Drive Shaft (and Steering Column) Angle and Phasing

[Claude Rouelle]

In his Facebook page FSAE Advice and Support, Pat Clarke has posted an interesting video (Thank you Pat!) of the issue of equal - or no equal, that is often the problem - rotational speed between input (differential left or right hub) speed and output (wheel) speed depending the angle between the different parts of the drive shaft.

You can see it also at https://www.youtube.com/watch?v=Idk3BVDVHq4

The issue is similar with the steering column angle between the 3 different parts of the steering column: are the steering wheel speed and the steering rack pinion speed in phase? I have seen to many of such problems on Formula Student cars for example with teams designing a steering column with only one U joint. You need equal opposite angles.

Interesting video. Worth to watch it and learn a bit from it

I have attached a few (useful, I think) additional pictures.

[Drew Price]

That's a great illustration, thanks for sharing!

[MCoach]

The issue is similar with the steering column angle between the 3 different parts of the steering column: are the steering wheel speed and the steering rack pinion speed in phase? I have seen to many of such problems on Formula Student cars for example with teams designing a steering column with only one U joint. You need equal opposite angles.



For driveshafts...absolutely, otherwise vibration is guaranteed and failure is more likely.

However, why is this considered a problem that steering is not absolutely linear?

[Claude Rouelle]

In your opinion?

[NickFavazzo]

Could the use of out-of-phase joints be beneficial if the team desired non-linear steering...

[MCoach]

Could the use of out-of-phase joints be beneficial if the team desired non-linear steering...

The point I'm looking for.
It's been done with several production vehicles and it is not uncommon for industrial vehicles to phase the steering shaft to lower the on-center steering effort.

[Drew Price]

Ahh yes, the industrial vehicle with prodigious and intentional on-center steering "slop!" I have driven many of those.

[MCoach]

Ahh yes, the industrial vehicle with prodigious and intentional on-center steering "slop!" I have driven many of those.

Yep, those ones. Usually the slop is from the recirculating ball box or loose U joints.

[Claude Rouelle]

Nick, Why would you want a nonlinear steering? Claude

[BillCobb]

A couple of thoughts on this subject, but may be a repeat of a previous post somewhere in a thread far, far away.

Steering shafts out of sync, out of phase or out of plane can have a profound affect on anybody's vehicle dynamics for several reasons:

1) If you miss a shift and drop the clutch, a mistake by your powertrain artist can send you into the weeds.

2) If hot-glue is your version of 3D print welding, the out of plane steering forces and moments in your intermediate shaft will cause a failure mode to arise.

3) Frequency response transfer functions which describe yaw velocity and side-slip angle transient responses contain contain TWO numerator terms. Knowledgeable students and professionals ought to know that the complex zero order term describes the effects of steer angle or moment, and the 1st order
complex frequency term describes the effects of steer velocity or steer torque. That being said, a harmonic steer ratio (lumpy steering) can induce several useful as well as un-useful handling characteristics.

A) A nonlinear steering gain can be produced if designed and developed for the specific chassis in which the numerical ratio is 'fast' on-center (3:1) and slows down as you steer (8:1) and vice versa. Since steering wheel rim force is the inverse of this numerically (heavy on-center) the process makes a high tierod load car easier to drive (less work).
This is for the Displacement type driver. A Feeling driver won't like it because the feeling falls off.

B) The impact of steering velocity on car control is probably not in any books because its a Controls Theory lesson, not a bicycle model
demonstration or a Ouija Board paranormal manifestation. To study it seriously requires a transient response simulation not a shim study tool use by the team mechanics.
Then you would see that a lumpy ratio with steer angle increasing but steer velocity decreasing as can happen in a slalom for example)
can cause some very troubling and often mistaken conclusions. Lash or slop being words used to criticise the feelings about the car.
But worry not, you can't make a car go left by turning right, but it will feel like somebody is stealing your inputs. BTW, the parameters
that make up the transfer function numerator terms will tell you what vehicle parameters are the big players in this game.

C) Probably the worst condition that can arise is the placement of the lump(s) off to one 'side' of the steering realm. Driver reactions can
be 'lead' (steer correction) or 'pull' (moment correction). And it feels different left to right.

D) Calculation of total vehicle understeer requires the calculation of the reference steer gain ( based on steering wheel inputs).
So what exactly are the road wheel inputs (angles or moments) when the ratio is not constant? You have to integrate the nonlinear ratio
to get the correct values. BTW: If you ever test a vehicle to get steer versus Ayg data and it looks very asymmetric, the vehicle may
be perfectly symmetric but the steer commands to the vehicle may be the cause of the problem. Same for the response time plots (not so easily fixed).

E) In my professional career, it was possible to teach and train the Vehicle Development engineers to sense the traits of lumpy steering to the extent that a ratio test was not needed for them to detect and
correct it.

F) There have been a few vehicle manufacturers who have designed carlines with intentionally nonlinear steering ratios (either from a
nonlinear gear or special Cardan joint geometries. But, after testing ratios of thousands of cars in my career, the big surprises are that
some take special care to produce a flat and well behaved ratio (careful steering part placement and design for manufacturability), while several others (and these
makers are the ones that naive students, magazine writers, and car snobs drool over) are absolute crap when it comes to attention to details.

G) Given a chassis and tire combo, an engineering design process ought to produce an effective, intentional, buildable, analytic, repeatable
and driveable steering mechanism. You can study the effects with a (transient) simulation, watch in disbelief as the process reveals the
often puzzling frequency response dynamics and have an "Aha" moment.

H) Here's an example of how I have implemented this characteristic into one of several simulations I use. By setting just a few
parameters, you can pretty much describe all the good, bad, and ugly conditions that the Global Feel Good Car Guys have been able to
produce. I tuned in the kind of providence that a FSAE car might have... A little Midnight Oil, some Simulink and a brewsky or two could make a wonderful thesis paper.
1180

[BillCobb]

See ? And this is just a simple SIMULATION of the ISO On-Center (Weave) test. It uses approximate lateral acceleration computed as the
product of yaw velocity and forward speed.1181

Now, it really make not make a hellofa lot of meaning to a FSAE car, but so few of you will be racing professionals and so many may try to
become engineers in some field (Kubota, John Deer, Mahindra, ???) that this kinda of work is the foundation of a hiring, a career, and a retirement.

[Swiftus]

Excluding the actual lash, placing the slowing peaks out at the extremes of steering might actually have the effect of linearizing the relationship between degrees of a turned input and degrees of output because of the sin-connection in the tie rods.

[BillCobb]

Except that the provisions for Ackermann, negative Ackermann or parallel steering makes that suggestion an unlikely possibility. (It's a
mistake for teams not to know which way to go in this case.

Here's a sample (again) from the real world of competitive analysis. BTW: given a database full of the fitting coefficients for these tests, it's pretty easy to know who knows what.
1183

[Claude Rouelle]

Out of the box observation here.

Many race cars, especially single seater should use pro (positive) Ackermann. You know that just looking at their tire models (expect is some specific case like some Michelin racing tires that do require Anti Ackermann).

However they don't for a simple reason: aerodynamics. Negative Ackermann could give significant aerodynamic downforce in corners that will more than compensate the little loss of mechanical grip.

[Swiftus]

@Bill - In the case of a single seater like an FSAE car which has a total of ~180º of steering, a lot of wonky behaviors are possible to implement which would otherwise be wacko in a more standard type of race car.

Putting a single u-joint in the steering shaft doesn't an effect on Ackermann, simply a scaling effect on the driver's feel of the steering - the geometry past the pinion in the rack would be identical in all cases.

It seems to be something which would be much more about driver feel and less about vehicle performance outright. For instance, on a circle track car which always has a 1/4 lock of steering on, one might want a dulled steering ratio when turning left compared to when turning right so they are better able to catch themselves from too much oversteer with some fast countersteer.

I'm thinking useful implementations of varying speed ratios, not what is appropriate for most situations.