06 September 2005 00:57

Overview for new Teams

Category: Pat´s Corner
By: Pat Clarke

This document is in no way an instruction sheet on how to design and build a car for Formula Student Germany. It is intended as an advisory document for new teams to allow them to build that all-important first car and avoid some of the more common pitfalls. It is very easy for a new team to get carried away trying to build their own complex "Formula One" car and lose sight of the objective. To get to the competition with a car that will allow them to complete the event and gain the satisfaction of competing in the best engineering competition in the world is what it is all about.

They will also get to leave the legacy of a complete project for those who come behind them to learn from or to evolve to the next level. It is important to realise that it is almost impossible to build a winning car at the first attempt, however it is all too easy to have no car for the competition due to being distracted along the way or having unattainable aspirations.

My favourite saying is "The trick is…there is no trick", and that applies especially to FS. The laws of physics are immutable, and if something seems to be reacting in a different way to what you expect, that is a message that you need to better understand the question before trying to answer it.

Usually though, you have to try an answer first before finding whether it is correct or not. That is when you run into my second favourite saying, "There is always more than one correct answer"! Your task then becomes finding the answer that best suits your situation.

It is important for a team to realise that it does not matter whether the Judges agree with your design decisions or not, as long as you can cogently argue in favour of your solution. After all, the final examination occurs on the last day of competition on the track, and there your solution will be validated by the stopwatch.

 

Design Suggestions

For a first year car, I recommend that a new team opt for a simple and straightforward steel tube space-frame chassis. It should be fitted with a conventional suspension system that is easy to understand and sort. The choice of wheel size, either 10" or 13" diameter is the subject of ongoing debate. Both sizes have their advantages, however would recommend that a team think about starting with a 13" wheel as this will allow more latitude in the design of their suspension package.

It cannot be over emphasised that the choice of tyres is the most significant decision made in the design process. Tyre choice is important because all forces of acceleration, turning and braking are reacted through the tyre contact patch.

FS cars are limited by the formula and course designs, to a top speed of little more than 100kph, so traction is generated by well-controlled tyre/track interface with little aero download assistance available. A team should finalise its tyre choice very early in the design process.

Competitive FS cars have evolved a kind of sameness as a result of evolution. Teams pick the best features of preceding designs, either theirs or from another team. As a result, and applying Darwin’s theory, the good cars are starting to feature very similar designs.

Overall designs may be similar, however, as the old saying goes, the devil is in the detail. Judges will look at the overall design, and whether they agree or not with the evolution theory, will then look closely at the details of your design and construction. So a new team would be well advised to choose a conservative evolutionary design for their first car and then really sweat the details.

 

Planning

Before anything is bought, before a sketch is made or a tube is cut, a detailed plan must be developed.

If the tyres are the most important technical component, then planning and project management is the most important logistical component.

The one thing all winning teams have in common is a good plan and a competent manager. The best piece of advice I can give the team manager is that this is not a personality contest. The ability to see through the project and carry it out means that there will be differences of opinion between team members. This happens! A good team manager will take this in his stride, find a solution and continue to drive the project forward.

An important aspect of the initial technical planning is ergonomics. The car will not be successful if it is difficult to drive. Your prototype is supposed to be for production and should in theory be adjustable so all and sundry can fit in and drive it. Realistically, the only people it is required to fit are your team members, so the ergonomics should be designed around those drivers. Take look at a production-racing car such as a Formula Ford, and copy the ergonomic layout.

Forget about the rest of that car design as it was built to a totally different set of parameters. Circuit racecars are optimised for straight-line performance and relatively open corners. The layout of the typical FS course will consist of tight corners of various shapes. There will be no real straight, and the top speed will be limited by course design to little more than 100kph.

A successful FS car will therefore have exemplary handling characteristics, be predictable to drive and easy on its tyres. The constant reversal of forces, cornering, braking and acceleration will exact a toll on the driver and chassis as well as the tyres!

With all this in mind, I will now talk about a set of design parameters suitable for a first time FS car.

 

Basic Design Parameters

I will start with a few design requirements. The car must meet all the requirements of the Formula Student rules! This is not Formula One where you are free to interpret the rules as you please as long as you can afford very expensive lawyers to argue your case.

The FS rules are simply written as they are, and the officials will not be swayed by your interpretive skills in reading these rules. If there are any doubts, refer the question to the FSDe organisers for a ruling before you proceed!

The rules also require some common sense interpretation. If there is anything dangerous about your car in the opinion of the judges, you simply will not be permitted to drive it, regardless of how ‘legal’ it may be.

FS encourages the use and development of new technology. Sometimes this will bring forward technology not foreseen by the rules committee. An example of this in recent years was ‘fly by wire’ throttle and steering controls.

An innovation may require a decision from the Rules Committee before the car is allowed to compete, and so any team contemplating some ‘avant garde’ design features would be well advised to bring them to the attention of the Rules Committee in plenty of time for a decision to be made, and any required alterations to the car completed and tested.

Be assured the committee will maintain your confidences and will not leak your design features to another team.

Finally, a car that looks good will always get a positive reaction from the judges, so concentrate on the details and make sure the car looks right. Judges like elegant engineering solutions, so the old truism that "Something that looks right usually is right" has relevance here. Paint is cheap and elbow grease is free, but inspiration is priceless.

 

Dynamics

Front of the car

The dynamic characteristics of the FS competition require a car that turns into corners accurately and maintains good cornering speed. Camber control when the chassis rolls is critical. Camber gain as the suspension compresses is desirable, but the designers should take into account the camber changing effects of Steering Axis Inclination and Caster as the car is steered. SAI results in a positive camber change as the wheel is steered. Caster will generate some negative camber on the loaded wheel on turn in. It follows that a good FS design has plenty of Caster and not much SAI.

Running a car with lots of Caster will result in significant diagonal weight transfer when turning into a corner or at any time steering lock is applied. On turn in, this diagonal weight jacking will unload the inside rear wheel so a team running without a locking differential of some sort may suffer traction difficulties.

On the other hand, this weight transfer may unload the inside rear wheel to such a degree that a differential may be dispensed with as in go-kart designs. Steering feel and weight as a result of caster angles can be adjusted by altering the ‘trail’. Trail is the position of the axle centerline relative to the steering axis. Altering trail will change the transverse weight transfer whilst not affecting the camber angle change.

A little positive steering offset will give good feedback to the driver, too much will quickly wear him out and may even hurt his hands from kickback in the event of contact. Too little offset may make it difficult for the driver to place the car due to lack of feel. A balance may need to be struck between wheel offset and SAI to achieve good steering ‘feel’.

‘Bump Steer’ should be avoided over the range of suspension travel. Any decision affecting rack position, Ackerman etc, should immediately be checked for any bump steer side effect.

Positive Ackerman geometry will result in an increase in toe out when the steering is turned away from straight ahead. Negative Ackerman is usually only seen on cars that encounter very high-speed corners and where aerodynamic stability in those corners is paramount.

Does it follow that low speed cars on a tight track need positive Ackerman? You bet it does! And it is unlikely the amount of Ackerman you think you need will be enough. Increase it by at least 50% and you might be in the ballpark! Be aware that the position of the steering rack will alter the Ackerman effect, and some teams actually ‘tune’ their toe out on turns with an adjustable rack position.

Camber on a FS car should never go positive on a loaded wheel in cornering. Judges will watch for this, and I have actually seen a judge accurately predict the SAI and Caster angles on a car by watching it on the track.

Rolling a loaded tyre onto its outer edge will not only reduce the grip, but will quickly ruin the tyre. Excess negative camber is not so damaging. Generally, radial ply tyres respond to more negative camber than cross ply tyres and are more forgiving.

Designers should be familiar with the effects of Camber Thrust, the tendency of a tyre to run towards the positive camber angle. Cars that spend a relatively long time traveling in a straight line may have their toe angles adjusted to reduce the scrub losses induced by Camber Thrust, but on a FS car this effect can effectively be ignored.

Stiff suspension rates, aggressive ‘anti dive’ and ‘anti squat’ geometry and ‘third springs’ are usually seen to best advantage on cars where control of the aerodynamic platform is paramount. More compliant suspension rates are better suited to FS cars and will enhance the mechanical grip as long the wheel angles are controlled.

Excessive dive under brakes can be controlled with a third spring or bump stop, but rearward weight transfer under acceleration may well be advantageous. Excessive amounts of ‘Anti-dive’ geometry in the front suspension may result in a very ‘wooden’ feedback to the driver, and so should be avoided.

Compliant suspension may result in changes of roll centres, both vertically and laterally as the suspension works. A mobile roll axis will send confusing feedback to the driver, making accurate control difficult. Judges are aware of this and will quiz the design team on the dynamic positions of the roll centres and roll axes. Pay attention to the migration of the roll centres under all conditions.

Low roll axes and compliant springing may require the fitting of anti roll bars, and it is important to understand the side effects of the transverse weight transfer of these devices. In any effect, ARBs are a good way to fine-tune a car for various circuits and conditions. Whether a team decides to fit ARBs to their car or to omit them, they can expect to be quizzed on this issue by the Judges.

A powerful force generated in a FS car is front wheel brake torque. Care should be taken to properly react this force into the chassis, avoiding steering changes or ‘tramp’.

 

Rear of the car

At the rear of the car the forces are different but the basics are the same. Toe control is important so aim to have a wide toe control base.

Try to avoid bump or roll steer, both of which may make the car undriveable in the heat of competition. Bump steer toe out will make the car very unstable on turn in.

Drive shafts and CV joints should be aligned in all planes when the car is loaded. Provision should be made to accommodate drive shaft ‘plunge’ as the suspension works. Tripod joints are probably the best choice.

Thought should be given to the rear brake arrangement relative to the differential type used. A single inboard brake may be used, but this will not be effective if the differential type does match this type of brake arrangement. Judges will quiz you on this! If outboard brakes are used, care must be taken to ensure the brake torque is properly reacted into the frame.

Locked or locking differentials usually contribute to the generation of understeer, the bugbear of FS car handling. This can be relieved to a degree with proper understanding and implementation of the diagonal weight transfer discussed when talking about the front of the car.

 

Safety

There are stringent safety requirements written into the FS rules, and you can rest assured there will be no relaxation of these standards by scrutineers or officials. You should be aware that these rules do not necessarily cover every area of every car, and the judges may well disallow a car on safety grounds because of some feature that is not covered in the rules. This is for your own safety as well as their ‘Duty of Care’ under law.

At all times during the design, build and preparation of the car, ask yourself "Would I like to be driving this if something goes wrong?". If something seems like a good idea safety wise, then it probably is. If there is ever any doubt give the Committee a call and they will help you through the process.

Although ‘Anti-intrusion’ bars are not mandated on suspension A arms, I would recommend them. A relatively minor bump can push the A arm into the drivers space and injure their feet or legs.

 

Basic Design Checklist

     

  1. All load-paths should be direct and obvious to the judges. Judges
  2. love isosceles triangles and hate voids and indirect load-paths.

  3. Never load a threaded rod end in bending. Apart from it
  4. being poor design, Judges hate this and hate seeing it again and again

    year after year.

  5. Chassis stiffness should be such that the suspension can
  6. effectively work. If the suspension spring rate is such that the

    chassis flex becomes the de facto suspension, all your calculations go

    out the window, rapidly followed by handling and road holding.

  7. Weight is bad! Remember the immortal words of the late Colin
  8. Chapman, "Add lightness and simplificate". (By the way, Mr Chapman also

    said "Any suspension will work if you don’t let it"…..but Judges watch

    out for that!)

  9. Cars with aggressive caster angles are self-adjusting with regard
  10. to corner weights. Therefore it is an absolute waste of time attempting

    to adjust corner weights unless the chassis is square, in proper

    alignment, on a flat and level surface and with tyre diameters equal

    front and rear.

  11. Push rod or pull rod suspension is a good idea for the following reasons.
    •  

    • It is possible to adjust the ride height or chassis attitude without altering spring preload, and vice versa.
    • By using a rod and bellcrank operation of the suspension
    • components, the motion ratio can be increased to permit more effective

      damper travel for minor wheel movements.

    • Unsprung weight may be decreased and the mass of the suspension components can be located to lower the CG.
    •  

  12. Never forget it is ‘Wheel rate’ that is important, not ‘Spring
  13. rate’. Work out a simple mathematical equation for the wheel/spring

    travel ratio to allow easy calculation of the effects of spring or

    bellcrank ratio changes. Beware of bellcranks with aggressive

    multiplication ratios as these make the car very sensitive to minor

    adjustments.

  14. Roll control devices (ARBs) are a good idea. If not needed
  15. they can always be disconnected, however, such devices are invaluable

    for fine-tuning the handling to suit track or weather conditions.

  16. Ensure there is an adequate toe control base at the rear of
  17. the car, and that the components are stiff enough to prevent unwanted

    dynamic toe change. The judges will check for this using the

    old-fashioned ‘Manual Labour’ method.

     

     

  1. Things will flex under load, therefore it is a good idea to use spherical bearings at both ends of all suspension units.
  2. You will not be permitted to compete with a noisy muffler.
  3. Noise can be a difficult thing to measure, so a good safety margin is

    recommended. The muffler is not a good place to save weight!

     

  4. Under stress it is incredible how much pressure a frightened
  5. driver can apply to the brake pedal. Judges will look for flexing in

    the pedal mountings. The brake pedal should not go ‘over-centre’ when

    the brakes are applied. Ideally, at full application the pedal should

    be just before a perpendicular load is applied to the push rod. Judges will also look for proper operation of the brake balance

    bar or for one master cylinder bottoming out and the full effort then

    being transferred to the other.

  6. Test, test and test some more. Plan to have your car built in
  7. plenty of time to allow testing. Things will break, and the competition

    is not the place for that to happen. This is part of the management

    plan we spoke about earlier. It is a critical element for successful

    teams.

     

In the mean time, all potential drivers should get as much seat time in go karts as is practicable. Not only will this give the drivers experience; it will permit the team manager to choose the best potential drivers. It also is a good team building exercise, and can even be used as a fund raising exercise.

 

Finally

Remember this is a learning experience, not a motor racing event. Sure, we all look forward to driving and the final competition, but the real winners are the competitors who learn something. Time and time you will hear bystanders, judges etc remark, "I wish there had been something like this when I was at Uni". They all see the benefits as well as the enjoyment. It rounds out your education, whether it is in engineering, electronics, management, marketing or any of the other disciplines pertinent to the competition. It is a killer achievement to put in your CV, and will greatly enhance your employment opportunities.

Don’t lose sight of the big picture and the opportunities the competition and its sponsors have given you.

 

 


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