Variable Steering Ratio & steering arm forces

[Kealan O Carroll]

I'm currently working on some design concepts for cable steeringhttp://fsae.com/groupee_common/emoticons/icon_smile.gif

The first problem I have is with the FMEA. One of the Failure Modes will obviously be a cable failure, i'll include a nice beefy FOS into the system to help eliminate this, but the problem is I don't know the baseline force on the cable / tie rod. It's not the castoring force from trail i'm worried about, its stuff like hitting a curb mid corner / landing a wheel into a pot hole etc...Any ideas on where to start to work out this kind of force ?

Secondly, The geometry is made so we have zero bump steer and theoretically perfect ackerman from lock to lock. Now I could introduce a variable steering ratio, relatively easily, but is it really a good thing ?
I was originally thinking it could be kept nice and low for low steering inputs, to keep it steady on the straights, but having a rack that speeds up as the steering angle increases changes the whole feel of the car mid-corner, where we're fighting to maximise grip. It could be argued that drivers should simply get used to how it feels I guess, but for marketing purposes it would probably be seen as something that would make the racing community see it as an unstable / unpredictable car mid-corner...

What do ye think of this ?

Second year team from Ireland so we're still in the youthful stages!

[EPMPaul]

If tou want to figure out if a variable steering ratio is worth it, you d probably need to simulate it and for that you ll need some tire data whch you can get from the ttc and start using that. then you re going to have a damn good idea of what your driver wants. That's the hard bit. What gives good feel to the car isn t super easy to do or to figure out with just data output. usually takes a seriously good ass sensor to figure that one out and define it in something other than a mumbo jumbo of various weird anecdotes. good place to start looking concerning variable steering ratios is bishop engineering they have a paper hanging around on the internet. then you re on your own... pretty sure you could find a paper somewhere where someone had the same idea.

for the heavy loadcase, a simple search for pat s corner should be sufficient o give you a good idea

last thing you need to ask yourself if you're a second year team is "is it really worth investigating with the ressources you have or can potentially obtain?".. a lot of excellent teams get away with linear ratios and steering racks
it s not the best engineering validation but benchmarking can be used as a sanity check

[Z]

Kealan,

Firstly, cable steering has a lot of potential benefits, and is well worth considering.

IMO, compared with most FSAE systems, it is (potentially!) simpler, lighter, cheaper, faster to build, can have less friction and backlash, and easily gives variable rates for each wheel (= easily adjusted "ackermann"). However, you only get all these benefits if the system is properly integrated into the rest of the car (ie. holistic design, rather than tacking something on to a pre-existing dog's breakfast... http://fsae.com/groupee_common/emoticons/icon_smile.gif).

On the down-side, you have very little to copy (= no "benchmarks"), so you have to find all the pitfalls yourself.
~~~o0o~~~

Calculating loads on the cables is not difficult. You identify any forces acting on the wheel assembly that are offset from the steer-axis. These can be in X, Y, or Z directions, and include gyroscopic couples.

As a rough baseline, there are many FSAE cars that have steering tie-rod forces about equal to the max horizontal force at the tyreprint (eg. a car with "steering offset" ~= "steer-arm length" has tie-rod force ~= braking Fx force). But as you said, the hard part is estimating the size of forces due to bumps, potholes, etc.

So I suggest you simply make the cables a bit stronger than the rest of the suspension (maybe x2, given the experimental nature?). So, for whatever loading condition you can imagine, you calculate that the wishbone will buckle, or BJ bolt will shear, etc., at half of the cable failure load. If all goes well, then fit thinner cables next year.

In general, it is a good idea to have all parts fail at about the same load, except for a particularly cheap part, the "fuse", that fails at a slightly lower load.
~~~o0o~~~

Ease of "variable ratio" is a big advantage of cables. This is a real performance benefit, because "linear steer" (= parallel steer, or zero ackermann) is a BIG DISADVANTAGE in FSAE conditions.

A good simulation will give you a good idea of potential performance gains from the right amount of non-linearity (= +/- ackermann) in the steering (ie. how much faster the car goes through corners). But I doubt you will learn much about driver "feel". However, the ease of swapping cable pulleys makes it easy to test different variable ratio set-ups. Then just ask the driver how it feels...

I assume you are thinking of some sort of easy to machine egg-shaped pulleys? By comparison, variable ratio R&Ps, which require difficult to machine teeth profiles, are much harder to do.

For a young team, I suggest you think about a simple, robust system with "ideal" ackermann for the first year. Then next year test with more extreme variable ratios.

Z

[Jay Lawrence]

We had a Bishop variable rack in 2007 I believe. We ended up going away from it the next year (a. because Bishop sponsorship was no longer on the cards and b. because it couldn't be justified).

In my opinion, it is hard to justify something that provides a noticable difference during steady state cornering when (at FSAE-A at least) there aren't really any steady state corners. At least that's how it feels from the seat. Given the tightness of our tracks, you find yourself quite busy behind the wheel and in that situation you want something that feels light and predictable. But as Z said, you can make components for both and test them.

My 2c

[PatClarke]

Kealan,

I suggest you request clearance from the Rules Committee before you proceed too far with design of a cable steering system.
There have been many requests in the past and all have been rejected, the latest being from a very high profile team last year!

Pat

[TMichaels]

Pat,
a clarification has been made in the FSAE2013 rules set allowing cable steerings, see T6.5.7

[PatClarke]

Thanks Tobi.

I must read the new rules properly =]

Pat

[Kealan O Carroll]

Originally posted by TMichaels:
Pat,
a clarification has been made in the FSAE2013 rules set allowing cable steerings, see T6.5.7

Submission and approval of an FMEA are required though http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z, the advantages that I see which made me look into cable steering are:

1) Lighter
2) Cheaper
3) Smaller packaging than Steering rack / column
4) Adjustable Ratios via pulley swaps
5) Zero Bump Steer
6) Potential for Zero degrees slop / freeplay

The main disadvantage im facing is no benchmarking alright !
I did up an excel sheet which worked out the steering angle, wheel angle and required "Tie Rod" Movement per steering wheel input (For perfect ackerman) then used the table of results to model a Cam shaped pulley in SolidWorks.

If you imagine the pulleys on the steering column as half cylinders, all of the pulleys will only take up about R=50mm x Height = 100mm

I Like the idea of overdesigning the cables so they're no longer the weakest link.
I'll FEA the suspension system to failure, then do some tensile testing on cables and see what size we need

Pat, I'm working on an FMEA at the moment for it, I'f that is submitted and approved I presume the system is then deemed OK ? Provided it passes safety scrutiny ?

I suppose the big question is:
Do I design the baseline cams for perfect ackerman with a constant steering wheel : wheel angle ratio,
or
Perfect ackerman with a variable steering wheel : wheel ratio ?
It depends on driver feedback but that can't be simulated and we don't have any test data...

[AxelRipper]

I suppose the big question is:
Do I design the baseline cams for perfect ackerman with a constant steering wheel : wheel angle ratio,
or
Perfect ackerman with a variable steering wheel : wheel ratio ?
It depends on driver feedback but that can't be simulated and we don't have any test data...

And no one else has any test data either. There's the biggest thing with so-called "innovation." You're breaking relatively new ground here. It sounds like a good idea, but you truly won't know unless you test it.

As one of my professors told me: "You have to make stuff to make stuff."

Just quick thinking about it while reading through this thread here first thing in the morning, seems as though if designed right you'll have only pure tension in the steer cables, which makes the sizing of them easy to do (relatively). The biggest issue I can think of is maintaining constant tension in the cables through the entire motion of the suspension. If you can get around that, as Z pointed out, the ability to change ratio or have variable ratio is relatively easy.

[Kealan O Carroll]

Converting the excel program into solidworks to avoid having to manually plot every point on the pulley...im on dimension 1 of 90

=( ( SIN ( ( "D4@Sketch1" / 2 ) * ( ( 3.141 ) / 180 ) ) ) * ( "D2@Sketch1" ) ) / ( SIN ( ( ( 3.141 ) - ( ( "D4@Sketch1" / 2 ) * ( ( 3.141 ) / 180 ) ) ) / 2 ) )-( ( SIN ( ( "D1@Sketch1" / 2 ) * ( ( 3.141 ) / 180 ) ) ) * ( "D2@Sketch1" ) ) / ( SIN ( ( ( 3.141 ) - ( ( "D1@Sketch1" / 2 ) * ( ( 3.141 ) / 180 ) ) ) / 2 ) )

Is there any way of getting solid works to recognise a pattern and continue it ?
Much like grabbing the bottom corner of an excel cell and dragging it down a column ?