10 July 2010 12:57

Steve’s Box (of tricks)

Category: Pat´s Corner


A guest article by Steve Fox, Chief Design Judge, FSG. Pat’s overheated keyboard needed a little cooling down time. So until his new liquid cooled keyboard comes in, I agreed to contribute a little something to keep you all ahead of the curve…

Cockpit Control Forces – or –
How Robust Do Driver Controls Really Need To Be?

I look forward to FSAE/FS competitions, not only for the obvious reasons (you get to see a lot of neat, different cars, and talk to a lot of incredibly bright engineering students), but also because it is the one time of year when all the Design Judges, Tech volunteers, Rules Committee members, and the other visionary individuals who make FSAE/FS what it is, are gathered together.  We use this opportunity to talk over ideas face to face and gain a better view of the ‘Big Picture’ so to speak.  One of the topics that came up over dinner one evening, this past May, was sub-system robustness, or how strong does a steering/brake/accelerator/gearshift (you pick one) system really need to be?

This is a topic which all FSAE/FS volunteers (Design Judges & Scrutineers in particular) are sensitive to because we, on occasion, have actually broken student built cars.  Make no mistake about it, when this happens, we feel very badly that we have broken something which students have put long hard hours of design/build/test time into.  However, we are also quick to point out that we would much rather see the suspect part break standing still, rather than out on the track where someone (be it a student driver, or worse yet, an innocent spectator) could be injured or worse.  Unfortunately, some students get upset, and feel that they are being unfairly singled out, because: “We tested that ____ (insert one: brake pedal, steering shaft, accelerator pedal, gear shift, etc) beyond reasonable limits, and it didn’t break for us.”

This article will look at each of the different sub-systems that a driver exerts force to operate and help you determine what the reasonable forces for normal operation are.  It is not my intention to tell you how to design your car.  This is just one example of how to go about designing a reasonably robust car.  This article will (hopefully) help you identify what level of forces your car should be able to withstand without driver induced damage.  When determining how strong any sub-system really needs to be, a common saying we use in motorsports is: “Never underestimate the strength of a scared driver!”

Steering System

There are two types of steering forces we will discuss.  The first will be steering wheel torque, or how much force can a driver put into the steering system through ‘normal’ actuation of the steering wheel.  The second steering system force we will talk about is lateral (radial) force, or how much can a driver pull up, down, or sideways on the steering wheel column.  We will not discuss axial force imparted during the dynamics of a collision.  Axial analysis is beyond the scope of this article.

First, let’s look at the most common types of steering system failures in FSAE/FS cars.  A recurring problem we see is a lack of proper steering column support.  When a steering wheel can be firmly pulled up, down or sideways, and the Design Judge or Scrutineer can detect noticeable compliance, you definitely have a problem.  
This is usually due to the fact that the steering column only has one support, and the support (usually a metal sleeve) does not support the shaft over a sufficient length.  A better design would have two supports per shaft.  The farther apart the supports are, the less compliance there will be in the finished assembly.  We also see all types of bearings/sleeves supporting the shaft, ranging from very well engineered radial ball bearings, to all types of bushings, made from all types of materials, to nothing more than a metal sleeve substantially bigger than the steering shaft (with all of the excess compliance that comes with that sloppy design!).

We also see all types of steering shaft couplers and u-joints which are under-designed, worn out, improperly installed or simply should never be used in a steering system in the first place.  The most common type of coupler failure we see is the notorious shaft in a metal sleeve with a cross drilled hole with a pin/bolt through it.  Typically the shaft is not hardened, the hole is too big, or the holes don’t line up properly.  This results in a sacrifice of adequate cross sectional area, which causes a shearing failure, or at the very least, lots of extra compliance.

Another common failure we see is adhesive failure of a shaft and coupler which has been bonded together with some sort of ‘super dooper epoxy’.  Your steering system is no place for bonded joints.  The better way to couple any of your steering components together is with splined joints.

I am not recommending this, but this was Pat’s karting solution (back when he raced).  If the kart had a tubular column (most do) Pat turned up a 5 cm (2”) long aluminum plug for the top of the shaft.

Steering System Torque / Force

So, how much torque / force does your steering system normally see, and how much torque / force should it reasonably be able to withstand?

Normal steering operating torque / forces depend on a number of variables.  You can build a 136kg (300 lb) car with very narrow, high pressure tires, at zero scrub radius, with zero KPI, and zero caster, lots of steering ratio, and the steering input force will be very small indeed, maybe as small as 1.3-2.7 Nm (1-2 ft.lb.).  This car would be worthless to try and drive competitively as it would have a ‘dead’ feel, and give the driver no feedback to his steering inputs.  The opposite extreme would be a very heavy car with wide, low pressure tires, lots of scrub radius, lots of KPI, caster, and a low steering ratio.  You better start working out now if you are going to drive ‘that’ car for any distance…