happen...

No spigot plus tapered wheel nuts stressing the centre every time the wheel is fitted eventually led to failure. Think it is just an isolated example? Well look at this one...

Not only do these wheel failures indicate a DNF, but loose wheels put the driver and bystanders in danger.

While I am on the subject, the shear load on the wheel studs at the hub face should be through the shank, not the thread of the stud. By definition, this means that wheel bolts (with the taper under the head, are not advisable on two grounds.

What a street car manufacturer does is not necessarily a good idea for your FSG car!

5.    Steering geometry

The introduction of the foot well template a few years ago has made designing a workable steering solution in a short wheelbase car very difficult. The usual solution used to be to mount the rack level with and just behind the upper wishbone. This permitted good steering geometry and made including Ackermann geometry fairly simple. Unfortunately, it also placed the rack just above the driver’s shins where, at worst, it could break his legs in a crash and at best, could snag the drivers legs, delaying his escape if he needed to get out in a hurry... like if the car was on fire!

The usual post template solution is to mount the rack on the floor parallel with the lower wishbone mounts. Sometimes the rack is mounted very high in the chassis with bell-cranks to drop the inner tie rod points to the level of the top wishbone pivots. Both these solutions cause some difficulties with steering column geometry and resultant poor steering feel. Designing a good steering system has become quite a challenge, one of the many in FSG.

A challenge that some teams fail catastrophically! Mounting the steering rack anywhere that causes a bending load when the car is steered is one of the new mortal sins! We have had such steering solutions fail catastrophically during design judging! The bending load on rod ends at the ends of the rack simply necked them off! Don’t load rod ends in bending anywhere!

I have seen several cars with steering racks feeding rearward facing steering arms at acute angles! Not only is the bending moment huge, but there is a significant chance of the steering linkage inverting!

A good hit on a cone with the wheel shown would turn the steering ‘inside out’, locking the rack and causing the driver to lose control!

6.    Cooling system connections

I see many cars where the cooling system is clearly an afterthought. Here, I will ignore the obvious requirement of getting cool air through the radiator and talk about the connections in the system instead. Some truths about the plumbing in the cooling system.

  • Water is heavy
  • Rubber hoses are heavy
  • Hose clamps are heavy and sometimes unreliable
  • Abrupt changes in flow direction or changes in cross sectional area create turbulence and restrict coolant flow, and...
  • Every joint is a potential DNF caused by a coolant leak.

Where possible, the radiator should be mounted as close as possible to the engine to reduce the length of the hose runs. There is no need for two radiators as the donor bike probably didn’t need two. A radiator full of water is a very heavy component!

Straight or gently curved aluminium pipe of the appropriate diameter is what should be used, connected to the engine and radiator by a minimal number of rubber hoses. The pipe ends should be beaded to prevent the hoses blowing off and good quality hose clamps should be used. Good quality hose clamps do not come from a hardware store!

I have seen cars with lots of pieces of pipe connected by lots of pieces of rubber hose.   Each joint requires two hose clamps and on one car recently, I counted 28 hose clamps in the cooling system alone. I was not surprised when that car DNF’d in the Enduro with a coolant leak!

When rubber gets hot it gets soft. If your hose clamps have a simple folded metal bridge, relying on the tightening pressure to keep it in place, when the rubber softens the bridge can distort, allowing the thread on the band to slip and the clamp to loosen!

That is what has happened in the illustration. The piece of rubber hose was used to put a bend in the pipe. When the rubber got hot, the clamp slipped a notch (see the angle of the tail) and the unbeaded aluminium pipe popped out of the rubber hose. Instant failure on a pressurised cooling system!

7.    Wheel or upright compliance

Look at this next illustration (thanks Claude). I know that the suspension geometry cannot permit this amount of positive camber at that degree of chassis roll.  It comes either from wheel centre flex or compliance in the upright assembly.

Unless the wheel centres are adequately stiff they can significantly contribute to camber loss and the associated loss of grip. Of course, a flexing wheel centre will fail eventually...they are GTB*!

Teams should consider the effect of using centre-lock wheel nuts on wheels. They leave a larger area of the wheel centre unsupported and so, unless designed properly, can contribute to wheel compliance. There is no requirement for fast wheel changes in FSG, so the cost and complication of centre-locks is questionable.

And a debate for another day, which direction should the nuts tighten on centre-lock wheels?

* GTB = Design Judge’s code for ‘Going To Break’
So, that’s Pat’s seven deadly sins this week. Next week they might be a different seven.

Design Error of the Month

Why would I need a Design Error of the month? I have already listed enough design errors to carry me through to the next blog.

Ah, what the heck, how about another Rod End in Bending with ‘inside out’ steering?

The next event coming up is FSAE Australasia in about a month where Claude Rouelle and I will be Chief Design Judges along with Ron Tauranac (Google him!) so after that I might have some more stuff to tell you.

So until next time,
keep safe,