Negative consequences of very large steering work product in a passenger car?

[Charles Kaneb]

The steering work function is the amount of work the driver has to do on the wheel (torque integrated over angle or force integrated over distance) to steer the car through a maneuver.

Large steering work function values allow large trail values and manual steering. The downside at first is obvious - the driver has to put in more work at the wheel. She'll get tired faster than in an easier-to-steer car.

However, if the driver drives the car regularly enough, her upper body strength will improve - humans have inverse fatigue properties to materials if recovery's allowed between overloads. Have teams with too-heavy steering noticed a training effect?

[Owen Thomas]

I am not entirely sure what you mean by "training effect". I assume you mean an increase in driver strength thus reducing the drawbacks of overly heavy steering. We have not had excessively heavy steering in the past, but I can provide an anecdote.

At the beginning of this season, we were having a discussion about steering forces - namely if we should reduce the diameter of the steering wheel. I was in favour of this, as it would increase driver feedback (we are all terrible drivers so the more feedback the better). I mentioned that the steering felt quite light, and that I never experienced any kind of fatigue. This was met with disagreement from others who had driven the car, who said the steering was fine and at the end of an endurance run they could feel some strain. Now, it is worth mentioning that I enjoy rock climbing, and so have relatively strong forearms; I also grip the wheel very lightly. Obviously, this discrepancy got me thinking about driver sensitivity.

The point of this story is that while driver strength and technique has an effect on muscle fatigue, I am unable to confirm if it is related to force sensitivity. One would think that as strength goes up, sensitivity to small fluctuations in force may go down. I see this as a problem.

[Charles Kaneb]

Originally posted by Owen Thomas:
I am not entirely sure what you mean by "training effect". I assume you mean an increase in driver strength thus reducing the drawbacks of overly heavy steering.

Yes. Exactly that.

[Drew Price]

I don't think most teams get enough track testing time to bulk up their arms enough to make much difference.

A car with overly heavy steering will quickly tire even a moderately physically fit driver. Like under 10 minutes quickly. I have driven a few, and I don't work out regularly, but I am in good health.

The tradeoff is not worth it in my opinion unless your drivers are on some sort of training regimen, which is excessive for FSAE.

My $0.02.

[Claude Rouelle]

Charles.

"SHE will get tired" and HE won't?

[Charles Kaneb]

Claude, I've seen proportionally more women who could drive the last five laps of a kart race as fast as the first five than men. Alexandria Gamache dominated the F1 Outdoors Ironman 150 (100 miles) for three years running, and I have never seen Haley Kaminski or Kristina Vorndran fade at the end of a race.

Reality ain't the same as perception - I think smaller drivers (men and women) are more willing to complain about too-heavy controls than the big bastards, even when we're dragging him out of the kart because he can't climb out or when he gets black-flagged because he went off from nearly blacking out.

[Claude Rouelle]

I have seen formula student cars losing 5 then 10 then 15 seconds a lap (out of about 55 seconds / lap normal "cruise" lap time ) in their last laps of their endurance stint simply because the steering was too heavy and/or the cockpit / seating was really uncomfortable.

I remember a German Formula student driver showing me his ribs, elbow and wrist swollen and or bruised (literally blue / violet) the day after the UK competition.

Ergonomy is a factor often too ignored in FS car design

Two advises

1. Try to limit your steering torque to 10 Nm maximum 12 Nm. On a passenger car the designers try to limit it to 5 to 8 Nm (depending how "sporty" these cars are) and that is with power steering 99% of the time. That steering is calculable with simple FBD from tire Mz, lateral and longitudinal tire forces, mechanical trail and scrub radius, upright and steering kinematics, steering rack design and ratio. A basic Excel spreadsheet will do. Make it basic 2D and then 3D for the sake of it but you won't see big differences. Do not forget to take both tires into account (no kidding, I have seen students making this mistake)

2. Simple test. In the shop, driver in the car with racing shoes, helmet gloves, belt the whole real think. Turn the steering full lock left. Start the stop watch full lock right. Back to full lock left. Repeat. The goal is to do that 30 times in less than one minute. Not for heavy smokers! I know the speed is 0 and the tire friction is not the real one but if you do not pass this test in the shop I bet you will have issues on the race track unless you hire Arnold Schwarzenegger as your driver.

[Z]

Originally posted by Charles Kaneb:
Large steering work function values allow large trail values and manual steering.
Charles,

Two points.
1. Why have a "large trail value"? I generally suggest starting with "centrepoint" steering (vertical steer-axis passing through wheelprint centre) and then maybe adding small amounts of Trail and Castor (Offset can be adjusted via wheel type, or wheel-spacers, etc.). This gives light steering, so low "work function".

2. I guess the main reason you might start with large Trail is if you have to use an existing upright which has massive Offset and SAI (= KPI), so you have to add lots of Castor to compensate for KPI-induced-camber-loss, so you then also end up with lots of Trail... So other than building new uprights, or building up the drivers, what to do?

Hmmm... Why not forget about "manual steering", and go for POWER-STEERING (!!!)?

Yes, I know brown go-carts are not supposed to have PS, but this is one of the very few complications that I think is actually rationally justifiable on a performance/points basis (much better than, say, pushrods-and-rockers!).

COST - Take an ElectricPS off one of the many larger quad-bikes (= ATVs) that nowadays have these (smallish integrated unit of a few kg). These vehicles have similar steering hand travel to FSAE (less angle lock-to-lock, but longer lever-arm), but probably bigger steering loads from the off-road bumps, so the EPS should be strong enough for FSAE conditions.

Or take HydraulicPS from any small car (almost all of which have PS these days!). So a smallish pump-tank that is belt, chain, or direct-driven from engine or transmission, some hoses, the valve-unit, and a double-acting ram or rotary actuator. All these are readily available, inexpensive from junk yards, and compact and light-weight.

BENEFITS - Toss your R&P, UJs, (or Bevel Gears if you use them), and use direct go-kart style steering. Namely, steering-wheel -> steering-shaft-with-valve-unit -> Pitman-Arm. The Pitman-Arm geometry means that you now have about 90 degrees lock-to-lock, or maybe 120 degrees at most. So you have very fast steering, which I reckon is a real benefit. But, unlike on go-karts, this steering is also very light (because it is powered!).
~~~o0o~~~

Bottom line is that with the addition of some complication from EPS or HPS, you can have significant simplification of the steering linkage. But most importantly you end up with VERY FAST and VERY LIGHT STEERING! http://fsae.com/groupee_common/emoticons/icon_smile.gif

The drivers can sit back comfortably, make only small, light-load hand movements, and their arms shouldn't be tired at the end of a 30 hour marathon, let alone the 30 minute FSAE Endurance.

In fact, if it wasn't for the overly restrictive steering-wheel Rules, then I would suggest side-stick steering! Ie., a vertical "pistol grip" at the end of the right arm-rest, which is rocked side-to-side for steering. It works ok on all those fighter planes... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

[Adambomb]

Originally posted by Z:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Charles Kaneb:
Large steering work function values allow large trail values and manual steering.
Charles,

Two points.
1. Why have a "large trail value"? I generally suggest starting with "centrepoint" steering (vertical steer-axis passing through wheelprint centre) and then maybe adding small amounts of Trail and Castor (Offset can be adjusted via wheel type, or wheel-spacers, etc.). This gives light steering, so low "work function".

2. I guess the main reason you might start with large Trail is if you have to use an existing upright which has massive Offset and SAI (= KPI), so you have to add lots of Castor to compensate for KPI-induced-camber-loss, so you then also end up with lots of Trail... So other than building new uprights, or building up the drivers, what to do?

Hmmm... Why not forget about "manual steering", and go for POWER-STEERING (!!!)?

Yes, I know brown go-carts are not supposed to have PS, but this is one of the very few complications that I think is actually rationally justifiable on a performance/points basis (much better than, say, pushrods-and-rockers!).
</div></BLOCKQUOTE>

Having spent a pretty fair amount of time in FSAE cars, I'll have to respectfully disagree on a few points. A tale of two cars:

Smokey, our 2007 car (more or less the Toyota Camry of FSAE cars because it's not the fastest, but just starts and drives whenever you want it to http://fsae.com/groupee_common/emoticons/icon_wink.gif ): Weighs 450 lbs, 50/50 weight distribution. 2.375" scrub radius, 3º caster, 0.5" trail, 20.5x7R13 tires. This car is a workout. You get pumped up in one lap, are really feeling it by 3, are tired by 5, and absolutely thoroughly exhausted to the point of barely being able to drive by the end of an endurance run.

CR-16, our 2010 car: Weighs 375 lbs, 40/60 weight distribution, 1.5" scrub radius, 5º caster, 1" trail, 20.5x6R13 front, 20.5x7R13 rear. Steering has nice feedback, but not at all tiring. Sort of hard to describe, like it's light on sweepers but the feedback comes in strong on transients. You can sit in it at a full stop, on concrete, warm tires, and saw it lock to lock without much effort. At design judging one of the judges commented on the ability to do this, and made some excuse about the tires just not being scrubbed in yet or something. I replied, "no, this is why it's so light," and without showing any visible strain picked up a front tire, while he was still strapped in.

If we can do that with a steel frame car with 13" tires and SLA suspension, it should be a piece of cake with a brown go-cart!



... The Pitman-Arm geometry means that you now have about 90 degrees lock-to-lock, or maybe 120 degrees at most. So you have very fast steering, which I reckon is a real benefit. But, unlike on go-karts, this steering is also very light (because it is powered!).
~~~o0o~~~


Personally I think that would be wayyy too fast of a ratio. For the past 10 years or so we've had about 270º, and I really wouldn't want to go any quicker than that. You must bear in mind that most of our drivers are terribly ham-fisted, and getting them to calm down their steering inputs is a big challenge.

One other point, I'm not experienced with the quad power steering, but I do know that most passenger car steering is terribly numb.

[Mike Cook]

Interesting topic.

I remember the steering on our older cars to be really light. Maybe it was because I sucked at driving. It might be because of the 13" tires.

However, between switching to 10" tires and other things steering effort has caused serious problems. We looked at power steering units off of atvs. We purchased one and I think they will work if you design enough space in your cockpit for one. It might have trouble passing a template rule. The weight penality is probably about 5-10 lb. It can draw a lot of power and I was unsure whether this would kill our battery since the single cylinder stators put out so little power to begin with. There definitly are some design issues that would need to be solved.

In the end, we revisited our steering geometry. We made some simple changes to reduce our caster and trail. This helped reduce our effort by about 50%. The original setup for reference was pretty bad. Steady state torque was probably equal to the high side of what Claude suggested. I imagine transient effort was much higher (factor of 2). I ran out of energy after about 4 endurance laps last year.

As we reduced trail, a couple interesting behaviors became evident. The first problem was that the steering feedback became quite diminished. Yes there was steering effort, but it did not change much with lateral acceleration. The steering felt very overdamped. It seemed hard to steer in or out of the corner. I would most likely attribute this to friction in the steering rack. It is difficult to pinpoint because the steering rack feels fine unloaded.

The second issue is that since we run such aggressive steering ratios, You can only do so much with trail reduction before the steering effort becomes reversed at the limit. And even with the steering effort approximately zero at the limit, the steering effort will still be quite peaky during slaloms or other transient events.

The third, and somewhat related issue is that as you decrease your trail towards zero, suspension compliance will play a large role with your steering effort. As you brake into a corner and you get caster deflection, your trail will become negative and the steering will pull into the corner. This is much worse for 10" wheels vs 13" wheels as your ball joint span is reduced and any suspension deflection will result in much larger trail differences.

In summary:

-Design steering with highest steer ratio you can get away with. Max steering wheel angles of +- 110 degrees at least.
-Reduction of friction in steering system very important
-Structurally stiff suspension important, especially reducing caster deflection under braking
-Power steering a realistic possibility, but will require some design work to integrate properly
-10" tires seem worse than 13" in my experience
-Higher tire pressures can help with reduction of steer effort.

[Mike Cook]

A few more points. [You can tell it is getting close to competition because I'm getting all pumped...]

Despite our absurdly high aero loads, I've never really had trouble steering our cars on really SCCA autocross courses. And were talking +- 2.75 g. Obviously it comes down to steering power, not effort. The worst thing in my opinion is the minimum sized slaloms. I believe they are 25ft. If your car is understeering through them it will make for a really long day. The number one thing you can do to help reduce steer effort is to free up the car through slaloms. You want it loose enough that it feels like the car will drive itself through the slaloms. This needs to be balanced with high speed slaloms too, but this is why having adjustable dampers can be a huge help.

Mike

[Z]

Adambomb,

"If we can [have light steering] with a steel frame car with 13" tires and SLA suspension, it should be a piece of cake with a brown go-cart!"

Yes, I am quite sure that a brown go-kart with the right steer-axis geometry (ie. close to "centrepoint") could have very light manual steering, even with only +/- 45 degrees available at the steering-wheel. After all, normal go-karts with massive Castor + Offset (which demand that the driver physically lifts the inner-rear-wheel at each corner) can manage with this sort of quick steering ratio.

My main concern is what happens when the aero-undertray is added... http://fsae.com/groupee_common/emoticons/icon_smile.gif I have no doubts that drive-on-the-ceiling, 3G-levels of downforce are possible on the Skid-Pad. But this means that without active-aero (that bleeds off DF) the car will literally have over a TON of downforce at the higher FSAE speeds. So now steering loads may become a problem (though not certain).

Furthermore, if, say, active-aero is fitted to keep downforce = weight, then the car should now be travelling 40% faster than normally. The slalom cones are now going past rather quickly. With even more downforce (implying 4,5,6+G?? cornering) the slalom becomes a blur.

Bottom line is that I reckon this high-G performance is possible, but on Autocross style courses things like "steering feel" are not really important. All that the driver needs, and has time for, is to make small, fast, hand movements (eg. see "side-stick" below), and then adjust according to subsequent car behaviour.
~~~~~o0o~~~~~

Mike,

"-Design steering with highest steer ratio you can get away with...
-Reduction of friction in steering system very important
-Structurally stiff suspension important, ...
-Power steering a realistic possibility, ...
-Higher tire pressures can help with reduction of steer effort."

I just wanted to restress some of your above points. Any "Steering/Suspension Guys" currently in FSAE should be particularly concerned with reduction of friction, and increase of stiffness.
~~~~~o0o~~~~~

Just as a BTW, my interest in side-stick steering came from an early involvement in "Pedal Car Racing" (http://www0.sun.ac.za/isr/sites/default/files/2012/yasmin/TrapkarreInfoBookletWeb.pdf). (<- It seems that this is still happening these days, although in my day (mid-1970s) all the races were 6 hours long! Oh, yes, ... you kids sure have it easy these days... http://fsae.com/groupee_common/emoticons/icon_smile.gif)

These Pedal Cars originally had normal steering-wheels, but the benefit of quick driver changes soon led to the almost universal adoption of side-stick steering. The cars+driver are significantly lighter than in FSAE, the speeds are similar (often fast, downhill sections, with right-angle corner at bottom), the cornering Gs are less (but enough to quickly shred the skinny tyres), but overall the side-stick steering, with ~90 degrees lock-to-lock (or less), works very well.

Z

PS. The Tech Rules in that Pedal Car link would be a good example for a "new, improved" FSAE Rules set. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif http://fsae.com/groupee_common/emoticons/icon_biggrin.gif http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Flight909]

There are a interesting article on this available on the FSG website: http://www.formulastudent.de/a...ves-box-of-tricks/1/ (http://www.formulastudent.de/academy/pats-corner/advice-details/article/steves-box-of-tricks/1/)