Automobile Steering Systems

You have undertaken an ambitious project.

i have seen that in normal steering systems, the axes of rotation of all wheels shd meet at a single point

That is only true in a simplified theory that does not account for the slip angles at which each tire operates under the design cornering load. In cornering, the outside wheel is more heavily loaded, so operates at a greater slip angle. Then the Ackermen affect geometry no longer applies.

However, if you are interested in a low speed vehicle, and are implementing 4 wheel steering to (for example) require less clearance from the curb in city driving, the the Ackerman effects should be taken into account.

If your intention is to improve handling in a more general sense, then you would do well to buy one of the existing four wheel steering cars (such as an old Honda Prelude) and experiment with it. When four-wheel steering has been implemented it is usually implemented in two modes. In one (for highway passing, for example) both axles steer in parallel. In the other, they steer in opposite directions, so that the front and rear wheels follow nearly the same path across the ground (whereas in standard steering, the rear wheels travel inside the fronts.) As you can imagine, this requires sophisticated controls. The Honda system was generally well-received by the motoring press, but it was not a hot seller.

You should be aware that for a modern car, four wheel steering could be a confounding issue for stability control systems.

But for low-speed use, agricultural use, industrial vehicles, etc, then yes, it makes sense to have all wheels following arcs with a common center. You would need to sketch out the actual angles for the vehicle in question. Whether or not you will have to modify front steering angles will depend upon existing geometry.

ThanQ blink...

u said about those slip angles... can i know what all factors cause slip angle??..

are there anyways of controlling or reducing slip angles at various steer angles???

in fact how does the wheel come back to its normal ackerman mode after the cornering is over???...

This article explains slip angle as it relates to tires.

in fact how does the wheel come back to its normal ackerman mode after the cornering is over???...

In two wheel steering, when cornering is over, (or the cornering g rate is reduced to near zero), the the slip angle is near zero for both the inside and outside wheel. Then in a very slow turn, the tire tread and wheels are aligned and Ackerman effect keeps the tires from "scrubbing" -- they both travel around the same center. There is no mechanical change in mode -- it has to do with tire distortion under cornering load.

To go from just beginning to understand the basics of steering to a safe functioning 4 wheel steering system is a major undertaking. For a manufacturer working with steering specialists, it costs many millions of dollars. I'd suggest buying a used four wheel steering car, and experimenting with it if you are so inclined. You might not like the response.

What problem are you hoping to solve by going to four wheel steering?

well... i have no plans to buy any vehicle with 4 wheel steering as of now... just wish to work on such a topic to know more about it and later if it proves to be worthy enough.. i d like to use it in my car...

and as such i don mean to solve any problem with 4 WS... just wish to expand my knowledge base... i had no idea as to where and how to start... but now i m getting informative concepts to think about in my leisure time... atleast that would be a good use of my time!

and thank u for that article link.. :)

A four corners steering system is not uncommon now many construction machines and some farm machinery has used it over the years as well. I have had a fair amount time operating some machines that had this form of steering and at low speeds and in tight quarters it is very effective and practical.

But there is a problem with developing the necessary mental perceptions of what the back end of the machine does when turning that are not familiar or in any way similar to driving a passenger vehicle or machine with only front wheel steering. You run over and into a lot of stuff with the back end while turning sharp. When you turn left the back end swings out to the right and vice versa. Imagine driving your vehicle back wards in tight traffic conditions or at moderate speeds. Your whole perception of how the vehicle handles and how you need to turn and make the necessary obstacle avoidance maneuvers is considerably different.

Chevy had this option on their pickups a number of years ago and apparently it caused a considerable amount of rear fender and property damage due to the rear end swing out effect when turning sharp so most people just turned it off.

I never had the opportunity to drive a passenger vehicle equipped with the four wheel steering but the few who have done it said at highway speeds the crab steer effect when passing can give an uncomfortable sensation that feels like you are sliding across the road at times. I understand that feeling from driving a few construction machines in road gear and it does not feel like driving a car and it does tend to cause unintentional over steer and driver induced stability issues. I suspect that because of the different handling feeling and perceptions plus the added costs involved in putting it on a vehicle is likely why its not popular. There is little gained in its application on a passenger vehicle.

Too many people now have poor driving skills because they simply can not mentally picture how their vehicle handles and moves even though they can see where the front of the vehicle is going when they turn.

Thank u for ur response!!

Even i had the same fear of this weird driving characteristic of a 4 wheel steered vehicle... but hope to cope up with it in a few hours of driving... actually i wish to use this system in my car for driving along hill routes.. where hair-pin bends are common.. anyways i ll post more threads when i encounter the next doubt!

with regards

Rahul

Why?

I regularly drive hill routes with hairpins. I always use a 2 wheel steered vehicle and that does fine.

It seems to me that you are overcomplicating things and I don't see the benefit that you are going to get our of it.

well commentator... thats perhaps right.. i m complicating things... because i just wish to get to learn more from what i do... i have not started my work anyways... i have another option of implementing such a system in vehicles that wont be taken to roads.. just for the purpose of knowing in depth the actual technology behind and how it responds to various inputs we give..

:)

Compared to a construction site a hair pin turn on a mountain is an open parking lot. To take full advantage of four wheel steering you need to regularly be in situations where you need to turn around in an area less than twice the size of your vehicle.

IE, do a full 180 degree turn around in the middle of a two lane paved road without stopping or backing up or dropping off the edge of the pavement itself. Or to drive into your garage going forward and then drive out of your garage going forward.

As far as learning the feel I have many hours in those types of four wheel machines and I still tend to run over stuff at times and I am actively thinking about where I am going.

You can spend your time and money building one butI rather doubt you will ever get past the DOT inspection to legally have it on the road. They are fussy about modified steering systems.

Plus when you do eventually get into an accident with it your insurance company will look at your vehicle as "heavily modified" if it does not have the legal DOT approval and you are screwed regardless of who's fault it was.

dear tcmtech,

this one was really informative.. as u said i also intend to adopt 4 wheel steering only occasionally.. such as to maneouvre in narrow lanes.. turn inside them without sweating myself out... so can this be added as an option to the existing system of the vehicle.. such that by pulling a lever or at the press of a button the 4 wheel system is engaged.. in the required mode.. ie same direction steer or opposite direction steer... one problem which (at this stage) i feel is the uncertainity of the steering position at which we engage the system... can u tell me where i shd refer for such topics... or if u can directly help me out i ll be really happy...

also do u feel that i m wasting my time thinking about such an issue?

See if you can rent or borrow one of the vehicles* which offered such steering, and drive some of those routes. Unless you see great advantages over normal two-wheel steering, and you still see lots of room for improvement, consider putting your efforts elesewhere. These companies spent millions of dollars designing and testing the systems; it will NOT be easy to obtain better results on a less than unlimited budget - and it may not be physically possible. If the vehicle DOES help, consider buying it, and thinking about some other problem to solve - put your abilities where they will do the most good for yourself and for the world. And if you have questions about those problems, please come back here to CR4!

* see http://en.wikipedia.org/wiki/Steering#Production_cars_with_active_four_wheel_steering

thanQ for replying..

actually i just thought about his idea during my leisure time.. i just want to do something creative and something benefecial to the industry... steering being my area of prime interest i just thought about this one... though i try drawing those wheel sketches of a 4 wheel steering n 2 wheel steering.. i do agree with wat u say... and i ll be happy to know from u which are the areas i should probably focus on to bring in something new n good... can u help me by telling few topics??... related with what r the currently faced problems of auto industry.. etc.,

I fully understand the "thinking about ideas in leisure time" - I do it a lot! And some work out great, while others fail completely, or prove to be already known by others As an engineer, I think about problem solving as part of my work, too, with much the same mix of results. I get enough of the right to keep myself employed! But I'm not in the automotive business, nor have I ever worked in that industry. I've had an interest in cars from very early, though, and have had close relatives who WERE in that field (brother, cousin, two uncles). It gives me some knowledge as to what has been done, but isn't much help on knowing what they're trying to solve right now. And because I only "know" you through this forum, I couldn't begin to guess what should be on a list that would help you figure out where to apply yourself. I may not be able to do that for someone I've worked with for years, though sometimes I can make good guesses.

But if you're like me, your ideas will come in a variety of fields, and the good ones can be unexpected, and not the result of deliberate effort to solve a particular problem. Trying to provide a list of where to look, or what to do, is not really likely to help you. More general advice is to remain observant: you will see problems every day, and some will be frustrating but apparently insoluble, while others will almost TELL you how to develop a solution. Many will go into your personal knowledge bank, and pop up from time to time, while others will suddenly return to your conscious thought with a possible pathway accompanying them.

When the difficulty you observe is of that category where you almost immediately see a solution, ACT upon it! Sketch it, write it down, build a quick-&-dirty prototype, whatever is appropriate. Get a couple of these, and you may have the income to let you think on your own time and be an inventor - or you may simply need to continue observing everything around you. I've gotten patents in widely different fields, simply because I was exposed to problems within the industry where I was working at a particular time. I've had several others in still other arenas, which got as far as having patents applied for, but withdrawn when another company was discovered to have made application a bit earlier. It happens. Still others got started, but the company went bankrupt, and they were dropped. Again, it happens. So far, none have made me enough money to retire; in part that's because ones developed while working for a corporation meant that the company get the profit, and I got an extremely modest award. But I knew that those were the conditions when I signed on; no regrets.

Do NOT confine your problem-finding to what you do for work, or for recreation, or that you read about just because you're interested. I've sent off sketches & photos of a prototype this past week to a patent attorney for something I thought up after seeing someone struggling to perform his task as I was walking by; I'll be making a prototype of another idea this weekend for something that I noticed while sitting upon a toilet. This one is [probably] not patentable, but it may be salable to some large-volume manufacturers. And one, or both, may already have been invented - but so far I can't see any products on the market, nor any evidence that anyone else has tried, so....

To repeat earlier advice: when you've got an idea or concept, but not enough knowledge to continue developing it, come to CR4 and ask questions!

Nice to hear about u... even i do think about a lot of things... but have not sketched those ...being a UG student i just have a theoretical know how of things.. considerable practical exposure too is there... but hope to take those leisure time thoughts more seriously here after... after all such thoughts become tomorrow's technologies!!!

As u said this is the right place to be in if one is in need of knowledge... I will keep scrolling these pages at CR4 again n again...

Cheers!!!

GA. Great advice, Ron!

NO, you are trying to mix apples and oranges. Four wheel steering typically requires that the rear wheels respond differently from the front ones. For instance, you might need for the rear wheels to actually steer opposite the front wheels to achieve the smallest turning radius, or you could have the rear wheels turn the same direction as the front and actually move like a crab walks. In either case, the movement between the front will be at differing degrees. What you are trying to accomplish is make a very basic mechanical action behave like something that requires special engineering.

TMF

yes TMF,

i m aware that in a 4 wheel steering tha angle turned by the rear wheels is much less compared with the front wheels..whether in tha same direction or not... actually my question was whether such a system could be implemented without disturbing the present angle of turn at the front wheels... i have a doubt if the outer front wheel angle should be reduced from the previous maximum...

and i have not started on this project... i just wish to acquire knowledge on this issue from the learned.. and later see if its worth doing.

good day TMF.