08 October 2007 13:31

Pat's Column - October

Category: Pat´s Corner
By: Pat Clarke

Well, FSG 07 is finished, congratulations to Stuttgart, and it is time to start planning for next year.

The first thing when planning a new project is to read the rules. There are always subtle changes, but for 2008, some significant changes have been made which will materially change most designs.  These rules changes have been posted at the official FSAE website (http://students.sae.org/competitions/formulaseries/rules/).  I would suggest that teams who intend building evolutionary cars should read these new rules carefully as many 2007 designs will not meet the 2008 rules, and indeed, the additional changes planned for 2009.  As usual, there will be some supplementary rules issued for FSG in order to adapt the FSAE rules to our competition. The FSG committee is very careful that these changes do not make a car ineligible for other competitions.

From my perspective, FSG 07 was the best FS/FSAE event ever.  After a damp start, the weather was kind and the event proceeded without any hassles. The support from the German Automotive Industry was top notch, with their HR head-hunters seeking accomplished young engineers.

Again, the quality of the cars was as good as or better than any other event.  The top three in Design were the equal of any of the best cars in the world. At the other end of the spectrum we had some unfinished projects, as usual, as the teams ran out of time and/or money. Even here there is a valuable educational lesson to be learned about the irreversible nature of time and money. This rule is even more important in the commercial world that comes after education.

There were lots of interesting technical items to be seen on the cars, both on and off the track. There were also some technical shortcomings to be seen on the track. From my position as the commentator, I was able to see the cars clearly on the course. Two shortcomings stood out clearly.

  1. Many cars, from teams that should know better had their outside loaded wheel go into positive camber under cornering load.  Not only does this give poor road-holding and destroy the tyres in short order, but the feedback to the driver is confusing making it very difficult to maintain concentration and consistency, especially in the Enduro event.
    There can be several reasons for this positive camber, poor design or construction, flexible chassis or suspension components. The most likely in many cases was wheel flexing!  This is certainly a 'Bad Thing'. Apart from the issues listed above, one has to include the inevitable fatigue related wheel failure.


  2. On the bumpy course, damper performance on some cars was clearly lacking. The modern Mountain bike shocks used by many teams are clearly unsuitable in standard condition.  The compression damping lock used in these shocks to prevent pedal energy being wasted by compressing the spring will make these dampers very harsh in bump on FSG cars.  Watching some cars tramp under brakes and on acceleration made it clear that the designers had not really understood what their damper curves were, or even what damping curves they desired.  Some cars fitted with air-spring bike dampers were faring even worse. As the air temperature increases, the pressure increases raising the spring rate, yet the damping rates remain constant (or even decrease with heat).

The ratio of compression to rebound damping should approximate the ratio of un-sprung to sprung weight, probably somewhere in the order of 1 to 3, so dampers with very high compression damping need to be modified for successful use on FS cars, especially on bumpy tracks.

We saw a spectacular engine failure from the Birmingham team. I think the teams official reason was "Electrical Failure (conrod knocked the starter off)". I am sure this failure was really related to oil surge.

Every team out there who did not have a dry sump arrangement has an oil surge problem!  They may well alleviate it by baffling the sump, overfilling the crankcases or some other scheme, but the fact remains they have an oil surge problem.  All it takes to generate a massive engine failure is to uncover the oil pickup pipe for a split second or to ask the oil pump to circulate a milkshake of oil and air. Run bearings or seizures will wreck the engine very quickly.

The best answer is to build a dry sump system as this evacuates all the engine oil. The oil can then be de-aerated and cooled before being delivered to the pressure pump for delivery to the heavily loaded parts of the engine. A couple of other significant benefits can be realized with a dry sump system.

  1. A significant amount of engine heat can be dissipated via the oil and an oil cooler, thereby reducing the load on the cooling system, and:
  2. Evacuating the crankcases can significantly reduce the parasitic pumping losses in the engine resulting in more power at the wheels.

If time and budget constraints do not permit the application of a dry sump, there is a very acceptable alternative, one I have never seen on a FS/FSAE car.  This is a pressurized oil accumulator that maintains a supply of pressurized oil to the engine under all conditions.  These are usually known as 'Accusumps' (a trade name) and are easily constructed, or can be purchased from most race supply shops. Google will quickly find one for you.

An animation of how Accusump works can be seen at this site www.racerpartswholesale.com/accusump1.htm
An additional benefit of Accusump is that it provides lubrication to a turbocharger after the engine has been shut down, thereby prolonging turbo life.

Pats Design Error of the Month.

Teams are regularly criticized about having insufficient toe base at the rear of the car. This picture shows a typical example.

The toe control is reacted over such a small base that involuntary rear steer is unavoidable.  Such a car will be unstable whenever any load is applied to the rear tyre.

 

A wider toe base as shown here should give good stability.....shouldn't it?

Well, no, actually it will not!  Note the toe is reacted through the two bolts close together at the top of the picture.  I originally had this picture because it showed a sheared stub axle, but later inspection showed the toe control mistake.

That's it for this month.  Next column will cover some design ideas for teams planning a new car for 2008.

So, until next time, keep safe

 

Pat