Steering System

[Mostafa Habib]

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Hi everyone ,
I have a question regarding the steering system
I need to know whether this angle affects the system and how ?
And how to calculate the proper angle ?
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Mostafa Habib
Zagazig University - Egypt
ZUFS Team

[MCoach]

Yes, it looks like this:

http://en.wikipedia.org/wiki/Universal_joint#/media/File:UJoint2.png

Now a question back to you. What is considered proper with respect to your goals? Is proper 0% deviation for in/out angles desirable or is a non-linear gain preferential for your application?

Source:
http://en.wikipedia.org/wiki/Universal_joint#Equation_of_motion

[Claude Rouelle]

Mostafa,

1. You U joint or Constant Velocity (CV joint) catalog should give you the maximum angle you can use.

2. If you want to avoid a phase shift between your steering wheel angle and you wheel steering angle (that would send a wrong car behavior signal to the driver) you need TWO U joints. Google is your best friend for that kind of search. Here is an example

https://www.google.com/search?q=steering+wheel+angle+phase+with+wheel+ste ering+angle&espv=2&biw=1920&bih=947&tbm=isch&tbo=u&source=univ&sa=X&ei=J4EiVeHPFoXXsAXL3oFo&ved=0CB0QsAQ&dpr=1#imgdii=_&imgrc=OQY4OLbiwkF0nM%253A%3BMTG0gAyiZH7sbM%3Bhttp% 253A%252F%252Fimg.photobucket.com%252Falbums%252Fv 68%252Fmotorbreath460%252FGeometry.jpg%3Bhttp%253A %252F%252Fforums.vwvortex.com%252Fshowthread.php%2 53F4389788-Diagnosing-steering-U-joint-wear-on-steering-shaft%3B493%3B331

3. Do not fall in the trap of many FSAE / FS teams which do not design, manufacture and install at least one steering column support between the steering wheel and the steering rack.

[ritwikdas18]

How to factor in rolling resistance in steering design? Is it related to effect of front tire steering angle and slip angle on increased rolling resistance. Can someone explain it in detail?

[Claude Rouelle]

Ritwik Das,

The answer to your question is in 2 or 3 posts that were discussed on this forum in the last few weeks.

Try to make your own search before asking such kind of questions

Claude

[BillCobb]

Since this is one of my favorite subjects, excuse me while I spew on it for a while.

Nonlinear overall steering ratio characteristics can be a troublesome situation for vehicle dynamics processes. To be correct, we really mean that a 'steering ratio' is the first derivative of a function relating the average of the roadwheels angle turning to steering wheel turning. Measurements of this function are done routinely for virtually all handling test vehicles ever done. This information is needed to calculate the understeer of a vehicle given test results containing steering wheel and lateral acceleration data, (among several other important channels).

One generally thinks of the 'steer ratio' as the quotient of two simple protractor readings, but this is ridiculous now days because of the realities of the findings. First of all, the yaw dynamics of a vehicle are activated by not only steering angle but also steering angular velocity. So, it is possible to concoct an input relationship to a steering gear which has increasing steer angle but decreasing steer velocity. In cases like this, drivers, even professionals, have been known to exit a car and check for 'lash' in the steering system, because that's what it feels like. So, considerable energy and time is spent designing and building (in the vehicle assembly process) a system which avoids variation in the overall ratio.

Back to the Cardan issues: Most production vehicles have a double Cardan steering intermediate shaft between the column and the steering gear input shaft. Done mainly for safety and driver comfort issues, the double jointed shaft still can be screwed up from its (usual) constant velocity specification. That's because in order to be a true constant velocity, the input and output shaft must be co-axial (they must be parallel in all views). If not (generally because of assembly carelessness, you get a nonlinear ratio, which we call "lumpy Steering". To fix this problem, you can go to a CV type joint, either the double cross used in rear drive prop shafts, or the multi-ball joints used (for example in front wheel drive prop shafts). For example: a CV joint from a Honda or Yamaha ATV can be made into an excellent steering shaft if you address lash.

On the other hand, several vehicle manufacturers have deliberately produced a nonlinear steering ratio to change the feel of their cars. One that I worked on produced a high numeric ratio straightahead that got lower as you steered in order to make up for the loss of steering gain due to increasing limit understeer and to create a form of "Valley Feel". Another manufacturer consistently produces a low numeric ratio on-center that increased as you steered more. Keep in mind that steering effort torque effects are the opposite of the ratio, so the quick ratio getting slower produces an decreasing steering feel signal to the driver. Either method is directed to the type of driver: displacement command or effort command. (BTW: an evaluation driver that can not tell the difference between a car with these differences installed in it has no place in a vehicle development program).

Here are plots of typical overall steer ratio characteristics I've tried to describe. All three are "10:1 Boxes" as we say. A simple test can be run using steer angle and roadwheel angle transducers with the steered wheels on grease plates or air bearings. You run the test and process the data. I have a function in Matlab that describes the full effect which you see here. This is installed as a feature in a handling simulation I routinely use. Producing the road wheel angles from such a function requires angular integration of this function, chaps ! Keep in mind that there are also nonlinear steering gear boxes which give additional complexity beside that of the steering shaft effect.

Now, use of a single Cardan type joint can be used to improve your FSAE car if done properly. Now you have to analyze and decide what's best for your team.

[Tim.Wright]

There is at least one manufacturer that I know of who don't bother centering the cardan error about 0deg...

[Mostafa Habib]

Coach ,
Thank you for your response
Let me say something
This is always the problem , i mean every question faced by numbers of questions
What about if i have little or no experience and so i came to here to find an answer to what i found it difficult.
What i want to say is i cant find a specific answer for ur question so let me search about it to find the proper design for me

[Mostafa Habib]

Cloude
Thank you about ur notes
but about the 3rd one i cant understand what do you mean

[Claude Rouelle]

Mostafa,

You cannot NOT have a steering column support between the steering column and the steering rack if the steering column is in more than one part. When you will be moving the steering wheel you will see some of the steering column part moving up and down left and right. That woud produce an elasticity that would make the diver really uncomfortable and apprehensive. You need to avoid this by putting a some sort of bracket that holds a spherical joint or a rod end in which the steering column will go through.

Get it?

Be careful that this bracket does not compromise the cockpit template.

[Claude Rouelle]

To elaborate on Bill Cobb (excellent) post this is simple test you can do on your own FS / FSAE car.


Turn the steering wheel 45 degrees and measure the steering rack movement. Make a note of it. Try 45 degrees of steering angle more and look at the steering rack displacement again. Are the numbers the same? If not is it on purpose? If not where does the discrepancy come from? Is it from kinematics design, wrong steering column parts assembly or is there compliance?

Try yo make smaller steering angle and bigger number of increments by measuring the angle accurately. The digital inclinometer won't be good enough unless you mathematically take into account the steering wheel inclination angle. I have seen some guys "stealing" the disk that engine builders usually put on their crankshaft while they assemble their engine: that can be useful for accurate measurement of the steering wheel angle

Of course the steering wheel angle (deg) / steering rack movement (mm) ratio is not the steering ratio. The toe link angle Vs the chassis longitudinal axis (and the variation of this angle in steering), the Ackermann, caster angle and king pin angles and trails etc.. play a role too in the steering ratio. But starting with the steering wheel angle (deg) / steering rack movement (mm) ratio is a good beginning.

[DougMilliken]

C-THRU (brand name) used to make a low priced 10" full circle protractor, in clear plastic --
I have a couple of these, bought long ago to make quick steering wheel angle measurements.

Couldn't find the full C-THRU catalog online, maybe someone has better google skills?
I did find this low cost 8" protractor which should be OK --
http://www.artistsupplysource.com/product/29489/alvin-8-circular-protractor-0-09-thick-clear-plastic/

Amusingly google images turned up a full circle protractor in Grads -- 400 divisions to the circle!

[BillCobb]

Put an I-Pad Air-2 under each wheel (or spindle) and a Smart Phone on the steering wheel rim. Couple them using Screen Mirroring or WI-FI Direct and App them into a test processing procedure.

"Welcome to the New World, Captain Ramius" ....

[DougMilliken]

"Welcome to the New World, Captain Ramius" ....
Or do it the quick way with a jury-rigged pointer (I spring clamp onto the sun visor on street cars),
full circle protractor taped on the steering wheel, and a couple of layers of cardboard under
the tires for turn-plates (dust in between the two pieces of cardboard is an OK solid lubricant).
Or use real turn plates (Weaver plates) if you have them.

Depending on steering ratio, a point at every 10 or 20 degrees of steering wheel angle is enough
to give reasonable plots of individual front wheel steering ratios. Make sure to run up and down to get
the hysteresis.

[rwstevens59]

Doug Milliken,

Might I suggest the large diameter degree wheels used by engine builders. Usually made from aluminum plate.

Sorry, BillCobb, some of the old caveman leopard skins still work and are real simple.

Ralph

[BillCobb]

Doug Milliken,

Might I suggest the large diameter degree wheels used by engine builders. Usually made from aluminum plate.

Sorry, BillCobb, some of the old caveman leopard skins still work and are real simple.

Ralph

Sure these methods will help you verify that you are close. In fact, just measure the quotient of steer arm length to pinion radius. That's your 'steer ratio'.

But methods using protractors probably won't help you diagnose the secondary Cardan joint angle error. And THAT'S what the OP seemed to be asking about. Will your method help you understand why the car feels different turning left than right? Or, will you hunt (forever) for a tire conicity or residual aligning torque problem or perform unneeded 4 wheel alignment checks or buy expensive steering gears and joints to fix a "lash" problem that won't go away?

[{'Good'}, {'fast'}, {'cheap'}]. You only get to pick two.

[rwstevens59]

Maybe pertinent, maybe not.
https://m.youtube.com/#/watch?list=FLUE9S-F6SnwCVdDDybIifTA&v=gmV4qwLfOMY

[Claude Rouelle]

Doug,

"Or use real turn plates (Weaver plates) if you have them."

Nope. Been there, done that. On a "normal" car (race or passenger car of 1000 Kg or more) I have seen the tire slipping in rotation compared to the so called low resistance turning plate. So imagine with a light FSAE / FS car....

"Make sure to run up and down to get the hysteresis." Good point!

Claude

[Claude Rouelle]

Rwstevens59,

What is the youtube link for a regular computer, not a mobile phone?

[rwstevens59]

Claude Rouelle,
https://www.youtube.com/watch?v=gmV4qwLfOMY

[rwstevens59]

BillCobb,

I believe the YouTube video, while a gross exaggeration of the variable velocity effect, does show your reference of 'lumpy steering'. I hope that is correct and it does demonstrate why it would be hard to detect given only partial revolution of the steering wheel and a much lower angular velocity.

Ralph

[DougMilliken]

https://www.youtube.com/watch?v=gmV4qwLfOMY
Good find, nice demonstration with the driveshaft.

Claude -- we had ancient Snap-On Weaver plates. I had to flush the dirt out of them and re-lube with a bit
of light oil. Then they worked fine, we could go a few cycles left and right to "settle" the hysteresis and after
that the data repeated quite well. I don't remember (was a long time ago), but I think we made our own
steer angle measurement device that was better than the crude pointers built into the plates.

Bill -- of course the method I described is fast and cheap. It's not going to find small Cardan joint angle errors but the angles shown in the sketch at the start of this thread were pretty big. A fish scale on the steering wheel isn't going to set any records for steering torque accuracy either, but it was plenty good enough to show that a certain fancy sports car had asymmetric boost -- iirc, the peak steering force at the rim was ~7 lbs in one direction and ~11 lbs the other. I only had about 15 minutes with that car (part of a magazine test), and no instrumentation budget.

If we are going to model a car in detail we go to a K&C rig and get good data, and then we write a big check.

[rwstevens59]

Poor mans turn plates.
http://www.arkansaspontiacs.org/techstories/turnplates/turnplates.html

[DougMilliken]

I don't like these plexi turn plates, they have a fixed axis of rotation -- which is hard (impossible) to align with the kingpin. Weaver plates I'm familiar with are free to rotate and also translate (X & Y motion).

[rwstevens59]

DougMilliken,

Agreed. But, with some thought they might be improved to allow some lateral movement. Good Weaver plates or equivalent are quite expensive for the amateur or occasional use.

Ralph

[Claude Rouelle]

Ralph,

EXCELLENT VIDEO! Thank you!!!

Not related at all but this also an excellent video, this one about differential. Wish I had youtube when I was trying to understand vehicle dynamics and vehicle components...

https://www.youtube.com/watch?v=yYAw79386WI

I DO like this kind of video that demystifies apparently incredibly complex systems.

Claude

[rwstevens59]

Claude Rouelle,

Ok, I'm hooked. Found several more good videos of the same vintage listed after watching the one you posted.

Being an old tool & model maker you will appreciate my affection for old school hard models. Oh, I have embraced our digital tools but there is still a lot to be said for something you can hold and feel the forces and motions.

Thanks for sharing.

Ralph